Modelling the hydrogeology and managed aquifer system of the Chalk across southern England

Abstract

Six regional recharge and groundwater models have been recently developed of the Chalk and Upper Greensand from Dorset to Kent. Updated Chalk stratigraphy and mapping have improved understanding of geological structure and the development of preferential groundwater flow pathways along hardground horizons. Where shallow dipping folds bring these into the zone of active groundwater flow, extensive ‘underdrainage’ may result in marked differences between surface and groundwater catchments. Hardgrounds and marls are also associated with spring discharges, as are some faults and the clay formations that underlie or confine the aquifer system. Higher specific yield within the Upper Greensand helps support summer baseflow, as do local groundwater discharges from augmentation schemes, watercress and fish farm operations. The aquifer system has been successfully modelled using the ‘variable hydraulic conductivity with depth’ version of MODFLOW. Depths of secondary permeability development have been distributed according to ground and groundwater level data. Interfluve–valley contrasts overlie a base hydraulic conductivity set according to the formation saturated at the water table and enhanced by active hardgrounds. Local parameter overrides may also be needed. The Wessex Basin conceptual and numerical model is described before summarizing similarities and contrasts from the other five regional model areas.

Diagnostic tests for conceptualizing transport in bedrock aquifers

“…Test method 1 can be applied before any subsurface data are collected. Flow accretion surveys can be carried out by measuring discharge at frequent intervals along streams to identify losing and gaining reaches to help inform hydrostratigraphic permeability variation (Arnott et al, 2009;Soley et al, 2012). Test methods 2-5 are derived from head data from wells, and Worthington (2009) gives a number of examples.…”

Groundwater abstraction impacts on river flows: predictions from regional groundwater models

“…A full justification of the credibility of the models used for the many examples presented here is beyond the scope of this paper, although two are described in more detail elsewhere in this volume (Shepley & Soley 2012;Soley et al 2012). Some general comments on the process of building and refining a model to the point where it can be used for the prediction of abstraction impacts on flow are, however, appropriate.…”

“…This may be initially parameterized according to an understanding of the length and width of the channel and permeability of the stream bed or other near-surface materials that are not explicitly represented as groundwater model layers. However, measured patterns of downstream flow accretion or loss along streams collated for several models (including those covering the Chalk of southern England described in Soley et al 2012) suggest that rates of groundwater discharge into streams (when groundwater levels are high) are often much greater than rates of leakage through the stream bed back to the aquifer during drier periods. In order to achieve a credible representation of this behaviour, the bed elevations built into the MODFLOW Stream Package are often brought closer to the stream stage during refinement so that leakage rates are limited to match measured losses as soon as heads fall below the bed bottom.…”

Introduction: groundwater resources modelling: a case study from the UK

“…Consequently, the value of this work should be judged against the results in these areas as well as the science and the degree of innovation that has been generated. Some of the papers, such as Soley et al (2012b) on the southern Chalk, provide broad overviews of the improvement in understanding that has been obtained from the modelling programme. The driving forces for the modelling are discussed in Whiteman et al (2012a), as well as the benefits forthcoming from the work.…”