Combining unsaturated and saturated hydraulic observations to understand and estimate groundwater recharge through glacial till

Although there has been much previous research into various aspects of the flow mechanisms through glacial till, an integrated analysis of the flow system from the ground surface to the aquifer is lacking. This paper describes such an approach with reference to a detailed field study of the hydraulic processes controlling groundwater recharge through lodgement till in Shropshire, UK. A fieldsite was instrumented with tensiometers and piezometers at a range of depths through the profile, and the geology investigated in detail through field and laboratory testing. The median matrix hydraulic conductivity of the 6m thick till is found to be around 2×10⁻¹⁰ m/s on the basis of laboratory measurements. Using the barometric efficiency of the till derived from on-site pressure responses, the specific storage for the till is found to be in the range 2×10⁻⁶-6×10⁻⁶ m⁻¹ and approximately 3×10⁻⁶ m⁻¹ for the underlying Permo-Triassic sandstone, the regional aquifer. The hydraulic data indicate that till water table responses to rainfall occur during the summer period even when large tensions are present higher in the profile. This is thought to be due to preferential flow through hydraulically active fractures in the till, which were observed in a test pit dug on-site. The field evidence indicates that the fractures are usually infilled with a variety of materials derived and transported from clasts within the till. The bulk hydraulic conductivity of the till seems to be greatly enhanced by these features and it is shown on the basis of hydraulic testing and numerical modelling that the bulk hydraulic conductivity of the till is orders of magnitude greater than that of the till matrix and reduces with depth below ground surface. The paper furthers understanding of the hydraulic processes contributing to recharge through till and makes the link between the detail of these processes and simplified models of recharge estimation, which may be needed for larger scale water resource studies. The results are relevant also to contaminant migration studies and aquifer vulnerability assessments.