Chemical weathering and runoff chemistry in a steep headwater catchment

We present here deductions about the location, rate, and mechanisms of chem ical weathering in a small catchment based on a catchment-scale sprinkling experiment. In this experiment demineralized water was applied at an approx imately steady rate in the CB1 catchment in the Oregon Coast Range to reach and maintain a quasi-steady discharge for a period of 4 days. Because of n early steady flow conditions within the catchment, the contribution to solu te fluxes from soil and bedrock could be partitioned. One half of the solut e flux from the catchment derived from colluvial soil, and one half from we athering in bedrock. This implies more intense weathering in the thin collu vium mantling the catchment than in the thick underlying weathered bedrock. The annual solute flux from the catchment, scaled to the annual runoff fro m the catchment is 32 +/- 10 t km(-2) year(-1), equivalent to published che mical denudation rates for nearby rivers with drainage areas 10(6) times gr eater than the experiment site. Soil waters sampled during the sprinkling e xperiment had steady compositions following a period of transient water flo w conditions, implying steady-state chemical evolution in the soil. The wat ers leached 'organic' anions from shallow depths in the soil, which solubil ized aluminium and iron, indicating that podzolization is occurring in thes e soils. Carbonate dissolution appears to be an important source of solutes from the bedrock, despite being present as only a minor phase in the rock. Water balance suggests that the residence time of water in the catchment i s about 2 months, and that typical 24 h storms displace only a fraction of the stored water. A consequence is that runoff chemistry is dominated by ol d water, which imposes strong limits on the variability of runoff compositi on. Copyright (C) 2001 John Wiley & Sons, Ltd.