Cations and microbial indicators: strong relationships in waters of urban/mixed land use watersheds of Southwest, VA

Background The salinity and composition of salts in freshwater streams, rivers, and waterbodies varies substantially, often impacted by human urban, agricultural, and mining land uses. While extreme fluctuations in salinity have been shown to influence both microbial communities and biogeochemical cycles, the differential effects of specific ion species at low salinity levels is poorly understood. The objective of this study was to examine the relationship between water chemistry and microbial water quality indicators. We collected weekly grab samples from nine sub-watersheds in Southwest Virginia. Samples were measured for standard physical and chemical properties: dissolved oxygen, temperature, specific conductance, pH, calcium, magnesium, potassium, chloride, fluoride, sulfate, nitrogen species, phosphorus, and dissolved organic carbon. In addition, three types of microbial fecal indicators were measured: total coliforms, E. coli, and HF183 (a human specific genomic marker). Results The relationships within and between water chemistry and water quality indicators are complex and frequently co-correlated. Concentrations of traditional biogeochemical elements (N, P, C) were less strongly related to water quality indicators than were Ca, Mg, Na in watersheds. Ca and Mg were strongly correlated with total coliforms, r2 = 0.88 and r2 = 0.86 respectively. While potassium is very strongly related to E. coli (r2 = 0.96). Currently, we cannot reasonably explain these relationships by the land use composition or common sources within the landscape. The human specific fecal indicator was not well correlated with other microbial water quality indicators, and yet found ubiquitously across the developed watersheds and most strongly correlated with sodium concentrations (r2 = 0.84). The results suggest that 1) wastewater via subsurface flowpaths may more broadly impact surface water chemistry and quality than expected, and 2) that cation chemistry may influence the microbial community and serve as a mediator of watershed biogeochemical cycling.