Extractability and Plant Uptake of Trace Elements from Drilling Fluids

Land disposal of drilling fluids is a common practice following completion of an oil or gas well. Drilling fluids are formulated from components containing a variety of trace elements, but little is known about the bioavailability of these elements following application to soil. A greenhouse study was conducted to determine the yield and trace element content of Swiss chard (Beta vulgaris L.) and ryegrass (Lolium perenne L.) grown in two soils containing 0, 200, or 500 g kg⁻¹ of three water-based drilling fluids. The drilling fluids differed only in the source of barite (density regulating component). The barites used varied in trace element content. Addition of all drilling fluids increased soil pH, electrical conductivity of saturation extracts (ECₛₐₜ), and concentrations of total and extractable trace elements. Growth of chard was only depressed by amending soil with the drilling fluid containing high levels of trace elements, while ryegrass growth was reduced with the 500 g kg⁻¹ application rate of all drilling fluids and the 200 g kg⁻¹ application rate of the drilling fluid with high levels of trace elements. Soil analyses suggested that chard and ryegrass yields were likely reduced by high Zn and ECₛₐₜ, respectively, in drilling fluid-treated soils. The concentrations of Zn, Cu, Cd, Pb, and As in chard and ryegrass tissues were directly related to total, DTPA-extractable, and acetic acid-soluble forms of these elements in soil. The application to soils of drilling fluids prepared from barite containing low concentrations of trace elements resulted in lower levels of trace elements in plant tissues than was obtained with control soils, because drilling fluid additions, increased soil pH and, thereby, decreased metal availability. Similar concentrations of Ba, Hg, and Cr were found in plant tissues grown in control soils and soil-drilling fluid mixtures. This finding suggests that Ba, Hg, and Cr in drilling fluids are not plant-available. Trace element availability to plants was similar for the three types of barites studied. Thus, the concentration in plants was a direct function of the amount of trace elements added to soil in the drilling fluids.