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 measu rements and fortnightly spot-sampled water quality data were used to examin e temporal variability and extremes in river water quality in an upland Sco ttish river: the River Dee at Mar Lodge. An empirical relationship was esta blished for Gran-alkalinity by multiple regression against how and conducti vity for the fortnightly data, Applying this relationship to the continuous data, an estimate of continuous Gran-alkalinity was calculated. The contin uous Gran-alkalinity record was used as (1) a conservative tracer in a simp le 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 st ream conductivity; and (3) to calculate the excess partial pressure of carb on dioxide in stream water. The episodic variations in pH, weathering and s ea-salt conductivity and excess partial pressures of carbon dioxide (EpCO(2 )) associated with high flow events in the River Dee suggest that hydrologi cal pathways play an important role in determining stream chemistry. The re sults of the hydrograph separation indicate that groundwater provides an im portant contribution to stream flow, and that there are large and hydrologi cally active stores of groundwater within the upper River Dee catchment. Se a-salts have an important influence on stream conductivity, particularly wi th the onset of storm runoff following summer drought periods. This suggest s that sea-salts are concentrated in the upper soil horizons by dry deposit ion and/or evapotranspiration, EpCO(2) behaves non-conservatively and shows marked diurnal variability under low-flow conditions during summer, induci ng diurnal pH variations, and Indicating the importance of within-river bio logical processes. This study emphasises the very intermittent nature of wa ter quality extremes with stream spates and the diurnal nature of biologica lly-induced responses. Fortnightly sampling programmes do not capture the r ange of high how extremes, and with sampling undertaken during the working day, biological extremes occurring at night are also missed, introducing bi as. This study shows the value of continuous measurements for infilling asp ects of these intermittent extremes. However, it is also noted that the rel atively simple patterns of response observed from the continuous measuremen ts may well belie a much more varied response at the sub-catchment and hill slope scales, as local chemical and hydrological heterogeneities do occur. Thus. the two-component mixing model used is, in practice, based on catchme nt integrated values for a range of soil water and groundwater endmembers. (C) 2001 Elsevier Science B.V. All rights reserved.