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Figure 1: The location of sites used for sample collection in Nueces Bay.

The Texas oil boom of the early 20th century transformed the fortunes of the state. Yet, the gusher age has also left a lasting and damaging impact on the region’s ecology.

Until 1993, the discharge of petroleum brine—a byproduct of oil extraction containing hydrocarbons and dissolved minerals—occurred routinely along the Texas coast. In 1979, for example, it was estimated that more than 32 million gallons of petroleum brine were released daily into Texan tidal waters.

One area where the legacy of petroleum brine discharge is still being heavily felt is Nueces Bay—a shallow microtidal bay in Southwestern Texas. Despite a ban on the release of petroleum brine into U.S. surface waters in 1993, the area remains severely ecologically affected, with species such as oysters still unable to grow over two decades after the last of the effluent was released.

The inability of the bay to fully recover from the effects of historical petroleum brine discharge has puzzled environmentalists for years. Studies fail to elucidate the source of toxicity in the bay, thus minimizing the success of any restoration efforts and proper evaluation of strategies to enhance habitat health. However, new research conducted by a Texas A&M University-Corpus Christi team may potentially shed light on this ecological mystery. Their findings, obtained using high resolution accurate mass spectrometry, suggest an underlying ecological problem that has been in the making for decades.

There’s something in the water
To gain insight into the causes of Nueces Bay’s ecological problems, the Texas A&M-Corpus Christi team decided to probe the chemistry of the water. Here, the team needed an approach that would quickly and accurately identify all of the organic components present in the samples collected.

The group collected water samples at 15 sites along two transects situated in the north and south of the bay (Figure 1). The organic compounds present in the water samples were removed by solid phase extraction.

The team leveraged the analytical speed and mass accuracy of the latest Thermo Scientific Orbitrap technology to comprehensively identify the organic components present in the water samples. They also needed to ensure their approach captured all of the chemical components in the water samples. Using an untargeted, full scan acquisition approach, the team was able to detect between 1,800 and 4,000 unique compounds, depending on sampling location.

One of the most striking findings was the disparity in the distribution of compounds between the two transects. A greater number of unique molecular formulas was detected at sampling points in the northern transect relative to the southern transect. While the number of unique carbon-, hydrogen- and oxygen-containing molecules was similar for both regions, the number of unique phosphorus-containing compounds was significantly higher in the water samples collected at sampling sites along the northern transect (Figure 2). In some cases, more than five times as many organophosphorus compounds were detected.

To further understand these findings, the team used van Krevelen plots to determine the chemical nature and origin of the molecules detected. These diagrams are two-dimensional graphical representations that can reveal the chemical identity of compounds based on the ratio of oxygen atoms to carbon atoms (O/C) and ratio of hydrogen atoms to carbon atoms (H/C).

As well as a high abundance of oxygenated hydrocarbons at sampling stations located close to former discharge sites, consistent with the bay’s history of extensive petroleum brine release, the plots revealed a more surprising result. A large proportion of phosphorus-containing compounds with low H/C ratios were also detected, indicative of aromatic organophosphorus molecules. These compounds tended to fall outside the range for naturally occurring phospholipids and phosphopeptides in the van Krevelen diagram, suggestive of manmade origin.

A toxic combination
Abdulla’s team believes the high abundance of organophosphorus compounds at sampling sites located along the northern transect originates from the agricultural land-use in areas to the north of the bay (see the brown colored open land in Figure 1). Many pesticides and agricultural products have historically been rich in phosphorus-containing compounds. Due to surface and/or subsurface agricultural runoff, these compounds have subsequently found their way into the bay.

Yet many of the phosphorus-rich pesticides identified by the team have now been banned, so what could explain their reappearance? The answer may still lie in the bay’s history of petroleum brine discharge.

In addition to hydrocarbons, petroleum brine is rich in iron—an element with a high affinity for organophosphorus and phosphates. The team believes a large proportion of the pesticide molecules present in the agricultural runoff would have become adsorbed on the bay sediment particles that have become enriched in iron during petroleum brine discharge. Over time, and with sedimentation, these insoluble organophosphorus containing iron deposits would have become buried beneath layers of sediment and hidden away for decades.

The reason for the recent reappearance? It could be due to changes in microbial activity. Bacteria present in the sediment are known to use a number of metabolic pathways to oxidize the organic matter they use to live and reproduce. While oxygen is their preferred oxidizing agent, once supplies run low they typically move onto other oxidants such as ferric iron (Fe3+), which they reduce to ferrous iron (Fe2+). In the oxygen-starved sediment at the bottom of Nueces Bay, the team believes the previously insoluble ferric iron complexes are being converted to the more soluble ferrous form, resulting in the release of the adsorbed organophosphorus molecules into the water.

Trapped by the iron-containing sediment and buried for decades, it appears these now-banned pesticides and their degradation products are being slowly released into the waters of Nueces Bay. The team’s findings suggest that the ecological problems currently observed in the bay are a result of ecological damage decades in the making, which are only now beginning to surface, thanks to the latest advances in full scan high resolution accurate mass instrumentation.

Looking to the future
To more fully understand the extent of the brine contamination problem, additional work is underway over the wider Nueces Bay area.

 “Using liquid chromatography coupled with high resolution accurate mass spectrometry, we plan to comprehensively identify most of the organophosphorus compounds present in the bay’s waters,” explains Abdulla.

The team hopes to determine location-specific concentrations to obtain a more complete picture of the situation. Abdulla and his team also want to understand how resilient the pollutants are to degradation.

“Given the large number of unique phosphorus-containing molecular formulae—between 800 and 1000 in some locations—it’s likely that many of the identified compounds are degradation products rather than original pesticides,” Abdulla says.

As these compounds are exposed to sunlight upon being released into the water from the iron-containing sediment, investigations into the compounds’ photooxidation degradation pathways are underway.

Thanks to the latest advances in high resolution accurate mass spectrometry, Abdulla’s team has cast light on an ecological problem that has been buried beneath Nueces Bay for many decades. It’s hoped the team’s full scan approach may be used to comprehensively profile environmental pollutants present in other surface waters.

Figure 2: The number of unique phosphorus-containing compounds at sampling sites 1 to 15.
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