AN ASSESSMENT OF EXCESS CARBON-DIOXIDE PARTIAL PRESSURES IN NATURAL-WATERS BASED ON PH AND ALKALINITY MEASUREMENTS

Methods of estimating excess partial pressures of carbon dioxide in ri ver waters using pH and Gran alkalinity measurements are considered us ing data from several UK lowland rivers covering a spectrum of industr ial, urban and agricultural catchments. Two simple equations are shown to be eminently suitable except for the most demanding accuracies at pH values greater than 9 when carbonate and hydroxide ions as well as calcium complexes with bicarbonate, carbonate and hydroxide become lar ger. The most basic of these equations, which simply allows for the av eraged effects of temperature and ion activity, is EpCo(2) = (Alk(Gran in mu Eq/l units) + 10(6-pH)/6.0. The second equation, which allows f or variation in temperature and average ionic strength, is EpCO(2) = ( 0.95Alk(Gran in mu Eq/l units) + 10(6-pH))*10(6-pH)/(6.46-0.0636 t de grees C) Within this equation, the 0.95 term represents the average fa ctor which converts the chemical concentration of monovalent ions into chemical activities and t degrees C is temperature in degrees Celsius . For more demanding situations, such as at high pHs, the following eq uation is suggested EpCO(2) = (0.95Alk(Gran in mu Eq/l units) + 10(6- pH) + 10(6+pH+pKwater)) 10(6-pH)/(6.46-0.0636*t degrees C)*(1+2.38*10 (pH-pK2)) As in the previous case, the 0.95 term allows for activity c oncentration relationships for univalent ions. For this equation, pK w ater and pK2 represent minus the logarithm of the equilibrium constant s for the respective reactions H2O=H++OH- and HCO3-+Co-3(2-), where pK water = -6.0843+4471.33/(273+t degrees C)+0.017053(273+t degrees C) a nd pK2=-6.498+2902.39/(273+t degrees C)+0.02379(273+t degrees C). The (1 + 2.38 10(pH-pK2)) term allows for the effect of the carbonate io n contribution to the alkalinity and it is the ratio of the carbonate ion to the sum of carbonate and bicarbonate with both the numerator an d denominator in units of equivalent concentration. The 2.38 term conv erts the ratio of carbonate to bicarbonate from an activity to an equi valent concentration ratio. Comparisons of measured EpCO(2) for the Ri ver Frome agree well with estimates based on the above equations and t his adds to the confidence of the methodologies above pH 6. For pH < 6 , other methods are required owing to interferences from organic acids and aluminium. (C) 1998 Elsevier Science B.V.