Using Peristaltic Bailing to Obtain Ground Water Samples in Push Probe Profiling of a Subsurface BTEX Plume

Rapid vertical profiling of BTEX concentration in ground water was needed to guide permanent monitoring well installation and soil coring in a study of natural microbial attenuation of a dissolved BTEX plume. The plume is located in a sand aquifer at the site of a crude oil pipeline leak near Cass Lake, MN. Because depth to the water table (>10 m) made typical peristaltic pump sampling impractical, a method dubbed “peristaltic bailing” was devised. At each BTEX profiling location, a hole was augered to ~30 cm above the water table. A push probe consisting of a 2 cm drive point with a 15 cm screen mounted on galvanized pipe was lowered inside the auger and driven to the sampling depth. Teflon tubing with 0.6 cm diameter was fed into the pipe until the bottom end reached the screen. A peristaltic pump attached to the Teflon tube at the surface was pumped until the tube filled with water to a height about 8 m above the water table in a few seconds. Water was then “bailed” by lifting the Teflon tube until the bottom of the tube was above the water table. The peristaltic pump pulled the slug of water that was in the tube (~60 cc) into a sample bottle at the surface. The procedure was repeated several times to purge the push probe prior to sampling. This method proved to be effective in obtaining samples for BTEX screening by head space gas analysis using a field GC. High concentrations of benzene (4346 μg/L) were detected 120 m downgradient from the pipeline leak. Concentrations dropped to 10 μg/L at 217 m downgradient. Estimated flow velocity is 6.3 m/year, which yields a benzene attenuation rate estimate of 280 μg/l/yr in the anoxic plume.