Fluoride and nitrate contamination of groundwater in the Loess Plateau, China: Sources and related human health risks.

Fluoride (F) and nitrate (NO) in groundwater have caused serious health problems worldwide. However, in the Chinese Loess Plateau where groundwater is the primary source of drinking water, previous studies have rarely reported the health risks from fluoride and nitrate in groundwater. Therefore, we collected 105 groundwater samples (78 from shallow aquifers and 27 from deep aquifers) from the western district of the Loess Plateau for physicochemical and isotopic analysis to investigate the sources of F and NO in groundwater and associated health risks. Fluoride concentration in 73.1% of shallow groundwater and 22.2% of deep groundwater exceeds 1.5 mg/L, while NO content in 76.3% of shallow groundwater and 51.9% of deep groundwater surpasses 50 mg/L. High-F groundwater is associated with HCO-Na, SO-Na·Mg and Cl-Na·Mg types water. Fluorine-bearing minerals dissolution, cation exchange, calcite precipitation, evaporation, and anthropogenic activities contribute significantly F in groundwater. Mixing with shallow groundwater is an important source of F in deep groundwater. The NO content is highest in Cl type water, followed by SO type and HCO type water. NO mainly originates from soil organic nitrogen (SON), chemical fertilizers (CF), and manure and sewage (M&S). Nitrification is the dominant transformation process of nitrogen nutrients in groundwater. The hazard index (HI) values for shallow groundwater are 0.203-9.232 for adults, 0.253-11.522 for teenagers, 0.359-16.322 for children, and 0.507-23.043 for infants, while those for deep groundwater are 0.713-5.813 for adults, 0.890-7.254 for teenagers, 1.261-10.277 for children, and 1.780-14.508 for infants. Approximately 96.2% of shallow groundwater poses non-carcinogenic risks to infants and children, followed by 92.3% to teenagers, and 89.7% to adults. All deep groundwater poses non-carcinogenic risks to infants and children, followed by 92.6% to teenagers, and 74.1% to adults. This study is helpful to develop strategies for the integrated management of high fluoride or nitrate groundwater in arid areas.