Water table condition

The relative importance of pressure and gravitational heads depends on whether the water formation is in a free water table condition or in a confined aquifer condition. 

In principle, the recovery of LNAPL is similar in mechanical operation to production of a low-pressure, water-driven reservoir. Almost all documented petroleum remediations have been characterized by subsurface conditions under water table conditions (i.e., the top surface of the fluids are at atmospheric pressure). Few cases of confined aquifer situations have been reported in the literature, and although the mechanical recovery procedures are slightly different, the economic considerations are similar. 

For over a decade, LNAPL occurrence has been investigated beneath an active refinery site in southern California. Numerous monitoring wells along with LNAPL samples have been used to evaluate the extent and character of LNAPL occurrence. LNAPL was found to occur as five pools. The main pools each consist of individual accumulations of distinct product types occurring under both perched and water table conditions. Two different recovery and mitigation strategies have been utilized. In relatively high permeability zones, a system of two-pump recovery wells was used to recover fluids recovered water is reinjected without treatment. In relatively low permeability zones, a system of one-pump recovery wells was used. In the latter case, recovered water is treated prior to disposal. 

Groundwater beneath the site occurs within the Gage aquifer under perched and water table conditions. Beneath the northern half of the main refinery shallow groundwater appears to occur only under water table conditions. Beneath the southwestern portion and southern half of the main refinery, groundwater occurs under perched conditions upon and within the middle unit. Groundwater is present under water table conditions within the lower unit. 

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. 

The site is situated on a glacial outwash plain, and surface deposits consist of fill and undisturbed gravel and sand outwash containing minor silt and peat. Underlying the outwash at a depth of 20 or more feet is glacial till. Groundwater occurs under water table conditions within the highly permeable outwash deposits at a depth of 8 to 12 ft across the site. 

Groundwater depth Shallow water table conditions at 12 to 15 ft below the ground... 

Concentrations of the marker compound (Figure 10) found during the high water table conditions for these three sites discussed above did not appear to correlate with extract color. The marker compound analytical data, however, contain uncertainty due to apparent chromatographic artifacts. The presence of colored compounds in the extractant, however, does provide an independent assessment of the possible interconnection between surface water and groundwater in this locale. The fact that the color patterns of the two houses compared were different also demonstrates potentially different pathways of flow that are reaching these two houses despite the close proximity of their wells. 

Improves moisture retention of soils. In wetlands during the period low water table conditions... 

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