STOCHASTIC-ANALYSIS OF SOLUTE TRANSPORT IN HETEROGENEOUS, VARIABLY SATURATED SOILS

Statistical moments of solute plumes from small sources in variably sa turated, heterogeneous porous media are analyzed by using a newly deve loped, efficient high-resolution Monte Carlo technique. In agreement w ith previous theoretical work, it is illustrated that the prediction o f such solute plumes is associated with large uncertainties for dimens ionless travel times, t', exceeding 40, particularly predictions of pl umes in highly heterogeneous soils (sigma(y)(2) > 2). Uncertainty abou t the travel path of the plume center contributes significantly to ove rall concentration uncertainty as flux fields become more variable. It is shown that the concentration coefficient of variation at the cente r of the plume initially increases but stagnates or decreases at later times. For highly heterogeneous soil flux conditions or for the commo n case of soils with strongly anisotropic conditions, analytical model s underestimate transverse spreading of the mean concentration plume a t any given time, while overestimating longitudinal spreading. At iden tical mean plume displacement distances, analytical models underestima te both transverse and longitudinal spreading and overestimate the var iance of solute flux (breakthrough curve) by up to a factor 4. As an a lternative to the statistical analysis of solute flux, we propose to a nalyze statistical properties of time associated with peak solute flux and with first exceedance of a given solute flux level.