Analysis of climate variability and change in observational groundwater quality data

Ascott, M.; Gooddy, D.; Mulcahy, A.. 2023 Analysis of climate variability and change in observational groundwater quality data. Nottingham, UK, British Geological Survey, 43pp. (CR/23/017N) (Unpublished)

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Abstract/Summary

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.

Item Type:	Publication - Report
Funders/Sponsors:	British Geological Survey, Environment Agency
Additional Information. Not used in RCUK Gateway to Research.:	This item has been internally reviewed, but not externally peer-reviewed.
Additional Keywords:	Groundwater, GroundwaterBGS
Date made live:	14 Sep 2023 12:37 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/535818

Item Type:

Publication - Report

Funders/Sponsors:

British Geological Survey, Environment Agency

Additional Information. Not used in RCUK Gateway to Research.:

This item has been internally reviewed, but not externally peer-reviewed.

Additional Keywords:

Groundwater, GroundwaterBGS

Date made live:

14 Sep 2023 12:37 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/535818