Geoenvironmental Engineering: Honoring David E. Daniel

This paper summarizes available liquids management data for double-lined mixed low-level radioactive waste (MLLW) landfills at four U.S. Department of Energy (DOE) sites. Information is provided on the design of the landfill liner systems, and performance data (i.e., leachate collection system and leakage detection system flow rate and liquid chemical constituent data) are presented and discussed. Data are presented for three distinct facility periods: (i) the initial period of operation; (ii) the active period of operation; and (iii) the post-closure period. For three of the facilities, the performance data are analyzed to calculate hydraulic containment efficiencies of the liner systems and to draw conclusions as to whether the liner systems are performing as expected. The results are also compared to previous data and information developed by the two lead authors for a 2002 U.S. Environmental Protection Agency technical resource document (TRD). Based on the data presented, all four facilities are performing very well in providing containment and collection of leachate. Performance metrics for the facilities are consistent with those presented in the 2002 TRD. The public availability of data for several of the facilities is limited; more intensive liner system monitoring and information dissemination at DOE MLLW facilities is recommended.

The use of an exposed geomembrane/geosynthetic clay liner (GM/GCL) composite is an attractive strategy for the closure of solid waste landfills as well as lining of liquid impoundments and canals. While these materials have had considerable attention in both laboratory and field evaluations, there are two issues which need additional investigation. Regarding the GCL component, avoidance of adjacent panels separating from one another is an issue. This is addressed in the paper wherein seven seaming methods have been investigated and quantified accordingly. Regarding the GM component, the issue is durability in an exposed atmosphere. This paper presents a 10-year evaluation of six different geomembranes at elevated temperatures until half-life of strength and elongation have been reached. Using these data, laboratory half-lives have been determined as well as predicted lifetimes in the field. Phoenix, Arizona was arbitrarily selected in this regard. The presented data resulting from both the GCL separation issue and the GM lifetime prediction method suggest that exposed GM/GCL composite systems can be used for at least 50-years, if not longer. To be sure there are other issues (wet/dry cycles and ion exchange for CCLs and defects and field damage for GMs) but only these two are addressed herein.

This paper presents results of a case study on the application of the acceptable placement range method for construction of low hydraulic conductivity (k) clay liners for waste containment systems. A field investigation was completed to identify potentially suitable low k clay liner material. Based on laboratory testing of the proposed liner material, a range of moisture contents and dry unit weights of compacted material that conformed to the regulatory hydraulic conductivity requirement of k ≤ 10^-7 m/s was developed. A test pad was constructed, field tested, and sampled to verify that the compacted material achieved the hydraulic conductivity requirements. Upon verification that the proposed acceptable placement range and the proposed construction methods provided suitable low k clay liner material, full-scale liner construction commenced. This case study demonstrates that the use of the acceptable placement range method provides for a relatively straight forward means to determine if proposed liner materials are suitable for use, provides an easy-to-use compaction specification, and facilitates the use of variable material types within the same project. The field density test results can be used to demonstrate that construction was completed such that the hydraulic conductivity requirements were achieved.

Over the past 30 years, important lessons have been learned about the stability of waste containment systems through failures and successes in design, testing, construction and operation. This paper presents five case histories to illustrate the following lessons: specifications about test conditions are as important as the test method; the input to a stability analysis is as important as the method of analysis; quality control requirements need to be consistent with design requirements; very small fluid pressures can be problematic when total stresses are very small; and the stability of containment systems may be most critical during construction or waste placement. The overarching theme of what we have learned over the past 30 years is that good engineering requires judgment, not prescriptive requirements.

Rain gardens are stormwater control measures (SCMs) that control runoff volume and mitigate pollutants through infiltration and evapotranspiration. Many states have developed manuals to aid in the design and construction of SCMs, including rain gardens. Delaware’s manual, the Delaware urban runoff management model, recommends that the following soil mixture be used in rain gardens to achieve percolation within 48 hours of the rain event: 50% construction sand, 20-30% topsoil, and 20-30% mulch. The engineered soils that were designed to meet the Delaware soil mix criterion in two failed rain gardens and two samples of native soils in a tract development in Glasgow, Delaware were examined for this paper. The soils were classified according to the USDA (common in stormwater design) and USCS (common in geotechnical design) classification systems. In addition, the saturated hydraulic conductivity and the soil water characteristic curves were determined. Soil samples from both failed rain gardens were classified as clayey sands (SC), while the native soil samples were classified as silty sands (SM). Soils in the failed rain gardens also had lower hydraulic conductivities compared to the native soils. The presence of clays in the engineered soils of the failed rain gardens impaired their ability to infiltrate, resulting in an unusually long residence time of the captured stormwater. This case study highlights the importance of testing native soils at a proposed site and re-using them if their infiltration capacity is appropriate. In this case, the acceptable native soils were replaced with an inadequate soil mixture resulting in an inferior performance and added expense.

The constant-head constant-volume (CHCV) method for measuring hydraulic conductivity of less permeable (hydraulic conductivity ≤ 10^-6 m/s) saturated porous materials consists of a closed-loop of liquid that passes through the specimen, tubing, a mercury-water reservoir, and a capillary tube. Use of a closed-loop forces inflow to equal outflow throughout the test, resulting in faster equilibration. The closed loop contains two liquids: water and mercury. A constant head drop is applied by displacing a column of mercury and flow rates are measured by monitoring the rate of displacement of a water-mercury interface in a horizontal capillary tube. Tests were conducted on a variety of media having very different stiffness and hydraulic conductivity using the CHCV device and a conventional open system. Nearly identical hydraulic conductivities were measured using both methods, but much less testing time was required with the CHCV device.

Vertical cutoff walls are frequently used for in situ containment of groundwater contaminant plumes or for secondary containment at waste disposal sites. Amending backfills for soil-bentonite (SB) cutoff walls with reactive media to increase attenuation of target contaminants has been proposed as a method for enhancing such containment. The purpose of this paper is to describe the physical and chemical characterization required for assessing the use of an SB backfill amended with 5% of a commercially available, high cation exchange capacity zeolite (chabazite) for the purpose of enhancing the attenuation capacity of the backfill for containment of metals. Physical characterization included slump tests, consolidation tests, and laboratory hydraulic conductivity (k) tests in both oedometers and flexible-wall permeameters. Chemical characterization included batch equilibrium adsorption tests (BEATs) and long-term column tests. The same characterization was performed for the unamended backfill to provide a basis for comparison of the effect of the zeolite amendment. The results indicated: (a) no significant effect on the physical properties of the backfill due to the zeolite amendment, including an ability for the amended backfill to achieve a suitably low k (< 3.0×10^-10 m/s), (b) enhanced adsorption capacity for the metal, potassium (K⁺), and (c) significantly greater attenuation of K⁺ for the amended backfill based on the column test results. The simulated performance of a hypothetical, 1-m-thick SB vertical cutoff wall comprising the zeolite-amended backfill indicated substantially improved attenuation of K⁺ relative to the simulated performance based on the unamended backfill, with an increase in containment period ranging from 63 to 85 yr for a source concentration, C_o, of 1,000 mg/L K⁺ and 18 yr for a C_o of 10,000 mg/L K⁺. The results of this study provide a proof of concept for the use of zeolite-amended backfills in SB cutoff walls for enhanced metals attenuation.

The common use of active clays, such as bentonites, as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical models that take into account the physical, chemical and mechanical coupled phenomena. The elementary particle (platelet) of a bentonite is characterized by a high total specific surface (S) and a permanent or fixed negative electric surface charge (σ) that interacts with the ions in solution at different concentration (c_s) in the pore water. This interaction is the primary factor that determines the final fabric of the solid skeleton and, consequently, most of the bentonite properties in terms of both mechanical behavior and solute transport. The tactoid formation or flocculation is the primary phenomenon that influences bentonite behavior and can be quantified with appropriate state parameters, such as the effective specific surface, S_eff, the average number of platelets per tactoid, N_l,AV, and the electric fixed-charge concentration of the solid skeleton, c_sk,0. On the basis of the aforementioned state parameters, a theoretical hydro-chemico-mechanical framework has been developed. The validity of this framework has been tested, within the present paper, by comparison of its predictions with some of the available experimental results on bentonites with more than 70% montmorillonite content, in terms of hydraulic conductivity, swelling pressure and osmotic efficiency versus void ratio and ion concentrations of the pore solution.