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This page provides information on <span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentrations'''</span> of total total suspended solids (TSS) in urban stormwater runoff. For a discussion of TSS in stormwater runoff, including information on sources, fate, and water quality impacts, see [[Total Suspended Solids (TSS) in stormwater]].
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This page provides information on <span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentrations'''</span> of total <span title="small solid particles which remain in suspension in water as a colloid or due to the motion of the water,suspended solids can be removed by the sedimentation because of their comparatively large size."> '''suspended solids'''</span> (TSS) in urban stormwater runoff. For a discussion of TSS in stormwater runoff, including information on sources, fate, and water quality impacts, see [[Total Suspended Solids (TSS) in stormwater]].
  
 
==Objective==
 
==Objective==
<span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''Event mean concentration'''</span> (EMCs) are used in several models for predicting water quality impacts from stormwater runoff and stormwater treatment practices. This page provides summary information that can be used for selecting appropriate EMCs. For a discussion of event mean concentrations, see [[Stormwater pollutant concentrations and event mean concentrations]]. For a discussion of event mean concentrations, see [[Stormwater pollutant concentrations and event mean concentrations]].
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<span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''Event mean concentrations'''</span> (emcs) are used in [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_models,_calculators_and_modeling models] for predicting water quality impacts from stormwater runoff and stormwater <span title="a practice, device or structure designed to treat stormwater runoff (i.e. remove pollutants from stormwater). These include structural practices such as rain gardens, and non-structural practices such as street sweeping."> '''treatment practices'''</span> or <span title="any practice that reduces, eliminates, or prevents pollution at its source"> '''pollution prevention'''</span> practices. Pollutant loads, which are typically used to assess water quality impacts, including establishing <span title="the amount of a pollutant from both point and nonpoint sources that a waterbody can receive and still meet water quality standards"> [https://stormwater.pca.state.mn.us/index.php?title=Total_Maximum_Daily_Loads_(TMDLs) '''total maximum daily loads''']</span> (TMDLs), are a function of pollutant concentration and volume of runoff. It is therefore important to accurately determine appropriate event mean concentrations when assessing water quality impacts from stormwater runoff.
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This page provides summary information that can be used for selecting or calculating appropriate emcs for total suspended solids.
  
 
{{alert|Many factors affect total suspended solids concentrations in stormwater. If you are unfamiliar with the concept of event mean concentrations, We recommend you first read [[Stormwater pollutant concentrations and event mean concentrations]]|alert-info}}
 
{{alert|Many factors affect total suspended solids concentrations in stormwater. If you are unfamiliar with the concept of event mean concentrations, We recommend you first read [[Stormwater pollutant concentrations and event mean concentrations]]|alert-info}}
  
 
==Methodology==
 
==Methodology==
We conducted a review of literature to develop the EMCs shown on this page. Nearly all studies provided summary information; we therefore did not analyze raw data with the exception of data from [https://www.capitolregionwd.org/monitoring-research/ Capitol Region Watershed District] (see discussion below). We compiled the summary information into a spreadsheet and conducted simple statistical analysis of the information.
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We conducted a review of literature to develop the EMCs shown on this page. Nearly all studies provided summary information; we therefore did not analyze raw data with the exception of data from [https://www.capitolregionwd.org/monitoring-research/ Capitol Region Watershed District] (see discussion below) and the [http://www.bmpdatabase.org/nsqd.html National Stormwater Quality Database]. We compiled the summary information into a spreadsheet and conducted simple statistical analysis of the information.
  
 
Data from the following studies were used to generate emcs for total <span title="small solid particles which remain in suspension in water as a colloid or due to the motion of the water,suspended solids can be removed by the sedimentation because of their comparatively large size."> '''suspended solids'''</span>.
 
Data from the following studies were used to generate emcs for total <span title="small solid particles which remain in suspension in water as a colloid or due to the motion of the water,suspended solids can be removed by the sedimentation because of their comparatively large size."> '''suspended solids'''</span>.
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*[https://erams.com/co-stormwater-center/wp-content/uploads/2017/09/Nutrient_Sources_Literature_Review-2017-6-5RefUpdate.pdf Nutrient Sources in Urban Areas – A Literature Review]. Report summarizing multiple studies in Colorado. Land uses include residential, mixed, commercial, and open space.
 
*[https://erams.com/co-stormwater-center/wp-content/uploads/2017/09/Nutrient_Sources_Literature_Review-2017-6-5RefUpdate.pdf Nutrient Sources in Urban Areas – A Literature Review]. Report summarizing multiple studies in Colorado. Land uses include residential, mixed, commercial, and open space.
 
*[https://link.springer.com/article/10.1007/s12665-014-3682-y Contribution of surface runoff from forested areas to the chemistry of a through-flow lake]. Forested land use in Poland.
 
*[https://link.springer.com/article/10.1007/s12665-014-3682-y Contribution of surface runoff from forested areas to the chemistry of a through-flow lake]. Forested land use in Poland.
 +
*[https://www.sciencedirect.com/science/article/pii/S004313540100375X Brezonik and Stadelman], (2002). Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentrations from watersheds in the Twin Cities metropolitan area, Minnesota, USA.
 +
 +
In addition to the above sources, we compiled water quality monitoring data from 10 storm sewer outfalls in the [https://www.capitolregionwd.org/monitoring-research/ Capitol Region Watershed] in Minnesota. The data period for each outlet varied but generally spanned the period from about 2005 to 2019. The following information was compiled for each monitoring location.
 +
*Date
 +
*Total suspended solids in mg/L.
 +
*Sample type, which included runoff samples during precipitation events, snowmelt samples, and <span title="Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of streamflow delayed shallow subsurface flow".> '''baseflow'''</span> samples for those locations where groundwater contributed to flow.
 +
 +
We also downloaded the [http://www.bmpdatabase.org/nsqd.html 2015 National Stormwater Quality Database]. The dataset includes information from across the U.S. We selected only data from Region 1, which includes Minnesota, for analysis. Four land uses included commercial, industrial, residential, and open space, with the number of samples for each land use varying.
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For both of these data sets, we conducted simple statistical analyses.
  
 
==Recommended event mean concentrations for total suspended solids==
 
==Recommended event mean concentrations for total suspended solids==
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<table class="infobox" style="border:3px; border-style:solid; border-color:#FF0000; text-align: right; width: 450px; font-size: 100%">
 
<table class="infobox" style="border:3px; border-style:solid; border-color:#FF0000; text-align: right; width: 450px; font-size: 100%">
 
<tr>
 
<tr>
<th><center><font size=3>'''Pollutant loads in stormwater runoff equal the pollutant concentration times the runoff volume. Thus, when calculating pollutant loading, it is necessary to consider not only the event mean concentration but factors affecting the volume of runoff. For most models and calculations, this requires adjusting curve numbers or runoff coefficients to account for differences in directly connected impervious surface between different land uses. There may be other adjustments to volume, such as accounting for interception by trees. See the discussion [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_and_dissolved_phosphorus_in_stormwater_runoff#Accounting_for_differences_in_pollutant_loading Accounting for differences in pollutant loading].'''</font size></center></th>
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<th><center><font size=3>'''Pollutant loads in stormwater runoff equal the pollutant concentration times the runoff volume. Thus, when calculating pollutant loading, it is necessary to consider not only the event mean concentration but factors affecting the volume of runoff. For most models and calculations, this requires adjusting curve numbers or runoff coefficients to account for differences in directly connected impervious surface between different land uses. There may be other adjustments to volume, such as accounting for interception by trees. See the discussion [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_suspended_solids_in_stormwater_runoff#Accounting_for_differences_in_pollutant_loading Accounting for differences in pollutant loading].'''</font size></center></th>
 
</tr>
 
</tr>
 
</table>
 
</table>
 
</div>
 
</div>
  
The following table summarizes results from our literature review. The table includes a range of values observed in the literature. Note this range does not represent a statistically-derived range but instead is based on a combination of data analysis and best professional judgement. For example, we did analyze the data for outliers, but also omitted entire studies if we felt the data were not representative of conditions likely to be encountered in Minnesota. To see the full range of values compiled from the literature, open the Excel spreadsheet containing the data.
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Emcs for TSS vary by land use. This section provides recommended emcs for different land uses. A discussion of [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_suspended_solids_in_stormwater_runoff#Factors_affecting_total_suspended_solid_emcs_in_stormwater_runoff factors affecting emcs] and [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_suspended_solids_in_stormwater_runoff#Adjusting_event_mean_concentrations potential adjustments to emcs] are provided in separate sections below.  
  
 
===Residential land use===
 
===Residential land use===
Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of land use, typically low-, medium-, or high-density residential. Most studies do not define criteria for dividing residential land use into these subcategories. Various definitions can be found in the literature. We use the following definitions.
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[[File:Residential land use 2.jpg|300px|thumb|alt=image of residential land use|<font size=3>Example of residential land use</font size>]]
*Residential: land use where the current or intended use includes, but is not limited to, housing (single and multiple dwellings), educational facilities, day care, agricultural land, correctional facilities, custodial care or long term health care.
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 +
Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of residential land use, typically low-, medium-, or high-density residential. Most studies do not define criteria for dividing residential land use into these subcategories. Various definitions can be found in the literature, including the following.
 +
*Residential: "Residential land use means any real property or portion thereof which is used for housing human beings. This term includes property used for schools, day care centers, nursing homes, or other residential-style facilities or recreational areas." ([https://www.lawinsider.com/dictionary/residential-land-use Law Insider] accessed December 31, 2019).
 
*High-density residential: More than 10 units per acre; can include multiple-occupant dwellings
 
*High-density residential: More than 10 units per acre; can include multiple-occupant dwellings
 
*Medium-density residential: 1-10 dwellings per acre; can include multiple-occupant dwellings
 
*Medium-density residential: 1-10 dwellings per acre; can include multiple-occupant dwellings
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===Commercial land use===
 
===Commercial land use===
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[[File:Commercial land use 1.jpg|300px|thumb|alt=image of commercial land use|<font size=3>Example of commercial land use</font size>]]
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"Commercial land use is the use of land for commercial purposes including building offices, shops, resorts and restaurants as opposed to construction of a residential house" ([https://www.reference.com/business-finance/commercial-land-use-d186d8d0a4ae4e72 Reference, accessed December 24, 2019)]. Commercial areas considered in this analysis do not include areas used for commercial crop production.
 
"Commercial land use is the use of land for commercial purposes including building offices, shops, resorts and restaurants as opposed to construction of a residential house" ([https://www.reference.com/business-finance/commercial-land-use-d186d8d0a4ae4e72 Reference, accessed December 24, 2019)]. Commercial areas considered in this analysis do not include areas used for commercial crop production.
  
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===Industrial land use===
 
===Industrial land use===
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[[file:Industrial land use 1.jpg|300px|thumb|alt=imageindustrial area|<font size=3>Example of an industrial area</font size>]]
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We used the following studies in our analysis.
 
We used the following studies in our analysis.
 
*NSWD; n=84; median=70
 
*NSWD; n=84; median=70
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===Open space===
 
===Open space===
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[[File:Undeveloped land use 1.jpg|300px|thumb|alt=image of undeveloped land|<font size=3>Example of open space land use</font size>]]
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Open space consists of land that is undeveloped. Typically it will not contain buildings or other built structures. Many open spaces are accessible to the public. Open space generally consists of green space (land that is partly or completely covered with grass, trees, shrubs, or other vegetation). Abandoned parcels lacking structures may be considered open space, but it is generally more accurate to include these areas in the land use that existed prior to the parcel being vacant, or including it in adjacent land use categories. The following references were used to generate a recommended value a TSS emc for open space.
 
Open space consists of land that is undeveloped. Typically it will not contain buildings or other built structures. Many open spaces are accessible to the public. Open space generally consists of green space (land that is partly or completely covered with grass, trees, shrubs, or other vegetation). Abandoned parcels lacking structures may be considered open space, but it is generally more accurate to include these areas in the land use that existed prior to the parcel being vacant, or including it in adjacent land use categories. The following references were used to generate a recommended value a TSS emc for open space.
 
*NSQD; n=6; median = 20.5
 
*NSQD; n=6; median = 20.5
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===Transportation corridors, highways, and freeways===
 
===Transportation corridors, highways, and freeways===
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[[File:Transportation land use 1.jpg|300px|thumb|alt=imagetransportation area|<font size=3>Example of transportation land use</font size>]]
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This land use includes major transportation corridors where the land use is exclusively transportation. These areas are typically highly impervious and may include only small vegetated areas consisting of swales or medians, and relatively small right-of-way areas. This land use does not include arterial streets in residential, commercial, and industrial areas. The following references were used to generate a recommended value a TSS emc for open space.
 
This land use includes major transportation corridors where the land use is exclusively transportation. These areas are typically highly impervious and may include only small vegetated areas consisting of swales or medians, and relatively small right-of-way areas. This land use does not include arterial streets in residential, commercial, and industrial areas. The following references were used to generate a recommended value a TSS emc for open space.
 
*China median=86.72
 
*China median=86.72
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*1 acre of transportation; emc = mg/L
 
*1 acre of transportation; emc = mg/L
  
Overall emc = (( * 10)/31) + ((10 * )/31) + ((10 * )/31) + ((1 * )/31) =  mg/L
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Overall emc = (73 * 10/31) + (75 * 10/31) + (93 * 10/31) + (1 * 10/31) =  80.5 mg/L
  
 
'''NOTE''': To calculate loads for a mixed land use, a curve number or runoff coefficient must be calculated based on the impervious surface for each of the land uses.
 
'''NOTE''': To calculate loads for a mixed land use, a curve number or runoff coefficient must be calculated based on the impervious surface for each of the land uses.
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==Factors affecting total suspended solid emcs in stormwater runoff==
 
==Factors affecting total suspended solid emcs in stormwater runoff==
Concentrations of TSS show considerable variability within land uses. Mean concentrations for Region 1 of the National Stormwater Quality Database are 50% greater than median concentrations for commercial, industrial, and residential land uses, indicating data are skewed higher concentrations. The mean for open space was only 7% greater than the median, indicating more uniform TSS concentrations.
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Concentrations of TSS show considerable variability within land uses. Using data from Region 1 of the National Stormwater Quality Database, mean concentrations are 50% greater than median concentrations for commercial, industrial, and residential land uses, indicating data are skewed toward higher concentrations. The mean for open space was only 7% greater than the median, indicating more uniform TSS concentrations.
  
Several factors affect concentrations of total suspended solids in stormwater runoff. The following bullet list summarizes some of the most important factors.
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Several factors affect concentrations of total suspended solids in stormwater runoff. The following bullet list summarizes some of the most important factors. Note these are general conclusions and not applicable to all local situations.
*'''Rainfall intensity and depth, including the maximum intensity and timing of this maximum'''. Initially emcs increase with rainfall intensity and depth, but at some intensity emcs begin to decline due to dilution. (Gong et al., 2016; Acharya and Piechota, 2010)
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*'''Rainfall intensity and depth, including the maximum intensity and timing of this maximum'''. Most studies show emcs increase with rainfall intensity and depth during the initial period of runoff, but at some intensity emcs begin to decline due to dilution (Gong et al., 2016; Acharya and Piechota, 2010). Other studies show little or no effect of rain intensity (Schiff et al., 2016). During the latter part of a runoff event, TSS emcs and rain intensity are inversely related (Schiff et al., 2016).
*'''Interval between runoff events'''. As the number of antecedent dry days between runoff events increases, pollutants build up on impervious surfaces, resulting in greater TSS loading when runoff does occur. The effect on emcs is less certain and appears to vary with climate. The effects of antecedent dry days appears to be smaller in humid and sub-humid climates compared to arid and semi-arid climates (Gong et al., 2016; Acharya and Piechota, 2010; Li et. al, 2015)
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*'''Interval between runoff events'''. As the number of days between runoff events increases, pollutants build up on impervious surfaces, resulting in greater TSS loading when runoff does occur. The effect on emcs is less certain and appears to vary with climate. This effect appears to be smaller in humid and sub-humid climates compared to arid and semi-arid climates (Gong et al., 2016; Acharya and Piechota, 2010; Li et. al, 2015).
*'''Length of runoff event'''. Typically, pollutant concentrations decrease after an initial peak associated with first flush. Studies suggest that, for runoff events lasting roughly 40 minutes or more, depending on intensity, TSS concentrations reach a relatively stable or slowly decreasing concentration that is 25-50% of the peak concentration (Li et. al, 2015).
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*'''Length of runoff event'''. Typically, pollutant concentrations decrease after an initial peak associated with <span title="the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm"> '''first flush'''</span>. Studies suggest that, for runoff events lasting roughly 40 minutes or more, depending on intensity, TSS concentrations reach a relatively stable or slowly decreasing concentration that is 25-50% of the peak concentration (Li et. al, 2015; Schiff et al., 2016; Stenstrom and Kayhanian, 2005).
*Nature of watershed contributing to runoff and impervious connectedness (Gong et al., 2016). This effect relates to the phenomenon of <span title="the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm"> '''first flush'''</span> in which the greatest pollutant loading occurs in the early stages of runoff (Acharya and Piechota, 2010).
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*'''Nature of watershed contributing to runoff and impervious connectedness'''. This effect relates to the phenomenon of <span title="the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm"> '''first flush'''</span> in which the greatest pollutant loading occurs in the early stages of runoff (Gong et al., 2016; Acharya and Piechota, 2010).
 
**First flush is more pronounced in smaller watersheds.
 
**First flush is more pronounced in smaller watersheds.
 
**First flush is more pronounced when the length of time between runoff events increases.
 
**First flush is more pronounced when the length of time between runoff events increases.
**First flush is less pronounced when there is greater directly connected impervious surface, since runoff can reach a specific discharge point from greater distances.
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**First flush is less pronounced when there is greater directly <span title="a subset of impervious cover, which is directly connected to a drainage system or a water body via continuous impervious surfaces."> '''connected impervious'''</span> surface, since runoff can reach a specific discharge point from greater distances.
 
**First flush is more pronounced with higher rainfall intensities in the early part of a runoff event.
 
**First flush is more pronounced with higher rainfall intensities in the early part of a runoff event.
**First flush is less pronounced when there is treatment up in a watershed, including <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span>.
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**First flush is less pronounced when there is treatment up in a watershed, including <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span>
*Construction activity within the watershed
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*'''Land use'''. Data on this page illustrates differences in TSS emcs between land uses. Several factors affect concentrations within a specific land use.
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**Emcs increase in areas with extensive lawns on compacted soils and where lawns are directly adjacent to impervious surfaces.
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**Emcs increase in areas with greater loading of organic debris, such as leaves and yard clippings. This is associated with seasonal effects.
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**Emcs increase in areas with heavy vehicle traffic. Vehicle debris may be an important component of TSS in these areas.
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**Emcs will be elevated in areas receiving winter applications of sand and deicers.
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*'''Construction activity within the watershed'''. Extensive construction activity can increase emcs during the construction period, particularly in watersheds with well-connected impervious surface. Construction activity (e.g. individual residences) not covered an <span title="The National Pollutant Discharge Elimination System (NPDES) addresses water pollution by regulating point sources that discharge pollutants to Waters of the United States (WOTUS)"> '''NPDES'''</span> permit will contribute more sediment on a per area basis due to less stringent or no <span title="practices designed to prevent or minimize erosion> [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices '''erosion protection''']</span> or <span title="practices designed to prevent or minimize loss of eroded soil at a site"> [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices '''sediment control''']</span> practices.
  
 
==Adjusting event mean concentrations==
 
==Adjusting event mean concentrations==
 +
Several factors affect emcs, as discussed above. Emcs can and should be adjusted when supporting data exist. Local monitoring data should be used to support different emcs than those recommended on this page, but the following guidelines may be used to adjust emcs.
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*In areas where sand and/or deicers are applied, adjust emcs upward if calculating pollutant loads for winter and early spring.
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*In areas with extensive tree canopy, adjust emcs upward if calculating pollutant loads for fall.
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*Adjust emcs upward if there is extensive construction activity occurring during the period when pollutant loads are calculated.
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*Adjust emcs as appropriate if certain management activities, such as street sweeping, are implemented.
  
 
==Effect of emc on pollutant loading==
 
==Effect of emc on pollutant loading==
[[File:Emc sensitivity tss.png|400px|thumb|alt=graph showing TSS sensitivity to changes in emc|<font size=3>TSS loading for 3 different TSS emcs (30, 54.5, and 100 mg/L) anf five land uses (1 acre of impervious with no pervious, and 1 acre of impervious with 1 acre of pervious turf on either HSG A, B, C, or D).</font size>]]
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[[File:Emc sensitivity tss.png|400px|thumb|alt=graph showing TSS sensitivity to changes in emc|<font size=3>TSS loading, in pounds, for 3 different TSS emcs (30, 54.5, and 100 mg/L) and five land uses (1 acre of impervious with no pervious, and 1 acre of impervious with 1 acre of pervious turf on either HSG A, B, C, or D).</font size>]]
  
To assess the effect of changing the TSS emc, we ran several scenarios using the [https://stormwater.pca.state.mn.us/index.php?title=MIDS_calculator Minimal Impact Design Standards Calculator]. For each model run we assumed the 31.9 inches of precipitation annually. We varied the emc as follows
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To assess the effect of changing the TSS emc, we ran several scenarios using the [https://stormwater.pca.state.mn.us/index.php?title=MIDS_calculator Minimal Impact Design Standards Calculator]. For each model run we assumed  
 +
31.9 inches of precipitation annually. We varied the emc as follows
 
*30 mg/L
 
*30 mg/L
 
*54.5 mg/L (MIDS Calculator default)
 
*54.5 mg/L (MIDS Calculator default)
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We varied land use as follows
 
We varied land use as follows
 
*1 acre of impervious
 
*1 acre of impervious
*1 acre of impervious and 1 acre of turf on hydrologic group soil (HSG)A soil
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*1 acre of impervious and 1 acre of turf on hydrologic group soil (HSG) A soil
 
*1 acre of impervious and 1 acre of turf on B soil
 
*1 acre of impervious and 1 acre of turf on B soil
 
*1 acre of impervious and 1 acre of turf on C soil
 
*1 acre of impervious and 1 acre of turf on C soil
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==Accounting for differences in pollutant loading==
 
==Accounting for differences in pollutant loading==
Pollutant loads are a function of pollutant concentrations in runoff and the volume of runoff. Consequently, when calculating pollutant loads it is necessary to adjust both the emcs and volume of runoff. Volumes are typically calculated using curve numbers or runoff coefficients. The MPCA Simple Estimator, for example, employs a default runoff coefficient of 0.8 for commercial areas, compared to 0.44 for residential areas. The tables below may be used to determine the proper curve number or runoff coefficient. Percent impervious can be converted to a curve number using the following formula.
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Pollutant loads are a function of pollutant concentrations in runoff and the volume of runoff. Consequently, when calculating pollutant loads it is necessary to adjust both the emcs and volume of runoff. Volumes are typically calculated using <span title="The SCS curve number method is a widely used method for determining the approximate amount of runoff from a rainfall even in a particular area. The curve number is based on the area's hydrologic soil group, land use , treatment and hydrologic condition."> '''curve numbers'''</span> or <span title="The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)."> [https://stormwater.pca.state.mn.us/index.php?title=Runoff_coefficients_for_5_to_10_year_storms '''runoff coefficients''']</span>. The [https://stormwater.pca.state.mn.us/index.php?title=Guidance_and_examples_for_using_the_MPCA_Estimator MPCA Simple Estimator], for example, employs a default runoff coefficient of 0.8 for commercial areas, compared to 0.44 for residential areas. The tables below may be used to determine the proper curve number or runoff coefficient. Percent impervious can be converted to a curve number using the following formula.
  
 
<math> Curve number = (Impervious * 98) + ((1 - impervious) * (open space curve number in good condition for the specific soil)) </math>
 
<math> Curve number = (Impervious * 98) + ((1 - impervious) * (open space curve number in good condition for the specific soil)) </math>
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| Pervious acres
 
| Pervious acres
 
| emc (mg/L)
 
| emc (mg/L)
| Total phosphorus load (lb/yr)
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| Total TSS load (lb/yr)
| TP export (lb/ac/yr)
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| TSS export (lb/ac/yr)
 
|-
 
|-
| Residential (40% canopy)
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| Residential (>40% canopy)
 
| 30
 
| 30
 
| 0.90
 
| 0.90
 
| 2.1
 
| 2.1
| 0.44
+
| 80
| 3.65
+
| 663.8
| 1.22
+
| 221.3
 
|-
 
|-
| Residential (10% canopy)
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| Residential (<10% canopy)
 
| 30
 
| 30
| 0.60
+
| 0.6
 
| 1.4
 
| 1.4
| 0.26
+
| 70
| 1.44
+
| 387.2
| 0.72
+
| 193.6
 
|-
 
|-
 
| Commercial
 
| Commercial
Line 297: Line 330:
 
| 0.85
 
| 0.85
 
| 0.15
 
| 0.15
| 0.20
+
| 75
| 1.09
+
| 408.8
| 1.09
+
| 408.8
 
|-
 
|-
 
| Industrial
 
| Industrial
Line 305: Line 338:
 
| 0.72
 
| 0.72
 
| 0.28
 
| 0.28
| 0.24
+
| 93
| 1.13
+
| 447.8
| 1.13
+
| 447.8
 
|-
 
|-
 
| Open space
 
| Open space
Line 313: Line 346:
 
| 0.10
 
| 0.10
 
| 0.90
 
| 0.90
| 0.19
+
| 21
| 0.34
+
| 37.6
| 0.34
+
| 37.6
 
|-
 
|-
| colspan="7" style="text-align: center;"| '''Total phosphorus load with adjusted emcs = 7.65 pounds/yr'''
+
| colspan="7" style="text-align: center;"| '''Total suspended solids load with adjusted emcs = 1945.2 pounds/yr'''
 
|-
 
|-
 
| MIDS unadjusted
 
| MIDS unadjusted
Line 323: Line 356:
 
| 3.17
 
| 3.17
 
| 4.83
 
| 4.83
| 0.30
+
| 54.5
| 7.77
+
| 1410.7
| 0.97
+
| 176.3
 
|}
 
|}
  
Line 334: Line 367:
 
*B soils with turf
 
*B soils with turf
 
*5 acres of residential consisting of the following
 
*5 acres of residential consisting of the following
 +
**3 acres of residential land use with high tree canopy coverage (> 50%) and 30% impervious
 +
**2 acres of residential with low tree canopy coverage (<10%) and 30% impervious
 
*1 acre of commercial land and 85% impervious
 
*1 acre of commercial land and 85% impervious
 
*1 acre if industrial land and 72% impervious
 
*1 acre if industrial land and 72% impervious
Line 339: Line 374:
  
 
EMCs are as follows.
 
EMCs are as follows.
*Residential = 73 mg/L
+
*The recommended TSS emc for residential land use is 73 mg/L. We assumed the following for the two residential areas described above.
 +
**For the high canopy area, we assumed 80 mg/L
 +
**For the low canopy area, we assumed 70 mg/L
 
*Commercial = 75 mg/L
 
*Commercial = 75 mg/L
 
*Industrial = 93 mg/L
 
*Industrial = 93 mg/L
 
*Open space = 21 mg/L
 
*Open space = 21 mg/L
 +
 +
Total load with the variable land uses (1945.2 pounds) is much greater than the default MIDS scenario (1410.7 pounds). This is primarily due to the higher emcs and partly due to the higher impervious acreages in the variable land use scenario. The effect of impervious acreage is shown, for example, by reducing the percent impervious for industrial land use from 72% to 50%. This results in a total load of 348 pounds, or a reduction of about 100 pounds (22.3% reduction for a 22% change in impervious). This example also demonstrates the importance of accurately identifying land use within a modeled area.
  
 
==References==
 
==References==
 
*Acharya, A., and T. Piechota. 2010. [https://www.researchgate.net/publication/267200251_Characterization_of_First_Flush_Phenomenon_in_an_Urban_Stormwater_Runoff_A_Case_Study_of_Flamingo_Tropicana_Watershed_in_Las_Vegas_Valley Characterization of First Flush Phenomenon in an Urban Stormwater Runoff: A Case Study of Flamingo Tropicana Watershed in Las Vegas Valley]. World Environmental and Water Resources Congress 2010: Challenges of Change. American Society of Civil Engineers. p 3365-3375. 10.1061/41114(371)347.
 
*Acharya, A., and T. Piechota. 2010. [https://www.researchgate.net/publication/267200251_Characterization_of_First_Flush_Phenomenon_in_an_Urban_Stormwater_Runoff_A_Case_Study_of_Flamingo_Tropicana_Watershed_in_Las_Vegas_Valley Characterization of First Flush Phenomenon in an Urban Stormwater Runoff: A Case Study of Flamingo Tropicana Watershed in Las Vegas Valley]. World Environmental and Water Resources Congress 2010: Challenges of Change. American Society of Civil Engineers. p 3365-3375. 10.1061/41114(371)347.
 +
*Baldys III, S., T.H. Raines, B.L. Mansfield, and J.T. Sandlin. 1998. [https://www.semanticscholar.org/paper/Urban-stormwater-quality%2C-event-mean-and-estimates-Baldys-Raines/c8ffa3331750ead92c7ee74c2c46234fbe2572b9 Urban Stormwater Quality, Event-Mean Concentrations, and Estimates of Stormwater Pollutant Loads, Dallas-Fort Worth Area, Texas, 1992–93]. U.S. Geological Survey Water-Resources Investigations Report 98–4158.
 +
*Bannerman, Roger T., Andrew D. Legg, and Steven R. Greb. 1996. [https://pdfs.semanticscholar.org/850e/ccf19feb2157c170ebdada940d30f9426711.pdf Quality Of Wisconsin Stormwater, 1989-94]. U.S. Geological Survey. Open-File Report 96-458.
 +
*Bannerman, R.T., D. W.Owens,R. B.Dodds, and N. J. Hornewer. 1993. [https://www.fws.gov/southwest/es/Documents/R2ES/LitCited/4TX_Sal/Bannerman_1993_Pollutants_in_stormwater.pdf Sources of Pollution in Wisconsin Stormwater]. Wac Sci tech 28:3-5. pp. 241-259.
 +
*Bartley, Rebecca, and William Speirs. 2010. [https://ewater.org.au/uploads/files/Water%20quality%20review_Bartley%20and%20Speirs_Final.pdf A review of sediment and nutrient concentration data from Australia for use in catchment water quality models]. eWater Cooperative Research Centre Technical Report.
 +
*Brezonik PL, Stadelmann TH.. 2002. [https://www.esf.edu/EFB/mitchell/Brezonik&Stadelmann2002.pdf Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentrations from watersheds in the Twin Cities metropolitan area, Minnesota, USA]. Water Res. Apr;36(7):1743-57. DOI: 10.1016/s0043-1354(01)00375-x
 +
*California Regional Water Quality Control Board. 2014. [https://www.waterboards.ca.gov/losangeles/water_issues/programs/stormwater/municipal/watershed_management/dominguez_channel/DominguezChannel_WP.pdf ENHANCED WATERSHED MANAGEMENT PROGRAM WORK PLAN FOR THE DOMINGUEZ CHANNEL WATERSHED MANAGEMENT AREA GROUP].
 +
*Capitol Region Watershed District. 2016. [https://issuu.com/capitolregionwd/docs/may_18__2016_board_packet_142422444f549a/256 May 18, 2016 Board Packet].
 +
*Erickson, A.J., P.T. Weiss, J.S. Gulliver, R.M. Hozalski. [http://stormwaterbook.safl.umn.edu/pollutant-removal/analysis-individual-storm-events Analysis of individual storm events, Stormwater Treatment: Assessment and Maintenance]. Accessed December 31, 2019.
 
*Gong, Y., X. Liang, X. Li, J. Li, X. Fang, and R. Song. 2016. [https://www.mdpi.com/2073-4441/8/7/278/htm Influence of Rainfall Characteristics on Total Suspended Solids in Urban Runoff: A Case Study in Beijing, China]. Water 2016, 8(7), 278; https://doi.org/10.3390/w8070278  
 
*Gong, Y., X. Liang, X. Li, J. Li, X. Fang, and R. Song. 2016. [https://www.mdpi.com/2073-4441/8/7/278/htm Influence of Rainfall Characteristics on Total Suspended Solids in Urban Runoff: A Case Study in Beijing, China]. Water 2016, 8(7), 278; https://doi.org/10.3390/w8070278  
 +
*Hallberg, M., G. Renman. 2008. [http://www.pjoes.com/Suspended-Solids-Concentration-in-Highway-r-nRunoff-during-Summer-Conditions,88100,0,2.html Suspended Solids Concentration in Highway Runoff during Summer Conditions]. Pol. J. Environ. Stud. 17(2):237–241.
 +
*Jung, Jae-Woon, Ha-Na Park, Kwang-Sik Yoon, Dong-Ho Choi, Byung-Jin Lim. 2013. [https://link.springer.com/article/10.1007/s13765-013-3128-3 Event mean concentrations (EMCs) and first flush characteristics of runoff from a public park in Korea]. Journal of the Korean Society for Applied Biological Chemistry. October 2013, Volume 56, Issue 5, pp 597–604.
 +
*Kieser and Associates. 2008. [http://kieser-associates.com/uploaded/pawpaw_urban_buildout_report_063008.pdf Urban and Build-out and Stormwater BMP Analysis in the Paw Paw River Watershed]. For the Southwest Michigan Planning Commission. 36 p.
 +
*Kim, Lee-Hyung. 2003. ''Determination of event mean concentrations and first flush criteria in urban runoff''. Environ. Eng. Res. 8:4:163-176.
 
*Li D, Wan J, Ma Y, Wang Y, Huang M, Chen Y. 2015. [https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118776 Stormwater Runoff Pollutant Loading Distributions and Their Correlation with Rainfall and Catchment Characteristics in a Rapidly Industrialized City]. PLoS ONE 10(3): e0118776. doi:10.1371/journal.pone.0118776
 
*Li D, Wan J, Ma Y, Wang Y, Huang M, Chen Y. 2015. [https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118776 Stormwater Runoff Pollutant Loading Distributions and Their Correlation with Rainfall and Catchment Characteristics in a Rapidly Industrialized City]. PLoS ONE 10(3): e0118776. doi:10.1371/journal.pone.0118776
 +
*Maniquiz, Marla C. , Soyoung Lee, Lee-Hyung Kim. 2010. Multiple linear regression models of urban runoff pollutant load and event mean concentration considering rainfall variables. Jour Environ. Sci. 22:6:946-852.
 +
*Maniquiz, Marla C., Jiyeon Choi, Soyoung Lee, Hye Jin Cho, Lee-Hyung Kim. 2010. [https://pdfs.semanticscholar.org/e24b/670b01617980cf9458bcf868c8feaebded74.pdf Appropriate Methods in Determining the Event Mean Concentration and Pollutant Removal Efficiency of a Best Management Practice].  15:215-223.
 +
*McKee, Paul W., and Harry C. McWreath. 2001. [https://pubs.usgs.gov/wri/wri01-4253/pdf/wri01-4253.pdf Computed and Estimated Pollutant Loads, West Fork Trinity River, Fort Worth, Texas, 1997]. U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 01–4253.
 +
*Olson, Chris, Tyler Dell, and Jason Brim. 2017. Nutrient Sources in Urban Areas – A Literature Review. Prepared for the City of Fort Collins, Colorado.
 +
*Rhee, Han-Pil, CG Yoon, S.J. Lee, and JH Choi. 2012. [https://www.researchgate.net/publication/263627021_Analysis_of_Nonpoint_Source_Pollution_Runoff_from_Urban_Land_Uses_in_South_Korea Analysis of Nonpoint Source Pollution Runoff from Urban Land Uses in South Korea]. Environmental Engineering Research 17(1):47-56. DOI: 10.4491/eer.2012.17.1.047.
 
*Schiff, Kenneth C., and Liesl L. Tiefenthaler. 2011. [https://onlinelibrary.wiley.com/doi/full/10.1111/j.1752-1688.2010.00497.x Seasonal flushing of pollutant concentrations and loads in urban stormwater]. Jour. Amer. Water Works Assoc. 47:1:136-143
 
*Schiff, Kenneth C., and Liesl L. Tiefenthaler. 2011. [https://onlinelibrary.wiley.com/doi/full/10.1111/j.1752-1688.2010.00497.x Seasonal flushing of pollutant concentrations and loads in urban stormwater]. Jour. Amer. Water Works Assoc. 47:1:136-143
 
*Schiff, Kenneth C., and Liesl L. Tiefenthaler. 2016. [https://www.mdpi.com/2073-4441/8/8/320 Effects of Rainfall Intensity and Duration on the First Flush from Parking Lots]. Water. 8(8), 320. https://doi.org/10.3390/w8080320
 
*Schiff, Kenneth C., and Liesl L. Tiefenthaler. 2016. [https://www.mdpi.com/2073-4441/8/8/320 Effects of Rainfall Intensity and Duration on the First Flush from Parking Lots]. Water. 8(8), 320. https://doi.org/10.3390/w8080320
 +
*Sharma, D., R. Gupta, R.K. Singh, and A. Kansal. 2012. [https://www.researchgate.net/publication/257799126_Characteristics_of_the_event_mean_concentration_EMCs_from_rainfall_runoff_on_mixed_agricultural_land_use_in_the_shoreline_zone_of_the_Yamuna_River_in_Delhi_India Characteristics of the event mean concentration (EMCs) from rainfall runoff on mixed agricultural land use in the shoreline zone of the Yamuna River in Delhi, India]. Applied Water Science. 2:55-62.
 +
*Smullen, James T., Amy L. Shallcross, Kelly A.Cave. 1999. [https://www.sciencedirect.com/science/article/abs/pii/S0273122399003121 Updating the U.S. Nationwide urban runoff quality data base]. Water Science and Technology. Volume 39, Issue 12, Pages 9-16. https://doi.org/10.1016/S0273-1223(99)00312-1.
 +
*Stein, Eric D., Liesl L. Tiefenthaler and Kenneth C. Schiff. 2008. [http://ftp.sccwrp.org/pub/download/DOCUMENTS/AnnualReports/2008AnnualReport/AR08_015_027.pdf Comparison of stormwater pollutant loading by land use type]. Southern California Coastal Water Research Project, AR08-015-027.
 +
*Stein, Eric D., Liesl L. Tiefenthaler and Kenneth C. Schiff. 2007. [http://ftp.sccwrp.org/pub/download/DOCUMENTS/TechnicalReports/510_pollutant_loading.pdf SOURCES, PATTERNS AND MECHANISMS OF STORM WATER POLLUTANT LOADING FROM WATERSHEDS AND LAND USES OF THE GREATER LOS ANGELES AREA, CALIFORNIA, USA].  Technical Report 510.
 +
*Stenstrom, M.K., and M. Kayhanian. 2005. [https://www.researchgate.net/publication/288208061_First_flush_phenomenon_characterization First Flush Phenomenon Characterization]. Prepared for: California Department of Transportation. 81 p.
 +
*U.S. Environmental Protection Agency. 1983. Results of the Nationwide Urban Runoff Program—Executive summary: U.S. Environmental Protection Agency, Water Planning Division, National Technical Information Service PB84–185545, 24 p.
 +
*U.S. Environmental Protection Agency. 1983. [https://www3.epa.gov/npdes/pubs/sw_nurp_vol_1_finalreport.pdf Results of the Nationwide Urban Runoff Program: Volume I – Final Report]. National Technical Information Service Number PB84-185552.
 +
*University of Wisconsin at Milwaukee. [https://www.mmsd.com/application/files/6214/8192/3796/1103620Phosphorus20Speciation.pdf Phosphorus Speciation and Loads in Stormwater and CSOs of the MMSD Service Area (2000 – 2008) Final Report]: February, 2011
 +
*Wang, Shumin, Qiang Hea, Hainan Aia, Zhentao Wanga, and Qianqian Zhang. 2013. [https://www.sciencedirect.com/science/article/pii/S1001074211610322 Pollutant concentrations and pollution loads in stormwater runoff from different land uses in Chongqing]. Journal of Environmental Sciences. Volume 25, Issue 3, Pages 502-510. https://doi.org/10.1016/S1001-0742(11)61032-2.
 +
*Washington District Department of the Environment. 2014. [http://dcstormwaterplan.org/wp-content/uploads/AppD_EMCs_FinalCBA_12222014.pdf Selection of Event Mean Concentrations (EMCs)].
 
*Yang, Yun-Ya, and Gurpal S. Toor. 2018. [https://www.nature.com/articles/s41598-018-29857-x  runoff driven phosphorus transport in an urban residential catchment: Implications for protecting water quality in urban watersheds]. Scientific Reports. 8:1-10
 
*Yang, Yun-Ya, and Gurpal S. Toor. 2018. [https://www.nature.com/articles/s41598-018-29857-x  runoff driven phosphorus transport in an urban residential catchment: Implications for protecting water quality in urban watersheds]. Scientific Reports. 8:1-10
*University of Wisconsin at Milwaukee. [https://www.mmsd.com/application/files/6214/8192/3796/1103620Phosphorus20Speciation.pdf Phosphorus Speciation and Loads in Stormwater and CSOs of the MMSD Service Area (2000 – 2008) Final Report]: February, 2011
+
 
*Lee-Hyung Kim. 2003. Determination of event mean concentrations and first flush criteria in urban runoff. Environ. Eng. Res. 8:4:163-176.
+
<noinclude>
*Jae-Woon Jung, Ha-Na Park, Kwang-Sik Yoon, Dong-Ho Choi, Byung-Jin Lim. 2013. [https://link.springer.com/article/10.1007/s13765-013-3128-3 Event mean concentrations (EMCs) and first flush characteristics of runoff from a public park in Korea]. Journal of the Korean Society for Applied Biological Chemistry. October 2013, Volume 56, Issue 5, pp 597–604.
+
[[category:pollutants]]
 +
</noinclude>

Revision as of 12:33, 8 April 2020

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image
Summary information - total suspended solids (TSS) concentrations in stormwater runoff
Land use Recommended emc TSS (mg/L)
Commercial 75
Industrial 93
Residential 73
Freeways/transportation 87
Mixed 76 or calculate
Open space 21
Conventional roof < 10


This page provides information on event mean concentrations of total suspended solids (TSS) in urban stormwater runoff. For a discussion of TSS in stormwater runoff, including information on sources, fate, and water quality impacts, see Total Suspended Solids (TSS) in stormwater.

Objective

Event mean concentrations (emcs) are used in models for predicting water quality impacts from stormwater runoff and stormwater treatment practices or pollution prevention practices. Pollutant loads, which are typically used to assess water quality impacts, including establishing total maximum daily loads (TMDLs), are a function of pollutant concentration and volume of runoff. It is therefore important to accurately determine appropriate event mean concentrations when assessing water quality impacts from stormwater runoff.

This page provides summary information that can be used for selecting or calculating appropriate emcs for total suspended solids.

Information: Many factors affect total suspended solids concentrations in stormwater. If you are unfamiliar with the concept of event mean concentrations, We recommend you first read Stormwater pollutant concentrations and event mean concentrations

Methodology

We conducted a review of literature to develop the EMCs shown on this page. Nearly all studies provided summary information; we therefore did not analyze raw data with the exception of data from Capitol Region Watershed District (see discussion below) and the National Stormwater Quality Database. We compiled the summary information into a spreadsheet and conducted simple statistical analysis of the information.

Data from the following studies were used to generate emcs for total suspended solids.

In addition to the above sources, we compiled water quality monitoring data from 10 storm sewer outfalls in the Capitol Region Watershed in Minnesota. The data period for each outlet varied but generally spanned the period from about 2005 to 2019. The following information was compiled for each monitoring location.

  • Date
  • Total suspended solids in mg/L.
  • Sample type, which included runoff samples during precipitation events, snowmelt samples, and baseflow samples for those locations where groundwater contributed to flow.

We also downloaded the 2015 National Stormwater Quality Database. The dataset includes information from across the U.S. We selected only data from Region 1, which includes Minnesota, for analysis. Four land uses included commercial, industrial, residential, and open space, with the number of samples for each land use varying.

For both of these data sets, we conducted simple statistical analyses.

Recommended event mean concentrations for total suspended solids

Pollutant loads in stormwater runoff equal the pollutant concentration times the runoff volume. Thus, when calculating pollutant loading, it is necessary to consider not only the event mean concentration but factors affecting the volume of runoff. For most models and calculations, this requires adjusting curve numbers or runoff coefficients to account for differences in directly connected impervious surface between different land uses. There may be other adjustments to volume, such as accounting for interception by trees. See the discussion Accounting for differences in pollutant loading.

Emcs for TSS vary by land use. This section provides recommended emcs for different land uses. A discussion of factors affecting emcs and potential adjustments to emcs are provided in separate sections below.

Residential land use

image of residential land use
Example of residential land use

Studies from the literature frequently provide concentrations for residential land use or occasionally for different types of residential land use, typically low-, medium-, or high-density residential. Most studies do not define criteria for dividing residential land use into these subcategories. Various definitions can be found in the literature, including the following.

  • Residential: "Residential land use means any real property or portion thereof which is used for housing human beings. This term includes property used for schools, day care centers, nursing homes, or other residential-style facilities or recreational areas." (Law Insider accessed December 31, 2019).
  • High-density residential: More than 10 units per acre; can include multiple-occupant dwellings
  • Medium-density residential: 1-10 dwellings per acre; can include multiple-occupant dwellings
  • Low-density residential: one dwelling per 1-5 acres; can include multiple-occupant dwellings

Note that residential land uses can include other land uses, such as commercial and industrial. Many studies therefore classify land uses as mixed or urban, even though a specific land use may dominate a particular area.

Because of the variable and arbitrary manner in which residential land use is classified, we provide a single recommended value for event mean concentrations in residential land uses. We provide additional discussion below so that users can adjust this recommended value depending on local conditions. We used the following references for generating a recommended value for residential land use.

  • Wisconsin; 10 sites; n=25; median=46
  • Korea; 6 sites; n=23; median=63.3
  • LA; 2 sites; 4 samples; median = 91.2
  • NURP median = 101
  • Dallas median = 78
  • China median = 68
  • NSQD 250 sites; median = 92 (region 1)
  • Line = 42

We chose these studies because they contained large amounts of data and they were located in humid and sub-humid areas of the U.S. The median of the above 7 values is 73 mg/L.

Information: The recommended event mean concentration for total suspended solids in residential areas is 77 mg/L

Commercial land use

image of commercial land use
Example of commercial land use

"Commercial land use is the use of land for commercial purposes including building offices, shops, resorts and restaurants as opposed to construction of a residential house" (Reference, accessed December 24, 2019). Commercial areas considered in this analysis do not include areas used for commercial crop production.

We used the following studies in our analysis. Commercial

  • NSQD; n=164; median = 97.15
  • NURP median = 69
  • Dallas median = 42
  • L.A.; n=5; mean = 49.6
  • China Median = 81
  • Harper = 87.7

The median concentration from these studies is 75 mg/L.

Information: The recommended event mean concentration for total suspended solids in commercial areas is 75 mg/L

Industrial land use

imageindustrial area
Example of an industrial area

We used the following studies in our analysis.

  • NSWD; n=84; median=70
  • Dallas median = 104
  • Korea median = 78.8; 6 sites
  • LA median = 92.2; 6 sites
  • Harper = 93.3
  • Line = 170

The median TSS concentration from these studies is 93 mg/L. TSS concentrations do not appear to vary much across different industrial land uses, with the primary sources likely being road salt and atmospheric deposition. However, the following may contribute to higher TSS loads in industrial areas.

  • Cleaning and washing operations
  • Heavy vehicle traffic
  • Specific industries such as food processing plants, meat packing plants and lockers, metal finishing facilities, and industries that generate or handle animal waste (including human sources)
Information: The recommended event mean concentration for total suspended solids in industrial areas is 93 mg/L

Open space

image of undeveloped land
Example of open space land use

Open space consists of land that is undeveloped. Typically it will not contain buildings or other built structures. Many open spaces are accessible to the public. Open space generally consists of green space (land that is partly or completely covered with grass, trees, shrubs, or other vegetation). Abandoned parcels lacking structures may be considered open space, but it is generally more accurate to include these areas in the land use that existed prior to the parcel being vacant, or including it in adjacent land use categories. The following references were used to generate a recommended value a TSS emc for open space.

  • NSQD; n=6; median = 20.5
  • NURP median = 70
  • Harper = 11.1

Parks and recreation areas are generally included in open space.

Information: The recommended event mean concentration for total suspended solids in open space, urban parks, and urban recreations areas is 21 mg/L

Transportation corridors, highways, and freeways

imagetransportation area
Example of transportation land use

This land use includes major transportation corridors where the land use is exclusively transportation. These areas are typically highly impervious and may include only small vegetated areas consisting of swales or medians, and relatively small right-of-way areas. This land use does not include arterial streets in residential, commercial, and industrial areas. The following references were used to generate a recommended value a TSS emc for open space.

  • China median=86.72
  • Fort Worth median =90; n=27
  • New Hampshite median = 55.54; n=27
  • LA median = 87.54; n=39
  • Harper = 50.3

The median value from these studies is 87 mg/L.

Information: The recommended event mean concentration for total suspended solids in transportation areas is 87 mg/L

TSS concentrations from transportation corridors are highly variable depending on inputs. The primary inputs include road salt, sediment, and vehicle-related wastes, including oil. The recommended value should be adjusted based on vehicle traffic and likely suspended solids sources and inputs.

Roofs

Information: The recommended event mean concentration for total suspended solids in runoff from conventional (non-green) roofs is less than 10 mg/L

Mixed land use

  • DC median = 47.25; 4 sites
  • Australia; n=49; median=105
  • Korea median = 153.3
  • Korea median =76; n=45
  • WI median = 188; 2 sites
  • NURP median=67
  • Capitol Region Watershed District (9 outfalls) median = 97 mg/L
  • Sullen median = 54.5
  • L.A. median = 65.1

Overall median = 76 mg/L

An emc can be calculated if the total area of interest (Atotal), the area of each land use in the area of interest, and the emc for each land use in the area of interest are known.

Site emc = Σ1n ((AArea 1 * emcArea 1)/ (Atotal) + ... ((AArea n * emcArea n) / (Atotal)

where A = area in acres.

Example calculation

  • 10 acres of residential; emc = mg/L
  • 10 acres of commercial; emc = mg/L
  • 10 acres of industrial' emc = mg/L
  • 1 acre of transportation; emc = mg/L

Overall emc = (73 * 10/31) + (75 * 10/31) + (93 * 10/31) + (1 * 10/31) = 80.5 mg/L

NOTE: To calculate loads for a mixed land use, a curve number or runoff coefficient must be calculated based on the impervious surface for each of the land uses.

Summary table for event mean concentrations by land use

Event mean concentrations for total suspended solids.
Link to this table

Land cover/land use Range (mg/L) Recommended value (mg/L) Notes
Commercial 42-164 75 If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads
Industrial 70-170 93
  • If applicable to models being used, adjust curve numbers/runoff coefficients when calculating loads
Residential 42-101 73
High-density/Multi-family residential Calculate
  • Insufficient information to recommend a specific emc
Medium density residential Calculate
  • Insufficient information to recommend a specific emc
Low density residential Calculate
  • Insufficient information to recommend a specific emc
Freeways/transportation 50-90 87
Mixed 47-188 76 or calculate
  • Residential land use was the primary land use in most studies that cited values for mixed land use
  • If the study area can be delineated into specific land uses and impervious area for each land use is know, we recommend calculating the emc
Parks and recreation Use value for open space or calculate
  • emc will be a function of vegetative cover
Open space 11-70 21
Conventional roof <20
Institutional 17-140 80
Forest/shrub/grassland 26-140 72 Sediment concentrations from forested areas vary widely with factors such as slope and forest condition. Concentrations may be very high, but the annual volume of runoff is typically much less than non-forested areas.
Open water and wetlands see Notes (next column)
  • If data exist, use the TSS concentration for the water body of interest
  • If data for a specific water body do not exist, use data from similar lakes in the area
Cropland (row crops) 50-160 Literature review was not adequate to recommend an emc
Pasture 75-150 84 Concentrations are a function of intensity of use.


Factors affecting total suspended solid emcs in stormwater runoff

Concentrations of TSS show considerable variability within land uses. Using data from Region 1 of the National Stormwater Quality Database, mean concentrations are 50% greater than median concentrations for commercial, industrial, and residential land uses, indicating data are skewed toward higher concentrations. The mean for open space was only 7% greater than the median, indicating more uniform TSS concentrations.

Several factors affect concentrations of total suspended solids in stormwater runoff. The following bullet list summarizes some of the most important factors. Note these are general conclusions and not applicable to all local situations.

  • Rainfall intensity and depth, including the maximum intensity and timing of this maximum. Most studies show emcs increase with rainfall intensity and depth during the initial period of runoff, but at some intensity emcs begin to decline due to dilution (Gong et al., 2016; Acharya and Piechota, 2010). Other studies show little or no effect of rain intensity (Schiff et al., 2016). During the latter part of a runoff event, TSS emcs and rain intensity are inversely related (Schiff et al., 2016).
  • Interval between runoff events. As the number of days between runoff events increases, pollutants build up on impervious surfaces, resulting in greater TSS loading when runoff does occur. The effect on emcs is less certain and appears to vary with climate. This effect appears to be smaller in humid and sub-humid climates compared to arid and semi-arid climates (Gong et al., 2016; Acharya and Piechota, 2010; Li et. al, 2015).
  • Length of runoff event. Typically, pollutant concentrations decrease after an initial peak associated with first flush. Studies suggest that, for runoff events lasting roughly 40 minutes or more, depending on intensity, TSS concentrations reach a relatively stable or slowly decreasing concentration that is 25-50% of the peak concentration (Li et. al, 2015; Schiff et al., 2016; Stenstrom and Kayhanian, 2005).
  • Nature of watershed contributing to runoff and impervious connectedness. This effect relates to the phenomenon of first flush in which the greatest pollutant loading occurs in the early stages of runoff (Gong et al., 2016; Acharya and Piechota, 2010).
    • First flush is more pronounced in smaller watersheds.
    • First flush is more pronounced when the length of time between runoff events increases.
    • First flush is less pronounced when there is greater directly connected impervious surface, since runoff can reach a specific discharge point from greater distances.
    • First flush is more pronounced with higher rainfall intensities in the early part of a runoff event.
    • First flush is less pronounced when there is treatment up in a watershed, including pretreatment
  • Land use. Data on this page illustrates differences in TSS emcs between land uses. Several factors affect concentrations within a specific land use.
    • Emcs increase in areas with extensive lawns on compacted soils and where lawns are directly adjacent to impervious surfaces.
    • Emcs increase in areas with greater loading of organic debris, such as leaves and yard clippings. This is associated with seasonal effects.
    • Emcs increase in areas with heavy vehicle traffic. Vehicle debris may be an important component of TSS in these areas.
    • Emcs will be elevated in areas receiving winter applications of sand and deicers.
  • Construction activity within the watershed. Extensive construction activity can increase emcs during the construction period, particularly in watersheds with well-connected impervious surface. Construction activity (e.g. individual residences) not covered an NPDES permit will contribute more sediment on a per area basis due to less stringent or no erosion protection or sediment control practices.

Adjusting event mean concentrations

Several factors affect emcs, as discussed above. Emcs can and should be adjusted when supporting data exist. Local monitoring data should be used to support different emcs than those recommended on this page, but the following guidelines may be used to adjust emcs.

  • In areas where sand and/or deicers are applied, adjust emcs upward if calculating pollutant loads for winter and early spring.
  • In areas with extensive tree canopy, adjust emcs upward if calculating pollutant loads for fall.
  • Adjust emcs upward if there is extensive construction activity occurring during the period when pollutant loads are calculated.
  • Adjust emcs as appropriate if certain management activities, such as street sweeping, are implemented.

Effect of emc on pollutant loading

graph showing TSS sensitivity to changes in emc
TSS loading, in pounds, for 3 different TSS emcs (30, 54.5, and 100 mg/L) and five land uses (1 acre of impervious with no pervious, and 1 acre of impervious with 1 acre of pervious turf on either HSG A, B, C, or D).

To assess the effect of changing the TSS emc, we ran several scenarios using the Minimal Impact Design Standards Calculator. For each model run we assumed 31.9 inches of precipitation annually. We varied the emc as follows

  • 30 mg/L
  • 54.5 mg/L (MIDS Calculator default)
  • 100 mg/L

We varied land use as follows

  • 1 acre of impervious
  • 1 acre of impervious and 1 acre of turf on hydrologic group soil (HSG) A soil
  • 1 acre of impervious and 1 acre of turf on B soil
  • 1 acre of impervious and 1 acre of turf on C soil
  • 1 acre of impervious and 1 acre of turf on D soil

The results, illustrated in the adjacent graph, indicate a small effect of soil. Changing the emc within a specific land use scenario, however, results in significant changes in loading. The change in loading is linear and equal to the following.

  • impervious: 6.18 lbs/acre/yr increase in TSS load for each 1 mg/L increase in TSS
  • A soil: 7.16 lbs/acre/yr increase in TSS load for each 1 mg/L increase in TSS
  • B soil: 7.48 lbs/acre/yr increase in TSS load for each 1 mg/L increase in TSS
  • C soil: 7.61 lbs/acre/yr increase in TSS load for each 1 mg/L increase in TSS
  • D soil: 7.81 lbs/acre/yr increase in TSS load for each 1 mg/L increase in TSS

This exercise illustrates the importance of selecting an appropriate emc.

Accounting for differences in pollutant loading

Pollutant loads are a function of pollutant concentrations in runoff and the volume of runoff. Consequently, when calculating pollutant loads it is necessary to adjust both the emcs and volume of runoff. Volumes are typically calculated using curve numbers or runoff coefficients. The MPCA Simple Estimator, for example, employs a default runoff coefficient of 0.8 for commercial areas, compared to 0.44 for residential areas. The tables below may be used to determine the proper curve number or runoff coefficient. Percent impervious can be converted to a curve number using the following formula.

\( Curve number = (Impervious * 98) + ((1 - impervious) * (open space curve number in good condition for the specific soil)) \)

where impervious is given as a fraction (not a percent).

For example, if an area on B soils is 50 percent impervious, the curve number is given as (0.5 * 98) + ((1 - 0.50)(61)) = 79.5.

Curve numbers for urban and agricultural areas. Source: [USDA Urban Hydrology for Small Watersheds - TR-55. USDA Urban Hydrology for Small Watersheds - TR-55].
Link to this table

Cover type and hydrologic condition Soil Group A Soil Group B Soil Group C Soil Group D
Open space poor condition (<50% cover) 68 79 86 89
Open space average condition (50-75% cover) 49 69 79 84
Open space good condition (>75% cover) 39 61 74 80
Impervious surfaces 98 98 98 98
Commercial (85% impervious) 89 92 94 95
Industrial (72% impervious) 81 88 91 93
Residential (65% impervious) 77 85 90 92
Residential (30% impervious) 57 72 81 86
Residential (12% impervious) 46 65 77 82
Pervious, no vegetation (newly graded) 77 86 91 94
Fallow with residue cover 74-76 83-85 88-90 90-93
Row crop, no residue 67-72 78-81 85-88 89-91
Row crop with residue 64-71 75-80 82-87 85-90
Pasture, good condition 39 61 74 80
Pasture, poor condition 68 79 86 89
Meadow 30 58 71 78
Woods, good condition 32 58 72 79
Woods, poor condition 57 73 82 86


Runoff coefficients for different soil groups and slopes. Coefficients are for recurrence intervals less than 25 years. Source: Hydrologic Analysis and Design (4th Edition) (McCuen, 2017).
Link to this table

Land use Soil Group A Soil Group B Soil Group C Soil Group D
0-2% 2-6% >6% 0-2% 2-6% >6% 0-2% 2-6% >6% 0-2% 2-6% >6%
Residential (65% impervious) 0.25 0.28 0.31 0.27 0.30 0.35 0.30 0.33 0.38 0.33 0.36 0.42
Residential (30% impervious) 0.19 0.23 0.26 0.22 0.26 0.30 0.25 0.29 0.34 0.28 0.32 0.39
Residential (12% impervious) 0.14 0.19 0.22 0.17 0.21 0.26 0.20 0.25 0.31 0.24 0.29 0.35
Commercial 0.71 0.71 0.72 0.71 0.72 0.72 0.72 0.72 0.72 0.72 0.72 0.72
Industrial 0.67 0.68 0.68 0.68 0.68 0.69 0.68 0.69 0.69 0.69 0.69 0.70
Streets 0.70 0.71 0.72 0.71 0.72 0.74 0.72 0.73 0.76 0.73 0.75 0.78
Parking 0.85 0.86 0.87 0.85 0.86 0.87 0.85 0.86 0.87 0.85 0.86 0.87
Open space 0.05 0.10 0.14 0.08 0.13 0.19 0.12 0.17 0.24 0.16 0.21 0.28
Cultivated land 0.08 0.13 0.16 0.11 0.15 0.21 0.14 0.19 0.26 0.18 0.23 0.31
Pasture 0.12 0.20 0.30 0.18 0.28 0.37 0.24 0.34 0.44 0.30 0.40 0.50
Meadow 0.10 0.16 0.25 0.14 0.22 0.30 0.20 0.28 0.36 0.24 0.30 0.40
Forest 0.05 0.08 0.11 0.08 0.11 0.14 0.10 0.13 0.16 0.12 0.16 0.20


There are numerous studies summarizing TSS exports for different land uses. Examples include the following.

  • Shaver et al. (2007) reported export rates of 1000 lb/ac/yr for commercial, 670 lb/ac/yr for industrial, 420 lb/ac/yr for high-density residential, 250 lb/ac/yr for medium density residential, and 65 lb/ac/yr for low density residential land use.
  • Using the data from the National Stormwater Quality Database, median TSS export is 242 lb/ac/yr for commercial, 193 lb/ac/yr for industrial, 76 lb/ac/yr for residential, and 35 lb/ac/yr for open space.
  • Baldys et al. (1998) reported TSS export of 4430 lbs/mi2/yr for industrial land uses, compared to 2000 lb/mi2 for commercial land use and 1440 lb/mi2/yr for residential land use in the Dallas-Fort Worth area

The studies illustrate the importance of estimating runoff volume, since loading from commercial areas, for example, is greater than from residential areas even though the emc for commercial areas is lower (0.20 mg/L compared to 0.325 mg/L for residential).

Example using the Minimal Impact Design Standards (MIDS) Calculator

Example MIDS Calculator with and without adjusted emcs
Land use % impervious Impervious acres Pervious acres emc (mg/L) Total TSS load (lb/yr) TSS export (lb/ac/yr)
Residential (>40% canopy) 30 0.90 2.1 80 663.8 221.3
Residential (<10% canopy) 30 0.6 1.4 70 387.2 193.6
Commercial 85 0.85 0.15 75 408.8 408.8
Industrial 72 0.72 0.28 93 447.8 447.8
Open space 10 0.10 0.90 21 37.6 37.6
Total suspended solids load with adjusted emcs = 1945.2 pounds/yr
MIDS unadjusted 39.6 3.17 4.83 54.5 1410.7 176.3

The following example illustrates how a variable land use setting may be modeled using the MIDS Calculator.

Site conditions.

  • 31.9 inches annual precipitation
  • B soils with turf
  • 5 acres of residential consisting of the following
    • 3 acres of residential land use with high tree canopy coverage (> 50%) and 30% impervious
    • 2 acres of residential with low tree canopy coverage (<10%) and 30% impervious
  • 1 acre of commercial land and 85% impervious
  • 1 acre if industrial land and 72% impervious
  • 1 acre of open space and 10% impervious

EMCs are as follows.

  • The recommended TSS emc for residential land use is 73 mg/L. We assumed the following for the two residential areas described above.
    • For the high canopy area, we assumed 80 mg/L
    • For the low canopy area, we assumed 70 mg/L
  • Commercial = 75 mg/L
  • Industrial = 93 mg/L
  • Open space = 21 mg/L

Total load with the variable land uses (1945.2 pounds) is much greater than the default MIDS scenario (1410.7 pounds). This is primarily due to the higher emcs and partly due to the higher impervious acreages in the variable land use scenario. The effect of impervious acreage is shown, for example, by reducing the percent impervious for industrial land use from 72% to 50%. This results in a total load of 348 pounds, or a reduction of about 100 pounds (22.3% reduction for a 22% change in impervious). This example also demonstrates the importance of accurately identifying land use within a modeled area.

References