Assessment of Groundwater Decontamination Processes around a Dismantled Septic Tank Using GIS and Statistical Analysis

Abstract

Septic tanks without proper construction and insulation entail a significant risk to the environment. In this study, the environmental impacts of a permeably designed septic tank on shallow groundwater contamination are investigated, and changes in water quality in the period after its elimination in 2014 are assessed. For the purpose of evaluating the pollution level of the site, 10 monitoring wells were installed around the septic tank in 2012 and long-term monitoring was carried out. Analytical measurements revealed a significant level of groundwater contamination in the operational period of the septic tank. Extremely high concentrations of NH4+ (>90 mg/L) were observed in the closest monitoring wells, and in most of the wells, concentrations exceeded the relevant contamination limit. δD and δ18O isotopic ratios of monitoring wells within 1 m from the septic tank indicate continuous recharge of sewage water originating from deeper aquifers. The groundwater dome resulting from the wastewater discharge exceeded 1.1 m, within a distance of 25 m. Statistical analyses also revealed significant changes in water quality depending on the monitoring well location from the septic tank. In the period after the septic tank elimination, considerable changes have been detected. Following the cessation of the wastewater discharge, the groundwater dome around the septic tank disappeared; therefore, differences in groundwater levels have decreased from more than 1 m to a few cm. Significant positive changes were detected in the water quality parameters investigated after the dismantling of the septic tank. Five years after the cessation of the pollutant supply, concentrations still exceeded the contamination limit in most of the monitoring wells, indicating slow decontamination processes with a permanently high level of pollution.