Hydrogeochemical Evolution Of High Arsenic Groundwater In The Hetao Basin, Inner Mongolia

The Hetao basin is located in the Midwestern part of Mongolia, where high Asgroundwater seriously affected health of local residents. Many people suffered fromendemic arsenicosis. Some lost the ability of working, and some faced death. Arsenicpoisoning has limited the development of local economy and the society. Thus, it isvery important to avoid drinking high-arsenic water in order to protect the drinkingwater safety for local residents.According to three-year investigation of the groundwater samples, thedistribution of high As groundwater and their geochemical characteristics have beenfigured out. The high As groundwater was characterised by high pH, low redoxpotential and the high concentrations of Fe and Mn. Concentrations of SO₄^2-and NO3-were relatively low because of reduced conditions. Arsenic（III） was the predominantAs species in high As groundwater. Besides, high F concentration was also observedin high As groundwater, which was not found in other high As groundwater areas.Although Eh, sulfide or ammonia was normally used to correlate with Asconcentrations, they are only indicators for low redox potentials and can not be usedfor quantatively describing As mobilization in the aquifers, The quantitativerelationship between the reducing intensity of SO₄^2-and As concentrations in thegroundwaters was established.A typical flow path was selected in the study area to study the spatial andtemporal evolution of high-arsenic groundwater. Sediments were collected from thedrilling boreholes and analyzed for chemical components. Long-term monitoring dataof multilevel samples at four locations were used to evaluate the influence ofwater-rock interaction on the concentration of As. Both groundwater As and F comefrom the aquifer sediment. Hydrogeochemical processes of these two elements weredifferent. Arsenic was mobilized into groundwater in reduction conditions, andconcentrated because of the evaporation and concentration effect and desorption.Besides, As concentrations varied with fluctuations of groundwater levels, which washigher with high groundwater levels. However, F-concentration was affected by theevaporation and mineral precipitation. Low Ca2+concentrations led to high F-concentration.In groundwater irrigation areas, the extraction of groundwater resulted inchanges of redox conditions and adsorption of As from groundwater to sediments.During the seasons without irrigation, the groundwater level increased leading to lowredox potential, whern As concentrations increased. In the surface water Irrigationareas, the groundwater level increased because of the recharge from surface water,which makes fluoride and arsenic concentrations decreased in the sallowest aquifers. In summer, the fluoride concentration increased due to the evaporation. However, theelevated levels of arsenic results from the change of redox conditions whichaccelerates the speed of arsenic release to groundwater. The impact of groundwater bythe upper aquifers is greater than lower ones. Concentrations of S2-, NH3-N and Fe2+showed good correlations with As, which further proves the correlation between theredox environment and the release of As.