The effect of onsite wastewater treatment systems on nitrogen in urban watersheds of metropolitan Atlanta, Georgia
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High density onsite wastewater treatment systems (OWTSs) can cause contamination of both surface water and groundwater. However, the effect of OWTS on water quality at the watershed-scale is not well understood. We investigated the effect of OWTS on stream N concentrations during base flow in 24 watersheds in Metropolitan Atlanta, ranging in area from 0.18 to 8.8 km2. We found that total nitrogen (TN) and nitrate (NO3-) concentrations increased linearly with increasing OWTS density above a threshold of about 75 OWTS km-2. Dual-isotope analysis of NO3- revealed that stream NO3--N originated predominately from OWTS in high density (HD) watersheds and from a combination of animal waste and perhaps organic N in low density (LD) watersheds. The effect of OWTS on NO3- load in an urban watershed where most of the homes use OWTS was simulated using the Soil Water Assessment Tool (SWAT). The model showed satisfactory daily stream flow and NO3- 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. OWTS caused an average increase in NO3- load of about 23% at the watershed scale and 29% at the sub-basin outlet with the highest density of OWTS. Failing OWTS were estimated to be 1% of the total systems and did not have a large impact on NO3- load. The NO3- load was 74% of the total N load in the watershed. Results from detailed monitoring using automated samplers in four headwater streams with a gradient in OWTS density demonstrated that the highest base-flow and storm-flow TN concentrations occurred in watersheds with the lowest density of OWTS and the highest density of OWTS, respectively. There were significant positive correlations between base flow NO3- concentrations and OWTS density. TN loads were also the highest at extremes of OWTS densities. This was due to the pattern in N concentrations and higher water yields at low and high OWTS density. These findings confirm the importance of OWTS on water quality in urban watersheds and can be used in other Piedmont regions where nitrogen Total Maximum Daily Loads (TMDL) are being developed.