Water table Weathering

Nickel laterite New Caledonia Shallow, water table Weathering and changing Ej -pH at the water table... 

Copper ore minerals maybe classified as primary, secondary, oxidized, and native copper. Primaryrninerals were concentrated in ore bodies by hydrothermal processes secondary minerals formed when copper sulfide deposits exposed at the surface were leached by weathering and groundwater, and the copper reprecipitated near the water table (see Metallurgy, extractive). The important copper minerals are Hsted in Table 1. Of the sulfide ores, bornite, chalcopyrite, and tetrahedrite—teimantite are primary minerals and coveUite, chalcocite, and digenite are more commonly secondary minerals. The oxide minerals, such as chrysocoUa, malachite, and azurite, were formed by oxidation of surface sulfides. Native copper is usually found in the oxidized zone. However, the principal native copper deposits in Michigan are considered primary (5). 

Modeling of the transport of the long-lived nuclides, especially U, require knowledge of the input at the water table as a boundary condition for aquifer profiles. There are few studies of the characteristics of radionuclides in vadose zone waters or at the water table. Significant inputs are likely to occur to the aquifer due to elevated rates of weathering in soils, and this is likely to be dependent upon climatic parameters and has varied with time. Soils may also be a source of colloids and so provide an important control on colloidal transport near recharge regions. 

An SVE pilot test was conducted at a service station site that had experienced a release of gasoline. Some LNAPL had been removed, but high concentrations of vapor persisted across the site. The site was underlain by silty clay, which overlies weathered and highly fractured shale at a depth of approximately 7 ft. The water table occurs within the upper portion of the shale at a depth of approximately 11 ft. 

According to Table 5.3, in the drainage area of Lake Cristallina, about 4 pmol of plagioclase, 2 pmole of epidote, 1 pmole of biotite, and 1 pmol of K-feldspar per liter of runoff water are weathered per year. This amounts to 19 meq of cations per m2-year. 

Although Eh (redox potential) can affect both the adsorption/desorption reactions that can occur as well as the secondary phases that can form during CCB weathering, Theis Richter (1979) stated that their studies of ash disposal ponds showed that oxidizing conditions rather consistently prevail in the active leaching zone (i.e., the unsaturated zone above the water table). 

Laterite bauxite Jamaica Shallow, water table drainage helped by karst Accumulation as residual deposit accompanying weathering... 

The failure to detect anomalous Hg above these buried deposits can at least in part be related to features of the weathering environment. In some of the case histories it was determined that the present water table was above the depth of oxidation of sulphides with the result that active sulphide oxidation had ceased. In others it was apparent that the low levels of Hg restricted the development of a halo. 

Figure 3.7 Schematic representation of changes in element abundances in a lateritic weathering profile affected by the establishment of a water table. This illustrates how an iron-enriched and silica depleted zone may develop within the weathering profile.

Cooke, R.U. (1994) Salt weathering and the urban water table in deserts. In Robinson, D.A. Williams, R.B.G. (Eds) Rock Weathering and Landform Evolution. Chichester Wiley, pp. 193-205. 

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