A talaj fixált ammónium-ion tartalmának hatása a talajvíz tisztulási folyamataira a szennyezőforrás felszámolását követően települési környezetben | Effects of fixed ammonium ion content of the soil on groundwater purification processes after the elimination of the pollution source in municipal environment

Growing NH₄⁺ content of groundwater results in increasing exchangeable and fixed ammonium ion content of the soil. NH₄⁺ bond in the soil may go again into solution parallel with the dilution of the soil solution but at a slower rate than fixing. This process influences significantly the NH₄⁺ content of the soil. In settlements with no sewerage system the high NH₄⁺ content of sewage flowing out of uninsulated septic tanks may increase the fixed NH₄-N content of the soil that could have a significant effect on the quality of groundwater even after the potential disappearance of pollution sources. In this study the effects of the fixed NH₄-N content of the soil around an uninsulated residential septic tank on the purification processes of the groundwater were investigated. The septic tank in the study area was dismantled in 2014 after 27 years of operation as a sewerage system was constructed. When the tank was still in operation in 2012 and 2013, very high, 55–75 mg l⁻1 NH₄⁺ content was measured in the water of the monitoring well 1 metre from the tank in the course of seasonal sampling. When sewage outflow was terminated in 2014 concentrations decreased right away but even 5 years after pollutant supply was stopped, concentrations (35–57 mg l⁻1) highly exceeding the pollution limit (0.5 mg l⁻1) were measured. Considering this very high concentration, it can be assumed that great amount of NH₄⁺ is still released into the groundwater. In order to prove this, the exchangeable and fixed NH₄-N and NO₃-N contents of the soil were determined by 20 cm down to a depth of 4 metres (2019). The measurements indicated the significant accumulation of exchangeable and fixed NH₄-N in the zone between 220 and 400 cm. Highest fixed NH₄-N concentrations of 457 mg l⁻1 were found between 220 and 240 cm suggesting that sewage outflow was most intense at this depth. Slow decrease in concentrations can be observed in deeper zones but concentrations higher than 350 mg l⁻1 were measured between 220 and 380 cm. Based on correlation analyses, the quantity of fixed NH₄-N shows no correlation with the soil texture thus it can be stated that the vertical pattern of NH₄-N content is determined dominantly by sewage outflow and its depth. In the unsaturated zone of the borehole a significant accumulation of NO₃-N was also identified. The maximum of NO₃-N was found in the zone between 100 and 140 cm. The peak nitrate calculated for NO₃⁻ ion with a value >1300 mg kg⁻1 is 2.5 times the limit set for the nitrate content of the geological medium. Based on the results, exchangeable and fixed NH₄-N contents in the soil are still very high, 5 years after sewage outflow was stopped. The continuous solution of this component still contributes to the high NH₄⁺ content of the groundwater. As a result, the contaminated soil in the immediate environment of the septic tank is still a pollution source.