The eutrophic Lake Eichbaumsee, a ~1 km long and 280 m wide (maximum water depth 16 m) dredging lake southeast of Hamburg (Germany), has been treated for water quality improvements using various techniques (i.e., aeration plants, removal of dissolved phosphorous by aluminum phosphorous precipitation, and by Bentophos^® (Phoslock Environmental Technologies, Sydney, Australia), adsorption) during the past ~15 years. Despite these treatments, no long-term improvement of the water quality has been observed and the lake water phosphorous content has continued to increase by e.g., ~670 kg phosphorous between autumn 2014 and autumn 2019. As no creeks or rivers drain into the lake and hydrological groundwater models do not suggest any major groundwater discharge into the lake, sources of phosphorous (and other nutrients) are unknown. We investigated the phosphorous fluxes from sediment pore water and from groundwater in the water body of the lake. Sediment pore water was extracted from sediment cores recovered by divers in August 2018 and February 2019. Diffusive phosphorous fluxes from pore water were calculated based on phosphorus gradients. Stable water isotopes (δ²H, δ¹⁸O) were measured in the lake water, in interstitial waters in the banks surrounding the lake, in the Elbe River, and in three groundwater wells close to the lake. Stable isotope (δ²H, δ¹⁸O) water mass balance models were used to compute water inflow/outflow to/from the lake. Our results revealed pore-water borne phosphorous fluxes between 0.2 mg/m²/d and 1.9 mg/m²/d. Assuming that the measured phosphorous fluxes are temporarily and spatially representative for the whole lake, about 11 kg/a to 110 kg/a of phosphorous is released from sediments. This amount is lower than the observed lake water phosphorous increase of ~344 kg between April 2018 and November 2018. Water stable isotope (δ²H, δ¹⁸O) compositions indicate a water exchange between an aquifer and the lake water. Based on stable isotope mass balances we estimated an inflow of phosphorous from the aquifer to the lake of between ~150 kg/a and ~390 kg/a. This result suggests that groundwater-borne phosphorous is a significant phosphorous source for the Eichbaumsee and highlights the importance of groundwater for lake water phosphorous balances.

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富营养化人工湖的磷供应：沉积与地下水源

富营养化的艾希鲍姆湖（Eichbaumsee）长约1公里，宽280 m（最大水深16 m），位于汉堡（德国）东南部的疏lake湖中，已经通过各种技术（例如曝气设备，去除溶解的磷）进行了水质改善处理。由铝磷沉淀，然后通过Bentophos ^®（Phoslock Environmental Technologies，澳大利亚悉尼），过去15年间的吸附。尽管采取了这些措施，但并未观察到水质的长期改善，并且在2014年秋季至2019年秋季之间，湖水的磷含量持续增加，例如，增加了约670公斤磷。水文地下水模型没有表明有任何主要的地下水排入湖泊，磷（和其他养分）的来源尚不清楚。我们研究了沉积物孔隙水和湖泊水体地下水中的磷通量。沉积物孔隙水是从2018年8月和2019年2月潜水员回收的沉积物芯中提取的。孔隙水的扩散磷通量是根据磷梯度计算的。稳定水同位素（δ² H，δ ¹⁸ O）在湖水进行测定，在湖周围银行间隙水，在易北河，并在三口地下水井靠近湖泊。稳定同位素（δ ² H，δ ¹⁸ O）水质量平衡模型来计算水流入/流出到/从湖。我们的结果表明，孔隙水中的磷通量在0.2 mg / m ² / d和1.9 mg / m ² / d之间。假设所测得的磷通量在整个湖泊中具有暂时性和空间代表性，则从沉积物中释放出约11 kg / a至110 kg / a的磷。该量低于观察到的2018年4月至2018年11月之间湖水磷的增加量〜344 kg。水稳定同位素（δ² H，δ ¹⁸ O）的组合物表明含水层和湖泊水之间的水交换。基于稳定的同位素质量平衡，我们估计磷从含水层流向湖泊的流量约为150 kg / a至390 kg / a。该结果表明，地下水中的磷是艾希鲍姆湖的重要磷源，并突出了地下水对湖水磷平衡的重要性。