Flow Source Forensics: Using Multiple Lines of Evidence to Identify and Quantify Dry-Weather Flow Sources
The San Diego Regional MS4 Permit requires responsible agencies to identify, prioritize and effectively eliminate the anthropogenic non-permitted sources of persistent non-storm water (dry weather) flows within their respective MS4 systems. In order to identify these sources, the County of San Diego has developed a robust program of high-quality low-flow monitoring coupled with stable isotope, mineral, and key indicator sampling and analysis. The County is using the multiple-lines-of-evidence (MLOE) approach to identify and investigate the sources of non-stormwater flows.
Recently, the County began evaluating several approaches to hydrograph separation to quantify the proportions of baseflow and quickflow (assumed to be anthropogenic flows). Hydrograph separation an urbanized watershed using existing approaches presents several challenges since existing approaches are designed for natural watersheds that are hydraulically quite different than the urbanized MS4 system. The County evaluated multiple techniques from the literature and determined a recursive digital filter produced the most plausible, repeatable hydrograph separation. Using this approach, the County quantified are slowly varying flows (such as baseflow or interflow) entering the MS4 through cracks, poor seam connection, or other permeable points with the conveyance and quickly varying flows entering the MS4 through curb inlets, inlet grates, etc. The quickly varying flows are assumed to be from anthropogenic sources such as over-irrigation, car washing, pool draining, or other potentially illegal discharges, which the County may be responsible for eliminating.
The results of this hydrograph separation analysis were combined with field knowledge (from illegal discharge detection investigations) and the results of stable isotope and geochemistry analysis of water samples collected from the MS4 and from potential sources. The data were employed in a MLOE approach to identify, investigate and eliminate the controllable sources of non-stormwater flows in the MS4. For instance, where the proportion of flows that are quickly varying help quantify flows generated from over-irrigation, car washing, pool draining, etc., stable isotope analysis provides another way to quantify the proportions of imported water (e.g., tap water) and local water (i.e., rain derived) within a given MS4 discharge. Concentrations of fluoride, chlorine, nitrate, TDS and other indicators will then further corroborate conclusions made regarding potential sources.
This presentation summarizes the results of the County’s 2018 MLOE study and assessment along with recent hydrograph separation results. These tools may be useful for other agencies faced with similar dry weather source quantification and reduction requirements.
This presentation addresses the conference theme by showcasing a set of unique methodologies that combine traditional source ID steps with newer techniques including stable isotope analysis and applying hydrograph separation techniques designed for natural systems to MS4 discharges. Use of these newer techniques can help better quantify proportions of the flows attributable to various dry weather sources including the potentially illegal discharges to the MS4 that must be eliminated in order to effectively reduce pollutant loads to receiving waters.