Quantifying solute transport at the Shale Hills Critical Zone Observatory

We collected and analyzed Br breakthrough curve (BTC) data to identify the parameters controlling transport from a series of soil cores and a field-scale tracer test at the Shale Hills Critical Zone Observatory (SH-CZO) in central Pennsylvania. The soil cores were retrieved from a continuous hole that extended through the soil profile to quantify also how solute transport behavior changes with depth and weathering. Additionally, we performed a fieldscale doublet tracer test to determine transport behavior in the weathered shale bedrock. Hydraulic conductivity and porosity were as low as 10 ^-15 m s ^-1 and 0.035, respectively, in the shale bedrock and upward of 10 ^-5 m s ^-1 and 0.45, respectively, in the shallow soils. Bromide BTCs demonstrated significant tailing in soil cores and field tracer experiments, which does not fit classical advection-dispersion processes. To quantify the behavior, numerical simulation of solute transport was performed with both a mobile-immobile (MIM) model and a continuous-time random walk (CTRW) approach. One-dimensional MIM modeling results yielded low mass transfer rates (<1 d ^-1) coupled with large immobile domains (immobile/mobile porosity ratio of 1.5 ^-2). The MIM modeling results also suggested that immobile porosity was a combination of soil texture, fractures, and porosity development on shale fragments. One-dimensional CTRW results yielded a parameter set indicative of a transport regime that is consistently non-Fickian within the soil profile and bedrock. These modeling results confirm the important role of preferential flow paths, fractures, and mass transfer between more- and less-mobile fluid domains and advance the need to incorporate a continuum of mass transfer rates to more accurately quantify transport behavior within the SH-CZO.