Water table encountered

FIGURE 3.19 Schematic showing flow paths encountered in artesian and water table wells. 

When contaminant air rises above the water table into the vadose zone, the VOCs are captured by soil-venting extraction, escape to the atmosphere, or are treated as they encounter indigenous bacteria present in that zone. 

Test borings revealed that the fill material consists predominantly of thinly laminated silt with a relatively high clay content. Occasional discontinuous layers of fine sand were encountered at random locations, both horizontally and vertically. The water table gradient under nonpumping conditions was toward the canal with an approximate gradient of 0.011 ft/ft. Slug tests indicated an effective permeability of 1 x 10-5 cm/s. 

A water table piezometric surface contour map is presented in Figure 12.18. The elevation of this surface varies from 10 to 40 ft below sea level across the facility. Groundwater occurring under water table conditions is first encountered between 30 and 60 ft in depth. The regional direction of flow of shallow groundwater is toward a depression in the piezometric surface of the Gage aquifer located about... 

Site No. 3, situated in the southern portion of the West Coast Basin, is underlain by, in descending stratigraphic position, the Semi-perched, Gage, Lynwood, and Silverado aquifers (see Figure 12.25). Most pertinent to LNAPL hydrocarbon occurrence is the Semi-perched aquifer, which is encountered under water table conditions. 

Fig. 2.1 A schematic section through typical wells at the upper part the soil and rocks contain air and water in pores and fissures, forming the aerated zone. Below occur rocks with only water in their pores and fissures, forming the saturated zone. The top of the saturated zone is the water table, recognizable in wells as the depth at which water is encountered.

Seasonal variations in the tritium concentration in two shallow tribal wells in the Kalahari are seen in Fig. 10.6. Variations in tritium were noticed to follow variations in the water table. This observation provided insight into the local recharge mechanism. The tritium-rich recharge of the rainy season formed an upper layer in the aquifer. As abstraction, mainly during the dry season, advanced lower water layers were encountered, having been stored in the aquifer for several years. The seasonal tritium peak was observed to arrive at the wells several months after the rain peak. Thus the recharge water moved in the aerated zone not through conduits, but in a porous... 

Answer 7.4 Date of drilling, water horizons encountered as drilling progressed, their temperature and composition, intervals of loss of circulation fluids, depth of the well, the lithological and stratigraphic profile, perforation intervals, initial hydraulic head and temperature of the pumped water, rise of the water table after the water was first encountered by the drillers (indicating confinement), and more. This is indispensable information that justifies every effort needed to get the drillers reports. 

A further demonstration of the importance of fundamental properties of both pollutants and water bodies is provided by the behaviour of chemicals upon reaching a groundwater aquifer. Soluble chemicals, such as nitrate, move in the same direction as groundwater flow. A poorly soluble liquid which is less dense than water, such as petrol, spreads out over the surface of the water table and flows in the direction of the groundwater. Poorly soluble liquids which are denser than water, such as various chlorinated solvents, sink below the water table and may flow separately along low permeability layers encountered at depth in the aquifer and not necessarily in the same direction as that of the overlying groundwater. ... 

If the density of the bulk product is greater than that of water, the product will continue to migrate downward through the water table aquifer under the continued influence of gravity. Downward migration ceases when the product is converted to residual saturation or when an impermeable surface is encountered. Polychlorinated biphenyls and other chlorinated organic solvents are usually denser than water and are characterized as DNAPLs. 

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. 

Only three wells out of 15 has encountered water-table horizons ( 1, 3,11), and vulnerability assessment of unconfined conditions has been made by numerical scores and TOT (Tables 4 and 5). 

One substitute for DBCP is 1,2-dichloropropane (1,2-D). This compound has been found in several wells in the extreme Northern part of California and also in an area between Fresno and Bakersfield. In particular, 1,2-D has been found in areas where the water table is shallow. Usually the concentrations are around 0.1 to 5 ppb but levels as high as 30 ppb have been encountered (19). Studies sponsored by the National Toxicology Program have indicated possible carcinogenic activity in laboratory animals (20). The manufacturer of 1,2-D has voluntarily withdrawn D-D, the formulation that contains 1,2-D, from sale in California. 

Product le ng from an underground storage tank will migrate downward until it encounters the water table, where it will then flow with the groundwater, leaving a long trail of contaminated soil. Above the water table, some product will be absorbed on the soil particles and in the pore space between the soil particles. If the soil is later saturated by water, product stored in the pore spaces may be released, causing a reappearance of the free product and movement of the material into previously unaffected soil. 

As the water table is lowered as a result of drainage, the reduced cations and other base cations present in soil pore water are transported downward. The center of the soil aggregate in the drained portion of the soil may remain anaerobic, whereas the surface of the soil aggregate becomes aerobic, as a result of oxygen diffusion. The dissolved Fe and Mn + are transported through the soil matrix until they encounter another aerobic zone, where they are oxidized and precipitated. This process is generally heterogeneous, and leads to soils with a matrix of one color and mottles of another (Schwertmann, 1993). 

An area of concern with regard to 2,A-D which is commonly encountered, is the drift of 2,A-D into surface water to be used for drinking water. Table XI shows a calculation of risk involving drinking water which has begn subject to direct spray from aircraft at the rate of 20 mg/ft and, given the assumptions shown, results in a margin of safety of about 700. 

The density of a hydrophobic substance enables it to sink or float in water. Density values are especially important for liquids migrating through the un-saturated zone and encountering the water table as free product. Generally, liquids that are less dense than water float on the water table. Conversely, organic liquids that are more dense than water... 

Quick conditions brought about by seepage forces are frequently encountered in excavations made in fine sands that are below the water table, for example, as in cofferdam work. As the... 

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