Vadose Zone Monitoring System for Site Characterization and Transport Modeling
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CHARACTERIZATION AND TRANSPORT MODELING J. B. SISSON, A. L. SCHAFER, and J. M. HUBBELL Integrated Earth Sciences Dept., Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID 83415-2107, [email protected]
ABSTRACT Monitoring the vadose zone below buried waste provides an early warning of contaminate transport toward the groundwater. To quantify the transport mechanisms, vadose zone hydraulic characteristics and the physical variables need to be
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Inner o :t ,kpi obtained. We have designed and implemented a Vadose Zone Monitoring System (VZMS) to transduce monitor or sample the 3 state variables of the vadose zone, water potential, water content and dN3-w lWa 0oou cotramlt reseitoir c POR chemical concentration. The state variables are monitored using an Advanced Tensiometer (AT), a Water table borehole water content sensor (BWCS) and a vacuum lysimeter, respectively. This system was installed at the Savannah River Site (SRS) E-Area disposal site, where low level wastes have been disposed of in shallow trenches. Figure 1.Components and installation of the advanced tensiometer. The system has operated for several months providing nearly continuous water content and water potential data. The vacuum lysimeters were activated on a quarterly schedule. Installation details and an example data set are presented to illustrate the effectiveness of the VZMS, and demonstrate the utility of the VZMS as an indicator of contaminant transport.
INTRODUCTION Current regulations require that groundwater be monitored for contaminant presence. Once contamination is detected in the groundwater above regulatory limits, the contaminant must be removed or contained. Achieving removal or containment becomes a combined legal and technical challenge. If the contaminant migration could be detected in the vadose zone above the aquifer before seriously impacting groundwater, engineering solutions could be designed and implemented before incurring regulatory penalties. To achieve this end, we have designed and implemented a VZMS. Its primary objective is to provide early detection and warning of contaminant transport, allowing time for engineering solutions to be evaluated, optimized, and implemented prior to contaminants impacting groundwater. 161 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
MATERIALS AND METHODS In the absence of large temperature gradients, contaminant transport in the vadose zone is governed by 3 state variables: water potential, water content and contaminant concentration. Water potential is the amount of energy needed to remove water from the soil and the gradient in water potential determines the direction of water movement. Water content determines the fraction of the porous media available for transport, the hydraulic conductivity, internal surface area available for chemical interaction, and ultimately the pore water velocity along transport paths. The chemical transport rate is a function of chemical concentration, the internal surface area it in
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