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r` • 'w.es made to the facility at 9:00 AM. At approximately 6:30 AM on No- vember 13, the vapor detection system went into alarm mode indicating a po- tential leak In the system. The chart records graph showed that combusti- ble vapors were initially detected at 4:45 AM on November 13. Inspection of the graph indicated that the vapors began to dissipate at approximately 11:00 AM on November 13. which pointed to an isolated single dose event as the cause of alarm. Had the alarm been due to a continuous leak from a gasoline tank rupture, the level of combustible vapors detected by the system would have been expected to remain constant or continue to in- crease. Since this was not the case. further investigation was undertaken to locate the source. Inspection of the fill pipe access boxes detected a strong gasoline odor in the soil at the bonom of one of the boxes. Since the bottoms of the boxes were not sealed at that time to prevent overfill, it appears that a small amount of gasoline was spilled during the fulling of the tank and seeped into the surrounding tank back- fill. The gasoline vapors then migrated laterally and were detected by the sen- sors 19 hours and 43 minutes after the spill. The system went into the alarm mode 22 hours after the spill took place. The vapors subsequently dissi- pated sufficiently to allow the system to reset, and no further episodes have occurred. The leak detection episode at the fa- cility provided firsthand evidence of the viability of combustible vapor monitoring as a leak detection method for underground storage tanks contain- ing volatile materials, whether they are hazardous substances, chemicals or waste. And the following conclusions are pertinent to the utilization of the vapor detection systems as an alter- native to groundwater monitoring for leak detection of underground storage tanks: 1. Very small leaks can be de- tected by monitoring vapor con- centrations in soil vapor wells. In this particular case. probably less C; than ot.e gallon was spilled during the felling operations. 2. Combustible vapors migrate rapidly through the soils. partic- ularly in granular backfill. In this can, the vapors migrated 13 feet in 19.75 hours or 0.66 feet per hour. The subsurface configurs- Con of the tanks and vapor wells are shown in Figure 2. 3. Based on the migration rates, an effective radius of influence for a vapor soil detection system in a 24-hour period would be approx- imately 15 fat. 4. Due to the sensitivity of the va- por detection system, all pathways to the subsurface must be sealed to prevent infiltration of the sur- face contamination. These path- ways include: a. Bottom of all fill pipe access boxes: b. Bottom of all monitoring or soil vapor well access boxes-. c. Any major cracks and joints in the concrete or asphalt pave- ments. 3. In areas where the groundwater surface is at considerable depths (greater than 30 feet), it is imper- ative that an alternative to ground- water monitoring be used to provide the earliest possible de- tection of tank leaks and therefore minimize contamination of the subsurface. For tanks which store motor fuel and other volatile haz- ardous materials or waste, a vapor detection system is a viable option which has been proven in field ap- plications. (Editor's Comment: The coming dec- ade will require massive resting, re- medial and removal etpenditures to meet the expected promulgation of Federal Standards for Underground Storage Tanks in 1987 and 1988.) About the Author: HMWM is indebted to Mr. Loyd Nelson. Hazardous Ma- terials tic Waste Manager of the Barks- dale Control Division of Transamerica Delaval and Mr. !ay Dablow, Etec- utive Vice President of Hydro -Fluent Inc. who engineered the vapor moni- toring system. Techniques And Practices For Managing UndergTound Storage Tanks Firms currently using or planning to use underground storage tanks, service station operators, design engineers. environmental and safety engineers, and fire prevention specialists - all will benefit from this reference manual. It's a guide with more than 200 pages on installation, leak testing, tank lining, and tank abandonment for underground storage tanks. Charts and illustrations aid in understanding safe procedures. A quick reference index and spiral binding make in -the -field use practical. Special sections address: Installation requirements Consensus standards and industry practices • Currosion protection Testing methods • Owner/operator liabilities Spill clean-up techniques The $59.95 price includes all shipping and handling charges. To order your copy, send check or money order for $59.95 to: Materials & Waate Mana`emcm Ma`auindJuly-Aust 1986 L.A. Weaver Co. 308 E .cones St. Raleigh, NC 27501 (919) 832-8242 Circle No. 163 on Rcafer inquiry Card 33