MICROBIOLOGICAL EFFECTS ON GEOCHEMICAL AND HYDROLOGICAL PROCESSES IN VARIABLY SATURATED (VADOSE) SUBSURFACE SYSTEMS

Understanding chemical fate and transport in the vadose zone is critical to protect groundwater resources and preserve ecosystem health. However, prediction can be challenging due to the dynamic hydrologic and biogeochemical nature of the vadose zone. Additional complexity is added by subsurface structural heterogeneity. This study uses repacked soil column experiments to demonstrate how microbial activity influences geochemical cycling and hydrologic flow. Three “short” laboratory soil columns were constructed to evaluate the effects of soil layering: a homogenized medium-grained sand, homogenized organic-rich loam, and a sand-over-loam layered column. In addition, two “long” columns were constructed using either gamma-irradiated (sterilized) or untreated sediments to evaluate the effects of both soil layers and the presence of microorganisms. The long columns were packed identically; a medium-grained sand matrix with two vertically separated and horizontally offset lenses of organic-rich loam. In all 5 columns, downward and upward infiltration of water was evaluated to simulate rainfall and rising water table events respectively. In-situ colocated probes were used to measure soil water content, matric potential, Eh, major anions, ammonium, Fe²⁺, and total sulfide. Enhanced biogeochemical cycling was observed in the short layered column versus the short, homogeneous columns, and enumerations of iron and sulfate reducing bacteria were 1-2 orders of magnitude greater. In the long columns, microbial activity caused mineral bands and produced insoluble gases that impeded water flow through the pores of the sediment. Capillary barriers, formed around the lenses due to soil textural differences, retarded water flow rates through the lenses. This allowed reducing conditions to develop in the lenses, evidenced by the production of Fe²⁺ and S^2-. At the fringes of the lenses, Fe²⁺ oxidized to form Fe(III)-oxide bands that further retarded water flux. No such mineral bands developed in the sterilized column. As a consequence, water content in the lenses of the sterilized column was half that of the other column and flow rates through the lenses were an order of magnitude lower. Results provide a specific example of the direct impact of biogeochemical cycling on water flow in the vadose zone and vice versa.