Characteristics Of Organic Matter In Arsenic-rich Aquifer Sediments Of The Hetao Basin, Inner Mongolia

High arsenic groundwater has been found in the Hetao basin of Inner Mongolia. Two boreholes(BH02 and H02) were drilled in the western part of the basin to investigate sources of organic matters in aquifer sediments and its roles in arsenic mobilization. The arsenic-rich mechanism, type and origin of organic matter as well as biogeochemical significance were revealed in this study based on data reviews, field survey, field sampling and laboratory analysis of the samples. Specific conclusions have been drawn as follows.1. Results of inorganic analysis showed arsenic contents and their controlling factors in sediments. The content of As was from 4.44 to 28.89 μg/g with the average of 12.07 μg/g. The content of Fe2O3 was from 1.43 to 6.86% with the average of 3.56%, and the content of Mn O from 0.03% to 0.135% with the average of 0.06%. Strong positive correlations between arsenic and iron/manganese were observed(R=0.88 for arsenic and iron, R=0.83 for arsenic and manganese). The adsorption of arsenic by ferric and manganic oxyhydroxides was probably the most important factor for arsenic enrichment in sediment.2. Total organic carbon and organic carbon isotope were characterized. The total organic carbon content was low with TOC contents between 0.03% and 1.65% in the sediments. Strong positive correlation between arsenic and TOC content was observed with the correlation coefficients of 0.57. The δ13C value varied from-24.24 ‰ to-32.3‰, close to the δ13C values of terrestrial plants, which suggested that the terrestrial plants were the main sources of organic matter in sediments of the study area. The contents of TOC and the δ13C were closely related to lithology. Moreover, contents of TOC and δ13C in clay sediments were higher than those in sand samples.3. The origin and biogeochemical significance of organic matter were evaluated by the biomarker analysis of sediment organic matter. Long chain n-alkanes were relatively rich in all samples with obvious odd-even predominance. The main source of organic matter was the herbal high plants, and the secondary source was the woody plants.The presence of UCM and the low odd-even predominance for long chain n-alkanes in some sediments provided the evidence for the invasion of petroleum organic matter. Characteristics of hopanes and steranes also showed the origin of mature organic matter. In addition, the n-alkanes originating from petroleum were readily used for reduction of Fe/Mn oxides by microorganism, which would cause the release of arsenic. Although the organic matter content in the sandy samples was low, they contained mature organic matter(petroleum), which was bioavailable and led to arsenic mobilization.