What’s in my MS4 Discharge? A New Procedure for Determining Groundwater-Based Flows in Dry Weather
Dry weather discharges into MS4s can occur due to a variety of sources and may contribute or mobilize pollutants such as bacteria and nutrients to receiving waters. For example, these discharges may be attributed to sewage (e.g., overflows, sewer exfiltration, illicit connections), recycled water (e.g., distribution system leaks, irrigation), potable water (e.g., leaks, irrigation, wash water, pool draining), and/or natural waters (e.g., groundwater, springs).
The MS4 Permit for the San Diego Region requires permittees to eliminate most dry weather MS4 discharges. The County of San Diego (County) implements an illicit discharge detection and elimination (IDDE) program to identify and abate anthropogenic discharges including sewage, recycled (reclaimed) water, and potable water. However, naturally occurring groundwater seepage is an allowable non-stormwater discharge. Thus, efficient and conclusive differentiation of MS4 discharge water sources is critical to the County and other MS4 agencies for permit compliance demonstration.
The County wished to develop a methodology for identifying contributions to dry weather MS4 discharges that are caused by groundwater as opposed to the anthropogenic sources listed above. A literature review was first conducted on available methods for distinguishing such flows. Groundwater, potable water, and recycled water data were then compiled from various local sources and analyzed to identify analytes and associated concentration ranges that reflect each water source. Water quality data and watershed boundaries, groundwater basins, and water district service areas were analyzed using GIS to identify analytes that were more useful in certain areas within the County.
As a result, a Groundwater Detection Manual was developed describing sampling and analysis procedures for the identification of groundwater in MS4 discharges in the County. This Manual presents an approach for distinguishing groundwater discharges from other anthropogenic sources, using a tiered approach for sampling in which low-cost conventional methods are first used. These methods involve outfall sampling and comparing the results for basic analytes (i.e., fluoride, nitrate, boron, total residual chlorine, surfactants, and TDS) with expected ranges for the potential water sources. If the results for the low-cost conventional methods are not conclusive in determining the source of the discharge and further evidence is required, slightly more expensive analytes are considered (i.e., trihalomethanes), followed by more advanced ion analyses and then finally isotope analyses. This tiered approach allows for a cost-effective means of executing the Manual’s procedures. Also, a field “cheat sheet” was created to concisely guide staff in the selection of sampling analytes and concentration ranges for each water source.
The County now has a robust, scientifically-based, and transparent methodology for determining the source of non-stormwater discharges across hundreds of MS4 outfalls, for demonstration of compliance with MS4 permit discharge prohibitions and other requirements. This approach may be reproduced for other agencies, using expected concentration ranges based on area-specific data.
This presentation will summarize the approach outlined in the Manual, including how the tiered approach allows for net cost savings. The interactive GIS tool to aid in execution of the Manual will also be presented.
The audience will be invited to participate through a Q&A session to discuss the utility of the Manual for other example applications. This presentation is directly related to the conference’s theme as it outlines new available guidance for efficient and cost-effective MS4 discharge source determination, which is a highly effective means of protecting downstream receiving waters.
This presentation is intended to be second in a three-part series. Linked with:
Data to Doorsteps, Norris, Scott
Isotope Isolation, Messina, Alex