Sources of stream base flow in blanket peat covered catchments

[Display omitted] •Streams draining blanket bogs have variable chemistry.•Base flow chemistry reflects substrate geochemistry.•Levels of stream water mineralisation are inversely proportional to discharge.•Hydrogeological data suggest near constant inputs of substrate water.•Variations in stream water chemistry reflect variable contributions from shallower sources. A two year hydrological characterisation programme of relatively intact blanket peat-covered catchments, underlain by geochemically distinct substrates, investigated the influence of sub-peat materials on the runoff regimes of the first order and second order streams draining them. Semi-continuous monitoring of flow and water quality revealed a strong inverse relationship between stream flow rates and specific electrical conductance (SEC), with runoff chemistries during storm flow proving comparable in all three catchments. By contrast, differences in water quality between catchments became more pronounced at lower flow, where increased ion concentrations in stream water reflected reactions with peat substrate materials. Water table monitoring in peat at all three sites revealed groundwater levels to vary by less than 0.6 m over the investigation period. Findings suggest runoff consists of near constant inputs of higher specific electrical conductance peat substrate discharge, mixing with variable proportions of less mineralised and more compositionally variable bog water. Although making up less than 5% to total runoff, the contribution of more mineralised waters plays an important role in maintaining stream flow, particularly during prolonged dry periods, and may help explain the aquatic biodiversity encountered in blanket peat covered areas.