Reevaluation of Large-Scale Dispersivities for a Waste Chloride Plume: Effects of Transient Flow

Daniel J. Goode^a and Leonard F. Konikow
U.S. Geological Survey, Reston, Virginia 22092, USA
^anow at USGS, 111 Great Valley Pkwy, Malvern PA 19355, USA

ABSTRACT

This paper investigates the effects of transient ground-water flow on dispersion of a waste chloride plume in the basaltic aquifer beneath the Idaho (USA) National Engineering Laboratory. In an early application of numerical modeling techniques to the two-dimensional simulation of field-scale plumes, previous investigators identified longitudinal and transverse dispersivities using an independently calibrated steady-state flow model and matching contours of observed and simulated concentrations. The unusual result of calibrated transverse dispersivity (140 m) being significantly larger than longitudinal dispersivity (90 m) has been attributed to spatial heterogeneity, the fractured nature of the aquifer, and to the use of a two-dimensional model. New calibrations of the solute-transport model are performed on point concentration observations using a flow model incorporating transient recharge conditions that cause significant fluctuations in velocity. Under transient flow conditions, lowest calibration errors are achieved with significantly larger dispersivities than previously hypothesized, and with the longitudinal component larger than the transverse component. Unfortunately, the sensitivity of the model calibration error to dispersivity is low. Incorporating transient flow in this two-dimensional porous-media model does not significantly improve our understanding of the processes controlling chloride transport at this site.

Citation:
Goode, D.J., and L.F. Konikow, 1990, Reevaluation of large-scale dispersivities for a waste chloride plume: Effects of transient flow: p. 417-426 in Kovar, Karel, (ed.), ModelCARE 90: Calibration and Reliability in Groundwater Modelling (Proceedings of the conference held in The Hague, September 1990), IAHS Publication no. 195.

Keywords: Idaho National Engineering Laboratory, National Reactor Testing Station, Chloride, Tritium, MOC, basalt, lava, waste injection, parameter estimation, fractured rock

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