A modeling feasibility study was conducted on an in-situ arsenic binding process for an aquifer in Washington State. The site is a production facility for arsenic-based pesticides. An assessment of a geochemically- engineered design for re-precipitation of iron arsenate solids as an arsenic sulfide mineral phase was completed. The modeling of the re-precipitation mechanism was completed using a coupled geochemical speciation and mass transfer model and a mass transport model in two dimensions, specifically constructed for this evaluation. Hydrochemical reactions were simulated in a stream tube representative of the three-dimensional flow system. The stream tube was divided into flow cells, and within each cell, the mass transfer code PHREEQE was used to model mass transfer. Equilibrium was not obtained in most cells given the relatively short residence times in each cell. The non-equilibrium condition was simulated by modifying the equilibrium constants proportional to reaction time and available reactive mass in each cell. The amount of modification was determined from batch experiments on the soils. A second-order reaction relationship was determined from the batch testing and used for the predicting the degree of reaction completion. The modeling enabled simulation of several very different starting solutions at the point of recharge to the contaminated aquifer in order to promote in-situ re-precipitation.
SSP&A has over fifteen years of experience in the application of chemistry to environmental problems from hazardous waste disposal, industrial contamination, mining and milling effluents, acid mine drainage, and in-situ remedial design for hazardous-waste cleanup.
Services in environmental chemistry include:
- Chemical fate modeling of subsurface contaminants;
- DNAPL investigation and in-situ solubilization stategies;
- Molecular and isotopic fingerprinting of contaminants;
- Waste-water remediation modeling;
- Chemical speciation modeling for chemical fate;
- Coupling of environmental chemistry simulation with numerical groundwater flow and transport models.
Feasibility Study of an In-Situ Arsenic Binding Process
Complementary Inverse & Forward Modeling of Acidic Seepage Formation; Porphyry Copper Mine, Western US
For a private client involved in litigation, SSP&A employees developed a mixing model for the origin of acidic seepage flowing at several million gallons per day near the base of an alkaline tailings pond dam in an area of historic mining adjacent to present-day heap-leach mining operations. From chemical analyses of the acidic seepage, natural drainage, tailings pond, and leach pad operations-related waters, a mass-balance model was derived for the acid drainage using the codes WATEQ4F and NETPATH (inverse modeling approach). The mass-balance model indicated that the acidic seepage originated from a mixture of tailings pond water and leach pad effluent. The results of the mass-balance model were then used in mass- transfer simulations using PHREEQE (forward modeling approach). Tailings pond water and leach pad water were mixed incrementally and supersaturated secondary minerals were allowed to precipitate. The predicted chemical composition was in good agreement with the observed chemistry of the acidic seepage, thus confirming the thermodynamic validity of the mass balance model.
RCRA Investigation of Former Refinery Site Casper, Wyoming
For a RCRA Facility Investigation (RFI) of this former BP-Amoco refinery, SSP&A provided technical support in geochemistry, groundwater, and geology, and participated in Collaborative Process meetings with Wyoming State agencies regarding Soda Lake, a former discharge point for refinery effluent. SSP&A's initial activities involved the development of a site conceptual model of groundwter for the Soda Lake region; siting of numerous groundwater-monitoring wells in both Cretaceous sedimentary formations and unconsolidated sandy soils; evaluation of the water balance and selenium biogeochemical cycle in the lake; and, development of a probabilistic model for predicting future selenium concentrations in lake water and sediments under different management alternatives.
Soda Lake was found to be a classical discharge lake, with a deeper groundwater underflow component. A goal of the RFI was to demonstrate how the Remedy Decision data were of sufficient quality and quantity to evaluate the nature and extent of contamination present or probable at the site and to support a Risk Assessment and a Corrective Measures Studies. Based SSP&A’s studies, the shallow groundwater flow system was found to discharge from the total lake periphery into Soda Lake, which lies in an enclosed basin. No groundwater was found to migrate down-gradient toward the North Platte River. This conclusion was based on (a) SSP&A’s water-balance modeling, which demonstrated that all water loss from the Soda Lake basin is via evaporation; (b) groundwater contour maps and hydrographs documenting that groundwater flows radially into Soda Lake; and (c) chemical analyses of groundwater and lake water indicating that net groundwater flow is toward Soda Lake.
Predictive Water-Quality Model; Abandoned Mine, Western US
For a private client involved in litigation, a model capable of predicting changes in water chemistry with time in an abandoned mine was developed by SSP&A. The purpose for the modeling task was to evaluate options for treatment of contaminated surface water and for partitioning of financial responsibility among the PRPs. The geochemical model involved two steps. First, the inflow of chemically-distinct water from various sources was modeled as a time-dependent mixing process using both PHREEQE and inflow data derived from a groundwater flow and transport model. The model allowed acid sulfate water to accumulate and to react with aluminosilicate minerals in the walls and floor of the mine and allowed for secondary minerals to form. Second, to simulate slow weathering reactions which remove protons from the water by silicate hydrolysis, the code MPATH was used. MPATH allows a kinetic treatment of dissolution and precipitation reactions in water-rock systems and thermodynamic treatment of complexation, ion association, and aqueous redox reactions. Because of the kinetic formulation of MPATH, slow processes can be modeled explicitly as a function of time. For example, simulations to 1000 years initially were made to determine the time scale of the weathering process. It was found that the time scale for neutralization of the acidity of the surface water and concomitant removal of heavy metals from the water could range from several decades to a few hundred years depending on the rate of deposition of windblown dust particles to the water surface.
This project is currently ongoing.
Agrico Chemical Company Pensacola, Florida
The Agrico Chemical Company (Agrico) site in Pensacola, Florida, was listed on the National Priorities List in 1989. Superphosphate fertilizer and sulfuric acid were produced at the site starting in the early 1900s. The acidic process wastewater containing fluoride was discharged to ponds. One public supply well was shut down in the late 1950s due to low pH. At the time, operations at the Agrico site were suspected as a source of the low pH. SSP&A provided expert services for Agrico in class action lawsuits. The originally proposed class included about 6000 residential properties. SSP&A used the concentrations of fluoride measured in groundwater downgradient from the Agrico site to define the extent of impacts from past operations that took place at the Agrico site. The plume delineation and groundwater flow regime determined by SSP&A were central to a legal motion that resulted in a substantial reduction of the initial class size. This project included detailed geochemical and hydrologic modeling, as well as an evaluation of the origin, fate, and transport of radionuclides (uranium, thorium, radium) in the subsurface environment. The lawsuits were settled in March 2004 to the satisfaction of our client.
Santa Cruz Atacamite/Chrysocolla Deposit, Arizona
For SAIC and the US Bureau of Mines Twin Cities Research Center, employees of SSP&A developed an acid-attenuation process model for use in modeling the fate of sulfuric acid lixiviant injected deep underground for in-situ mining of copper. The ore body of interest is the Santa Cruz atacamite/chrysocolla deposit in Arizona. The chemical dissolution reactions and precipitation reactions associated with rate-controlled leaching of the atacamite-bearing ore were successfully simulated. The focus was the net consumption of acidity in the injectate with dissolution of ore followed by the re-precipitation of metal hydroxides along the migrating-metals front. The mass transfer model PHREEQE was employed in the reaction modeling to solve the mass action equations. The results were employed in the mass transport model SWIFTII to simulate the migration of H+ ions. To simulate the neutralization of H+ ions along the flow path, acidity was 'decayed' in the mass transport model at a rate commensurate with the modeled uptake by the atacamite and chrysocolla ores.
L Bar Uranium Mine, New Mexico
A hydrochemical model of the fate of acidic seepage from beneath a 50-acre tailings impoundment was developed and applied for predicting the movement of heavy metals. The site was actively leaking sulfuric acid milling waste water to the underlying sandstone aquifer. Migration of the metal-laden milling water through the aquifer was impacting the quality of the fresh-water aquifer. Acid-attenuation modeling demonstrated the natural attenuation of most metals and re-precipitation of various secondary mineral phases at the reaction front. The aquifer contained several acid-reactive minerals at significant concentrations. The lack of attenuation of dissolved nickel and uranium lead to the installation of several groundwater extraction wells along the property boundary. Particle tracking using SWIFTII and PTRACK software guided the installation of the extraction wells to provide reasonable assurance of complete capture of the plume of metals.
Uranium Mine Water Purification
For a private client, SSP&A employees have developed a thermodynamic model for the attenuation of uranium, radium, and molybdenum from uranium mine discharge waters. Application to date has been at southwest Texas mines. The attenuation model involves ion complexation and mineral precipitation reactions and is solved using the USGS chemistry model PHREEQE. The thermodynamic stabilities of the mineral phases are calibrated to batch-test data. With the model in hand, scavenging powders can be custom-blended for the specific compositions of waters encountered at each mine site. The treatment of water is currently completed in a portable water-treatment system rated at several hundred gpm. The treatment process is currently undergoing EPA SITE program assessment.