Groundwater-lake interactions on water quality in the nearshore zone of the Great Lakes: A regional scale modeling approach

加拿大拥有五大湖大约一半的海岸线。近岸地区尤其容易出现水质恶化和环境问题，例如由粪便污染造成的藻华和海滩封闭。保持五大湖的健康给加拿大带来了巨大的经济利益：据估计，每年仅与娱乐有关的活动的经济价值就超过20亿加元。虽然地下水排放对河流流量的重要性在五大湖周围的流域得到了充分认识，但直接向湖泊输入地下水的限制仍然很差，据估计，在某些地区，地下水排放占河流流量的百分之几到50%以上。地下水排放进一步将营养物质和污染物从陆地输送到近岸地带，可能造成严重的生态影响。确定地下水排放作用的主要挑战是：(1)准确估计向湖的流量和地下水的化学成分，(2)解释近岸地表水和湖水混合带发生的生物地球化学转变，以及(3)发展预测土地利用变化、水管理、气候变化和湖泊水位变化如何调节地下水排放和相关物质通量的能力。虽然对沿海海洋环境中海底地下水排放的研究正在迅速增加，但对大型内陆水体的研究却少得多。由于海岸线的长度和异质性，以及大湖沿岸大部分未测量的盆地，区域地下水输入的估计需要创新的数据同化和建模方法。拟议的探索基金研究将创建和使用地下水流动和反应模型框架，以填补目前的知识空白。该模型将应用于五大湖的加拿大一侧，并将重点放在大量营养物质（氮、磷和硅）从陆地向湖泊的转移上，这可以说是淡水水体面临的主要人为压力之一。区域地下水流模型计算将与营养元素的生物地球化学反应模块相结合。将特别注意地下水和湖水交汇的近岸地下反应区，以及对人类影响情景的分析（施肥、沿海开发、抽取地下水、湖泊水位变化）。模拟结果将为评估地下水排放对五大湖水质和生态系统健康的影响奠定基础，从而为监测和水管理及治理制定准则。

Canada has about half of the Great Lakes’ shoreline. Near-shore areas are especially prone to deterioration of water quality and environmental problems, such as algal blooms and beach closures caused by fecal contamination. Maintaining the health of the Great Lakes brings large economic benefits to Canada: the economic value of recreation-related activities alone has been estimated at more than $2 billion per year. While the importance of groundwater discharge for river flow is well recognized for watersheds around the Great Lakes, direct input of groundwater to the lakes remains poorly constrained, with estimates ranging from a few percent to over 50% of river discharge in some areas. Groundwater discharge further transports nutrients and pollutants from land to the near-shore zone, with potentially severe ecological impacts. Key challenges in determining the role of groundwater discharge are (1) to accurately estimate the lake-ward flow and chemical composition of the groundwater, (2) to account for the biogeochemical transformations taking place in the near-shore mixing zone between ground- and lake-water, and (3) to develop the capability to forecast how land-use changes, water management, climate change and lake water level changes modulate the discharge of groundwater and the associated material fluxes. While the number of studies on submarine groundwater discharge in coastal marine environments is rapidly increasing, much less research is being carried out for large inland water bodies. Because of great length and heterogeneity of the shoreline, and the large fraction of ungauged basins bordering the Great Lakes, estimations of regional groundwater inputs require innovative data assimilation and modeling approaches. The proposed Discovery Grant research will create and use a groundwater flow and reaction model framework to fill the current knowledge gap. The model will be applied to the Canadian side of the Great Lakes, and focus on the land to lake transfers of macronutrients (N, P and Si), which are arguably one of the major anthropogenic pressures on freshwater bodies. Regional groundwater flow model calculations will be coupled to biogeochemical reaction modules for the nutrient elements. Particular attention will be given to the near-shore subsurface reaction zone where groundwater and lake water meet, and the analysis of human-impact scenarios (fertilizer application, coastal development, groundwater extraction, lake level changes). The modeling results will set the stage for assessing the implications of groundwater discharge for water quality and ecosystem health of the Great Lakes and, hence, formulating guidelines for monitoring and water management and governance.