Episodic Increases In Stream Acidity, Catchment Flow Pathways And Hydrograph Separation

Abstract

A positive correlation between stream discharge and stream acidity is common in areas affected by Surface Water Acidification (SWA). This has led to conceptual models of SWA in which low flow is less acid because of slow weathering reactions during a long transit time through the catchment. Acid storm runoff was considered to be primarily precipitation routed through the catchment too rapidly for weathering to occur. The predictions of SWA models are sensitive to the proportions of short and long transit time water reaching the stream. Those proportions, however, are poorly defined by the rainfall and runoff data customarily used to calibrate hydrological models. Isotope Hydrograph Separation (IHS) divides storm runoff into components of new rainwater and "pre-event water" which was in the catchment prior to a storm event. Such data can be used to improve the calibration of SWA model hydrology, but IHS usually shows pre-event water rather than precipitation to be the major component of storm runoff. If IHS is correct, then models of SWA need to be revised to include a larger proportion of pre-event water in acid storm runoff. This may alter the predictions of SWA models significantly. Doubts about the validity of IHS, though, made it unclear whether that revision was warranted. IHS is based on several assumptions, one of which is that pre-event water can be assigned a single, static δ18O value. The possibility of spatial variability in the δ18O of pre- event water, coupled with changing flow pathways during the storm hydrograph, made that assumption questionable. The rapidity with which stream discharge and chemistry react to rainfall could also be taken to suggest that IHS overestimates the component of pre-event water in the storm hydrograph. This thesis seeks to test whether the results of IHS are adequate grounds for revising the hydrological concepts upon which the understanding of SWA is based. Using detailed field investigations at Loch Fleet in Scotland and the Svartberget Research Catchment in northern Sweden, the validity of IHS is investigated, together with the argument that the response of both streamflow and chemistry during acid episodes is too rapid to be effected largely by the displacement of pre-event water. Spatial variability was found in the δ18O of pre-event water. That reduces the accuracy of IHS, but the qualitative conclusion that pre-event water is usually the major source of storm runoff is not disproven. Hydrological and chemical mechanisms consistent with a large component of pre-event water were also found. The key to the rapid hydrological response is the feedback relationship between inputs of water to the catchment and the capacity of the catchment to transmit that flow through the soil matrix to the stream along "spate-specific" flow pathways. These conductive flow pathways drain quickly after the input of water to the catchment ceases, and they are reactivated when new inputs of water arrive. The acidity of storm runoff at Svartberget results from organic acids/complexes that enter runoff when spate-specific flow pathways intersect organic-rich, riparian soils. This thesis concludes that the large component of pre-event water in storm runoff indicated by IHS cannot be dismissed as erroneous and should be incorporated into the conceptualization of episodic acidity. The key feature of that revised hydrology would be spate-specific flow pathways along which storm runoff is concentrated and from which it acquires a chemical "signature". The short transit times of water along spate-specific flow pathways relative to the longer mean transit times of water in the catchment as a whole suggest that the response of streamwater quality to changes in atmospheric deposition will appear first during periods of high flow. The strength of those conclusions owe much to the integration of observations from a two-dimensional hillslope flow system at Svartberget with the hydrochemical dynamics of the runoff from that hillslope.