C0

Ct = (25.68)[e-(00555)(90) ] = 0.17 pg / L This represents about 99.8% removal. 3.3.2 Removal Performance

The land treatment systems are the only natural treatment systems that have been studied extensively to determine the removal of priority-pollutant organic chemicals. This is probably due to the greater concern about groundwater contamination with these systems. Results from these studies have been generally positive. As indicated previously, the more soluble compounds such as chloroform tend to move through the soil system more rapidly than the less soluble materials such as some PCBs. In all cases, the amount escaping the treatment system with percolate or effluent is very small. Table 3.6 presents removal performance for the three major land treatment concepts. The removals observed in the SR system were after 1.5 m of vertical travel in the soils indicated, and a low-pressure, large-droplet sprinkler was used for the application. The removals in the OF system were measured after flow on a terrace about 30 m long, with application via gated pipe at the top of the slope at a hydraulic loading of 0.12 m3-m-hr. The SAT data were obtained from wells about 200 m downgradient of the application basins.

The removals reported in Table 3.6 for SR systems represent concentrations in the applied wastewater ranging from 2 to 111 pg/L and percolate concentrations ranging from 0 to 0.4 pg/L. The applied concentrations in the overland-flow system ranged from 25 to 315 pg/L and the effluent from 0.3 to 16 pg/L. Concentrations of the reported substances applied to the SAT system ranged from 3 to 89 pg/L, and the percolate ranged from 0.1 to 0.9.

The results in Table 3.6 indicate that the SR system was more consistent and gave higher removals than the other two concepts. This is probably due to the use of the sprinkler and the enhanced opportunity for sorption on the organic matter in these finer-textured soils. Chloroform was the only compound to appear consistently in the percolate, and that was at very low concentrations. Although they were slightly less effective than SR, the other two concepts still produced very high removals. If sprinklers had been used in the OF system, it is likely that the removals would have been even higher. Based on these data, it appears that all three concepts are more effective for trace organic removal than activated sludge and other conventional mechanical treatment systems.

Quantitative relationships have not yet been developed for trace organic removal from natural aquatic systems. The removal due to volatilization in pond and free water surface wetland systems can at least be estimated with Equations 3.19 and 3.24. The liquid depth in these systems is much greater than on an OF slope, but the detention time is measured in terms of many days instead of minutes, so the removal can still be very significant. Organic removal in subsurface flow wetlands may be comparable to the SAT values in Table 3.6, depending on the media used in the wetland. See Chapters 6 and 7 for data on removal of priority pollutants in constructed wetlands.

In a modification of land treatment, Wang et al. (1999) have demonstrated the successful removal by hybrid poplar trees (H11-11) of carbon tetrachloride (15 mg/L in solution). The plant degrades and dechlorinates the carbon tetrachloride and releases the chloride ions to the soil and carbon dioxide to the atmosphere. Indian mustard and maize have been studied for the removal of metals from contaminated soils (Lombi et al., 2001). Alfalfa has been used to remediate a fertilizer spill ( Russelle et al., 2001).

In microcosm studies, Bankston et al. (2002) concluded that trichloroethylene (TCE) could be attenuated in natural wetlands which would imply that similar results would be expected in constructed wetlands. The presence of broad-leaved cattails increased the rate of mineralization of TCE above that observed by the indigenous soil microorganisms.

3.3.3 Travel Time in Soils

The rate of movement of organic compounds in soils is a function of the velocity of the carrier water, the organic content of the soil, the octanol-water partition coefficient for the organic compound, and other physical properties of the soil system. Equation 3.25 can be used to estimate the movement velocity of an organic compound during saturated flow in the soil system:

where

K = Saturated permeability of soil (ft/d; m/d), in vertical or horizontal direction.

Kv = Saturated vertical permeability (ft/d; m/d). Kh = Saturated horizontal permeability (ft/d; m/d).

G = Hydraulic gradient of flow system (ft/ft; m/m), equal to 1 for vertical flow.

= ÀH/ÀL for horizontal flow (ft/ft; m/m); see Equation 3.4 for definition. n = Porosity of the soil (%, as a decimal); see Figure 2.4.

p = Bulk density of soil (lb/in.3; g/cm3). Oc = Organic content of soil (%, as a decimal). Kow = Octanol-water partition coefficient.

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