Groundwater and nutrient dynamics in heterogeneous agricultural catchments

在爱尔兰和苏格兰的农业区，地下水越来越被认为是水和养分输入河流的主要途径[1]。地下水还起到缓冲作用，在干旱期间维持河流流量，稀释通过地表路径输送的高营养浓度。人们认识到，地下水的养分输入与地下水停留时间呈负相关。了解地下水在集水区的路径和停留时间是改善农业管理和预测集水区对气候变化的恢复力的关键。传统的概念模型过分简化了浅层地下水系统与深层地下水系统之间的关系，前者具有短流程、年轻和高营养负荷，而后者具有长流程、年长和低营养负荷。然而，爱尔兰和苏格兰集水区的特点是一个复杂的地下组成的冰川沉积物上覆断裂基岩，高度异质性，这使得该模型复杂化，通过创建深，快速的优先流动路径，通过高渗透性的冰川沉积物和断裂带。此外，由冰川存款厚度的变化所决定的含水层限制条件也影响地下水补给和营养衰减过程。这两个反过来又导致复杂的空间分布，时间和规模的水和养分投入流。该项目旨在使人们更好地了解水文地质控制对农业集水区地下水和营养物质流入溪流的重要性。它将建立在使用地球物理数据来模拟项目团队开发的地下水路径和停留时间的最新进展的基础上[2]，并辅之以对地下水和地表水的高分辨率监测，包括营养物质和同位素（稳定和放射性），以进一步限制路径和停留时间。 该研究将使用北方爱尔兰的格伦伯恩集水区，该集水区由研究小组广泛监测[3]。集水区的特点是基岩并置风化/破碎的杂砂岩和固结不良的砂岩，两者都被排水不良、空间异质的冰碛覆盖。它配备了16个多深度钻孔和一个河流流量计。它还受益于广泛的、多尺度的地球物理数据集的可用性。研究将涉及; 1/绘制溪流和钻孔中流域尺度的营养物分布图（土壤、浅层和深层含水层）以及稳定同位素和放射性同位素取样; 2/利用分布式地下水数值模型，以多尺度地球物理数据得出的含水层特性为参数，模拟地下水流路径和停留时间的流域尺度分布（钻孔、地面和TELLUS航空勘测）; 3/评估径流/营养物分布与地下水排放/停留时间之间的关系; 4/测试减少营养物和优化缓解措施的土地使用管理方案。 该奖学金将作为与AFBI（农业食品和生物科学研究所）Rachel Cassidy博士长期合作的一部分实施，该研究所将提供额外的监督和安置。学生将受益于水文地质学和地下水流建模（UoA，QUB）的前沿培训;水文地球物理学（UoA，AFBI）;水文地球化学（QUB）;和先进的营养监测和分析以及将研究成果转化为政策指导（AFBI）的培训。他们还将受益于更广泛的、全QUADRAT范围的培训课程，包括目前在研究地点举办的实地地球物理学课程。

In Irish and Scottish agricultural regions, groundwater is increasingly recognised as a major pathway for water and nutrient inputs to streams [1]. Groundwater also serves as a buffer, maintaining river flow during dry periods and diluting high nutrient concentrations delivered via surface pathways. It is acknowledged that nutrients inputs from groundwater are inversely correlated with groundwater residence times. Understanding groundwater pathways and residence time in catchments is key to improving agricultural management and to predicting catchment resilience to climatic change. Traditional conceptual models oversimplify the relationship between the shallow groundwater system with short flow paths, young age and high nutrient load on the one hand, and deeper groundwater with longer flow paths, older age and lower nutrient load on the other hand. Irish and Scottish catchments however are characterised by a complex subsurface composed of glacial deposits overlying fractured bedrock, both highly heterogeneous, which complicates this model by creating deep and rapid preferential flow paths through highly-permeable glacial deposits and fractured zones. In addition, variable aquifer confining conditions, dictated by variability in glacial deposit thickness also influences groundwater recharge and nutrient attenuation processes. Both in turn result in complex spatial distribution, timing and magnitude of water and nutrient inputs to streams. This project aims at providing a better understanding of the importance of hydrogeological controls on groundwater and nutrient inputs to streams in agricultural catchments. It will built on recent advances in using geophysical data to model groundwater pathways and residence times developed by the project team [2], complemented by high-resolution monitoring of groundwater and surface water including nutrients and isotopes (stable and radioactive) to further constrain pathways and residence times. The research will use the Glen Burn catchment in Northern Ireland extensively monitored by the team [3]. The catchment is characterised by a bedrock juxtaposing weathered/fractured greywacke and poorly-consolidated sandstone, both overlain by poorly-draining, spatially heterogeneous, glacial-till. It is equipped with 16, multi-depth boreholes, and a river discharge gauge. It also benefits from availability of extensive, multi-scale geophysical datasets. The research will involve; 1/mapping catchment-scale nutrients distribution in streams and boreholes (soil, shallow and deep aquifer) as well as stable and radioactive isotope sampling; 2/modelling catchment-scale distribution of groundwater flow paths and residence times using a distributed groundwater numerical model parameterised with aquifer properties derived from multi-scale geophysical data (borehole, ground and TELLUS airborne surveys); 3/assessing the relationship between streamflow/nutrient distribution, and groundwater discharge/residence time; 4/testing land-use management scenarios for nutrient reduction and optimisation of mitigation measures. The studentship will be implemented as part of a long-established collaboration with Dr Rachel Cassidy, AFBI (Agri-Food and Biosciences Institute), which will provide additional supervision and placements. The student will benefit from cutting-edge training in hydrogeology and groundwater flow modelling (UoA, QUB); hydrogeophysics (UoA, AFBI); hydrogeochemistry (QUB); and advanced nutrient monitoring and analysis as well as training in translating research outcomes into policy guidance (AFBI). They will also benefit from wider, QUADRAT-wide training courses, including the field geophysics course currently run on the study site.