The geochemistry of a small upland catchment in north-east Scotland
A study has been made of a 2 km2 catchment at Peatfold, Glenbuchat in north-east Scotland. The catchment is underlain by basic igneous rock, predominantly quartz-biotite-norite, which has been deeply weathered in Tertiary times and subsequently affected by glacial and periglacial processes such as solifluction of glacial drift. Vegetation is predominantly heather moorland. The chemistry and mineralogy of the soils and parent material was investigated in detail. Precipitation and river water chemistries were studied through a one year period, which involved collection of weekly and monthly precipitation samples and regular weekly and storm river water samples. River samples were analysed for pH, sodium, potassium, calcium, magnesium, chloride, sulphate, phosphate, nitrate, nitrite, ammonia, bicarbonate, iron, aluminium, manganese, silicate and total organic carbon. Precipitation was acidic (mean pH 4.3), dilute (mean total dissolved solids 13-7 mg 1-1) and dominated by hydrogen ion, sodium, chloride and sulphate. Its composition varied greatly, but distinct seasonal trends as a result of varying influence from oceanic, industrial and terrestrial sources were observed. River water was neutral (mean pH 7.0), more concentrated (mean total dissolved solids 70 mg 1-1) and less variable in composition. The latter was dependent upon hydrology with calcium, magnesium, silicate and bicarbonate increasing in concentration at low discharges and hydrogen ion, potassium, transition metals and total organic carbon increasing in concentration at higher discharges. Since hydrology has influenced soil type, variations in river composition related to soil type being drained could be detected. Seasonal trends in composition were also observed. Inputs in precipitation and outputs in river water were calculated for all species for one year. All species had a net output except hydrogen ion, nitrate, nitrite, ammonia, sulphate and phosphate. Outputs of silicon (32 kg ha-1), calcium (22 kg ha-1) and bicarbonate (114kg ha-1) were large. The large net output of chloride (35 kg ha -1) indicates the importance of dry deposition of aerosols of oceanic spray. When this is allowed for, the calculated rate of outputs of elements due to weathering in kg ha-1 is: Si > Ca > Mg > Na > K > Fe > Al.