Characterizing Groundwater Quality, Recharge and Distribution under Anthropogenic conditions

Awareness concerning sustainable groundwater management is gaining traction and calls for adequate understanding of the complexities of natural and anthropogenic processes and how they affect groundwater quality and availability. This research project aimed to investigate different qualitative impacts on groundwater, and provide assessment methodologies that can be employed to ensure sustainable quantitative groundwater use. The study site, situated in north-eastern Switzerland, included groundwater systems located in minor and major alluvial deposits associated both with high elevation and plains regions. The Thur River catchment presents a study area that is both well investigated and of a large enough size (~1700 km2) to be considered a mesoscale catchment. Specific objectives of this research project included 1) the determination of the major controls on groundwater contamination, 2) the assessment of the spatiotemporal variability of groundwater recharge, in a manner applicable to other catchments, and 3) the monitoring of groundwater in an adaptive and event-based manner. These research objectives were addressed by first reviewing historic and current qualitative threats to groundwater concerning anthropogenic processes and how they affect groundwater quality globally. Secondly, the spatiotemporal groundwater recharge in the mesoscaled Thur catchment was estimated, based on open-source data and software. Finally, variabilities of source water contributions to aquifers located in the Thur catchment were determined using environmental tracer data from sampled sites. The structure of this dissertation spans three different scales ranging from global, to mesoscale, and to localized sampling, and covers subsequent different levels of data availability. In order to illustrate the many qualitative threats to groundwater resources, some of the key contaminants originating from anthropogenic activities, namely agricultural, urban, and industrial, were presented in the form of a review. Furthermore, a selection of case studies describing the continued concerns of both established, as well as new and emerging contaminants were presented. As demands on groundwater continue to increase, in addition to groundwater quality, it is also imperative to consider the sustainable management of groundwater quantity. For this, knowledge concerning a catchment’s spatiotemporal groundwater recharge, and the dominant water sources controlling surface-groundwater dynamics, is vital. In many regions however, groundwater recharge remains challenging to quantify, whether due to hydrogeological process complexities or limited observation data. Gridded components from readily available remotely sensed and ground-based data, including precipitation, actual evapotranspiration, and hydrological discharge data (separated into quick- and baseflow), were used to generate spatiotemporal groundwater recharge maps over a 20-year period (2000 - 2019) using open source software. Results from the gridded groundwater recharge estimates agreed well with estimates from other studies, and overall, recharge was shown to account for 29% of total precipitation in the Thur catchment. Results highlighted the importance of precipitation to groundwater recharge, with prolonged periods of drought having a negative effect on groundwater recharge, while periods of above average rainfall had a buffering effect on the Thur catchment’s groundwater resources. River water represents a connection between the surface and sub-surface environment. Between 2017 and 2020, Thur River water was collected during extreme events (high- and low flow), and analysed for its conservative tracer (δ18O and δ2H) content. These event-sampled surface waters were analysed in conjunction with groundwater and rainwater samples to characterize the spatiotemporal water dynamics in the Thur catchment. Using a cluster and three end-member mixing analysis, the spatiotemporal variability of different source water components contributing to the Thur catchment’s aquifers were identified. Source water variabilities of aquifers were found to be dependent on both elevation and geology, with different water sources dominating different regions of the Thur catchment. A surface water-type was found to be a dominant source in the high elevation regions, while a groundwater-type dominated the middle elevation regions, and a rainwater-groundwater mix the low elevation regions. A clear shift towards groundwater signatures during dry event conditions was observed in the event-sampled river water, with groundwater making up an overall average of 30% of the surface water sampled in the Thur catchment. This research project highlighted key contaminants that originate from anthropogenic activities, as well as the complexities involved in understanding the underlying physical processes and factors governing groundwater quality, including climate, geological settings, topography, and land use. Pertinent case studies emphasized persistent uncertainties concerning contaminant degradation processes, contaminant pathways, and subsequent contamination risks to groundwater quality. Where groundwater quantity is concerned, this study demonstrated the value of remotely sensed data in estimating the spatiotemporal recharge of a mesoscale catchment, in particular where observation data is limited, but also emphasised the importance of continued ground-based monitoring networks. Finally, investigations demonstrated the variability of river and groundwater source components in the Thur catchment, where event-based samples provided a relatively inexpensive insight into the surface-groundwater characteristics of a mesoscale catchment.