Climate and Seasonal Temperature Controls on Biogeochemical Transformations in Unconfined Coastal Aquifers
Climate and Seasonal Temperature Controls on Biogeochemical Transformations in Unconfined Coastal Aquifers
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DOI：
10.1029/2021jg006605
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发表时间：
2021-11
期刊：
影响因子：
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通讯作者：
C. Cogswell;James W. Heiss
中科院分区：
文献类型：
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作者：
C. Cogswell;James W. Heiss
沿海含水层发生了一系列生物地球化学反应，这些反应改变了沿海生态系统中地下水源养分、金属和其他化学负荷。温度是对微生物介导的反应的有力控制；因此，沿海含水层的化学反应性可能随着地下水温度的变化而在空间和时间上发生变化。在这项研究中，我们利用变密度地下水流、热传输和反应传输模型研究了全球地下水和海面温度控制以及季节性温度变化对沿海含水层生物地球化学过程的影响。耦合模型表明，随着地下水温度从 5°C 升高到 35°C，沿海含水层的硝酸盐去除效率从 5% 提高到 88%，而海洋温度对去除效率的影响可以忽略不计。基于每月地下水和海洋温度测量的瞬态模拟表明，反硝化和氨化热点在温暖的淡水地下水团中季节性向海迁移。反应热点被冬季注入的较冷地下水隔开。由于温暖和寒冷地下水之间的浮力效应，反应热点和硝酸盐羽流沿着水平流动路径垂直振荡。瞬态模型和温度等效稳态模型之间的比较表明，稳态模型充分捕获了年平均 NO3− 去除量，但忽略了局部反应瞬态和羽流几何形状的变化。考虑到全球不同的热状况，敏感性分析提供了沿海含水层反应潜力的一阶估计。这些发现对于区域尺度地下水养分通量的估计和预测气候变暖下沿海含水层的反应性具有重要意义。
Coastal aquifers are host to a range of biogeochemical reactions that alter groundwater‐derived nutrient, metal, and other chemical loads to coastal ecosystems. Temperature is a strong control on microbially mediated reactions; thus, chemical reactivity in coastal aquifers may vary spatially and temporally with changes to groundwater temperature. In this study, we investigated the influence of global groundwater and sea surface temperature controls and seasonal temperature variability on biogeochemical processing in coastal aquifers using variable‐density groundwater flow, heat transport, and reactive transport models. The coupled models showed that nitrate removal efficiency in coastal aquifers increased from 5% to 88% as fresh groundwater temperature increased from 5°C to 35°C, while ocean temperature had a negligible effect on removal efficiency. Transient simulations based on monthly groundwater and ocean temperature measurements showed that denitrification and ammonification hotspots migrated seaward seasonally within warm fresh groundwater masses. The reaction hotspots were separated by colder groundwater emplaced during winter months. The reaction hotspots and nitrate plumes oscillated vertically along horizontal flow paths due to buoyancy effects between warm and cold groundwater. Comparison between transient and temperature‐equivalent steady‐state models suggests that steady‐state models adequately capture mean annual NO3− removal, but neglect local reactive transience and changes to plume geometry. The sensitivity analysis provides a first‐order estimate of the reactive potential of coastal aquifers considering globally diverse thermal regimes. The findings have implications for regional‐scale estimates of groundwater nutrient fluxes and for predicting coastal aquifer reactivity in a warming climate.