Baseflow buffering of streamwater acidity in five mid-Wales catchments

Abstract

Streamwater pH and concentrations of calcium, bicarbonate and aluminium have been monitored in two forest and three grassland catchments at the Plynlimon area of upland mid-Wales. Baseflow in all the streams is characterised by waters bearing calcium and bicarbonate and which are low in hydrogen ions and aluminium. Stormflow shows more complex chemical variation. Stormwaters are acidic, low in calcium and bicarbonate, and contain aluminium; except in the stream draining the mineralised grassland catchment. At this latter site, streamwater pH is relatively high and constant, even at high flows, and aluminium is not detected. Limited buffering of streamwater acidity is also observed in the streams draining the mineralised forest catchment and the agriculturally improved grassland catchment. There is little difference in the means and ranges of acidity between the stream draining the unmineralised part of the forest and the stream flowing from the unimproved unmineralised semi-natural grassland catchment. Mean concentrations of aluminium are almost three times higher in the forest stream. The streamwater data are discussed in relation to the chemistry of soil waters and vegetation canopy throughfall in the forest and grassland systems. The significance, to water quality, of the hydrological pathways operating in the two systems is also examined. The study shows that the presence of CaCO 3 in the bedrock, even on a small scale, and liming of the land in the catchment can both have a marked effect on stream water acidity. This result is important when remedial action in areas subject to catchment acidification is considered and demonstrates that it is difficult to generalise about catchment responses to acidic atmospheric inputs. The results also show that for catchments with similar soils and bedrock geology, those under coniferous forest do not necessarily have more acid streamwater than those under semi-natural grassland vegetation. Aluminium concentrations in the stream are not simply determined by Al(OH) 3 solubility controls as streamwater concentrations of aluminium for the semi-natural grassland catchment are only one third those for the forest catchment, even though soil types are the same.