| Long term changes in the water quality of
rainfall, cloud water and stream waters draining acidic and acid sensitive
moorland and forested catchments at Plynlimon, mid-Wales,
are examined for the period 1983 to 2001. Atmospheric inputs of chloride and
sulphate are influenced by the relative inputs of clean
maritime and polluted land based air masses. There is no systematic increase or
decrease over time for chloride and non-sea-salt sulphate.
Rather, there is a decadal scale process possibly representative of the
influence of the North Atlantic Oscillation that affects
the maritime and pollution climate of the Atlantic seaboard of the UK. Over 17
years of study, there may be a small decrease in non-sea-salt
sulphate of about 10 μeq l-1 and a small improvement in acid
neutralising capacity of about 20 to 30 μeq l-1 in rainfall. There
is a clear improvement in cloud water chemistry with
respect to pollutant components (ammonium, nitrate, non-sea-salt sulphate) and
acidity (acid neutralising capacity improved by about 300 μeq l-1)
through the study period. Many of the changes in cloud
water chemistry are similar to rainfall over the same
period except the magnitude of change is larger for the cloud water. Within the
streams, there is some evidence for reductions in acidity
as reflected by acid neutralising capacity becoming less negative. For one
stream, deforestation occurred during the sampling period
and this led to large increases in nitrate and smaller increases in aluminium
midway through the study period. However, the climate and
hydrological variability largely masked out other changes. The current analysis
provides only a start to identifying trends for such a
complex and variable environmental system. The need for strong statistical tools
is emphasised to resolve issues of: (a) hydrological induced
water quality variability, (b) changing soil and groundwater "endmember"
chemistry contribution to the stream and (c) the non-linear patterns
of change. Nonetheless, the analysis is enhanced by examining trends in
chemistry for yearly averages and yearly average low catch and
high catch rainfall and cloud water events as well as low and high flow stream
chemistry. This approach allows trends to be examined within
the context of endmember mixing.
Keywords: Calcium, aluminium, ammonium, pH, Gran alkalinity, ANC, nitrate,
chloride, sulphate, Plynlimon, cloud, mist, rainfall, stream, acidification,
North Atlantic Oscillation, trends |