SEPTIC IMPACT ON STREAM WATER QUALITY: EVALUATING EMERGING CONTAMINANTS AS EFFLUENT TRACERS

Septic systems are extensively used to treat domestic wastewater in the rural and outlying suburban areas of the Chesapeake Bay watershed; however, the role of septic system input in the nitrogen (N) budget is difficult to quantify. Moreover, N is present in watersheds as both organic and inorganic N, but the majority of past research has focused on the source and fate of only inorganic N, despite organic N being active in transformation pathways. Dissolved organic N (DON) is a major component of N in waste entering septic systems and a significant component of dissolved N in freshwaters. The uncertainty in distinguishing sources of N highlights the need to be able to identify septic impacted freshwater systems and to better characterize septic effluent.

We are developing a tracer for septic wastewater using a suite of analyses, including 1) characterization of dissolved organic matter (optical properties, untargeted mass spectrometry, and targeted mass spectrometry to measure anthropogenic emerging contaminants), 2) isotopic methods (δ¹⁸O and δ¹⁵N of nitrate, δ¹³C and δ¹⁵N of dissolved organic matter), and 3) traditional nutrient and water quality measurements. In a year-long survey in Calvert County, MD, a county in the Chesapeake watershed where most homes are served by septic systems, we compare samples from reference sites, sites with predominantly traditional septic systems, and sites with predominantly best available technology (BAT) nitrogen-removing septic systems. Preliminary results from six months of sampling between October, 2016 and May, 2017 show elevated total dissolved N (TDN) and dissolved inorganic N (DIN) at septic sites (avg. 2.12 mg N/L TDN, 1.80 mg N/L DIN) in comparison to reference sites (avg. 0.47 mg N/L TDN, 0.31 mg N/L DIN) and that DON is an important component of TDN (avg. 32%). We present results in context of land use and measured concentrations of the contaminants sucralose, triclosan, and methyl-, ethyl-, propyl-, and butyl- parabens.