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

Abstract

In 1995, bulk precipitation was sampled at weekly intervals from 25 collectors located across Wales. The precipitation chemistry was dilute, acid (overall mean pH 4.88), and dominated by seasalts with a sodium to chloride ratio 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. Precipitation chemistry varied spatially with a decrease in the proportion of seasalts but an increasing proportion of excess sulphate with distance eastwards. Seasonal maximum concentrations of seasalts occurred in the first 3 months of the year whilst maximum excess sulphate, nitrate and ammonium concentrations were observed during the summer months. Maps of modelled wet plus dry deposition at 20-km square resolution showed that the acid-sensitive uplands of Snowdonia, the Cambrian mountains and the Brecon Beacons received large inputs of excess sulphur (15–25 kg S ha −1 year −1). Total inorganic 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 ktonnes of inorganic nitrogen equivalent to 9 and 11%, respectively, of the UK total deposition (based on 1992–94 figures). Nitrogen deposition contributed 60% of the total acidifying input to Wales. Since the first survey in 1984, the overall mean total sulphate concentration in rainfall has declined by about 28% to 2 mg litre −1; comparable to changes at long-term monitoring sites. Nitrate concentrations also declined by 18% to 0.23 mg N litre −1 whilst there was no statistically significant change in ammonium concentrations. Between 1984 and 1995, wet deposition of total sulphur to Wales decreased by 22% from 36 to 28 ktonnes, whilst wet deposited inorganic nitrogen declined from 27 to 22 ktonnes. These intensive regional data from the far west of Europe provide valuable insight into current deposition trends and the relative importance of sulphur and nitrogen deposition to acidification.