Groundwater and nutrient dynamics in heterogeneous agricultural catchments

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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.