316
within a 30 km radius of the Ixtoc 1 wellhead. This estimate was thought to be conservative due to restricted sampling and, in particular, a lack of samples from sediments
beneath the course of the oil as it was carried westward by the Gulf Loop Current.
TPH is a broad and non-specific measure of petrocarbon contamination, and no
universal analytical method for measuring it exists; commonly, organic extracts of
sediments are analyzed by gas chromatography-flame ionized detection, but methods may differ in terms of solvents used, sample purification, and chromatographic
method. Nevertheless, TPH has proven useful in assessing contamination shortly
after an oil spill. An early time-series study documented an increase in TPH near the
Ixtoc 1 wellhead from 1978 to 1980, followed by a decline to pre-spill values by
1981 (Botello and Villanueva 1985). In cases with weaker spatial and temporal connections between source and spill, however, TPH is less useful because it provides
little information about the composition or provenance of the hydrocarbons detected.
Further, the analytical window of TPH is relatively narrow, only enabling detection
of hydrocarbons that are amenable to gas chromatography and excluding large and
polar residues of petroleum and its weathering products. The rapid attenuation of
TPH observed after the Ixtoc 1 spill (Botello and Villanueva 1985) was likely the
result of combined removal mechanisms (e.g., microbial respiration and abiotic
degradation) and transformation processes that created polar moieties and insoluble
residues not readily extracted or measured using standard TPH protocols. Following
the DWH spill, such transformation products have been well studied using Fourier
transform ion cyclotron resonance-mass spectrometry (FTICR-MS) and thin-layer
chromatography-flame ionized detection (Aeppli et al. 2012; Radović et al. 2014).
Evidence for similar transformation of Ixtoc 1 hydrocarbons in southern GoM environments is presented in Sect. 19.5.3 and in Radović et al. (2020).
Polycyclic aromatic hydrocarbons (PAHs) have greater forensic potential than
TPH, but they may be rapidly biodegraded; for instance, Bagby et al. (2017) estimated half-lives of alkylated phenanthrenes to be <6 years in northern GoM sediments containing Macondo well oil residues from the DWH blowout. Only PAH
with >4 rings had half-lives >10 years, and alkylated dibenzothiophene half-lives
were as short as ~160 days. PAH concentrations, like those of more labile oil compounds such as n-alkanes, are thus likely to be of limited utility in tracing spilled oil
over decadal time scales. Ratios of specific compounds that degrade at similar rates,
however, can remain useful even as their concentrations decline.
Boehm and Fiest (1980) reported a predominance of petrogenic over pyrogenic
PAHs and abundant dibenzothiophenes (DBT) in sediments containing putative Ixtoc
1 oil. Ratios of alkylated phenanthrenes (three-ringed PAHs with two or three methyl
groups) and alkylated dibenzothiophenes were considered most diagnostic after the
spill (Boehm et al. 1983). These metrics provided evidence of Ixtoc 1 oil on the seafloor 50 km from the wellhead and helped to identify Ixtoc 1 oil residues in coastal
Texas sediments. Recently, Lincoln et al. (2017) used the same ratios to screen southern GoM seafloor sediments for residual traces of Ixtoc 1 petroleum input.
PAH and TPH measurements were also used in subsequent investigations of
hydrocarbon contamination in sediments near zones of petroleum production in the
Bay of Campeche (e.g., Marchand et al. 1982; Botello et al. 1991; Alpuche-Gual
and Gold-Bouchot, 2014) and its coastal regions (e.g., Vazquez et al. 1991; GoldS. A. Lincoln et al.
within a 30 km radius of the Ixtoc 1 wellhead. This estimate was thought to be conservative due to restricted sampling and, in particular, a lack of samples from sediments
beneath the course of the oil as it was carried westward by the Gulf Loop Current.
TPH is a broad and non-specific measure of petrocarbon contamination, and no
universal analytical method for measuring it exists; commonly, organic extracts of
sediments are analyzed by gas chromatography-flame ionized detection, but methods may differ in terms of solvents used, sample purification, and chromatographic
method. Nevertheless, TPH has proven useful in assessing contamination shortly
after an oil spill. An early time-series study documented an increase in TPH near the
Ixtoc 1 wellhead from 1978 to 1980, followed by a decline to pre-spill values by
1981 (Botello and Villanueva 1985). In cases with weaker spatial and temporal connections between source and spill, however, TPH is less useful because it provides
little information about the composition or provenance of the hydrocarbons detected.
Further, the analytical window of TPH is relatively narrow, only enabling detection
of hydrocarbons that are amenable to gas chromatography and excluding large and
polar residues of petroleum and its weathering products. The rapid attenuation of
TPH observed after the Ixtoc 1 spill (Botello and Villanueva 1985) was likely the
result of combined removal mechanisms (e.g., microbial respiration and abiotic
degradation) and transformation processes that created polar moieties and insoluble
residues not readily extracted or measured using standard TPH protocols. Following
the DWH spill, such transformation products have been well studied using Fourier
transform ion cyclotron resonance-mass spectrometry (FTICR-MS) and thin-layer
chromatography-flame ionized detection (Aeppli et al. 2012; Radović et al. 2014).
Evidence for similar transformation of Ixtoc 1 hydrocarbons in southern GoM environments is presented in Sect. 19.5.3 and in Radović et al. (2020).
Polycyclic aromatic hydrocarbons (PAHs) have greater forensic potential than
TPH, but they may be rapidly biodegraded; for instance, Bagby et al. (2017) estimated half-lives of alkylated phenanthrenes to be <6 years in northern GoM sediments containing Macondo well oil residues from the DWH blowout. Only PAH
with >4 rings had half-lives >10 years, and alkylated dibenzothiophene half-lives
were as short as ~160 days. PAH concentrations, like those of more labile oil compounds such as n-alkanes, are thus likely to be of limited utility in tracing spilled oil
over decadal time scales. Ratios of specific compounds that degrade at similar rates,
however, can remain useful even as their concentrations decline.
Boehm and Fiest (1980) reported a predominance of petrogenic over pyrogenic
PAHs and abundant dibenzothiophenes (DBT) in sediments containing putative Ixtoc
1 oil. Ratios of alkylated phenanthrenes (three-ringed PAHs with two or three methyl
groups) and alkylated dibenzothiophenes were considered most diagnostic after the
spill (Boehm et al. 1983). These metrics provided evidence of Ixtoc 1 oil on the seafloor 50 km from the wellhead and helped to identify Ixtoc 1 oil residues in coastal
Texas sediments. Recently, Lincoln et al. (2017) used the same ratios to screen southern GoM seafloor sediments for residual traces of Ixtoc 1 petroleum input.
PAH and TPH measurements were also used in subsequent investigations of
hydrocarbon contamination in sediments near zones of petroleum production in the
Bay of Campeche (e.g., Marchand et al. 1982; Botello et al. 1991; Alpuche-Gual
and Gold-Bouchot, 2014) and its coastal regions (e.g., Vazquez et al. 1991; GoldS. A. Lincoln et al.