Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Hydraulic properties of biologically initiated rock crust on sandstone
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It is well known that the presence of biologically initiated rock crust (BIRC in further text) increases the resistance of slightly cemented sandstone to erosion. As moisture and its transport are critical for many weathering processes, this study is focused on the effect of BIRC on hydraulic properties of sandstone. Changes in water vapor permeability, saturated hydraulic conductivity and capillary water absorption (both in laboratory and in situ) due to BIRC were measured and its influence on the weathering processes was discussed. Hydraulic properties of sandstone surface with BIRC were compared to those of its subsurface since we believe that this is the only way to quantify correctly the effect of BIRC.
Our research clearly shows that the studied BIRC considerably changes the hydraulic properties of sandstone surfaces. It significantly reduces saturated hydraulic conductivity (˜28 times, Table 2, Fig. 6) and capillary water absorption - measured on samples in laboratory, and in situ using the Karsten tube (6, resp. 5.4 times, Table 2) - which affects both the infiltration rate and moisture content. Therefore, BIRC is sufficient to influence most of the weathering processes controlled by moisture content and flux. However, it was found that the presence of BIRC has no measurable effect on water vapor diffusion rate. To our knowledge, this is for the first time that capillary water absorption measurements in a laboratory were compared to measurements in situ using the Karsten tube.
It was also found that BIRC only occurs on surfaces of the volumetric moisture content over ˜1 % (Table 1). Biocolonization of sandstone surfaces is thus controlled by sandstone moisture content. It can appropriately explain why, after clearing the trees which kept the humid microclimate, microorganisms on sandstone surfaces die out. Therefore, environmental factors affecting sandstone moisture content such as exposure to sun radiation and rain may control - via the occurrence/absence of BIRC - the erosion rate of sandstone surfaces.References
