Hydrological pathways and solute chemistry of storm runoff at Dargall Lane, southwest Scotland

Abstract

The hydrology and hydrochemistry of an acidic moorland catchment is described on the basis of the results obtained from field observations and intensive sampling of surface water and rainwater during selected storm events. At runoff, the prevailing input of soil water of low neutralizing capacity explains the usually low pH condition of stream water (mean pH 4.84). In addition to the high levels of sulphate (up to 7040 ppb), the chemistry of runoff is dominated by Na (mean 3437 ppb) and Cl (mean 6627 ppb). The mean concentrations of Ca (523 ppb) and Mg (521 ppb) show the low buffering capacity of the catchment. The main short-term effect of storms is a dilution of the pre-storm stream concentration of major cations. Only K levels increase at higher flow. Despite the flashy runoff, the catchment is capable of delaying stream water changes of solute chncentration, especially in the case of Cl. Although good correlation were found between most cation ppairs in stream water, a poor correlation was found between Na and Cl. Cation exchange mechanisms operating within the organic soil horizons are likely to be the main control of surface water chemistry during runoff.A chemical separation technique based on the conservative tracer acid neutralization capacity is used to separate the hydrograph into ȁsoil waterȁ and ȁdeep waterȁ components. It provides a good approximation to the hydrochemical processes prevailing during storm episodes.