The stable isotope ratios of groundwater sulfate (³⁴S/³²S, ¹⁸O/¹⁶O) are often used as tracers to help determine the origin of groundwater or groundwater contaminants. In agricultural watersheds, little is known about how the increased use of sulfur as a soil amendment to optimize crop production is affecting the isotopic composition of groundwater sulfate, especially in shallow aquifers. We investigated the isotopic composition of synthetic agricultural fertilizers and groundwater sulfate in an area of intensive agricultural activity, in Ontario, Canada. Groundwater samples from an unconfined surficial sand aquifer (Lake Algonquin Sand Aquifer) were analyzed from multi-level monitoring wells, riverbank seeps, and private domestic wells. Fertilizers used in the area were analyzed for sulfur/sulfate content and stable isotopic composition (δ¹⁸O and/or δ³⁴S). Fertilizers were isotopically distinct from geological sources of groundwater sulfate in the watershed and groundwater sulfate exhibited a wide range of δ³⁴S (−6.9 to +20.0‰) and δ¹⁸O (−5.0 to +13.7‰) values. Quantitative apportionment of sulfate sources based on stable isotope data alone was not possible, largely because two of the potential fertilizer sulfate sources had an isotopic composition on the mixing line between two natural geological sources of sulfate in the aquifer. This study demonstrates that, when sulfate isotope analysis is being used as a tracer or co-tracer of the origin of groundwater or of contaminants in groundwater, sulfate derived from synthetic fertilizer needs to be considered as a potential source, especially when other parameters such as nitrate independently indicate fertilizer impacts to groundwater quality.

中文翻译：

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受农业肥料影响的浅层含水层中硫酸盐的同位素表征

地下水硫酸盐的稳定同位素比( ³⁴ S/ ³² S, ¹⁸ O/ ¹⁶O) 通常用作示踪剂，以帮助确定地下水或地下水污染物的来源。在农业流域，人们对硫作为土壤改良剂以优化作物生产的增加使用如何影响地下水硫酸盐的同位素组成知之甚少，尤其是在浅层含水层中。我们在加拿大安大略省的一个农业活动密集的地区调查了合成农业肥料和地下水硫酸盐的同位素组成。来自无承压表层砂含水层（阿冈昆湖砂含水层）的地下水样本从多级监测井、河岸渗漏和私人家用井中进行分析。对该地区使用的肥料进行了硫/硫酸盐含量和稳定同位素组成（δ ¹⁸ O 和/或 δ ³⁴S)。化肥与流域地下水硫酸盐的地质来源在同位素上不同，地下水硫酸盐显示出大范围的 δ ³⁴ S (-6.9 to +20.0‰) 和 δ ¹⁸O (-5.0 到 +13.7‰) 值。仅基于稳定同位素数据对硫酸盐源进行定量分配是不可能的，主要是因为两个潜在的肥料硫酸盐源在含水层中两个天然硫酸盐地质源之间的混合线上具有同位素组成。这项研究表明，当硫酸盐同位素分析被用作地下水来源或地下水中污染物的示踪剂或辅助示踪剂时，需要考虑将来自合成肥料的硫酸盐作为潜在来源，尤其是当其他参数如硝酸盐独立地表明肥料对地下水质量的影响。