From wells to waves: Evaluating fecal contamination sources using FIB and MST markers along Texas coastal waters

Fecal contamination in coastal waters poses significant public health risks, yet identifying specific sources remains challenging due to complex transport mechanisms and multiple potential hosts. This study investigated fecal contamination sources along Texas coastal waters in Chambers and Galveston Counties using conventional fecal indicator bacteria (FIB) enumeration and molecular microbial source tracking (MST) markers. Over 19 months (November 2021-May 2023), 224 water samples were collected from four transects that included groundwater wells, beach surface water, and pore water from saturated beach sand. Samples were analyzed for culturable enterococci using IDEXX Enterolert kits and four qPCR-based MST markers: human-associated (HF183), dog-associated (BacCan), avian-associated (GFD), and general enterococci (Entero1). Culturable enterococci results revealed that over 65 % of surface water samples exceeded safe swimming thresholds (>104 MPN/100 mL). The GFD marker showed the highest detection frequency across all sample types (84 % well water, 97 % beach water, 86 % pore water), suggesting substantial avian fecal inputs. Dog-associated contamination was moderately prevalent (38 % well water, 67 % surface water), while the human-associated marker was detected infrequently (9 % of surface water samples). However, reliance on a single human marker may underestimate sewage inputs in this barrier island environment characterized by aging septic systems. Multivariate analysis revealed that samples clustered more strongly by water type than by spatial proximity, suggesting similar contamination patterns within each environmental medium. The study demonstrates the value of MST approaches while highlighting the need for comprehensive toolbox strategies incorporating multiple human-associated markers and complementary tracers to better characterize contamination sources and inform targeted management for coastal water quality protection.