HR: 1340h
AN: H43F-0545    [Abstracts]
TI: Time-Lapse Monitoring of Density-Driven Free Convection Using 3-D Electrical Resistivity
AU: * Stevens, J D
EM: joel.stevens@mail.utexas.edu
AF: The University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712 United States
AB: Density-driven free convection, a potentially important groundwater transport process, has been monitored at Padre Island National Seashore to determine if this phenomenon can develop under natural environmental conditions. To date, this process has not been conclusively detected in the field. To monitor the development of inverted density gradients and the subsequent onset of free convection on Padre Island, this study utilizes both hydrogeology and geophysics. Groundwater measurements of hydraulic head and water chemistry along with time-lapse 3-D electrical resistivity surveys were collected at two-to-three week intervals for a period of six months. To ensure electrical resistivity measurements accurately reflect the lithology and pore-fluid properties, core samples, shallow monitoring wells, piezometers, multi-level sampling wells and data loggers verified the resistivity data. Ground-truth, time-lapse, 3-D electrical resistivity measurements revealed that the sites at Padre Island National Seashore increased in salinity and developed distinct inverted density gradients during the study period. Hydraulic head data showed horizontal gradients followed topography down-slope towards the shoreline, while the vertical hydraulic gradient was upward, suggesting the sites act as groundwater discharge zones. Additionally, water table levels decreased during the course of the study of mid-spring to mid-fall. This implies that evaporation is the source of the measured salinity increase and density inversions. Notably, these density inversions did not overcome forces of diffusion and dispersion because concentration gradients were not sufficiently large enough to develop density-driven flow and evaporation-driven density inversions develop too slowly for this type of solute transport. These findings suggest that density-driven free convection may not develop in natural, homogeneous groundwater systems where inverted density gradients develop slowly, such as Padre Island. Rather, free convection can only occur when a density inversion with a high concentration gradient develops quickly enough to avoid mixing, thus allowing for the onset of density-driven flow. These findings suggest there may be a critical rate of density inversion development.
DE: 1832 Groundwater transport
DE: 1835 Hydrogeophysics
DE: 3094 Instruments and techniques
SC: Hydrology [H]
MN: Fall Meeting 2005