Bacteria Source Identification Study in Upper Malibu Creek Watershed: Findings and Implications for TMDL Compliance
The Upper Medea Creek and Upper Lindero Creek drainage areas are located in the Upper Malibu Creek Watershed in Ventura County, and are subject to a bacteria TMDL. The drainage areas consist predominantly of urban and undeveloped land uses. The goals of the dry weather bacteria source identification study included identifying sub-drainages contributing the highest loads of E. coli, identifying anthropogenic inputs of fecal pollution (e.g. human, dog, irrigation runoff, etc.), and estimating the relative contributions of natural vs. anthropogenic sources of fecal pollution.
This presentation will summarize the methods used in the study, including desktop surveys, sewer mapping, field surveys and flow mapping, targeted sampling and tiered analyses of E. coli, IC/ID markers, host-specific DNA markers and chemical sewage indicators. Recommendations for the most effective methods will be presented.
For instance, flow mapping using field measurements and deployment of level loggers was very useful in delineating areas contributing surface flow to the compliance stations. Approximately 80% of the urban land in both drainage areas did not directly drain to the compliance station, because of soil infiltration and/or evapotranspiration following discharge to the creeks. These areas were considered not to contribute significantly to E. coli at the compliance stations, and were therefore not selected for subsequent sampling. Also, alternative indicators including IC/ID markers (ammonia) and organic sewage chemicals (caffeine, cotinine, sucralose, carbamazepine) were of limited use in determining the origin of fecal pollution, due to their low specificity and/or sensitivity. However, sucralose and carbamazepine were identified as potential tracers for reclaimed water, which has relevance for identifying sources of nutrients.
The results describing the sources of E. coli will be presented concisely and in integrated fashion, using custom maps and simple graphs. The presentation will focus on delineating E. coli source areas, identification of E. coli hot spots, and identification of fecal sources of E. coli, where possible. Interestingly, patterns of E. coli concentrations and fecal sources were very different in outfalls compared to creek locations. E. coli concentrations in creeks were consistently lower than in outfalls and storm drains, even though most, if not all, creek flow consisted of urban runoff. Human and dog genetic markers were detected with low to medium frequency, and almost exclusively at storm drain locations, while bird markers were detected frequently and exclusively at creek locations. Correlation analyses suggested that dog waste was a significant source of E. coli at some storm drain locations, while birds were major sources of E. coli at some creek locations, and may even cause exceedances. The latter was corroborated by bird (mostly duck) sightings.
The presentation will also include recommendations for source control and structural Best Management Practices, specific to the two drainage areas. However, given that birds are significant in-stream sources of E. coli, eliminating E. coli contributions from urban outfalls may not result in a corresponding reduction of bacteria exceedances. The County of Ventura is taking these findings into consideration for updating its strategy towards TMDL compliance, but the results are likely relevant for other similar watersheds.