Delineation of contaminant sources and denitrification using isotopes of nitrate near a wastewater treatment plant in peri-urban settings

Abstract

Distinguishing sources of groundwater contamination in regions with multiple potential sources can be challenging using conventional markers. In this study, isotopes of nitrate (δ15NNO3 and δ18ONO3) were examined in conjunction with other hydrochemical parameters to better distinguish sources of groundwater contamination, where intensive agriculture occurs adjacent to a wastewater treatment plant (WWTP). High nitrate concentrations were found in groundwater both within the WWTP site and surrounding market garden farms (maximum of 99 mg/L and 78 mg/L nitrate as N, respectively). Ranges and median δ15NNO3 values showed clear differences between sample groups. In groundwater close to the WWTP, δ15NNO3 and δ18ONO3 values ranged from 10.4 to 41.2‰ and −0.5to 21.3‰, respectively, indicating predominantly sewage-sourced nitrate, while samples within market gardens showed evidence of mixed fertilizer (manure and synthetic) sourced nitrate, with δ15NNO3 and δ18ONO3 values between 7.2 and 29.8‰ and 0.4 to 15.1‰, respectively. Nitrate interpreted to be derived from the WWTP was also typically associated with elevated ammonia as N (median concentration of 17 mg/L) and SO4 (median concentration of 350 mg/L). These distinctive signatures allowed for clearer delineation of the extent and overlap between different contaminant plumes than otherwise possible. Geochemical conditions in groundwater surrounding the WWTP appear to promote denitrification, evident through enrichment in δ15NNO3 and δ18ONO3 and reduced nitrate concentrations between sampling rounds (locally). However, isotopic signatures in market garden areas showed no evidence of denitrification, and groundwater exhibited conditions likely to preserve nitrate (e.g. dissolved oxygen levels >2 mg/L). There is limited evidence of nitrate contamination currently impacting a nearby groundwater dependent ecosystem (Tootgarook Swamp), located down-gradient from the WWTP. This research demonstrates that a combination of hydrochemical and isotope data can help resolve sources of groundwater contamination and characterise nutrient degradation behaviour in settings with multiple inputs.