Low concentrations of compounds that have the potential to disrupt the endocrine system of aquatic organisms have been identified in surface waters worldwide. These compounds, known as endocrine disrupting compounds (EDCs), include steroidal hormones, surfactants, plasticizers, and pharmaceuticals. Bed sediments below streams are ubiquitously coated by biofilms, generally referred to as the epilithon in freshwater ecosystems. Biogeochemical cycling in the epilithon is an important component of stream ecosystem function as nutrients and organic matter are absorbed and utilized by the biotic consortia. Therefore, evaluating the potential of this matrix to attenuate EDCs is an important part of understanding their fate in surface waters.

This study focused on the role of the epilithon in attenuating steroidal hormones and alkylphenols, primarily focusing upon the removal processes of sorption and biodegradation. Results indicated that the epilithon is an effective matrix for steroidal hormone and alkylphenol sorption and that biofilms play an important role in increasing the sorptive capacity of stream bed surfaces. Sorption to the epilithon was primarily controlled by organic matter interactions dominated by hydrophobic partitioning. Linear sorption experiments determined organic matter partition coefficients (K_om, L kg^-1) in the epilithon for 17β-estradiol (10^2.3-2.8 ), 17α-ethynylestradiol (10^2.5-2.9), 4-nonylphenol (10^3.4-4.6), 4-nonylphenolmonoethoxylate (10^3.5-4.0 ), and 4-nonylphenoldiethoxylate (10^3.9-4.3). Slower biodegradation rates were observed in the epilithon as compared to the sediments, and therefore steroidal hormones and alkylphenols will tend to become concentrated in the epilithon of streams subject to continual loading (e.g., downstream of wastewater treatment effluents).

A tracer study on the Redwood River in southwestern Minnesota was performed to evaluate the importance of the epilithon and to quantify fate of steroidal hormones and alkylphenols. In-stream decay coefficients were determined for 17β-estradiol, estrone, and 4-nonylphenol (3.1 d^-1, -0.78 d^-1, -1.5 d^-1, respectively). These results indicate that although 17β-estradiol was removed, estrone and 4-nonylphenol were being produced in the evaluated stream reach. Sorption to the epilithon and bed sediments was found to the primary mechanism of removal for 17β-estradiol. Estrone was produced in the evaluated reach as a result of oxidation from 17β-estradiol, and 4-nonylphenol was being produced from chemical transformation of nonylphenolpolyethoxylates and nonylphenolpolyethoxycarboxylates.