Spatial and temporal variations in critical loads for rivers in N.E. Scotland: a validation of approaches

Previously, critical load maps for UK surface freshwaters have been created by applying the steady state water chemistry model to a spot sample of water from a lake, from each 10 km square. If no lake is present, as was the case for many squares in the north east of Scotland, a sample from a river thought likely to be most sensitive to acidification was used. This study evaluates the effectiveness of this approach for protecting all the waters of the complex river network of a major catchment in N.E. Scotland, under high and low flow regimes. Samples were collected and analysed over a period of one year, at intervals of two weeks, from 59 sub-catchments of the River Dee catchment. Critical load values were calculated for every sample, and also using mean annual concentrations of Na +, Ca 2+, Mg 2+, K + and Cl −. In addition, to investigate discharge effects, they also were calculated for each site for base and high flows, using stream water chemistry corresponding to the three lowest and the three highest discharge levels respectively. Only two rivers were insufficiently protected under base flow conditions, whereas 20 apparently were not sufficiently protected under high flow conditions. It is concluded that the system used in the UK is generally very effective for protection of rivers, but only under low flow conditions. More work is needed to assess the biological impacts of episodic critical load exceedance in upland rivers. A more recently suggested approach for classifying UK river water sensitivity to acidification from the Na + dominance of the solute base cations in water samples is also shown to be effective for this river system.