Reductive dissolution of iron oxyhydroxides and reduction of arsenic are often invoked as leading causes of high dissolved As levels in shallow groundwater of Bangladesh. The second of these assumptions is questioned here by comparing the behavior As and phosphate (P), a structural analogue for As (V) which also adsorbs strongly to Fe oxyhydroxides but is not subject to reduction. The first line of evidence is provided by a detailed groundwater time-series spanning two years for three wells in the 6-9 m depth range showing removal of As(III) from shallow groundwater during the monsoon without of loss of P. The data indicate a loss of ~90% of the dissolved As from groundwater in the intermediate well relative to a level of 3 μmol/L As predicted by conservative mixing between groundwater sampled from the shallower and the deeper well. In contrast, P concentrations of ~30 μmol/L in the intermediate well closely match the prediction from conservative mixing. Reduction therefore appears to inhibit the release of As to groundwater at this site relative to P instead of enhancing it. A re-analysis of existing groundwater As and P data from across the country provides a broader context for this finding and confirms that, without reduction, elevated concentrations of As would probably be even more widespread in shallow aquifers of Bangladesh. Without providing definite proof, X-ray absorption spectroscopy of sediment from the time-series site and elsewhere suggests that the loss of As from groundwater may be coupled to precipitation of As sulfide. Further study is needed to assess the implications of these observations for shallow aquifers that have been subjected to increased withdrawals for irrigation in recent decades.

中文翻译：

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孟加拉国浅层地下水中砷和磷酸盐脱钩的证据及其潜在影响

氢氧化铁的还原溶解和砷的还原通常被认为是孟加拉国浅层地下水中高溶解砷水平的主要原因。这些假设中的第二个在这里通过比较 As 和磷酸盐 (P) 的行为受到质疑，磷酸盐 (P) 是 As (V) 的结构类似物，它也强烈吸附到 Fe 羟基氧化物但不会被还原。第一条证据是由 6-9 m 深度范围内三口井跨越两年的详细地下水时间序列提供的，表明季风期间从浅层地下水中去除了 As(III)，而 P 没有损失。数据表明相对于 3 μmol/L 的水平，中间井中地下水中约 90% 的溶解砷损失正如通过从较浅井和较深井采样的地下水之间的保守混合所预测的那样。相比之下，中间孔中~30 μmol/L 的 P 浓度与保守混合的预测非常匹配。因此，相对于磷，还原似乎抑制了该地点地下水中砷的释放，而不是增强。对全国现有地下水 As 和 P 数据的重新分析为这一发现提供了更广泛的背景，并证实，如果不减少，孟加拉国浅层含水层中 As 浓度的升高可能会更加普遍。在没有提供明确证据的情况下，来自时间序列地点和其他地方的沉积物的 X 射线吸收光谱表明，地下水中砷的损失可能与硫化砷的沉淀有关。