Measurable levels of endocrine disrupting compounds (EDCs) and pharmaceutically active compounds in environmental waters have been reported in numerous studies and have been shown to cause sexual and developmental abnormalities in vertebrate and invertebrate aquatic species at environmentally relevant concentrations. Specifically, compounds such as steroid hormones and nonionic surfactant metabolites are of particular interest because of their prevalence, their high level of potency at low concentrations, their ubiquitous presence in waste waters, and their potentially harmful effects on the environment. Although a few studies indicate that some of these EDCs could be partially removed or transformed during sewage and septic treatment, many of these compounds are detectable in the environment and in drinking water supplies. The complex interactions that may occur during transport through soil-water systems after discharge from a septic or other on-site disposal system are little understood. It is hypothesized that the relatively high concentration of nonionic surfactant metabolites discharged from septic systems will impact the soil-water equilibrium of steroid estrogens, allowing them to spend more time in the aqueous phase and thereby increasing their rate of transport towards groundwater. Results will be presented from ongoing research that is investigating the effect of competitive sorption between nonylphenols (endocrine active metabolites of nonionic surfactants) and human steroid estrogens in batch studies. Analytical methods include the analysis of both aqueous and solid phases via isotopic dilution and gas chromatography/mass spectrometry, as well as the use of a yeast estrogen screen assay (YES) to monitor overall estrogenicity of aqueous fractions.