Estimating The Amount Of Free Product In The Subsurface

The first steps in the remediation of a site where an LNAPL spill has occurred is to try to limit the movement of contaminant plumes and to remove from the subsurface as much free product as possible. As long as free product is present, it continues to partition into the sorbed, dissolved, and vapor contaminant plumes, continually feeding their growth. Only after the mobile free product has been removed from above the water table can remediation of the contaminant plumes be effective.

LNAPL free product in the subsurface is generally detected and measured by its accumulation in wells. To design a program for removing free product, one must obtain a reliable estimate of the volume of free product that must be removed. However, the relationship between the thickness of free product that accumulates in a well and the thickness of free product distributed above the water table is easily misinterpreted. This relationship is influenced by soil texture, fluctuations of the water table level, and the thickness of the free product layer.

Figure 5.6 illustrates some of the factors that affect free product accumulation in a well. In the subsurface away from a well, liquids are influenced by capillary attractions that draw them into small pore spaces and interstices. Where no LNAPL free product is present, three forces determine the aquifer water table elevation:

1. Gravity pulls water downward.

2. Water pressure in the aquifer acts upward against gravity.

3. Capillary forces at the interface between the saturated and unsaturated zones also act upward against gravity.

Within a well, there are no upward acting capillary forces affecting the liquid levels. Only the balance between gravity and water pressure in the aquifer determines the water level in a well.

FIGURE 5.6 Thickness of LNAPL accumulated in a well compared to thickness in adjacent subsurface.

If LNAPL is present, it also develops a capillary fringe at its interface with the unsaturated zone. Capillary fringes occur at the upper boundaries of both the water table and the free product layer. In the capillary fringes, liquids are drawn upward against gravity and are largely immobile, especially in horizontal directions. The thickness of the capillary fringes depends on the soil texture. In coarse soils and sands with few capillary-size pore spaces and interstices, capillary fringe layers may be only a few millimeters thick; in fine soils and sands, they may be several meters thick.

Where LNAPL free product floats in contact with the water table, the water level is lowered by the weight of LNAPL (see Figure 5.7). Where the LNAPL free product layer is thin, it lies largely above the water capillary fringe because LNAPL cannot easily displace water from this region. Where the LNAPL layer is thick, its greater weight makes it penetrate farther into, or even through, the water capillary fringe, moving the free product-water interface even lower.

When an appropriately screened well passes through an LNAPL free product layer into the saturated zone, water and free product flow into the well from the surrounding subsurface. Liquid movement into the well occurs from the water and free product regions below their respective capillary fringes, where liquid mobility exists. LNAPL from the mobile zone around the well flows into the wells, and the additional weight of LNAPL lowers the water level in the well to below the normal water table in the aquifer. LNAPL flows into the well until the top level of LNAPL in the well is the same as the top of the mobile zone in the surrounding soil.

Within a well, where no capillary forces exist, the weight of LNAPL lowers the LNAPL-water interface farther than in the surrounding subsurface. LNAPL will continue to flow into the well, lowering the water table, until the upward pressure of aquifer water balances the weight of LNAPL. The end result is that LNAPL accumulates in a well to a greater thickness than in the surrounding subsurface, where capillary forces buoy up both the water level and free product layer. The upper level of LNAPL in the well is lower than the upper level in the surrounding subsurface by the thickness of the LNAPL capillary fringe. The LNAPL-water interface in the well is lower than in the subsurface by an amount that depends on the soil texture and the thickness of the subsurface mobile layer of LNAPL. This behavior is illustrated in Figures 5.7 and 5.8.

figure 5.7 Comparison of LNAPL thickness in wells with thickness in adjacent subsurface.
figure 5.8 Effect of soil texture on LNAPL thickness in a well.

Equation 5.2 may be used to calculate the water table level in the adjacent subsurface from well measurements where LNAPL is present. Use of Equation 5.2 is necessary for evaluating and plotting groundwater elevations when LNAPL is present in the wells.

WTE = WEwell + (LNAPL density x LNAPL thickness in well)

where: WTE

well

= water table elevation in subsurface adjacent to the well. = water elevation at the water/LNAPL interface in the well.

Effect of LNAPL Subsurface Layer Thickness on Well Thickness

Referring to Figure 5.7, in the subsurface near wells MW-2 and MW-3, the weight of LNAPL free product is not sufficient to force water downward through the capillary fringe. Near well MW-1, where the LNAPL is thicker because it has formed a dome, the weight of LNAPL is great enough to force water downward through the capillary fringe and below the original water table. Within the wells, there are no capillary forces acting upward on the water. In wells MW-2 and MW-3, the LNAPL thickness in the wells is greater than the LNAPL thickness in the surrounding soils because the absence of capillary forces in the wells reduces the net upward forces acting on the water, and the water level is lower. In well MW-1, where LNAPL has pressed down through the capillary fringe, upward forces on the water are due only to aquifer pressure and are the same in the well and in the surrounding soil. Thus, the LNAPL thickness and the water level in well MW-1 are the same as in the surrounding subsurface.

Effect of Soil Texture

Soil texture determines the magnitude of the capillary forces that exert upward forces on subsurface water and LNAPL. Capillary forces are much larger in fine-grained soil than in coarse-grained soil. Consequently, the difference between LNAPL thickness in a well and LNAPL thickness in the adjacent subsurface is greater in fine-grained soil. The effect of soil texture is illustrated in Figure 5.8.

Details for calculating the recoverable volume of LNAPL free product in the subsurface from well measurements have been published.67 8 Computer programs are also available for this as well as other related calculations.

Rules of Thumb

1. Measured LNAPL thickness in a well typically exceeds the corresponding LNAPL thickness in the surrounding subsurface by a factor of 2 to 10, because LNAPL above the water table flows into the well and depresses the well water level.

2. The LNAPL thickness ratio, hwell/hs , generally increases with decreasing soil particle size, increasing capillary fringe thickness, and increasing LNAPL density.

3. A crude estimate, ignoring the soil properties, of LNAPL thickness in the adjacent subsurface can be made from hweil/hs = (LNAPL density)/(water density - LNAPL density). (5.3)

4. Since fuel LNAPL (gasoline and diesel) generally has a density of 0.7-0.8 g/cm3, Equation 5.3 gives hwell/hs = 0.7/0.3 to 0.8/0.2, or 2.3 to 4.0.

Effect of Water Table Fluctuations on LNAPL in Subsurface and Wells

Water table fluctuations promote vertical spreading of LNAPL in the subsurface and influence the thickness of LNAPL that collects in monitoring wells. When the groundwater table rises

• Some floating LNAPL free product is driven up into the unsaturated zone. Sorbed residual LNAPLs in the formerly unsaturated zone can be remobilized by dissolving into the free product, causing further lateral spreading.

• Some free product remains trapped in pore spaces below the water table within the saturated zone.

• The mobile free product layer above the water table becomes thinner.

When the groundwater table falls

LNAPL free product floating on the water table moves downward as the water table drops, leaving behind an immobilized fraction of free product as sorbed residual LNAPL that is retained in the newly unsaturated zone above the lowered water table. The mobile free product layer above the water table may become thicker because free product formerly trapped below the water table is free to migrate downward to the new water table.

FIGURE 5.9 Spreading of LNAPL into a "smear zone" because of water table fluctuations.

INITIAL WATER TABLE RAISED WATER TABLE LOWER WATER TABLE

INITIAL WATER TABLE RAISED WATER TABLE LOWER WATER TABLE

FIGURE 5.10 Effect of fluctuating water table on LNAPL accumulation in a well.

The rising and falling of the water table leaves behind a "smear zone" of contamination that lies partially in the saturated zone and partially in the unsaturated zone. This behavior is illustrated in Figure 5.9.

Effect of Water Table Fluctuations on Well Measurements

LNAPL spills are often first detected by the appearance of a free product layer above the water in downgradient wells. If the well free product layer diminishes during a remediation program, it is tempting to believe that the cleanup effort is working successfully. An increase in the well free product thickness often initiates a search for new LNAPL sources. However, unless fluctuations in groundwater depth are taken into account, basing such conclusions on changes in the free product layer thickness in wells can lead to serious errors.

When groundwater rises, the thickness of the free product layer in wells generally decreases because a portion of the free product becomes trapped below the water table and becomes immobile, thinning the mobile free product layer. When the water table falls, free product formerly trapped in the saturated zone becomes mobile again and can accumulate as a free product layer over the water table where it is free to flow into wells. This behavior is illustrated in Figure 5.10.

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