A Novel Method for Mapping Critical Loads Across a River Network: Application to the River Dart, Southwest England

Abstract

To date, estimates of freshwater critical loads have beenbased on a single sample site within a given area, in theUK the `most sensitive' surface water in each 10 km gridsquare. The critical loads obtained are thus highlydependent on the sites chosen, and at a relatively coarsespatial resolution. To produce a higher resolutioncritical load assessment, the PEARLS (Prediction ofAcidification and Recovery on a Landscape Scale)procedure has been used to estimate critical loads acrossa large (248 km2), partially acid-sensitivecatchment in Southwest England. PEARLS utilises availablesoils and land-use databases, and sampled streamchemistry data, to derive characteristic runoffcompositions for a set of landscape types. Mixingequations are then used to calculate runoff chemistry,and subsequently critical loads, throughout the streamnetwork. Results show major spatial variability, withcritical loads lowest in streams draining peat-moorlandheadwaters, and generally increasing downstream asagricultural land contributes an increasing proportion ofrunoff. The 5th percentile freshwater critical loadfor the catchment is estimated at 0.29 keq H+ ha-1yr-1,and critical loads are exceeded for around 40% of totalstream length. The PEARLS methodology provides a novelopportunity to assess the spatial variability infreshwater critical loads, and to provide estimates ofexceedance at whole catchment scale. It has potentialapplication in the assessment of surface watersensitivity to acidification across wider areas in the UKand elsewhere.