Abstract

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 field-scale 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.

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