There is increasing attention on minimizing the risk of land-based drilling activities to the terrestrial environment. As a result, the selection of base oils used in drilling fluids can be based upon optimal environmental properties. However, information into what role different site conditions such as temperature and soil type have on terrestrial risks is lacking. In the present study, different base fluids were tested for terrestrial toxicity using aging conditions and soil types to determine how toxicity could be impacted by site conditions. Four base oils were tested including Diesel; a low toxicity mineral oil (LTMO), and two synthetic, gas-to-liquids (GTL) based paraffins, GTL C 10-C22 (GTL 1) and GTL C11-C24 (GTL 2). These base oils were tested in two different soil types (sandy loam and silt loam), with three different ageing treatments of fresh spike, aged 90 days at 10°C and aged 90 days at 30°C, to determine the influence of climate and soil aging on terrestrial toxicity. Terrestrial toxicity bioassays conducted included: earthworm, spring tail and cricket bioassays, germination bioassays with three plant types (alfalfa, wheatgrass, and saltbrush), soil respiration, and microbial community analysis. In both soil types, the synthetic base oils had lower toxicity scores in freshly spiked soil compared to LTMO and Diesel. Aged base oil soil treatments had lower toxicity scores than freshly spiked soil with differences in profiles observed between soils aged at different temperatures. Soil type had the greatest impact on the results of earthworm and microbial diversity tests. Earthworm survival was greater in silt loam soil likely due to the increased organic carbon content of this soil over the sandy loam soil tested. Overall this research found that assessment of base oil performance against a variety of toxicity endpoints yields a greater understanding of the environmental risk of the base oil drilling fluid. This research also demonstrated the importance of testing materials in field soils. Many toxicity studies on base fluids are conducted using synthetic soil which has a higher organic carbon content than many natural soil types and thus may under predict overall terrestrial risk.