Mar 8, 2018 - Low Flows and Abstraction Management. Thursday ... water supply boreholes in this area) is at the expense
South East Section
Low Flows and Abstraction Management Thursday 8th March 2018 6.30pm to 8.00pm The Institution of Civil Engineers, One Great George Street, London (Nearest tube: Westminster)
The Chiltern Chalk Streams and Groundwater Abstraction – Can this relationship survive? Dr Ilias Karapanos, Affinity Water This presentation discusses the impact of groundwater abstraction from public water supply boreholes on Chalk rivers across the Chilterns. Modflow groundwater models have been developed by the Environment Agency, based on the original Alleviation of Low Flows (ALF) projects of the late 1980’s and conceptual understanding established over 25 years of investigation work following on from various low flow studies. These groundwater models represent the Chalk as a single layer aquifer with the addition of varying vertical and horizontal hydraulic conductivity to allow for heterogeneity. In some locations, the river bed conductance has been altered to allow for varying surface water/groundwater interactions in specific reaches based on field measurements. This supports the assumption that the groundwater abstracted (mainly from the public water supply boreholes in this area) is at the expense of river flows under all background groundwater level conditions. This led to the implementation of an abstraction reduction programme over the last couple of decades which is still ongoing. Following the implementation of these abstraction reductions aiming to leave more water in the environment and benefit the chalk streams especially under low flow conditions, it has been observed so far that there is very little to no benefit at all during low flow conditions. This has led to further data interrogation and analysis with the inclusion of groundwater level data from more recent nested piezometers in the Chalk. However, other factors, such as changing rainfall patterns and land use influencing the amount and location of recharge, river modifications, re-alignment and flood management also have an influence on river flows and their resulting ecology, have not been taken into account. A few case studies across the River Ver are presented, using water level measurements in observation boreholes installed at different depths in the chalk aquifer and compared to river flow data at the catchment gauge. Following changes in abstraction patterns at the public water supply boreholes under different background groundwater level conditions, it is clear that there is dual piezometry in the Chalk aquifer in at least two locations. This triggered a further geological study to bring together all the geological and hydrogeological evidence required to ascertain the dual piezometry effect. The outcome of this study shows that there is geological control in the New Pit Chalk units with the marl seams and the hard bands controlling the leakage between the various Chalk layers. This new conceptual understanding has led to targeted investigation that will prove the spatial extent of this phenomenon with the aim to change the conceptual model of how the Chilterns Chalk has to be simulated. Once data is collected, this may lead to the redefinition of the CAMS strategy in this area, which will in turn influence water resources position in the future.
Details of second speaker to follow There is no charge to attend this meeting and both BHS members and non-members are welcome. No registration is required. Enquiries: Victoria Crellin, Tel.: (01372) 756813 Email:
[email protected] 