Groundwater nitrate pollution in the recharge zone of a regional Quaternary flow system (Wielkopolska region, Poland)

The present study examines the behaviour of nitrate in the recharge zone of a regional Quaternary flow system. The presence of contaminated groundwater with high nitrate content in the shallow part of the flow system was documented. Tritium analyses confirmed that these contaminants can migrate downward. A high downward gradient exists in the study area, extending into the regions of groundwater extraction. In the unconfined part of the flow system, which is the most vulnerable to pollution, a high concentration of nitrate was found to occur at great depth. However, denitrification processes limit nitrate migration. As a result, in the deeper parts of the flow system in regions under confined conditions, an absence of nitrate was observed, and a higher sulphate concentration and total hardness were evident. The denitrification was also confirmed by the existence of a high gaseous N-2 concentration. It was documented that denitrification occurs in both the confined and unconfined parts of the flow system, but the potential for denitrification is higher in the confined parts (leading to the disappearance of nitrate in deep aquifers). Autotrophic denitrification supported by sulphide compounds was indicated as the dominant denitrification process.