Methods for the Analysis of 26 Million VOC Area Measurements during the Deepwater Horizon Oil Spill Clean-up

Abstract

The NIEHS GuLF STUDY is an epidemiologic study of the health of workers who participated in the 2010 Deepwater Horizon oil spill response and clean-up effort. Even with a large database of approximately 28 000 personal samples that were analyzed for total hydrocarbons (THCs) and other oil-related chemicals, resulting in nearly 160 000 full-shift personal measurements, there were still exposure scenarios where the number of measurements was too limited to rigorously assess exposures. Also available were over 26 million volatile organic compounds (VOCs) area air measurements of approximately 1-min duration, collected from direct-reading instruments on 38 large vessels generally located near the leaking well. This paper presents a strategy for converting the VOC database into hourly average air concentrations by vessel as the first step of a larger process designed to use these data to supplement full-shift THC personal exposure measurements. We applied a Bayesian method to account for measurements with values below the analytic instrument's limit of detection while processing the large database into average instrument-hour concentrations and then hourly concentrations across instruments on each day of measurement on each of the vessels. To illustrate this process, we present results on the drilling rig ship, the Discoverer Enterprise. The methods reduced the 26 million measurements to 21 900 hourly averages, which later contributed to the development of additional full-shift THC observations. The approach used here can be applied by occupational health professionals with large datasets of direct-reading instruments to better understand workplace exposures.

Keywords: Deepwater Horizon oil spill; direct-reading instruments; high volume data; volatile organic compounds.

Figure 1.

Steps to developing the THC:VOC relationship for prediction of THC from VOC direct-reading area measurements. This paper presents Steps 1 and 2. VOC TWA: time weighted average over hours of THC personal sample; Daily TWA: time weighted average over a day.

Figure 2.

Flowchart of procedures in estimating VOC exposure of hourly averages on a vessel. Gray squares are databases that were analyzed. We ran 12-h datasets through this algorithm using parallel computing.

Figure 3.

VOC concentrations within the 12:00–13:00 h on 6/28/2010 for selected instruments. The horizontal gray band represents the limit of reporting (LOR = 0.05 ppm). Each instrument that operated on this vessel on any day was given a number, 1–31. Only operational instruments (out of 35 total instruments) are presented. No modeling was performed, and censored values were imputed for these plots as the LOR. The VOC ppm scales for instruments 2, 7, and 8 are 0–12 ppm. Instruments 10–12 have a scale of 0–16 ppm. The remaining instruments have a scale of 0–4 ppm.

Figure 4.

VOCs AM estimates by instrument and hour on the Enterprise on 6/28/2010. Some instruments had higher levels of VOC while others had low levels of VOC likely due to the instruments’ locations on the vessel (i.e. inside in the living quarters versus the open deck).