Calcite saturation in the River Dee,NE Scotland

The spatial and temporal variations in calcite (calcium carbonate) solubility within the Dee basin of NE Scotland were assessed using water chemistry data gathered from a network of 59 sites monitored for water quality from June 1996 to May 1997. Calcite solubility, expressed in terms of a saturation index (SI_calcite), was determined from measured streamwater pH, Gran alkalinity and calcium concentrations and water temperature. In general, the waters of the Dee system are undersaturated with respect to calcite, though the saturation index is higher during the summer months indicating a dependency on flow conditions and biological activity. Under low-flow conditions, the streamwaters are dominated by water derived from the lower soil horizons and deeper groundwater stores and therefore, ions such as Gran alkalinity and calcium are at their highest concentrations as they are derived mainly from bedrock weathering. The influence of biological activity on the carbonate system is also evident as the observed pH and estimated EpCO₂ values indicate strong seasonal patterns, with the highest pH and lowest EpCO₂ values occurring during the summer low-flow periods. Only at three sites in the lowland region of the catchment, during the summer low-flow period, are the waters oversaturated. As such, the Dee system represents an extreme ‘end-member’ case when compared to many UK rivers that span both under- and oversaturated conditions during the year. Regression analysis highlights a systematic change in the SI_calcite–pH relationship in a broad east-west direction across the Dee system. At sites draining the relatively impermeable upland areas, the regression of SI_Calcite against pH gives a straight line with a gradient in the range 1.6–2.4. Correspondingly, under the most extreme alkaline conditions found at sites draining lowland agricultural areas, a straight-line relationship exists but with a gradient of unity. It is concluded that these changes in the SI_calcite–pH relationship are due to variations in the bicarbonate system induced by the flow conditions and biological activity. Given the waters are undersaturated, then calcite precipitation and hence phosphorus co-precipitation cannot occur within the water column.