Tracing nitrate origins and transformation processes in groundwater of the Hohhot Basin's Piedmont strong runoff zone through dual isotopes and hydro-chemical analysis

Abstract

Nitrate, which poses a serious threat to the drinking water supply, is one of the most prevalent anthropogenic groundwater contaminants worldwide. With the development of the chemical industry, the nitrate pollution of groundwater in the Piedmont strong runoff zone of the Hohhot Basin, which is the main groundwater extraction area, is becoming increasingly severe. The special hydrogeological and complex pollution conditions in the study area make it difficult to identify nitrate sources and transformation processes. In order to identify the results more accurately, this study combined water chemistry, multivariate statistical analysis and isotope tracer methods to determine the sources and transformation processes of nitrate in the study area. The results showed that the groundwater in the eastern part of the study area (ESA) was clearly affected by anthropogenic activities, and its nitrate was mainly from nitrification of ammonia in industrial wastewater, nitrate in industrial wastewater (the sum of the two contributions was 62.2 %), and nitrate in manure (20.5 %). The hydrogeochemical characteristics of groundwater in the western part of the study area (WSA) are the same as those of natural groundwater in the Piedmont strong-runoff zone. The nitrate in groundwater in the WSA was mainly derived from soil nitrogen (63.8 %) and ammonia fertilizer (28.8 %). Nitrification and denitrification occurred only locally in the aquifer of the study area and were more pronounced in the ESA. Meanwhile, the transformation processes of nitrate in groundwater in the ESA and WSA was significantly influenced by contamination with chlorinated hydrocarbon volatile organic compounds and hydrogeological conditions, respectively. These findings provide a scientific basis for the development of groundwater pollution prevention measures in the study area and guide the traceability of nitrate in groundwater in areas with similar hydrogeological and pollution conditions.