| Summary: | <p> This project explored the potential fate and transport of seawater-based drilling fluid used in the Antarctic Drilling Program (ANDRILL) South McMurdo Sound project (SMS). The SMS drilling reported a loss of 5.6 × 10<sup> 5</sup> liters of drilling fluid to the surrounding formation throughout a borehole depth of 1139m. The introduction of these drilling fluids raise concerns of potential contamination to a pristine, isolated environment. The volume of fluid lost to the subsurface is unrecoverable and will only break down through natural attenuation processes, such as biodegradation. The objectives of this study are to estimate the extent of fluid migration laterally from the borehole and to determine when biodegradation of the water-based drilling fluid is effectively occurring. Variable density groundwater flow modeling (SEAWAT) was used to simulate the environment around the borehole. Applying stresses similar to the drilling events produced an estimate of how far fluid will be transported as drilling fluid is being circulated. Results show the fluid to migrate up to 7.5m into the subsurface. Additionally, laboratory microcosms were set up to incubate drilling fluid samples at various temperatures (5, 25, and 50°C) under aerobic and anaerobic conditions. Experimental data collected over 188 days was analyzed to evaluate the time frame when biodegradation of drilling fluids occurred. Carbon isotope fractionation (<sup> 13</sup>C/<sup>12</sup>C) was used to determine the ability of the drilling fluids to be used as a food source. Biological data observed changes in microbial growth using DNA quantification, and changes in microbial communities using Biolog EcoPlates™. Results show a positive correlation between the increase of δ<sup>13</sup>C (‰) values and an increase in DNA (ng/µl) quantity. Data from geochemical and community changes indicate biodegradation of the drilling fluid occurred between time 40 and time 100. The methods employed to investigate fate and transport is a unique approach, and applied to these water-based drilling fluids for the first time in this study.</p>
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