Electrical resistivity structure of a raised mire: Malham Tarn Moss, Yorkshire Dales, U.K

Malham Tarn Moss is a raised mire that has grown over an impermeable layer such that the shape of its water surface represents near equilibrium between accumulating rainwater and lateral drainage within the mire. With a view to seeing how the water conductivity varies within the mire accompanying this drainage pattern and whether a uniform impermeable layer likely underlies the mire, an electrical resistivity survey was carried out in 2013. The survey involved deploying a 64-electrode system along a single 315-m north-south profile adjacent to a line of samples collected in boreholes in the 1960s. The derived resistivity values decrease from >200 m at the mire surface (attributed to rainwater) down to <50 m at 10 m depth below the surface, which borehole and other geophysical data suggests is within glacial till underlying the mire. This low value would be consistent with the presence of clay minerals such as occur in glacial till and/or the presence of high solute concentrations from water reactions with rocks. The resistivity minimum varies by only 10 m laterally, suggesting a uniform composition of the sealing basal layer. Below 30 m depth, where resistivities are >1000 m, which likely represent underlying bedrock, a lateral resistivity boundary suggests a deeper geological boundary that has not previously been recorded. Within the mire, the resistivity structure is laterally uniform, with 100 and 200 m contours rising towards the northern edge of the mire by only ~1 m relative to the surface. This is surprising if the hydraulic structure were to involve internal flow balancing rainwater input in a uniform-permeability structure and if low resistivities originate from diffusion of solutes entering the mire from below. We suggest that this observation can probably be explained by non-uniform permeability, with most lateral drainage occurring near the mire surface.