Enrichment of stream water with fecal indicator organisms from bottom sediments during baseflow periods

Authors: Pachepsky, Yakov; STOCKER, MATTHEW - UNIVERSITY OF PUERTO RICO; OLMEDA SALDANA, MANUEL - ORISE FELLOW; SHELTON, DANIEL

Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/2016
Publication Date: 1/6/2017
Citation: Pachepsky, Y.A., Stocker, M., Olmeda Saldana, M., Shelton, D.R. 2017. Enrichment of stream water with fecal indicator organisms from bottom sediments during baseflow periods. Environmental Monitoring and Assessment. 189(2):51-61.

Interpretive Summary: E. coli and enterococci concentrations are used to evaluate microbial quality of freshwater sources. Both organisms are known to survive in bottom sediments. Prior research has shown that there is a constant resuspension of bacteria from sediments into the water column, occurring during both base and high flow (rainfall) conditions. Consequently, the fluxes of E. coli and enterococci from sediment to water column can affect the reliability of these bacteria as a means for evaluating the efficacy of animal management practices since they occur independently of land use. The objective of this work was to measure the flux of E. coli and enterococci from bottom sediments to stream water. We applied a novel experimental design to measure changes in bacteria concentrations in a labeled “slug” of water moving along a first-order creek located in an agricultural setting. We found an approximate ten-fold increase in bacterial concentrations along the 600-m log stream reach due to bacterial resuspension during base flow. The results of this work can be useful to environmental protection specialists in that they indicate the need to account for the flux of indicator bacteria from bottom sediments to water column if the indicator organisms are used to assess the efficiency of animal management practices that are measured to protect freshwater sources from microbial contamination.

Technical Abstract: Fecal indicator organisms (FIOs) are generally believed to be present in surface waters due solely to direct deposition of feces or through transport in runoff. However, emerging evidence points toward hyporheic exchange between sediment pore water and the overlying water column during baseflow periods as a source of FIOs in surface waters. The objective of this work was to confirm that stream sediments are a source of FIOs by measuring their concentrations in the same section (or “slug”) of water during flow from inlet to outlet reaches while simultaneously monitoring the parallel movement of a conservative tracer. Concentrations of E. coli in the “slug” were significantly greater (P = 0.035, P = 0.001, and P = 0.001, respectively) at the outlet reach in all three replications while enterococci concentrations were significantly greater in two of three replications (P = 0.001, P < 0.001 and P = 0.602). FIO release rates across replications ranged from 36 to 57 cells m-2 s-1 and 43 to 87 cells m-2 s-1 for E. coli and enterococci, respectively. No diurnal trends were observed in indicator concentrations. The absence of surface water runoff as a FIO source during experiments indicates that FIO are released into stream water through hyporheic exchange during baseflow periods.