2001 Conference Proceedings

Numerical Investigations of Vadose Zone Transport of Saturated Sodium Thiosulfate Solutions

M. D. White, A. L. Ward
Pacific Northwest National Laboratory, Richland, Washington

Proceedings of American Geophysical Union 2001 Fall Meeting held in San Francisco, CA

Abstract

Compared with water, hypersaline liquid wastes (NaNO3 > 10 N) from the reduction-oxidation (REDOX) process at the Hanford site have elevated viscosity (μ > 1.2 cP), density (ρ > 1.4 gm/cm3), and surface tension (σ > 100 dyn/cm). Such liquids have infiltrated into the vadose zone at Hanford from leaking underground storage tanks. The migration behavior of saturated or hypersaline salt solutions through unsaturated soils is largely unknown. Laboratory tests with tank-waste simulants suggest that the elevated density, viscosity, and surface tension properties of these liquids can influence the wetting front behavior, altering its shape and migration rate. Conditions under which these mechanisms are active in the field and the extent to which they contribute to transport through the vadose zone are largely unknown, making it impossible to accurately predict the post-leak distribution of these fluids in the field. To investigate the effects of fluid properties on subsurface migration of hypersaline saline solutions, numerical simulations were conducted of a field-scale, tank-leak experiment. The field experiments consisted of five 4000-L injections, at a depth of 5 m, of saturated sodium thiosulfate brine (used as a surrogate for REDOX type wastes) over a 5-week period, followed by three 4000-L injections of Columbia River water. Pre-test modeling of river water injections at this Hanford field site predicted significant lateral spreading of the moisture plume and were confirmed by geophysical logging. A series of three-dimensional, multifluid (i.e., aqueous and gas phases) numerical simulations were conducted that systematically considered the effects of elevated density, viscosity, and surface tension, and reduced vapor pressure on vadose-zone transport. Hydrologic properties were determined from cores collected at the field site and calibrated using river-water injection experiments. Isothermal conditions were assumed for the simulations, however, the effects of saline concentration on water vapor migration were considered. Simulated distributions of water, thiosulfate and chloride ions (injected as sodium chloride at 0.5% concentration as a tracer with the sodium thiosulfate) are compared against field distributions of water, determined by neutron probe, and ion concentrations, determined from the analysis of soil cores.

NAPL Migration in Response to Hydraulic Controls at the Brooklawn Site near Baton Rouge, Louisiana

M. D. White, M. Oostrom
Pacific Northwest National Laboratory, Richland, Washington

Proceedings of Twenty First Annual American Geophysical Union Hydrology Days, Fort Collins, CO

Abstract

Petro-Processors of Louisiana, Inc. (PPI), a contract waste disposal company operating in East Baton Rouge Parish, Louisiana, disposed effluent from petrochemical processing industries at two waste disposal facilities, the Brooklawn and Scenic Highway sites, from 1964 to 1980. In July of 1980, the U.S. Justice Department and state and local governments jointly filed suit against PPI and several waste generators that used the site for disposal. Since mid-1980, the responsible industries along with regulatory agencies have conducted several investigations at the site. These investigations have detected chlorinated aromatic and nonaromatic hydrocarbons in samples from the soil, groundwater, and air at the both disposal sites and in surface water samples at the Brooklawn site. Organic contaminants have been detected off-site in water samples, sediments and fish. On February 16, 1984, a Consent Decree for site closure was finalized with the participation of all parties and the court. The consent decree specified that the potential responsible parties (PRP), represented by NPC Services Inc., implement a remedial investigation, design a remedial plan and conduct long-term monitoring at the Brooklawn and Scenic Highway sites.

An estimated 160,000 tons of organic contaminants in the form of dense nonaqueous phase liquids (DNAPLs) were disposed into lagoons at the Brooklawn site. A portion of the disposed liquids has migrated from the disposal lagoons into subsurface as DNAPL and dissolved organic contaminants. In conjunction with the remediation plan for the Brooklawn site, NPC Services, Inc. has been using hydraulic containment and source recovery to mitigate the spread of the NAPL plume. A matrix of pumping wells, within and surrounding the DNAPL plume, has been used for the removal of DNAPL and contaminated groundwater. Organics recovered from these wells are incinerated and groundwater is treated and pumped offsite toward the Mississippi River. Rates of DNAPL recovery from the source wells have declined considerably from the startup values in 1994, with some peripheral wells having negligible recoveries.

The objective of this investigation was to evaluate the effectiveness of the hydraulic containment strategy being implemented at the Brooklawn Site to control DNAPL migration toward a fresh water aquifer. The investigation comprised experimental and numerical components. Laboratory experiments on soil samples and pumped DNAPL from the Brooklawn site were conducted to determine hydrologic properties of the soils and physical and chemical composition of the liquid. Numerical simulations were conducted using a multifluid simulator for multiple realizations of a two dimensional cross-section through the Brooklawn site transecting the region of known DNAPL contamination. Multifluid flow behavior considered included three-phase retention and relative permeability characteristics, nonwetting fluid entrapment, and multiphase pumping. The principal objective of the simulations was to generate quantitative comparisons between various hydraulic control options, thus providing a stronger scientific rationale for future environmental management decisions at the site. Results indicate that under current conditions the pumping wells peripheral to the DNAPL plume do not significantly contribute to hydraulic control of DNAPL migration or source recovery.

Side-Slope Considerations for Above-Grade Earthen Covers

A. L. Ward, G. W. Gee, M. D. White
Pacific Northwest National Laboratory, Richland, Washington

Proceedings of the International Containment and Remediation Technology Conference and Exhibition, Orlando, FL. (2001).

Abstract

Above-grade earthen covers are often used at newly constructed waste sites and for waste left in place at toxic-waste landfills, both to minimize worker health risks and reduce excavation costs. Side slopes are critical to the overall performance of such covers and traditionally have been used to stabilize cover extremities. Because protective slopes may occupy over half the footprint of covers at small waste sites (< 5 ha), they can potentially influence the overall water balance. Yet, there is no consistent design standard to optimize hydrologic performance. A multi-year test, comparing two side slope designs, was recently initiated at the Department of Energy's Hanford Site where a field-scale prototype cover was placed over a radioactive waste trench. Results show that side slopes play an important role in a cover's water balance. Essentially all measured drainage from the cover was due to net infiltration into the coarse-gravel and rock side slopes. As much as 30 percent of annual precipitation intercepted by the side slopes has been captured and diverted through passive and active lateral drains. Numerical simulations with the STOMP simulator confirmed that advective loss from wind action on the rock surfaces reduces, but does not eliminate, drainage from the rock slopes. These data suggest that in arid or semiarid climates, consideration of slope design and the impacts of lateral diversion on adjacent waste sites must be a top priority. The ideal side slope design should minimize water accumulation and intrusion along the extremities. Design considerations might include water harvesting by terraced vegetated strips on side slopes.