William H. Albright

Field Evaluation of Alternative Earthen Final Covers

ABSTRACT Five methods to assess percolation rate from alternative earthen final covers (AEFCs) are described in the context of the precision with which the percolation rate can be estimated: trend analysis, tracer methods, water balance method, Darcys Law calculations, and lysimetry. Trend evaluation of water content data is the least precise method because it cannot be used alone to assess the percolation rate. The precision of percolation rates estimated using tracer methods depends on the tracer concentration, percolation rate, and the sensitivity of the chemical extraction and analysis methods. Percolation rates determined using the water balance method have a precision of approximately 100 mm/yr in humid climates and 50 mm/yr in semiarid and drier climates, which is too large to demonstrate that an AEFC is meeting typical equivalency criterion (30 mm/yr or less). In most cases, the precision will be much poorer. Percolation rates computed using Darcys Law with measured profiles of water content and matric suction typically have a precision that is about two orders of magnitude (or more) greater than the computed percolation rate. The Darcys Law method can only be used for performance assessment if the estimated percolation rate is much smaller than the equivalency criterion and preferential flow is not present. Lysimetry provides the most precise estimates of percolation rate, but the precision depends on the method used to measure the collected water. The lysimeter used in the Alternative Cover Assessment Program (ACAP), which is described in this paper, can be used to estimate percolation rates with a precision between 0.00004 to 0.5 mm/yr, depending on the measurement method and the flow rates.

Field Hydrology of Water Balance Covers for Waste Containment

Abstract A study was conducted at 12 sites across the United States to evaluate field-scale hydrology of landfill final covers using water balance methods to control percolation. The sites were located in climates ranging from arid to humid, with annual precipitation varying from 119 to 1,263 mm. Fifteen test sections were constructed with large ( 10 × 20 m ) drainage lysimeters for continuous and direct monitoring of the water balance over a period of 3–6 years. Monolithic and capillary barrier designs were used for water storage, and plant communities consisting of grasses, grasses and shrubs, or grasses and trees were used to promote evapotranspiration. Data from these test sections are analyzed along with data from 10 other sites in the literature to draw general inferences regarding the hydrology of water balance covers. Percolation ranges from 0 to 225 mm / year (0–34% of precipitation) on an average annual basis and is shown to be affected by annual precipitation, preferential flow, and sto...

Field Hydrology of Landfill Final Covers with Composite Barrier Layers

AstudywasconductedatsevensitesacrosstheUnitedStatestoevaluatethe fieldhydrologyof finalcoverswithacompositebarrier (a geomembrane over a soil barrier or a geosynthetic clay liner) for final closure of landfills. The water balance of each cover was monitored withalarge(10320m)instrumenteddrainagelysimeter.Withoneexception,thecoverslimitedtheaverageannualpercolationto,2.8mm/year (,0.4%ofprecipitation).Thegeomembranebarrieratonesite(Marina,California)waslikelydamagedduringconstruction;percolationatthis site averaged 30 mm/year (6.9% of precipitation). The annual percolation through the cover at the wettest site (Cedar Rapids, Iowa) ranged between0.1and6.2mm/year.Theannualpercolationataridandsemiaridsiteswastypicallynomorethanatrace(,0.1mm/year).Percolation fromalltestcoversgenerallywascoincidentwithhighwaterstorageinthesurfacesoillayerandlateral flowinthedrainagelayeronthesurface of the geomembrane barrier. Water balance predictions were made with the hydrologic evaluation of landfill performance model using site- specific input. Surface runoff was overpredicted and evapotranspiration underpredicted when as-built soil hydraulic properties were used as input. Better agreement was obtained when in-service soil hydraulic properties were used as input. The lateral flow was consistently overpredicted regardless of the hydraulic properties, and no correspondence existed between the predicted and measured percolations. DOI: 10.1061/(ASCE)GT.1943-5606.0000741.

Sustainable Covers for Uranium Mill Tailings, USA: Alternative Design, Performance, and Renovation

The U.S. Department of Energy Office of Legacy Management is investigating alternatives to conventional cover designs for uranium mill tailings. A cover constructed in 2000 near Monticello, Utah, USA, was a redundant design with a conventional low-conductivity composite cover overlain with an alternative cover designed to mimic the natural soil water balance as measured in nearby undisturbed native soils and vegetation. To limit percolation, the alternative cover design relies on a 160-cm layer of sandy clay loam soil overlying a 40-cm sand capillary barrier for water storage, and a planting of native sagebrush steppe vegetation to seasonally release soil water through evapotranspiration (ET). Water balance monitoring within a 3.0-ha drainage lysimeter, embedded in the cover during construction, provided convincing evidence that the cover has performed well over a 9-year period (2000–2009). The total cumulative percolation, 4.8 mm (approximately 0.5 mm yr−1 ), satisfied a regulatory goal of <3.0 mm yr−1 . Most percolation can be attributed to the very wet winter and spring of 2004–2005, when soil water content exceeded the storage capacity of the cover. Diversity, percent cover, and leaf area of vegetation increased over the monitoring period. Field and laboratory evaluations several years after construction show that soil structural development, changes in soil hydraulic properties, and development of vegetation patterns have not adversely impacted cover performance. A new test facility was constructed in 2008 near Grand Junction, Colorado, USA, to evaluate low-cost methods for renovating or transforming conventional covers into more sustainable ET covers.Copyright

Field Data and Model Predictions for a Monolithic Alternative Cover

Water balance data from a test section simulating a monolithic alternative cover were compared to predictions made with two numerical models: UNSAT-H and Vadose/W. Onsite data were used as model input to the greatest extent practical. More accurate predictions were obtained with Vadose/W than UNSAT-H. Surface runoff was overpredicted appreciably by UNSAT-H, which affected all subsurface hydraulic processes. In contrast, Vadose/W accurately predicted surface runoff, evapotranspiration, and the temporal variations in soil water sto rage. However, neither model predicted percolation accurately. Both models also failed to capture a key change in the transpiration pattern during the last winter-summer period of the study. Differences in the method used to simulate precipitation inten sity appear partly responsible for the difference in accuracy by which the two models predict surface runoff. Simulations were conducted with the lower boundary condition as a unit gradient or a seepage face to evaluate how this boundary condition affects predictions of the water balance. Essentially the same predictions were obtained regardless of the lower boundary that was used.

Using pilot test data to refine an alternative cover design in northern California.

Two instrumented test sections were constructed in summer 1999 at the Kiefer Landfill near Sacramento, California to test the hydraulic performance of two proposed alternative final covers. Both test sections simulated monolithic evapotranspiration (ET) designs that differed primarily in thickness. Both were seeded with a mix of two perennial and one annual grass species. Oleander seedlings were also planted in the thicker test section. Detailed hydrologic performance monitoring of the covers was conducted from 1999 through 2005. The thicker test section met the performance criterion (average percolation of <3 mm/y). The thinner test section transmitted considerably more percolation (average of 55 mm/y). Both test sections were decommissioned in summer 2005 to investigate changes in soil hydraulic properties, geomorphology, and vegetation and to collect data to support a revised design. Field data from hydrologic monitoring and the decommissioning study were subsequently included in a hydrologic modeling study to estimate the performance of an optimized cover system for full-scale application. The decommissioning study showed that properties of the soils changed over the monitoring period (saturated hydraulic conductivity and water holding capacity increased, density decreased) and that the perennial grasses and shrubs intended for the cover were out-competed by annual species with shallower roots and lesser capacity for water uptake. Of these changes, reduced ET from the shallow-rooted annual vegetation is believed to be the primary cause for the high percolation rate from the thinner test section. Hydrologic modeling suggests that the target hydraulic performance can be achieved using an ET cover with similar thickness to the thin test section if perennial vegetation species observed in surrounding grasslands can be established. This finding underscores the importance of establishing and maintaining the appropriate vegetation on ET covers in this climate.

ENHANCEMENTS TO NATURAL ATTENUATION: SELECTED CASE STUDIES

In 2003 the US Department of Energy (DOE) embarked on a project to explore an innovative approach to remediation of subsurface contaminant plumes that focused on introducing mechanisms for augmenting natural attenuation to achieve site closure. Termed enhanced attenuation (EA), this approach has drawn its inspiration from the concept of monitored natural attenuation (MNA).

Evaluation of Soil Manipulation to Prepare Engineered Earthen Waste Covers for Revegetation.

Seven ripping treatments designed to improve soil physical conditions for revegetation were compared on a test pad simulating an earthen cover for a waste disposal cell. The field test was part of study of methods to convert compacted-soil waste covers into evapotranspiration covers. The test pad consisted of a compacted layer of fine-textured soil simulating a barrier protection layer overlain by a gravelly sand bedding layer and a cobble armor layer. Treatments included combinations of soil-ripping implements (conventional shank [CS], wing-tipped shank [WTS], and parabolic oscillating shank with wings [POS]), ripping depths, and number of passes. Dimensions, dry density, moisture content, and particle size distribution of disturbance zones were determined in two trenches excavated across rip rows. The goal was to create a root-zone dry density between 1.2 and 1.6 Mg m and a seedbed soil texture ranging from clay loam to sandy loam with low rock content. All treatments created V-shaped disturbance zones as measured on trench faces. Disturbance zone size was most influenced by ripping depth. Winged implements created larger disturbance zones. All treatments lifted fines into the bedding layer, moved gravel and cobble down into the fine-textured protection layer, and thereby disrupted the capillary barrier at the interface. Changes in dry density within disturbance zones were comparable for the CS and WTS treatments but were highly variable among POS treatments. Water content increased in the bedding layer and decreased in the protection layer after ripping. The POS at 1.2-m depth and two passes created the largest zone with a low dry density (1.24 Mg m) and the most favorable seedbed soil texture (gravely silt loam). However, ripping also created large soil aggregates and voids in the protection layer that may produce preferential flow paths and reduce water storage capacity.