The effects of gravel extraction on groundwater hydrology
The study which was carried out around gravel pits at Stanton Harcourt (Oxfordshire) and Rinwood (Hampshire) falls into three sections. The first involves an analysis of the groundwater character- istics of the gravels. This includes an estimation of the permeability of the gravels (using single-well dilution methods) and an analysis of the recharge mechanisms operating in them. The latter is based upon observed relationships between groundwater fluctuations and hydrometeorological factors. Secondly, the hydrological effects of gravel extraction and dewatering are analysed. By monitoring groundwater levels, the nature and extent of the zone of drav/dcvm around the gravel pits is determined. Induced recharge from rivers and recirculation of water from surface ditches into the pits, as a result of dewatering, is identified. Estimates of the proportion of induced recharge in groundwater are then made using chemical evidence. A mathematical model of gravel pit dewatering is developed. The influence of hydraulic conductivity and the initial pumping rate in determining the effects of dewatering is established by a series of sensitivity analyses using this model. As a practical example, the model is used to predict the effects of dewatering a new site near Stanton Harcourt. The extent of the drawdown around each stage in its development is determined, and this is used to evaluate the effects on private sources and agriculture in the surrounding area. Thirdly, the effects of one type of rehabilitation project (i.e. lake formation) are investigated. A method for predicting the final lake level is explained. Deviation of the observed lake levels from those predicted is explained in terms of the sealing effect of fine sediment plus chemical and biological processes. The effects of lake formation on groundwater levels and flow are described, and the changes in water quality produced by the flow of groundwater through a lake are examined.