Occurrence Of High Salinity And High Arsenic Groundwater Inwestern Part Of Hetao Basin And Thermodynamic Model For Arsenic Mobilization

Groundwater arsenic occurs in many parts of China, especially in the Hetao basin, Inner Mongolia, where lots of local people are affected. High salinity groundwater also occurs in this basin with semi-arid and arid climate. It is not only unsuitable for drinking but harmful to crops as irrigation water. Groundwater with high salinity and high arsenic were investitaed to figure out the factors controlling their enrichment and the relationship between these two. Furthermore, thermodynamic model was applied to quantify geochemical processes related to As enrichment. The main findings in this study are shown as follows.Br/Cl ratio, stable isotopes and relationships between major ions showed limited effect of direct evaporation on groundwater salinity.The main mechanism for salts accumulation in groundwater includes chemical weathering and dissolution of minerals and evaporates in sediments, which contributes to 80% salinity in groundwater. Groundwater chemistry is dominated by silicate weathering and also affected by dissolution of evaporates. Na Cl, Ca SO4 and Na2SO4, which would be the main evaporates in sediments, play an important role in controlling groundwater salinity.High arsenic groundwater occurs in both shallow and deep groundwaters. It shows a gradual increase from alluvial fans(< 10 mg/L) to flat plain(300-400 mg/L). In vertical profiles, arsenic tends to increase along depth at first 50 m, while shows a decrease trend after 50 m. The occurrence of high As groundwater is controlled by reductive dissolution of Fe minerals as the decomposition of organic matter continues, leading to release of adsorbed As to groundwater. Arsenic concentration increases during the reduction of Fe and SO42-, and reaches the maximum at the reduction of SO42-, indicating that SO42- reduction enhances the As mobilization. This may be attributed by the reduction of Fe minerals by HS- produced by SO42- reduction.Although high arsenic and high salinity groundwaters show similar distribution, there are two quite different processes controlling their occurrence. The longer groundwater residual time benefits both of them. The dissolution of SO4 salts not only increases the salinity of groundwater but also contributes to the As mobilization as it affects reduction processes related to As release. Ca minerals precipitated as CO2/HCO3- produced by organic matter decomposition, leading to the formation of high sodium adsorption ratio(SAR) groundwater.Model results show that only 0.01 mol Fe(OH)3 and 6.42 mg/L organic matter were used to mobilize As to the current state, and 11 m M Fe S,4.5 m M Ca CO3 were precipitated in this process. Considering large amount of organic matter and active Fe minerals in sediments, groundwater arsenic will stay a quite long time. The abstraction of groundwater stimulates groundwater flow, leading to adverse effects of high arsenic groundwater on fresh water.