Laboratory, Field, and Modeling Analysis of Solute Transport Behavior at the Shale Hills Critical Zone Observatory

We collected and analyzed breakthrough curve (BTC) data to identify the parameters controlling transport from a series of undisturbed fully saturated soil cores and a field test at the Shale Hills Critical Zone Observatory in central Pennsylvania. The soil cores were collected in a continuous hole extending across the soil profile vertically at one location to quantify how solute transport behavior changes with physical and chemical weathering. Additionally, we performed a field scale doublet tracer test to determine transport behavior within the weathered shale bedrock. Hydraulic conductivity and porosity are as low as 10-15 m/s and 0.035, respectively, in the shale bedrock and range as high as 10-5 m/s and 0.45, respectively, in the shallow soils. Bromide BTCs demonstrated significant anomalous tailing in soil cores and shale bedrock, which do not fit classical advection-dispersion model equations. To quantify the behavior, numerical simulation of solute transport was carried out with both a mobile-immobile (MIM) model and a continuous-time random walk (CTRW) approach. 1-D MIM modeling results on the soil cores yielded low mass transfer rates (<1/d) coupled with large immobile domains (1.5 2 im m θ θ − ) and revealed that solutes were transported within only 30-40% of the total pore space. MIM modeling results also suggested that immobile porosity is a combination of soil texture, fracture spacing, and porosity development on shale fragments. Similarly, the field scale doublet tracer test between boreholes indicated fractures are controlling transport and the surrounding shale matrix has a large potential to store and retard solute movement. 1-D CTRW results yielded a parameter set indicative of a transport regime that is consistently non-Fickian across the vertical length of the soil profile, identified solute tracer velocities are up to 50 times greater than the average fluid velocity, predicted that anomalous transport behavior could extend for significant periods of time, and identified the need to incorporate a continuum of mass iv transfer rates to accurately predict and describe the observed tailing behavior. These modeling results confirmed the important role of preferential flow paths, fractures, and mass transfer between more-and less-mobile fluid domains, and established the need to incorporate a mass transfer process that utilizes a distribution of mass transfer rates.