Sources And Mobilization Processes Of Arsenic In The Shallow Aquifers Of The Datong Basin

The bulk geochemistry of sediment samples from three boreholes (each around 50 m deep) drilled at the arsenic contaminated (As content >1060μg/L) aquifers of Datong Basin in Shanxi Province, northern China, indicates that the average bulk concentrations of major and trace elements of the sediments are similar to those of the average upper continental crust. The average content of arsenic (18.7 mg/kg) is higher than that of modern unconsolidated sediments (5-10 mg/kg). However, the abundance of elements varies with grain size, with higher concentrations in finer fractions of the sediments (clays and silts). The NH2OH-HCl extracted iron is closely correlated with extracted arsenic, suggesting that iron oxyhydroxides may be the major source of arsenic. Geochemical and environmental magnetic studies were carried out to identify the relationship of iron hydroxides/oxides with the arsenic in aquifer sediment and how it relates to the occurrences of high arsenic groundwater. Arsenic and iron contents showed strong association, however, weak correlation was observed between contents of phosphorus and iron and arsenic. This may be the result of competitive adsorption of phosphorus and arsenic on Fe-hydroxides/oxides. The strong correlations between arsenic contents and magnetic parameters suggest that the ferrimagnetic minerals are the dominated carrier of arsenic though there are some antiferromagnetic minerals present in the aquifer sediment sample. The results of chemical extraction experiments also suggest the arsenic co-exists with the reducible iron oxides such as maghemite and hematite in aquifer. And results of microcosm experiments show that the mobilization of arsenic from sediments at Datong is probably mainly related to changes in redox conditions. Moderately reducing conditions are favorable for release of arsenic into groundwater at Datong.To better understand the sources and mobilization processes responsible for arsenic enrichment in groundwater in the central part of Datong Basin where serious arsenic poisoning cases have been reported, hydrochemical characteristics of the groundwater and the geochemical and mineralogical features of the aquifer sediments were studied. The aqueous arsenic levels are strongly depth-dependent in the study area and the high arsenic concentrations are found at depths between 15 m to 60 m, with a maximum up to 1820μg/L The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that the mobilization of arsenic is related to reductive dissolution of Fe oxide/hydroxides and/or desorption from the Fe oxide/hydroxides at high pH (above 8.0). The bulk chemical results of sediments show that the arsenic and iron contents are moderately correlated, suggesting the association of arsenic with iron-bearing minerals. Sequential-extraction experiment results show that solid-phase arsenic is similarly distributed among the different pools of reservoir in the aquifer sediments. Strongly adsorbed arsenic and arsenic co-precipitated in solids are the dominant species of arsenic in the solid-phase. Geochemical studies using chemical analysis, X-ray diffraction and scanning electron microscopy on the magnetically separated fractions demonstrate that iron oxides/hydroxides with residual magnetite and chlorite, illite, iron oxide/hydroxides-coated quartz and feldspar, and ankerite are the dominant carriers of arsenic in the sediments. The major processes of arsenic mobilization should be desorption and reductive dissolution targeting Fe-rich phases in the aquifer sediments under reducing and alkaline conditions.Hydrochemical and sulfur isotope investigation helps understand the release mechanism of arsenic from aquifer into groundwater. The correlation between arsenic contents and values of proxies for redox conditions (Eh, U, and SO₄^2-) all indicate that the occurrences of high arsenic groundwaters are related to the reducing conditions of the aquifer system. The dissolved arsenic is dominated by inorganic species, including arsenite (As(III)) and arsenate (As(V)) in the waters. Distribution of As(III) and As(V) was primarily regulated by redox potential (Eh). Theδ³⁴S_[SO4] values of groundwater samples from the study area have a wide range (from -2.5 to +36.1‰), suggesting that the microbiological reducing of sulfate could occur within the aquifer. The springs which were regarded as the major recharge sources of groundwater in this area haveδ³⁴S_[SO4] values of +8.7 and +9.7‰, respectively. The δ³⁴S_[SO4] values fit within the typicalδ³⁴S values of atmospheric SO₄^2-, suggesting that atmospheric SO₄^2- is a major source of groundwater sulfate. Besides, the correlation betweenδ³⁴S_[SO4] and Eh and U indicates the reduction of sulfate in groundwater. The positive correlation betweenδ³⁴S_[SO4] and total arsenic supports that reducing processes are responsible for arsenic mobilization from aquifer into the groundwater. However, a continuous enrichment ofδ³⁴S_[SO4] was not observed with the increase of the total arsenic. Moreover, there are two high arsenic groundwater samples that have lowδ³⁴S_[SO4] values (-2.5 and +8.5‰, respectively). This can not be explained by microbial reducing processes which should result in a continuous enrichment ofδ³⁴S_[SO4] values of residual sulfate. Instead, it may be attributed to microbial oxidation under anaerobic conditions. Therefore, it can be concluded that apart from reducing processes, microbial anaerobic oxidation process in the presence of Fe-Mn oxides/hydroxides is also an important factor for the mobilization of arsenic in shallow groundwater in Datong Basin.Biomarker and hydrochemical characteristics of geogenic arsenic-contaminated aquifers at the Datong basin were analyzed to understand the impact of natural organic matter (NOM) biodegradation on arsenic enrichment in groundwater. The hydrochemical characteristics of high arsenic groundwater from the Datong Basin indicate that the mobilization of arsenic is related to reductive dissolution of Fe and Mn oxyhydroxides under the impact of NOM biodegradation in the aquifer. Meanwhile, the elevated concentration of bicarbonate alkalinity is another important factor to mobilization of arsenic into groundwater. Biomarker analysis reveals that all the sediments contain petroleum-sourced hydrocarbons, which may have undergone biodegradation, as suggest by the dominance of C25-C31 n-alkanes, C29 sterane and the distribution pattern of hopanes. The presence of unresolved complex mixtures (UCMs) in all samples also indicates the effect of biodegradation. At some depths (5.4-11.8 m; 31-33.2 m and 40-48.4 m below the land surface), the samples have low n-alkanes content and no odd-over-even predominance, suggesting that indigenous microbes within the aquifer can preferentially remove the petroleum-sourced n-alkanes. The bioavailable organic carbon is very important to promote the microbial activity and subsequent arsenic release from the aquifer to groundwater.In generally, clay minerals and iron oxides/hydroxides are the dominant sources of arsenic for groundwater. High pH and low Eh condition is favorable for the mobilization of arsenic from aquifer sediments into groundwater. Arsenic can be mobilized into groundwater either via desorption from the surfaces of clay minerals and iron oxides/hydroxides in alkaline environment, or via dissolution driven by the reduction of As-bearing Fe(III) oxides/hydroxides at low Eh conditions. Moreover, microbial activities are also important in controlling the mobilization of arsenic.