Effect of Hydrofracking Fluid on Colloid Transport in the Unsaturated Zone.

Hydraulic fracturing is expanding rapidly in the US to meet increasing energy demand and requires high volumes of hydroffacking fluid to displace natural gas from shale. Accidental spills and deliberate land application of hydroffacking fluids, which return to the surface during hydroffacking, are common causes of environmental contamination. Since the chemistry of hydroffacking fluids favors transport of colloids and mineral particles through rock cracks, it may also facilitate transport of in situ colloids and associated pollutants in unsaturated soils. We investigated this by subsequendy injecting deionized water and flowback fluid at increasing flow rates into unsaturated sand columns containing colloids. Colloid retention and mobilization was measured in the column effluent and visualized in situ with bright field microscopy. While <5% of initial colloids were released by flushing with deionized water, 32-36% were released by flushing with flowback fluid in two distinct breakthrough peaks. These peaks resulted from l) surface tension reduction and steric repulsion and 2) slow kinetic disaggregation of colloid floes. Increasing the flow rate of the flowback fluid mobilized an additional 36% of colloids, due to the expansion of water filled pore space. This study suggests that hydroffacking fluid may also indirectly contaminate groundwater by remobilizing existing colloidal pollutants. [ABSTRACT FROM AUTHOR]