Utility of radium isotopes for evaluating the input and transport of groundwater-derived nitrogen to a Cape Cod estuary
Utility of radium isotopes for evaluating the input and transport of groundwater-derived nitrogen to a Cape Cod estuary
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DOI：
10.4319/lo.2001.46.2.0465
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发表时间：
2001-03-01
影响因子：
4.5
通讯作者：
Andrews, JE
中科院分区：
文献类型：
--
作者：
Charette, MA;Buesseler, KO;Andrews, JE
由于人口的快速增长，人为来源的氮对好望角沿海池塘的水质产生了不利影响。这些河口的“新”氮的一个主要来源是地下水，地下水在流向海岸线的路径上拦截了化粪池。人们曾多次尝试对这一过程进行量化;然而，地下水的排放往往是不均匀的，因此很难使用传统技术（如渗透仪）进行研究。在马萨诸塞州的Waquoit湾，我们测试了一种基于镭的方法，镭在含水层流体中天然富集，有四种同位素，半衰期从4天到1600年不等。进入海湾的地下水盐度低，含有几个数量级更大的镭和溶解的无机氮（DIN）相对于周围的海湾水。使用镭的质量平衡方法，我们计算出的海底地下水通量类似于37，000 m（3）d（-1），与根据降雨计算的含水层补给率相当。根据地下水DIN含量估算，河口直接输入的DIN约为2100 molNd（-1）。然而，这个氮通量与文献值相比，DIN通量从人口稠密的subestuaries小。此外，我们的研究结果表明，地下水通量的DIN被同化的植物生物量在夏季，但可能会出口到沿海沃茨水域在冬季。
Because of rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape God. A major source of "new" nitrogen to these estuaries is groundwater, which intercepts septic tank fields in its flow path to the coastline. Many attempts have been made to quantify this process; however, groundwater discharge is often patchy in nature and is therefore difficult to study by use of traditional techniques such as seepage meters. In Waquoit Bay, MA, we tested an approach based on radium, which is naturally enriched in aquifer fluids and has four isotopes with half-lives ranging from 4 d to 1600 yr. Groundwater entering the bay was low in salinity and contained several orders of magnitude greater radium and dissolved inorganic nitrogen (DIN) relative to ambient bay water. Using a mass-balance approach for radium, we calculated a submarine groundwater flux of similar to 37,000 m(3) d(-1), which compared well with aquifer recharge rates calculated from rainfall. From the DIN content of the groundwater, we estimated that similar to 2100 mol N d(-1) was directly input to the estuary. However, this nitrogen flux was small in comparison with literature values for DIN fluxes from the heavily populated subestuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from the embayment to coastal waters during the winter months.