Real exposure: field measurement of chemical plumes in headwater streams.

In fluvial systems, organismic exposure to nonpoint source pollutants will fluctuate in frequency (exposure events), intensity (concentration), and duration. The reliance on lethal concentrations and static exposure in many laboratory studies does not adequately represent nor address exposure to in situ chemical plumes of fluvial habitats. To adequately address field exposure in a laboratory setting, one needs an understanding of the physics of chemical transmission within moving fluids. Because of the chaotic nature of turbulence, chemical plumes introduced to fluvial systems have a spatial and temporal microstructure with fluxes in chemical concentration. Consequently, time-averaged static exposure models are not ecologically relevant for the major reason of in situ distribution. The purpose of this study was to quantify in situ chemical distribution and dispersion within two physically different streams. Dopamine was introduced as a chemical tracer mimicking groundwater runoff. Chemical fluxes and stream hydrodynamics were simultaneously measured using a microelectrode and an acoustic Doppler velocimeter, respectively, at three heights of three downstream locations at each research site. Fine-scale measurements of the dopamine plume microstructure showed that organisms could be exposed to chemical fluctuations where concentrations are significantly greater than the overall time-averaged concentration. These measurements demonstrate that rather than relying on static exposure, standards for pollution must consider the concept of exposure being interdependently linked to flow of the fluid medium. The relationship between fluid dynamics, pollution exposure, and organism physiology are complex and must be evaluated in ways to mimic natural systems.