TCMA Chloride Management Plan - TCMA Chloride Conditions
Contents
TCMA Chloride Conditions
Chloride data across the TCMA was compiled and assessed to support the development of the CMP. As part of the TCMA Chloride Project, the MPCA worked with local partners to develop and implement a chloride monitoring program. The objective of the monitoring program was to better inform an understanding of chloride conditions across the TCMA, including seasonality, trends over time, and the potential for high chloride concentrations in the deepest part of lakes. Seventy-four lakes, 27 streams, and eight storm sewers were monitored as part of this effort from 2010-2013. The Chloride Monitoring Guidance for Lakes and Chloride Monitoring Guidance for Streams and Stormsewers were developed by the MPCA and local experts from the TCMA Chloride MSG and can be found on the MPCA’s TCMA Chloride Project website. The monitoring guidance provides recommendations on sample collection, times of the year to sample, as well as guidance for monitoring high risk waters. In addition to data collected in 2010-2013 following the TCMA Chloride Project monitoring program, chloride data from a host of other sources and timeframes were compiled. The data were collected by several local organizations including the MPCA, the United States Geological Survey (USGS), Capitol Region Watershed District (CRWD), Metropolitan Council Environmental Services (MCES), Minneapolis Park & Recreation Board (MPRB), Minnehaha Creek Watershed District (MCWD), Mississippi Watershed Management Organization (MWMO), Ramsey County Environmental Services, Ramsey-Washington Metro Watershed District (RWMWD), Rice Creek Watershed District, Scott County Watershed Management Organization, and Three Rivers Park District. A large portion of the data were compiled and submitted to the State of Minnesota’s Environmental Quality Information System database (EQuIS). All data collected by Metropolitan Council are available on their Environmental Information Management System (EIMS) database, and data collected by USGS are available on their water-quality data for the Nation database: waterdata.usgs.gov/nwis/qw.
The impacts of climate change create uncertainty related to winter salt application and chloride levels in TCMA waters in the future. Predictions provided by the United States Global Change Research Program for the TCMA area include warmer winter temperatures by 5 - 6 degrees Fahrenheit, longer freeze-free seasons increasing by 20-30 days, greater winter precipitation, and the likelihood of more frequent extreme events (Kunkel et al. 2013). On the one hand, these predictions of climate change may result in reduced salt use. On the other hand, more frequent snow events, more extreme events, and potentially more frequent ice storms may result in greater needs for deicing roads. Continued monitoring of climate change and chloride concentrations in the TCMA waters, tracking of salt use on all paved surfaces, and an adaptive process will be needed to restore and protect the TCMA waters from chloride impairments with the prospects of a changing climate.
The remainder of this section will present an overview of the assessments conducted based on the available data, including determinations of impairment, time and spatial trends in chloride concentrations, the TMDLs developed for impaired waters, and waters showing a high-risk for future impairment.
Condition Status
This section describes the current status of water resources within the TCMA with respect to applicable chloride criteria. The status of surface waters including lakes, wetlands and streams is presented first. The status of groundwater resources is presented second.
Surface Water
The MPCA’s approach to determining whether or not a stream, lake, or wetland is impaired by chloride relies on an assessment of available data. The MPCA conducted an assessment for chloride in the TCMA waterbodies in 2013. Two or more exceedances of the chronic criterion of 230 mg/L within a three-year period are considered an impairment. One exceedance of the acute criterion of 860 mg/L is considered an impairment. The 2013 TCMA chloride assessment resulted in 29 new chloride impairments (6 streams, 19 lakes, and 4 wetlands) added to the 2014 draft impaired waters list, resulting in a total of 39 chloride impairments in the TCMA. Shingle Creek and Nine Mile Creek were previously listed as impaired with completed chloride TMDLs. Approximately 11% of the 340 waterbodies assessed were determined to be impaired. An additional 38 (11%) were classified as high risk and 11% did not have enough data. High risk was defined as a waterbody having one sample in the last 10 years that was within 10% of the chronic criteria (207 mg/L). An interactive map showing assessed, impaired, not impaired, and high risk waters is on the MPCA Chloride Project website (MPCA Chloride Project Website Map of Assessments and Impairments). The assessed lakes, wetlands, and streams are shown on Figure 1. The highest density of impairments is in the heavily urbanized area in Hennepin and Ramsey Counties, though three streams in the outlying suburban areas are also impaired by chloride. The chloride causing impairments in the streams in the outlying areas of the metro is largely effluent from the WWTPs, rather than deicing salt.
It is important to keep in mind that of the over 1,000 lakes, wetlands and streams in the TCMA, less than one-third had chloride data to make an assessment of impairment/attainment of water quality criteria. Also, of those waters with adequate data to make an assessment, only 30% were part of the TCMA Chloride Project monitoring program, which was developed to collect samples at critical times of the year and critical locations. As a result, data used to evaluate water quality conditions in waters not part of the TCMA Chloride Project monitoring program, may not have been representative of critical conditions. Critical times of the year for collecting chloride samples are typically during the winter snowmelt runoff (February through March) and during low flow periods, and critical areas for collecting chloride samples in a lake are near the bottom.
The impaired lakes, wetlands, and streams were compared by the concentrations of chloride ranked from highest to lowest concentrations. These rankings are presented in Figure 2 and Figure 3. These figures are not a direct reflection of the 303(d) listing assessment; they are intended to make a relative comparison of the extent of impairment across impaired waters. The values presented in these figures were calculated by identifying the maximum chloride concentration measured in a waterbody on individual sampling days, then averaging all individual sampling day maximums that exceeded the standard of 230 mg/L from 2003-2013. These figures indicate the variability in one waterbody or watershed to the next by the severity of the impairment. These rankings can be used by chloride users to prioritize management activities by area. Since only a portion of the TCMA waters have chloride monitoring data, the rankings can also be used to determine specific areas that are close to impaired and high-risk watersheds for further monitoring and higher levels of management.
Groundwater
Chloride concentrations in shallow groundwater are increasing, likely as a result of the application of deicing salt. This correlation is observed in a recent study by the MPCA, The Condition of Minnesota’s Groundwater, 2007-2011, which found that chloride concentrations were higher in wells sampled in urban areas, where salt is more commonly applied in winter months, compared to wells sampled in areas that were undeveloped (Table 2).
Average chloride concentrations in groundwater based on land use
Link to this table
| Land Use | Chloride (mg/L) |
|---|---|
| Residential | 45 mg/L |
| Commercial/Industrial | 60 mg/L |
| Undeveloped | 15 mg/L |
The median chloride concentration in sand and gravel aquifers in the TCMA was 86 mg/L, compared to a median concentration of 17 mg/L in sand and gravel aquifers outside the TCMA. Twenty-seven percent of sand and gravel monitoring wells in the TCMA had chloride concentrations greater than 250 mg/L, the secondary maximum contaminant level set by the EPA (Figure 4). Very few monitoring wells outside the TCMA (about 1%) had chloride concentrations exceeding 250 mg/L.
Chloride Sources
Chloride enters the TCMA lakes, streams, wetlands, and groundwater from a variety of sources. The relative significance of each source of chloride is dependent on the watershed. For highly developed urban areas, winter maintenance activities are typically the primary source. In less developed areas where point source discharges exist, the municipal wastewater treatment facilities may be the primary source of chloride, which in most cases is due to water softening. A conceptual model diagram of the primary anthropogenic sources is shown in Figure 5. A chloride budget for the TCMA estimated that only 22%-30% of the chloride applied in the TCMA was exported out of the TCMA via streamflow in the Mississippi, Minnesota, and St. Croix Rivers (Stefan et al. 2008). Therefore, 70%-78% of the applied chloride remains in TCMA lakes, wetlands, and groundwater and it may also be stored in soil-water where infiltration is slow. Since chloride is an element and does not breakdown over time, the high percentage retained in the TCMA suggests that chloride will continue to accumulate and eventually make its way to the deep aquifers. This implies that, on average, chloride concentrations in the TCMA waterbodies are increasing with time. If the chloride loading remains steady, the concentrations will level out when equilibrium develops between loadings and transport out of the TCMA. By the same token, if loadings are reduced sufficiently and persistently, the chloride concentrations in the TCMA waterbodies will begin to decrease and will continue to decrease until a new equilibrium is reached. Each of these sources is briefly described below.
As shown in Figure 6, land use in the TCMA is largely urban in the core of Minneapolis and St. Paul with a transition to rural and agricultural moving outward through the suburbs. The primary source of chloride may shift locally depending on land use. Section 2.3 discusses the correlation of road density and chloride concentrations in surface waters.
Winter Maintenance Activities
Winter maintenance activities include snow and ice removal. Application of deicing and anti-icing chemicals, primarily salt, is common. Salt is applied to a variety of surfaces including roads, parking lots, driveways, and sidewalks. Runoff from salt storage facilities is another potential source of salt. The St. Anthony Falls Laboratory at the UMN developed an inventory of salt uses in the TCMA for a Minnesota Department of Transportation (MnDOT) Local Roads Research Board study (Sander et al. 2007). The inventory estimated the total amount of salt used for winter maintenance activities in the TCMA, based on purchasing records, to be 349,000 tons per year. Estimates of use by various entities are shown in Figure 7.
