OBTAINING HIGH-RESOLUTION SITE DATA
nificant. Spatial and temporal variations in mass flux are caused by variations in both contaminant concentrations and groundwater-flow magnitude and direction, which typically vary widely for most dissolved plumes (Guilbeault, Parker, and Cherry, 2005). In contrast, mass discharge (Md) can vary only over time at transects since there is only a single value for an entire transect (The Interstate Technology & Regulatory Council, 2010).
TREATMENT DESIGN, IMPLEMENTATION, AND MONITORING
Obtaining quantitative, high-resolu- tion data to fully characterize the site and prepare an accurate CSM also resulted in a remedial design that was extremely successful. It is essential for in-situ treatment to know where the solute “is” and where it “isn’t”. To be effective, especially “cost effective”, in a complex setting such as this site, the design needs to be pin-point accurate and the implementation needs to be surgical or the vagaries and nuances of solute occurrence can cause solute persistence and result in project failure.
The design of the BOS 100® rem- edy was based on carbon adsorption (Freundlich isotherms) and iron demand of VOCs. Slurry loading is a volumetric calculation based on grid dimension (typ- ical grid spacing was 5- to 15-foot cen- ters) and solute concentration. Detailed cross sections and high-resolution soil and groundwater data were used to select target injection locations.
Implementing Sophisticated Remedy Design. A variety of imple-
Figure 9-Proper implementation of design results in accurate placement of slurry.
mentation challenges were overcome because high-resolution data were col- lected. As previously discussed, a com- plex system of preferential thin, narrow, convoluted pathways through the sedi- mentary units carried extremely-high solute concentrations within deposits that had little to no apparent impact. To overcome this complex distribution net- work, the high-resolution data were used to place the remedy with “surgical” preci- sion within the impacted matrix (Figure 9 above). In situations where DNAPL was present and very high loadings were required, guar (and a de-linking agent) was added to increase slurry density, i.e., approximately double the amount of BOS 100® could be suspended and injected into the native matrix.
Slurry distribution was occasional- ly inadequate due to preferential flow steering slurry away from a targeted area or because a very flat hydraulic gradient (4.88E-4 m/m) within much of
the treatment area caused unpredict- able solute distribution. Radial injection grids (using 1-, 2-, and 3-meter radii) were often used for re-treatment in lieu of Cartesian grids to overcome these challenges.
As previously discussed, other dis- tribution challenges were overcome by using a high-pressure, high-flow-rate pump. An assortment of specialized injection tips were used to manage the flow from the pump. A typical slurry vol- ume of from 120 to 190 liters was injected at rates ranging from approximately 10 to 16 ls-1. Injection durations ranged from approximately 10 to 15 seconds. The injectate exited the tips at velocities that ranged from 60 to over 100 meters per second and was directed at select locations with precision. The effective- ness of this injection technique to dis- tribute the BOS 100® slurry is shown in Figure 10 on the following page).
Mass-flux calculations along the transect shown on Figure 7 are presented in Table 2. The estimated pre- treatment mass discharge was 138 kilograms per year (kg/y), based on a cross-sectional area of approximately 165 m2. The estimated post-treatment mass discharge was 6 kg/y, a mass reduction of over 95 percent due to the BOS 100® treatment.
14 TPG •
Oct.Nov.Dec 2017
Performance Monitoring Methodologies. There were a num- ber of performance-monitoring methods used during the implementation and clo- sure phases of the project. For example, groundwater samples were collected before, during, and after slurry injec- tions to monitor remedy performance. It is important to be “nimble” while in the field and next-day laboratory results for the samples allowed for design revisions to be implemented quickly. Mass dis- charge was periodically calculated along selected transects (Table 2) to monitor mass reduction, i.e., diminished plume strength, over time and to ensure that “zero” mass flux was maintained at dis- crete locations along the property bound- ary. Continuous soil borings were also completed and sampled after treatment in DNAPL areas to confirm that solute mass was significantly reduced/elimi-
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