The Vowchurch gravel aquifer pipeline crossing : hydrogeological evaluation of impact

The Brecon to Tirley gas pipeline includes a crossing of the River Dore, its flood plain and associated shallow gravel aquifer at Vowchurch in Herefordshire. The aquifer is used as a public supply resource and concern has been raised regarding the likely impact of the pipeline on the aquifer and the integrity of the public supply boreholes. A detailed assessment has been undertaken to identify the likely impact on the water table and hence the integrity of the abstraction borehole performance, which: 1. investigates the hydrogeological impact of the pipeline during construction, and 2. investigates the post-construction long-term effect of trenching and pipe laying. A detailed evaluation of the geological framework, in which the gravel aquifer is a part, showed that the aquifer could receive direct rainfall recharge through the overlying granular till and fine-grained overbank deposits. A conceptual groundwater flow model was created which identified that secondary recharge was taking place from the River Dore in the upper part of the aquifer with loss back to the river at the lower end of the aquifer. All available data on the aquifer, surface water, meteorological data and land use information were gathered together and used to assess and quantify the recharge processes in the catchment. Numerical modelling of the original pumping test data at Vowchurch yielded formation constants for the aquifer. These data were then incorporated into a numerical groundwater flow model using the object oriented ZOOM suite of modelling software to replicate the available historical data for the aquifer. The best fit model has been used to run a variety of ‘what if’ scenarios. The effect of drawing the water table down with a well point system during construction, to maintain a dry 2m deep trench, will cause a likely drop in the groundwater head at the Vowchurch pumping station of up to 0.5 m. This head reduction will be apparent from about one day after the well point system is switched on until the pipe is laid and the trench backfilled some two to three days later. The trench will only need to be dewatered in the 200 m section nearest to and west of the River Dore. Should any turbidity be created in the aquifer by trenching it is likely to be lost as the groundwater travel time from the trench area to the pumping station is at least 100 days. The impact of the completed pipeline on groundwater flow and groundwater heads is minimal. The damming effect of the pipeline on the water table is very small, as is the effect of varying the permeability of the backfill material. Although the relationship with the River Dore will be very slightly modified, that with the Slough Brook remains unchanged as it is perched in the vicinity of the pipeline. The river crossing is through overbank silts and will neither penetrate the till nor the gravel aquifer. It is not expected to impact the aquifer. Trenchless crossings of either the River Dore or the Slough Brook would involve a risk of penetrating the aquifer and, therefore, pose a risk of contaminating it. The trench option for both crossings thus posses the least risk to the aquifer.