Use of continuous water quality records for hydrograph separation and to assess short-term variability and extremes in acidity and dissolved carbon dioxide for the River Dee, Scotland.

A combination of continuous (15-min) pH, conductivity and temperature measurements and fortnightly spot-sampled water quality data were used to examine temporal variability and extremes in river water quality in an upland Scottish river: the River Dee at Mar Lodge. An empirical relationship was established for Gran-alkalinity by multiple regression against flow and conductivity for the fortnightly data. Applying this relationship to the continuous data, an estimate of continuous Gran-alkalinity was calculated. The continuous Gran-alkalinity record was used as (1) a conservative tracer in a simple two-component mixing model to determine the relative proportions of near-surface runoff and deeper groundwater contributing to stream flow; (2) to deconvolute the contribution of weathering and sea-salt contributions to stream conductivity; and (3) to calculate the excess partial pressure of carbon dioxide in stream water. The episodic variations in pH, weathering and sea-salt conductivity and excess partial pressures of carbon dioxide (EpCO2) associated with high flow events in the River Dee suggest that hydrological pathways play an important role in determining stream chemistry. The results of the hydrograph separation indicate that groundwater provides an important contribution to stream flow, and that there are large and hydrologically active stores of groundwater within the upper River Dee catchment. Sea-salts have an important influence on stream conductivity, particularly with the onset of storm runoff following summer drought periods. This suggests that sea-salts are concentrated in the upper soil horizons by dry deposition and/or evapotranspiration. EpCO2 behaves non-conservatively and shows marked diurnal variability under low-flow conditions during summer, inducing diurnal pH variations, and indicating the importance of within-river biological processes. This study emphasises the very intermittent nature of water quality extremes with stream spates and the diurnal nature of biologically-induced responses. Fortnightly sampling programmes do not capture the range of high flow extremes, and with sampling undertaken during the working day, biological extremes occurring at night are also missed, introducing bias. This study shows the value of continuous measurements for infilling aspects of these intermittent extremes. However, it is also noted that the relatively simple patterns of response observed from the continuous measurements may well belie a much more varied response at the sub-catchment and hillslope scales, as local chemical and hydrological heterogeneities do occur. Thus, the two-component mixing model used is, in practice, based on catchment integrated values for a range of soil water and groundwater endmembers.