Aaron I. Packman

Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface-subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed

Biofilm cells are less susceptible to antimicrobials than their planktonic counterparts. While this phenomenon is multifactorial, the ability of the matrix to reduce antibiotic penetration into the biofilm is thought to be of limited importance studies suggest that antibiotics move fairly rapidly through biofilms. In this study, we monitored the transport of two clinically relevant antibiotics, tobramycin and ciprofloxacin, into non-mucoid Pseudomonas aeruginosa biofilms. To our surprise, we found that the positively charged antibiotic tobramycin is sequestered to the biofilm periphery, while the neutral antibiotic ciprofloxacin readily penetrated. We provide evidence that tobramycin in the biofilm periphery both stimulated a localized stress response and killed bacteria in these regions but not in the underlying biofilm. Although it is unclear which matrix component binds tobramycin, its penetration was increased by the addition of cations in a dose-dependent manner, which led to increased biofilm death. These data suggest that ionic interactions of tobramycin with the biofilm matrix limit its penetration. We propose that tobramycin sequestration at the biofilm periphery is an important mechanism in protecting metabolically active cells that lie just below the zone of sequestration.

Stream-subsurface exchange processes are important because of their role in controlling the transport of contaminants and ecologically relevant substances in streams. Laboratory flume experiments were conducted to examine solute exchange with gravel streambeds. Two morphologies were studied: flat beds and beds covered by dune-shaped bedforms. High rates of exchange were observed with flat beds under a wide range of stream flow conditions, indicating that there was considerable turbulent coupling of stream and pore water flows. The presence of bedforms produced additional exchange under all flow conditions. The exchange with bedforms could be represented well by considering solute flux caused by bedform-induced advective pumping. Pumping exchange was enhanced by inertial effects, including non-Darcy flow and turbulent diffusion. For the flat bed case, dye injections showed that exchange also occurred by a combination of advective pore water flow and turbulent diffusion near the stream–subsurface interface. The relative effects of advective and diffusive transport processes could not be separated due to the complex nature of the induced flows in the gravel bed. However, exchange was found to scale with the square of the stream Reynolds number in all cases. Comparison of these results with those obtained with coarser and finer sediments demonstrated that the exchange rate is also proportional to the square of the characteristic bed sediment size. These scaling relationships can be used to improve interpretation of solute transport observed in natural rivers.