Hydrologic control of chemical disequilibria in soil and surface waters, Sogndal, Norway

Abstract

Two small (96,290 and 7,220 m 2) gneissic bedrock, alpine watersheds with thin (< 50 cm) Haplumbrept soils were used to quantify the role of antecedent soil moisture conditions in fractionating unsaturated zone flow. Soil solutions were collected with 66 lysimeters of two types, one proven to collect primarily gravitational (macropore) flow and one proven to collect capillary (micropore) flow. Irrigation experiments over a 4-year period used 30-mm events. The manipulated catchment received pH 3.4 water, acidified with sulfuric acid and LiBr tracer, while the control catchment was irrigated with unacidified water with LiBr. Waters were applied only to the terrestrial parts of the watersheds and not directly to the lakes. Antecedent soil moisture conditions ranging from 18 to 53 wt% water produce significant variations in soil solution flowpath in both watersheds. These variations, in turn, create significant differences in the compositions and relative volumes of macropore, micropore and surface waters. Tracer results from the manipulated catchment demonstrate that soil waters reach the lake sooner under the driest conditions and that flows are restricted to the upper 20 cm of soil under wetter conditions. Under progressively drier conditions, the pH of all soil waters (macropore and micropore, shallow and deep) decreases, while inorganic monomeric Al (IMAL), Ca and sulfate all increase in deep soil waters, but remain relatively constant in shallow waters. Saturation indices for aluminum hydroxide phases are constant, averaging −0.14 ± 0.11 fro microcrystalline gibbsite, only in shallow soil solutions. Deeper macropore and micropore solutions show increasing undersaturation with drier conditions. Drier antecedent conditions increase the importance of macropore flow, reduce the amount of flow along the organicmineral soil interface, decrease the contact and residence time of soil waters resulting in a faster release to lakes of more acidic waters with higher IMAL concentrations. Thus, precipitation events occurring during prolonged dry periods may create more toxic conditions than those observed during the “acid slug” events during early spring runoff.