Characterisation of sedimentary structure and hydraulic behaviour within the unsaturated zone of the Triassic Sherwood Sandstone aquifer in North East England

Abstract

A study of the sedimentological framework and permeability characteristics of the Sherwood Sandstone has been undertaken together with a detailed investigation of moisture migration in the vadose zone at a single field site. Sedimentary structure and likely permeability variations were studied by use of laboratory grainsize analysis, logging of nearby outcrops, borehole geophysics and Ground Penetrating Radar (GPR). GPR is ideal for vadose zone hydrogeological applications as the majority of the features imaged are visible to radar as a result of variations in capillary, held moisture, and the amount of capillary retention is controlled by the size of the pore throats in the sediments, which directly influences their permeability properties. Combined use of GPR, reconstruction of sedimentary facies and quantification of permeability characteristics has provided detailed 3D models of the sedimentary subsurface. Time Domain Reflectometry (TDR) was used to monitor water movement within the Sherwood Sandstone at a site near Selby in Yorkshire, creating a vertical and lateral profile of groundwater movement within the unsaturated zone. Moisture content has also been monitored using a neutron probe, and a commercially-available portable packer system, which have provided verification of the accuracy of the custom-made TDR system. The TDR installations consisted of automated arrays of TDR probes, permanently installed upon borehole packers at varying depths, and these have provided moisture content data of a high temporal resolution. The TDR system has allowed monitoring of seasonal moisture variation under natural rainfall loading, and the results have been interpreted in order to gain a better understanding of groundwater migration at a different scale to data previously available. The bulk of the rock in the Sherwood Sandstone aquifer study area consists of relatively permeable medium-grained sandstones. However, results suggest that vertical flow in the unsaturated zone may be impeded by the presence of relatively impermeable fine sandstone units, which correspond to bar top and slack water environments, and occasional mudstone layers representing overbank deposits. This restriction to vertical flow may cause localized perched aquifer conditions, which provide sufficient hydrostatic head to initiate horizontal migration in the overlying rock. Modelling of real rainfall events suggests that 25% of the water present in the perched aquifer layers undergoes lateral or bypass flow (i. e. it drains laterally rather than through the fine-grained layers). In the saturated zone, the horizontal flow of groundwater in the Sherwood Sandstone aquifer is likely to be dominantly via the relatively coarse, trough-stratifed sandstone layers, so that a low proportion of the total aquifer porosity may provide a route for rapid contaminant transport