U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Evapotranspiration Covers

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Application • Mining

Evapotranspiration covers have been installed in arid climates to prevent the infiltration of water to mine tailings and subsequent mobilization of metals and sulfate in acid rock drainage. During precipitation events, the covers are designed to store moisture, subsequently releasing the moisture to evaporation and transpiration following the precipitation event. By so doing, the cover prevents infiltration from penetrating into the underlying tailings. Such covers are often termed "store and release" covers.

The following resources offer examples of the use of ET covers at mining sites.

AA Leach Pad Cover Design: A Successful Reclamation Project at a Nevada Gold Mine
Zhan, G., M. Milczarek, J. Keller, and J. Giraudo.
International Mine Water Association, 2013 Annual Conference, Golden, CO. 237-242, 2013

The AA Leach Pad at the Barrick Goldstrike Mine was reclaimed using an ET cover designed to limit meteorological infiltration through the leach pad. Eleven years of monitoring data indicate that the cover is limiting net percolation through the pad to less than 1% of precipitation. Results from the cover monitoring study provide an overview of ET cover system requirements and performance in an arid environment.

Cover System Performance in a Semi-Arid Climate on Horizontal and Sloped Waste Rock Surfaces.
O'Kane, M., D. Porterfield, A. Weir, and A. Watkins. ICARD 2000: Proceedings of the 5th International Conference on Acid Rock Drainage, May 21-24, 2000, Denver, Colorado. 11 pp, 2000

BHP Iron Ore initiated a program in January, 1995 at their Mt. Whaleback operation in Newman, Western Australia to develop a decommissioning plan for the waste rock material. The primary research program includes the development of technology for the long term performance of the waste rock dumps with respect to vegetation, slope stability, surface runoff, erosion, and water infiltration. This paper evaluates field performance of cover systems constructed on a horizontal and a sloped waste rock surface. The cover system is constructed using suitable run-of-mine waste material to minimize closure costs. The moisture is subsequently released to the atmosphere as evapotranspiration. Rainfall entering the waste material is buffered due to the presence of the cover material thereby significantly reducing net percolation to the underlying waste rock. The objective is to control acid rock drainage by preventing moisture movement into and through the waste rock material. Two years of field data are presented to illustrate low percolation rates to the underlying waste rock and key performance characteristics of the moisture store and release cover system design. Field data collected to date demonstrates that a moisture store and release cover system constructed with suitable run-of-mine waste material has good potential as a final acid rock drainage control cover system at the Mt. Whaleback site. The performance of the cover system on a sloped surface was significantly altered as compared to placing the cover system on a horizontal surface.

Determination of the Reasons for Deterioration of the Rum Jungle Waste Rock Cover.
Taylor, G., A. Spain, A. Nefiodovas, G. Timms, V. Kuznetsov, and J. Bennett. International Network for Acid Prevention, 119 pp, 2003

The 18 year old cover Rum Jungle in Northern Territory, Australia is one of the oldest cover systems for which the design, construction, and continuous monitoring date are well documented. Water infiltration through the covers increased significantly in the last few years, and this project was designed to use the data history to understand the reasons for the deterioration performance. The Australian Nuclear Science and Technology Organisation (ANSTO) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) were contracted to undertake the work. The study found that the cover materials no longer met the original design specifications, particularly for permeability, which increased by several orders of magnitude. Findings from the study indicated that the increased permeability may be attributed to (1) a shortage of cover material during construction resulting in bare patches; (2) a combination of biological and physical process, such as galleries formed by termites and ants, (3) root growth from the pasture grasses and the few volunteer trees, and (4) an extensive system of shrinkage/dessication cracks.

Evaluation of the Long-Term Performance of Dry Cover Systems.
O'Kane Consultants, Inc. International Network for Acid Prevention, Report 684-02, 195 pp, 2003

The objectives of dry cover systems are to minimize the infiltration of water and provide an oxygen diffusion barrier to minimize the influx of oxygen. Apart from these functions, dry covers are expected to be resistant to erosion and provide support for vegetation. This report is the culmination of a two-phase project examining the long-term performance of dry covers for reactive mine waste. Phase 1 consisted primarily of a literature study. Information was compiled regarding; the processes affecting the long-term performance of dry cover systems, the numerical models capable of predicting long-term performance, laboratory characterization of cover materials, and field performance monitoring practices. Phase 2 included collecting and analyzing detailed performance monitoring data for five mine sites located in Canada, Australia, and the United States. Field saturated hydraulic conductivity tests were completed at the four North American sites to examine changes in saturated hydraulic conductivity of the cover system material with time. A calibrated numerical model, selected from the Phase 1 study, was developed for three of the five sites. The calibrated model allowed examination of cover system performance at these sites and provided information regarding key processes and characteristics that will control long-term performance.

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Long-Term Performance of a Reclamation Cover: The Evolution of Hydraulic Properties and Hydrologic Response.
Kelln, C.J., S.L. Barbour, A. Elshorbagy, and C. Qualizza. Unsaturated Soils 2006: Proceedings of the 4th International Conference on Unsaturated Soils, 2-6 April 2006, Carefree, Arizona. ASCE, Reston, VA. Geotechnical Special Publication No. 147, ISBN: 0784408025, 2006

The performance of a prototype reclamation cover constructed over saline-sodic shale overburden was tracked over a six-year period. The test cover, constructed on a 5H:1V slope, was comprised of a thin layer (approximately 20 cm) of a peat-mineral soil mixture overlying a thicker layer (approximately 80 cm) of `secondary' (glacial lacustrine or till). The primary objective of the cover design was to provide sufficient moisture storage for vegetation while mitigating the upward diffusion of salts from the underlying pyritic shale. In situ measurements of saturated hydraulic conductivity were conducted using a Guelph permeameter. Hydrologic measurements included soil moisture, matric suction, soil temperature, surface runoff, and interflow. The major ion chemistry of both surface runoff and interflow waters were measured. In situ measurements of hydraulic conductivity over the six-year period demonstrated that the cover developed secondary structure within four years of placement, likely due to repeated freeze/thaw and wet/dry cycles. The hydrologic response data and interflow chemistry suggests that the mechanisms responsible for the rapid delivery of precipitation to the base of the cover during spring melt are controlled by interactions between soil conditions and climate. Preferential flow occurs when a threshold wetting is achieved and when the ground is either frozen or the matric suctions are low. It also appears that the relative contribution of 'event' and 'pre-event' water evolves as interflow proceeds. Lateral interflow appears to be an important mechanism for mitigating the upward diffusion of salts into the cover. [evapotranspirative covers]

Long-Term Performance of a "Store/Release" Cover over Potentially Acid Forming Waste Rock in a Semi-Arid Climate.
Williams, D.J., D.J. Stolberg, and N.A. Currey. Unsaturated Soils 2006: Proceedings of the 4th International Conference on Unsaturated Soils, 2-6 April 2006, Carefree, Arizona. ASCE, Reston, VA. Geotechnical Special Publication No. 147, ISBN: 0784408025, 2006

The reclamation of potentially acid forming waste rock is site specific, being a function, among other factors, of the rock types, the dumping and storage method employed and, importantly, the climatic setting. The "store/release" cover developed to manage acid rock drainage from mineralised waste rock dumps at Kidston Gold Mines' open pit operations in the semi-arid, seasonal, sub-tropical climate of North Queensland, Australia, has been monitored for up to 9 years. The paper describes the philosophy behind the store/release cover design and its adaptation over time to suit Kidston's conditions. The results of monitoring of the store/release cover system over 9 years are presented, and estimates are made of the water balance of the store/release cover for a range of annual rainfall totals and of the overall water balance of the waste rock dumps. The Kidston story is a valuable case study of a successful approach to remediating an identified source of acid rock drainage in a semi-arid climate, which has actively engaged all Stakeholders. [evapotranspirative covers]

Performance Evaluation of the Engineered Cover at the Lakeview, Oregon, Uranium Mill Tailings Site.
Waugh, J., G. Smith, B. Danforth, G. Gee, V. Kothari, and T. Pauling. Waste Management 2007 Conference, 25 February - 1 March, Tucson, Arizona. Paper 7499, 15 pp, 2007

The cover of the Lakeview, Oregon, disposal cell relies on a compacted soil layer (CSL) to limit radon escape and water percolation into underlying tailings. The design created habitat favorable for growth of woody plants that sent roots through the CSL. The mean saturated hydraulic conductivity (Ksat) of the CSL, measured at 17 locations, was 3.0 x 10-5 cm s-1, 300 times greater than the design target. The highest Ksat values were measured near the top of the CSL at locations both with and without roots; the lowest Ksat values were measured deeper in the CSL. Water flux meters (WFMs) installed in 2005 to directly measure percolation flux show significant percolation through the cover. Three WMFs began recording percolation in mid-November, 7 days after the start of a prolonged precipitation event, and continued until early June 2006. Percolation flux during this period ranged between 3.1 x 10-5 and 8.5 x 10-5 cm s-1. The cumulative percolation was greater than total precipitation during the period, probably because of a water-harvesting effect. The WFMs were strategically placed in downgradient positions on the cover top slope where water likely accumulated in a sand drainage layer. Routine monitoring at Lakeview shows that the ground water remains protected. LM plans to evaluate potential effects of high percolation rates in covers to ensure that disposal cells remain protective for the long term.

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Preferential Flow in a Reclamation Cover: Hydrological and Geochemical Response.
Kelln, C., L. Barbour, and C. Qualizza. Journal of Geotechnical and Geoenvironmental Engineering 133(10):1277-1289(2007)

Evapotranspirative covers used for waste containment or land reclamation strategies are intended to function in perpetuity. Pedogenesis of the cover materials caused by biophysical processes may lead to the development of macroporosity (i.e., preferential flow paths), which will alter the hydrological response from the intended design function. Hydrometric and geochemical data were used in this study to examine the contribution of preferential flow to the hydrological response of a reclamation cover on saline-sodic shale mine overburden, in a cold semiarid environment. The hydrometric data suggest that infiltration occurs along preferential flow paths when the ground is frozen or when wet antecedent soil moisture conditions develop prior to precipitation events. Interflow is initiated during the spring snowmelt when the cover thaws and water migrates from the preferential flow paths into the soil matrix, causing a perched water table to form on the cover-shale interface. The cessation of interflow coincides with a recession of the perched water table and an increase in matric suction within the cover in response to elevated evapotranspiration demands. The chemistry and stable isotope signature of the interflow demonstrates that these waters are initially composed of fresher snowmelt water, flowing along preferential flow paths, which then transition to pre-event water dominated by higher concentration water from within the soil matrix. A numerical simulation demonstrates that macroporosity imposes a significant control on the discharge rate and cumulative volume of interflow.

Vegetative Cover for Phosphogypsum Dumps: A Romanian Field Study.
Petrisor, I.G., K. Komnitsas, I. Lazar, A. Voicu, S. Dobrota, and J.M. Kuperberg. 2001 International Containment & Remediation Technology Conference & Exhibition, 10-13 June 2001, Orlando, Florida.

Intensive mining and processing activities along the Romanian Black Sea Coast have resulted in the production of millions of tons of waste that has been disposed without any treatment in tailing dumps. At Navodari, 20 km north of Constanta harbor, over 3,000,000 m3 of phosphogypsum, deposited in three stacks, represents a permanent threat to the surrounding environment and human population. High levels of toxic metals and radionuclides, along with elevated sulfate concentrations, are examples of problems associated with these stacks, according to a complex environmental characterization and risk assessment study. Consequently, vegetative cover might substantially reduce the risk of contaminant migration, under the coastal climate of high rainfall and strong winds. In this paper, a field research effort to cover phosphogypsum residue with suitable vegetation is presented. Several remediation schemes, based on greenhouse experiments have been successfully deployed in the field. These schemes used different combinations of soil amendments and plant species. Periodic investigations on plant growth and metal uptake, as well as enzymatic activities in the substrates were carried out. On the basis of this 18-month experiment, an efficient rehabilitation scheme is proposed.

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