Bedrock water, composition

Table II summarizes analytical data for dissolved inorganic matter in a number of natural water sources (J3, 9, J 9, 20, 21). Because of the interaction of rainwater with soil and surface minerals, waters in lakes, rivers and shallow wells (<50m) are quite different and vary considerably from one location to another. Nevertheless, the table gives a useful picture of how the composition of natural water changes in the sequence rain ->- surface water deep bedrock water in a granitic environment. Changes with depth may be considerable as illustrated by the Stripa mine studies (22) and other recent surveys (23). Typical changes are an increase in pH and decrease in total carbonate (coupled), a decrease in 02 and Eh (coupled), and an increase in dissolved inorganic constituents. The total salt concentration can vary by a factor of 10-100 with depth in the same borehole as a consequence of the presence of strata with relict sea water. Pockets with such water seem to be common in Scandinavian granite at >100 m depth.

Fig. 5.20 gives the water composition of four lakes at the top of the Maggia valley in the southern alps of Switzerland. Although these lakes are less than 10 km apart, they differ markedly in their water composition as influenced by different bedrocks in their catchments. All lakes are at an elevation of 2100 - 2550 m. The small catchments are characterized by sparse vegetation (no trees), thin soils and steep slopes. 

Comparison of water composition of four lakes influenced by different bedrocks in their catchments. Drainage areas of Lake Zota and Lake Cristallina contain only gneiss and granitic gneiss that of Lake Piccolo Naret contains small amounts of calcareous schist that of Lake Val Sabbia exhibits a higher proportion of schist. 

Figure 5.14. Water composition of four lakes in southern Alps of Switzerland. The difference in composition is caused by the geology of the bedrocks in the catchment areas. Lakes Zota and Cristallina are situated within a drainage area of gneissic rocks. The other two lakes are in catchment areas that contain calcite and dolomite.

Figure 10.18. Effect of pH on residual metal concentration in fresh waters. Dissolved zinc is plotted against pH. (a) Zinc in relatively undisturbed major rivers including the Yangtze (Chiang Jiang) and tributaries of the Amazon and Orinoco, (b) Zinc normalized to total dissolved solids for the same set of major rivers, (c) Zinc in pH-adjusted aliquots of Mississippi River water (April 1984, 103 mg liter suspended load, pH 7.7) the adjusted aliquots were allowed to equilibrate overnight before filtration and analysis. (From Shiller and Boyle, 1985.) (d) Zinc in different mountain lakes in the southern parts of the Swiss Alps. These lakes are less than 10 km apait, so that the atmospheric inputs can be considered to be uniform over this scale, but their water composition (pH) is influenced by different bedrocks in their catchments. A similar dependence on pH has also been observed for Cd and Pb but this dependence is less pronounced with Cu(II) when solute complex formation counteracts adsorption (data 1983-1992). (From Sigg et al., 1995, in press.)...

The geochemical balance of a 103 acre watershed underlain by silicate bedrock was investigated. Base flow composition of the stream water was essentially constanty but flood flows showed a decrease in concentration of silica, bicarbonate, and sodium and an increase in sulfate, magnesium, calcium, and potassium. Laboratory experiments indicate that fresh rock or soil reacts rapidly with distilled water and achieves a composition similar to the stream water, suggesting control of water composition by reaction with the silicate minerals. The aluminosilicate minerals react with CO charged water to form kaolinite, releasing cations and silica to solution. The products of weathering are removed as particulate matter (0,28 metric tons per year) and dissolved material (1,5 metric tons per year). 

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

The final composition of stream water is the product of the weathering reactions and related processes outlined above. However, the chemical processes are influenced and controlled by an intricate combination of environmental factors that are characteristic for each drainage system. Therefore, the composition of the bedrock in an area and the residual material left at the surface as soil and subsoil exert a strong influence on the chemical composition of mnoff from the area. The reactions of water with this material are the ultimate geological control and are the source of soluble weathering products. 

Groundwater composition in granitic bedrock at great depth and the artificial standard waters used... 

Seismic refraction can be used to define many natural and geohydrologic conditions Including number and thickness of layers, layer composition and physical properties, depth to bedrock or water table, and anamalous feature. 

The primary objectives of mass-balance studies are (i) quantify the mass fluxes into and out of watershed systems (ii) interpret the reactions and processes occurring in the watershed that cause the observed changes in composition and flux (iii) determine weathering rates of the various minerals constituting the bedrock, regolith, and soils of the watershed and (iv) evaluate which mineral phases are critically involved in controlling water chemistry to help develop models of more general applicability (i.e., transfer value). 

Rais well (1984) showed that the base-cation composition of glacial melt waters does not reflect that of the lithology of the bedrock. The predominant cation is always Ca, even on acid igneous and metamorphic bedrocks. This is because the dissolution kinetics of Ca from trace carbonates, which are ubiquitous in most... 

The chemical composition of glacial melt waters described above is the result of a series of reactions that are controlled first by reaction kinetics, and then by microbial activity. The following is a summary of the principal reactions in glaciated terrain, first on bedrock that is primarily composed of silicates and aluminosilicates. 

Figure 6 Sr and Nd isotopic compositions of granitoid bedrock, minerals, waters, and sediments from the Strengbach Catchment, France. A mixing calculation is shown with the percentage apatite in apatite-plagioclase mixtures. Sediments are dominated by plagioclase, whereas the waters and sediment leachate derive 10-30% of their Nd and Sr from the trace mineral apatite (source Aubert et al., 2001).

Most speleothem crystals are translucent rather than transparent. This is because of the presence of myriad tiny bubbles trapped in the growing calcite. The bubbles provide a sample of the water from which a particular bit of calcite grew. Measurements can also be made on the drip water from actively growing speleothems. The calibration is important because of the complicated pathway taken by the infiltration water from its origin as rainfall, through soil water, to water dissolving limestone at the bedrock interface, to drip water and finally to incorporation in the speleothem. If speleothem isotope ratios are to provide useful climatic information, it must be shown that the isotope record incorporated in the speleothem accurately reflects the isotope composition of the original rainfall. 

Table 1. Composition of ground water (August-October, 1997) contaminated by leachate from the Area 4 landfill, and uncontaminated ground water in bedrock and alluvial deposits adjacent to the Area 4 landfill. (Concentrations in milligrams per liter unless otherwise noted)...

The first point of discussion is the influence of the bedrock nature on the chemical composition of waters. We can see in Table II and in Fig. 6 that waters in the chloritic schist or argillaceous sand areas do not differ from those in the granitic area. This is not surprizing, in spite of the well-known control of rock mineral on the groundwater composition (M. Schoeller, 1962 Tardy, 1969) in fact, in the whole bioclimatic sequence considered, the lateritization processes have destructed all the primary minerals, except quartz, and the weathering minerals are always the same kaolinite, iron oxide and oxy-hydroxide, the stabilities of which in the surface are very great. 

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