Consideration of Natural Sources in a Bacteria TMDL—Lines of Evidence, Including Beach Microbial Source Tracking

Total Maximum Daily Load (TMDL) stipulations remained unmet at a southern California beach despite a suite of management actions carried out since 2001, prompting exploration of a Natural Sources Exclusion (NSE) provision within the TMDL. Quantitative Microbial Source Tracking (MST) was employed from 2012 to 2015 to inventory sources of natural and anthropogenic fecal indicator bacteria (FIB). Data suggested FIB exceedances could be traced to gulls based on gull marker prevalence and correlations with FIB concentrations in seawater, sand, and eelgrass. In contrast, human marker concentrations and a tracer dye test did not indicate prevalent human sources. Exponential decay of gull marker in sand amended with live Catellicoccus marimammalium suggested that measured marker reflected fecal inputs versus growth outside the host. Improved water quality was coincident with a 2013 bird exclusion structure, consistent with NSE. However, load allocation needed for TMDL reconsideration was hampered by variable ratios of FIB, MST markers, and pathogens measured in seawater and in gull, cat, and raccoon feces. Quantitative Microbial Risk Assessment is a suggested path forward because such models can incorporate distributions from a combination of FIB sources and communicate criteria in terms of human health risk.