Assessing differential transport of chemical and biological constituents in groundwater impacted by a municipal waste water treatment system

Abstract

An increasing number of homes and subdivisions rely on septic systems and decentralized wastewater systems to treat domestic sewage. These treatment systems can release poorly treated wastewater if regular maintenance is not performed or maximum loading capabilities are exceeded. To better quantify signs of wastewater impacts on groundwater, this study sampled groundwater down-gradient of an aerated sewage lagoon which discharges high concentrations of waste to groundwater. The intent is to better understand how chemical and biological constituents originating from wastewater behave and move through groundwater. Groundwater samples were collected in four locations down-gradient from the infiltration beds every other week from June - August 2012. Samples were analyzed for chloride, boron, nitrate-N, total coliforms, Escherichia coli, and Bacteroides species using AllBac and HF183 primers. The Bacteroides genus is abundant in the gut and specific to the guts of warm blooded mammals, the species targeted by AllBac is present in all mammals and HF183 is specific to humans. It was hypothesized that chloride and boron would behave similarly and conservatively moving down-gradient and nitrate-N would behave non-conservatively due to its ability to denitrify under reduced conditions. It was also hypothesized that Bacterodies species (AllBac and HF183), would be a better microbiological indicator of wastewater. Findings from this study indicated strong relationships between chloride and boron as they move down-gradient from the source (R 2=0.9901, p = 0.003) and weaker relationships between chloride and nitrate-N (R 2=0.4371, p = 3.8e-6). Additional evidence indicated nitrate-N may be behaving non-conservatively as it moves downgradient due to denitrification in areas with dissolved oxygen levels <0.5 mg/L, but it may be behaving conservatively further down-gradient after denitrification has occurred. The microbiological analyses indicated that Bacteroides may not be a better indicator organism in areas with low Bacteroides presence. However, Bacteroides (AllBac) had a strong relationship with total coliforms (R 2=0.785, p=1.25e-4) and E. coli (R 2=0.750, p=6.57e-10) indicating similar behavior in groundwater. Findings from this study allow for a better understanding of the transport of these biological and chemical constituents in groundwater, and further elucidation would be possible through development of a groundwater model.