Abstract

The Shale Hills Critical Zone Observatory (SHCZO) team has found that soil chemistry does not correlate with variability in pore fluid chemistry, suggesting the presence of macropores. Because of such heterogeneity, it is often difficult to relate short-term event chemistry to what we know about the chemistry of waters in catchments. Additionally, it is not clear what role the shale bedrock has on flow and transport of solutes within the catchment. We have been conducting tracer tests at the laboratory and field-scale to move toward describing short-term flux and solute transport behavior with the goal of integrating behavior over geologic time clarify the relationship between soil chemistry and pore fluid data. In field sites where such high permeability contrasts exist, what roles do flow and transport play in long-term fate of solutes? What is the importance of the interface between the shale bedrock and the regolith above? Is the shale bedrock “impermeable”? To improve characterization of permeability of the consolidated shale, we drilled four 17-m deep bedrock wells at the SHCZO and have collected a suite of borehole logs. From the drilling and data collected within the new wells, we can make the following conclusions: that there is a “slow drilling” zone around 6-7 m below land surface, above which is highly weathered shale that is reddish in color, beneath which is largely unfractured blue-grey shale. The natural gamma data similar indicate a higher percentage of clays with depth than in the top 6 m, which corresponds with data from Jin et al. (submitted, Geochimica et Cosmochimica Acta) that shows variability in shale bedrock density down about 6 m. Pump and slug test indicate an effective hydraulic conductivity of the Rose Hill Shale in the drilled boreholes on the order of 10-6 m/s, although hydraulic conductivity of the shale bedrock matrix estimated in a triaxial compression chamber is approximately10-15 m/s. In field-scale and lab-scale tracer tests, observed transport behavior appears inconsistent with the standard advective-dispersive model. Results from a conservative NaBr tracer test conducted in 10-cm undisturbed soil columns from the SHCZO include concentration histories that show long tailing behavior and non-Gaussian breakthrough, indicative perhaps of dual-domain solute transport between preferential pathways and a less permeable matrix. A numerical model of the soil column indicates than a mass transfer rate of approximately 1/hr with a mobile-domain porosity of 0.3 and an immobile-domain porosity of 0.35 can explain the data. The total porosity is consistent with previously published estimates of total porosity. At the field scale, a NaBr tracer test conducted within the Rose Hill Shale shows similar behavior, and mass transfer is needed to explain those concentration histories. These data indicate that solutes transfer between the highly permeable macropores and fractures and into the soil/shale matrix, and that diffusion is a transport property of concern in predicting solute transport behavior over the long term at the SHCZO. Both soils and shale material at this site show preferential pathways that may be indicative of dual-domain solute transport behavior.
 

Citation

Singha, K., Kuntz, B., Toran, L. (2009): Exploring Lithologic Controls on Solute Transport at the Shale Hills Critical Zone Observatory. AGU Annual Fall Conference Proceedings.

This Paper/Book acknowledges NSF CZO grant support.