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Bacteria Growth, Persistence, and Source Assessment in Rural Texas Landscapes and Streams: Final Report

L. Gregory, R. Karthikeyan, T. Gentry, D. Harmel, K. Wagner, R. Lopez

Bacteria water quality impairments are the most common water quality issue in Texas and are a considerable source of impairments nationally. Fecal indicator bacteria such as Escherichia coli (E. coli) and enterococci derived from birds and mammals are used as a measure of a waterbody’s ability to support contact recreation. Relationships between monitored levels of E. coli and enterococcus have been established with human contraction of a gastrointestinal illness from pathogenic organisms and serve as the basis for water quality standards that protect contact recreation. Stakeholder processes are often undertaken to improve the quality of impaired waters, define pollutant sources, and develop strategies to reduce bacteria loading to streams. Questions are often asked during these processes regarding the fate and transport of these bacteria in various environmental settings, the distribution of E. coli sources across watersheds, and how they respond to changes in water quality. Past research conducted has worked to address these questions; however, additional work is warranted.

Re-created stream mesocosms were used to develop an improved understanding of E. coli fate and transport in the environment under controlled treatment conditions. Nutrient amendments that mimic increases in nutrient concentrations seen from nonpoint source pollutant loadings and wastewater effluent loadings were applied to determine if E. coli concentrations would change as a result of the amendments and alter growth or decay relative to a control mesocosm. No E. coli growth response was observed in any trial, and no significant differences in decay rates were observed either. This suggests that a single nutrient addition to a stream environment is not sufficient to produce a growth response in the ambient E. coli community.

Soil and runoff samples collected from three controlled land uses were processed to enumerate E. coli and allow individual colonies to be isolated and fingerprinted for bacteria source tracking (BST). E. coli source contributions to native prairie, managed hay pasture, and cultivated cropland sites were determined using 7-way source identification splits. In all cases, wildlife were found to be the primary E. coli contributor. Unexpectedly, cattle and humans were identified as sources of E. coli in runoff and soils from some of the sites. Cattle are not actively stocked nor have they been stocked at any of these sites for at least three years, and no known sources of human fecal deposition have occurred in these watersheds. This demonstrates the complex diversity of E. coli in unimpacted environments and the potential for bacteria to be translocated by transmission vectors.

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