Spatial and temporal variations in critical loads for rivers in NE 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 wat er from a lake, from each 10 km square. If no lake is present, as was the c ase for many squares in the north east of Scotland, a sample from a river t hought likely to be most sensitive to acidification was used. This study ev aluates the effectiveness of this approach for protecting all the waters of the complex river network of a major catchment in N.E. Scotland, under hig h and low Row regimes. Samples were collected and analysed over a period of one year, at intervals of two weeks, from 59 sub-catchments of the River D ee catchment. Critical load values were calculated for every sample, and al so using mean annual concentrations of Na (+)(,) Ca2 + Mg-2 (+), K (+) and Cl-. In addition, to investigate discharge effects, they also were calculat ed for each site for base and high flows, using stream water chemistry corr esponding to the three lowest and the three highest discharge levels respec tively. Only two rivers were insufficiently protected under base flow condi tions, whereas 20 apparently were not sufficiently protected under high Row conditions. It is concluded that the system used in the UK is generally ve ry effective for protection of rivers, but only under low flow conditions. More work is needed to assess the biological impacts of episodic critical l oad exceedance in upland rivers. A more recently suggested approach for cla ssifying UK river water sensitivity to acidification from the Na (+) domina nce of the solute base cations in water samples is also shown to be effecti ve for this river system. (C) 2000 Elsevier Science Ltd. All rights reserve d.