Impacts of a mature forestry plantation on blanket peatland runoff regime and water quality

A lack of information concerning the hydrology and hydrogeology of intact blanket bogs limits current understanding of how their alteration to mature forestry plantations impacts stream flow and associated water quality. An integrated hydrological/ hydrogeological monitoring programme compared processes operating in a relatively intact blanket peat-covered catchment with conditions encountered in an adjacent area under closed canopy plantation forestry. Groundwater monitoring revealed contrasting water level regimes and deeper summer water tables in the afforested area, with forest groundwater also having more elevated specific electrical conductance (SEC) and containing higher concentrations of dissolved organic carbon (DOC). Near-simultaneous pairwise runoff sampling at the relatively intact catchment and afforested catchment outlets demonstrated no significant difference in DOC concentration. Conversely, water samples from the afforested catchment outlet displayed significantly greater SEC; this arose in part because of higher concentrations of dissolved calcium and magnesium, discharging via artificial drainage. Comparison of base flow runoff SEC with peat groundwater samples revealed significant contrasts in ionic signature and greater levels of mineralisation in surface water, pointing to contributions of deeper water, derived from inorganic substrate materials. Study findings indicate that disturbance to the ground in that part of the catchment under plantation forestry has led to greater variations in hydrological supporting conditions for aquatic ecosystems. Comparable conditions have been observed instreams flowing through plantation forestry in similar physical settings elsewhere. Study findings suggest that plantations on deep peat can adversely stream water quality and this may impact on a water body’s Water Framework Directive status.