Dissolved plume

NAPL pool and dissolved plume delineation and containment ... 

The issue of dissolved fractions, in terms of aquifer restoration objectives, would be more appropriately addressed at a phase of the remediation program after the source of the dissolved contamination is controlled. Of course, containment of the dissolved plume remains a priority throughout the program. For large-scale recovery programs, requiring treatment prior to reinjection would place an excessive economic burden on the overall remediation effort without technical justification, at least during the course of NAPL recovery. 

Plume containment involves stabilization of the dissolved plume hydraulically downgradient. Containment can be achieved by several means, including natural-gradient renovation with minimal pumping and treatment directly within the source area pump-and-treat scenarios, which rely on aquifer hydraulics and solute transport mechanisms and consist of the select placement of extraction wells or natural flushing with capture. 

The lateral extent of the dissolved hydrocarbon plume has continued to decrease, although no significant decrease in dissolved benzene has occurred since January 1992, nor is anticipated to occur in the future as a result of continuation of conventional pump-and-treat (Figure 13.11). Aerobic biodegradation has also been documented to exist since 1992, and is ongoing as indicated by the inverse relationship between dissolved BTEX and DO. The dissolved plume is also situated hydraulically upgra-dient from a relatively larger LNAPL pool and dissolved hydrocarbon plume separation of plumes has historically been maintained and has continued to be maintained. 

The combined ozone/H202 process was applied in situ to remediate a contaminated plume on a site in Delaware, containing MTBE, BTEX, fBA and TAME [97]. The authors report that most of the dissolved plume was remediated in the first 3 months of system operation. 

URS/Radian proposed the use of phytoremediation to treat groundwater eontaminated with dissolved phase CHC at a ehemieal plant. The State of Louisiana regulatory ageney reeom-mended standard pump treat teehnology. However, Radian was able to eonvinee the State to use a new more eost effeetive remedy. Hybrid Poplar trees were planted to aehieve hydraulie eontainment and phytoextraetion of the entire dissolved plume. The shallow groundwater and long growing seasons were ideal for this remedial approaeh. Radian was eventually able to elose the site and obtain a no further aetion letter from the State of Louisiana. 

The dimensions of subsmface transport and transformation coupled with the properties of intermediate byproducts further complicate risk assessments as well as the application of technologies to manage potential health or environmental impacts. For example, contaminant characteristics as well as associated risks differ significantly between source areas and the dissolved plume. Fortunately, predictive models incorporating fate and transport considerations have been developed to cope with these complications given adequate source and site characterization data. These models are widely distributed and are discussed in depth in Section III. 

Groundwater pollution problems, however, are modeled using a two dimensional analysis. A typical aquifer has area dimensions that are much hirger than the vertical dimension. Therefore, chemictils dissolved in the groundwater aclticvc ertical uniformity a short distance from the source and the chemical plume will move in the lateral and longitudinal directions. 

Trace organic compounds and dissolved solids from the quench water entrained in the steam plume rising from the tower... 

If the plants do not take up the dissolved contaminant, the plume that emerges will be concentrated (i.e., the mass of contaminant in the plume will be the same, but the concentration remaining will actually be greater due to the reduction in water volume caused by the vegetation). This is a potential concern for phytoremediation of groundwater plumes or in created wetlands, where a relatively hydrophilic contaminant can be concentrated on the downstream side of the phytotechnology system. 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

Has the potential to cause a lateral spread of dissolved or separate phase contaminant plumes. 

Fig. 12. Scenario for the in-situ modification of subsoils or aquifers for pollution control. The organoclay formed by injecting a solution of long-chain quaternary ammonium cations (e.g. HDTMA) acts as a sorptive zone where organic contaminants dissolved in a plume from buried waste can be immobilised and degraded. After Xu et al. (1997).

Large regional-scale dissolved contaminant plumes have been reported in many parts of the country. In California, many of the alluvial basin aquifers (i.e., major groundwater basins) were found to be impacted by chlorinated solvents during the period between 1979 to 1981. Beneath one valley a dissolved TCE plume incorporated the upper 50 ft of the saturated zone of a water table aquifer, was over 3 miles wide, and extended hydraulically downgradient over 14 miles. 

AGE DATING OF NAPL POOLS AND DISSOLVED HYDROCARBON PLUMES... 

In all these cases, several characteristics of dissolved contaminant plumes and NAPL pools are assumed ... 

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