AQUIFER PHYSICAL, CHEMICAL AND MICROBIOLOGICAL HETEROGENEITY REVEALED BY HIGH SPATIAL RESOLUTION CHARACTERIZATION

Heterogeneity of physical and chemical properties of sedimentary aquifers is common in fluvial-deltaic plains across various spatial scales. At present, it remains difficult to estimate any water quality parameter with a degree of certainty relevant for management purpose. This problem is especially relevant for human health protection because highly toxic substance such as geogenic arsenic remains impossible to estimate or predict for individual wells used for drinking. To tackle this seemly intractable problem, this thesis takes a critical and previously rarely attempted step to simultaneously characterize groundwater and sediment samples at high spatial resolution. The study area is located in the northern Yinchuan Plain where elevated groundwater arsenic (As) is frequently detected in the shallow Holocene aquifer and is mobilized under iron-reducing conditions thus typical of most high-As groundwater aquifer in most regions of the world. In May 2021, 12 sediment cores (12 - 20 m) and 70 groundwater samples were obtained through AMS-Power-Probe drill machine and retractable groundwater sampler along a transect of 2.5 km with a depth of 20 m between two locations YCA (38°49'48.20'' N, 106°21'14.04'' E) and YCB (38°50'41.53'' N, 116°22'24.57''E). Groundwater samples were analyzed for 10 and 12 chemical parameters in the field and in the laboratory respectively. Sediment samples were analyzed for grain size, microbial communities and 19 chemical parameters. The key findings are:
First, the shallow phreatic aquifer in our study area is composed of multiple layers of alluvial and lacustrine deposits, with a fine to medium sandy layer in the middle and two silty-clay layers of 1.5 - 5 m above and below. We used an end-member mixing model based on δ18O and δD to trace and quantify the groundwater sources. Groundwater in YCA was mainly recharged by Sand Lake water (82.7 ± 12.1 %). Groundwater in YCB was primarily recharge by irrigation water pumped from deeper groundwater (69.7 ± 9.8 %). Irrigation water diverted from the Yellow River accounted for the majority of water source in area between YCA and YCB (67.2 ± 26.8 %). Mass balance based on water source quantification revealed that salinity leaching, and nitrogen pockets resulting from agricultural fertilization, could be significant pathways for the input of key electron acceptors (SO42- and NO3-). Besides, quantifying the groundwater source helps to trace the supply pathways for electron donor DOC, compensating for the invisibility of groundwater systems.
Second, we then provided sufficient evidence for the consistency between groundwater and sedimentary As distribution. High consumption degree of DOC with massive As release from sediment was observed in groundwater recharged by surface water with high bioavailable DOC. Contrary, little DOC consumption and weak As release from sediment occurred in groundwater mainly recharged by deeper groundwater with less bio-available DOC. This suggested the microbial mediation on regulating As release. Groundwater As showed a high spatial correlation with sedimentary As at the depth of > 7.5 m, where sedimentary HCl-extractable Fe(II)/Fe and phosphate-extractable As(III)/As implied reducing environment. This spatial cor-relation is particularly prominent in high HCO3- environment (R = 0.59~0.69，P < 0.05), indicating HCO3- (derived from microbial metabolism and carbonate minerals weathering) is a key driver for As release. With our unique evidences, it is finally clear that groundwater As heterogeneity is originated from sedimentary As heterogeneity, and further exacerbated by redox gradients.
Last, to further clarify the microbial mediation on the spatial heterogeneity of As release, microbial communities abundances in sediments was obtained by 16s rRNA sequencing. Abundance of arsenic reducing genera was positively correlated with groundwater DOC and NH4+ concentration, while abundance of sulfate reducing genera was negatively correlated with groundwater salinity. Enrichment of As reducing genera and deficiency of sulfate reducing genera were found within the groundwater As accumulation zone, recharged primarily by saline surface water. Such environment can not provide suitable growing conditions for sulfate reducing genera to generate sulfide for As interception, because salinity stress is adverse to the activity of sulfate reducing genera. Therefore, sedimentary As release into groundwater may be significantly stimulated by the enrichment of As reducing genera and lack of sulfide supply.

在不同空间尺度上，河流三角洲平原含水层的物理化学性质普遍具有空间非均质性。诸如地源性砷等剧毒物质的空间分布规律目前仍然无法预测，与管理目的相关的水质参数难以准确评估，这一问题攸关人类健康保护，亟待解决。为此，本研究选取典型高砷区域，采用高空间分辨率手段对地下水和沉积物样品同时进行表征。研究区位于银川平原北部，其全新世浅层含水层中已被检出超标地下水砷，在铁还原条件下有释放迁移趋势，具有全球高砷地下水含水层的典型特征。本研究在银川平原YCA（38°49'48.20''N, 106°21'14.04''E）和YCB（38°50'41.53'' N, 116°22'24.57'' E）两点间，利用AMS液压式直推钻机和伸缩式地下水采样器，分别获取12组连续原状沉积物岩芯和70个地下水样品，在野外和实验室条件下，对水样进行化学参数分析，对沉积物样品进行粒度、化学参数和微生物群落分析。主要发现如下:

首先，研究区潜水含水层由多层冲积湖相沉积物组成，中间为细砂至中砂层，上下为1.5~5米的粉质黏土层。采用基于δ¹⁸O和δD的端元混合模型对地下水水源进行了追踪和量化，结果表明：YCA处以沙湖水补给为主（82.7%±12.1%）；YCB处主要受来自深层地下水的灌溉补给（69.7±9.8%）；YCA和YCB两点间的地下水主要受引黄灌溉水补给（67.2±26.8%）。基于水源量化的质量衡算表明，灌溉盐碱淋溶和农业施肥产生的氮富集区可能是关键电子受体（SO₄^2-和NO₃^-）输入的重要途径。此外，量化地下水来源有助于追踪电子供体DOC的供应途径，弥补地下水系统的不可见性。

其次，本文为水相砷与固相砷空间分布的相似性提供了充分证据。在地表水（DOC生物可利用度高）补给区域中，DOC消耗程度高，并伴有沉积物砷的大量释放。与之相反，在深层地下水（DOC生物可利用度低）灌溉补给区域中，DOC消耗较少，沉积物砷释放程度较弱，指示了微生物在砷释放中的调节作用。在还原条件下（深度>7.5米），地下水砷与沉积物砷（盐酸可提取部分）表现出较高的空间相关性，且在高浓度HCO₃^-的环境中尤为突出（R = 0.59~0.69，P < 0.05），表明HCO₃^-是砷释放的关键驱动因素。因此，地下水砷空间非均质性一定程度上取决于沉积物砷的空间非均质性，并受氧化还原条件的影响进一步加剧。

最后，通过16s rRNA测序获得沉积物中微生物群落丰度，进一步阐明了微生物对砷释放的调节作用。砷还原属丰度与地下水DOC、NH₄⁺浓度呈正相关，硫酸盐还原属丰度与地下水盐度呈负相关。在地下水砷富集区中发现砷还原属的大量富集和硫酸盐还原属的缺失。砷富集区的地下水主要由盐度高的沙湖水补给，该环境不能为硫酸盐还原属提供适宜的生长条件（盐度胁迫），从而难以生成硫化物拦截水相砷。因此，本文认为砷还原属的富集和硫化物供给的缺失可能是促进沉积物砷释放的重要因素。

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