Impact of soil and groundwater heterogeneity on surface water chemistry in an upland catchment

The interaction of precipitation with catchment soils is commonly assumed to be the dominant control on the composition and quality of the resulting surface waters. A hydrochemical investigation of a small granitic upland catchment (NE Scotland) was undertaken to study the link between the spatial distribution of soils and the heterogeneity of surface water chemistry. The approach involved division of the study area into subcatchments with a spatial range of 4-124 ha. Mean concentrations of stream water solutes showed considerable spatial variability across the catchment. Although links existed between surface water hydrochemistry and soil distribution, the dominant soil types on an area basis did not necessarily control subcatchment hydrochemistry. Solute fluxes showed more pronounced heterogeneity than that reflected by concentrations alone. Groundwater inputs to streams were implicated from calculations of water budgets and were characterised by elevated geochemical solute concentrations; this enhanced the hydrochernical heterogeneity attributed to surface soil drainage and hence highlighted the complexities of subsurface flow pathways. One small tributary lower in the catchment (3% of the total area) from a groundwater-dominated source was influential in controlling the overall outflow chemistry from the whole catchment. Generally, whilst hydrochernical modelling often considers such catchments as homogeneous units, advances in understanding the hydrochemical functioning of catchments will only be made when the full range of catchment water source compositions is accounted for. Such heterogeneity makes it difficult for management decisions based on spatially averaged data to adequately predict and protect against degradation in water quality. (c) 2005 Elsevier Ltd All rights reserved.