Modeling the Effects of Onsite Wastewater Treatment Systems on Nitrate Loads Using SWAT in an Urban Watershed of Metropolitan Atlanta

Abstract

Onsite wastewater treatment systems (OWTSs) can be a source of nitrogen (N) pollution in both surface and ground waters. In metropolitan Atlanta, GA, >26% of homes are on OWTSs. In a previous article, we used the Soil Water Assessment Tool to model the effect of OWTSs on stream flow in the Big Haynes Creek Watershed in metropolitan Atlanta. The objective of this study was to estimate the effect of OWTSs, including failing systems, on nitrate as N (NO-N) load in the same watershed. Big Haynes Creek has a drainage area of 44 km with mainly urban land use (67%), and most of the homes use OWTSs. A USGS gauge station where stream flow was measured daily and NO-N concentrations were measured monthly was used as the outlet. The model was simulated for 12 yr. Overall, the model showed satisfactory daily stream flow and NO-N loads with Nash-Sutcliffe coefficients of 0.62 and 0.58 for the calibration period and 0.67 and 0.33 for the validation period at the outlet of the Big Haynes Watershed. Onsite wastewater treatment systems caused an average increase in NO-N load of 23% at the watershed scale and 29% at the outlet of a subbasin with the highest density of OWTSs. Failing OWTSs were estimated to be 1% of the total systems and did not have a large impact on stream flow or NO-N load. The NO-N load was 74% of the total N load in the watershed, indicating the important effect of OWTSs on stream loads in this urban watershed.

Assessment of Nitrogen Inputs into Hunt River by Onsite Wastewater Treatment Systems via SWAT Simulation

“…According to Jeong et al [11] and Hoghooghi et al [14], the contribution of groundwater to streamflow is relatively high during dry years, similar to the research findings of this study.…”

“…By representing the area under the septic system, using available sewer data and TN loads data, a more accurate model of anthropogenic nitrogen inputs into the Hunt River Watershed was able to be developed. This study showed the importance of groundwater contributions to streamflow and nitrogen loading and the findings are corroborated by research conducted by Jeong et al and Hoghooghi et al [11,14]. …”

“…Few studies have been conducted modeling nutrient inputs on the watershed scale using the Soil Water Assessment Tool (SWAT) due to complexities stemming from the uncertainty of differences in subsurface hydrology, soil and water dynamics, and the often lack of detailed information about OWTS [11]. Increased densities of OWTS have been shown to increase baseflow within a watershed system [12][13][14], which can have the effect of increasing nutrient loading into ground and surface waters, eventually entering into coastal waters. OWTSs have a biozone layered designed to filter out nutrients, preventing their leaching into the groundwater.…”

Frontiers in assessing septic systems vulnerability in coastal Georgia, USA: Modeling approach and management implications

“…The average time required for an septic system to malfunction typically is more than 10 years [ 47 ]. Almost 70% of the septic systems that listed as ‘new’ in the original inspection report are more than 10-year old (average = 20 year), which may not have accurately represented the real septic systems condition.…”