Abstract Water quality globally suffers from overuse and pollution, but desert areas with high evaporation rates and groundwater as the only water source have additional challenges. Namakwaland in the Northern Cape is an arid region with a sparse human population dependent largely upon stock farming, with minor mining and tourism. The complex Proterozoic metamorphic geology is overlain by Cenozoic deposits known for containing secondary uranium mineralisation in places. 86 samples of groundwater were taken over an area of 25 000 km2 and analysed for various parameters in the field and laboratory. The salinity of the water varies from fresh to sea water, averaging 2500 mg/L, with pH in the 7–9 range. Major ion abundances indicate marine aerosol as the main source of Cl, Na, Mg and K, with rock weathering being more responsible for Ca and HCO3. Irrigation water quality tests give mixed results, suggesting careful use of groundwater is possible in some locations. Nitrate is occasionally high, the random distribution indicating farm animals as the source. Fluoride averages 2.4 mg/L and is strongly geologically controlled, but also enriched through evaporation. Uranium, averaging 0.155 mg/L (5 times the guideline), has a complex distribution, poorly correlated to bedrock geology and shows two enrichment trends, one in tandem with other ions and one independent. These two trends are proposed to reflect enrichment through evaporation (other ions also increasing) or precipitation of secondary uranium minerals (limited correlation with other ions). These three parameters are uncorrelated, which emphasizes the variety and complexity of hydrochemical processes taking place. Given the U and other water quality risks, further work into cumulative exposure for plants, animals and humans is warranted.

中文翻译：

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南非 Namakwaland 高铀、氟化物和硝酸盐地下水的水化学演化

摘要 全球水质受到过度使用和污染的影响，但蒸发率高且地下水作为唯一水源的沙漠地区面临着额外的挑战。北开普省的 Namakwaland 是一个干旱地区，人口稀少，主要依赖畜牧业，很少有采矿业和旅游业。复杂的元古代变质地质被新生代矿床所覆盖，这些矿床以在某些地方含有次生铀矿化而闻名。在 25 000 平方公里的区域内采集了 86 个地下水样本，并在现场和实验室分析了各种参数。水的盐度从淡水到海水不等，平均为 2500 毫克/升，pH 值在 7-9 范围内。主要离子丰度表明海洋气溶胶是 Cl、Na、Mg 和 K 的主要来源，而岩石风化对 Ca 和 HCO3 负有更大的责任。灌溉水质测试给出了不同的结果，表明在某些地方可以谨慎使用地下水。硝酸盐偶尔会很高，随机分布表明农场动物是来源。氟化物平均为 2.4 毫克/升，受到强烈的地质控制，但也通过蒸发而富集。铀的平均浓度为 0.155 mg/L（标准值的 5 倍），分布复杂，与基岩地质相关性较差，并显示出两种富集趋势，一种与其他离子串联，另一种独立。提出这两种趋势是为了反映通过蒸发（其他离子也增加）或次生铀矿物沉淀（与其他离子的相关性有限）而引起的富集。这三个参数是不相关的，这强调了发生的水化学过程的多样性和复杂性。