Improved representation of regional groundwater flow in the Hertfordshire Chalk (UK) by merging four existing regional groundwater models

The Environment Agency required a single regional groundwater model to better represent flow in the Chalk aquifer in Hertfordshire (United Kingdom). Previously, four adjacent regional groundwater models were used; the South-West Chilterns, Vale of St. Albans, Cam and Bedford Ouse and Rib Ash Stort models. A single groundwater model reduces the impacts of boundary effects at the model joining locations and ensures a consistent approach to Chalk modelling in the area. The completed Hertfordshire model will be used to inform regulatory decisions, assess the impacts of abstraction on water bodies under the Water Framework Directive, inform Source Protection Zone delineation and inform water company source investigations. Bedrock geology in the study area comprises the White Chalk and the Grey Chalk subgroups, which are confined by the Lambeth Group and Thames Group in the south-eastern area of study. The study area includes major rivers such as the River Thames, the River Lee, the River Colne and their tributaries. Karstic streams are present in the River Colne catchment, causing water to flow via sinkholes eastwards into the River Lee catchment. The model simulates a total of 1,248 groundwater abstractions and discharges with a total fully licensed abstraction volume of 1,170Ml/d. Approximately 96% of groundwater abstracted from the Hertfordshire Chalk between 2011 and 2015 was for public water supply. A new recharge model was developed using the 4R (Routing of Rainfall to Runoff and Recharge) code (Entec UK Ltd, 2006) to simulate recharge and the runoff component of surface water flow. A modified version of the U.S Geological Survey groundwater model (MODFLOW-96) which allows variable conductivity with depth was used to simulate groundwater flow in the study area. Construction of the Hertfordshire groundwater model was undertaken by merging the component models into a single MODFLOW-VKD model, with an active model area of 4,060km2, making it one of the largest groundwater models in the UK. Component model parameters were retained, where appropriate. To ensure consistent routing and as a product of the new recharge model and new LIDAR data, a new stream file was produced. The Hertfordshire Chalk model was calibrated to 232 observation boreholes and 34 stream gauging stations in the area. Final calibration resulted in 73% of gauging stations showing either an improvement on the component model or simulating average flows within a 15% difference to the average observed flows. Almost all locations show a good calibration to stream flows; where the Hertfordshire model does not improve on the component model, the model results are still considered good in the context of a regional Chalk model. The model was deemed fit for purpose in all but one catchment, the River Beane catchment, though groundwater levels are simulated better than the component model.