ABSTRACT

Release of saline wastes into shallow ground waters at a now-closed commercial oil field waste disposal and storage site in south Louisiana has prompted an investigation into potential engineering problems and geologic features which permitted contaminant transport to occur. Within a few years after the site began operations in 1980-81, salt water contamination was noted in shallow monitoring wells surrounding the disposal units and storage pits. The ground waters had not only elevated levels of chloride, but also locally high levels of dissolved barium and arsenic. Problems in site operations which appear to have led to the release of wastes include the breaching of a brine pit liner during drag-line operations and the disposal of wastes in unlined pits. Once released into the ambient sediments, some contaminated waters travelled laterally outward along the interface between the phreatic and vadose zones and/or vertically downward through jointed clay beds. Other wastes may have travelled down improperly sealed monitoring well boreholes. Observed rates of waste migration have been significantly higher than those predicted from conventional Darcy calculations using measured water levels and assumed average sediment permeabilities. This discrepency is not surprising considering that the hydrodynamics of a variable-salinity fluid system cannot be adequately characterized using water level alone as a proxy for hydraulic force.

As a further complexity, data from two independent deep subsurface studies support the conclusion this waste disposal site is situated on an active growth fault. Present rates of movement on this fault cannot be determined from currently available shallow geologic information. But the potential for fault activity significantly reduces the geologic suitability of this site for waste disposal as a result of the potential enhancement of vertical sediment permeability through extension and the disruption of engineered barriers.