Isotope Isolation: Using Stable Isotope Analysis to Track Non-Stormwater Flow Sources
The San Diego Regional MS4 Permit requires responsible agencies to eliminate non-storm water (dry weather) flow within their respective MS4 systems. The County of San Diego has developed a robust system of high-quality low-flow monitoring coupled with a priority outfall surveys and outreach program to address this goal. Although these programs have been successful, the County continues to explore other options to evaluate discharges that may be allowable, such as local ground water infiltrating the MS4.
Indicators like conductivity can be useful, but are limited in the ability to quantify source contributions. The County is working with Wood Environment and Infrastructure Solutions, Inc. and San Diego State University to implement and evaluate an innovative approach to determine the contributions of local and imported water in the MS4 by analyzing stable isotope ratios of water (18O/16O and 2H/1H). This separation of local and imported sources is possible because water molecules containing the heavier 18O and 2H atoms rain out and are depleted during transport from the ocean to the land, resulting in precipitation that is increasingly 16O dominant with distance inland and with increasing elevation. If endmembers of local and imported water are adequately quantified, the δ18O of a given sample can be used to quantify the percentage of local and imported water. Using this information, the County can implement a cost-effective approach to achieving compliance by focusing resources on outfalls where dry weather flows are impacted by imported water being discharged to the MS4 through over-irrigation or other unallowable discharges.
This study was designed to define isotopic signatures for imported and local water, and compare samples collected from the MS4 to quantify the source contributions. Reference samples of purely imported water were collected from taps in the same water distribution network as the monitoring locations to reflect isotopic composition in these areas. Suitable reference sites for local water were difficult to find since most perennial streams in San Diego County are impacted by urbanization or are found at higher elevations where 18O can be depleted compared to lower elevations. Several locations were identified throughout the County but were not co-located with each sampling site. Samples were collected at 51 outfall monitoring locations in the beginning (May) and end (September) of the dry weather monitoring season. Additionally, 20 priority sites were sampled bi-weekly to more closely assess seasonal changes in water sources. Data from these samples was used to evaluate the composition of the MS4 discharges and County responsibility in decreasing those discharges. Based on the reliance of reference sample data for this approach, a robust reference data set will improve the reliability and defensibility of study results. Potential future adoption of this approach by other agencies in the region will help assessment of responsibility and bolster the dataset for defining the pure imported water and pure local water, resulting in a stronger regional tool.
This presentation summarizes the rationale and approach for this study, the outcomes, and how the results can be used by the County. The audience will participate through a Q&A session following the presentation.This presentation addresses the conference theme by showcasing a unique methodology to help identify dry weather flow sources and the magnitude of unallowable discharges the County must eliminate, thereby reducing pollutant loading to receiving waters. It also addresses the CASQA Vision by highlighting an efficient and effective means of management by identifying allowable dry weather discharges so the County can focus resources appropriately.
This presentation is intended to be third in a three-part series linked to two other presentations:
1) Data to Doorsteps, Norris, Scott
2) What’s in my MS4 Discharge, Wisniewska, Joanna