Factors affecting the chemistry of precipitation and river water in an upland catchment

Abstract

The chemistry of precipitation and river water was studied for one year in Glendye, a 41 km 2 moorland catchment in northeast Scotland. The precipitation was very dilute, weakly acidic, and highly variable in composition. River water was much less dilute, neutral, and less variable. Factor analysis was used to investigate the controls on water chemistry. This suggested three main processes affecting precipitation: aerosols of oceanic spray, which affected sodium, magnesium, chloride and total organic carbon (TOC) concentrations; emission of gaseous sulphur and nitrogen oxides from industrial processes and the burning of fossil fuels, which affected pH; wind-blown terrestrial dust. The factors affecting river water are quite different. The first factor represents cation-exchange and weathering reactions in the soil and affects calcium, magnesium, sodium, bicarbonate and silicon concentrations. The second factor affects the concentrations of iron, TOC, manganese and aluminium and represents the translocation of these elements down the soil profile and into the river at times of high discharge. The third factor affects the concentrations of ammonium and nitrate and reflects nitrogen demand and mineralisation in the soil. Phosphate, sulphate, potassium and chloride appear to vary independently, but all show low variability in river water compared with precipitation. The chemistry of river water from the catchment was also investigated during two storm events, and the results support the grouping of variables produced by factor analysis. The chemistry of the river water is thus controlled by processes in the soil, suggesting that nearly all the river water originates within the soil, and that direct surface runoff is of minor importance.