Runoff Chemistry at a Mini-Catchment Watered with “Unpolluted Precipitation”

Abstract

600 mm of artificial, “unpolluted” precipitation with a chemical composition of assumed preindustrial quality ([H+] = 9 μeql−1, [SO4] = 11 μeql−1) was sprayed on a “mini-catchment” (98 m2) over a period of 3½ weeks. The mini-catchment is a granitic bowl partially covered with shallow, acid soils. It is located in the Storgama area, Telemark county, southern Norway, an area which today receives acidic precipitation ([H+] = 54 μeql−1, [SO4] = 59 μeql−1). The treatment resulted in lowered values for [H+], [Al] and [SO4] in runoff within a week. Excluding the spring snowmelt, [H+] ≈ 50 μeql−1 (pH = 4.3) under natural conditions, whereas during the experiment [H+] levelled off at 30 μeql−1 (pH = 4.5). Correlation analysis suggests that SO4 and organic anions are related to the runoff acidity. The level of total organic carbon in runoff was, however, elevated compared to undisturbed conditions. Under the present natural conditions S04 is the most important anion. The amount of water-extractable sulfate in the soil is equivalent to about ½ of the present yearly input. The response in the sulfate output to a reduced input is difficult to predict because of processes which transform sulfur to and from various compounds in the soil and vegetation. Several years may be required before a new steady state is reached after a substantial decrease in input. Long-term experiments in which the catchment is shielded against natural precipitation are required to obtain more certain estimates of the response in runoff composition.