BACKGROUND High salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012-2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy. RESULTS Drill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1-3 mm/year. Most tubewells are screened at depths of 15-52 m in sediments deposited 8000-9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na-Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11-57 mg/L and As 2-258 ug/L, and low sulfur (S) 2-33 mg/L. CONCLUSIONS Groundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured (14)C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.

中文翻译：

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孟加拉国西南部地下水的盐分和砷来源。

背景技术在孟加拉国西南部的潮汐三角洲地区，地下水中的高盐度和砷（As）浓度是普遍存在的问题。为了确定溶解盐和砷的来源，在2012-2013年的干（五月）和湿（十月）季节从管井中收集了来自区域全新世浅层含水层的地下水样本。记录了13个钻芯，并在木材碎片上测量了27个放射性碳年龄，以表征地下地层特征。结果钻屑，矿坑裸露和区域研究表明，厚度大于5 m的地表泥浆盖覆盖着约30 m厚的夹层泥浆和细砂层上部单元，下部较粗的单元高达60 m厚，以干净的沙子为主，所有这些在床的连续性和厚度上都有明显的水平变化。整个全新世早期，这个较厚的下部单元以每年约2厘米/年的速度增生，局部下陷或压实率为1-3毫米/年。大多数管井在沉积的8000-9000 YBP的沉积物中筛选出15-52 m的深度。管井地下水样品的成分显示出高的空间变异性，表明混合受限，补给速率和流速低且在空间上可变。地下水为Na-Cl型，主要还原硫酸盐，比电导率（SpC）为3至29 mS / cm，高溶解有机碳（DOC）为11-57 mg / L，As 2-258 ug / L，低硫（S）2-33 mg / L。结论埋入潮汐通道水并随后与溶解的有机物发生反应，导致缺氧，含水三氧化二铁（HFO）还原，As迁移，硫酸盐（SO4）的还原和去除在浅层含水层中。在雨季引入不稳定的有机碳作为稻田肥料也可能导致HFO的减少和As的迁移。可变的现代补给发生在粘土盖被挤压或被潮汐通道破坏的区域，这可以解释先前测得的（14）C地下水年龄小于沉积年龄。从浅层含水层收集的样品中，孟加拉国政府的准则中砷含量超过46％，盐分含量超过100％。这可以解释先前测得的（14）C地下水年龄小于沉积年龄。从浅层含水层收集的样品中，孟加拉国政府的准则中砷含量超过46％，盐分含量超过100％。这可以解释先前测得的（14）C地下水年龄小于沉积年龄。从浅层含水层收集的样品中，孟加拉国政府的准则中砷含量超过46％，盐分含量超过100％。