Identifying Potential Exposure Pathways and Estimating Risk from Marcellus Shale Gas Development.

摘要

Concern over natural gas extraction across the U.S. and particularly from the Marcellus Shale formation, which underlies approximately two-thirds of the state of Pennsylvania, has been growing in recent years as natural gas drilling activity has increased. Identifying sources of concern and risk from shale gas development, particularly form the hydraulic fracturing process, is an important step in better understanding sources of uncertainty within the industry. Hydraulic fracturing is a well stimulation technique used in the production of natural gas from shale. While hydraulic fracturing has been in use for decades as a method for oil and gas recovery, recent advances in horizontal drilling techniques and fracturing fluid production have made previously unattainable natural gas reservoirs accessible and economically recoverable.;In the years after hydraulic fracturing came into widespread use in Pennsylvania, a large amount of data on flowback characteristics became available due to public and regulatory attention to the process. Chapter 3 examines and analyzes the constituents that make up flowback waters collected from drilling sites in the states of Pennsylvania, New York, and West Virginia. Flowback sampling data were collected from four different sources and compiled into one database with a total of 35,000 entries. Descriptive statistical analysis revealed high concentrations of chlorinated solvents, disinfectants, dissolved metals, organic compounds, radionuclides and TDS. Relative prioritization scores were developed for 58 constituents by dividing observed mean concentrations by the Maximum Contamination Level (MCL) guidelines for drinking water. The following constituents were found to have mean concentrations over 10 times greater than the MCL: Barium, Benzene, Benzo(a)pyrene, Chloride, Dibromochloromethane, Radium, and Thallium.;Regulatory inspection and violation reports also provide insight into the impact of natural gas extraction on the surrounding environment, human health, and public safety. Inspection reports for natural gas wells in Pennsylvania were collected from the Pennsylvania DEP Compliance Report from 2000 to 2014. Logistic regression analysis of 215,444 inspection records for 70,043 conventional and unconventional wells was conducted in order to compare the odds of violations occurring under different circumstances. The results in Chapter 4 revealed that, when inspected, conventional wells had 40% higher odds of having a violation, but unconventional wells had higher odds for environmental violations related to waste discharge as well as cementing and casing failures.;From there, a list of twelve failure scenarios of concern was developed focusing on specific events that may occur during the shale gas extraction process involving an operational failure or a violation of regulations to identify and prioritize potential failure scenarios for natural gas drilling operations through an elicitation of people who work in the industry. Illegal dumping of flowback water, while rated as the least frequently occurring scenario, was considered the scenario least protected by safety controls and the one of most concern to the general public. In terms of worker safety, the highest concern came from improper or inadequate use of personal protective equipment. While safety guidelines appear to be highly protective regarding PPE usage, inadequate PPE is the most directly witnessed failure scenario. Spills of flowback water due to equipment failure are of concern both in regards to the welfare of the general public and worker safety as they occur more frequently than any other scenario examined in this study.;In Chapter 6 of this study, the flowback data collected and the violation and failure scenario analyses conducted are used to develop potential exposure scenarios to wastewater from shale gas development. A risk assessment of occupational and residential exposure pathways to flowback water as carried out. Constituents of concern in flowback water were identified from the previous prioritization. The occupational cancer risk estimate for median concentrations did not exceed the target lifetime cancer risk of 10-6 except for benzo(a)pyrene, which exceeds the target risk level even at the 2.5 percentile value. The upper limit of cancer risk form exposure to heptachlor also exceeds 10 -6 in this model. Hazard quotient for barium in the same model exceeds 1 (1.7) and results in a total hazard index of 2. The residential risk assessment revealed that several carcinogenic compounds found in flowback water exceed target limits and significantly increase the risk of an individual developing cancer following chronic exposure. In general, exposure from the dermal pathway posed the greatest risk to human health. Considering non-carcinogenic effects, only barium and thallium exceed target limits, where the ingestion pathway seems to be of greater concern than dermal exposure. Exposure to radionuclides in flowback water, particularly through the inhalation pathway as they volatilize from the water to the air, poses a greater threat to human health than other contaminants examined in this assessment.