Investigation into the physical and chemical characteristics of the Abbey Arms Wood observation borehole, Delamere, Cheshire

In March 2002, a 150-m borehole was drilled in Abbey Arms Wood, Delamere, Cheshire UK. It was located in an embayment of the outcrop of the Helsby Sandstone Formation (part of the Sherwood Sandstone Group) and was cored throughout its entire length. The area is known to give rise to relatively high arsenic (10–50 μg L–1) groundwaters. The aims were (i) to provide a better understanding of the geology of the area in order to provide the Environment Agency and United Utilities with an improved understanding of the local hydrogeological conditions, and (ii) to determine the location and nature of possible sources of the arsenic in the local groundwater and to determine whether any engineering solutions could lead to a mitigation of the arsenic problem, e.g. by screening out high source zones. The results indicated that a perched water table existed at 26 m bgl and the main water table was at about 40 m. The underlying Wilmslow Sandstone formation was found at 48.9 m and below. The physical properties of the cored sandstone were constant throughout its length (typical porosity of 0.19–0.21; median plug permeability of 1000 mD). Flow logging indicated a major flow horizon at 105 m while CCTV and the caliper log indicated two large cavities in a similar position in the borehole. The chemical composition of the sediments was also quite uniform although some systematic differences with depth were noted. There was a somewhat greater concentration of elements associated with clay minerals (e.g. illite) in the Wilmslow Sandstone Formation compared with the Helsby Sandstone Formation. The top 50 m of the Wilmslow Sandstone also had a low Ca content, suggesting the absence of calcite. The arsenic content of the sediments ranged from 5–15 mg kg–1 and averaged 8 mg kg–1 (n = 60) and showed no trends with depth. Porewater spun from the core showed a steadily increasing trend in As concentration with increasing depth. This ranged from 8 μg L–1 at 10 m (unsaturated zone) to 30 μg L–1 at 150 m (saturated zone). There was no convincing evidence for specific sources of As or other types of mineralization. It appears that the high-As groundwaters cannot be avoided by screening out high-As zones. Restricting intakes to the uppermost parts of the saturated zone may reduce As concentrations but will reduce yields and may risk higher nitrate concentrations. The analyses of water samples referred to in this report were not carried out for the purposes of assessing the quality of water from each of the boreholes tested for drinking, cooking or other domestic, agricultural, or industrial purposes and do not amount to certification of potability in respect of groundwater in the region. If such information is required, specific tests should be carried out for this purpose.