HR: 0800h
AN: H51F-0421    [Abstracts]
TI: Transverse Dispersion: Small, but Significant, and Difficult to Determine
AU: * Cirpka, O A
EM: Olaf.Cirpka@EAWAG.CH
AF: Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Überlandstr. 133, Dübendorf, 8600 Switzerland
AB: Transverse dispersion plays a decisive role in the natural attenuation of continuously emitted contaminants that must mix with compounds from ambient flow in order to be degraded. Once the mass flux of reactant advected into the source zone is depleted, the only process to replenish the reactant is transverse dispersion, at least in steady-state stable flow. Various theoretical and experimental studies have shown that the length of such plumes is inversely proportional to the transverse dispersivity of the formation. Also in longitudinal mixing, transverse dispersion is important, since it leads to the mass transfer between finger-shaped plume fringes and the surrounding water, transferring longitudinal spreading to longitudinal mixing. According to first-order theory, transverse dispersion is hardly affected by heterogeneity. Transverse ensemble dispersion coefficients exhibit a maximum value at a travel distance of a few integral scales, reflecting the uncertainty in plume meandering. The large-time limit is hardly larger than the local coefficient. Transverse effective dispersion, which is more representative for solute mixing, does not show the intermediate peak. Transient flow may lead to enhanced transverse mixing because of spatial reorientation of the dispersion tensor and due to interactions between the fluctuating flow field and kinetic sorption. A simplistic analysis of transverse diffusion lengths in formations with high-permeability inclusions reveals that higher-order effects, not accounted for in first-order theory, may be important for transverse dispersion in heterogeneous domains. The contribution covers the significance of transverse dispersion for reactive transport, the effects of heterogeneity, and the difficulties in determining transverse dispersion coefficients. It is shown that transverse concentration profiles cannot be used to evaluate the coefficients unless the velocity distribution is known. Techniques based on reactive transport itself, such as inferring transverse dispersion coefficients from the length of plumes or from the net dissolution rate of a NAPL pool, appear to be most reliable.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1869 Stochastic hydrology
SC: Hydrology [H]
MN: Fall Meeting 2005