Short-term ionic responses as indicators of hydrochemical processes in the Allt a' Mharcaidh catchment, western Cairngorms, Scotland

Abstract

An attempt to identify chemical controls in a transitional catchment in the Cairngorm region of Scotland was made using data from two distinct sampling regimes. The first was based on twice-weekly dip samples with associated flow conditions and the second was based on 1-2h time-series sampling during hydrological events. Annual mean data from spot samples provided general information on alkalinity sources and acidification status, and flow/ion relationships suggeste a three-phase chemical response within the flow profile. Alkalinity and organics appeared to show the greatest response to flow and sulphate the smallest. Information collected from time series sampling showed significant differences in the chemical response on the rising limb of the hydrograph compared with that on the falling limb, particularly for pH and alkalinity. In the first, low-flow, response phase, the chemical changes are attributed to a simple dilution process with little or no influence from surface soil horizons. During the second and more complex intermediate flow phase, the chemical response is compatible with wetting and mixing processes in the soil horizons resulting in a more variable relationship between flow and pH/alkalinity. The delayed return of alkalinity, base cations and organics, during flow recession, to pre-event levels, indicates a reservoir of acid/organic-rich water which, until depleted, contributes a major fraction of the output water. In the third, high-flow phase, a state of chemical equilibrium appears with little or no change in ion concentrations for large changes in flow. This insensitivity reflects a relatively saturated system with established of major hydrological pathways which produce water of a similar chemistry. From an interpretive standpoint, the time-series sampling provides a more detailed picture of the chemical contributions of the different soil horizons; flow-related dip samples can be used to identify the flow regimes within which these processes occur. © 1990.