Groundwater and stream E. coli concentrations in coastal plain watersheds served by onsite wastewater and a municipal sewer treatment system Charles Humphrey, Algernon Finley, Michael O’Driscoll, Alex Manda and Guy Iverson ABSTRACT The goal of this study was to determine if onsite wastewater treatment systems (OWS) were influencing groundwater and surface water Escherichia coli concentrations in a coastal plain watershed. Piezometers for groundwater monitoring were installed at four residences served by OWS and five residences served by a municipal wastewater treatment system (MWS). The residences were located in two different, but nearby (<3 km), watersheds. Effluent from the four septic tanks, groundwater from piezometers, and the streams draining the OWS and MWS watersheds were sampled on five dates between September 2011 and May 2012. Groundwater E. coli concentrations and specific conductivity were elevated within the flow path of the OWS and near the stream, relative to other groundwater sampling locations in the two watersheds. Groundwater discharge in the OWS watershed could be a contributor of E. coli to the stream because E. coli concentrations in groundwater at the stream bank and in the stream were similar. Stream E. coli concentrations were higher for the OWS in relation to MWS watersheds on each sampling date. Water quality could be improved by ensuring OWS are installed and operated to maintain adequate separation distances to water resources. Charles Humphrey (corresponding author) Environmental Health Sciences Program, East Carolina University, 3408 Carol Belk Building, Greenville, NC 27858, USA E-mail: humphreyc@ecu.edu Algernon Finley Environmental Health Sciences Program, East Carolina University, 3400 Carol Belk Building Greenville, NC 27858, USA Michael O’Driscoll Department of Geological Sciences, East Carolina University, 204 Graham, Greenville, NC 27858, USA Alex Manda Department of Geological Sciences, Institute for Coastal Science and Policy, East Carolina University, 387 Flanagan, Greenville, NC 27858, USA Guy Iverson Coastal Resources Management Program, East Carolina University, 379 Flanagan, Greenville, NC 27858, USA Key words | coastal watersheds, fecal bacteria, groundwater quality, onsite wastewater INTRODUCTION Onsite wastewater treatment systems (OWS) are used by approximately 23% of US citizens, and each day an esti- mated 15 billion liters of OWS-treated effluent is discharged to the environment (United States Environ- mental Protection Agency ). OWS typically include a septic tank, drainfield trenches, vadose zone (aerated soil) beneath the trenches, and setback distances from the trenches to features of potential concern such as a well or stream. Effluent discharged from septic tanks to drainfield trenches contains elevated concentrations of pathogenic microorganisms including Cryptosporidium, Escherichia coli (E. coli), Salmonella, and Shigella (United States Environmental Protection Agency ; Lowe et al. ). If pathogen concentrations in septic tank effluent are not reduced in the vadose zone beneath the OWS trenches, then groundwater and possibly surface waters near the OWS may become contaminated and thus threaten public and environmental health. For example, studies by Scan- dura & Sobsey (), Conn et al. (), and Humphrey et al. () have shown that concentrations of fecal indi- cator bacteria including E. coli and/or enterococci concentrations in groundwater beneath OWS can exceed water quality standards, especially when the separation dis- tances from drainfield trenches to groundwater are less than 60 cm and the OWS are installed in sandy soils. Linkages have been established between fecal contamination of water resources and public health. For example, Borchardt et al. () discovered that, in central Wisconsin, high den- sities of OWS were associated with endemic diarrhea illness in children that drank groundwater from wells (near OWS). OWS have also been identified as a major contributing source of fecal bacteria to economically and ecologically 1851 © IWA Publishing 2015 Water Science & Technology | 72.10 | 2015 doi: 10.2166/wst.2015.411