Nonideal transport of reactive solutes in heterogeneous porous media 4. Analysis of the Cape Cod natural-gradient field experiment

One of the largest field studies of reactive-solute transport is the natura l-gradient experiment conducted at Cape Cod from 1985 to 1988. Major findin gs regarding the transport behavior of the reactive solute (lithium) were t hat the rate of plume displacement decreased with time (temporal increase i n effective retardation), the degree of longitudinal spreading was much gre ater than that observed for bromide for an equivalent travel distance, and the plume was asymmetric, with maximum concentrations located near the lead ing edges. The objective of our work was to quantitatively analyze the tran sport of lithium and to attempt to identify the factor or factors that cont ributed significantly to its observed nonideal transport. We used a mathema tical model that accounted for several transport factors, including spatial ly variable hydraulic conductivity and spatially variable, nonlinear, rate- limited sorption, with all parameter values obtained independently. The tra nsport behavior observed during the first 250 days, corresponding to a tran sport distance of 60 m, was predicted reasonably well by the simulation tha t incorporated spatially variable hydraulic conductivity; nonlinear, rate-l imited, spatially variable sorption; and uniform water chemistry. However, the larger degree of deceleration observed during the latter stage of the e xperiment (the final 20 m) was not. The larger deceleration was successfull y simulated by increasing 3-fold; the mean sorption capacity of the latter portion of the transport domain. Such a change in sorption capacity is cons istent with the potential impact on lithium sorption of measured changes in water chemistry (e.g., pH increase, reduction in resident Zn) that occur i n the zone through which the lithium plume traversed. The results of the an alyses suggest that nonlinear sorption and variable water chemistry may hav e been primary factors responsible for the nonuniform displacement of the l ithium plume, with rate-limited sorption/desorption having minimal impact. In addition, the asymmetry of the plume appears to have been caused primari ly by nonlinear sorption, whereas the enhanced longitudinal spreading appea rs to have been caused by the combined influences of spatially variable hyd raulic conductivity and sorption, nonlinear sorption, and rate-limited sorp tion/desorption. A comparison of the results of this analysis to those we o btained from an analysis of the Borden natural-gradient study reveals sever al similarities regarding the transport of reactive contaminants at the fie ld scale.