MODE OF OCCURRENCE AND ENVIRONMENTAL MOBILITY
OF OILFIELD NORM AT USGS RESEARCH SITE B,
OSAGE SKIATOOK PROJECT, NORTHEAST OKLAHOMA
Robert A. Zielinski*
James R. Budahn
U.S. Geological Survey
MS 973, Denver Federal Center
Denver, CO 80225
Voice: 303-236-4719
Fax: 303-236-3200
E-mail: rzielinski@usgs.gov
A field survey at this active E&P site using a hand-held radiation exposure meter indicated marginally elevated radioactivity (2-5 times background) at the exterior of a produced water holding tank, but not in sediments of an adjacent pit used to temporarily collect tank water overflows. Suspended solids in 2 liters of water drained from the bottom of the tank were collected by filtration. Radioactivity of the solids was measured by high-resolution gamma-ray spectrometry and derives from two isotopes of radium (226Ra and 228Ra) and their decay products. Treatment of the solids with concentrated HCl to dissolve abundant red iron oxides had little effect on radioactivity levels. SEM-EDAX observation of the remaining grains indicated abundant small euhedral (5-50 µm) barite grains of variable morphology that occur as individuals, as composite grains, and as coatings on 50-100 µm quartz grains. Barite is a well-documented scavenger of radium from solution and is a common host of radioactivity in oilfield scale and sludge. Less abundant and smaller (2-20 µm) barite grains were observed in a heavy mineral concentrate from surficial sediments of the overflow pit. Sampled tank and pit waters are chemically oversaturated with barite, based on mineral/solution equilibrium calculations. Regular depth-wise declines of excess (unsupported by thorium parent) 228Ra activity in a 28 cm core from the pit suggests closed-system decay of radium in a buried, insoluble host (barite?). The apparent average sedimentation rate in the pit based on 228Ra (t1/2= 5.76 years) decay is ~3-4 cm/year. Barite is a geochemically stable radium host that limits the solubility and environmental mobility of radium at this site.