Journal of Environmental Engineering

Subsurface barriers are an extremely promising remediation option to many waste-management problems. It is recognized that monitoring of the barrier is necessary to provide confidence in the ability of the barrier to contain the pollutants. However, the large size and deep placement of subsurface barriers make detection of leaks a challenging task. Therefore, typical geophysical methods are not suitable for the monitoring of an emplaced barrier's integrity. Perfluorocarbon tracers (PFTs) have been tested as a means of barrier verification at the Hanford geotechnical test facility, where a soil/cement barrier was emplaced around a buried drum. PFTs were injected beneath the drum for three days in the center of the barrier 3 m below grade. The concentration of PFTs in seven external and two internal monitoring wells has been measured as a function of time over a 17-day period. The data have been analyzed through numerical modeling to determine barrier integrity and PFT diffusion rates through the barrier. This paper discusses the experimental design, test results, data analysis, and modeling of PFT transport in the subsurface system.

The difficult task of locating and quantifying nonaqueous phase liquids (NAPLs) present in the vadose and saturated zones has prompted the development of innovative, nondestructive characterization techniques. The use of the interwell partitioning tracer's (IWPT) test, in which tracers that partition into the NAPL phase are displaced through the aquifer, is an attractive alternative to traditional coring and analysis. The first field test of IWPT was conducted in a hydraulically isolated test cell (3.5 by 4.3 m) to quantify the total amount of a complex NAPL (a mixture of JP-4 jet fuel and chlorinated solvents) trapped within a 1.5-m smear zone in a shallow, unconfined sand and gravel aquifer at Hill Air Force Base (AFB), Utah. Tracer breakthrough curves (BTCs) were measured in three extraction wells (EWs) following a tracer pulse (0.1 pore volume) introduction through four injection wells (IWs). The measured retardation of the partitioning tracer (2,2-dimethyl-3-pentanol) relative to the nonreactive tracer (bromide) was used to quantify the NAPL present. The EW data were used to estimate an average NAPL saturation of 4.6–5.4% within the test cell. NAPL saturations estimated by using measured concentrations in soil cores of two significant compounds present in the NAPL were 3.0 and 4.6%.

An analytical model has been developed to predict in-situ compositions and contaminant volumes from early monitoring of effluent gas concentrations of individual species during multicomponent soil vapor extraction. The model exploits the wave-like propagation of the evaporation fronts of individual species to present analytical expressions for the in-situ compositions, contaminant volumes, effluent concentrations, and recovery rates during discrete time intervals. The efficiency of soil vapor extraction systems also can be determined. Comparison of this theory to a previous experiment of through-flow venting of benzene, toluene, and o-xylene yielded excellent agreement. This suggests that if effluent concentrations of individual species are monitored in the field during soil vapor extraction operations, only minimal data are required at early times to yield predictions of in-situ compositions and contaminant volumes, as well as subsequent effluent concentrations and recovery rates.

A large-scale, natural-gradient tracer test in a heterogeneous aquifer at a site near Columbus, Miss. is simulated using three-dimensional (3D) hydraulic conductivity distributions derived from the borehole flowmeter test data. The simulated plume is more sensitive to the way the hydraulic conductivity field is interpolated from the measured data than it is to the dispersivity value. The transport model with longitudinal dispersivities in the range of 1–5 m can reasonably reproduce the observed plume to a certain concentration limit, but fails to reproduce the significant spreading of the tracer at diluted concentrations as observed in the field. This modeling study illustrates the formidable challenges in modeling and monitoring contaminant transport in very heterogeneous aquifers and points to the needs for new, innovative monitoring techniques that can be applied to characterize the spatial and temporal variabilities in the aquifer properties at scales suitable for detailed transport modeling.

A new method is discussed for garnering information from water level data. Using measured water level data, a series of local estimates of the hydraulic gradient are generated. By combining these estimates through a series of plots, significant insight can be gained regarding the regional gradient and local variability in the gradient. Utility of this approach for monitoring gradients and temporal changes in the gradient is discussed through application to experimental results from a constructed aquifer, field results from a site in northern Indiana, and numerical simulation. In each case, the estimator provided a measure of the regional gradient and local deviation from this regional gradient. It is shown that the shape of the plots generated is related to errors in measurement and local anomalies in the hydraulic gradient. It is suggested that the shape of the plot also provides insight into the relative scales of the measurement and the integral scale of the underlying transmissivity field.

Permeable ground-water treatment walls (PTWs) have been implemented as a means by which innovative ground-water treatment technologies can be applied in-situ. Though not widely addressed in the technical literature, the ground-water monitoring program for a PTW at a commercial site should consider several factors including: (1) design elements of the PTW; (2) the remediation process to be implemented through the PTW; (3) the distribution of contaminants in the affected aquifer; (4) ground-water sampling methods; and (5) regulatory issues. Also, the compliance monitoring well network within the PTW and sampling plan should be designed to assure that (1) design ground-water residence time goals within the PTW are achieved prior to sampling; (2) the uniformity of ground-water flow through the PTW is accounted for; and (3) ground-water samples are collected using techniques (e.g., micropurging) that reduce the potential for collecting nontreated ground water from down- or upgradient of the PTW. A case study illustrates the concepts used to develop a ground-water monitoring program for a PTW that was accepted by regulatory agencies for a commercial site.

Ground water flow and transport processes in fractured porous rocks present special challenges for obtaining representative samples. Designing appropriate monitoring programs for these systems requires an assessment of spatial and temporal variability (1) to define representativeness, and (2) to determine the most efficient and cost-effective methods for obtaining representative samples. Gas tracer studies, borehole tests, and geochemical sampling conducted at a fractured rock site in east Tennessee indicate large spatial and temporal variations in transport processes. Ground water flow as indicated by helium tracers moves predominantly along strike, perpendicular to hydraulic gradients except during storm events, demonstrating temporal variability in transport directions. Distinct vertical changes in water chemistry encountered in multilevel wells are indicative of discrete transport pathways in a poorly mixed system. Comparison of samples obtained from standard screened wells and multilevel wells show that screened wells mask these vertical variations in ground water composition and miss important transport pathways. Experimental results demonstrate the need for adequate characterization of spatial and temporal variations for appropriate placement and construction of monitoring wells, timing of ground water monitoring, and evaluation of exposure risk and contaminant flux in support of remedial decision making.

Long-term performance of a waste-containment facility liner system can be simply and inexpensively monitored in dry climates by relative humidity measurements. An increase in humidity as atmospheric air is circulated through a dry coarse layer within the liner system indicates imminent movement of water into the layer. Continued airflow will evaporate and remove water, thereby restoring the coarse layer to a dry condition. Thus, quantification of relative humidity and water removal rates can be directly used to assess liner system performance. The effectiveness of this approach is increased in dry climates, where the atmospheric air has a low initial relative humidity and leachate production is expected to be small. Simple calculations, numerical simulations, and medium-scale field tests indicate substantial airflow rates can be induced through a coarse layer within a liner system. Air can be circulated actively with blowers, or passively with wind-powered chimneys. System efficiency can be improved by incorporating a coarse layer that has significant primary storage.

A novel single-fiber optical sensor was developed to measure solute fluorescence and to estimate a correction factor to compensate for the signal attenuation caused by suspended particles. The latter was effected using a reference fluorescer mounted beyond the end of the fiber tip. This sensor is intended for monitoring of contaminants in model aquifers and is adaptable to field use. The suspensions tested were Ca-montmorillonite, polystyrene latex beads, and silica. A tracer dye was used as the test analyte so that sorption of the sample onto the suspended particles would be negligible. The relationship between the reference fluorescence attenuation and the analyte attenuation agreed approximately with geometric prediction and could be determined without knowledge of the particle size distribution or the type of particles used. The results indicated that corrected solute fluorescence intensities could be obtained by successively sending an analyte excitation wavelength and a reference excitation wavelength into the fiber-optic sensor.

A passive discrete-level multilayer ground-water sampler was evaluated to determine its capability to obtain representative discrete-interval samples within the screen intervals of traditional monitoring wells without purging. Results indicate that the device is able to provide such data for the inorganic contaminants and species analyzed in the study and provided more accurate estimations of contaminant distributions and mass in the vertical direction within a sandy Atlantic coastal plain aquifer, than “traditional” sampling methods.

A high-resolution passive soil vapor survey was recently conducted at a coastal military base in southern California where a chlorinated and aromatic hydrocarbon plume threatens San Diego Bay. An existing sampling grid was extended offshore the entire length of the contaminated site. PETREX sampling tubes were emplaced conventionally onshore and were emplaced offshore in sandy bay-bottom sediments by divers working in 3 to 17 m (10 to 55 ft) of water. Results of mass spectrometric analysis of passive vapor samples were consistent with submarine ground water discharge measurements made previously with seepage meters, yet also produced one unexpected result: evidence of in situ degradation. Tetrachloroethene (PCE), widespread onshore, was not detected offshore. Trichloroethene (TCE), the primary contaminant onshore, was detected in only three points offshore. Dichloroethene (DCE), present only at trace levels onshore, was the dominant constituent measured in the offshore plume. Results were confirmed by laboratory analyses of sediment samples. The study demonstrated the utility of extending the passive soil vapor technique offshore in up to 17 m (55 ft) of sea water in a heavily trafficked waterway with a strong tidal current.

Several down-hole logging and sampling methods were used in a 335 m (1,100 ft) deep water supply well to monitor the distribution of trichloroethylene (TCE) and groundwater flow in a deep, multilayered aquifer system. The vertical distribution of TCE was assessed by collecting discrete-depth groundwater samples from the open well casing under non-pumping conditions and from 5 m (18 ft) intervals using a straddle-packer assembly under pumping conditions. The distribution of groundwater flow under pumping and non-pumping scenarios was assessed using spinner logs, temperature and fluid resistivity logs, and a FloVision survey. The combined data indicated that: (1) water containing TCE is confined to the uppermost water bearing zone in the screened interval; and (2) the well provides a conduit for downward migration of this water under non-pumping conditions. Subsequent long-term monitoring with pressure transducers has shown that vertical flow reversals occur in the well on a seasonal basis. The approach we developed can be used to monitor chemical and groundwater flow distribution in other areas where water supply wells already exist.

A passive in situ headspace sampler has been developed for the collection of dissolved gases in ground water during tracer tests using dissolved, inert gases. The sampler is constructed of a semipermeable membrane and copper tubing equipped with a Schrader valve. Because the headspace in the sampler equilibrates with the dissolved gases in the ground water, no water is collected, which can be a great advantage in use at contaminated sites. The injection of the dissolved gases into the ground water is accomplished by diffusion through a semipermeable membrane. Oak Ridge National Laboratory routinely uses this technology for site characterization. The passive-diffusive technology also has potential as a low-cost, low-maintenance, and long-term method for containment assessment.

Subsurface vertical barriers have been used as components of containment systems to prevent or reduce the impact of contaminant sources on ground-water resources. A better understanding of the hydraulic head distribution associated with vertical barriers can enhance the ability of existing performance monitoring systems to detect breaches in physical containment systems and may aid in the design of new performance monitoring systems. Given the current regulatory interest in containment systems as either supplemental or stand-alone remedial alternatives and the lack of adequate performance monitoring strategies at most existing hazardous waste sites utilizing vertical barrier technologies, there is an immediate need for general guidelines for determining whether a containment system is functioning as intended. This technical note describes an approach for evaluating the performance of containment systems, based on relative spatial and temporal hydraulic head distributions.