Temporal variability of groundwater chemistry in shallow and deep aquifers of Araihazar, Bangladesh
Temporal variability of groundwater chemistry in shallow and deep aquifers of Araihazar, Bangladesh
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DOI：
10.1016/j.jconhyd.2008.03.007
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发表时间：
2008-07-29
影响因子：
3.6
通讯作者：
Ahmed, K. M.
中科院分区：
文献类型：
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作者：
Dhar, R. K.;Zheng, Y.;Ahmed, K. M.
在孟加拉国阿拉伊哈扎尔，在23年的时间里，每隔2-4周从37口监测井中采集样本，以评估地下水中砷和其他成分成分的时间变异性。监测井分6个井网，井深5-91m。砷、钙、铁、钾、镁、锰、钠、磷、S等元素的测定采用高分辨离子质谱仪，精密度在5%以上；氯离子的测定采用离子色谱法。在深部的浅井中，As和P的浓度变化一般为-lt；30%，而主要离子(Na、K、镁、钙和氯)和氧化还原敏感元素(Fe、Mn和S)的浓度随时间的变化最高可达+/-90%。在开采更深含水层的井中，地下水的浓度要低得多，变化幅度只有10%。在这些深层含水层中，主要阳离子的浓度也变化了10%。相比之下，深层含水层中对氧化还原反应敏感的铁、S和锰的浓度随着时间的推移变化高达97%。因此，在浅层和深层含水层的地下水中，As和Fe浓度的变化之间明显存在强烈的脱钩。时间序列数据与H-3/He-3和C-14测年确定的地下水年龄的对比表明，主要阳离子和氯离子浓度的较大季节或年际变化仅限于5年前的浅层含水层和补给地下水。尽管在这些非常年轻的水域中，对氧化还原敏感的成分有很大的变化，但砷的浓度没有相应的变化。这归因于溶质和固体As之间的快速平衡所产生的化学缓冲。在现有浅井地下水中砷平均含量分别为102mg/L(范围：5~648mg/L；n=118)和272mg/L(范围：10~48mg/L；n=65)的两个地点，砷浓度的系统性长期下降支持了冲刷可能会慢慢耗尽含水层As的观点。~gt；5年H-3/He-3年龄的浅层含水层水随时间的变化具有恒定的As：P摩尔比9.6，这表明了共同的动员机制。(C)2008爱思唯尔B.V.保留所有权利。
Samples were collected every 2-4 weeks from a set of 37 monitoring wells over a period of 23 years in Araihazar, Bangladesh, to evaluate the temporal variability of groundwater composition for As and other constituents. The monitoring wells are grouped in 6 nests and span the 5-91 m depth range. Concentrations of As, Ca, Fe, K, Mg, Mn, Na, P, and S were measured by high-resolution ICPMS with a precision of 5% or better; concentrations of Cl were measured by ion chromatography. In shallow wells < 30 in deep, As and P concentrations generally varied by < 30%, whereas concentrations of the major ions (Na, K, Mg, Ca and Cl) and the redox-sensitive elements (Fe, Mn, and S) varied over time by up to +/- 90%. In wells tapping the deeper aquifers > 30 mn often below clay layers concentrations of groundwater As were much lower and varied by < 10%. The concentrations of major cations also varied by < 10% in these deep aquifers. In contrast, the concentration of redox-sensitive constituents Fe, S, and Mn in deep aquifers varied by up to 97% over time. Thus, strong decoupling between variations in As and Fe concentrations is evident in groundwaters from shallow and deep aquifers. Comparison of the time series data with groundwater ages determined by H-3/He-3 and C-14 dating shows that large seasonal or inter-annual variations in major cation and chloride concentrations are restricted to shallow aquifers and groundwater recharged < 5 years ago. There is no corresponding change in As concentrations despite having significant variations of redox sensitive constituents in these very young waters. This is attributed to chemical buffering due to rapid equilibrium between solute and solid As. At two sites where the As content of groundwater in existing shallow wells averages 102 mu g/L (range: < 5 to 648 mu g/L; n=118) and 272 mu g/L (range: 10 to 485 mu g/L; n=65), respectively, a systematic long-term decline in As concentrations lends support to the notion that flushing may slowly deplete an aquifer of As. Shallow aquifer water with > 5 years H-3/He-3 age show a constant As:P molar ratio of 9.6 over time, suggesting common mechanisms of mobilization. (c) 2008 Elsevier B.V. All rights reserved.