Municipal Separate Storm Sewer System (MS4) permits must be consistent with Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs). Minnesota’s Phase 1 (Minneapolis and St. Paul) permit and Phase 2 MS4 General Permit require regulated MS4s to demonstrate that they meet the WLA. TMDL implementation plans offer information on strategies to reduce point (WLA) and nonpoint (LA) source pollution and can be a valuable resource for regulated entities that must achieve a reduction in loading from their MS4.
An implementation plan that has a regulated MS4 component should contain information on four broad topics.
This guidance provides a recommended list of elements that enhance the linkage between implementation plans and MS4 permits. The level of detail in each implementation plan will vary.
The first three chapters provide an overview of the TMDL, TMDL implementation plan, and MS4 Stormwater General Permit. Chapter 4 discusses the elements of a TMDL implementation plan that may be appropriate for MS4 stormwater.
Each of the four sections in Chapter 4 has introductory text that describes the potential elements for an implementation plan. The elements are summarized in a text box within each section of Chapter 4. Each of the elements is then discussed in greater detail in the body of the guidance. Some examples are included as insets.
Several terms in the guidance are highlighted with blue font. These terms are defined at the end of the main body of the guidance and prior to the Appendices. Recommended websites are also highlighted in blue. In an electronic version of this guidance, these terms and websites are hyperlinked.
Appendices A, B, and C provide case studies, example calculations and more detailed discussion of BMP strategies for specific pollutants. These appendices may be useful for MS4s seeking clarity to the discussion in this document.
This document provides guidance for developing TMDL implementation plans that have a regulated MS4 component. It is not policy. TMDL projects vary significantly. This document outlines a preferred approach to developing an implementation plan. It is in the stakeholders interest to participate in development of an implementation plan because the plan can serve as a road map for meeting permit requirements.
The reader may wish to familiarize themselves with other policy and guidance for MS4 stormwater. These can be found at .
This guidance will be updated as needed, particularly Appendix C, which focuses on pollutant-specific stormwater management strategies and BMPs.
This guidance document provides a discussion of components that could comprise a Total Maximum Daily Load (TMDL) implementation plan for Municipal Separate Storm Sewer System (MS4) stormwater. The document is primarily written for people who are involved in development of an implementation plan. It is intended for just the MS4 Stormwater portion of the implementation plan. This includes both regulated and non-regulated MS4 stormwater, although the focus of this document is on regulated stormwater.
A TMDL is the quantity of pollutant than can be discharged to a lake or stream and still have the lake or stream meet its designated use, typically determined by compliance with water quality standards. TMDLs are developed for lakes or streams that currently exceed the water quality standard for one or more pollutants. The TMDL is comprised of pollutant loads from nonpoint sources (the Load Allocation), loads from point sources, which includes sources that require or will require a National Pollution Discharge Elimination System (NPDES) permit (the Wasteload Allocation), and a Margin of Safety.
Pollutant loads from NPDES-permitted (regulated) MS4 stormwater must be included in the Wasteload Allocation (WLA). Federal law requires NPDES permits to be consistent with any TMDL WLA. When an MS4’s current load exceeds its WLA, the MS4 must implement practices to reduce that load. For more information on the relationship between TMDLs and NPDES-permitted stormwater, see the EPA memo “Establishing Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those WLAs” (November 22, 2002)(),a second revision to this memo (November 12, 2010) (), and the most recent revision to this memo (November 26, 2014)().
The TMDL implementation plan is completed concurrent with or shortly after completion of the TMDL. Following a period of implementing pollution control practices, an MS4 will meet the WLA. At this point, the lake or stream may reach water quality standards. For many impaired waters, there may be a considerable period of time between an MS4 meeting the WLA and the water meeting water quality standards. Even though a WLA has been met, an MS4 will need to maintain the BMPs that were used to meet the WLA.
Even if water quality goals in a receiving water are met, an MS4 must still meet the WLA as described in the TMDL. A new WLA can be established only if a TMDL is re-opened. Similarly, a change in the designated use of a receiving water would require a public notice process and a re-opening of the TMDL. More information on designated uses for lakes can be found in MPCA’s Lakes Protocol ().
An implementation plan is developed concurrent with or shortly after completion of the TMDL. Implementation plans provide a discussion of management activities intended to achieve the WLA. Implementation plans include information on monitoring and compliance schedules and are completed within one year of United States Environmental Protection Agency (EPA) approval of a TMDL. The EPA does not approve implementation plans.
The level of detail in implementation plans varies with extent of local involvement, scope of the TMDL, and information available during development of the TMDL.
EPA regulations, 40 CFR 122.44(d)(1)(vii)(B), state Effluent limits developed to protect a narrative water quality criterion, a numeric water quality criterion, or both, [must be] consistent with the assumptions and requirements of any available wasteload allocation for the discharge prepared by the State and approved by EPA pursuant to 40 CFR 130.7. Minnesota’s Municipal Separate Storm Sewer System (MS4) General Permit requires an MS4permittee to address all WLAs in TMDLs approved prior to the effective date of the permit. In doing so, they must determine if they are currently meeting their WLA(s). If the WLA is not being achieved at the time of application, a compliance schedule is required that includes interim milestones, expressed as best management practices, that will be implemented over the current five-year permit to reduce loading of the pollutant of concern in the TMDL. Additionally, a long-term implementation strategy and target date for fully meeting the WLA must be included.
The EPA, in a November 22, 2002 memo written by Robert H. Wayland and James Hanlon, indicates Best Management Practices (BMPs) are an appropriate form of effluent limit for MS4 stormwater: “in light of 33 U.S.C. §1342(p)(3)(B)(iii), EPA recommends that for NPDES-regulated municipal and small construction storm water discharges effluent limits should be expressed as best management practices (BMPs) or other similar requirements, rather than as numeric effluent limits.” MPCA advocates a BMP approach to meeting the WLA. With this approach an MS4 will implement BMPs and estimate load reductions based on the performance of the BMP. When the reductions meet the reduction required in the TMDL, the MS4 meets the TMDL requirement in the permit.
An MS4, through its SWPPP (Storm Water Pollution Prevention Program), demonstrates how it will meet a WLA. A SWPPP is essentially a compilation or summary of BMPs designed to minimize water quality impacts from MS4 stormwater runoff. The implementation plan may contain specific information about implementation activities that can be incorporated into an MS4 SWPPP.
The MPCA prefers TMDL implementation plans be as prescriptive as data allows for MS4 stormwater provided MS4s have input into development of the implementation plan. A well written implementation plan provides a guide for MS4s to follow in meeting permit requirements. Appendix A provides a summary of some TMDL implementation plans that have an urban stormwater component. Even though these case studies illustrate examples of reasonably good implementation plans, none link well with the MS4 permit. This illustrates the need for implementation plan guidance.
A good TMDL implementation plan for MS4 stormwater contains information on four general topics.
Each of these is discussed below.
A WLA represents a quantity of allowable pollutant that can be discharged to a receiving water. WLAs may be expressed in several ways. The most common expression is as a mass or quantity, such as pounds per day (e,g, for phosphorus) or number of organisms per day (e,g, for bacteria). This form of the WLA is necessary since TMDLs are expressed as a daily load. However, a WLA expressed in this form is problematic for most TMDLs and most MS4s. First, most TMDLs are determined using monitoring data for the receiving water. The TMDL thus represents the allowable pollutant entering a receiving water. This number may differ dramatically from the allowable pollutant load being discharged from a MS4 upstream of the receiving water because the pollutant delivery rate is almost never 100 percent. In an extreme example, a pollutant load determined at Lake Pepin does not represent the amount of pollutant discharged from Bemidji’s stormwater system that is necessary to meet the load requirement at Lake Pepin. Second, few MS4s monitor and model discharges from their stormwater system. Complying with a WLA expressed as a mass would require MS4s to invest in monitoring and modeling. This would come at the expense of implementing BMPs. Third, even when complex models are used to estimate loads from a MS4, the model may not consider all BMPs that can be counted toward the WLA. The model may also fail to account for illicit discharges.
The TMDL should provide clear reduction targets. It is preferable if the TMDL expresses the reduction as a percent from a well-defined baseline. If a baseline is not clearly identified in a TMDL, it can be determined if the allocation table(s) in the TMDL contain the current load. Baselines are discussed in section 4ii.
The implementation plan should contain the following three elements:
Each of these is discussed below.
MPCA’s stormwater program prefers that WLAs be expressed individually for each MS4. This allows for easier translation into the permit. However, a TMDL may provide a categorical wasteload allocation rather than an individual WLA. Individual WLAs should be given to each permitted MS4 unless one of the following three conditions applies:
Categorical WLAs allow flexibility in achieving the WLA since BMPs may be targeted to locations where they achieve the greatest reductions. Categorical WLAs make it difficult to determine what a MS4’s responsibility is in the permit, since they do not have their own individual WLA. It would be cumbersome to track several MS4s that are working together to achieve a categorical WLA. It is therefore desirable to translate categorical WLAs into meaningful target loads for an MS4. A target load is the load which an MS4 attempts to achieve through implementation of BMPs. Although somewhat analogous to an individual WLA, a target load is not enforceable through the permit.
The target load should be expressed as a percent reduction from a baseline. If it is expressed as a mass load, the current load should be provided so that a percent reduction can be determined. The baseline year must be included (see Section 4.ii). If the percent reduction is the same for all MS4s, this should be stated clearly in the TMDL or implementation plan.
If the TMDL or implementation plan do not express the WLA or target load as a reduction, MPCA’s stormwater program will determine a target load for each MS4. If progress is not being made toward this reduction, the stormwater program may ask that the TMDL be re-opened and individual WLAs assigned to the MS4s.
Target loads can be calculated using one of several approaches. Example approaches include the following.
Although a target load can be determined using one of the above approaches, the target load for individual MS4s can be modified as new data become available, provided the overall WLA does not change. Appendix B provides examples of how to calculate individual target loads for different TMDLs. MS4s should find these examples useful as they determine their target loads in implementation plans.
The implementation plan should include information about how the WLA was calculated, or it can refer to this information if it exists in another document. The following may be useful for an MS4, particularly if the MS4 intends on utilizing the same model to track progress toward achieving the WLA.
Since the stormwater program advocates an accounting process for achieving the WLA, with a percent reduction in loading being the target, it is important to define the baseline condition from which calculations will be made. The baseline must include a year but may include additional information, such as a summary of BMPs considered in deriving the WLA. Those BMPs cannot be applied toward the WLA.
For TMDLs developed with short-term monitoring data, such as two or three years, the baseline year will be one of those years. When long-term monitoring data is used it is difficult to determine the appropriate baseline year. TMDL authors, TMDL project managers, Stormwater Program staff, and stakeholders should work together to determine an appropriate baseline year. When a baseline is not clearly defined in the TMDL or implementation plan, Stormwater Program staff will determine the baseline.
Examples of baselines from TMDLs developed in Minnesota are provided below. In general, more recently-approved TMDLs provide clearer target loads and baselines.
All TMDL reports illustrate the study area in the final report. However, it is not always clear how the boundaries of the study area were determined. The implementation plan should describe the method used to delineate the watershed. For example, the DNR minor watershed GIS coverage is often used to delineate watersheds, including lakesheds. This approach may not accurately reflect a lakeshed, creating the potential for implementation of BMPs outside the actual lakeshed. In addition, many MS4s have modified drainages because of their stormwater sewer system. These modified drainages may not be reflected in the TMDL.
A WLA for MS4s will be met through implementation of BMPs. The implementation plan should include a stormwater management strategy that lays out an adaptive management approach to meeting the WLA, an overview of the adaptive management approach, and an overview of the general implementation strategy. The implementation plan may also contain general information on BMPs (e.g. removal efficiency, cost, maintenance requirements, resource requirements, and applicability), a summary of existing stormwater management strategies in the watershed, a summary of funding needs and mechanisms, and trading guidelines if applicable. These are discussed below.
MS4s should develop a stormwater management strategy, which is a general approach used to meet the WLA . The strategy consists of five general approaches to meeting a specific pollutant load. MS4s will typically employ more than one of these, but the priority for each will vary for different MS4s.
The implementation plan should describe the general strategy that will be employed to meet the TMDL. An example is shown in the inset below.
Implementation of BMPs is expensive and environmental impacts of BMP implementation are uncertain. MS4s should employ an adaptive management approach to implementing BMPs. Using this approach, BMPs are selected, implemented, and evaluated. This may occur across multiple permit or funding cycles. Monitoring may occur concurrent with implementation. The TMDL implementation plan should provide a framework for the adaptive management approach. For example, an MS4 may choose to address the TMDL WLA by implementing Low Impact Development in newly developing areas. This may occur over a period of several permit cycles. The implementation plan could provide a framework for linking development with progress toward the WLA, including linking long-term development plans to likely load reductions. The inset below continues with the example from the previous section.
BMP selection and implementation is the tool by which an MS4 will achieve its WLA. There is considerable information in the literature on BMP pollutant removal efficiency, design, cost, and maintenance requirements. This information is scattered through the literature and represents a wide variety of environmental conditions that may not necessarily apply to Minnesota. Currently we lack standardized numbers that reflect pollutant removal efficiencies for specific BMPs. The MPCA, in conjunction with others, is developing standardized numbers for a wide range of BMPs. MPCA anticipates that as these numbers become available, they will be incorporated into the permit, either directly or by reference.
The implementation plan offers an opportunity to guide MS4s in selecting BMPs. For example, an implementation plan may indicate that infiltration BMPs, such as rain gardens, should be implemented to achieve a WLA. But because we lack standardized numbers for pollutant removal, the implementation plan should not provide specific information about those BMPs, such as where they should be located. An exception may occur when a very detailed model was used to determine current loading. To date, such modeling has not occurred. The Silver Lake and Medicine Lake TMDLs are examples where detailed modeling information could be used to identify BMP categories or target general areas where BMPs would be most effective. Appendix C provides guidance for BMP selection for bacteria, phosphorus, TSS, and chloride.
The following inset contains an example from the Lake Independence Nutrient TMDL. Note the task includes a description, cost, timeline, responsible party, expected phosphorus reductions, and specific BMPs. Additional information that could be included is maintenance requirements, resource requirements, and if possible, likely implementation scenarios, such as utilizing specific BMPs in certain types of land use.
Local water management organizations often lead the development of TMDLs. This is particularly true in the Twin Cities Metro area. These organizations typically have water management plans that lay out strategies for managing stormwater. Examples include watershed plans, county plans, city plans, and any other local water plans. Generally these plans are more detailed than TMDL implementation plans and because they are developed locally, they often serve as better tools for managing stormwater. In some cases, stormwater management plans, if fully implemented, will meet TMDL WLAs.
TMDL implementation plans should therefore include information from local water management plans. The implementation plan should include strategies and BMPs from these local plans. The implementation plan should quantify, to the extent possible, how local management strategies and BMPs will make progress toward the WLA. NOTE: The MPCA does not approve BMP load reductions provided in implementation plans. These estimates of load reduction, however, may be viewed as reasonable estimates that a MS4 can use in developing a stormwater management strategy to address a WLA.
Local stormwater management strategies may address goals other than impaired waters (e.g. flood control, nondegradation, specific watershed or receiving water goals, etc.). Ideally, the implementation plan would provide a discussion of how these strategies interrelate, and incorporate these strategies into the Stormwater Management Strategy. The discussion could include suggestions for interagency coordination. The process of identifying existing strategies and authorities can be used to determine the party responsible for implementing the different components of the stormwater management strategy.
The implementation plan may contain additional information on local requirements that may be related to stormwater. Examples include development plans, zoning rules, plan review, model ordinances, and so on. This information may be linked to implementation activities identified in the implementation plan. For example, development plans and zoning rules can be used to identify areas where Low Impact Development might be most effective in reducing pollutant loads.
The implementation plan should include a discussion of current funding and future funding needs, as well as explain the strategies to meet those needs. A discussion could include: 1) currently funded projects that will help meet the TMDL; and 2) proposed BMP projects, cost estimates for those projects, and strategies to secure those funds. Local water management plans often contain this information.
In addition to local government funding sources, several state and federal loan and grant programs may be available to pursue. This includes the Clean Water Legacy Act funds, Clean Water Revolving Fund, TMDL Grants, and Phosphorous Reduction Grants. In addition, although EPA 319 funds cannot currently be used to help an MS4 meet its permit requirements, 319 funds may be available for education and innovative BMPs. Further information on implementation funding can be found at .
The inset below provides a summary of some language from the Lake Independence Nutrient TMDL. Specific costs were discussed in the inset example in Section 4.B.iii. The discussion in the inset could be expanded in the implementation plan to include information on existing stormwater utilities for the cities in the watershed and funding mechanisms for the watershed organization.
MPCA is currently writing a rule for water quality trading. Trading by MS4s will be a viable option to meet WLAs, but conditions under which trading is allowed have not been defined. If trading is an option for meeting the WLA, the implementation plan should summarize the conditions necessary for trading.
One possible important application of trading is when internal loading is an important component of the TMDL. Internal loading is part of the LA. Therefore an MS4 will not receive credit toward its WLA if it implements BMPs to reduce internal loading, such as alum treatments. This could disincentivize MS4s from implementing BMPs that improve water quality. To date, no TMDLs have addressed this issue, but trading appears to be a viable option for promoting BMPs that would typically not be used by an MS4 to meet its WLA.
A MS4 will have to track progress toward achieving the WLA. Tracking progress by MS4s will occur through the permit (i.e. the SWPPP and Annual Reports). The implementation plan may reference the permit, but methods for tracking should be consistent with permit requirements. Water quality monitoring may be conducted by MS4s or other entities to track progress and evaluate effectiveness of BMPs. Currently water quality monitoring is conducted primarily by the two Phase 1 cities and by watershed management organizations. The MPCA does not envision monitoring requirements for the 2013 Phase 2 permit. The implementation plan may, however, provide a summary of water quality monitoring currently being conducted within the study area.
The implementation plan should contain a target date for achieving the TMDL. The implementation plan may also contain a general compliance schedule, including rough timelines for implementing BMPs.
Some TMDLs have very large reduction requirements and include multiple sectors (e.g. wastewater, agriculture). Examples include the Minnesota River turbidity TMDL and the Lower Cannon River turbidity TMDL. It is unreasonable to expect one sector to achieve reductions exceeding 50 percent of current loading while other sectors make no progress toward achieving their reduction goals. MPCA advocates a phased implementation approach for these situations.
With a phased approach, target reductions are set at specified timelines. All sectors are responsible for meeting their requirements within the timeline. If one sector does not meet its requirement while others do, those sectors meeting their requirements will not be required to make further reductions until all sectors have achieved the goal for a particular phase.
If the implementation plan provides a phased approach and compliance schedule, four factors should be considered.
This appendix contains information on implementation from completed TMDLs and TMDL implementation plans.
The Ballona Creek and Estuaries TMDL encompasses a relatively small watershed in an urban area. Two other TMDLs have been completed or are underway for the study area. The Implementation section of the metals TMDL was reviewed for this TMDL. The WLA is expressed as a concentration for dry and wet weather conditions. Consequently, monitoring is necessary to evaluate progress toward the TMDL. Following are notes from the review of this TMDL.
The Implementation section of the Ballona Creek TMDL report demonstrates the following concepts that can be incorporated into a TMDL implementation plan.
This TMDL provides individual WLAs for three permitted MS4s. Two additional MS4s were given de minimus WLAs. The following information is from the TMDL implementation plan.
The Implementation section of the Lake Independence TMDL report demonstrates the following concepts that can be incorporated into a TMDL implementation plan.
This TMDL provides the WLA as a percent reduction in loading for a group of MS4s. All MS4s are given the same reduction. The implementation plan provides a suite of BMPs that, if implemented, are expected to achieve the WLA. All MS4s are expected to implement the same BMPs. The following information is from the TMDL implementation plan.
Below are several examples of how to determine a target load from a TMDL.
This TMDL provides individual WLAs, expressed in lbs/year, for MS4s. The WLA is therefore the target load. The figure below illustrates WLAs for three MS4s in the Lake Independence watershed. The WLAs are 356 lbs-P/year for Independence, 16 lbs/year for Loretto, and 231 lbs/year for Medina. Existing (current) loads are included in the allocation table, as are load reductions. These can be translated into percent reductions if needed. This is the preferred form of the WLA since the WLA for each MS4 is clearly defined.
This TMDL provides a categorical WLA, in lbs-P/day, for MS4s. The implementation plan indicates the overall WLA for permitted MS4s is 147 lbs/per day during the summer low flow period. This load is the total cumulative acceptable load for the ten permitted MS4s in the study area.
The Hydrologic Simulation Program Fortran (HSPF) model was used to calculate urban stormwater loads. All MS4s were treated equally. Consequently, the WLA can be divided among the ten MS4s based on either area or population contribution of each MS4. Since the model lumped all urban pervious area together, including nonpermitted MS4s, it would be difficult to determine the area contribution of each MS4. Using 2000 U.S. Census Bureau populations, the individual MS4 target loads can be determined by multiplying the categorical WLA (147 pounds) by the fraction of permitted MS4 population for each MS4. The results are illustrated in the table below. Thus, individual target loads during the summer low flow period are 12.4 lbs-P/day for Fairmont, 37.0 for Mankato, and so on.
This TMDL also provides the WLA as a uniform 30 percent reduction for all MS4s. Since no BMPs were assumed in the model, the baseline condition would include no BMPs. The TMDL report does not clearly define a baseline year. However, the Scenario used to set the allocations was based on 1988. The target load for MS4s could therefore be a 30 percent reduction from a no-BMP condition in 1988.
This TMDL concluded that a 71 percent reduction in chloride load was needed across the watershed to achieve the water quality standard in Shingle Creek. The assumption was that each permitted MS4 would implement the same suite of BMPs and these would result in a 71 percent reduction in chloride loading. However, the implementation plan provides a summary of BMPs but does not indicate which BMPs need to be implemented to reach the 71 percent reduction goal.
The preferred method for establishing a target load would be to have a reduction target, such as 71 percent, for each MS4, along with a summary of any progress the MS4s have made to date. The TMDL assigns a categorical WLA, however, with no indication of progress to date for the different MS4s.
The TMDL also expressed the WLA as a quantity at each of five different flow duration intervals. The primary source of chloride is application of salt to roads. Consequently, one way to calculate a target load is to partition the categorical WLA among each MS4 based on their proportion of road miles. The result of this calculation is shown below for the five different flow duration intervals.
The Shingle Creek TMDL does not clearly identify a target load for the MS4s. The default would be to assign a load reduction of 71 percent to each MS4 from the monitoring year, which was 2002. This approach is not equitable to MS4s that have implemented more BMPs than others.
To date, most fecal coliform TMDLs have used Flow Duration Curves to derive WLAs. Loads are assigned to the Margin of Safety and wastewater. The remaining load is divided among the remaining sectors based on an area distribution. For example, if permitted MS4s make up 10 percent of the watershed, they receive 10 percent of the load that remains after the Margin of Safety and wastewater loads are assigned. Loads are typically given as a categorical load, in number of organisms per day across five flow regimes.
Concentrations of fecal coliform in urban stormwater exceed the water quality standard (200 colonies or MPN/100 ml) for all urban land uses. The tables below illustrate concentrations from urban areas. Even runoff from urban roofs exceeds the water quality standard.
Calculating and tracking loads of fecal coliform from an MS4 would be difficult. There is little information in the literature useful for calculating load reductions for specific BMPs. Furthermore, BMPs other than infiltration or disinfection are not likely to achieve the water quality standard. Consequently, the following should be considered in the TMDL implementation plan.
WLAs for lake phosphorus TMDLs are generally expressed in pounds per day. These TMDLs are generally for small watersheds. WLAs may be derived using a variety of models. These can roughly be divided into watershed delivery models and in-lake models. The two types of models can be used in conjunction. Some TMDLs may consider different precipitation regimes.
Phosphorus WLAs for streams and rivers may be expressed as an allowable load in pounds per day or as a required load reduction. These TMDLs are generally for large watersheds and phosphorus may be a surrogate for the actual impairment (e.g. dissolved oxygen). Models used for these TMDLs are generally sophisticated and consider both watershed loading for a variety of land uses and in-stream processes.
Because of differences in scale and modeling approaches, TMDL implementation plans will differ for lake and stream phosphorus TMDLs. The following discussion focuses on lake TMDLs. Many of the principles discussed for turbidity TMDLs (below) are applicable for stream and river phosphorus TMDLs and the reader is referred to that section for these TMDLs.
Phosphorus WLAs will often be difficult to achieve, particularly in urban areas that are fully developed. BMPs can be expensive and many are marginally successful in reducing pollutant loads. BMP treatment train approaches may need to be implemented along with aggressive education and prevention strategies. Internal loading in lakes may contribute significantly to the lake impairment and we therefore may not see immediate water quality improvements as BMPs are implemented.
Many BMPs exist for reducing phosphorus loads. Because required phosphorus reductions are typically large, BMP options are varied, and structural BMPs are expensive, a phased approach is recommended. An example might be the following:
TSS is a pollutant of concern in rivers and streams. Watersheds are typically larger than the watersheds for lake TMDLs. An important source of TSS in impaired streams is often in-stream erosion, which is typically related to altered hydrology.
Much of the discussion for phosphorus applies to TSS. These TMDLs will be addressed over multiple permit cycles and will involve implementation of expensive BMPs. The BMPs are similar to those for phosphorus with the exception of the prevention BMPs. In addition, BMPs that address hydrologic processes should be considered.
The primary source of excess chloride in urban areas is road salt applied during the winter months. Chloride is a conservative chemical and readily moves with water. The primary cause of impairments appears to be short-term thaw events that occur in the winter. Because flow in urban streams is typically very low in the winter, these melt events represent a significant load to the streams.
Groundwater can also be an important source of chloride to urban streams. Highest elevations of chloride in groundwater are in areas where road salt is applied. Concentrations may be more than three times the water quality standard in these areas. In urban areas less impacted by road salt, concentrations are well below the water quality standard.
Currently there are only a small number of urban streams listed as impaired. However, the belief is that most urban streams will eventually be listed. TMDLs for these impairments are completed at the small watershed scale, such as a Shingle Creek or Nine Mile Creek.
Prevention BMPs will be most effective for chloride. BMPs that improve local hydrology, such as use of rain gardens and infiltration devices, will be effective if the BMPs are properly placed. Because chloride is a conservative chemical in water, implementation of BMPs may have relatively impacts low or high on the impaired water. The following BMPs are available for addressing chloride impairments.
The MPCA will attempt to address chloride impairments through specific permit requirements rather than through an accounting and tracking approach. Consequently implementation plans can list all these BMPs. It would be useful if the implementation plan provided an inventory of which BMPs are currently being implemented and which are being considered.