ARSENIC SPECIATION IN BLUE CRAB (CALLINECTES SAPIDUS) AND SEDIMENT FROM A LOUISIANA FRESHWATER MARSH OILFIELD
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Arsenic is ubiquitous in the environment occurring from both natural and anthropogenic sources. The major route of exposure to arsenic is via ingestion of tainted water and food—primarily rice and seafood. This is especially true for U.S. Gulf Coast populations where the seafood consumption rate is higher than the rest of the nation. Oilfields in the U.S. Gulf of Mexico Basin are some of the top producers of hydrocarbons in the world. Produced water is a byproduct of the oil and gas brought to the surface; the U.S. Environmental Protection Agency (USEPA) and U.S. Geological Survey (USGS) have estimated that in the U.S. for every one barrel of oil, ten barrels of water are produced. Historically produced water was discharged directly into marshes and unlined pits, contaminating surface sediments and waters with arsenic and other contaminants. Since arsenic toxicity is highly dependent upon species and physicochemical properties, there is a need to accurately determine the arsenic species present in a freshwater marsh oilfield ecosystem in a subsistence fishing community. The inorganic arsenites (As (III)) and arsenates (As (V)) are known to be highly toxic and carcinogenic. As (V) dominates in surface waters and sediments, while two organic forms arsenobetaine (AsB) and arsenocholine (AsC)—regarded as non–toxic—are the species primarily found in seafood. Previous research has shown that monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) are metabolites of the inorganic arsenicals. It has been a long–held belief that this transformation was a mechanism of detoxification. However, MMA and DMA are classified by the World Health Organization’s (WHO) International Agency for Research on Cancer (IARC) as Group 2B possible human carcinogens and they likely form toxic intermediates. It is therefore important for human and ecological health risk assessments to determine how arsenicals behave and metabolize in an aquatic food chain. Arsenic bioaccumulation is dependent upon the organism, its trophic level, and ecosystem, (i.e., marine, freshwater, estuarine). The goal of this study was to determine the arsenic species present in the sediment and blue crab (Callinectes sapidus) from an oilfield situated in a freshwater marsh located in south central Louisiana and how metabolic processes impact arsenic speciation and the potential transfer of arsenic within the food chain. Sediment samples and blue crabs were collected from 13 locations across the East White Lake Oilfield, including three locations on the easternmost edge of the lake. Composited sediment samples and three crabs from six locations—three lake and three oilfield—were analyzed. The arsenic species As (III), As (V), MMA, and DMA in the sediment and three blue crab tissue types (meat, shell, and hepatopancreas) were determined by ion chromatography coupled with inductively coupled plasma mass spectrometry (IC/ICP–MS). Complex chemical reactions and interactions determine the mobility of arsenic in sediment, water, and biological systems. As (V) was the most stable and predominant species found in the sediment (91.3%). There was no MMA detected in the sediment, while 5.5% was DMA and 3.1% was in the As (III) form. All four species were found in the crabs in the following order of predominance: 88.3% DMA, 6.7% As (V), 4.7% As (III), and 0.2% MMA. While arsenic was found in all three tissue types, the highest levels were found in the hepatopancreas. The distribution of arsenic species in the sediment and crabs in the East White Lake oil and gas field illustrates the complexity of the arsenic cycle in this freshwater marsh ecosystem. It also illustrates the importance of speciating arsenic in multiple tissue types of the blue crab since local residents are potentially ingesting arsenic from the entire organism, not just the edible portions.