Long-term changes in the water quality of rainfall, cloud water and stream water for moorland, forested and clear-felled catchments at Plynlimon, mid-Wales

摘要

Long term changes in the water quality ofrainfall, cloud water and stream waters draining acidic and acid sensitivemoorland and forested catchments at Plynlimon, mid-Wales,are examined for the period 1983 to 2001. Atmospheric inputs of chloride andsulphate are influenced by the relative inputs of cleanmaritime and polluted land based air masses. There is no systematic increase ordecrease over time for chloride and non-sea-salt sulphate.Rather, there is a decadal scale process possibly representative of theinfluence of the North Atlantic Oscillation that affectsthe maritime and pollution climate of the Atlantic seaboard of the UK. Over 17years of study, there may be a small decrease in non-sea-saltsulphate of about 10 μeq l-1 and a small improvement in acidneutralising capacity of about 20 to 30 μeq l-1 in rainfall. Thereis a clear improvement in cloud water chemistry withrespect to pollutant components (ammonium, nitrate, non-sea-salt sulphate) andacidity (acid neutralising capacity improved by about 300 μeq l-1) through the study period. Many of the changes in cloudwater chemistry are similar to rainfall over the sameperiod except the magnitude of change is larger for the cloud water. Within thestreams, there is some evidence for reductions in acidityas reflected by acid neutralising capacity becoming less negative. For onestream, deforestation occurred during the sampling periodand this led to large increases in nitrate and smaller increases in aluminiummidway through the study period. However, the climate andhydrological variability largely masked out other changes. The current analysisprovides only a start to identifying trends for such acomplex and variable environmental system. The need for strong statistical toolsis emphasised to resolve issues of: (a) hydrological inducedwater quality variability, (b) changing soil and groundwater "endmember"chemistry contribution to the stream and (c) the non-linear patternsof change. Nonetheless, the analysis is enhanced by examining trends inchemistry for yearly averages and yearly average low catch andhigh catch rainfall and cloud water events as well as low and high flow streamchemistry. This approach allows trends to be examined withinthe context of endmember mixing. style="line-height: 20px;">Keywords: Calcium, aluminium, ammonium, pH, Gran alkalinity, ANC, nitrate, chloride, sulphate, Plynlimon, cloud, mist, rainfall, stream, acidification, North Atlantic Oscillation, trends