Effects of nitrogen loading from domestic wastewater on groundwater quality

Abstract

In this study the effects of nitrogen effluent from a permeable constructed sewage tank on groundwater quality were investigated. Sampling took place before and 1.5 years after the closure. Using a 3D hydrogeological model, the spatial distribution of dissolved inorganic nitrogen (DIN), comprising the species NH₄⁺, NO₂^- and NO₃^-, was modelled in the saturated zone and the amounts and changes in the ratio of NH₄^-N, NO₂^-N, NO₃^-N were determined. The first part of our general hypothesis, that the groundwater was heavily contaminated in the area surrounding the sewage tank, was clearly verified, since every investigated nitrogen compound exceeded the contamination limit values; but the second part of our hypothesis, that the degree of contamination would significantly decrease after the sewage tank was taken out of use has not been confirmed, since the amount of nitrogen present in inorganic forms increased in the modelled zone. The increase in DIN and the relatively high concentration of NH₄⁺ (35 mg/L) in the immediate vicinity of the tank can be explained by the fact that organic matter (OM) accumulated over the decades provided a constant supply of inorganic nitrogen forms.