Fecal coliform and Enterococci relationships to rainfall and runoff in a rural watershed reveal intrinsic differences between these fecal pollution indicators

Fecal coliforms and Enterococci, the two most commonly used fecal pollution indicators, were enumerated on a weekly basis for eight years at a USGS real time water station located on the Tangipahoa River in southeast Louisiana. This river provides drainage for 520 sq. mi. and receives both domestic and agricultural runoff. The fecal pollution data set was used to construct a nonparametric multiplicative regression hydrologic model that was able to forecast fecal coliform concentrations (cross-validated R2 = 0.57); however, Enterococci forecasting was unsuccessful. This result was surprising as it was expected that both fecal pollution indicators would have similar routes of entry in receiving waters. It was also found that Enterococci exceeded recreational water quality standards (33cfu / 100ml) in 91% of samples and fecal coliforms exceeded water quality standards (200cfu / 100ml) 46% of the time. The differing conclusions that could be reached depending upon the fecal pollution indicator used pose a dilemma from a public health perspective and the underlying causes of these discrepancies were investigated. The role of measurement uncertainty stemming from sampling error (spatial and temporal) and lab error (operator and systematic error) in both fecal coliform and Enterococci hydrologic modeling was examined. Measurement error between the two indicator species did not account for differences between hydrologic modeling results. Alternative possibilities for the observations included differential survival and environmental reservoirs (growth/regrowth) of each indicator organism. Time series analysis demonstrated distinct trends for each indicator organism. Fecal coliforms had peak densities in the winter months, corresponding to higher rainfall, runoff, and lower temperatures. Enterococci had relatively stable densities, with a slight increase in density in the summer months. These trends also show that Enterococci to fecal coliform ratios are relatively greater in summer months. The trend of Enterococci is inversely related to river discharge, as opposed to fecal coliforms, which is proportional to river discharge. The differing results of fecal pollution indicators in this system highlight the need for appropriate modeling techniques to describe water quality in recreational river systems.