Dissertation Title: Simulating Bioclogging Effects on Dynamic Riverbed Permeability and Infiltration
Research Questions for my PhD
1) How does dynamic permeability affect infiltration in losing rivers? In gaining rivers? In disconnected rivers?
2) Can dynamic permeability affect local water levels, river discharge, and regional water budgets during drought years and wet years?
Russian River in Sonoma County California
Reduction in riverbed permeability due to biomass growth is a well-recognized yet poorly understood process associated with losing connected and disconnected rivers. Although several studies have focused on riverbed bioclogging processes at the pore-scale, few studies have quantified bioclogging feedback cycles at the scale relevant for water resources management, or at the meander-scale. At this scale, often competing hydrological-biological processes influence biomass dynamics and infiltration. Disconnection begins when declines in the water table form an unsaturated zone beneath the river maximizing seepage. Simultaneously, bioclogging reduces the point-scale infiltration flux and can either limit the nutrient flux and reduce bioclogging, or preferentially focus infiltration elsewhere and enhance bioclogging. These feedbacks are highly dependent on geomorphology and seasonal patterns of discharge and water temperature. To assess the mutual influences of disconnection, biomass growth, and temperature changes on infiltration in a geomorphologically complex river system, we built a 3D numerical model, conditioned on field data, using the reactive-transport simulator MIN3P. Results show that in disconnected regions of the river, biomass growth reduced vertical seepage downward and extended the unsaturated zone length; however these changes were contingent upon disconnection. Mid-way through the seasonal cycle, biomass declined in these same regions due to limited nutrient flux. Seepage and biomass continued to oscillate with a lag correlation of 1 month. Connected regions, however, showed the largest infiltration rates, nutrient fluxes, and concentrations of biomass. Despite the reduction in conductivity from biomass, flow remains high in connected regions because the feedback between bioclogging and infiltration is not as pronounced due to the sharpening hydraulic gradient. Bioclogging ultimately shapes the pattern of flow, however geomorphology dominates the strength of connection. Recognition of the feedbacks between geomorphological patterns and heterogeneous biomass on meander scale hydrological processes can lead to better estimates of local water volumes and capacities, especially when these systems are used as municipal and public water supply sources.
Definition: Dynamic permeability
A change in hydraulic conductivity over time from sediment, biomass, detritus, etc.
Definition: Bioclogging effects on conductivity:
Growth of bacteria and accumulation of cells in the pore-space from decaying algae on the surface.
Bank Filtration Systems
The coupling between bioclogging-induced riverbed dynamic permeability and disconnection processes could have significant ramifications for practical management of water resources. In particular, riverbank filtration systems consist of high capacity pumping wells that are located adjacent to or beneath rivers. Taking advantage of natural filtration processes, these system are used around the world as an inexpensive, reliable way to treat surface water for public uses [Tufenkji et al., 2002]. The groundwater-surface water interface as a conduit to the wells provides valuable ecological services including natural attenuation of contaminants [Buss et al., 2009]. Physical, mechanical, chemical, and biological clogging are all processes reported to occur during riverbank filtration, and it is typical for riverbed permeabilities to be lower than aquifer permeabilities in these settings [Schubert, 2006; Buss et al., 2009; Jaramillo, 2012]. Well production has been documented to be dependent on river water depth, previous pumping rates, sediment deposition, biological clogging, and aquifer/riverbed anisotropy [Zhang et al., 2011].