PH, ALUMINUM AND CALCIUM CONCENTRATIONS OF STREAMS DRAINING ACIDIC SOILS ON CALCAREOUS TILL AT KERSHOPE, CUMBRIA

The pH of a forested stream draining acidic soils on calcareous till w as measured at frequent intervals along its upper 3 km on several occa sions covering a range of flows. The drainage ditches and stream headw aters had a pH close to 4.0 whatever the flow rate. The pH of the main stream rose for the first 0.5 km and remained fairly constant thereaf ter. This constant pH value was 7.5, 6.0 and 5.5 for low, intermediate and high flow rates, respectively, but was about 4.9 on two occasions of high flow rate. In two tributaries the pH increased with distance downstream. The calcareous till was the major influence on stream chem istry except during floods when the acidic soil had a stronger influen ce. Correlations between pH and log(flow) ranged from -0.7 to -0.9 exc ept at some points at the head of the stream which had weaker correlat ions. Flow rate exerted a small influence on pH (e.g. pH = 4.50 - 0.23 3 log(flow)) near the head of the stream and a large influence on it ( e.g. pH = 8.22 - 1.149 log(flow)) further down. The aluminium concentr ation of the stream water fell from 0.6 to 1.0 mg l-1 in the drainage ditches to less than 0.4 mg l-1 where the stream left the forest. The calcium ion concentration increased from less than 5 mg l-1 in the dit ches to around 10 mg l-1 where the stream exited. On one occasion of v ery high flow, however, calcium concentration remained around 4 mg l-1 along the sampled section of the stream.