PROCESSES CONTROLLING THE CHEMICAL COMPOSITION OF NATURAL WATERS IN THE HREPPAR-LAND AREA
3. WATER COMPOSITIONS
The major element composition of cold and thermal waters in the Hreppar-Land area is given in Table 1. The waters include streams, cold water springs as well as geothermal waters from springs and drillholes covering temperatures from less than 20 °C to a little over 150°C. One sample (no. 9067) is from a small lake and one (no. 9058) from a pond forming in peat soil after a heavy rainfall one night and collected the following morning. This sample was, therefore, expected to be clostest to rainwater in composition.
The dissolved solids content of stream waters and cold springs lies in the range of 62-160 ppm. Geothermal waters are higher in dissolved solids, the value increasing with temperature (Fig. 2A). In the cold waters the most abundant component is silica, but the main cation is sodium and the main anions chloride and bicarbonate. Potassium, calcium, magnesium and iron generally constitute a significant proportion of the dissolved substances in these waters. The relative amount of silica is slightly higher in the thermal waters than in the cold waters. Sodium is the main cation and relatively considerably more abundant than in the cold waters. On the other hand calcium, iron and magnesium, particularly the latter two, are depleted in the geothermal waters in comparison with the cold waters and so is potassium in the warm waters but hotter waters possess similar or higher potassium concentrations than the cold waters. Aluminium and fluoride are low in the cold waters but increase in concentration with rising temperature. Sulphate and bicarbonate are the main anions in the geothermal waters although chloride is often only a little less abundant.
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Fig. 2. Relationship between temperature and (A) the dissolved solids content and (B) the pH of natural waters in the Hreppar-Land area.
The pH of the surface waters lie between 6.5 and 7.6 and most of them are a little above 7 (Table 1). Warm waters have, on the other hand, significantly higher pH, up to 10, but with increasing temperature the pH falls again (Fig. 2B). The pattern of total carbonate variation with temperature is a mirror image of that of pH.
The chloride content of the of cold waters lies in the range 8.2-14.1 ppm (Table 1) whereas in the geothermal waters the range is 11.4-49.4 ppm (Fig. 3A). Generally speaking chloride concentrations are positively related to temperature as are boron concentrations (Fig. 3B). In the cold waters Cl/B ratios are siilar to those of seawater (4350) or somewhat lower. With increasing temperature these ratios become progressively lower and in the hottest waters they are some 50-60.
100 120 140 160
Fig. 3. (A) Chlorine-boron and (B) boron-temperature relationships in natural waters in the Hreppar-Land area. Circles denote cold groundwaters and surface waters and dots thermal waters. In Fig. 3B squares indicate slightly thermal waters (<30°C) and the filled diamond the geothermal water in Thjörsärdalur which is associated with silicic volcanics. Other waters are associated with basaltic rocks. The line in Fig. 3A was obtained by a linear regression through the data points. It has a slope of 50.6 (the Cl/B rock ratio) and an intercept with the Y-axis of
11.3 ppm Cl. The curve in Fig. 3B is a second order polynom (B (ppm) = 1.680xlO-5t2 + 1.628xlO-3t - 6.247xlO-3, where t is in °C) describing well the boron-temperature relationship.
Data points for which local names are given are discussed specifically in text
Hydrogen sulphide is not present in the cold and warm waters. It is detectable in some waters in the range 50-60 °C. Above this temperature there is a clear, approximately linear increase in H2S concentrations with temperature, some 0.4 ppm for every 10° C
The geothermal water in Thjorsardalur, which is located within an extinct central volcano (sample no. 7913 in Table 1), is anomalous compared to other such waters in the Hreppar-Land area, particularly in respect to low silica concentrations and high calcium, sulphate and fluoride. The hot water at Thjorsardalur is associated with silicic volcanics.
This association accounts for the elevated fluoride concentrations but not for the unusually high calcium and sulphate, whether comparison is made between geothermal waters associated with basalts in the area or with such waters issuing from silicic volcanics in other areas in Iceland. One may speculate that anhydrite is present as a hydrothermal mineral of a fossil high-temperature geothermal system within the central volcano and that its dissolution explains the anomalous calcium and sulphate concentrations. Indeed the water is just anhydrite saturated. In the discussion that follows the characteristics of the Thjorsardalur water will not be specifically discussed but emphasis will be made of the chemical characteristics of the remaining waters which are associated with basaltic rocks.
The water sample collected from a pool in vegetated soil by Sandlaskur (sample no.
9058 in Table 1) represents rainwater with very short residence time with the soil. The pool formed during heavy rainfall one night and was sampled the following morning. Indeed the pH of the water is practically the same as that of pure rain water and the silica concentration is very low. The ratio of Na to Cl is also similar to that of seawater but the ratios of other major elements (K, Ca, Mg, AI, Fe, SO4 and F) to Cl, as well as B, are considerably higher
-90 -12
Fig. 4. Relationship between ÔD and OigO in cold and thermal waters from the Hreppar-Land area. Circles denote cold waters, open squares water <40°C, half filled squares 41-80°C waters and dots waters >80°C. The line represents the meteoric line: SD = 8518Q + 10.
than in seawater indicating some reaction with the soil that must have occurred within a period of few hours.
The overall compositional change associated with the evolution of geothermal waters from their parent cold waters involve an increase in the dissolved solids content caused by an increase in the concentrations of Si, Na, SO4, Cl, F, B and Al. At the same time a decrease in Ca, Mg and Fe occurs. K and CO2 also tend to undergo a decrease from cold to warm waters but they show an increase in the hot waters.
The deuterium content of natural waters in the Hreppar-Land area lie in the range -55 to -94 %o (Fig. 4). All the cold and warm (<40 °C ) waters, except two, possess 02H values between -54 and -65%o. The two exceptions include the spring forming the source of the stream of Klofalœkur (no. 9047) and a spring at Krökur (no. 9048) where the values are -74 and -67%o , respectively. These two springs are close to the eastern volcanic belt in South-Iceland and emerge from below very permeable post-glacial lava fields (Fig. 1).
Groundwater flow through the lavas from a distant source inland is hydrologically probable, so the water in these springs is most likely not local.
Hot waters in the area have lower ô 2H-values than the cold waters, or in the range -67 to -94%o . Precipitation today with 0 2H values in this range falls in the area to the north of Hreppar-Land as far inland as the top of the Hofjökull ice-cap in central Iceland [4].
The hottest waters in the area show a significant oxygen shift from the meteoric line whereas the cold waters plot close to that line (Fig. 4).