Linear Relationships Between Total Hydrocarbons and Benzene, Toluene, Ethylbenzene, Xylene, and n-Hexane during the Deepwater Horizon Response and Clean-up

Abstract

Objectives: Our objectives were to (i) determine correlations between measurements of THC and of BTEX-H, (ii) apply these linear relationships to predict BTEX-H from measured THC, (iii) use these correlations as informative priors in Bayesian analyses to estimate exposures.

Methods: We used a Bayesian left-censored bivariate framework for all 3 objectives. First, we modeled the relationships (i.e. correlations) between THC and each BTEX-H chemical for various overarching groups of measurements using linear regression to determine if correlations derived from linear relationships differed by various exposure determinants. We then used the same linear regression relationships to predict (or impute) BTEX-H measurements from THC when only THC measurements were available. Finally, we used the same linear relationships as priors for the final exposure models that used real and predicted data to develop exposure estimate statistics for each individual exposure group.

Results: Correlations between measurements of THC and each of the BTEX-H chemicals (n = 120 for each of BTEX, 36 for n-hexane) differed substantially by area of the Gulf of Mexico and by time period that reflected different oil-spill related exposure opportunities. The correlations generally exceeded 0.5. Use of regression relationships to impute missing data resulted in the addition of >23 000 n-hexane and 541 observations for each of BTEX. The relationships were then used as priors for the calculation of exposure statistics while accounting for censored measurement data.

Conclusions: Taking advantage of observed relationships between THC and BTEX-H allowed us to develop robust exposure estimates where a large amount of data were missing, strengthening our exposure estimation process for the epidemiologic study.

Keywords: Deepwater Horizon; n-hexane; BTEX; correlations; determinants of exposure.

Figure 1.

Benzene, toluene, ethylbenzene correlations with total hydrocarbons (THC) by time period. Correlations reflect all outside areas within a particular group. The error bars represent the 95% credible interval and the points represent the median posterior estimate.

Figure 2.

Xylene and n-hexane correlations with total hydrocarbons (THC) by time period. Correlations reflect all outside areas within a particular group. The error bars represent the 95% credible interval and the points represent the median posterior estimate. n-Hexane was not observed in all time periods (leading to missing measurements that we later imputed) and is therefore only shown for TP1b-3. Within land, n-hexane had too few measurements in TP2 to develop a relationship (N = 5) and had too few measurements to develop separate relationships for Beaches/Marshes and Ports/Docks. These n-hexane land correlations include near shore measurements due to the lack of observed n-hexane measurements on land. The correlations for Ports/Docks in TP5 are not shown due to limited measurements (N = 11).

Figure 3.

Benzene and toluene correlations with total hydrocarbons (THC) by area. Correlations reflect all time periods and outside measurements. The error bars represent the 95% credible interval and the points represent the median posterior estimate.

Figure 4.

Ethylbenzene and xylene correlations with total hydrocarbons (THC) by area. Correlations reflect all time periods and outside measurements. The error bars represent the 95% credible interval and the points represent the median posterior estimate. n-Hexane is not shown since priors were not developed by area within other vessels or land categories.