Trace element geochemistry of groundwater in a karst subterranean estuary (Yucatan Peninsula, Mexico)
Trace element geochemistry of groundwater in a karst subterranean estuary (Yucatan Peninsula, Mexico)
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DOI：
10.1016/j.gca.2014.01.037
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发表时间：
2014-05-01
影响因子：
5
通讯作者：
Morales-Ojeda, Sara M.
中科院分区：
文献类型：
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作者：
Gonneea, Meagan Eagle;Charette, Matthew A.;Morales-Ojeda, Sara M.
地下河口内的微量元素循环经常改变地下水的化学特征，并可能最终控制与海底地下水排放相关的沿海海洋的总化学负荷。在全球范围内，喀斯特地貌占所有海岸线的12%以上。这些地区的地下河口渗透性很强，导致降水迅速渗透，地下水迅速输送到海岸，主要的碳酸盐矿物很容易溶解。我们研究了尤卡坦半岛碳酸盐岩岩溶地下河口内钡（Ba）、锶（Sr）、锰（Mn）、铀（U）、钙（Ca）和镭（Ra）的化学循环，其特征在于陆地地下水透镜覆盖海洋地下水入侵，并通过海岸泉活跃地排出海底。陆地地下水钙（1-5 mmol kg（-1））和碱度（3-8 mmol kg（-1））的富集程度超过了补给降水与方解石之间的平衡预测值，这可归因于地下水有机质的呼吸作用以及随后方解石、白云石和石膏的溶解作用。结果表明，地下水Sr/Ca、Mn/Ca、Ba/Ca和Ra/Ca的实测值与方解石平衡溶解预测值吻合较好，说明地下水微量元素含量主要受矿物溶解作用的影响。在活跃的排泄泉中，陆地地下水和海洋之间的后续混合以Sr、Mn、Ba和Ca的保守混合为特征，而U是可变的，Ra显示出大的富集（盐度：1.9-34.9，Ba：60-300 nmol kg（-1），Sr：15-110 μ mol kg（-1），U：0.3-35 nmol kg（-1），Mn：0.3-200 nmol kg（-1），Ca：4.3-12.9 mmol kg（-1），226 Ra：18-2140 dpm 100 L-1）。通过天然井取样的深层地下水，局部溶解特征，典型的是Ba、Sr、Ca、Mn和Ra升高，而海洋地下水中不存在U，这是由于有机质降解和氧化还原过程（包括硫酸盐还原）后含水层基质的溶解增强所致（盐度：0.2-36.6，Ba：7-1630 nmol kg（-1），Sr：1.3-210 μ mol kg（-1），U：0.3-18 nmol kg（-1），Mn：0.6-2600 nmol kg（-1），Ca：2.1 15.2 mmol kg（-1），Ra-226 20-5120 dpm 100 L-1）。然而，没有证据表明，在这个反应带的深层海洋地下水通过春季排放与沿海海洋交换的春季地球化学。根据镭示踪剂计算的海底地下水总排放率为40-95 m（3）m（-1）d（-1），陆地排放占总量的75 +/- 25%。全球岩溶地下河口化学负荷估算表明，Sr和U通量分别占海洋输入总量的15-28%和7.33%（8.2- 15.3moly（-1）和4.0- 7.7moly（-1））。来自岩溶地下河口的~（226）Ra输入量是河流输入量的34-50倍（6.7-9.9 × 10 ~（16）dpm y ~（-1））。(C)2014爱思唯尔有限公司版权所有。
Trace element cycling within subterranean estuaries frequently alters the chemical signature of groundwater and may ultimately control the total chemical load to the coastal ocean associated with submarine groundwater discharge. Globally, karst landscapes occur over 12% of all coastlines. Subterranean estuaries in these regions are highly permeable, resulting in rapid infiltration of precipitation and transport of groundwater to the coast, and the predominant carbonate minerals are readily soluble. We studied the chemical cycling of barium (Ba), strontium (Sr), manganese (Mn), uranium (U), calcium (Ca) and radium (Ra) within the carbonate karst subterranean estuary of the Yucatan Peninsula, which is characterized by a terrestrial groundwater lens overlying marine groundwater intrusion with active submarine discharge through coastal springs. Terrestrial groundwater calcium (1-5 mmol kg(-1)) and alkalinity (3-8 mmol kg(-1)) are enriched over that predicted by equilibrium between recharging precipitation and calcite, which can be accounted for by groundwater organic matter respiration and subsequent dissolution of calcite, dolomite and gypsum. There is a close agreement between the observed terrestrial groundwater Sr/Ca, Mn/Ca, Ba/Ca and Ra/Ca and that predicted by equilibrium dissolution of calcite, thus the trace element content of terrestrial groundwater is largely determined by mineral dissolution. Subsequent mixing between terrestrial groundwater and the ocean within the actively discharging springs is characterized by conservative mixing of Sr, Mn, Ba and Ca, while U is variable and Ra displays a large enrichment (salinity: 1.9-34.9, Ba: 60-300 nmol kg(-1), Sr: 15-110 mu mol kg(-1), U: 0.3-35 nmol kg(-1), Mn: 0.3-200 nmol kg(-1), Ca: 4.3-12.9 mmol kg(-1), 226 Ra: 18-2140 dpm 100 L-1). The deep groundwater sampled through cenotes, local dissolution features, is typified by elevated Ba, Sr, Ca, Mn and Ra and the absence of U within marine groundwater, due to enhanced dissolution of the aquifer matrix following organic matter degradation and redox processes including sulfate reduction (salinity: 0.2-36.6, Ba: 7-1630 nmol kg(-1), Sr: 1.3-210 mu mol kg(-1), U: 0.3-18 nmol kg(-1), Mn: 0.6-2600 nmol kg(-1), Ca: 2.1 15.2 mmol kg(-1), Ra-226 20-5120 dpm 100 L-1). However, there is no evidence in the spring geochemistry that deep marine groundwater within this reaction zone exchanges with the coastal ocean via spring discharge. Total submarine groundwater discharge rates calculated from radium tracers are 40-95 m(3) m(-1) d(-1), with terrestrial discharge contributing 75 +/- 25% of the total. Global estimates of chemical loading from karst subterranean estuaries suggest Sr and U fluxes are potentially 15-28% and 7 33% of total ocean inputs (8.2-15.3 mol y(-1) and 4.0-7.7 mol y(-1)), respectively. Radium-226 inputs from karst subterranean estuaries are 34-50 times river inputs (6.7-9.9 x 10(16) dpm y(-1)). (C) 2014 Elsevier Ltd. All rights reserved.