HGC Masthead


Winter 1998
In This Issue:

Chemical Incompatibility of Bentonite with DNAPLs

Acid Rock Drainage: Challenge for Mining

Limitations of Hydraulic Containment for Plume Management


Chemical Incompatibility of Bentonite with DNAPLs

Bentonite clay has many environmental applications where a hydraulic barrier is desired. For example, bentonite can be used to construct slurry walls, annular seals in wells, liners, covers and to backfill exploratory borings. Bentonite-based barriers are sometimes constructed at hazardous waste sites to separate dense non-aqueous phase liquids (DNAPLs) from the surrounding groundwater. A recent laboratory investigation identified structural incompatibility between bentonite seals and DNAPLs (McCaulou and Huling, 1998). Bentonite seals constructed in double-ring permeameters deteriorated and hydraulically failed when exposed to trichloroethylene (TCE), methylene chloride (MC), and creosote. These results indicate that current guidelines for constructing wells (U.S.E.P.A. RCRA, 1992) and slurry wall systems at hazardous waste sites may be inadequate with regard to DNAPLs.

The following summary highlights the observed effects of DNAPLs on bentonite seals under different exposure scenarios.

Figure 1 - TCE moved through desiccation cracks that permeated the bentonite barrier Permeability to Saturated Aqueous Concentrations of TCE, MC, and Creosote
Hydration and exposure of bentonite with water containing saturated concentrations of TCE, MC, and creosote did not alter its hydraulic conductivity relative to water. These results indicate that bentonite structures in heavily-contaminated groundwater, but outside the DNAPL zone, may resist chemical desiccation and hydraulic failure. However, bentonite barriers containing soluble concentrations of these compounds may be a long-term source of dissolved organic compounds to the surrounding groundwater after the designed remediation period.

DNAPL Immersed Bentonite Pellets and Subsequent Water Hydration / Permeation
Bentonite pellets immersed in DNAPL retained their rigid shape, did not swell, and did not form a hydraulic barrier. However, when the DNAPL was removed and replaced with water, the DNAPL-wetted pellets imbibed water to swell and form a competent hydraulic barrier (Figure 4). However, discrete zones of DNAPL remained in the bentonite. These results indicate that bentonite pellets exposed to DNAPL retain their swelling potential and will retain some of the DNAPL as a source of organic compounds after hydration.

Figure 2 - Methylene Chloride moved through desiccation cracks Water Hydrated Bentonite Permeated with DNAPL
Competent hydraulic barriers constructed with bentonite pellets, hydrated with water, and exposed to liquid TCE, MC, and creosote deteriorated (Figures 1, 2, and 3). Deterioration involved the formation of desiccation cracks up to 5 mm wide. Cracks were largest at the top of the barrier and were not preferentially located at the sidewall interface. Vertical cracks initiated at the surface intersected with other near horizontal cracks that appeared to connect to other vertical cracks below. The intrinsic permeability of water-hydrated bentonite permeated with DNAPL was 46 to 2,640 times greater relative to water. The network of cracks facilitated the rapid, preferential flow of DNAPL through the clay.

TCE Permeability of Bentonite Grout and Sand Mixtures to TCE
Silica sand is expansively inert, yet, 50%, 75%, 83%, 90%, and 95% (wt silica sand/wt bentonite) mixtures with bentonite grout were insufficient to prevent desiccation cracking and/or hydraulic failure. Results indicate that the addition of sand to bentonite decreases the resistance to water flow and the bentonite remains vulnerable to chemical incompatibility mechanisms.

Figure 3 - Creosote created deep desiccation cracks Effect of Hydraulic Head on Permeability
Visible cracks formed in bentonite exposed to TCE in less than 1 week at 25 feet of total head, approximately 1 month at 5 feet of total head, and in 3 months under hydrostatic conditions (1 inch TCE) indicating that the rate of desiccation crack propagation was positively correlated with pressure. However, formation of desiccation cracks under hydrostatic conditions indicates that the processes that initiate chemical desiccation are dependent on chemical reactions and not solely dependent on hydraulic pressure.

Additives Which May Decrease Incompatible Effects Between DNAPLs and Bentonite
Presently, additives to bentonite are under evaluation to minimize the effects of incompatibility. Additives being tested include organophilic clays, gelling agents, and surfactants. Test results and data interpretation have not been completed.

Figure 4 - A competent, water-permeated bentonite barrier Conclusions
These laboratory investigations point to the possibility that DNAPL may compromise the integrity of various types of bentonite barriers. The incompatibility between bentonite and these DNAPLs indicate that the structural integrity of hydraulic barriers may be compromised at the critical locations they were designed to hydraulically isolate.

The laboratory conditions under which these investigations were performed do not, however, replicate field conditions. Caution should, therefore, be used in extrapolating these results to field conditions.

References:
McCaulou, D. R., and S. G. Huling, 1998, Compatibility of Bentonite and DNAPLs, Groundwater
Monitoring and Remediation, (to be submitted, January 1998)

U.S.E.P.A. RCRA Groundwater Monitoring Technical Guidance Document. 1992.
Washington D.C. EPA/530-R-93-001.


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Acid Rock Drainage: Challenge for Mining

The technological challenge of preventing and controlling acid rock drainage (ARD) was the central topic of the Fourth Annual International Conference on Acid Rock Drainage (ICARD) in Vancouver, B.C.

Although ARD can occur naturally where sulfide minerals are exposed to weathering, the conference focused on prevention and mitigation of acidification and metal contamination that can result when ground or surface water contacts metal sulfide-bearing rock exposed by mining. The conference emphasized hard rock metal mining, but the information reported was applicable to coal mining, quarrying, and industrial processing; the fundamental physical and chemical processes controlling the environmental fate and transport of acidity and metals are the same.

The conference proceedings were organized into five sections: special presentations by ICARD sponsors, prediction, prevention, treatment and control, and monitoring and restoration. Several ICARD sponsors provided examples of how their companies proactively use geochemical methods of ARD prediction to classify waste rock and develop waste-management strategies for ARD prevention in planning new mining operations.

Papers were presented on a wide range of topics: geochemical and mineralogical characterization methods for predicting future chemical loading from rock, tailings, and sludge; evaluations of cover designs to reduce or eliminate moisture and oxygen transport; subaqueous disposal methods; the limnology and geochemistry of pit lakes; and treatment of solids and liquids using chemical and biological methods, to name just a few.

The diversity of topics in the proceedings exemplifies the great variety of options and issues facing the mining industry worldwide. Although significant progress has been made in the prediction and control of ARD over the last ten years, no standard approach exists to evaluate the potential for or the mitigation of ARD. Only by careful analysis of site-specific physical, chemical, and operational conditions can comprehensive options be developed to cost-effectively prevent or control ARD.

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Limitations of Hydraulic Containment for Plume Management

Figure A - Ideal Hydraulic Containment Zone for a Single Well So-called "pump and treat" is a generally accepted approach for controlling the migration of contaminants in groundwater. The approach is based on the concept, illustrated in Figure A, that a pumping well creates a capture zone in which all groundwater eventually flows to the well. The edge of the capture zone is defined by a bounding streamline. The bounding streamline is a hydraulic barrier in the sense that no macroscopic flow of water occurs across the bounding streamline under steady flow conditions (Bear, 1979). Thus, in theory, the well establishes a hydraulic capture zone around the area of contaminated soil or groundwater and prevents dissolved contaminants from leaving the capture zone. In practice, hydraulic containment may not prevent plume migration for several reasons: failing to consider the effects of hydrodynamic dispersion; failing to consider the effects of short-term variations in natural groundwater flow velocities; and failing to consider the effects of variations in groundwater pumping rates.

Figure B - Bounding Streamline and Pore-Scale Fluid Paths Hydrodynamic Dispersion
In the simplest case, the movement of contaminants in groundwater is controlled by two processes: advection, which is the movement of dissolved contaminants with and in the same direction as the bulk groundwater flow; and hydrodynamic dispersion, which is the spreading of dissolved contaminants due to both small-scale variations in flow directions and molecular diffusion. Hydrodynamic dispersion results in the spreading of contaminants from zones of higher to lower concentration and transverse to the direction of bulk groundwater flow.

This process is illustrated in Figure B which shows the large-scale position of the bounding streamline zone and the small-scale pathways that water and contaminants actually follow. Even though the average direction of flow is toward the well, the actual paths of water flow back and forth across the bounding streamline. If the water flowing inside the capture zone near the bounding streamline is contaminated, the contaminants will cross the bounding streamline and be outside the capture zone. Due to the simultaneous action of molecular diffusion, which results in an irreversible movement of contaminants from zones of higher to lower concentrations, not all of the contaminant mass that leaves the capture zone at one location will return. This process can result in a “bleed” of contaminants out the capture zone, as illustrated in Figure C.

Figure C - Containment Bleed from Containment Zone Many pump-and-treat systems work because the wells are pumped at a high enough rate that the capture zone encompasses not only the contaminated portion of the aquifer, but also uncontaminated groundwater. In such cases, the groundwater flowing near the edge of the capture zone is either uncontaminated or contains very low concentrations of contaminants. Contaminant bleed out of the capture zone is thus limited by a buffer zone of relatively clean water. Even in this best case, however, the principles of mass transport discussed dictate that some contaminant mass, albeit small, will eventually disperse out of the capture zone.

Problems with contaminant bleed may become significant, however, when the capture zone is established so as to just barely encompass the contaminated portion of the aquifer. In this case, contaminated water will be flowing near the boundaries of the capture zone and significant bleed may occur due to hydrodynamic dispersion.

Variations in Groundwater Flow and Pumping
The location of the bounding streamline of the capture zone is determined by the ratio of groundwater pumping to the background rate of groundwater flow. At a particular site, the background rate of flow may be affected by short-term variations in recharge, seasonal variations in groundwater levels, and pumping by third parties. At sites near rivers, the rate and even direction of groundwater flow may vary significantly depending on river stage. These uncontrolled factors will cause the position of the bounding streamline to change with time unless appropriate adjustments are made to the pumping. An analysis of the influence of these factors will be presented in an upcoming issue of the Gradient.

References:
Bear, J. 1979. Hydraulics of Groundwater. McGraw-Hill, NY.

Gary R. Walter, Ph.D.

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