Effect of uncertainty of low permeable unit hydraulic parameters on prediction of long term contaminant transport.

Abstract:

Characterization of local-scale hydraulic parameters of aquifers with complex internal architecture still is a challenger. For low permeable units the uncertainty of estimated hydraulic conductivity and diffusion coefficient values typically could be an order of magnitude and more. For long term migration prediction the role of low permeable units is very important in overall contaminant subsurface spreading.

To quantify the role of uncertainty of parameters of low permeable units on migration processes at the waste injection site we developed using 3-D TP/MC approach series of high-resolution lithological models of injection aquifer that includes about 40% of discontinuous clay hydrofacies. The difference between the models is in the used for simulation characteristic lengths of clay hydrofacies due to uncertainty of experimental variogramms.

Using core samples we estimated in lab hydraulic conductivity and diffusion coefficient of clay. Next we simulated with the developed models pure advective and advective-diffusion vertical contaminant transport with estimated in lab and found in previous studies parameters of clay and compared the simulation results. The comparison shows that the role of hydraulic and diffusion parameters of clay hydrofacies strongly depends from its characteristic length, which controls the portion of advective flow through the clay units in overall flow. The difference between advective and advective- diffusion results decreases with decreasing the horizontal characteristic length of clay for broad range of hydraulic and diffusion parameters of clay. Thus developing of realistic model of internal aquifer lithological heterogeneity and analysis with this model future migration is the essential step effecting on the program of site hydraulic characterization.