Real Exposure: Field Measurement of Chemical Plumes in Headwater Streams

Abstract

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.

Mapping Dynamic Exposure: Constructing GIS Models of Spatiotemporal Heterogeneity in Artificial Stream Systems

“…Defining exposure as the peak area appeared to create distributions that shared some similarities with distributions of both peak magnitude and peak frequency (Murlis et al 2000;Justus et al 2002). This aligns with that both the peak frequency and the size of those peaks contribute to the total area of peaks observed in a given location (Edwards and Moore 2014).…”

“…Depending on the peak characteristic being described, degrees of exposure are distributed differently throughout the stream. The definition being used to model toxicant pulses throughout a stream environment can create dramatic differences in which portions of the stream are identified as undergoing the highest degrees of exposure (Edwards and Moore 2014;Harrigan and Moore 2017). As previously noted, defining exposure as either the peak area or the peak magnitude resulted in much steeper gradients between high and low levels of exposure than when exposure was defined as the peak frequency.…”

“…Because chemicals introduced via runoff and groundwater enter streams at different points in the water column, they will encounter vastly different flow regimes (Vogel 1994;Nikora 2010;Steele et al 2018). As such, these two modes of entry are likely to result in different patterns of chemical transport (Edwards and Moore 2014;Lahman and Moore 2015). In fluids, the transport of chemicals is primarily a product of two distinct processes: advection and diffusion.…”

“…Variation in the degree of turbulent flow gives rise to variation in patchiness in the structure of chemical plumes. This patchiness translates into variability in exposure pulses for any organisms that encounter the plume (Wolf et al 2009;Edwards and Moore 2014). Any factors that affect flow dynamics throughout a stream habitat will alter the patchiness of chemical plumes, ultimately shaping exposure (Ludington and Moore 2017).…”

“…Under a static exposure paradigm, organisms are exposed to a chemical at a consistent concentration over a given time-period (Watanabe et al 2018;Gordon et al 2012). However, organisms in natural systems rarely encounter constant and consistent exposure to toxicants (Ashauer and Escher 2010;Edwards and Moore 2014). Pulsed exposure paradigms have introduced temporal variation into models of toxicant exposure.…”