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Distribution of Heavy Metals in Waste Drilling Fluids Under Conditions of Changing pH¹

ABSTRACT

A laboratory study was conducted to determine the influence of pH on the distribution of arsenic (As), barium (Ba), chromium (Cr), and lead (Pb) in three different oil and gas well drilling fluid wastes. Samples were obtained from disposal pits and equilibrated in the laboratory under controlled conditions of pH. A sequential extraction procedure was then used to fractionate the selected elements into the fractions of aqueous, KNO₃-extractable, H₂O-extractable, NaOH-extractable, EDTA-extractable, and HNO₃-extractable phases. The chemical fractions present at each pH were then compared, thus providing insight into the potential of mobilization of As, Ba, Cr, and Pb into ground- or surface waters. The majority of each of the elements studied was found in the NaOH-extractable, EDTA-extractable or HNO₃-extractable fractions except for one waste which contained a major portion of the total barium in the KNO₃-extractable fraction. Generally, decreasing pH caused a shift from the more stable HNO₃-extractable fraction toward less stable EDTA-extractable, NaOH-extractable, or KNO₃-extractable fractions. In no case was there a substantial release to the aqueous phase with changing pH. The significance of these results is that with pH changes to be expected in the natural environment there is not likely to be a major release of As, Ba, Cr, and Pb from drilling fluid waste disposal pits. The lower pH values (< 4) which might produce some impact are unlikely to occur in the environment because the wastes themselves have a large neutralizing capacity.

Key Words: sequential extraction • drilling mud • disposal pits

¹ Contribution of Environmental and Ground Water Inst., School of Civil Engineering and Environmental Science, College of Engineering, Univ. of Oklahoma, Norman, OK 73019.

² Research Chemist, Shell Development Company, Westhollow Research Center, Houston, Texas; Sun Company Professor of Ground Water Hydrology and Director, Environmental and Ground Water Inst., Univ. of Oklahoma, Norman, OK, respectively.

Received for publication March 25, 1985.

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