Confirmation of the Impact of Onsite Wastewater Treatment Systems on Stream Base-Flow Nitrogen Concentrations in Urban Watersheds of Metropolitan Atlanta, GA

Abstract

Wastewater and lawn fertilizer potentially contribute to degraded water quality in urban watersheds. Previously we described a study from 2011 to 2012 in which we examined the effect of the density of onsite wastewater treatment systems (OWTS) on nitrogen concentrations in 24 small streams in metropolitan Atlanta. Our objective in this study was to confirm that the impact on water quality that we observed was due to OWTS and not lawn fertilizer. We sampled the same 24 streams again in 2013 and 2014, representing watersheds ranging in area from 0.18 to 8.8 km. We conducted regression analysis of the effect of OWTS and season, used dual-isotope analysis (nitrogen and oxygen in nitrate) to identify sources and determine the effect of denitrification and mixing, and conducted stream walks to identify areas where animals had access to the streams. Twelve streams were characterized as high-density (HD, more than 75 systems km) OWTS and 12 as low-density (LD, less than 75 systems km) OWTS. Water samples were collected three times a year under base-flow conditions, from November 2011 to July 2014, and analyzed for nitrate (NO-N), ammonium (NH-N), and total Kjeldahl nitrogen. Total nitrogen and NO-N concentrations increased linearly with increasing OWTS density above a threshold of about 75 OWTS km. Dual-isotope analysis of NO showed that stream NO originated predominantly from OWTS in HD watersheds and from a combination of animal waste and perhaps organic N in LD watersheds. Stream walks showed that livestock had access to some of the LD streams with high N concentrations. Our results confirm that HD OWTS can significantly degrade water quality at the watershed scale.