Mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the Huhhot Basin, Inner Mongolia

Abstract

Observed As concentrations in groundwater from boreholes and wells in the Huhhot Basin of Inner Mongolia, northern China, range between <1 μg l −1 and 1480 μg l −1. The aquifers are composed of Quaternary (largely Holocene) lacustrine and fluvial sediments. High concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells (well depths ranging between <10 m and 400 m). Populations from the affected areas experience a number of As-related health problems, the most notable of which are skin lesions (keratosis, melanosis, skin cancer) but with internal cancers (lung and bladder cancer) also having been reported. In both the shallow and deep aquifers, groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low-lying central part of the basin. High As concentrations occur in anaerobic groundwaters from this low-lying area and are associated with moderately high dissolved Fe as well as high Mn, NH 4, dissolved organic C (DOC), HCO 3 and P concentrations. Many of the deep groundwaters have particularly enriched DOC concentrations (up to 30 mg l −1) and are often brown as a result of the high concentrations of organic acid. In the reducing groundwaters, inorganic As(III) constitutes typically more than 60% of the total dissolved As. The highest As concentrations tend to be found in groundwater with low SO 4 concentrations and indicate that As mobilisation occurs under strongly reducing conditions, where SO 4 reduction has been an active process. High concentrations of Fe, Mn, NH 4, HCO 3 and P are a common feature of reducing high-As groundwater provinces (e.g. Bangladesh, West Bengal). High concentrations of organic acid (humic, fulvic acid) are not a universal feature of such aquifers, but have been found in groundwaters from Taiwan and Hungary for example. The observed range of total As concentrations in sediments is 3–29 mg kg −1 ( n=12) and the concentrations correlate positively with total Fe. Up to 30% of the As is oxalate-extractable and taken to be associated largely with Fe oxides. The release of As into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the Fe oxide minerals. The association of dissolved As with constituents such as HCO 3, DOC and P may be a coincidence related to the prevalent reducing conditions and slow groundwater flow, but they may also be directly involved because of their competition with As for binding sites on the Fe oxides. The Huhhot groundwaters also have some high concentrations of dissolved U (up to 53 μg l −1) and F − (up to 6.8 mg l −1). In contrast to As, U occurs predominantly under the more oxidising conditions along the basin margins. Fluoride occurs dominantly in the shallow groundwaters which have Na and HCO 3 as the dominant ions. The combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved As concentrations observed as the sediments are likely to contain newly-formed and reactive minerals and have not been well flushed since burial.