Journal of Hazardous, Toxic, and Radioactive Waste

In situ solidification/stabilization (ISS) projects require a significant amount of characterization, sampling, and bench-scale testing in the design or feasibility phase to ensure a successful project. During this phase the proposed construction methods need to be considered, taking into account such things as slurry proportions, untreated soil type/density, treated soil consistency, and soil/contaminant variability. As the project moves from the preconstruction phase into the construction phase, the results of the design phase are used to refine key project objectives which may include target improvements for permeability, strength, and/or leachability. A quality control/quality assurance monitoring program, which may include a combination of process controls, in-situ testing, and laboratory testing on grab samples, is then developed to confirm that the key project objectives are achieved. Process controls provide immediate feedback but generally do not directly measure the target properties. Many of the available in situ testing methods were not developed for ISS mixtures and are therefore limited for use in this application. Finally, many of the laboratory tests conducted on field collected grab samples, specifically leachability tests, require long lead times and therefore provide limited real-time feedback. In order to account for the advantages and disadvantages of each monitoring method, the quality control/quality assurance monitoring program should include a combination of short and long turnaround testing to be used collectively to predict the long-term performance of the improved material.

A phased stabilization/solidification (S/S) treatability study was performed on nonaqueous phase liquid (NAPL)–impacted soils at the Quanta Resources Superfund site, where naphthalene was the key constituent of concern (COC) from a mass perspective. Conventional synthetic precipitation leaching procedure (SPLP) testing was used to characterize the baseline leaching of the untreated soil. SPLP and two semi-dynamic leaching (SDL) methods [American Nuclear Society (ANS)/American National Standards Institute (ANSI) 16.1 and EPA Method 1315 modified for organics] were used to assess leaching reduction of the S/S treatments to demonstrate a 90% reduction in total COC leaching, as required by the Record of Decision. As expected, SPLP testing of treated samples produced virtually no change in leaching behavior, whereas a 5-day ANS/ANSI 16.1 was effective in a screening mode to identify successful mix designs for long-term (67-day) testing via EPA Method 1315M. Total pozzolan doses of 8% by weight or greater satisfied the UCS and hydraulic conductivity criteria, and leaching reductions of 90% or more were realized for the majority of the COCs per EPA Method 1315M.

The Gowanus Canal superfund site is located in Brooklyn, New York. The site is impacted by nonaqueous phase liquid (NAPL) from historic manufactured gas plant (MGP) sites and other sources. One of the primary objectives for the selected remedy included eliminating the migration of NAPL into the canal. Accordingly, the amount of NAPL present in sediment samples was quantified and physical testing was conducted to determine a threshold amount of NAPL that could be potentially mobilized into the canal as a result of upwelling groundwater. Undisturbed core samples of native sediment were collected and analyzed for NAPL, water, and air pore fluid saturation (PFS) using the Dean-Stark extraction method, and the total petroleum hydrocarbon (TPH) content. Sediment samples were found to contain NAPL saturations ranging from nondetectable ($<0.12\%$) to 52%. A relatively good correlation ($R^2=0.77$) was observed between the Dean-Stark and TPH results when compared on an equivalent mass basis. The NAPL mobility was evaluated for induced groundwater fluxes ranging from average existing site conditions to a factor of 10 higher ($13–130\mathrm{cm}/\mathrm{day}$). Consistent NAPL mobility was observed when the NAPL saturation exceeded 21%. However, when the NAPL saturation results were converted to TPH on a mass basis (milligrams of NAPL per kilogram of sediment), the results were a poor indicator of NAPL mobility.

The Gowanus Canal Superfund site is located in Brooklyn, New York. The sediment of the 2.9-km (1.8-mi) long canal is impacted at multiple locations by nonaqueous phase liquids (NAPLs) mainly from three historic manufactured gas plant (MGP) sites with measured NAPL saturations up to 50%. A stabilization/solidification (SS) treatability study was completed using 15 mix designs in a tiered testing suite that included unconfined compressive strength, hydraulic conductivity, and contaminant leach testing by equilibrium (as per U.S. EPA standard methods) and semidynamic leach methods, where semidynamic leach testing was modified with polydimethylsiloxane liners. Mix designs containing 5–10% (by dry weight) of a cement blend paired with 0.5% bentonite, or 1–4% organoclay added to wet NAPL-impacted sediments were characterized by unconfined compressive strength values greater and hydraulic condictivity values lower than 345 kPa (50 psi) and $1\times {10}^{-6}\mathrm{cm}/\mathrm{s}$, respectively. Leaching reductions for naphthalene (key contaminant) ranged from 89 to 93% for all mix designs compared to the untreated baseline conditions. Moreover, it appeared that the chief contributor to leaching reduction was a decrease in the hydraulic conductivity value, not the inclusion of a hydrophobic organic sorbing phase.

Carbon dioxide (${\mathrm{CO}}_2$) presents an interesting alternative to air as a gas for in situ sparging because it acts as a weak acid that can lower groundwater pH. Through a proof of concept or pilot-scale test, in situ sparging of gaseous ${\mathrm{CO}}_2$ was demonstrated to be an effective means to neutralize a caustic brine plume (CBP) and to reduce concentrations of mercury and other heavy metals. The plume exhibits high pH levels ranging from 10.5 to 12.0, densities as high as $1.06\mathrm{g}/\mathrm{mL}$, and mercury ranging from 50 to $\mathrm{1,000}\mathrm{\mu g}/\mathrm{L}$. The plume lies at the base of a moderately permeable aquifer at depths ranging from 9.1 to 15.2 m (30 to 50 ft) below ground surface. The proof of concept test involved 2 sparge wells and 13 monitoring wells installed at varying depths and radial distances up to 30.5 m (100 ft). Monitoring wells were equipped with data-logging field electrodes to continuously monitor pH and automatic data loggers to monitor water level mounding and collapse during intermittent sparging operations. The pilot test demonstrated that pH within an aquifer volume of approximately $184{\mathrm{m}}^3$ ($\mathrm{6,500}{\mathrm{ft}}^3$) could be reduced to near-neutral pH. A radius of influence (ROI) of at least 6.1 m (20 ft) was achieved in the deep aquifer. In the intermediate and shallow portions of the aquifer, ROIs were significantly larger. Mercury levels declined by 80–90% in monitoring wells where the pH was decreased to near 7. In addition, significant declines in silica, arsenic, and chromium concentrations were also observed. Based primarily on the successful pilot test, a full-scale ${\mathrm{CO}}_2$ sparging system to treat the 8-acre CBP is underway.

In situ sparging of gaseous ${\mathrm{CO}}_2$ was demonstrated through laboratory and pilot-scale testing to be an effective means to neutralize a caustic brine plume (CBP) and to reduce levels of mercury and other heavy metals in groundwater. The CBP exhibits high pH levels ranging from 10.5 to 12, densities as high as $1.06\mathrm{g}/\mathrm{mL}$, high dissolved silica concentrations, and mercury ranging from 50 to $\mathrm{1,000}\mathrm{\mu g}/\mathrm{L}$. The CBP lies at the base of a moderately permeable aquifer at depths ranging from 30 to 50 ft below ground surface. The pilot test involved a single sparge well and 13 monitoring wells screened at varying depths and radial distances up to 100 ft. The pilot test demonstrated that pH within an aquifer volume of approximately $184{\mathrm{m}}^3$ ($6,500{\mathrm{ft}}^3$) could be reduced to near neutral pH. Sparging caused intermittent mounding of groundwater levels, particularly in the piezometric surface of the basal portion of the aquifer, where the sparge well was screened. The piezometric surface in the basal portion of the aquifer rose to several feet above the ground surface during active sparging and then rapidly declined upon cessation of sparging. However, the rise in the overlying groundwater table was considerably muted by the lower vertical hydraulic conductivity of the aquifer. At its peak during the sparging, the groundwater table rose to within approximately a foot of the surface. Pre- and post-sparging aquifer testing indicated that the transmissivity of the aquifer was reduced by approximately 75% and the storativity was substantially increased. These changes in aquifer properties are believed to be primarily associated with the residual saturation of ${\mathrm{CO}}_2$ gas from the sparging and exsolution of ${\mathrm{CO}}_2$ gas from the groundwater in areas immediately adjacent to ${\mathrm{CO}}_2$ channels where groundwater is expected to have near saturation levels of ${\mathrm{CO}}_2$.