InterPore2025

Hydraulic boundary conditions control feedbacks between fluid flow, biofilm growth, and dissolved oxygen in groundwater

20 May 2025, 12:05

15m

Oral Presentation (MS05) Microbial Dynamics in Porous Media: Advances in Biofilms, Biogeochemistry, and Biotechnology MS05

Speaker

Hamidreza Ahadiyan (Boise State University)

Description

Biofilms are sediment-attached microbial communities that fuel numerous reactions in groundwater. Biofilm clogging of pores, or bioclogging, instigates dynamic feedbacks between fluid transport, oxygen demand, and microbial growth and decay that are poorly understood. Here, we present results from microfluidic experiments to demonstrate that these feedbacks are controlled by the hydraulic conditions driving flow. The microfluidic chambers (micromodels) were patterned after a homogenous sand and integrated with an optode sensor to measure dissolved oxygen (DO). Bacillus subtilis, a model biofilm-forming soil bacterium, was grown by flowing an oxygenated nutrient-rich solution through the micromodel. Two types of experiments were conducted, each with identical initial conditions but different boundary conditions: constant flow rate (Q) vs. constant pressure gradient ($\mathrm{\Delta }$P). Bulk permeability was quantified over time by relating measured flow and pressure to Darcy’s Law. Microscopy was used to monitor spatial maps of biomass and DO.

For both conditions, biofilm patches formed uniformly at early times. Coalescence of patches caused permeability to decrease 30-fold and average DO to decline to anoxic conditions (i.e., DO <2 mg/L) in the first 24 h. Distinct pseudo-steady state behavior emerged over the remainder of the 48 h experiments that differentiated the two boundary conditions. Experiments at constant Q promoted frequent permeability fluctuations and flow channelization into preferential flow paths (PFPs) that maintained a fully connected pore network. DO concentrations correlated strongly with PFP location, with concentration declining along PFPs and spatial maps of DO responding to changes in PFP location. In contrast, biofilm fully clogged pores near the micromodel inlet in constant $\mathrm{\Delta }$P experiments, which restricted DO availability and caused biofilm to slough in DO-depleted locations. Sloughing was followed by a regrowth period characterized by slow changes to permeability, biomass, and bulk DO. Our results demonstrate that hydraulics fundamentally control the dynamics of bioclogging and therefore determine the spatio-temporal heterogeneity of redox conditions that determine the transformation of redox-sensitive elements in groundwater.