Analysis of vertical transportation mechanism of VOCs from horizontal hydraulic fracturing wastewater

Abstract

Return water including high concentration salts and toxic organic compounds will be stored on the surface in storage tanks and/or ponds after hydraulic fracturing operation. The potential contamination risk associated with hydraulic fracturing wastewater disposal is several orders of magnitude larger than the other pathways. Even in a bestcase scenario, it was very likely that an individual well would release at least 200 m3 of contaminated fluids (Rozell and Reaven, 2012). Wastewater spills via faulty storage could lead to groundwater contamination, volatile to atmosphere and/or be adsorbed in the solid phase creating a risk of negative impacts on the environmental compartments, water, soil, air, and on human health. An analytical 1D vertical transport model of Volatile organic compounds (VOCs), grouped into nine families according to their similar behavior, through vadose zone to groundwater is settled by assuming linear, equilibrium partitioning between vapor, liquid and absorbed chemical solid phase. The evaluated maximum concentration of each organics in storage tanks as representative example of surface storage was used. The transportation results shown that the higher value of external recharge rate and water solubility of organic compounds, the faster the organic compounds are dissolved and migrate to the aquifer. In this model, Henry’s constant and recharge rate are both key parameters in the assessment of the cumulative mass percentage in atmosphere. Under the same conditions, comparison of the curves of gas percentage from different compounds allows to understand and conclude that to a greater number of substances with a high number of Henry’s constant greater volatility into the gas phase. For a certain VOCs, a minor amount of constitutes in atmosphere is produced in moist area compared with the gas mass percentage value in the arid zone.