OBTAINING HIGH-RESOLUTION SITE DATA


the elemental iron serves as an electron donor (being oxidized in the process) and the aliphatic chloroethenes/chloro- ethanes serve as the principal electron acceptors. The final step is the genera- tion of end-product hydrocarbons (eth- ene or ethane) which, due to very high vapor pressures and temporary London dispersion forces, escape the matrix and allow for “fresh” contaminant to be adsorbed by the carbon catalyst.


Figure 2-Matrix/Geologic Column


at or near the interface. Impacts did not extend into the underlying claystone bedrock. Solute transport was dictated primarily by physical properties of the DNAPL, (e.g., density) and by matrix characteristics.


Matrix characteristics which had the biggest impact on site remediation were the anisotropy and heterogeneity caused by variances in density and grain size in the alluvial sands. Other challenges were caused by a very small gradient (4.88E-4 m/m) that resulted in solute distribution and flow direction being poorly predictable. Over the course of history, it did not take much to “tip the table one way or the other”, so flow direction, hence, distribution patterns changed frequently.


Remedial Technology: The select- ed remedy was in-situ treatment using an immiscible, activated carbon solid injectate, BOS 100®. Each carbon grain is impregnated with elemental iron (Figure 3) such that carbon adsorp-


Deficiencies such as slow or incom- plete treatment due to insufficient residence time are avoided in that the initial contaminant is sequestered in the BOS 100® (as are kinetically-generated derivatives) during the cleanup cycle. The resident solutes are then reduced to innocuous end products via adequate contact with the impregnated iron.


Initial Challenges. Initial treatment was only marginally successful because of data gaps inherent to using standard site-characterization procedures. The conventional practice of collecting only a few soil samples and basing treat- ment strategy on a limited network of monitoring wells did not provide the detail necessary to generate an accurate conceptual site model (CSM). In addition to needing higher-resolution data in the source area, it was imperative to shift the design focus to (saturated) soil impacts rather than basing injection loadings on dissolved-phase groundwater concentra- tions only.


Another challenge (apparent as more- focused data was obtained) was the fact that most of the dissolved-phase plume was contained within granular alluvi- um. Remedy delivery is less of a problem when the slurry is a miscible fluid. In this case, however, the remedy is a solid that is mixed in water as a slurry. The well-graded matrix of the native forma-


tion tended to strain out the granular BOS 100® thereby leaving large globular masses of injectate, rather than a more lenticular distribution. Ostensibly, a sand unit cannot be fractured so it was difficult to propagate the remedy outward via hydrofracturing. These dis- tribution woes were overcome by using a high-pressure (14.00E6 Pascals), high- flow-rate [up to 16 liters per second (ls-1)] pump to “jet” the granular carbon-based slurry into the formation. Mechanical mixing was achieved by “fluidizing” the sandy matrix (Figure 4 below).


CHARACTERIZATION APPROACH AND METHODS


As the project advanced, more data were gathered to the point where a “high resolution” image of the site evolved that was used to generate a detailed and accurate CSM. Abundant soil and groundwater samples were collected in the DNAPL areas and throughout the dissolved-phase plume.


In DNAPL areas, the soil data were used to characterize the sorbed and dissolved-phase impacts in saturated samples and the groundwater data were used to characterize the extent of desorp- tive partitioning. Care was taken to prevent vertical migration during prob- ing/drilling.


Within the dissolved-phase plume, groundwater data were used to evaluate plume strength and solute distribution due to advective and dispersive trans- port. In all cases, soil and groundwa- ter concentrations were used to design appropriate slurry loadings for treat- ment.


Figure 3-BOS 100® Particle


tion properties are coupled with the dechlorination process of iron (a step- wise function that produces a variety of byproducts, i.e., “daughters”). Based on applicable half-cell reactions and asso- ciated Gibbs Free Energy of Reaction,


www.aipg.org Figure 4 -Mechanical Mixing


Continuous Soil Cores. Direct- push technology (DPT) was used to complete 186 soil borings throughout the DNAPL and dissolved-phase plume areas (Figure 5 on the following page). DPT (also known as “direct drive,” “drive point,” or “push technology”) refers to a growing family of tools used to perform subsurface investigations by driving, pushing, and/or vibrating small-diam- eter, hollow steel rods into the ground. Sampling tools attached to the end of the rods can be used to collect soil, soil-gas, and groundwater samples. DPT rods can also be equipped with probes that pro- vide continuous in-situ measurements of subsurface properties, e.g., geotech- nical characteristics and contaminant distribution via MIP (United States Environmental Protection Agency, August 2005).


Oct.Nov.Dec 2017 • TPG 11


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