Mitchell and Boult
FSC Malham Tarn
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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 Ωm at the mire surface (attributed to rainwater) down to 1000 Ωm, 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 Ωm 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.