Investigating septic system impacts on a rural subwatershed using fluorescent dissolved organic matter and contaminants of emerging concern

Abstract

Septic systems introduce partially treated wastewater, which contains contaminants of emerging concern (CECs) like pharmaceuticals, antibiotics, UV filters, and hormones, into nearby surface water via groundwater transport. In addition to CECs, septic effluent contains wastewater-derived fluorescent dissolved organic matter (FDOM), which we hypothesized could be employed to quickly and economically identify sections of streams impacted by septic effluent. We used fluorescence excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) in conjunction with CEC measurements to characterize septic wastewater and identify potential EEM and EEM-PARAFAC based indicators of septic effluent. The potential indicators were applied to spatially resolved samples from a rural subwatershed with variable septic density and no municipal wastewater infrastructure. We proposed three ratiometric FDOM parameters as effective indicators of septic effluent: the area-normalized ratio of tryptophan-like to humic acid-like fluorescence (R2/R5); the ratio of wastewater EEM-PARAFAC components with tryptophan-like and humic-like fluorescence (W2/W1); and the ratio of North Branch EEM-PARAFAC components with microbially derived protein-like and humic-like fluorescence (C4/C1). All three ratiometric indicators were considered conservative and well correlated with concentrations of the artificial sweetener sucralose in areas with medium (41 ? 80 tanks/km2) and high (>80 tanks/km2) septic density. In medium septic density areas, C4/C1 and W2/W1 were also well correlated with the pharmaceutical carbamazepine. The absence of correlations between FDOM parameters and CEC concentrations in areas with low septic density suggests that 40 tanks/km2 serves as a threshold for measurable impacts of septic systems on nearby surface waters. For areas above this septic density threshold, the identified ratiometric FDOM indicators can provide an economical method for evaluating the impacts of septic systems on water quality. The indicators were employed to evaluate the complex pathways involved with septic effluent transport to streams via preferential groundwater seeps during baseflow and subsurface mobilization during a storm event. Overall, these findings confirmed the potential for using FDOM-based parameters to identify and assess the impacts of septic effluent on nearby surface water quality.