United States Environmental Protection Agency Environmental Monitoring Systems Laboratory Las Vegas NV 89193 Research and Development EPA/600/S6-87/005 Aug. 1987 AEPA Project Summary Processes Affecting Subsurface Transport of Leaking Underground Tank Fluids Scott W. Tyler, Michael R. Whitbeck, Marsha W. Kirk, John W. Hess, Lome G. Everett, David K. Kreamer, Barbara H. Wilson, and Jeff van Ee This document presents the funda- mental theories and an understanding of the processes controlling migration and fate in the subsurface of material released from an underground storage tank. Processes affecting the migration of fluids from a leaking underground stor- age tank and their effects on monitoring methods are reviewed by experts. An understanding of these processes is critical to an understanding of the methods that monitor underground storage tanks for material released from the tank in the environment surrounding the tank. Soil heterogeneities and the potential for multiphase flow will lead to high monitoring uncertainties if leak detec- tion systems rely on liquid sampling alone. Vapor transport is also affected by these properties, although to a lesser degree. More research is needed, how- ever, to better understand the physics of vapor transport. The processes of adsorption, partitioning, and microbial alteration of fluids in the subsurface may have strong effects on the uncer- tainty of monitoring systems. Fate pro- cesses have received less attention than liquid and vapor transport processes and will require significantly more re- search before the effects are fully understood. This Project Summary wag developed by EPA's Environmental Monitoring Systems Laboratory, Las Vegas, NV, to announce key findings ol the research project that la fully documented In a separate report ol the same title (tee Project Report ordering Information at back). Introduction Estimates of the number of under- ground storage tanks in the United States range from 3 to 5 million. Estimates of the number of leaking tanks vary, but increasingly more incidents of leaking tanks are being reported. These tanks may contaminate soil and ground water to the extent that the environment and human health is adversely impacted. The Solid Waste Disposal Act was amended by the Hazardous and Solid Waste Amendments of 1984 (HSWA; Public Law 98-616) to provide for the development and implementation of a comprehensive regulatory program for underground storage tanks (USTs). Tanks containing hazardous waste are subject to regulation under Subtitle C of the Resource Conservation Recovery Act of 1976. Tanks containing petroleum and hazardous substances are subject to regulation under Subtitle I of the Haz- ardous and Solid Waste Amendments; regulations for these will be proposed in 1987 and will become final in 1988. Regulations for tanks containing haz- ardous waste were promulgated by the EPA (40 CFR Part 264) on July 14, 1986. Many approaches exist in the preven- tion, detection, and cleanup of leaked product from USTs. One approach is to monitor the environment outside the tank to determine if product is entering the environment from the tank, piping, or surface spills. The Environmental Moni- toring Systems Laboratory in Las Vegas (EMSL-Las Vegas) is conducting research to evaluate the feasibility of monitoring outside the tank for leaks from the tank. The environment outside a tank is sub- ject to a number of factors that may ------- affect and complicate detecting a leak from a tank system (Figure 1). These factors affect the transport and fate of product from the tank to the external leak detection system. Studies of these factors, particularly factors affecting organic and petroleum products, have been initiated in recent years in response to an in- creasing number of reported cases where the environment or humans have been exposed to material released from USTs. In addition, recent state and Federal laws and regulations on ground-water con- tamination have served to increase the need of researchers to understand the processes affecting the subsurface trans- port and fate of leaking underground tank fluids. Approach The demand for technical guidance that has been created by new regulations has created a need to develop and disseminate information quickly. Initial steps taken by EMSL-Las Vegas to evaluate external leak detection systems have included surveys of the literature. Federal, state and local agencies, industry, and academia. The results from these surveys will be made available in a variety of reports and technical notes. The first effort to develop information on the processes affecting the subsurface transport of product from leaking USTs consisted of several experts being asked to summarize the existing knowledge on: liquid flow, vapor flow, soil surface and interfacial effects of product in the environment around a tank, and the implications of subsurface biological activity in the monitoring of USTs. Four experts in these areas covered their respective fields, and a team of hydrogeologists from the Desert Research Institute coordinated the experts in their examination of the problems posed by external leak detection monitoring of USTs. Each expert was asked to refer to the available literature and to describe in layman's terms the present state of knowledge in the subject area. Each ex- pert was also asked to address those areas where the understanding of the processing affecting subsurface transport of leaking undergound tank fluids was weak and where further research was required. The conclusions reached by each expert were consolidated and summarized in the trial section of the full report where the advantages and complications of a number of monitoring approaches were listed against four fate and transport processes. The monitoring approaches were: active and passive liquid monitoring, 2 Top Soil •*• Vadose Zone -+• Capillary Fringe •* Water Table Sampling Points \ Pea Gravel «- Aquifer *- Clay «- Drinking Water Aquifer Spill Figure 1. The hydrogeologic environment around underground storage tanks can be complicated even under idealized circumstances. Tab/* 1. Parameters Affecting the Transport and Fate of Organics in the Soil and Ground Water Soil Contaminant Environmental Multiphase permeability Residual saturation Pore size distribution Fracture density Wettability Soil texture Porosity Variability of soil properties Porosity Water content Soil structure & variability Permeability to air Moisture content Organic content Clay content Soil surface area Pore water chemistry Soil gas diffusion Colonization potential Oxygen concentration Methane concentration Contaminant velocity Fluid Transport Parameters Density Viscosity Solubility Surface tension Vapor Transport Parameters Volatility Vapor diffusivity Distribution coefficients Temperature Precipitation Depth to water table Biological activity Recharge Temperature Barometric changes Water table fluctuations Surface Chemistry Parameters Solubility Temperature Concentration Pressure Microbiological Parameters Nutrient loading Toxicity Solubility pH Temperature Recharge & groundwater transport ------- active and passive vapor monitoring, and surface and borehole geophysics. Conclusions The monitoring of leaks from outside the tank is complicated by a number of factors (Table 1), and no one monitoring approach will be applicable for all ap- plications. The assessment of those factors in evaluating the'performance of external leak detection monitoring sys- tems is not easy. Uncertainty in flow directions because of variations in con- ductivity, water content, texture, etc. is a major problem in obtaining predictable performance from an external leak detec- tion sensing system. Since an increase in the distance from the leak to the sensor also increases the number of hetero- geneities encountered and, hence, in- creases the uncertainty in flow direction, it is important to locate the sensor close to the leak source. Tank installations may be engineered to reduce further the heterogeneity near the tank, and this would make the sensor location less sensitive. Active samplers, i.e., samplers that pump the environment, appear to be less affected by transient spills; however, further research is needed to develop sensor criteria. The present state of knowledge is limited in the transport and fate processes affecting leaked organic products from USTs. Further research is needed to understand those processes. Scoff W. Tyler, Michael R. Whitbeck, Marsha W. Kirk, and John W. Hess are with the Desert Research Institute, Reno, NV 89506; Lome G. Everett is with Kaman Tempo, Santa Barbara, CA 93102; David K. Krearner is with Arizona State University, Tempe, AZ 85287; Barbara H. Wilson is with the University of Oklahoma, Ada, OK 74820; and the EPA author Jeff van Ee (also the EPA Project Officer, see below) is with the Environmental Monitoring Systems Laboratory, Las Vegas, NV 89193-3478. The complete report entitled "Processes Affecting Subsurface Transport of Leaking Underground Tank Fluids," (Order No. PB 87-201 521/AS; Cost: $13.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at' Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency P.O. Box 93478 Las Vegas, NV 89193-3478 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S6-87/005 (, 0000329 PS u R 230 S _ CHICAGO STtfET IL 60604 i-Jinlhmiiuiiuulnliin ------- |