Abstract In 1998, as part of the process to assess the suitability of a site near Sellafield, Cumbria, for a deep geological repository for solid intermediate level and some solid low-level radioactive waste, United Kingdom Nirex Limited (Nirex) commissioned a study into the seasonal fluctuations in groundwater pressure observed in many of the monitoring boreholes in the area. Many of the monitoring zones in the deep boreholes at Sellafield show some response in groundwater pressure to annual variations in recharge. These seasonal fluctuations were quantified in terms of amplitude and lag over two full recharge cycles (1994–1996). The extremely detailed monitoring array installed by Nirex at the potential repository zone gives a unique opportunity to observe in detail the attenuation of the recharge signal, as it is propagated vertically downwards through the Sherwood Sandstone Group and into the underlying basement rocks. Use of an analytical approach to model the data provides constraints on values of the vertical diffusivity of the strata. The values of hydraulic formation parameters derived by this methodology are broadly consistent with results from borehole testing, though somewhat higher. This may in part be due to the large scale associated with the cyclic recharge signal. The attenuation of the seasonal fluctuations in three dimensions (3D) throughout the study area provides information at a scale suitable for use in constraining regional flow models.

Abstract Neighbouring groundwater models often have large areas of overlap to avoid boundary issues in hydrogeological assessments and such overlap or artificial boundaries can lead to inconsistent representations of aquifers and processes. This paper presents the aggregation of six adjacent models spanning East Anglia, England, into one model without internal boundaries. This study principally discusses the effect of, and difficulties arising from model edge boundaries. In addition, a review of conceptual and numerical discontinuities at model boundaries is included and a more consistent and robust modelling approach over the whole area is demonstrated. The large, joined model is used to delineate groundwater divides, assess their transient migration, review edge boundary implications for water balances and investigate abstraction impacts without the influence of internal static boundaries. Computer codes developed in conjunction with this study facilitate joining adjacent models and, conversely, splitting of the joined model back into models at the scale of the original component models using simulated groundwater divides, or to smaller submodels incorporating edge boundary conditions calculated from the parent model.

Abstract The Lincolnshire Chalk aquifer on the east coast of England is used regionally for water supply. However, groundwater abstraction can exceed recharge during times of drought, resulting in saline intrusion. Following hydrogeological investigations in the 1970s and 1980s, a regional groundwater model was developed to underpin the tactical management of the aquifer. Although it is a coarse, single-layer, constant-density model, routine regulatory use of the model since 1988 has enabled a sustainable level of groundwater abstraction to be reached and brought saline intrusion under control. Abstraction rates have been proactively adjusted to suit prevailing conditions using model forecasts. Forecasts are evaluated against model-based thresholds, set by comparing model output (e.g. aquifer–estuary flux) with observed data (e.g. groundwater salinity). The success of the model in managing groundwater resources is attributed to: field-based hydrogeological investigations informing the model design; collaboration between regulators and abstractors, which has built confidence in the model results; commitment to updating the model; and flexibility in the supply network through conjunctive use of ground and surface water. The model has also been applied for purposes not considered during development and has therefore provided value for money for the UK water industry over a 30 year period.

Abstract The Cam and Ely Ouse Chalk aquifer has been an important source of public water supply for over 100 years. In response to growing demand for water in the area in the 1970s and 1980 s, the National Rivers Authority developed the Lodes–Granta scheme to provide augmentation water to key rivers, subject to low flows. However, during the droughts in the late 1990s, the River Granta, which derives baseflow from the Chalk aquifer, was dry in some locations for several months. In response, the Environment Agency and Cambridge Water carried out investigations into the impacts of abstraction on the flow and ecology of the Granta and agreed to restrict abstraction from two operational groundwater sources during low flow periods. However, these abstraction restrictions could potentially result in a shortfall within the relevant public water supply zone under some climatic conditions and so Cambridge Water was considering increasing abstraction from an alternative source of groundwater within the catchment to retain the level of resilience of its supply. The Environment Agency was concerned that use of this abstraction could pose a risk of deterioration of the ecological status of the water body under the EU Water Framework Directive. This paper describes the investigations undertaken to assess the risk of deterioration and shows how these are being used to manage this risk going forward.

Abstract This paper explains the background and development of the Environment Agency National Groundwater Modelling System (NGMS) to integrate recharge functionality with the existing groundwater modelling (Modflow functionality). The Environment Agency groundwater models were originally developed primarily as a tool for making high-level strategic decisions but their use for short-term extreme event scenarios, such as drought or flood, has been relatively limited. This functionality has been constrained by the format of the rainfall/recharge input datasets. Undertaking scenario runs based on change in climate and weather has only been possible by direct manual alteration of those input datasets, which is not always practical on a day-to-day basis. Full implementation of recharge models into NGMS changes this, allowing the recharge models to be run in the NGMS environment and output to be generated. The fusion aspect of the process involves that output being processed in such a way that it can be used by NGMS as input into a Modflow scenario run. This process is explained using the example of a recent drought scenario in the Wessex Basin groundwater model.

Abstract Groundwater models are used to manage water resources in carbonate aquifers throughout the UK. These models often utilize non-linear modifications of equivalent porous media (EPM) assumptions to simulate the observed head-flow relationships. This paper describes the application of similar principles to time-variant water balance modelling of a karstified aquifer: the Carboniferous Limestone in SW England. The non-linear model successfully simulates the large changes in bulk hydraulic conductivity required to adequately model flows from two karstic spring catchments. Model results illustrate that the rate of drainage from the vadose zone is affected by the extent and thickness of soil cover. In a catchment with mixed soil cover, vadose zone drainage is shown not to be significant in sustaining summer low flows at the spring. The performance of the model is limited in some cases: summer storm flows in particular are poorly simulated. The limitations suggest that the true response of the system is dependent on recharge intensity (rate) as well as antecedent water levels and that the EPM assumption of good connection between conduits and the diffuse flow zone is not always valid in this aquifer. This may have implications for groundwater model design in other carbonate aquifers.

Abstract The Lichfield Permo-Triassic Sandstone aquifer is a strategic groundwater resource for the UK West Midlands. However, the flows of a number of streams have been heavily affected by abstraction. The catchment of one of these, the Leamonsley Brook, is under-drained by the Hanch Tunnel, a 5 km-long water transfer tunnel that is more than 150 years old. The Hanch Tunnel was excavated below the water-table for its entire length and is only partly lined. Regional-scale simulation of the aquifer was undertaken using a version of the USGS MODBRNCH code, which had been previously adapted to enable simulation of both open channel and pressurized pipe flow systems (the ‘Adit Code’). This paper reports on the simulation of the Hanch Tunnel, with an emphasis on the representation of this tunnel, and its effects on the groundwater flow in the Leamonsley Brook catchment. Critical aspects to consider when implementing the Adit Code are identified and discussed. The value of the application of the Adit Code for options appraisal to resolve the low-flow conditions of the Leamonsley Brook is explored.