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Adapting stormwater criteria for receiving waters

The State of Minnesota has many different kinds of mandated special watershed and water resource designations that directly influence how stormwater is managed at a site. When these are combined with the even more numerous water resource designations created by localities and watershed organizations (see for example, [[References for Unified Sizing Criteria|WCWC, 2003 and EOR, 2000]]), there is a great deal of potential for overlap and confusion. Indeed, in many regions of the state there can be more area designated and managed for specially protected waters than for regular ones.

This page presents a condensed framework for managing stormwater when these [https://stormwater.pca.state.mn.us/index.php?title=Sensitive_waters_and_other_receiving_waters sensitive waters] need additional protection. Please note there is a focus on additional stormwater management practices that can be used to supplement protection of sensitive receiving waters. Some of these waters currently have limited protections under state or local programs, while others do not. The material in this section is offered as guidance when further stormwater management is deemed necessary or desirable by state or local decision makers.

In addition to the eight specific [[Summary of suggested stormwater criteria for MN receiving waters|“special waters”]] mentioned in the state CGP, there are several specifically protected waters that may warrant supplemental protection relative to stormwater. These include calcareous fens, all DNR designated Public Waters, many kinds of wetlands, shoreland/floodplain areas, areas with active karst, drinking water source areas, impaired waters and the Mississippi River Critical Area. On-line [[Minnesota maps|maps and lists]] exist to help designers and reviewers determine if their development project is located in a special water of the state.

The many different local and state receiving waters noted above that could be addressed by supplemental stormwater management fall into five basic groups:
*lakes;
*trout resources;
*drinking water source areas;
*wetlands; and
*impaired waters.

Some of these groups may be further divided into management subcategories, as shown in the right column of the table below. These subcategories will each be discussed in greater detail.

{{:Classification of receiving waters used in this manual}}

The table below compares the main stormwater management criteria and considerations for all five groups listed in the above table. The text following the table discusses the details of application for each of the receiving water classes.

{{:Summary of suggested stormwater criteria for MN receiving waters}}

==Lakes==
Research has shown that development can increase eutrophication, bacteria and turbidity levels in lakes. According to a national survey of 3,700 urban lakes, more than 80 percent were found to be either eutrophic or hyper-eutrophic ([[References for Unified Sizing Criteria|U.S. EPA, 1980]]). Urban and urbanizing lakes receive higher phosphorus loads than non-urban lakes because urban watersheds, particularly those under construction, produce higher unit area phosphorus loads from stormwater runoff, compared to other watersheds (Caraco and Brown, 2001). 

Impacts of eutrophication on lake and reservoir quality include the following ([[References for Unified Sizing Criteria|Brown and Simpson, 2001]]):
*nuisance algal blooms in the summer;
*reduced dissolved oxygen in the bottom of the lake;
*fish kills due to low dissolved oxygen;
*taste and odor problems with drinking water;
*formation of THMs (trihalomethanes)and other disinfection byproducts in water supplies;
*increased cost to treat drinking water;
*reduced water clarity;
*decline in fish community (more rough fish,fewer game fish);
*blockage of intake screens by algal mats;
*reduced quality of boating, fishing and swimming experience;
*decline in lakefront property values;
*floating algal mats and/or decaying algal clumps; and
*increased density of aquatic weeds in shallow areas.

From a stormwater management standpoint, lakes can be divided into three management categories based on their current trophic status and sensitivity to additional phosphorus loads.
*Most-Sensitive Lakes are normally defined as being oligotrophic.
*Sensitive Lakes are considered to be mesotrophic or slightly eutrophic.
*A third category of all other lakes would include eutrophic and highly eutrophic lakes that are not generally categorized as sensitive because of their relatively poor quality. These lakes should be treated under regular stormwater programs.

The lake designation is normally made by the local or regional lake management authority, although the state may do so for certain special waters such as trout lakes or lake trout lakes. Often, the lake management designation has already been made by the local, watershed, regional or state agencies, or perhaps even by a university or local educational institution. If no designation has been made, the local review authority should consult available data on water clarity, phosphorus content and algal abundance (using Chlorophyll-a as a surrogate measure). If none of these data exist, the local review authority may want to collect lake monitoring data to make a designation. Future phosphorus loadings should also be considered when making a stormwater management designation for a lake.

As a general rule, all surface water drinking supplies, such as water supply reservoirs and river intakes should be managed using the same stormwater sizing criteria as Sensitive Lakes, given the importance of controlling bacteria, toxic pollutants and turbidity that can threaten drinking water quality. The ensuing section presents stormwater guidance for most-sensitive and sensitive lakes, including enhanced sizing criteria and recommendations for BMP design and selection.

===Most-sensitive lakes===
The following adjustments to the basic sizing criteria are recommended for lakes designated as most-sensitive:

*'''Recharge''': Highly recommended

*'''Water Quality''': Highly recommend a site-based phosphorus load reduction if the lake in question is a state-designated “special water” under the [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html CGP] or falls under a similar local regulatory designation; also highly recommended for non-designated lakes with documented oligotrophic quality.

Under this criterion, designers would demonstrate that no increase in total phosphorus (TP) loads will occur at a site from pre-development to post-development conditions using a site-based TP load calculation. The designer could use the [[The Simple Method for estimating phosphorus export|Simple Method]] (Schueler, 1987) or equivalent to compute pre-development and post-development TP loads at the site and determine the pollutant removal requirement (in pounds). The designer would then propose a series of BMPs that maximize the amount of phosphorus removal at the site to reach the desired condition. This criterion provides a major incentive to design for maximum phosphorus removal which is essential for managing most-sensitive lakes. Site-based phosphorus reductions have been adopted by several communities in Minnesota, which vary between no change in phosphorus load to as much as a 25 percent reduction from pre-development conditions. The step-wise [[The Simple Method for estimating phosphorus export|computational approach]] is outlined below.
*Calculate site imperviousness before and after development.
*Calculate the pre-development phosphorus load.
*Calculate the expected post-development pollutant load.
*Calculate the pollutant removal requirement.
*Identify feasible BMPs and calculate load removed (using standard [[BMP pollutant removal for phosphorus|BMP removal efficiencies]]).
*Select off-site mitigation option, if needed.

If a designer cannot meet the total removal requirement, they could be allowed to pay an offset fee that is equivalent to the cost of removing an equivalent amount of phosphorus elsewhere in the watershed.

'''Channel Protection''': Highly recommended if the site drains to a direct tributary stream to a lake.

'''BMP Selection''': The following BMP design and selection guidance is recommended for lakes designated as most-sensitive.

The foremost concern is to choose BMPs with a proven ability to reliably [[BMP pollutant removal for phosphorus|remove high levels of phosphorus]]. Soluble phosphorus is of particular interest since it is most readily available for algal uptake. Therefore, any BMP employed to protect most-sensitive lakes protection should have a moderate to high capability to remove total and soluble phosphorus.

[[Infiltration trench|Infiltration practices]] tend to have the highest phosphorus removal, but are not always be feasible due to soil constraints or lack of the 3 foot separation distance between the bottom of the infiltration device and the seasonally saturated water table. Pond systems are generally a reliable removal option for both soluble and total phosphorus. Filters are fairly effective at removing total phosphorus, but exhibit little or no capability to remove soluble phosphorus. This can be explained by the fact that most sand filters have no biological or chemical processes to bind soluble phosphorus. The addition of organic matter or binding agents to sand filters may show promise in boosting removal, but early monitoring of experimental filters have yet to demonstrate this result conclusively ([[References for Unified Sizing Criteria|Schueler, 2000a]]).

[[Stormwater wetlands|Wetlands]] have a highly variable capability to remove both soluble and particulate forms of phosphorus. The variability can be explained in part by internal phosphorus cycling within the wetland, sediment release, and vegetative dieback during the non-growing season ([[References for Unified Sizing Criteria|Schueler, 1992]]). Factors such as soil pH, oxygen conditions, nutrient saturation and presence of calcium, magnesium or iron in the soil can also make a big difference in whether phosphorus is removed or released. The best design variation for phosphorus removal in the stormwater wetland group is the pond-wetland system (e.g., wetland with a relatively large portion of its storage devoted to a deep pool).

===Sensitive lakes===
The following recommends that stormwater ponds and constructed wetlands discharging to sensitive lakes be sized larger to increase the retention time for additional phosphorus removal. These designs are more conservative than the MPCA sizing rule and could be considered by local authorities interested in greater protection for sensitive lakes. Recommended adjustments to the standard stormwater sizing criteria for Sensitive Lakes are:

:'''Recharge''': Highly recommended.
:'''Water Quality''': Adjusted pond sizing. 

The MPCA water quality [[Water quality criteria|sizing Rule 2]] should be applied to size stormwater ponds (e.g., ponds located within special waters). If the site has more than 30 percent impervious cover, the Walker Rule (see below) presents a size option that should result in similar TP load reductions in ponds (see [[File:Issue paper D - unified sizing criteria for Minnesota.pdf]] for more discussion). The Walker Rule was developed in the upper Midwest to maximize retention time needed within a pond to promote maximum algal uptake of phosphorus and subsequent settling between storm events. The Walker Rule seeks to attain an average pond retention time of about two weeks. Based on the distribution of storm events in the upper Midwest, [[References for Unified Sizing Criteria|Walker]] (1987) recommended all storage via a permanent pool storage volume equivalent to 2.5 inches multiplied by the site runoff coefficient. Based on the Minnesota [[Rainfall frequency maps|rainfall frequency]] spectrum, the Walker Rule would capture about 98 percent of all runoff producing events each year, resulting in very little bypass of untreated runoff. In addition, runoff from many storm events is retained within the pond over several storm cycles to help improve phosphorus uptake. The pond designer should allocate total storage to the permanent pool under the Walker Rule. The total storage in acre-feet needed under the Walker Sizing Rule is provided using the following equation.

'''I. Walker Rule'''

<math>V_{wq} = 3630 RA</math>

where
:V<sub>wq</sub> = required water volume in cubic feet;
:R = runoff for design storm (inches) and is given by
<math>2.5 FI +((2.5-0.2S)^2/(2.5+0.8S)) (1.0 - FI)</math>;
:A = area, in acres;
:FI = Fraction of total impervious cover of site;
:S = (1000/''CN'') - 10;
:CN = curve number; and
:3630 = conversion factor (to cubic feet).

The MPCA water quality sizing [[Water quality criteria|Rule 3]] should be applied if the designer is using a BMP other than a pond (e.g., non-BMPs draining to special waters), keeping in mind that a minimum water quality storage volume of 0.2 watershed inches is recommended for pre-treatment, regardless of site impervious cover.

:'''Channel Protection''': Limited application. 
::Channel protection is recommended if a development site drains to a tributary stream to a lake. 
::Channel protection is not needed if the site discharges directly to the lake.
:'''BMP Selection''': The following guidance on BMP design and selection is offered for Sensitive Lakes:
::Designers should only apply BMPs that have a total phosphorus removal rate exceeding 50 percent.
::Four kinds of BMPS are not recommended in sensitive lakes: media filters, wet vegetated swales, micropool extended detention ponds, and extended detention wetlands. If these BMPS are used, they need to be combined with more effective BMPs in a treatment train. By contrast, infiltration, wet ponds, and bioretention have high phosphorus removal rates, and are strongly encouraged in Sensitive Lakes.

{{:Recommended BMPs for Sensitive Lakes}}

In addition, designers and plan reviewers should evaluate every BMP to look for ways to maximize phosphorus removal (see table below). For example, the use of multiple treatment pathways is encouraged (e.g., directing runoff to a filtering or infiltration BMP, and then routing it a wet pond).

{{:Stormwater Design Recommendations to Enhance Phosphorus Removal}}

Also, as a general rule, no BMPs should be located inside the shoreline buffer, as defined by the local reviewing authority.

==Trout streams==
Trout populations are threatened by stream habitat degradation, stream warming, possible chloride toxicity, and other impacts associated with upland development. Trout are very sensitive to increases in water temperature. The optimal temperature range for adult trout is from about 57°F to 65°F. Generally, adult trout can survive warmer temperatures if cool water refuge is present in the form of ground water upwelling or springs. Juvenile trout, fry and eggs are much more susceptible to warm water temperatures and are not able to tolerate temperatures much above 68°F ([[References for Unified Sizing Criteria|Emmons & Olivier Resources, 2000]]). Stream warming also harms trout by reducing dissolved oxygen available for fish and aquatic life. Increased temperatures can also increase the metabolic rates of aquatic organisms and increase their sensitivity to other pollutants, parasites, and diseases ([[References for Unified Sizing Criteria|SSL SWCD, 2001]]).

<!--{{:BMP pollutant removal for phosphorus}}-->

The reduction in streamside forest cover removes much of the mechanisms that keep a stream cool. The heating of impervious surfaces by solar radiation also warms precipitation that runs over them and potentially into a stream. A series of monitoring studies have documented the stream warming effect in urban trout streams ([[References for Unified Sizing Criteria|Roa-Espinosa et al. 2003; SSL SWCD, 2001; Johnson, 1995; Galli, 1990]]).

Sedimentation is also a major concern for trout. Construction runoff, channel erosion and road sand all increase sediment loads which can impair streambed habitat in trout streams. Excess sediment can affect the productivity of a trout stream in several ways. Sediment can impede trout respiration by clogging gill plates. In addition, sediment deposition can destroy spawning habitat and harm the benthic organisms upon which the trout feed.

Road salt may also significantly impact trout habitat. Chloride is one of the main components of road salt, and is extremely soluble in water. As a result, there is virtually no way to remove chloride once it gets into either surface or groundwater. Chloride levels are the highest in late winter as initial melting occurs from snow containing significant amounts of road salt and stream flows are lowest. The chloride from the salt can be toxic in trout streams during some meltwater events. For more detained discussion see [[Cold climate impact on runoff management]].

The following adjustments to the basic sizing criteria are recommended to protect trout streams.

:'''Recharge''': Highly recommend infiltration as part of stormwater control.
::It is highly recommended that all excess runoff volume above that produced from the pre-development 2- year, 24-hour storm event should be infiltrated for designated trout streams, where soils conditions permit. The state [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html CGP] contains this method as one option to meet mandated temperature control for designated trout streams.

:'''Water Quality''': Discourage use of ponds/wetlands
::Use MPCA water quality [[Water quality criteria|Sizing Rule 3]] for non-pond BMPs volume determination for special waters, and infiltrate and/or filter this volume at the site regardless of soil conditions (e.g., bioretention, dry swales, infiltration, and better site design practices). Discharge from ponds or wetlands with standing water to trout streams is discouraged. If they are used, they should be sized according to MPCA [[Water quality criteria|Sizing Rule 2]], incorporate temperature controls, and have an extended detention time no longer than 12 hours.
:'''Channel Protection''': Highly recommended
::Given the importance of trout habitat, it is highly recommended that channel protection criteria be applied to all trout streams. If soils do not permit infiltration of the channel protection volume, then designers should provide 12 hour extended detention of 1-year, 24-hour runoff volume in a thermally acceptable pond option. Note that CGP allows up to 24 hours, but 12 is recommended in the Manual. Release of the 1-yr, 24-hour volume in 12 hours should be compared with the ½ 2-yr pre-development peak matching method described previously to determine which approach would result in less heating of the stored water.
:'''BMP Selection''': There are quite a few do’s and don’ts when it comes to BMP design for trout streams.

Designers should look for ways to incorporate the following design features into their BMPs:
*consider using forested wetland design (see Capiella 2005b);
*maximize use of better site design techniques;
*use infiltration and bioretention to the maximum extent possible;
*construct BMPs “off-line”;
*shade pilot and outflow channels and micropools by planting trees and shrubs;
*plant trees to the maximum extent possible in the stormwater practices and buffers;
*manage buffers to maximize forest cover and shading in riparian areas;
*pre-treat roadway runoff to reduce sediment and road sand discharges to streams; and
*develop and test roadway spill management plans.

Designers should ensure that each BMP does not have
*a large, unshaded permanent pool or shallow wetland;
*extensive and unshaded pilot and outflow channels within the BMP;
*an extended detention time longer than 12 hours;
*extensive exposed riprap or concrete channel;
*an on-line or in-stream location;
*a location within the forested buffer; and
*infiltration practices that are undersized or lack pre-treatment.

==Drinking water source areas==

===Groundwater===
This group includes any groundwater recharge areas that supply water used for drinking water supply. The management goal is to maintain groundwater recharge while preventing the possibility of groundwater contamination. Groundwater is a critical water resource, as many residents depend on groundwater for their drinking water, and the health of many aquatic systems depends on steady recharge to maintain surface water bodies throughout the year. For example, during periods of dry weather, groundwater sustains flows in streams and helps to maintain the hydrology of wetlands. Because development creates impervious surfaces that prevent natural recharge, a net decrease in groundwater recharge rates can be expected in urban watersheds. Thus, during prolonged periods of dry weather, stream flow sharply diminishes. In smaller headwater streams, the decline in stream flow can cause a perennial stream to become seasonally dry.

Urban land uses and activities can also degrade groundwater quality if stormwater runoff is directed into the soil without adequate treatment. Certain land uses and activities are known to produce higher loads of metals and toxic chemicals and are designated as [[Potential stormwater hotspots|potential stormwater hotspots]] or “PSHs”. Soluble pollutants, such as chloride, nitrate, copper, dissolved solids and some hydrocarbons can migrate into ground water and potentially contaminate wells. Stormwater runoff should never be infiltrated into the soil from sites designated as a PSH. Below is a list of business operations at potential stormwater hotspots (adapted from [[References for Unified Sizing Criteria|MDE, 2000]]). Note that road surfaces are not always considered PSHs unless a history of contaminated water has occurred.
*vehicle salvage yards and recycling facilities;
*outdoor liquid container storage;
*vehicle service and maintenance facilities;
*outdoor loading/unloading facilities;
*vehicle and equipment cleaning facilities;
*public works storage areas;
*fleet storage areas (bus, truck, etc.);
*facilities that generate or store hazardous materials;
*industrial sites;
*commercial container nursery;
*marinas (service and maintenance);
*large parking lots;
*transportation routes and fueling areas;
*large chemically managed turf areas.

Stormwater hotspots commonly occur as commercial, industrial, institutional, municipal, or transportation-related operations that produce higher levels of stormwater pollutants, and/or present a higher potential risk for spills, leaks or illicit discharges. Runoff from these operations may contain soluble pollutants which cannot be effectively removed by current BMPs and can contaminate ground water quality.

Typical sources of nutrients, metals, hydrocarbons, toxins and other pollutants that can be generated from PSH are summarized in the table below. It should be noted that not all of these operations or activities will actually generate pollution at an individual stormwater hotspot. In fact, many industrial operations are highly regulated under state and federal programs. There are, however, many small or unregulated facilities (such as gas stations or auto salvage yards) that are of concern because of the potential for release of toxic material to stormwater.

{{:Stormwater Pollutants Associated With Common Operations at Potential Stormwater Hotspots}}

The management goal in groundwater drinking water source areas is to prevent possible groundwater contamination by preventing infiltration of untreated hotspot runoff. At the same time, recharge of unpolluted stormwater is needed to maintain flow in streams and wells during dry weather. As such, structural BMPs alone should not be relied upon as a sole stormwater management strategy at a PSH. A stormwater pollution prevention plan for a PSH should also incorporate a combination of
*good housekeeping;
*preventive maintenance;
*spill prevention and clean-up;
*employee training;
*inspections;
*record-keeping; and
*chemical use restrictions.

The following adjustments to the standard stormwater sizing criteria are recommended to protect the quality of groundwater drinking water source areas.

'''Water Quality''': Enhanced sizing and pre-treatment. 
MPCA water [[Water quality criteria|quality sizing Rules 2 or 4]] should be applied to development sites within groundwater drinking water source areas, depending on whether a pond or non-pond BMP option is being considered. A minimum of 0.2 watershed-inches of effective pre-treatment is recommended for non-pond BMPs to remove pollutants prior to any infiltration or soil filtration.

'''Recharge''': Encouraged in limited situations. 
Infiltration is encouraged at residential subdivisions to increase groundwater recharge through rooftop disconnections and other better site techniques. Commercial and institutional rooftops can also be disconnected as long as they are not a potential stormwater hotspot. No infiltration or recharge of runoff from potential stormwater hotspot operations should be allowed to reduce the risk of groundwater contamination. Caution on the source of infiltrating water should be exercised in all cases.

'''BMP Selection''': The following guidance on BMP design and selection is offered to protect groundwater drinking water source areas.
*In general, infiltration of clean runoff from residential and non-residential rooftops is encouraged with acceptable pre-treatment.
*Stormwater ponds, wetlands, bioretention, and filters are effective surface treatment
*No infiltration from PSHs, especially those with potentially high chloride levels and/or vulnerable ground water resources.
*Minimum setbacks from wells, septic systems, sinkholes and wellhead protection zones in conformance with state and local regulations (contact Minnesota Department of Health) and plans.
*Avoid pooling or infiltrating stormwater in active [[Karst|karst]] areas.

Additional BMP design criteria for groundwater protection are presented in the following table.

{{:BMP design considerations for ground water aquifer protection}}

==Surface water==
There is a large portion of Minnesota residents served by drinking water obtained from a surface water source. The supplies for the St. Cloud, Minneapolis and St. Paul metropolitan areas are obtained mostly from the Mississippi River; St. Paul’s supply is supplemented by both small stream flow and ground water. Several other cities throughout the state are also supplied by smaller rivers such as the Minnesota/Blue Earth, Red Lake and Red Rivers, by Lake Superior or by large abandoned quarries in the Iron Range. In each of the river source areas, protection of the surface water source reaches far beyond the local border to the entire watershed feeding the supply intake. For the quarries, inflow occurs primarily from groundwater sources that must be protected as noted in the previous section. Lake Superior itself requires attention, as do the tributary streams that feed it.

Each of the surface water sources is preparing or has prepared a source water protection plan in which they identify potential pollutants of interest and the likely source of those pollutants. They also must put together a plan to protect the source of water. This plan, as is the case for the Mississippi River communities, can stretch far upstream (or up-gradient for groundwater) to areas not under the control of the served communities. This severely limits the direct control that the supplied communities have over pollution generating activities. Fortunately, a willingness to help protect these drinking water source areas has led to multi-community cooperative protection efforts.

The pollutants mentioned in the previous groundwater section certainly all apply to surface water sources. In addition, surface water suppliers have to be concerned about such things as sediment, phosphorus, nuclear waste (Mississippi River suppliers), any cargo hauled through the watersheds on rail or roads, or on the water in barges, [[Potential stormwater hotspots|PSHs]], fire-fighting runoff and a myriad of other potential surface water contaminants. All of the precautions mentioned in the previous section for groundwater source areas should also be applied to surface waters that provide drinking water.

The management goal in surface water drinking water source areas is to prevent possible source contamination by preventing any potential contaminant from reaching either the stream or river providing the water or any groundwater inflow that will eventually feed a surface water source. [[Pollution prevention]] and emergency response become primary BMP approaches for source waters. The list of focal BMPs remains similar to the groundwater list noted previously with the addition of good watershed management to control pollutants associated with nonpoint sources.

The following adjustments to the standard stormwater sizing criteria are recommended to protect the quality of surface water drinking water source areas.

'''Water Quality''': Enhanced sizing and pre-treatment. 
MPCA water quality [[Water quality criteria|sizing Rules 2 or 4]] should be applied to development sites within surface water drinking water source areas that are determined in a source water protection plan to be critical to maintaining the quality of the source water. A minimum of 0.2 watershed-inches of effective pre-treatment is recommended for non-pond BMPs to remove pollutants prior to any infiltration or soil filtration.

'''Recharge''': Encouraged for watersheds, with caution for groundwater feeding a surface water source.

Infiltration is encouraged within watersheds upstream of drinking water intakes from surface water. Protective measures consistent with the previous groundwater supply section are encouraged for groundwater feeding surface water sources.

'''BMP Selection''': Supplemental BMPs should follow those suggested for Sensitive Lakes. The following guidance on BMP design and selection is offered to protect surface water source areas.
*A pollution prevention plan is essential for the entire area draining to the surface water intake.
*Stormwater ponds, wetlands, bioretention, and filters are effective surface treatment.
*No infiltration or direct runoff in the vicinity of the intake from PSHs, especially those with potentially high chloride levels and/or vulnerable ground water resources.
*An emergency response plan should be prepared for spill response in areas critical to supply protection.

==Wetlands==
For a long time, wetlands were viewed as wastelands that were better drained or filled. It is estimated Minnesota has lost nearly 42 percent of its original wetland acreage ([[References for Unified Sizing Criteria|MN SWAG, 1997]]). Wetlands are now recognized as performing many important watershed functions and services, and their direct disturbance is closely regulated. there are several state, local and federal aspects of wetland [[Regulatory information|regulation]] and management.

Naturally occurring quantities of runoff with seasonal fluctuations are essential for the maintenance of a wetland, and moderate amounts of nutrients and sediment in the runoff can increase a wetland’s productivity. However, excessive stormwater runoff has the potential to alter the hydrology, topography, and the vegetative composition of a wetland ([[References for Unified Sizing Criteria|U.S. EPA, 1993]]). For example, an increased frequency and duration of inundation can degrade native wetland plant communities or deprive them of their water supply.

Stormwater inputs can also cause changes in water or soil chemistry that can degrade wetlands. This is a particular concern for wetlands with a narrow pH range such as acidic sphagnum bogs and alkaline calcareous fens ([[References for Unified Sizing Criteria|MN SWAG, 1997]]). Calcareous fens are the rarest wetland plant community in Minnesota, and as such are specially protected. These fens are peat-accumulating wetlands dominated by distinct groundwater inflows having specific calcium carbonate chemical characteristics. Flows are circum-neutral to alkaline, with high concentrations of calcium and low dissolved oxygen content. The water chemistry creates a unique environment for a disproportionately large number of rare, threatened, and endangered wetland plant species compared to other plant communities in the Great Lakes region (MN SWAG, 1997). Changes in wetland water quality can alter the nature of the plant community, encouraging invasive species, and reducing sensitive species that are preferred by fish, mammals, birds, and amphibians for food and shelter ([[References for Unified Sizing Criteria|U.S. EPA, 1993]]).

Stormwater runoff inputs can exceed the water depths and frequency/duration of inundation prevalent in natural wetlands. Deposition of sediment carried by urban stormwater can have the same effect, causing replacement of diverse species with monotypes of reed-canary grass or cattails, which are much more tolerant of sedimentation and fluctuating water levels. [[References for Unified Sizing Criteria|Schueler]] (2000b) reported that invasive or aggressive plant species are favored when water level fluctuation (WLF) is high (e.g., reed-canary grass). The result is low vegetative diversity and lower quality wildlife habitat values (MN SWAG, 1997). A modest change in WLF sharply decreases plant species richness, and amphibian species richness a study in the Pacific Northwest ([[References for Unified Sizing Criteria|Horner, et al., 1996]]). Some communities have used existing wetlands for stormwater treatment by increasing the depth of ponding on a permanent or temporary basis. The end result is the transformation of a natural wetland into a stormwater wetland, with the attendant loss of diversity and functional values.

Not all wetlands respond in the same way to the impact of stormwater runoff. In the context of this Manual, wetlands can be defined as Susceptible or Non-Susceptible to stormwater runoff, based on the MN SWAG (1997) wetland classification scheme. This classification provides a useful framework for managing stormwater inputs to different types of wetlands.

Highly susceptible wetland communities can be composed of dozens of plant species. The table below presents the MN SWAG classification of wetland types according to their presumed susceptibility to degradation by stormwater. Given this diversity of wetland types, it is not surprising that wetlands have a broad range of tolerance to stormwater runoff. Some wetlands (e.g. calcareous fens) are sensitive to any disturbance and will show signs of degradation with even low-level inputs of urban stormwater. Note that Susceptible Wetlands are defined as highly and moderately susceptible and Non-Susceptible Wetlands are defined as slightly and least in the table.

{{:Susceptibility of wetland types to degradation by stormwater input}}

The following adjustments to the standard stormwater sizing criteria are recommended to protect wetlands from the indirect impact of stormwater runoff. Note that wetlands are highly [[Regulatory information|regulated]] within the state and that all federal, state and local/watershed authorities should be consulted before any activity is initiated on any parcel of land that appears to be a wetland.

'''Recharge''': Highly recommended for Susceptible Wetlands.
Many Susceptible Wetlands are dependent on groundwater to maintain their natural hydrology so it is important to maintain recharge at a consistent rates in the contributing source area to the wetland. Recharge is also recommended for Non-Susceptible Wetlands that are dependent on groundwater.

'''Water Quality''': Recommend site based phosphorus load reduction. 
Site-based phosphorus load reduction for Susceptible Wetlands using the method described for Most-Sensitive Lakes are recommended to control nutrients. Site-based nutrient load reduction should be used for nutrient sensitive bogs and calcareous fens. No untreated stormwater discharges should be allowed to Non-Susceptible Wetlands, which are operationally defined as providing water quality volume according to MPCA [[Water quality criteria|sizing Rules 2 and 4]] (depending on the type of BMP chosen). Currently, the MPCA interprets the CGP as requiring a permanent pool in constructed stormwater wetland systems. While this seems appropriate for a [Stormwater ponds|pond]]/wetland system, it does detract from the bioretention character of other [[Stormwater wetlands|wetland BMPs]]. The application of a permanent pool to constructed stormwater wetland systems that behave as [[Bioretention|bioretention]] systems should be considered for change in the next CGP update.

In addition, Susceptible Wetlands should not be used for stormwater treatment. A Non-Susceptible Wetland should only be used for stormwater treatment if designers can demonstrate that it will restore wetland functional value, and only when approved by the local government unit acting as approving agency under the [https://bwsr.state.mn.us/wca-program-guidance-and-information Minnesota Wetland Conservation Act].

'''Channel Protection''': Limited. 
Channel Protection is recommended only when a channel is a direct tributary to a wetland.

'''Other''':Maintain wetland hydroperiod.
Designers should maintain the hydroperiod of Susceptible Wetlands following development to prevent detrimental impacts. Any wetlands present at the site should be investigated in the field to determine their wetland type and contributing hydrologic source area, and then determine if any additional runoff will be delivered to the wetland as a result of the proposed project. Based on this determination, a wetland will be classified as either [[Susceptibility of wetland types to degradation by stormwater input|Susceptible or Non-Susceptible]].

The following table presents hydroperiod guidelines for wetlands, developed by MN SWAG (1997) for use unless better site-specific data are available. The term “existing” means the existing hydrologic conditions. If there have been recent significant changes in conditions, it means the conditions that established the current wetland. Designers then model the effect of runoff discharge from the site on the wetland to ensure they conform to the storm bounce and inundation duration guidelines standards using infiltration, extended detention, diversion or other methods.

{{:Recommended hydroperiod standards for wetlands}}

'''BMP Selection''': Additional guidance on BMP design to protect wetlands is offered below.
*BMPs such as stormwater wetlands, infiltration systems, and bioretention are encouraged to treat runoff prior to discharge to a wetland.
*Direct pipe outfalls to wetlands should be restricted (e.g., not allowed, allowed if energy dissipated, or routed through a pre-treatment system).
*Stormwater should be routed around sensitive wetlands using a diversion or bypass system.
*Constrictions at wetland outlets should be avoided.
*Natural wetlands should not be used for stormwater treatment, unless they are severely impaired and construction would enhance or restore wetland functions; if natural wetlands are used in this manner, [https://www.revisor.mn.gov/rules/?id=7050 MN Rules 7050] establishes the sequence of avoid, minimize and compensatory replacement.
*The discharge of untreated stormwater to a wetland is prohibited.

==Impaired waters==
Under the [https://www.epa.gov/laws-regulations/summary-clean-water-act Clean Water Act], Minnesota administers water quality standards which consist of numeric and narrative criteria that protect the physical, chemical and biological integrity of surface waters in the state. These criteria are set to maintain seven designated or beneficial uses of water in the state. The state routinely monitors the quality of its waters to determine if they are meeting their designated uses. If monitoring indicates that water quality standards are not being met and/or designated uses are not being achieved, the state lists the water as being “impaired”. This, in turn, triggers the Total Maximum Daily Load (TMDL) provisions of the Clean Water Act.

A [http://www.pca.state.mn.us/water/tmdl/index.html TMDL study] consists of an analysis to determine what pollutant reduction is needed to achieve water quality standards, and is normally conducted at the watershed scale. A TMDL determines the amount of pollutants that a waterbody can receive from both point and nonpoint sources and still meet water quality standards (e.g., no impairment). Water quality sampling and computer modeling determine how much each pollutant source needs to be reduced to assure the water quality standard is met.

Impaired waters include streams and lakes that do not meet their designated uses because of excess pollutants or identified stressors. As of 2004, 916 lakes and 199 river and stream segments were listed as impaired waters for Minnesota ([[References for Unified Sizing Criteria|MPCA, 2004]]). Each listed water will ultimately require a TMDL based on the assessment. Currently, there are 14 pollutants causing water quality standard violations in some part of the state.
*sediment
*phosphorus
*nitrogen
*ammonia
*fecal coliform
*oxygen demand
*turbidity
*chloride
*DDT
*dieldrin
*mercury
*PCBs
*toxaphene
*dioxin

To date (fall 2005), only four final TMDLs and their corresponding implementation plans have been completed in Minnesota, so many listed waters currently lack a TMDL or are in the process of developing one.

While none of the completed TMDL implementation plans currently contain stormwater requirements, they may eventually be included if stormwater pollution is determined to be a significant source of the listed pollutant. Therefore, development projects that occur in a listed watershed may require a higher level of stormwater treatment, regardless of whether a TMDL has been completed or not. The main reason is that both municipal NPDES Phase I or II stormwater permits and individual construction general permits must be consistent with the load allocations and pollutant reductions contained in an approved TMDL. If stormwater runoff is likely to be a significant pollutant source within a listed watershed, the local review authority may elect to require higher levels of stormwater treatment to restore the impaired water.

Some general guidance on how to deal with stormwater pollutant loads at development sites located within listed waters is provided below.

First, the local review authority should check with MPCA to determine:
*whether any local waters as listed as impaired;
*which pollutant(s) is causing the impairment to be listed;
*the estimated watershed area to the receiving water;
*the timeframe under which the water will fall under the TMDL program; and
*whether stormwater is expected to be a significant source of the impairment for the indicated pollutant.

Second, if an impairment exists, the local reviewing authority should determine whether the indicated pollutant is considered computable or non-computable. In the context of stormwater, “computable” is defined as a pollutant for which enough data exist to perform a site-based pollutant load calculation that documents no increase or even a reduction in pollutant loading. By contrast, “non-computable” pollutants lack enough data to perform a reliable site based pollutant reduction calculation. See [[File:Issue paper E - receiving water based stormwater criteria final.pdf]] for determining pollutant computability. Computable pollutant must pass the following four tests:
*enough stormwater EMC data is available to characterize its average level in stormwater;
*stormwater concentrations are high enough to constitute a major source in the stormwater load allocation;
*sufficient BMP performance data are available to estimate expected removal for a range of stormwater practices; and
*stormwater removal rates are high enough to warrant performing the calculation.

Currently, only five pollutants meet all four criteria -- sediment, phosphorus, nitrogen, ammonia, and fecal coliform bacteria). A stormwater strategy to deal with computable and non-computable pollutants within listed is offered below.

'''Water Quality''': Computable pollutants
If a new development site is located in a watershed subject to a TMDL that has no remaining stormwater allocation, the local review authority may wish to adopt a “no net increase” policy for the listed computable pollutant (e.g., sediment, phosphorus, nitrogen, ammonia or fecal coliform). Pollutant removal calculations should be conducted on a site-by-site basis, using the general method proposed for the Most-Sensitive Lakes, adapted for the listed pollutant.

'''Water Quality:''' Non-computable pollutants
Since non-computable pollutants lack enough data to perform a site-based load reduction calculation, they can only be managed by increasing the V<sub>wq</sub> assuming that a higher level of pollutant reduction will occur within the BMP. In these situations, the local review authority may wish to require that development sites satisfy MPCA water quality volume [[Water quality criteria|sizing Rules 2 or 4]], depending on the type of BMP employed.

'''Channel Protection''': Recommend for waters listed for sediment or sediment related pollutant. 
Given the importance of channel erosion in the sediment budget of urban streams, it is advisable to require channel protection criteria in watersheds that are listed for sediment. In all cases, the local review authority should check with MPCA to determine what, if any, water quality or channel protection requirements need to be addressed as part of TMDL implementation.

'''BMP Selection''': The selection and design of specific BMPs to address impaired water pollutant reductions will be determined through the TMDL process.

[[Category:Level 3 - Best management practices/Specifications and details/Design criteria]]