Abstract
Abstract. Onsite wastewater treatment systems are common in rural and semi-rural areas around the world; in the US, about 25–30% of households are served by a septic (onsite) wastewater treatment system, and many property owners also operate their own domestic well nearby. Site-specific conditions and local groundwater flow are often ignored when installing septic systems and wells. In areas with small lots (thus high spatial septic system densities), shallow domestic wells are prone to contamination by septic system leachate. Mass balance approaches have been used to determine a maximum septic system density that would prevent contamination of groundwater resources. In this study, a source area model based on detailed groundwater flow and transport modeling is applied for a stochastic analysis of domestic well contamination by septic leachate. Specifically, we determine the probability that a source area overlaps with a septic system drainfield as a function of aquifer properties, septic system density and drainfield size. We show that high spatial septic system density poses a high probability of pumping septic system leachate. The hydraulic conductivity of the aquifer has a strong influence on the intersection probability. We find that mass balance calculations applied on a regional scale underestimate the contamination risk of individual drinking water wells by septic systems. This is particularly relevant for contaminants released at high concentrations, for substances that experience limited attenuation, and those that are harmful even at low concentrations (e.g., pathogens).
Highlights
In rural, semi-rural, and many suburban areas, septic systems are the primary method for wastewater disposal
The probabilistic spatial analysis demonstrates that domestic wells have a wide range of intersection probabilities with septic system leachate, controlled primarily by lot density and horizontal hydraulic conductivity of the aquifer
– When septic system density is very low (∼ one per 400 000 m2), the probability of intersection decreases to approximately 1 in 100 domestic wells pumping some portion of septic system leachate
Summary
Semi-rural, and many suburban areas, septic systems (onsite wastewater treatment systems, OWTS) are the primary method for wastewater disposal. Regulators use minimum requirements for horizontal setback distances between drinking water wells and drainfields, and minimum vertical separation distances between drainfields and the seasonally highest groundwater table These setback distances are intended to ensure a minimum residence time for septic tank leachate in the subsurface that allows decay, absorption and dilution of chemical and microbiological contaminants. In the second case, prescribed subsurface residence times may fail since setback distances and maximum septic system densities (see above) have primarily been estimated using coliform bacteria or nitrate as representative contaminants These studies generally did not consider other potential contaminants of septic system leachate with differing behavior in the aquifer. We compare results to mass balance-based septic system pollution risk estimates and discuss the broader implications of the results to understanding of potential septic system pollution of domestic drinking water
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