Although different barium compounds are known to exhibit vastly different toxicities, cleanup standards for barium in soil or sediment at environmental corrective action sites are typically based on total barium concentration without regard for the specific barium compounds present. However, the Texas Commission on Environmental Quality (TCEQ) has recently agreed that the insoluble barium compound barium sulfate (i.e., barite) is not a compound of concern with regard to human health, thereby eliminating the need for cleanup standards or site remediation for this compound. The TCEQ still requires that their barium cleanup standards be applied to any soluble barium compounds that may be present at a site. To allow the application of barium cleanup standards to soluble barium compounds while excluding barium sulfate, we have developed a protocol to measure the concentration of soluble barium in soil or sediment samples that may contain both barium sulfate and soluble barium compounds. In this paper, we present (1) a review of the toxicity and regulation of soluble barium compounds and insoluble barium sulfate, and (2) a description of our protocol to measure the concentration of soluble barium compounds in soil and sediment samples and the validation of this protocol.
Barite, the mineral form of barium sulfate, has been used for years throughout the oil and gas industry as a weighing agent during drilling. Barite, which has a density more than four times that of water, is mixed with water, mineral oil, or diesel oil, and other materials to form drilling mud, a fluid solution used to control downhole pressures while drilling. In addition to its high density, barite is insoluble and thus provides stability for the drilling mud. Other uses of barium sulfate include filler material in products such golf balls and automotive interiors and as a whitener for paints. Barium sulfate is also used in medical testing procedures.
Oil services companies that sell and store barite along with drilling mud and additives at their facilities often have areas of the facility where barite may have been spilled or released into the environment. Typical site assessments at these facilities conducted under a state voluntary cleanup program will show elevated levels of barium in the soil, which may require response actions. We have developed a protocol to identify barium sulfate in the environment and to differentiate it from other barium compounds with specific laboratory methods.
Toxicity and Regulation of Barium Compounds
Toxicity of Barium Compounds
The key physical and chemical properties of barium compounds are summarized in Table 1. Barium toxicity is mediated through the free barium ion.1 Exposure to insoluble barium sulfate does not cause significant toxicity because barium sulfate does not release free barium ions that can be absorbed into the lungs or intestine. The available human and animal data2-4 indicate that barium sulfate is poorly absorbed from the gastrointestinal tract with an absorption efficiency of 10-4 to 10-6. This compares to an absorption efficiency for soluble barium compounds of 0.03 to 0.95. This difference in absorption efficiency provides a clear mechanistic explanation for the observed difference in toxicity between barium sulfate and soluble barium compounds. For example, barium sulfate is a radio-opaque agent that is administered orally as a medical diagnostic at doses of >200 g to increase contrast during x-rays of the digestive system. Although this dose is approximately 100 times the acutely lethal dose of soluble barium, diagnostic barium sulfate typically causes only minor side effects of constipation or diarrhea. In addition, occupational exposure to high levels of barium sulfate dust can result in benign pneumoconiosis (baritosis) that usually resolves following termination of exposure. In contrast to barium sulfate, soluble barium compounds dissociate in water, releasing barium ions and corresponding anions. As a result, soluble barium compounds are readily absorbed by the lungs or intestines and have a much higher toxicity than barium sulfate. Soluble barium doses of 0.2 to 0.5 grams have been found to cause acute toxic effects in humans; in the absence of treatment, the lethal dose of soluble barium for adults is 3 to 5 grams.