ABSTRACT

Since the 1950's, crude oil has been seeping into the Colorado River adjacent to an oil field in Wharton County. The volume of oil discharging to the river has changed over time. Multiple seeps occur over a 600-ft length along the east bank of the river. Since 1936 oil-field features in a 0.03-mi² area immediately adjacent to the east bank of the river have included (1) numerous oil wells, one of which blew out in 1959, (2) six surface tanks that contained oil or salt water, and (3) as many as four former earthen pits reported to have contained crude oil, salt water, or drilling mud.

In 1999 the Bureau of Economic Geology (BEG) assessed the possible sources of oil by collecting continuous cores and installing groundwater-monitoring wells at 12 locations. Strata are clay and sand of the Beaumont Formation. Some fractures in the clay are moist and have reduction rinds. The measured plan view hydraulic gradient is toward the river. We found hydrocarbon impacts (elevated concentrations of ionizable compounds and total petroleum hydrocarbons) in six of the cores to depths as great as 45 ft, and free-phase product up to 1.3 ft thick in two monitoring wells located beneath and down gradient of the largest earthen pit and up gradient of the seeps. We hypothesize that oil from the pit migrated downward through fractures in the clay, has collected at the interface with the underlying water-bearing sand at a depth of 50 ft, and moves laterally toward the river.