Opening opportunities for high-resolution isotope analysis - Quantification of δ15NNO3 and δ18ONO3 in diffusive equilibrium in thin–film passive samplers

The fate of nitrate transported across groundwater-surface water interfaces has been intensively studied in recent decades. The interfaces between aquifers and rivers or lakes have been identified as biogeochemical hotspots with steep redox gradients. However, a detailed understanding of the spatial heterogeneity and potential temporal variability of these hotspots, and the consequences for nitrogen processing, is still hindered by a paucity of adequate measurement techniques. A novel methodology is presented here, using Diffusive Equilibrium in Thin-film (DET) gels as high-spatial-resolution passive-samplers of δ15NNO3 and δ18ONO3 to investigate nitrogen cycling. Fractionation of δ15NNO3 and δ18ONO3 during diffusion of nitrate through the DET gel was determined using varying equilibrium times and nitrate concentrations. This demonstrated that nitrate isotopes of δ15NNO3 and δ18ONO3 do not fractionate when sampled with a DET gel. δ15NNO3 values from the DET gels ranged between 2.3 ± 0.2 and 2.7 ± 0.3‰ for a NO3– stock solution value of 2.7 ± 0.4‰, and δ18ONO3 values ranged between 18.3 ± 1.0 and 21.5 ± 0.8‰ for a NO3– stock solution of 19.7 ± 0.9‰. Nitrate recovery and isotope values were independent of equilibrium time and nitrate concentration. Additionally, an in situ study showed that nitrate concentration and isotopes provide unique, high-resolution data that enable improved understanding of nitrogen cycling in freshwater sediments

Using 18O/2H, 3H/3He, 85Kr and CFCs to determine mean residence times and water origin in the Grazer and Leibnitzer Feld groundwater bodies (Austria)

Two groundwater bodies, Grazer Feld and Leibnitzer Feld, with surface areas of 166 and 103 km2 respectively are characterised for the first time by measuring the combination of δ18O/δ2H, 3H/3He, 85Kr, CFC-11, CFC-12 and hydrochemistry in 34 monitoring wells in 2009/2010. The timescales of groundwater recharge have been characterised by 131 δ18O measurements of well and surface water sampled on a seasonal basis. Most monitoring wells show a seasonal variation or indicate variable contributions of the main river Mur (0–30%, max. 70%) and/or other rivers having their recharge areas in higher altitudes. Combined δ18O/δ2H-measurements indicate that 65–75% of groundwater recharge in the unusual wet year of 2009 was from precipitation in the summer based on values from the Graz meteorological station. Monitoring wells downstream of gravel pit lakes show a clear evaporation trend. A boron–nitrate differentiation plot shows more frequent boron-rich water in the more urbanised Grazer Feld and more frequent nitrate-rich water in the more agricultural used Leibnitzer Feld indicating that a some of the nitrate load in the Grazer Feld comes from urban sewer water. Several lumped parameter models based on tritium input data from Graz and monthly data from the river Mur (Spielfeld) since 1977 yield a Mean Residence Time (MRT) for the Mur-water itself between 3 and 4 years in this area. Data from δ18O, 3H/3He measurements at the Wagna lysimeter station supports the conclusion that 90% of the groundwaters in the Grazer Feld and 73% in the Leibnitzer Feld have MRTs of 20 m) with relative thicker unsaturated zones. The young MRT of groundwater from two monitoring wells in the Leibnitzer Feld was confirmed by 85Kr-measurements. Most CFC-11 and CFC-12 concentrations in the groundwater exceed the equilibration concentrations of modern concentrations in water and are therefore unsuitable for dating purposes. An enrichment factor up to 100 compared to atmospheric equilibrium concentrations and the obvious correlation of CFC-12 with SO4, Na, Cl and B in the ground waters of the Grazer Feld suggest that waste water in contact with CFC-containing material above and below ground is the source for the contamination. The dominance of very young groundwater (<5 years) indicates a recent origin of the contamination by nitrate and many other components observed in parts of the groundwater bodies. Rapid measures to reduce those sources are needed to mitigate against further deterioration of these waters

Impacts of climate and land use change on groundwater quality in England : a scoping study

In 2019, the Environment Agency (EA) carried out a risk assessment following the government Climate Change Committee's methodology. This work highlighted groundwater quality as one area where the quality of the EA’s plan was weak, and progress in managing the risk was poor. In the same year, the UK parliament declared a climate emergency, and the COP26 summit in 2021 has brought climate change and the need to adapt into even stronger focus. This prompted the need for further evidence gathering and in 2021 the EA commissioned BGS to undertake a scoping study to explore the impacts of climate and land use change on groundwater quality. The study had the following objectives: (1) To determine what key risks to groundwater quality may be associated with climate change, (2) what adaptation and mitigation measures may be needed, (3) how EA groundwater quality monitoring may need to change in the future associated with climate change and (4) what are the research and evidence gaps associated with the impacts of climate and land use change on groundwater quality. The study addressed the aims above through a number of desk-based activities which are detailed in this report. A review of the findings of UKCP18 has illustrated the potential changes to the climate of England over the 21st century as context for changes in groundwater resources and quality. Air temperature, evapotranspiration and sea level are all predicted to increase throughout the 21st century. Whilst the direction of change in annual precipitation is unclear, wetter winters and drier summers are predicted, with greater magnitude extreme winter rainfall events. No published work has evaluated the impact of climate change based on UKCP18 data on groundwater recharge and levels. A review of previous studies using UKCP09 and other climate projections has shown limited consistency in the direction of change in long term average groundwater recharge and levels in England. There is some consistency in changes to seasonality in groundwater recharge and levels, with increased recharge and levels in winter, decreased recharge and levels in summer. There is limited evidence for changes in extremes (increasing high winter groundwater levels). A review of international literature related to climate change and groundwater quality has shown an overall worsening of groundwater quality over the next 50 – 80 years, although the trajectory of change for individual parameters is highly uncertain. Some parameters have a high level of confidence in a relationship with climate variables (e.g. shallow groundwater temperature and air temperature, sea level rise and salinity in coastal aquifers). However, for many components of climate change and water quality parameters, our understanding of relationships is near non-existent and speculative. A workshop was held on “Groundwater Quality into the Future” as part of this study. The purpose of the workshop was to gather input from both Environment Agency and external stakeholders regarding the key issues related to future groundwater quality, and the priorities for adaptation, management and research. This workshop identified uncertainty in impacts of climate change on groundwater quality, the need for holistic approaches to management of groundwater in the terrestrial water cycle, and the need for continued monitoring as cross cutting themes. A number of focus areas were also identified: nutrients, emerging substances, changing rainfall characteristics, changing temperature, groundwater rebound, urban development and construction, changing salinity and groundwater ecosystems. The potential impacts of climate change on groundwater quality are illustrated through five case studies – Brighton, Chichester, Birmingham, Eden and Dove. The case study areas cover a range of different hydrogeological (Chalk, Permo-Triassic sandstone and Carboniferous Limestone), geographical (north, south, inland, coastal) and land use settings (rural, urban). For each case study we discuss the hydrogeological conceptualisation and water quality issues of concern. We then present the results of UKCP18 (temperature, rainfall) and the derived products eFLaG (rainfall, evapotranspiration, groundwater recharge, groundwater levels) and GeoCoast (sea level rise), before providing a qualitative evaluation of the impacts of climate change on groundwater quality. Across all five case study areas, air temperatures are predicted to increase by up to 3°C. This could increase reaction rates for degradation of contaminants, but such increases may only be marginal. Increased sea levels are predicted to increase salinity in coastal aquifers. The direction of changes in long term average rainfall and recharge is uncertain, but the magnitude of changes is predicted to be small. There is generally a high confidence of increased rainfall and recharge seasonality and greater magnitude of extreme events in winter. This has the potential to result in spikes of pollutants, but this could also be offset by increased dilution. Land use change, and in the case of Birmingham, groundwater level recovery from historic over-abstraction, may have a greater impact on groundwater quality than changes in climate. On the basis of the literature review, case studies and input from stakeholders, an initial prioritisation of the potential risks to groundwater quality associated with climate change has been made. The relatively small increases in temperatures and changes in long term average rainfall and recharge make these a low priority. The local nature of increases in sea level affecting coastal aquifers make these a medium priority. The high confidence in changes in rainfall and recharge seasonality and extremes and impact through changes to leaching, spikes and dilution make these a relatively high priority. The highest priority risk is land use change, whether induced by climate change or otherwise. Land use change may change contaminant sources and pathways, and is both highly uncertain and has a potentially high impact on both groundwater and other components of the terrestrial water cycle. Building on the previous project tasks, a number of recommendations have been made regarding evidence gaps, monitoring approaches, regulation and adaptation measures. Specific recommendations are detailed in the table below and general recommendations are discussed herein. Further research is required to address the significant evidence gap related to how drivers of groundwater quality are likely to change in the future, and what the hydrogeological system response to changes in multiple, competing drivers may be. This is a large area of work and should be prioritised based on stakeholder needs. Subsequent work is required to consider the impacts of future changes in groundwater quality on downstream receptors, and what management strategies should be adopted. Recommendations for changes in groundwater quality monitoring detailed below are speculative at this stage given the high level of uncertainty associated with the impacts of climate change on groundwater quality. A key recommendation from the workshop was for better integration of groundwater resources and quality in regulation, as well as better integration of groundwater as a whole within the terrestrial water cycle and urban planning. Given the uncertainty regarding the impacts of climate and land use change on groundwater quality, “no regrets” adaptation measures are most appropriate at this time. These measures, detailed below will address groundwater quality needs under current climate and land use and in any future. However, as “no regrets” measures address current groundwater quality issues, future issues which are not currently a concern (e.g. the next generation of emerging contaminants) will not be impacted by these approaches. This highlights the importance of addressing the evidence gaps above through targeted research projects. Detailed project proposals to address these gaps are beyond the scope of this report and should be co-produced between the Environment Agency, BGS and other stakeholders

Geological structure as a control on floodplain groundwater dynamics

Groundwater in upland floodplains has an important function in regulating river flows and controlling the coupling of hillslope runoff with rivers, with complex interaction between surface waters and groundwaters throughout floodplain width and depth. Heterogeneity is a key feature of upland floodplain hydrogeology and influences catchment water flows, but it is difficult to characterise and therefore is often simplified or overlooked. An upland floodplain and adjacent hillslope in the Eddleston catchment, southern Scotland (UK), has been studied through detailed three-dimensional geological characterisation, the monitoring of ten carefully sited piezometers, and analysis of locally collected rainfall and river data. Lateral aquifer heterogeneity produces different patterns of groundwater level fluctuation across the floodplain. Much of the aquifer is strongly hydraulically connected to the river, with rapid groundwater level rise and recession over hours. Near the floodplain edge, however, the aquifer is more strongly coupled with subsurface hillslope inflows, facilitated by highly permeable solifluction deposits in the hillslope–floodplain transition zone. Here, groundwater level rise is slower but high heads can be maintained for weeks, sometimes with artesian conditions, with important implications for drainage and infrastructure development. Vertical heterogeneity in floodplain aquifer properties, to depths of at least 12 m, can create local aquifer compartmentalisation with upward hydraulic gradients, influencing groundwater mixing and hydrogeochemical evolution. Understanding the geological processes controlling aquifer heterogeneity, which are common to formerly glaciated valleys across northern latitudes, provides key insights into the hydrogeology and wider hydrological behaviour of upland floodplains

Groundwater recharge and age-depth profiles of intensively exploited groundwater resources in northwest India

Intensive irrigation in northwest India has led to growing concerns over the sustainability of current and future groundwater abstraction. Environmental tracers and measurements of groundwater residence times can help quantify the renewal processes. Results from 16 paired locations show the interquartile ranges for residence times in shallow alluvial groundwater (8–50 m deep) to be 1–50 years and significantly less than those from deeper groundwater (76–160 m deep) at 40–170 years. The widespread occurrence of modern tracers in deep groundwater (>60% of sites had >10% modern recharge) suggests that there is low regional aquifer anisotropy and that deep aquifers are recharged by a significant component of recent recharge via vertical leakage. Stable isotope and noble gas results at all depths conform to modern meteoric sources and annual average temperatures, with no evidence of significant regional recharge from canal leakage in this study area close to the Himalayas

Characteristics of high-intensity groundwater abstractions from weathered crystalline bedrock aquifers in East Africa

Weathered crystalline bedrock aquifers sustain water supplies across the tropics, including East Africa. Although well yields are commonly <1 L s−1, more intensive abstraction occurs and provides vital urban and agricultural water supplies. The hydrogeological conditions that sustain such high abstraction from crystalline bedrock aquifers remain, however, poorly characterised. Five sites of intensive groundwater abstraction (multiple boreholes yielding several L s−1 or more) were investigated in Uganda and Tanzania. Analysis of aquifer properties data indicates that the sites have transmissivities of 10–1,000 m2 day−1, which is higher than generally observed in deeply weathered crystalline bedrock aquifers. At four of the five sites, weathered bedrock (saprolite) is overlain by younger superficial sediments, which provide additional storage and raise the water table within the underlying aquifer. Residence-time indicators suggest that: (1) abstracted water derives, in part, from modern recharge (within the last 10–60 years); and (2) intensive abstraction is sustained by recharge occurring over several decades. This range of encountered residence times indicates a degree of resilience to contemporary climate variability (e.g. short-term droughts), although the long-term sustainability of intensive abstractions remains uncertain. Evidence from one site in Tanzania (Makutapora) highlights the value of multi-decadal groundwater-level records in establishing the long-term viability of intensive groundwater abstraction, and demonstrates the influence of intra-decadal climate variability in determining the magnitude and frequency of recharge

Derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection

For groundwater-surface water interactions to be understood in complex wetland settings, the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways. Discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground. Here a noninvasive geophysical imaging approach combining three-dimensional electrical resistivity tomography (ERT) and the novel application of gradient and isosurface-based edge detectors is considered as a means of illuminating wetland deposit architecture. The performance of three edge detectors were compared and evaluated against ground truth data, using a lowland riparian wetland demonstration site. Isosurface-based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces (i.e., peat/gravels and gravels/Chalk). The use of gradient detectors approach was unsuccessful, indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect. These findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities. In addition, ERT revealed substantial structures in the gravels related to the depositional environment (i.e., braided fluvial system) and a complex distribution of low-permeability putty Chalk at the bedrock surface—with implications for preferential flow and variable exchange between river and groundwater systems. These results demonstrate that a combined approach using ERT and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands

Integrated time-lapse geoelectrical imaging of wetland hydrological processes

Wetlands provide crucial habitats, are critical in the global carbon cycle, and act as key biogeochemical and hydrological buffers. The effectiveness of these services is mainly controlled by hydrological processes, which can be highly variable both spatially and temporally due to structural complexity and seasonality. Spatial analysis of 2D geoelectrical monitoring data integrated into the interpretation of conventional hydrological data has been implemented to provide a detailed understanding of hydrological processes in a riparian wetland. This study shows that a combination of processes can define the resistivity signature of the shallow subsurface, highlighting the seasonality of these processes and its corresponding effect on biogeochemical processesthe wetland hydrology. Groundwater exchange between peat and the underlying river terrace deposits, spatially and temporally defined by geoelectrical imaging and verified by point sensor data, highlighted the groundwater dependent nature of the wetland. A 30 % increase in peat resistivity was shown to be caused by a nearly entire exchange of the saturating groundwater. For the first time, we showed that automated interpretation of geoelectrical data can be used to quantify shrink-swell of expandable soils, affecting hydrological parameters, such as, porosity, water storage capacity, and permeability. This study shows that an integrated interpretation of hydrological and geophysical data can significantly improve the understanding of wetland hydrological processes. Potentially, this approach can provide the basis for the evaluation of ecosystem services and may aid in the optimization of wetland management strategies