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Submarine groundwater discharge to Tampa Bay: Nutrient fluxes and biogeochemistry of the coastal aquifer

机译：坦帕湾海底地下水排放：沿海含水层的营养通量和生物地球化学

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To separately quantify the roles of fresh and saline submarine groundwater discharge (SGD), relative to that of rivers, in transporting nutrients to Tampa Bay, Florida, we used three approaches (Darcy's Law calculations, a watershed water budget, and a ~(222)Rn mass-balance) to estimate rate of SGD from the Pinellas peninsula. Groundwater samples were collected in 69 locations in the coastal aquifer to examine biogeochemical conditions, nutrient concentrations and stoichiometry, and salinity structure. Salinity structure was also examined using stationary electrical resistivity measurements. The coastal aquifer along the Pinellas peninsula was chemically reducing in all locations sampled, and that condition influences nitrogen (N) form and mobility of N and PO_4~(3-). Concentrations of NH_4~+, PO_4~(3-) and ratio of dissolved inorganic N (DIN) to PO_4~(3-) were all related to measured oxidation/reduction potential (ps) of the groundwater. Ratio of DIN: PO_4~(3-) was below Redfield ratio in both fresh and saline groundwater. Nitrogen occurred almost exclusively in reduced forms, NH_4~+ and dissolved organic nitrogen (DON), suggesting that anthropogenic N is exported from the watershed in those forms. In comparison to other SGD studies, rate of PO_4~(3-) flux in the seepage zone (μM m~(-2) d~(-1)) in Tampa Bay was higher than previous estimates, likely due to 1) high watershed population density, 2) chemically reducing conditions, and 3) high ion concentrations in fresh groundwater. Estimates of freshwater groundwater flux indicate that the ratio of groundwater discharge to stream flow is ～20 to 50%, and that the magnitudes of both the total dissolved nitrogen and PO_4~(3-) loads due to fresh SGD are ～40 to 100% of loads carried by streams. Estimates of SGD based on radon inventories in near-shore waters were 2 to 5 times greater than the estimates of freshwater groundwater discharge, suggesting that brackish and saline SGD is also an important process in Tampa Bay and results in flux of regenerated N and P from sediment to surface water.
机译：为了分别定量淡水和咸水海底地下水排放（SGD）相对于河流的作用，在将营养物输送到佛罗里达州坦帕湾的过程中，我们使用了三种方法（达西定律计算，分水岭水预算和〜（222） （Rn质量平衡）估算Pinellas半岛的SGD率。在沿海含水层的69个位置收集了地下水样本，以检查生物地球化学条件，养分浓度和化学计量以及盐分结构。盐度结构还使用固定电阻率测量进行了检查。沿Pinellas半岛的沿海含水层在所有采样位置均化学还原，该条件影响氮（N）的形态以及N和PO_4〜（3-）的迁移率。 NH_4〜+，PO_4〜（3-）的浓度以及溶解态无机氮（DIN）与PO_4〜（3-）的比例均与地下水的氧化还原电位（ps）有关。淡水和含盐地下水中DIN：PO_4〜（3-）的比率均低于Redfield比率。氮几乎全部以还原态NH_4〜+和溶解性有机氮（DON）的形式存在，这表明人为氮以这些形式从流域输出。与其他SGD研究相比，坦帕湾渗流区PO_4〜（3-）的通量率（μMm〜（-2）d〜（-1））高于先前的估计，可能是由于1）高流域人口密度； 2）化学还原条件； 3）新鲜地下水中的高离子浓度。淡水地下水通量的估算表明，地下水排放与水流的比例为〜20％至50％，新鲜SGD引起的总溶解氮和PO_4〜（3-）负荷的幅度均为〜40％至100％流所承载的负载。基于近岸ra气清单的SGD估算值比淡水地下水排放量估算值高2到5倍，这表明淡咸盐水SGD也是坦帕湾的重要过程，并导致N和P的再生沉积到地表水。

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