Preliminary analysis of water and solute movement beneath a coniferous hillslope in Mid-Wales, U.K.

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Streams draining coniferous forests are often loaded with solutes such as hydrogen ion, sulphate, nitrate and aluminium. As a result, fish populations can be reduced and water quality may fall below recommended potable standards. The transport of ions into water-courses is governed by the movement of water. Within most temperate and tropical areas the stream discharge and chemistry, during periods of rapid runoff, is dominated by the exfiltration of water and solutes from stream-side soils. The movement of water to stream-side or ‘riparian’ areas remains, however, an enigma. This paper attempts to explain how the riparian area might be rapidly recharged during storm events. Two analytical techniques, the free-surface method and tangent-continuity method, are applied to hydrological properties monitored on a steep coniferous hillslope, during a selected storm event. Comparison of the ionic concentrations of waters within each component of the hydrological system, is used to verify the hydrological analysis. Perched water-tables developed within the basal zones of the O/Ah and Eag soil horizons of the steep podzolic hillslope, during all major storm events. Most of the rapid response within the riparian zone could be explained by lateral flow in these near-surface soil horizons, particularly in the saturated basal zones. This pathway is corroborated by the similarity of riparian zone and near-surface (or topsoil) chemistries. Relatively low concentrations of monomeric aluminium and relatively high concentrations of chloride, sodium and hydrogen ion were observed within these zones, compared with the subsoil (Bsl and B/C) horizons.