Evaluating in Situ Remediation Using Innovative Monitoring Technologies

Innovative monitoring probes based on a diffusion cell concept (Barber and Briegel, 1987) have been developed to provide in situ, undisturbed and on-line measurements of parameters of interest under variably-saturated conditions during remediation and contaminant monitoring. Probes for oxygen and volatile organic compounds (VOCs) have been tested in three field trials of in situ remediation; two carried out in Kwinana, Western Australia and the other at Hill Air Force Base (AFB), Utah. One trial involved bioventing to stimulate the microbial breakdown of residual diesel non-aqueous phase liquid (NAPL). The other two trials involved air sparging; one to remediate benzene, toluene, ethylbenzene and xylene (BTEX) contaminated groundwater, and the other to remediate a NAPL-contaminated aquifer. In situ oxygen probes provided on-line logging of oxygen concentrations at between 3 and 60 minute intervals, at numerous depths and locations over a fourteen-month period in the bioventing trial. Results compared well with data from conventional sampling and analysis by gas chromatography (GC). The probes were used to assess aeration efficiencies and to estimate oxygen consumption rates. Both oxygen and total BTEX compounds were monitored on-line using in situ oxygen and VOC probes during the Kwinana air sparging trial. The probes provided additional oxygen bio-utilisation rates and data on BTEX removal efficiency. Results suggest that the oxygen probes may be responding to both dissolved oxygen and entrapped air, which is common in air sparging trials. For the Hill AFB study, excellent correspondence was obtained for results form duplicate probes and GC analysis, even after nearly 1 year of emplacement within a NAPL-contaminated vadose zone. INTRODUCTION Remediation of environmental contamination is expensive and problematic, especially where non-aqueous phase liquid (NAPL) and other organic compounds contaminate groundwater and soil environments (Rao et al. 1996, Glass et al. 1997). A large proportion of the cost of remediation of a contaminated site is the cost of characterisation and monitoring. Also, inadequate monitoring capabilities often lead to a high level of uncertainty in clean-up efficiencies at sites. Better tools are required both for remediation and for monitoring the effectiveness of remediation strategies. Better monitoring would reduce costs, allow estimation of time frames for clean up, and enable timely modification of remediation strategies. This is especially so where volatile and semi-volatile organic compounds contaminate groundwater and soil environments posing significant health and environmental hazards. Accurate monitoring of such compounds is critical to understanding their movement, as well as 1997 INTERNATIONAL CONFERENCE ON GROUNDWATER QUALITY PROTECTION Remedial Technology and Management Policy for NAPL Contamination Taipei, Taiwan, ROC 2 their physical and biochemical properties. Monitoring oxygen is especially important where aerobic bioremediation is a key clean-up strategy. A major difficulty with the determination of organic compounds and gases in the environment, especially in soil and groundwater, is the reliability of sampling procedures and analytical methods. Sampling disturbs the target environment and recovering representative samples to determine groundwater quality remains a challenge (Barber and Davis 1987). Organic compounds can be easily lost by absorption or volatilisation during collection and processing (Barker et al. 1987, Davis et al. 1992, Barber and Davis 1994). Traditional methods for sampling groundwater often use submersible and gas operated pumps with synthetic polymer pump discharge lines (Scalf et al. 1981). Studies by Barcelona et al. (1985) and Reynolds et al. (1990) showed that volatile organic compounds were lost from a water phase by absorption into polymeric material, including teflon, and recommended avoidance of such materials when sampling groundwater for volatile organic compounds (VOCs). The only materials not to absorb VOCs were borosilicate glass and stainless steel. Measurement of dissolved oxygen concentrations also poses further difficulties because of potential contact with the atmosphere. To address these sampling and monitoring difficulties, innovative monitoring tools and instrumentation based on a diffusion cell concept have been developed by CSIRO Land and Water, Perth through the Cooperative Research Centre for Waste Management and Pollution Control, Australia. The diffusion cell provides in situ, undisturbed and approximately real time measurement of some of the major gases, as well as volatile and semi-volatile organic compounds of interest during remediation. Initially, Barber and Briegel (1987) used diffusion cells consisting of coiled teflon tubing installed in backfilled boreholes with nylon access lines to the ground surface to measure methane dissolved in groundwater. Barber et al. (1990) used diffusion cells to describe seasonal fluctuations and vertical variations of dissolved and gaseous oxygen concentration profiles under deep rooted natural vegetation. Patterson et al. (1995) and Davis et al. (1995) used a similar, but automated, on-line system to monitor dissolved oxygen concentrations during bioremediation of contaminated groundwater and soil. More recently these devices have been improved and modified to provide on-line, in situ monitoring instrumentation for organic pollutants in gaseous and aqueous environments (Environment Industry Review 1996, Barber 1997). In this paper, we give an overview of the general principles of the diffusion cell devices, and detail results of the use of these devices in three field trials of in situ remediation. One trial involved bioventing to stimulate the microbial breakdown of residual diesel non-aqueous phase liquid (NAPL) contaminating soil in the near-water table zone of an unconfined aquifer in Kwinana, Western Australia. The other two in situ trials involved air sparging; one in Kwinana, Western Australia to remediate gasoline contamination in groundwater, and the other at Hill Air Force Base (AFB) in Ogden, Utah to remediate a complex LNAPL (Light NAPL) mixture. All three trials sought to assess the effectiveness of the adopted clean up strategy. The latter trial was carried out in conjunction with Michigan Technical University under the auspices of the SERDP program, and overseen by the US EPA Ada laboratories in Oklahoma. For the trials, in situ and in many cases, real time measurements were carried out for parameters indicative of loss of the target contaminants and biodegradation processes. Key parameters targeted with the devices were oxygen, volatile organic gasoline compounds (such as benzene, toluene, ethylbenzene and xylene BTEX) and chlorinated compounds. 1997 INTERNATIONAL CONFERENCE ON GROUNDWATER QUALITY PROTECTION Remedial Technology and Management Policy for NAPL Contamination Taipei, Taiwan, ROC 3 GENERAL PRINCIPLE OF THE DIFFUSION CELL DEVICES The diffusion cell devices (Barber and Briegel 1987) consist of a gas-filled polymer tube which is commonly buried within contaminated soil or groundwater. Concentrations of VOCs and gases within the tube equilibrate with concentrations in the contaminated environment external to the tube. By sampling the gas phase concentration within the tube, changes in the external environment can be monitored. This allows minimal disturbance of the monitored environment and provides reliable samples. 0 25 50 75 100 125 0 500