Spatial and temporal variation of fecal indicator organisms in two creeks in Beltsville, Maryland

Authors: Stocker (ctr), Matthew; RODRIGUEZ-VALENTIN, JOSE - Universidad Interamericana De Puerto Rico; Pachepsky, Yakov; Shelton, Daniel

Submitted to: Water Quality Research Journal of Canada
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2016
Publication Date: 3/4/2016
Citation: Stocker, M.D., Rodriguez-Valentin, J., Pachepsky, Y.A., Shelton, D.R. 2016. Spatial and temporal variation of fecal indicator organisms in two creeks in Beltsville, Maryland. Water Quality Research Journal of Canada. 51(2):167-179.

Interpretive Summary: Microbial water quality is of paramount importance for recreation, irrigation, and other water uses. Fecal indicator bacteria are used to determine the potential presence of pathogenic microorganisms in waters. Monitoring of microbial water quality requires guidance on when and where water samples should be taken. We studied the effect of the time of the day, position across the stream, and sampling location along the stream on E. coli and enterococci concentrations at reaches of two streams located in agricultural settings in Maryland. In absence of rainfall and runoff from agricultural fields, indicator bacteria concentrations decreased from morning towards evening and significantly increased from the upstream to downstream position along the stream sections which were less than one mile long. The latter implies that the bottom sediment serves as a reservoir of indicator organisms in the absence of runoff during the base flow. Results of this work will be used by regulators and consultants concerned with the microbial quality of water and produce since they indicate the urgent need to account for the influx of indicator bacteria from bottom sediment while evaluating the need in BMPs to improve microbial water quality.

Technical Abstract: Evaluation of microbial water quality is commonly achieved by monitoring populations of indicator bacteria such as E. coli and enterococci. Monitoring data are utilized by water managers to predict potential fecal contaminations as well as a decision tool to improve microbial water quality. Both temporal and spatial variability of fecal indicator concentrations have been observed. For example, the occurrence of elevated levels of indicators in watersheds after storm events, due to runoff induced suspension of sediments, has been well documented. However, much less is known about variability under base flow conditions. The objective of this study was to quantify the variability of E. coli and enterococci in two small streams running through agricultural land use areas. Water at locations upstream and downstream relative to agricultural land was grab-sampled every second or third day for a month; samples were taken three times a day at three positions across the stream. Differences in E. coli and enterococci concentrations were not significantly different across the streams. Diurnal trends were observed at each of the sampling locations, although differences were not consistently statistically significant. Statistically significant differences in concentrations along reaches were noted for both creeks during the base flow periods when no runoff occurred. A hypothetical explanation is that indicator organisms were released from sediment due to the effect of groundwater influx into streams. Since the release of fecal indicator organisms from bottom sediments to the water column is currently thought to result from runoff induced suspension of sediment the possibility that organism release may also be mediated by ground water intrusion may affect our understanding and prediction of the role of bottom sediments in microbial quality of surface waters.