Lakes: alkaline, 162 evaporative

The Owens Lake dusts are derived from lake bed sediments containing abundant alkaline evaporative salts of sodium, chloride, carbonate/ bicarbonate, and sulfate, including, e.g., halite, natron, thermonatrite, mirabilitie, and trona (Saint Amand et al, 1986). In addition, the dusts contain a variety of silicates and other minerals derived from local alluvial material, and possibly some mine waste materials from the Cerro Gordo lead-zinc-silver mining district on the east end of Owens Lake. 

Evaporation of river water will not make seawater. Instead, evaporation of the nearly neutral Na -Ca -HCOi" river water produces a highly alkaline Na-HCO -COf water such as foimd in the evaporitic lake beds of eastern California (Carrels and MacKenzie, 1967). In addition, on comparing the amount of material supplied to the ocean with the amoimt in the ocean, it may be seen that most of the elements could have been replaced many times (Table 10-12). Thus some chemical reactions must be occurring in the ocean to consume the river flux. 

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

Garrels and Mackenzie wanted to test whether simple evaporation of groundwater discharging from the mountains, which is the product of the reaction of rainwater and CO2 with igneous rocks, could produce the water compositions found in the saline alkaline lakes of the adjacent California desert. They began with the mean of... 

Figure 24.3 compares the calculated composition of the evaporated water, concentrated 100-fold and 1000-fold, with analyses of waters from six saline alkaline lakes (compiled by Garrels and Mackenzie, 1967). The field for the modeled water overlaps that for the analyzed waters, except that Ca++ and Mg++ are more depleted in the model than in the lake waters. This discrepancy might be explained if in nature the calcite and sepiolite begin to precipitate but remain supersaturated in the fluid. 

Zone two can be defined by the absence of montmorillonite and by the tie-line mica-opal (Figure 37). Zone one, which contains montmorillonite shows the coexistence of feldspar and montmorillonite (Figure 37a). Trona and halite found in the sediments are considered to indicate higher alkalinity and alkali content of the pore fluids that effected the crystallization of the feldspar "facies" in zone two at the lake center. Here the evaporated fluids became more concentrated. 

The spring waters of the Sierra Nevada result from the attack of high C02 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the eartKs atmosphere should produce a highly alkaline Na-HCO.rCOA water a soda lake with calcium carbonate, magnesium hydroxy-silicate, and amorphous silica as precipitates. 

Finkelstein, D.B., Munhall, A., Pratt, L.M. and Bauer, C.E. (2004) A baseline study of evaporative water chemistry and microbial mat diversity from alkaline lakes in Warner Valley, Oregon. Abstracts with Programs. The Geological Society of America, 36(5), 87. 

Von Damm and Edmond (1984) utilized the lakes of the Ethiopian and northern Kenya rift zones to examine reverse weathering (the formation of authigenic clay minerals), because here evaporative concentration had not proceeded to the extent that salt precipitation interfered with a mass balance approach. They found that —60% of an alkalinity deficit could be accounted for by processes other than carbonate precipitation, and concluded that solute magnesium was lost as rapidly to clay as solute calcium was to carbonate. This situation, particularly in volcanic terrain, was also initially recognized at saline Lake Abert, Oregon, by Jones and VanDenburgh (1966). 

The dilute inflow to the East African basins acquires most of its alkalinity by the rapid hydrolysis of volcanic glass and lavas, producing high initial Na, SiOi, and HCO concentrations (Jones et al., 1977). Waters in the region are therefore nearly exclusively of the Na-COa or Na-COa-Cl type. Most of the other solutes are lost to carbonate or silicate precipitation (Jones et al., 1977 Beadle, 1981 Renaut and Tiercelin, 1994). Sulfate in East African waters is often removed from solution during evaporative concentration, probably due to reduction, especially in lake-marginal wetlands (Deocampo and Ashley, 1999). 

High arsenic concentrations can also occur in alkaline, closed-basin lakes. Mono Lake, California, USA has dissolved arsenic concentrations of (10-20) X lO pg with pH values in the range 9.5-10 as a result of the combined influences of geothermal activity, weathering of mineralized volcanic rocks, evaporation of water at the lake surface, and a thriving population of arsenate-respiring bacteria (Maest et al., 1992 Oremland et al., 2000). 

These results suggest that evaporative playa lake sediments and dusts generated from dry lake beds can be potentially significant sources of reactive, alkaline material with high levels of soluble, potentially toxic trace metals and metalloids. Further studies are needed to determine whether such dusts pose a substantial health hazard to those exposed to them on a regular basis. 

Weathering of silicate minerals usually supplies cations in addition to Ca and Mg. Such waters with [HCO ] > 2[Ca ] (residual alkalinity) tend upon evaporation to increase their pH values and concentrations of HCO and C03 and to decrease [Ca ]. Upon extensive evaporation, such waters acquire a composition similar to that found in natural soda lakes eventually alkaline brines of the Na-C03-S04-Cl type may be formed. 

For fresh waters there is a further restraint on pH rise the CO2 reservoir of the atmosphere. For a given pco2 the pH is a function of alkalinity. In order to raise the pH of a water in equilibrium with the atmosphere from 8 to 9, alkalinity must increase by nearly 5 meq liter (either by base addition or by evaporation). Hence only soda lakes, that is, lakes containing substantial amounts of soluble carbonates and bicarbonates, can attain high pH values for example, Sierra Nevada spring waters discharged to the east of the Sierra and evaporated in a plaza of the California desert. 

Tjeukemeer is an alkaline, humic-substance-rich, polder lake in the northern Netherlands (De Haan et al., 1979, 1981b). In winter the lake receives hu-Tiic-substance-rich water from the surrounding peaty polders. In summer, evaporation losses are compensated for by input of humic-substance-poor vater from the Ijsselmeer. De Haan (1972a) demonstrated that the fulvic -cids from Tjeukemeer could be separated into three different molecular -eight fractions. The relative proportion of each fraction appeared to de-rend on the type of water present. In winter, about 70% of the fulvic acid... 

High pH s (10 to 11) are caused by the dissolution of such minerals as nahcolite (NaHCO ) and natron (Na2C03 IOH2O), which form in evaporative alkaline lakes and dissolve according to reactions such as... 

Bicarbonate ion is usually the chief anion in freshwaters. In and on silicate rocks, the HCOj concentration is usually 50 to 200 mg/L, whereas in groundwaters that contact a few percent carbonate materials up to pure limestone and dolomite, bicarbonate levels are usually in the range of 200 to 400 ppm. Seawater contains 140 mg/L HCOj. Carbonate alkalinity (CO3 ) rarely exceeds 10 mg/L. Why The presence of caustic alkalinity (free OH ) at pH s above 10 usually indicates artificial contamination of a water by, for example, Ca(OH)2 (portlandite) from the setting of concrete at newly completed wells. Cg concentrations can reach 1000 ppm as HCO3 in sodium carbonate-bicarbonate brines found in evaporative, closed basin lakes. 

Eogenetic magnesite cement in sandstones is relatively rare because its formation requires pore waters to be enriched in Mg " " and depleted in Ca " ", S04 and Cl". These conditions may occur in arid climates in which marine pore waters evaporate and become successively saturated with respect to calcium carbonates, calcium sulphates and halite, such as in sabkha settings (Kinsman, 1969 Morad et al., 1995). Continental brines enriched in Mg + are also suitable for the formation of eogenetic magnesite due to the low sulphate and chloride ion concentrations. Most recent magnesite cements form in the fine-grained sediments of alkaline/saline lakes (Last, 1992 Warren, 1990) and, less commonly, in freshwater lacustrine sediments (Zachmann, 1989). 

This system is represented by a closed basin, made of impermeable rocks and filled in the past by a saline alkaline lake. Water in this case could not permeate downwards but only evaporate, so the deposit develops horizontally, instead of vertically as in the previous occurrence. Here pH and salinity in the fluids tend to increase, giving rise to brines, c.g., basic, alkali-rich solutions. Concentric zones of authigenic minerals are so formed, from an outer and upper ring of little altered glass and clay minerals, to zeolites, analcime and a finally alkali-feldspars. A good example for this type of occurrence is Lake Tecopa, California, where the zeolitic ring is constituted by phillipsitc, clinoptilolite and erionite, followed by the central feldspar zone [36]. 

Figure 18.5 Lake Natron in Africa s Great RiftVaiiey is a naturaiiy basic body of water. Water, iaden with dissoived sodium carbonate from surrounding voicanic rocks, drains into the lake but finds no outiet. Evaporation concentrates the mineral leaving a white crust on the surface and strongly alkaline water. 

Anoxic water samples, because they contain little in the way of particles, are far easier than aquifer materials to develop radioassays for the measurement of arsenate reduction. Arsenic speciation quantitatively changes from arsenate to arsenite with vertical transition from the surface oxic waters to the anoxic bottom depths of stratified lakes and fjords (55,56). This also occurs in Mono Lake, California (57), a transiently meromictic, alkaline (pH = 9.8), and hypersaline (salinity = 70-90 g/L) soda lake located in eastern California (Fig. 11). The combined effects of hydrothermal sources coupled with evaporative concentration have resulted in exceptionally high ( 200 fiM) dissolved arsenate concentrations in its surface waters. Haloalkaliphilic arsenate-respiring bacteria have been isolated from the lake sediments (26), and sulfate reduction, achieved with... 

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