EFFECT OF AERATION ON THE PHYSICAL AND BIOGEOCHEMICAL CONDITIONS IN A MARCELLUS PRODUCED WATER STORAGE POND

Development of deep horizontal drilling techniques has stimulated the recovery of natural gas from the Marcellus Shale formation in the Appalachian region, where thousands of wells are currently in production. Each hydrofracture requires millions of gallons of water amended with various chemicals to maximize gas extraction. Flowback water from the wells is typically high in salts dissolved from the formation and much of it is stored in surface ponds for further treatment and recycling. The storage of water can result in stagnant conditions and the generation of noxious odors, so aeration units are occasionally used to circulate and aerate the water. In this study, we examined the physical and biogeochemical changes in a produced water storage pond in Greene County, PA after 9 days of aeration.

Physical parameters (including temperature, conductivity, pH, oxidation-reduction potential (ORP), and dissolved oxygen (DO)) were measured and water samples collected from selected depths along defined transects by boat. In the laboratory, element concentrations were measured by ICP-OES, anions by ion chromatography, dissolved sulfide by iodometric titration, and dissolved iron by phenanthroline complexation. Bacteria were examined using direct staining and fluorescent in situ hybridization (FISH) methods.

All parameters measured were consistent with the existence of initially stratified conditions in the pond, with fresher water on the immediate surface, underlain by highly saline anoxic waters. Aeration resulted in the complete mixing of the water column and changes in the concentration of redox-sensitive chemical species. Water chemistry was dominated by Na (367 mM), Ca (119 mM) and Cl (508 mM) with smaller contributions from Mg (29 mM) and SO₄ (23.1 mM). Dissolved sulfide concentrations prior to aeration were very high (ca. 1 mM), but were negligible following aeration. Bacterial analyses suggested a shift from a population dominated by sulfate-reducing bacteria at depth to a more diverse one throughout the water column following aeration. The aeration method employed was therefore effective at disrupting the stratification of the water column and the reduction of the sources and causative agents of noxious dissolved sulfide odor from the pond.