ROCK SURFACE BIOFILMS REDUCE WATER INFILTRATION INTO POROUS STONE

Rock surface microbial communities play a poorly understood role in aquifer recharge dynamics. To understand microbial effects on hydrology, it is essential to consider microbial biofilms as the mode of growth exhibited by these communities. Microbes living within the upper 2-5 mm of the sandstone surface harden the stone slightly. This resistance, though seemingly slight–on the order of 1.5 kPa– is just enough to cause the formation of small scale features such as undercut flutes and rock visors. Mechanistically, this hardening results from two features of microbial growth, filamentous growth and extracellular polysaccharide (EPS) production. Using microcosm cultures grown on sand with and without filamentous cyanobacteria, we have started separating the contribution of filamentous growth and EPS to the surface hardening phenomenon. Microbes living in surface pore spaces will influence the movement of water into and out of the system. Monitoring the temperature of a natural system in the field, we have determined that the evaporation rate increases when a microbial biofilm is removed, resulting in a temperature decrease of 3-5oC during the afternoon hours. This indicates that the microbial community will retard the rate of evaporation. This process is best explained by pore clogging as the biofilm will act as a barrier to prevent the loss of water through evaporation. The reverse is also true when water is on the surface of the rock. We have found that intact, non-abraded biofilm samples demonstrate a high level of hydrophobicity, indicating that re-hydration of the surface microbial biofilm community is a relatively slow process. Even after re-hydration, movement of water into the pore spaces of the stone or sand sample is retarded compared to the same material without microbial film. Thus, the microbiota set up the conditions necessary for most of the precipitation impinging on the rock outcrops in the aquifer recharge area to become runoff. While the simple solution would be to set up ponds to collect the runoff and enter the sandstones, microbes will grow on the basin surfaces and clog the freshly opened pore spaces. Observations in the field indicate that many of the ephemeral pools within the Navajo Sandstone are lined with a layer of iron concrete. The source and transport of this iron is currently under investigation.