Within the realm of environmental monitoring, trace organic contaminants aretechnically challenging because of their low, and variable, aqueous concentrations andtheir generally hydrophobic character. The ability of these trace contaminants totransport across biological membranes to access the lipid phase of an organism raisesconcerns regarding biouptake and the health and viability of ecosystems. Consequently,the bioavailability of a contaminant to a target organism is a key factor in determiningregulatory endpoints for remediation and monitoring. As such, many research effortshave focused on mimicking the transport behavior of trace organics across biomembranesusing synthetic surrogates; semipermeable membranes have emerged as strongcontenders.A complicating factor rests with the operational definition of the aqueous phase of acontaminant in a natural water system. The aqueous phase may infer not only the freelydissolved chemical, but also compound that is associated with the dissolved or colloidalhumic substances that are present. Such interactions may impact the transport andfate of these pollutants within the system, specifically in relation to their permeabilityacross membranes. Further, interaction with natural organic matter (NOM) affects the rate of abiotictransformations for certain chemicals.In this research, a model system using semipermeable cellulose ester membranes wasused to simulate the exposure of the lipid phase of a biological system to an aqueousphase contaminated with a series of trace-level agrochemicals and industrial pollutants.The impact of dissolved NOM on the transport of the trace organic contaminants acrossthe membrane system was evaluated, including the influence of calcium ions.The variables that were incorporated into the experimental matrix included the relativeabundance of contaminant versus NOM, calcium content, buffered pH, and the length ofdialysis. Includes 13 references, tables, figures.