Spatial Distribution of Wastewater Microbial Indicators in Groundwater Beneath Two Large Onsite Wastewater Systems.

Unequal distribution of onsite wastewater system (OWS) effluent to drainfield trenches may lead to hydraulic failure and/or decrease wastewater treatment efficiency. Therefore equal distribution is important for OWS performance. The study objective was to assess the spatial distribution of microbial indicators (coliform, enterococcus, and E. coli) in groundwater beneath two large OWS. Both OWS used dual-alternating drainfields and were in operation for more than 12 years prior to the study. One system used a pump to distribution box system (D-box), while another used a low pressure pipe (LPP) distribution. Monitoring wells were installed up and down-gradient from the OWS drainfields. Monitoring wells (4) were also installed near the front of the trenches and 4 wells were installed near the end of the trenches for each system. The wells were evenly spaced across the two drainfields at each site. Water samples were collected 4 times from each of the wells, septic tanks, and nearby stream for coliform, E. coli and enterococcus analysis using the IDEXX method. The pH, electrical conductivity, and temperature of the samples were determined using field meters. Groundwater physical, chemical, and biological parameters at the front of the trenches were compared to characteristics near the end of the trenches for both systems to help determine if effluent was more uniformly distributed via LPP or D-box. Results indicate that both systems were effective at reducing indicator bacteria concentrations (all > 99%) before discharge to groundwater. Enterococcus concentrations were significantly higher in groundwater near the end of the trenches (in comparison to the front) for the OWS with D-box distribution, but not for the OWS with LPP distribution. Moderate to weak correlations between enterococcus and total coliform were observed at both sites. Overall, groundwater biogeochemistry was more similar beneath the LPP system in comparison to the pump to D-box.