Local chemical gradients can have a significant impact on bacterial po
pulation distributions within subsurface environments by evoking chemo
tactic responses. These local gradients may be created by consumption
of a slowly diffusing nutrient, generation of a local food source from
cell lysis, or dissolution of nonaqueous phase liquids trapped within
the interstices of a soil matrix. We used a random walk simulation al
gorithm to study the effect of a local microscopic gradient on the swi
mming behavior of bacteria in a porous medium. The model porous medium
was constructed using molecular dynamics simulations applied to a flu
id of equal-sized spheres. The chemoattractant gradient was approximat
ed with spherical symmetry, and the parameters for the swimming behavi
or of soil bacterium Pseudomonas putida were based on literature value
s. Two different mechanisms for bacterial chemotaxis, one in which the
bacteria responded to both positive and negative gradients, and the o
ther in which they responded only to positive gradients, were compared
. The results of the computer simulations showed that chemotaxis can i
ncrease migration through a porous medium in response to microscopic-s
cale gradients. The simulation results also suggested that a more sign
ificant role of chemotaxis may be to increase the residence time of th
e bacteria in the vicinity of an attractant source.