A natron source at Pikrolimni Lake in Greece? Geochemical evidence

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138 E. Dotsika et al. / Journal of Geochemical Exploration 103 (2009) 133–143 to form from evaporating water is calcite, and calcite precipitation continues until either Ca 2+ or CO 3 2− is exhausted. Waters with a low proportion of CO 3 2− TOT relative to Ca 2+ will be carbonate depleted after calcite formation, and will thus not yield Na-carbonate minerals upon further evaporation. So the elevated alkalinity/2[Ca 2+ ] ratio is a main requirement, according to Hardie and Eugster (1970),for the formation of significant quantities of Na-carbonate minerals. Also, according to Hardie and Eugster (1970) the appearance of sepiolite and gypsum determines the evaporative sequence. So, after the sepiolite precipitation the water can become carbonate-enriched and alkaline earth-poor or vice versa. If alkalinity is higher than 2[Ca 2+ +Mg 2+ ] then the sepiolite precipitation is not capable to modify the evolution of the solution along the path of alkaline facies (Risacher, 1992). The significantly high proportion of CO 3 2− TOT relative to 2[Ca 2+ +Mg 2+ ] in Pikrolimni area waters causes them to move along the path, in the Hardie–Eugster model, of alkaline faces, forming Na–CO 3 minerals. However, because the ratio of alkalinity to Ca 2+ and Mg 2+ controls the types of evaporite minerals formed, and specially the deposition of trona mineral, it is very important to determine the potential processes that affected the HCO 3 − contents in these waters. 5.2. Potential processes affected the concentration of HCO 3 − There are a number of processes that can affect the HCO − 3 contents in groundwater [these waters are rich in HCO − 3 (about 1.50–3 g/L)]. Eugster (1980) described in detail processes that can affect the concentration of alkalinity in groundwater: dissolution or precipitation of carbonate minerals, chemical weathering of silicates, redox reactions, especially the reduction of NO − 3 to NH + 4 and SO 2− 4 to HS − , microbial respiration or anaerobic decay and the conversion of CO 2 of deep origin to HCO − 3 in the aquifer. The dissolution/precipitation of carbonate mineral is a common control of HCO − 3 contents in groundwater. These waters show an important over-saturation with respect to calcite (Parkhurst and Appelo, 1999). In Fig. 3a the relationship Ca 2+ versus alkalinity (HCO − 3 +2CO 2− 3 ) is shown. For the Mg–HCO 3 -type waters it is observed that, from Pikrolimni village (Na–HCO 3 ) to boreholes near Pikrolimni Lake (Mg–HCO 3 ), increase of HCO 3 contents takes place: the Mg–HCO 3 groundwater that was sampled within the Holocene lacustrine sediments of Pikrolimni has HCO − 3 contents higher than 38 mmol/L. It is also observed that the HCO − 3 increases with relatively slight change in Ca 2+ , suggesting that carbonate mineral dissolution/ precipitation reactions are not an important control on HCO − 3 .Alsothe absence of chloride in the soda and Mg–HCO 3 type waters suggest that the dissolution of marine carbonates is not the only source of the carbon. Furthermore, these waters are plotted to the right of the 2(Ca 2+ )= (HCO − 3 ) equilibrium line (Fig. 3a). If the dissolution of the calcite was the dominant process producing CO 2 in the system then these waters would plot along the 2(Ca 2+ )=(HCO − 3 ) equilibrium line. Chemical weathering of silicates is another major influence on groundwater. The Na/HCO 3 ratio of dilute Na–HCO 3 inflow is very close to unity. When accompanied by the absence of Ca–HCO 3 groundwater, this shows that these Na–HCO 3 waters do not evolve from Ca–HCO 3 groundwater that undergoes normal ion exchange. Therefore the ratio Na/HCO 3 , very close to unity, suggests that the chemistry of soda water is controlled by simple weathering reactions like Na-feldspars: Na–AlSi 3 O 8 þ H 2 CO 3 þ 4; 5H 2 O→Na þ þ HCO − 3 þ 2H 4 SiO 4 þ 1=2Al 2 Si 2 O 5 ðOHÞ 4 : The hydrolysis of silicate minerals takes place rapidly producing bicarbonate rich water with high silica content (sample Keratea, nr. 21a). Probably this fresh water is from the deep cell of the local groundwater system and probably circulates in the bedrock. On the contrary, the Na/HCO 3 ratio of dilute Mg–Na–HCO 3 inflow (Fig. 3b) Fig. 3. Ca 2+ versus alkalinity of all groundwaters (a) and Na + versus HCO 3 − of Mg–HCO 3 groundwaters (b). Same symbols as Fig. 2. indicates that not only silicate alteration but also more processes are involved in the chemistry of these waters. The correlation between HCO 3 − and Mg 2+ indicates that basic rocks become a major target of weathering reactions in the Mg–HCO 3 − waters. In summary, as referred by Eugster (1980), the composition of inflow water depends largely on the minerals present in the watershed: basic and ultrabasic rocks are probable to give Mg-HCO 3 waters. Redox reactions are an additional possible control on HCO 3 − in particular the reduction of NO 3 − to NH 4 + and SO 4 2− to HS − ,bothofwhich produce CO 2 . So, concerning the reduction reactions as a possible control of HCO 3 − , although Eh was not measured, the high SO 4 2− and NO 3 − contents in these waters suggest that these reactions are not likely to be occurring. Also the microbial respiration or anaerobic decay process (potential sources of CO 2 that could be converted to HCO 3 − ) appears unlikely to apply significant control on the ground water chemistry because the material that would indicate large accumulation of organic matter in the borehole logs is absent. The final major reaction that could influence the HCO 3 − contents in the deep aquifer is the addition in the system of CO 2 of deep origin. The pCO 2 values of the most Mg–Na–HCO 3 type waters (10 − 0.5 –10 − 1 ) are higher than the atmospheric value (10 − 3.5 ) and the value of the soil (10 − 2.5 –10 − 1.5 ), suggesting that input of atmospheric/soil is not a major source and evidencing that gas is being added to the aquifer from the deep. The highest values are observed for the Mg–HCO 3 type waters emerging in the Holocene lacustrine sediments of Lake Pikrolimni. These waters present also the highest Na + ,Mg 2+ and HCO 3 − contents, from Pikrolimni area (Pikrolimni Village) to Lake Pikrolimni (borehole near Pikrolimni), suggesting probably that basic mineral hydrolysis driven by injection of deep (mantle or metamorphism source) CO 2 is acting to control the concentration of HCO 3 − in the lacustrine basin.
E. Dotsika et al. / Journal of Geochemical Exploration 103 (2009) 133–143 139 5.3. Brine evolution The plot of Na + ,SO 4 2− ,CO 3 2− ,HCO 3 − versus Cl − contents of the borehole sample in the thermal spa of Lake Pikrolimni and the domain of brine from Lake Pikrolimni are illustrated in Fig. 4. In these diagrams we also put published data for Lake Natron in Tanzania (Gueddari, 1984) and Lake Magadi in Kenya (Gueddari, 1984), where the natron actually precipitates and for Wadi Natrun which was the natron source in antiquity (Taher, 1999). According to all diagrams, it is concluded that the chemical composition of Lake Pikrolimni's water is similar to that of the lakes that precipitate natron today. Such solute concentrations support that the evaporative concentration is the dominant process in the chemical evolution of these lake waters. The Na–Cl relation (Fig. 4a) illustrates in general the stability of Na/Cl ratio in the lake brine. The Na/ Cl ratio of Lake Natron and Lake Magadi, as well as of Wadi Natrun which was natron source in antiquity, remains stable between diluted waters and concentrated brines. Comparatively to the waters of Lake Natron (Na/Cl ratio=3.3) and Magadi (Na/Cl ratio=2), the water of Lake Pikrolimni presents a lower Na/Cl ratio (Na/Cl ratio from 1.2 to 1.7), almost two times. The Na/Cl ratios of Lake Natron, Lake Magadi and Wadi Natrun tend to unity, close to Pikrolimni brines, only in the most concentrate brines (Gueddari, 1984) showing a very small deviation from the evaporative trend (from diluted to brine waters), until within the concentration range appropriate for trona saturation. Once trona precipitates, the Na + extraction from the solution enriches the brine in chloride and thus, Na/Cl ratio decreases. In fact in the diagram log Na/Cl versus Cl (Fig. 4b) the values of diluted waters and brines from Lake Magadi are given as well as the corresponding data from Lake Pikrolimni. As precisely referred by Eugster and Jones (1979), “from groundwaters to all but the most concentrates brines Na/Cl remains constant, because no fractionation occur. The most concentrated brines are saturated with respect to trona and precipitation clearly leads to a decrease in Na/Cl”. In Lake Pikrolini the most concentrated brine (8/2002) shows a decrease in Na/Cl in relation to the most groundwaters that are related to trona precipitation. Between groundwater and lake brines, a decrease of Ca/Cl, Mg/Cl and HCO 3 /Cl ratios is observed (Table 1). Carbonate species are subject to removal from solution by different mechanisms. In Fig. 4c, the greater than 1:1 slope of the regression line for Cl − versus CO 3 2− +HCO 3 − contents illustrates the gradual loss of carbonate species, accompanying evaporative concentration of Lake Magadi (Jones et al., 1977). The same is observed for the waters from Pikrolimni area. This loss is attributed to equilibration with the atmosphere [2HCO 3 − (aq) → CO 3 2− (aq)+CO 2 (gas)+H 2 O], calcite precipitation from diluted waters [Ca 2+ (aq)+HCO 3 − (aq) → CaCO 3 (sol)+H + ] and trona precipitation from saturated brine [3Na + (aq)+ HCO 3 − (aq)+CO 3 2− (aq)+2H 2 O → Na 2 CO 3 ,NaHCO 3 ,2H 2 O (sol)]. In most diluted lake waters, the carbonate loss is probably related to both degassing and carbonate precipitation. Loss to atmosphere is expected for pCO 2 greater than 10 −3.5 atm (Jones et al., 1977). In fact, in these diluted waters the pCO 2 is lower than 10 −3.2 . In the lake brines, the Fig. 4. Na + versus Cl − (a), log (Na/Cl) versus log Cl (b), Cl − versus CO 3 2− +HCO 3 − (c), and SO 4 2− versus Cl − (d). Black diamonds: Pikrolimni brines; white diamonds: groundwaters; black circles: Lake Natron springs (Gueddari, 1984); white circles: Lake Natron brines (Gueddari, 1984); crosses: Wadi Natrun (Shortland, 2004); white squares: Lake Magadi (Gueddari, 1984; Jones et al., 1977); white triangles: Wadi Natrun (Taher, 1999).
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A natron source at Pikrolimni Lake in Greece? Geochemical evidence

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