Considering Bacteria-Sediment Associations in Microbial Fate and Transport Modeling

Abstract

The development of a total maximum daily load (TMDL) for water bodies impaired by elevated microbial levels (the second leading cause of impairment nationally) requires an understanding of microbial transport processes at the watershed scale. Continuous monitoring of impaired water bodies can be expensive, and models are typically employed, but most current models represent bacteria as single discrete (“free” phase) organisms with near-neutral buoyancy, subject to first-order decay resulting primarily from predation or die-off. Studies indicate, however, that a significant fraction of microbes are associated with sediment particles, both in the water column and bed-sediments, associations that can impact microbial transport behavior and survival rates. This work incorporates considerations of microbial partitioning and its impact on survival into microbial fate and transport modeling using a well-characterized watershed. Agreement between observed and modeled instream microbial concentrations is comparable to, or better than, that seen in similar studies. Nonetheless, differences in instream concentration between model runs that consider microbe-sediment association (with attendant survival differences) and those that assume all microbes exist in the free phase are relatively small. A sensitivity analysis of relevant model inputs further indicates the minor effects of incorporating these considerations. The low settling velocities of small particles with which microbes typically associate and the dominance of other inputs related to wet weather microbial loadings, when compared with resuspension, result in the reduced significance of microbial partitioning as a factor in water quality modeling.