Vadose water

The often smooth top surface to the calcrete suggests that the calcrete formed by the precipitation of Ca (as carbonate) from sub-surface laterally and vertically flowing vadose water sourced from rainfall, which penetrates easily through the overlying sand. The sub-surface has been plugged with calcrete preventing vertical downwards penetration of vadose water. Instead, vadose water would move laterally until it evaporates or is removed by transpiration. Precipitation of calcium carbonate at the surface of the calcrete would occur, thus contributing to the laminated occurrence of the calcrete. 

Preliminary work (10) on the transition from oxidized surface sediment to reduced subsurface sediment in Milltown Reservoir showed that the redox transition occurs in the upper few tens of centimeters. Strong chemical gradients occur across this boundary. Ferrous iron in sediment pore water (groundwater and vadose water) is commonly below detection in the oxidizing surface zone and increases with depth. Arsenic is also low in pore water of the oxidized zone, but increases across the redox boundary, with As(III) as the dominant oxidation state in the reduced zone. Copper and zinc show the opposite trend, with relatively high concentrations in pore water of the oxidized surface sediment decreasing across the redox boundary. 

Vadose water moving by vadose flow through fractures has been sampled from the roofs of caves (Frantz, 1971) in the Town Hill and Walsingham formations. This water is lower in dissolved CO2 and contains less Ca2+ and HCO3 ... 

Thrailkill J. and Robl T.L. (1981) Carbonate geochemistry of vadose water recharging limestone aquifers. J. Hydrology 54, 195-208. 

One possible explanation is that it was deposited at comparatively great depth beneath the surface, where the rock mass is relatively poorly ventilated. Saturated percolation water in small voids could generally not give up its surplus CO2, except where close to larger, better ventilated, caverns. Consequently, the rocky mass around better - aired voids became impregnated with flowstone, whereas vadose water channels farther away remained free of flowstone. 

Figure 8.1 defines the different types of subsurface-waters. The zone above the water table is called the zone of aeration, vadose zone, or unsaturated zone. Moving downward in this zone we encounter first soil water or soil moisture, then intermediate vadose water, and then capillary water. Capillary water rises into rock and sediment pores above the water table because of capillary forces. The capillary zone may be as much as 30 ft (9 m) thick in clayey sediments. To the extent the unsaturated zone can truly be called the zone of aeration, then air and atmospheric oxygen are present and conditions are oxidizing. 

CARBONATE GEOCHEMISTRY OF VADOSE WATER RECHARGING LIMESTONE AQUIFERS... 

Vadose water from Lower Carboniferous rocks in Kentucky, U.S.A., was analyzed monthly in Mammoth Cave and in the caves and overlying soil at two other caves. Calcite saturation and equilibrium CO2 pressures of water were calculated after correcting for ion pairing and activity coefficients. Soil gas CO2 concentrations were determined directly. 

Three types of vadose water may thus reach the aquifer undersaturated with respect to calcite (1) vadose flows which transit the vadose zone rapidly (2) low-Ca vadose seepage which is closed to CO2 as it transits the vadose zone, and which may remain undersaturated with respect to calcite even after ventilation and (3) high-Ca vadose seepage which leaves the soil zone slightly undersaturated, probably because of insufficient residence time. 

The aqueous envelope of the Earth, including the oceans, freshwater lakes, rivers, saline lakes and inland seas, soil moisture and vadose water, groundwaters, and atmospheric vapor. 

High-grade ore may be stopped in its lateral movement by a permeable barrier or suddenly stranded by lowering of the groundwater-table. If it is also protected from the downward leaching of vadose water by an impermeable cap or suitably arid climate, the orebody can survive to be exposed by erosion at the surface. Even under these unusual conditions, however, it will normally be oxidized to a bright yellow before exposure. 

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