Formation Of High As Groundwater And Water-rock Interaction In Shallow Aquifers From The Hetao Basin, Inner Mongolia

The Hetao Basin of Inner Mongolia is a typical endemic arseniasis area, where high As groundwater has universally been found since 1990s. This study has investigated hydrogeochemistry and mechanism of As mobilization in the shallow aquifers, based on intensive data analysis, field works and lab experiments. The main achievements having been reached are shown as follows.1. The basin is one of the Cenozoic rift basins, which has experienced a series of paleo-climate change since Early Pleistocene, including wet cold, wet-warm, and arid (Holocene). A former lake was believed to develop at the early stage of late Pleistocene (around 120 ka BP), and vanish during the Holocene. With the vanishment of the lake, the early Yellow River occurred. It results in a complex sedimentary environment with fluvial and lacustrine sediments interlacing each other. 14C dating shows that shallow aquifer sediments (9-23 m below land surface) developed during late stage of late Pleistocene, during which climate was arid and the lake shrank. The lake water became a part of pore water in the sediment and is gradually evolved into modern shallow groundwater, which is the main supply water for residents'living.2. Chemical characteristics of high As groundwater have been evaluated. Arsenic concentrations in shallow groundwaters from the Hetao Basin of Inner Mongolia range between 0.6 and 572μg/L. They are characterized by high concentrations of dissolved Fe, Mn, HCO3-, and S2?, and low concentrations of NO3- and SO42-. Groundwater pH is near-neutral to weak alkaline (7.01-8.43). Redox potentials range from ?153 to 83 mV. In the reducing groundwaters, inorganic As(III) accounts for around 75% of total dissolved As. Fe concentrations are up to 5.90 mg/L3. Systemically investigation has been carried out for three representative boreholes. It indicates that high As groundwaters usually occur in shallow aquifers with dark grey or black fine sand, which deposited in the lacustrine reductive environment. Total As contents in the sediments are observed to be 7.3–73.3 mg/kg (average of 18.9 mg/kg). The total As is mildly-strongly correlated with total Fe and total Mn, while a quite weak correlation exists between total As and total S. It is found in the sequential extraction that chemically active As is mainly bound to Fe-Mn oxides, up to 3500μg/kg. The mobilization of As under reducing conditions is believed to include reductive dissolution of Fe-Mn oxides and reduction of adsorbed As.4. Effect of groundwater particles on As behavior has been performed. Chemical analysis of 43 groundwater samples shows concentrations of most elements (Ba, Co, Mn, Mo, Ni, V, Sr, Mg, Ca, Na, K) in unfiltered samples are well correlated with those in <0.45μm fraction, and the ratios between them are close to 1.00. Although saturation indices for calcite and dolomite are greater than 0, they would possibly exist as small size colloids in groundwater. Concentrations of Cd, Pb, Fe, and Al in <0.45μm fraction are mostly less than those in unfiltered fraction. Arsenic, Cu, Zn and Cr are grouped into another species. These elements could mainly be associated with organic-complexing colloids with particle size less than 0.45μm, although As, Zn and Cu would partially be carried by large-size inorganic colloids.5. R cluster analysis has been carried out in terms of 27 chemical compositions of 26 representative groundwater samples. Multiple linear regression was established between As and other components. Curve regression analysis shows that As(III) is a negative exponential of Eh.