--> Historic Oilfield Arsenic Usage and Pit Groundwater Models: An Example from Lake St. John Field, Louisiana


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Historic Oilfield Arsenic Usage and Pit Groundwater Models: An Example from Lake St. John Field, Louisiana


Old oilfield pits can leave a modern geochemical legacy within environmental media. Pit geochemical signatures reflect a field's unique environmental setting and the history of pit usage and abandonment. Some measured constituents, heavy metals for example, can have multiple possibilities as to their origin, whether naturally-occurring or modified by anthropogenic impacts. This study examines dissolved groundwater arsenic in oilfield pit areas. The presence and origin of arsenic in an oilfield region may be difficult to interpret given arsenic's multiple possible origins and complex geochemistry.

At Lake St. John (LSJ) Field, Concordia Parish, Louisiana, dissolved arsenic in shallow groundwater (8 ft to 22 ft below ground surface) occurs in former emergency pit areas associated with tank batteries and with the saltwater injection disposal system. The pit geochemical data are interpreted here as evidence that natural arsenic patterns have been modified to various degrees around the pits. This study specifically addresses the following—does a reductive dissolution model explain, or partially explain, an elevated dissolved arsenic shallow groundwater signature around the oilfield pits? If the model does not explain elevated arsenic (or even if it does), what additional evidence is available through the historic oil field operations concerning possible arsenic addition, specifically 1950s arsenic corrosion inhibitors?

Reductive dissolution of iron oxides and hydroxides is a popular model for groundwater arsenic-containing plumes. The model's reducing setting occurs due to the presence or introduction of oily waste, plant organics, peat/coal, or clays. The model has been applied in oil industrial or pipeline spill areas, where some studies interpret an oil impact on reducing conditions and increased arsenic, and other studies which do not find that relationship.

Under the model's reducing conditions, adsorbed natural arsenic is released into solution when oxidized iron and manganese species are dissolved. Expected groundwater model conditions include low oxidation-reduction potential (ORP) measurements, elevated arsenic and elevated iron. While this model may be applied to oilfield pit signatures, anthropogenic reasons for both the arsenic and the iron should be considered. Potential historic oilfield arsenic sources are related to corrosion control (and related dissolved iron), original produced waters, or weed control.