Analysis of climate variability and change in observational groundwater quality data

This report details Task 1 (“Evaluation of historical observational groundwater quality data”) of Phase 2 of the Environment Agency-BGS collaborative project “climate and land use change impacts on groundwater quality”. The objective of this task is to evaluate historical observational groundwater quality data held by the Environment Agency (EA) to determine the following: (1) the suitability of existing monitoring for future monitoring of long-term impacts of climate and land use change and (2) whether there is evidence for climate variability, and if possible, impacts of historical climate change in the observations. It was agreed in an EA-BGS kickoff meeting that this task would investigate climate variability and change in nitrate and groundwater temperature data. This task focusses on southeast England as a case study. Analysis of groundwater nitrate data held by the Environment Agency in WIMS has shown that a small number of sites meet the required time series length requirement for climate change impact monitoring in southeast England (30 years). The recent natural variability in climate combined with short record length means that any climate change impacts cannot be observed in the data provided. Cluster analysis has revealed different modes of temporal fluctuations in nitrate concentrations. The depth of groundwater flow system intercepted by the boreholes appears to control the long-term direction of change in groundwater nitrate concentrations. Non-linear and seasonal behaviour associated with climate variability are present in two clusters, which are weakly spatially coherent across the North and South Downs. Cross-correlation of nitrate time series with both raw and standardised indices of groundwater level and precipitation show that the extent of nitrate fluctuation appears to be controlled by precipitation and groundwater level fluctuation. This may be due to a combination of piston flow and changing groundwater flow paths. Under future climate change, nitrate fluctuations may change associated with the changing intersection of the water table and the legacy nitrate peak in the unsaturated zone. The timescales for land use change impacts on nitrate at the water table will vary substantially depending on the dominant process controlling nitrate fluctuations. Processes which represent a transfer of mass (bypass flow) will impact concentrations much more rapidly than processes representing a transfer of energy (piston flow). Analysis of groundwater temperature data for 20 boreholes has shown that, for 8 of 17 shallow boreholes with temperature data over 2012-2022, groundwater temperature trends are broadly consistent with current air temperature trends. 7 of these sites show increasing trends, with a mean trend of 0.66 °C/decade. Three deep interfluve sites show increases, with a mean trend if 0.38 °C/decade. It is likely that these trends are controlled by current and historical near-decadal trends in local air temperature for shallow and deep sites respectively. The remaining 8 shallow sites show inconsistent trends in comparison with local air temperature trends. For these sites it likely that in addition to air temperature trends, additional heat fluxes into the subsurface are occurring superimposed on changes in groundwater flow to the boreholes. The shallow sites show seasonal temperature fluctuations associated with propagation of air temperature signals, with seasonal range in groundwater temperature significantly negatively correlated with borehole depth. Three very shallow sites show diurnal fluctuations, although these fluctuations are below the accuracy of the sensors. The increases in groundwater temperature observed have some implications for other components of groundwater quality (e.g. biogeochemical cycles, stygofauna, pollutant (N, pesticide, LNAPL) degradation and for the role that groundwater discharges to surface water play in providing cold-water hydro-refugia to cold-water species during summer

Hydrological assessment and monitoring of wetlands

The physical and chemical characteristics which favour wetland plant communities, primarily high soil water levels and anaerobic soil chemistry, are related directly to the hydrology/hydrogeology of the wetland and often its surrounding catchment. Appreciation and successful management of a wetland therefore almost always requires an understanding of its hydrological functioning, including the influences on hydrological functioning which often lie beyond the designated boundary of the sit

Nitrate modelling workshop report

This report details the findings of a workshop held by the Environment Agency (EA) and British Geological Survey (BGS) about Nitrate Modelling on 7th February 2023. The workshop was attended by over 80 delegates and was a part of the “Impacts of climate and land use change on groundwater quality” project, which is a three-year collaborative project between the EA and BGS. The BGS nitrate modelling work, including a part of PhD study co-funded by Defra, BGS and Jilin University, and nitrate-related work of the EA, Water Companies and Natural England was presented at the sessions of “pollutant sources from the soils” and “nitrate transport and legacy in the groundwater system”. The delegates were divided into seven groups for breakout discussions. Based on the notes recorded from the seven groups, there are five key themes for future work related to nitrate in groundwater: • Improved conceptualisation of nitrate processes based on enhanced use and collection of data • Improved representation of processes in models • The need for modelling across different spatial and temporal scales • Training, knowledge, model and data exchange • Public engagement and action on-the-ground The discussions in this workshop provided some guidance for identifying the next steps for nitrate work covered by the three-year project, and provided a starting point for greater engagement with stakeholders

A conceptual model of the groundwater contribution to streamflow during drought in the Afon Fathew catchment, Wales

In 2022 BGS was commissioned by Dŵr Cymru Welsh Water (DCWW) to undertake desk and field investigations to develop a conceptual understanding of the contribution of groundwater to streamflow during drought in the Afon Fathew, Wales. This report details the findings of these investigations. In addition to a desk study, two field visits were completed to survey water features in the catchment, take samples for groundwater residence time indicators, and undertake a passive seismic (Tromino) geophysical survey. The results of the desk study and field visits were combined with flow accretion profile data to develop a conceptual model of groundwater flow to the Afon Fathew during drought, described herein. The Fathew is underlain by a bedrock of silty mudstones which are traditionally considered to be poor aquifers. In the Fathew catchment there is evidence from boreholes for local-scale groundwater flow in the bedrock within fractures and other discontinuities. An upper weathered layer, in combination with faulting and folding patterns, is likely to control the geometry and magnitude of bedrock groundwater flow systems and the location of springs. The residence time indicator data suggest that groundwater in the bedrock is over 40 years old. Estimated discharge from bedrock springs (< 2 l/s, 0.17 Ml/day) is very small relative to the total flow in the Fathew and tributary inflows. The Tromino has shown the superficial deposits in the catchment to be highly heterogeneous in the valley bottom. Changes in the likely permeability and areal extent of the superficial deposits going down the valley bottom correspond to changes in river flows in the Fathew based on the accretion profiles. The Fathew and its tributaries are losing over well drained alluvial gravels, and gaining over low permeability lacustrine and clay-ey alluvial deposits. The Fathew is likely to be hydraulically isolated from the Dysynni catchment. 60% of low flow inflows to the Fathew are coming directly from upland tributary inflows. where very limited superficial deposits are present. In these upland settings during dry periods it is likely that the majority of discharge is coming from baseflow from bedrock. Baseflow support to the Fathew during drought periods can be conceptualised as a two-phase system: (1) Discharge from the superficial deposits to the river, particularly associated with the down-catchment variability in the permeability and thickness of the deposits, (2) Discharge from the weathered bedrock aquifer into the river, from both springs and tributary inflows. The contribution of these two processes is likely to vary as drought conditions develop. Moreover, flows in springs and tributaries may contribute to downstream storage within the superficial deposits, which may complicate the deconvolution of the Fathew river flow hydrograph into different flow components. This temporal sequencing requires further investigation. Further work such as groundwater and surface water monitoring during dry periods and electrical resistivity tomography may be beneficial to constrain these uncertainties

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

Semiquantitative mapping of climate and land use change impacts on groundwater quality

This report details Task 2 (“Development of semi-quantitative national maps for assessment of potential impacts of Land Use and Climate Change and Groundwater Quality”) of Phase 2 of the Environment Agency-BGS collaborative project “climate and land use change impacts on groundwater quality”. This task has developed semi-quantitative maps of changes in contaminant source term risks associated with land use futures, accompanied by maps of metrics of changes in precipitation and temperature associated with climate change. The key findings of this task are as follows. An initial scoping workshop identified a very wide range of drivers, pressures, and groundwater quality variables of interest. It also identified a wide range of users of information, covering both technical and non-technical staff and a range of scales from national down to area and local. A literature review of relevant land use and climate change datasets showed that the land use futures developed under the SPEED project (Brown et al., 2022) associated with the Shared Socioeconomic Pathways (SSPs) were the most appropriate to use for this task, accompanied by climate change projections derived from UKCP18 and CHESS-SCAPE (Robinson et al., 2022). A semi-quantitative risk scoring methodology has been developed to link land use classes reported in the land use futures to contaminant source term risk. This methodology has been applied to land use futures under six scenarios and seven future time slices, with results summarised by EA areas and aquifers. Across the SSPs there is a divergence of changes in risk in some areas and commonality in others. Some common features across the SSPs include: stable land use (and limited change in contaminant risk) in eastern England associated with ongoing need for food production; afforestation (and reduction in contaminant risk) in southern England; urbanisation and intensification of arable land in northern England (an increase in contaminant risk). CHESS-SCAPE data has been processed to produce maps of changes in precipitation (seasonal mean and extreme temperature, number of wet days) and temperature (seasonal mean) metrics for 10-year timesteps to 2070 for 4 Representative Concentration Pathways (RCPs). Under RCP8.5 for 2070, this results in wetter winters (particularly in northern England and coastal southern England) and drier summers (particularly in southern England), with the largest increases in extreme winter precipitation in northwest England and on the south coast. The greatest rises in mean air temperature are in greatest temperature rises in Solent and South Downs and West Thames. The data generated in this task represent exploratory futures which are designed to support discussions with policymakers on the robustness of existing policies related to groundwater quality, and to inform spatial prioritisation of future work based on where risk is likely to be greatest. A next step would be to use the data generated in this task to inform the development of coupled water and pollutant models to quantitatively assess the impact of the land use futures on groundwater quality

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