Acid deposition in Wales: the results of the 1995 Welsh Acid Waters Survey

In 1995, bulk precipitation was sampled at weekly intervals from 25 collect ors located across Wales. The precipitation chemistry was dilute, acid (ove rall mean pH 4.88), and dominated by seasalts with a sodium to chloride rat io close to that of seawater. Seventy percent of the sulphate was from non- seasalt sources and the equivalent ratio of sulphate to nitrate was 1.8. Pr ecipitation chemistry varied spatially with a decrease in the proportion of seasalts but an increasing proportion of excess sulphate with distance eas twards. Seasonal maximum concentrations of seasalts occurred in the first 3 months of the year whilst maximum excess sulphate, nitrate and ammonium co ncentrations were observed during the summer months. Maps of modelled wet p lus dry deposition at 20-km square resolution showed that the acid-sensitiv e uplands of Snowdonia, the Cambrian mountains and the Brecon Beacons recei ved large inputs of excess sulphur (15-25 kg S ha(-1) year(-1)). Total inor ganic nitrogen deposition to much of Wales was between 20 and 25 kg N ha(-1 ) year(-1). In 1995, Wales received 32 ktonnes of excess sulphur and 42 kto nnes of inorganic nitrogen equivalent to 9 and 11%. respectively, of the UK total deposition (based on 1992-94 figures). Nitrogen deposition contribut ed 60% of the total acidifying input to Wales. Since the first survey in 19 84, the overall mean total sulphate concentration in rainfall has declined by about 28% to 2 mg litre(-1); comparable to changes at long-term monitori ng sites. Nitrate concentrations also declined by 18% to 0.23 mg N litre(-1 ) whilst there was no statistically significant change in ammonium concentr ations. Between 1984 and 1995, wet deposition of total sulphur to Wales dec reased by 22% from 36 to 28 ktonnes, whilst wet deposited inorganic nitroge n declined from 27 to 22 ktonnes. These intensive regional data from the fa r west of Europe provide valuable insight into current deposition trends an d the relative importance of sulphur and nitrogen deposition to acidificati on. (C) 1999 Elsevier Science Ltd. All rights reserved.