Hydrogeochemical processes in shallow quaternary aquifers from the northern part of the Datong Basin, China

Groundwater is the most important source of water supply in Datong city. However, the levels of shallow Quaternary groundwaters from urbanized areas have been declining continuously and groundwater quality deteriorating in recent years. Understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Datong. Mineral hydrolysis of alumino-silicate minerals such as plagioclase and clinopyroxene, is the primary process controlling the concentration of H4SiO4 in the study area. Speciation calculations using the geochemical modeling code PHREEQC indicated that hydrolysis of bedrock, mainly composed of basalt and metamorphic rocks, is the major hydrogeochemical process controlling groundwater chemistry. The study area can be divided into 3 hydrogeochemical zones: A. Recharge (unimpacted) zone, B. Intermediate (industry-impacted) zone, and C. Discharge (agriculture-impacted) zone. Ion exchange and industrial and/or agricultural contamination contribute to the increase of Na+ from Zone A to Zone C, where the concentration of NO3- is up to 461.5 mg/l with a mean value of 101.5 mg/l, indicating that agricultural practice seriously affects groundwater. Sulfate concentration in groundwaters in an alluvial fan at Datong is extremely high, up to 1172.9 mg/l, and shows a close relationship with the concentrations of trace elements, especially Ni and Co, indicating that coal mining is the main contamination source for groundwater from the alluvial fan, in addition to agricultural activities.