Input/export relationships of major ions in west Wales catchments

Abstract

Relationships between major ions entering and leaving catchments in west mid-Wales were examined. Data were obtained on inputs in rainfall and aerosol salt, the chemistry of upland streams draining unimproved hill pasture, and changes in the concentrations of ions in two rivers from their sources in the Plynlymon range to the west coast. Sea-derived salt is the dominant source of Na and Cl and weather conditions exert a major influence on the salt concentration in input water. On one occasion salt transported 15 km inland by westerly gales raised the concentrations of Na and Cl in throughfall in coniferous woodland to around 25 times that in average rainfall. The concentrations of all major ions increased from source to sea in the Rivers Leri and Ystwyth. The greatest increases were in Ca and Mg and although chlorite in the soil parent material is a significant source of Mg, about three-quarters of the Ca and half of the Mg in rivers at lowland sites originates from magnesian limestone applied as a liming material in agriculture. Losses of Ca from catchments were calculated to be much greater than the current Ca inputs through liming. Only 20–25% of the input of SO 4-S onto catchments is sea derived. There was no evidence for either the net accumulation or release of S from soils of the area.