| Summary |
Onsite wastewater treatment systems (OWTS) have been identified as a potential source of microbial contamination of groundwater. Microbial indicators, such as E.coli and Enterococcus, are transported from these systems to groundwater and may migrate in the subsurface. North Carolina Administration Code 15 A NCAC 18 A.1900 suggests that a 45 centimeter vertical separation distance should be maintained between the bottom of the drainfield and the top of the seasonal high water table (SHWT) in sandy soils and a 15 meter horizontal separation (setback distance) from the drainfield to private wells and surface water bodies is sufficient to protect water quality. The goal of this project was to examine if there was contaminant transport of E. coli and Enterococcus to surficial aquifers and surface water bodies via OWTS in eastern North Carolina. Densities of E.coli and Enterococcus were monitored in wastewater, drainfield groundwater, groundwater up-gradient and down-gradient from drainfield trenches, and drinking water samples. Septic tank effluent was sampled monthly, and groundwater was sampled bi-monthly from October 2009 through May 2010 at two residences in Washington, North Carolina. It was hypothesized that 1) the North Carolina 45 cm separation distance does not always prevent microbial contamination of groundwater, and 2) microbial contaminants from OWTS can migrate greater than the North Carolina 15 m setback distance. Results indicate that the unsaturated zone has the greatest control on microbial reduction, with nearly 99.7% reduction of E.coli at Site 1 and 98% reduction of E.coli at Site 2 occurring between the drainfield and the water table. There was 93% reduction of Enterococcus at Site 2; however, there was only 33% reduction of Enterococcus at Site 1 in the unsaturated zone. In piezometers located near the 15 m setback distance, the horizontal treatment efficiency (microbial density decline from drainfield groundwater to down-gradient piezometer) was 83% for E.coli and 98.5% for Enterococcus at Site 1. There was no reduction of both E.coli and Enterococcus in piezometers 13 and 17 m from the drainfield at Site 2. Even though significant reduction occurred, relative to tank effluent densities, there was evidence that microorganisms could leach to the groundwater and travel greater than 15 meters down-gradient. These data suggest that more conservative separation distance and setback rules could improve water quality in sandy surficial aquifers and adjacent surface waters. Specifically, increasing North Carolina's separation distance for sandy soils to 60 cm and setback distance to 30 m would probably reduce E.coli and Enterococcus to background groundwater levels. |
