Open pit mining has increased substantially throughout the western United States over the past 20 years. During active mining, groundwater seepage, rainfall, and surface runoff are continuously removed from the pit. When mining and dewatering stop, water from these sources may accumulate in the inactive pit resulting in a "pit lake". This process is illustrated in Figure 1. If the pit wall rock or bottom sediment contains oxidizable minerals, particularly sulfide minerals, the pit lake may turn acidic and contain elevated concentrations of metals and sulfate. The pit lake may then pose an environmental hazard to wildlife and adversely affect groundwater quality. Whether or not a pit lake develops unacceptable water quality depends on the nature of the wall rock, the hydrodynamics of the lake, and its hydrogeologic setting. Sulfide oxidation is the primary process responsible for acidification and other water-quality problems in pit lakes. Understanding the physical and chemical processes controlling the oxygen budget of the lake is the key to predicting and controlling future water quality in the lake. A typical oxygen budget for a pit lake is shown in Figure 2. The three primary oxygen sources to the lake are:

• re-aeration from the atmosphere,
• photosynthetic oxygen production, and
• transport of dissolved oxygen in groundwater seepage (Thomann and Mueller, 1987).

For most pit lakes, re-aeration will be the primary source of oxygen, although the other two sources may be important under special circumstances. The availability of dissolved oxygen in contact with oxidizable minerals is required for acidification to occur. Because these minerals are generally located in the wall rocks and bottom sediment, whereas oxygen enters the lake from primarily the surface, acidification is strongly controlled by hydrodynamic mixing in the lake. Water circulation within a deep pit lake is largely controlled by the vertical profile of water density. Stable stratification results when the densest water is at the bottom of the lake. Unstable conditions occur when dense water occurs above less dense water. The lake water density depends on water temperature and dissolved solids content. In most pit lakes, water temperature controls the density of the water. Water temperature depends on climate, radiative heating, and the temperature of groundwater seeping into the lake. Climate is usually the most important control on the thermal regime of the lake. A typical heat budget for a pit lake is given in Figure 3.

Lakes in areas with small annual temperature fluctuations will develop little temperature stratification. Oxygen and chemical transport in these lakes are largely controlled by wind-induced eddy currents, groundwater circulation, and molecular diffusion resulting in oxygen mass transfer to the lake bottom throughout the year. Thus, the entire wall rock, bottom sediment, organic matter, and any other life forms will have access to dissolved oxygen year round. The presence of oxygen-consuming reactions with minerals and organic matter at the lake bottom will produce an oxygen distribution which monotonically decreases with depth throughout the year.

Deep lakes in colder climates develop strong thermal stratification, and transport of oxygen to deeper portions of the lake is dominated by the seasonal stability of the stratification. When the thermal stratification is stable, relatively little oxygen transport occurs between the epilimnion and hypolimnion (see Figure 2). When the stratification becomes unstable (density of the epilimnion is greater than that of the hypolimnion), the lake may turn over carrying oxygenated, shallow water to the bottom and deep, mineralized water to the surface. In climates where the surface water temperature remains above 4^o C (39^o F), a lake may turn over once a year in the winter. In colder climates, where the surface water cools below 4^o C, lakes may turn over in both the fall and spring.

Most of the larger existing pit lakes contain water that does not meet drinking water standards, agricultural water quality standards, or aquatic life standards. Given the recent mining activity in the West, numerous pit lakes may be expected to form over the next 20 to 50 years. Because most pit lakes have yet to form, mathematical models represent the only way to estimate their ultimate environmental impact. The ability to predict the hydraulic structure of pit lakes may lead to development of operation strategies for controlling their chemistry. Hydro Geo Chem has initiated a research program to better understand the water quality and dynamics of this new type of human-made water body. Results of this research will be used to assess and mitigate the potential environmental effects of pit lakes.

Reference: Thomann, R.V. and J.A. Mueller. 1987. Principles of surface water quality modeling and control. Harper Collins, New York, NY.

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Efficiency of Air Sparging Trenches

This article is a continuation of a series of articles in the HGC Gradient on air sparging processes. Air sparging is a groundwater remediation process that removes volatile organic compounds (VOCs) from groundwater by injecting air into water-saturated sediments.

Two approaches to air-sparging are common. The first involves injecting air through wells into the contaminated portion of the aquifer, thus directly attacking the contaminant plume. The second approach creates a barrier to VOC transport by sparging downstream from the contaminated zone (see Pankow and others, 1993).

In the sparging barrier approach, a sparging zone is created using vertical wells, horizontal wells, or trenches to inject air at a location where the contaminated groundwater will move through the sparging zone (Figure A). To the extent that the design is successful, VOCs are removed from the groundwater as it passes through the sparging zone and the water is aerated to enhance biodegradation of less volatile compounds.

As illustrated in Figure B, trench sparging is conceptually quite simple, but is limited to relatively shallow depths. In principle, a trench sparging system should be highly effective and simple to operate because contaminated groundwater is physically and hydraulically constrained to move through the sparging zone.

Hydro Geo Chem, Inc. recently completed a detailed pilot study of the performance of a trench sparging system at a site in Arizona that was contaminated with gasoline-range petroleum hydrocarbons. The test was designed, in part, to determine the efficiency of the sparging trench in removing benzene, toluene, ethylbenzene, and total xylenes (BTEX) at various air injection rates.

The results of the test are summarized in Figure C which shows the concentrations of the target compounds measured in the trench with time, as the air injection rate was increased. The continuous lines in Figure C represent the concentrations of each compound predicted by a mathematical model of the sparging process.

The model accounts for the dimensions of the sparging trench, the air injection rate, the groundwater flow rate, the gas-water partitioning coefficient (Henry’s Law coefficient) of the compounds, and the dynamic sparging efficiency. The dynamic sparging efficiency represents the effect of deviations from complete chemical equilibrium between the contaminated water and the sparge air on the sparging rate. The model is used to calculate the dynamic sparging efficiency at each air injection rate by adjusting the efficiency factor to match the observed concentrations.

As indicated by the insert table in Figure C, the estimated efficiency factor for BTEX compounds decreased from 50% at the lowest air injection rate to 5% at the highest. The fact that the concentration of each compound could be matched using a consistent efficiency factor at a given injection rate indicates that the sparging efficiency was affected by physical rather than chemical factors. The decrease in sparging efficiency with increasing air injection rates is probably the result of channelized air flow reducing the air/water contact area and contact time. Even at the lowest efficiencies, 99% reductions in concentrations were achieved for BTEX. Similar reductions were obtained for gasoline-range total petroleum hydrocarbons.

Reference:

Pankow, J.F., R.L. Johnson, and J.A. Cherry. 1993. Air sparging in gate well in cutoff walls and trenches for control of plumes of volatile organic compounds (VOCs). Ground Water. vol. 31, no.4, pp. 654-663.

Jinshan Tang, M.S.

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RCRA Update in the Pipeline

The Notice for Proposed Rule Making (as opposed to the advance notice) for the Subpart S revisions was originally scheduled for mid-1997, but has been delayed until December 1997. At that time, only some of the revisions will be officially proposed, while others will be re-crafted. The final rulemaking is anticipated in December 1998. So, these rules aren’t a quick fix, but the start of a long process that should result in more efficient and cost-effective RCRA corrective actions.

Alternate Regulatory Acronyms:

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HGC Professional Named BTI "A-Team" Performer

Every year, BTI Consulting Group, Inc. interviews organizations that use environmental services about individuals and organizations that are most effective. This year, BTI conducted 630 open-ended, unprompted interviews of these customers, who, altogether, nominated 104 service providers in the US.



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