Jökull


Jökull - 01.12.1984, Side 40

Jökull - 01.12.1984, Side 40
river emerges from the glacier. The data show large scattering during the year but there may be seasonal variations with the lowest 6D values towards the end of the year. During the jökul- hlaup in 1972 low and high values were both recorded. Preliminary results of ö lxO values indicate a similar scattering (Sigfús J. Johnsen, pers. comm.). Samples taken at various times during the years 1982- 1983 varied from 6 lsO = -13.5 to -11 %o. The variation seems to be irregu- lar and neither seasonal nor showing a systematic change from normal flow to jökulhlaups. During the jökulhlaup in 1982 samples collected on the same day during the maximum discharge of the flood showed -13.5 to -12 %o. The scattering of the isotopic ratios may to some extent be due to inhomogeneity of the water mixture in the rivers during jökulhlaups. In 1972 estimates of dis- charge by the method of dilution of radioactive materials proved to be unsuccessful. The radioac- tive material did not mix completely in the river course (Páll Theodórsson pers. comm.). CHEMISTRY OF WATER IN THE GRÍMSVÖTN LAKE. The water that drains the Grímsvötn lake has obtained its original chemical composition from four components (with concentrations (C) and the flow of mass per unit time (M) given in parenthesis): geothermal water (Cgw, Mgw), geothermal vapour (Cgv, Mgv), ablation from the surface of the glacier (Ca, Ma), and ice melted inside the geothermal area (Q, Mj). Later this composition may have been modified by chemi- cal reactions in the lake and on the way to Skeidarársandur. The river water in jökulhlaups on Skeidarársandur is a mixture of water from the lake and river water from the normal drain- age basin outside the lake (with Cr and Mr). The concentration in jökulhlaups of substances that do not undergo chemical reaction in the lake can be expressed as Cj = k(CgWMgW+CgvMgv)/(Mgw+Mgv) + (l-k)(Ca Ma+Q Mi+Cr Mr)/(Ma+M|+Mr), (1) where k is a geothermal mass fraction defined as the ratio k=(Mgw+Mgv)/(Mgw+Mgv+Ma+Mi+Mr). (2) The geothermaí fraction k could be calculated if we knew the original chemical concentrations and the masses of the components. Not all of this information is available but the model is nevertheless useful as a guide in the discussion of the chemical analyses. For some substances we are able to conclude that the model does not apply and chemical changes must have occurred in the lake. For others the model is applicable and for one of them, silica, reasonable estimates can be made and k evaluated. Substances whose concentrations are changed by reactions in the Grímsvötn lake The observed concentrations of sodium, potas- sium, calcium and magnesium in jökulhlaups are not consistent with a mixture of meltwater and high-temperature geothermal water. They demonstrate that chemical changes have taken place within the lake. The model described by equation (1) does not apply for those substances. That such is the case is clear from comparison with known concentrations in deep water from other geothermal areas like Námafjall, Krafla and Nesjavellir (see Table 3). In those areas the concentration of sodium is much too low to account for the concentration in the jökulhlaups from Grímsvötn. The same applies for potassium although not as conclusively as for sodium. Sodium and potassium are the most soluble of the cations and their rates of leaching out of basaltic glass will be enhanced by the low pH in the lake. The pH of the lake water must be considerably lower than that of the melted ice. This is due to dissolution of acid gases from the geothermal fluid. Calcium does not exceed about 3 ppm in deep water at non-saline geothermal areas in Iceland (see Table 3) and that is considerably lower than in the normal discharge of the glacial rivers. Dissolution of Ca must have occurred in the highly carbonated water of the Grímsvötn lake. The magnesium concentration of water from the Grímsvötn jökulhlaups, (10-15 mg/kg), is similar to those of cold groundwater (3-12 mg/kg, NEA data). Experimental data (see Mottl and Seyfried 1980) and field data (Kristmannsdóttir 1980, Tómasson et al. 1977) show that Mg is precipitated at temperatures above 40 °C. The magnesium concentration of non-saline geoth- ermal water is generally in the range of 0.001-0.1 38 JÖKULL 34. ÁR
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