HR: 0800h
AN: H11B-0299    [Abstracts]
TI: On The Tailing Behavior Of The Backward Travel Time Probability Density (BTTPD) Of Contaminants In Alluvial Aquifer Systems
AU: * Zhang, Y
EM: yong.zhang@dri.edu
AF: Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512 United States
AU: Benson, D A
EM: Dave.Benson@dri.edu
AF: Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512 United States
AB: The shape of the backward travel time probability density (BTTPD) of groundwater contaminant is important to water-quality related problems, such as contaminant remediation and aquifer vulnerability assessment. The BTTPD in alluvial aquifer systems may have significant weight in one or both tails because of the anomalous transport of plumes, which is due to the preferential flow in networks of ancient stream channels and the sequestration of plumes in surrounding fine-grained aquitard materials. The behavior of BTTPD tails and the controlling factors were explored systematically using thousands of km-scale numerical models based on geologically constrained realizations of alluvial systems, generated with exponential transition probability (TP) geostatistics. The numerical models were discretized on a very fine scale (5*10*0.5m) to capture the interactions of numerous depositional facies. The numerical results show that although the BTTPD contains an early arrival peak, the early tail is Gaussian-like with a slope on a log-log plot larger than $\sim$6. The simulated backward location probability (BLP) indicates that the distribution of vertical displacements of solute particles only has a weak power-law exponent that varies with time, and the longitudinal movement of particles inside of channel is negatively skewed upstream. The sediment layering and the short correlation scale of channels on the vertical direction may limit the large jumps of particles vertically. In contrast, the simulated BTTPDs contain late-time tails varying from exponential to power-law, depending on the proportion of low-K materials, the distribution of the size of low-K materials, and the value of the local dispersion coefficient. A heavy late-time tail with a slope on a log-log plot of -2 to -3 was found for systems dominated by aquitards, while a weaker tail with a slope $<$-3 or an exponential tail was found for systems with abundant coarse-grained materials. The exponential density function of the 'sizes' of the low-K units simulated by the exponential-form TP gives rise to a late-time BTTPD that is a predictable function of the advection time, geostatistics of the low-K material, and the molecular diffusion coefficient.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting