Jökull


Jökull - 01.12.1986, Side 23

Jökull - 01.12.1986, Side 23
Fig. 12. Thickness of the freshwater layer in the western Reykjanes pen- insula. The freshwater in the western part of the Reykjanes peninsula floats in a thin layer on the seawater in the bedrock, like oil on water (Sig- urðsson 1985). Mynd 12. Þykkt ferskvatnslagsins. Á utanverðum Reykjanesskaga flýtur ferskvatnið á sjó í berginu, eins og olía á vatni (Freysteinn Sigurðsson 1985). 12), in Lágasvæði, in the middle of the western penin- sula, it is only about 50 m thick. Farther east the rocks in the freshwater layer include more hyaloclastites and the thickness of the freshwater layer •s greater (fig. 13). In the interglacial basalts at the coast north of Lágasvæði the thickness of the fresh- water layer remains close to 50 m. The Lágasvæði area is surrounded by less permeable rocks, where the freshwater layer is thicker, except in the permeable 'passes” leading to southwest and south. (Sigurðsson et al. 1978, Ingimarsson & Eliasson 1980, Sigurðsson 1985). The present picture of the thickness of the freshwater layer is drawn from various sources: 1 • Direct measurements in some few boreholes pene- trating through the freshwater layer. 2. Thickness calculated from the mean elevation of the groundwater table in a number of boreholes and the ratio between this elevation and the thick- ness measured in the above mentioned boreholes. 3- Approximately 100 geoelectrical soundings, that are very valuable because of their large number, despite the indirect nature of the method and the great range of error for single soundings. 4- Relative elevation of the groundwater table as esti- mated from groundwater currents indicated in the distribution of water quality parametres (tempera- ture, chemistry). The geoelectrical soundings were partly performed as Part of a survey for the freshwater and partly in a geothermal survey (Ágústsson & Þórarinsson 1979, Georgsson 1979, Georgsson & Tulinius 1983). The interpretations of the thickness of the freshwater layer with all four methods gave more or less the same results. From the eastem part of the peninsula much less data is available. The water level of the almost 100 m deep lake Kleifarvatn should represent approximately the true groundwater table. Kleifarvatn has no surface outflow and the water table fluctuates around or below 140 m asl. An elementary survey of the Straumsvík area indicates that the groundwater table in the mountains east of Kleifarvatn hardly rises much above 200 m asl. (Sigurðsson 1976). West of the lake geothermal fields cause widespread disturb- ances in the groundwater level. Alteration and infill- ings of secondary minerals reduce the permeability of the rocks, while differences in the density of the water, due to great differences in temperature, result in anomalous height of the groundwater level. Farther west the groundwater level probably falls abruptly, reaching about 3 m asl. at the western termination of the highlands (fig. 13). The groundwater level is also low in the lava covered lowlands along the northern coast of the peninsula. On the whole the knowledge of the elevation of the groundwatertable in the peninsula is still fragmentary and the picture of it, as drawn up here, must still be largely a guesswork. Elevation of the groundwater table, location of spring areas and distribution of water quality para- meters can be used to reconstruct the groundwater currents (fig. 10). They are hydrogeologically domi- nated, the differences in the infiltration playing a secondary role, because the differences in the perme- 21

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