Field and Laboratory Studies on the Effects of Bioturbation on Porosity and Permeability In the Vadose Zone In Continental Settings: How Ichnopedologic Fabric Shapes and Modifies Aquitards, Aquicludes, Aquifers, and Tomorrow's Reservoirs and Seals

At any one time the majority of continental settings are subaerially exposed and modified by pedogenic processes that include bioturbation, produced by soil biota as one of the five soil-forming factors. Resulting fabrics are ichnopedologic in nature and have unique biophysicochemical properties compared to sediments in subaqueous settings. Three major differences exist between subaerially and subaqueous settings: 1) bioturbation in subaerial settings is produced mostly in the vadose zone; 2) bioturbation is much more penetrative in subaerial than in subaqueous settings; and 3) fabrics in subaerial settings result from abiotic and biotic pedogenic processes, whereas fabrics in subaqueous settings result mostly from biotic processes. Similar differences exist between continental and marine ichnofabrics, with the major caveat that marine bioturbation was much more prolific in the early Paleozoic. Field studies in fluvial and periaquatic palustrine settings suggest that deep penetrative traces in otherwise tight media can produce leaky aquitards and aquicludes. Bioturbation of subaerially exposed overbank and crevasse-splay deposits result in homogenized ichnopedologic fabrics with a variety of macropores and macrochannels. Laboratory experiments using earthworms, masked chafer beetle larvae, and cicada nymphs reveal a variety of ichnopedologic properties produced by organism activity in the vadose zone. Air trapped in burrows during flooding of experimental setups suggests that diagenetic alteration of bioturbated media will be heterogeneous, likely producing better-cemented media and less permeable areas compared to areas with burrows. Differential compaction of backfilled sediment within burrows and heavily bioturbated areas strongly alters the porosity and permeability of the original deposits. Observed microbial growth along burrow walls may also enhance pathways of greater permeability by binding the burrow walls that maintain these pathways after burial.