Seasonal Variations Of Groundwater Arsenic Concentration In Shallow Aquifers At Jianghan Plain

The consumption of As-contaminated groundwater as drinking water affects the health of almost 20 million people in China and 73 thousand people, including 20 thousand children, in the Jianghan Plain, central China. Although a great deal of effort has been made in the typical areas of waterborne endemic arsenic poisoning in Yellow River basins of northern China, limited work has been conducted in the Jianghan Plain of the Yangtze River in southern China, where there are abundant wetland resource under humid monsoon climate conditions. Differences in climate, geological background and anthropogenic activities make investigations of high arsenic groundwater in the Jianghan Plain different from those in northern China. High levels of arsenic in the groundwater of Jianghan Plain impact safe supply of drinking water and food, and the well-being of wetland ecosystems.To understand the sources and major factors controlling arsenic concentration in groundwater at Jianghan Plain, and to explain seasonal variations in groundwater As concentrations, four related studies were conducted in this work:1) systematic survey of the hydrogeochemical characteristics of shallow groundwater and the distribution of high arsenic groundwater in the Jianghan Plain.2) evaluation of temporal variations of groundwater level and chemical composition for As and other constituents based on dynamic monitoring data at a field monitoring site at Jianghan Plain.3) study of the distribution and speciation of As in the shallow aquifer sediments from the field site with synchrotron radiation light source technology.4) laboratory experiments of the release and retention of arsenic coupled to periodic aeration of sediments collected from the field site.Groundwater from Jianghan Plain were HCO3-Ca and HCO3-Ca·Mg types with circum-neutral pH, moderate to high electrical conductivity and negative ORP values. The concentrations of DOC, HCO3-, NH4-N and dissolved Fe, and Mn were high, while those of NO3- and SO42- were low. About 58% of the 729 groundwater samples contained As concentrations exceeding the WHO guideline for drinking water (10 μg/L), while the As content of most samples was 10-400 μg/L, although it reached as high as 2620 μg/L. High As groundwater in the Jianghan Plain mainly occurred in 10-45 m deep aquifers along the Dongjing and Tongshun rivers, and in areas where the watercourse of the Yangtze River changes.In the field monitoring site, groundwater water tables fluctuated over the sampling period (May,2012 to April,2014), with higher water levels during the rainy season and lower water levels during the dry season. Arsenic concentration in 34 of the 39 monitoring wells showed temporal variations which were positively correlated to groundwater levels changes, with lower concentration corresponding to lower water level during dry season and higher concentration corresponding to higher water level during rainy season. Concentrations of redox-sensitive constituents Fe, Fe(Ⅱ), and sulfides in the field monitoring site varied both spatially and temporally. In parallel with changes in concentration of As, concentration of Fe, Fe(Ⅱ), and sulfides rose during the rainy season and dropped during the dry season.High As contents in the sediments mainly occurred between the depth of 15 m and 35 m below the land surface. Shallower (less than 5 m deep) clay and silty clay also showed higher As contents under the effects of anthropogenic activities. Extremely high As contents only occurred around 20 m deep. Results of arsenic speciation study on the sediment samples showed that arsenate was the predominant As species in the near surface silty clay (usually less than 2.5 m deep). As the conditions turned to be reducing, arsenite became the dominant species in the clay and silty clay layers, while in the silty to fine sand layers, arsenate remained as the main species. In the deep medium to coarse sand aquifers (more than 30 m deep), As-sulfides were the main species of arsenic.The spatial distribution of groundwater As is closely related to the distribution and speciation of sediments As. The main source of groundwater As is the As-bearing Fe-oxides deposited during the erosion and transport of the Yangtze river and lacustrine sediments. Arsenic originating from anthropogenic activities is another source for As in the near surface aquifers. Arsenic sulfides in the deep sediments are not the source of groundwater As, but may be the products formed during As releasing.Batch sediment incubations experiments were carried out under varying (imposed) redox conditions to probe the cause of seasonal release and retention of arsenic between sediments and groundwater. The results showed that aqueous As concentrations responded rapidly to changes between anoxia and O2 delivery in sediments in the Jianghan Plain. Arsenic release to solution prevailed under anoxic conditions while As was removed from solution under oxic conditions.Desorption of As from clay minerals and Fe-oxides, and dissimilatory Fe oxides reductive dissolution and dissimilatory As(V) are the main mechanisms for As releasing under reducing conditions. Anoxia is a prerequisite for As mobilization into groundwater from sediments at Jianghan Plain. More importantly, in situ aquifer sediments at depths>15 m could release As through multiple redox cycles without the need for addition of an exogenous electron donor. The low concentrations of reactive organic matter in the sediments are sufficient for arsenic mobilization via reductive processes at depth. Significant seasonal changes in As concentrations at Jianghan can be attributed to shifts in redox conditions due to changing of surface and groundwater levels and flow regime.