Article pubs.acs.org/est
Fate of Radium in Marcellus Shale Flowback Water Impoundments and Assessment of Associated Health Risks Tieyuan Zhang,†,‡ Richard W. Hammack,‡ and Radisav D. Vidic*,†,‡ †
Department of Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States
‡
ABSTRACT: Natural gas extraction from Marcellus Shale generates large quantities of flowback water that contain high levels of salinity, heavy metals, and naturally occurring radioactive material (NORM). This water is typically stored in centralized storage impoundments or tanks prior to reuse, treatment or disposal. The fate of Ra-226, which is the dominant NORM component in flowback water, in three centralized storage impoundments in southwestern Pennsylvania was investigated during a 2.5-year period. Field sampling revealed that Ra-226 concentration in these storage facilities depends on the management strategy but is generally increasing during the reuse of flowback water for hydraulic fracturing. In addition, Ra-226 is enriched in the bottom solids (e.g., impoundment sludge), where it increased from less than 10 pCi/g for fresh sludge to several hundred pCi/g for aged sludge. A combination of sequential extraction procedure (SEP) and chemical composition analysis of impoundment sludge revealed that Barite is the main carrier of Ra-226 in the sludge. Toxicity characteristic leaching procedure (TCLP) (EPA Method 1311) was used to assess the leaching behavior of Ra-226 in the impoundment sludge and its implications for waste management strategies for this low-level radioactive solid waste. Radiation exposure for on-site workers calculated using the RESRAD model showed that the radiation dose equivalent for the baseline conditions was well below the NRC limit for the general public.
1. INTRODUCTION Benefits of combining recent technology breakthroughs in horizontal drilling and hydraulic fracturing enabled economical recovery of oil and gas that is trapped in the low-permeability shales. Gas production from shales increased from 2% (0.01 trillion cubic meters, TCM) of the total U.S. natural gas production in 2000 to 23% (0.14 TCM) in 2010. The U.S. Energy Information Administration (EIA) projected that it will account for 50% (0.47 TCM) of total natural gas production in the U.S. by 2040.1 Environmental and public health concerns associated with natural gas extraction from Marcellus Shale have also increased significantly in recent years. Environmental concerns include land and habitat fragmentation, induced earthquakes, and impacts on air and water quality.2−9 Drilling and hydraulic fracturing of an unconventional gas well typically requires 3−6 million gallons of fracturing fluid, which is mainly comprised of water (∼90%), proppant (∼9%), and other chemicals (
