Spatial and temporal patterns of groundwater arsenic in shallow and deep groundwater of Yinchuan Plain, China

摘要

Groundwater is a strategic resource in Yinchuan Plain, with deep groundwater increasingly used for drinking water supply to reduce arsenic exposure of rural villagers who have relied on shallow tube wells for drinking. To understand the sources and mobilization processes for arsenic enrichment in groundwater, a well water and sediment geochemistry study was carried out. Considerable spatial variability of groundwater arsenic exists. Arsenic concentration in deeper groundwater (40-250 m) was generally less than 10 μg/L (n = 26) except for seven wells, with a mean of 7.0 μg/L (n = 33). For shallower depths (4-40 m). As concentration increased from <1 μg/L in the alluvial plain at higher elevation to 177 μg/L near the center of alluvial lacustrine plain at lower elevation. Two clusters of high arsenic, shallow groundwater run nearly parallel to the Yellow River present day and ancient courses as two narrow strips where groundwater flow is sluggish. Arsenic content in sediments ranged from 3.7 μg/g to 49.8 μg/g with an average of 9.2 μg/g, and was positively correlated with concentrations of sediment Cu, Fe2O3, Mn, Ba, Zn, F and organic C. Concentrations of arsenic between Dec. 2007 to Aug. 2011 in groundwater samples collected monthly vary more for the shallower depths (8 m: 76±29 μg/L; 12 m: 116±27 μg/L; 15 m: 88±26 μg/L; 20 m: 164±14 μg/L) than for the deeper depths (30 m: 35 ± 4 μg/L; 80 m: 33±5 μg/L). Temporal variability of As concentrations in shallow groundwater correlates with water levels, driven by irrigation to a large extent.