Nutrient Source Tracking to Support Modification of an Algae TMDL in the LA Region
Statewide, TMDL responsible parties are exploring means of modifying TMDL requirements based on new information. In the Ventura River Watershed (Watershed), an Algae TMDL identifies responsible parties including MS4s, general stormwater permittees, ranching, agriculture, and onsite wastewater treatment systems (OWTS). Working closely with the Regional Water Board, the Ventura County Environmental Health Division (VCEHD) sought to pursue a precedent-setting study to spatially define the primary contributing nitrogen sources in the watershed, to support TMDL modification. This study establishes a model for other regulators and dischargers to follow to achieve TMDL modification, and in particular when accounting for hydrologically complex groundwater-surface water interaction-based pathways.
In the Watershed, there are over 2,000 single family residential OWTS, most of which are permitted by the VCEHD. OWTS typically release treated wastewater effluent into unsaturated soil via leach fields, which disperse contaminants prior to reaching groundwater. However, this can potentially impact receiving waters if systems are not maintained or functioning properly, and can also contribute nitrate even when systems properly function. Nitrates can persist in shallow groundwater, which can then flow into surface waters and impact surface water quality.
The Algae TMDL requires all OWTS in the Watershed to reduce total nitrogen loading by 50%. Further, the entire Watershed is initially identified as requiring an Advanced Protection Management Program (APMP), which would require all OWTS to be upgraded to advanced treatment or connected to sanitary sewer, options that potentially represent high costs to private homeowners. However, the TMDL also allows for local agencies to perform an investigation to determine which OWTS are contributing nutrient loading to surface waters. Areas found not to be contributing could be removed from the APMP.
The objective of this State grant-funded study is to define the geographic extent of OWTS that are contributing substantive nutrient loads to the River. More specifically, the study investigates if groundwater nitrogen levels are elevated downgradient of OWTS areas, if those areas are also impacted by sewage indicators that would further support OWTS as a source, and if those impacted groundwaters are also impacting surface water nitrogen levels at upwelling locations.
These questions are addressed by sampling groundwater and instream surface water at representative areas downgradient of OWTS, in addition to control sites. It is anticipated that non-contributing OWTS may be removed from the APMP and no longer require enhanced nitrogen removal, as implemented through a future TMDL reconsideration.
GIS analysis and advanced forensic tools are used, including analysis of chemical sewage indicators and stable nitrate isotopes. Using a tiered approach, lower cost nitrogen compounds are first more broadly screens for, then advanced forensic tools receive targeted use. Results from representative areas are then extrapolated throughout the watershed based on relative risk of OWTS nitrogen impact to surface waters. The end product is a GIS map delineating high risk areas and recommendations for the management of OWTS in the Watershed.
The presentation will include an overview of the study objectives and approach, followed by sampling results and initial conclusions. The study design will be presented as a model for others to follow to similarly define the geographic sources of surface water pollutants through groundwater.
The audience will be invited to participate through a Q&A session on the utility of the study design for other example applications. This presentation is directly related to the conference’s theme as it outlines precedent for more targeted pollutant source identification and management, which is a cost-effective means of protecting downstream receiving waters.