Spatiotemporal variation of bacterial water quality and the relationship with pasture land cover

Pathogens are a major cause of water quality impairment and public health concern world-wide. In the United States, each state is tasked with developing water quality standards (WQS) to protect the designated use(s) of waterbodies. Several streams in the Illinois River Watershed in northwest Arkansas are currently listed as impaired due to elevated levels of pathogens. Our objective was to evaluate Escherichia coli (E. coli) numbers at 29 stream sites, compare these numbers to the applicable WQS, and investigate the relationship between E. coli numbers and land cover variables. E. coli numbers in samples collected at most sites were within allowable limits, although there were several instances of violations of the WQS. Violations were variable from year to year at some sites, and elevated levels of E. coli were spatially localized during baseflow. Violations also were positively related to pasture land cover in the drainage area, and particularly within the riparian buffer area. This relationship was non-linear, or threshold based, where there was a significant increase in the mean E. coli exceedances when riparian pasture land cover was greater than approximately 50%. These results can be used to identify specific stream reaches where E. coli numbers might be elevated and the implementation of best management practices can be geographically targeted.