Restricted (Penn State Only)
Cai, Zhang
Graduate Program:
Petroleum and Mineral Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
April 27, 2018
Committee Members:
  • Li Li, Dissertation Advisor
  • Li Li, Committee Chair
  • Jeremy M. Gernand, Committee Member
  • Hamid Emami-Meybodi, Committee Member
  • Nathaniel R Warner , Outside Member
  • Reactive transport modeling
  • Marcellus Shale flowback / produced waters
  • Mineralogy
  • Mineral spatial heterogeneity
  • Natural attenuation
  • Rock-water interaction
  • Geochemistry
Flowback and produced waters from Marcellus Shale gas extraction (MSWs) typically contain high levels of salinity and pollutants including trace metals, which raise public concerns on drinking water quality. Extensive studies have focused on evidences linking the potential water contamination to the shale gas development and the interactions of MSWs with minerals and different types of waters in batch systems. However, the natural attenuation and reactive transport of MSW chemicals in natural aquifers remains elusive due to the facing challenges: (i) the different time scales and magnitude of MSW release under various receiving water conditions, (ii) the complex aquifers composed of multiple minerals with differing reactivity, and (iii) the ubiquitous occurrence of spatial heterogeneity in natural subsurface. Numerical experiments indicates that in clay-rich sandstone aquifers, ion exchange plays a key role in determining the maximum concentration and the time scale of released cations in receiving natural waters. In contrast, mineral dissolution/precipitation play a minor role. The relative time scales of recovery Tau(rr), a dimensionless number defined as the ratio of the time needed to return to background concentrations over the residence time of natural waters, vary between 5-10 for Na, Ca, and Mg, and between 10-20 for Sr and Ba. In rivers and sand and gravel aquifers with negligible clay content, Tau(rr) values are close to 1 because cations are flushed out at ~ 1 residence time. These values can be used as first order estimates of time scales of released MSWs in natural water systems. Mineralogy regulates the types of reactions that occur and the extent of solute immobilization from MSW release. In the clay-rich column, trace metals are retarded by ion exchange but also are retained via mineral precipitation (~50-90%). In the calcite-rich column, trace metals are retained through precipitation and solid solution partitioning by 75-99%. In the quartz column, the trace metals are retained the least. As to spatial heterogeneity, we set up two two-dimensional heterogeneous cells with the same vermiculite-to-quartz mass ratio but different spatial patterns as compared to a “Uniform” column: the “1/4-zone” and “1/2-zone” cells have rectangular vermiculite clusters at a quarter and a half lengths of the cells, respectively, and the “Uniform” column has uniformly distributed vermiculite and quartz. Spatial heterogeneity regulates not only the extent, but also the dominant types of clay-MSW interactions. In comparison to Uniform media, heterogeneous media minimizes the vermiculite-MSW interaction with the decrease of trace metal (Mn, Cu, Zn, Pb, Cd) immobilization by 1-2 orders of magnitude. This implies the higher risk on drinking water quality in natural heterogeneous aquifers. Consequently, this study has significant implications on predicting natural attenuation and reactive transport of complex contaminants from MSW release in the natural subsurface.