The Cretaceous Chalk is a fractured aquifer with high permeability and low storage, that supplies about 60% of potable groundwater in the UK. Since the 1970s, groundwater quality in the Chalk aquifer in East Yorkshire focusing on the Kilham catchment area has been threatened by nitrate and pesticide contamination. This project sets out to better characterize the groundwater flow velocities, patterns, and regimes in the East Yorkshire Chalk aquifer to facilitate catchment and source protection zone delineation for aquifer protection. The aims were pursued via: conducting of single open well ambient flow dilution testing and reinterpretation of previous data including geophysical logs, well-to-well tracer tests; well hydraulic head and stream discharge hydrographs; conducting of time-series monitoring of springs and wells for temperature, SEC and major ion composition; re-analyses of archived pumping test data to investigate the hydraulic properties of faults and consequent development of conceptual model of aquifer recharge, throughput and outflow.
Discrete flow horizons were detected at all depths of investigation (up to 70 mgbl), but with the majority of flow features at shallow depth in promixity to the water table. Ambient flow pattern in wells is location specific, with strong upwards flows of up to 4 m/min in valley boreholes while boreholes at high elevation typically show downflow, and those at intermediate elevation show crossflows. Previous well-to-well and well-to-spring tracer tests are interpreted as indicating karstic flow via solutionally enlarged bedding features connected by vertical fractures, with groundwater velocities ranging between 40 – 480 m/d, connecting wells and springs 4.2 km apart. Novel approaches for estimating local groundwater velocities from single open-well dilution tests were further developed in this work and applied to the single-well data; these gave velocities in broad agreement with those from well-to-well tracer tests, validating the single well method.
Monitoring data showed that wells and spring temperatures fell in a narrow range (9.2 – 10.1 °C) reflective of average annual (10.3 °C) temperature suggesting sufficient residence time to enable groundwater temperature to equilibrate with that of the aquifer. Groundwater and spring waters are dominated by Ca2+ and HCO3-, with evidence of nitrate contamination (51 – 74 mg/L NO3-). Well water SEC is non-varying, whereas that for springs showed variation reflective of both degree of access of soil-derived CO2 and seasonal variations in soil biological activity and hence soil pCO2. Ca2+ and HCO3- concentration variation in the springs may also indicate seasonal switching between open and closed carbonate systems. Well hydraulic head and groundwater fed stream discharge hydrographs showed seasonal fluctuation, with well hydraulic head variations increasing towards recharge areas. The effects of faults on aquifer transmissivity could not be ascertained via re-analyses of archived pumping tests.
The findings from this work were used to develop both physical and chemical hydrogeological conceptual models for the catchment. The aquifer functions as a hydrogeologic continuum, limiting the utility of the individual source protection zones currently in use by the Environment Agency of England and Wales (EA), but supports the EA’s decision to demarcate the entire Chalk outcrop as nitrate vulnerable. Knowledge of ambient well flow velocities, and flow patterns are required for effective planning of future geochemical sampling, contaminant tracking, remediation activity and pumping tests campaigns. Such knowledge allows both depth specific sampling and vertical hydraulic characterisation of depth intervals in wells, which are vital to develop more accurate simulations of contaminant transport. Further hydraulic characterisation of fault zones is also recommended.
