API Publ 4701:2000 pdf download

API Publ 4701:2000 pdf download.BIOACCUMUL ATION: AN EVALUATION OF FEDERAL AND STATE REGUI ATORY
INITIATIVES.
Arsenic cycles readily among valence states. The form present in water will depend on several factors (API, 1998). High dissolved oxygen, pH, Eh (a measure of redox potential), and low organic material favor the formation of arsenate, the most common form of arsenic in water. Arsenite and arsenide formation are favored by the reverse of these conditions (API, 1998, Eisler, 1988). In addition, the type and degree of biological activity will also affect the form of arsenic present in the environment and biota. For instance, some anaerobic bacteria, found in soil, sediments and digestive tracts, reduce arsenate to arsenite (Cullen and Reimer, 1989).
Arsenic is primarily introduced into the aquatic food web through uptake of arsenate by phytoplankton. These primaiy producers can metabolize arsenate into a wide variety of hydrophoblc and water soluble derivatives. Commonly, arsenate is reduced to arsenite and subsequently methylated. primarily to methylarsonic acid arid dimethylarsinic acid (Phillips. 1990). This process is generally considered a detoxification mechanism. Methylated arsenic can be excreted, reducing toxicity within the organism. The excreted methylated arsenic can cycle back to arsenate in deep waters, most likely through bacterial demethylation (Phillips, 1990).
The most common water-soluble form of arsenic in higher marine organisms is arsenobetaine. Conversion of arsenic to arsenobetaine is also a detoxification mechanism (Phillips, 1990). Under normal conditions, the primary source of arsenic to humans is from seafood as arsenobetaine. While it is readily absorbed in the digestive tract, arsenobetaine is generally excreted without transformation and therefore, poses little toxic hazard (Phillips, 1990; Neff, 1997). Little research has been conducted to determine whether arsenic is present in freshwater higher organisms in a detoxified form; however, betaine is expected to be more prevalent in marine organisms because it is used for osmoregulation.
Inorganic arsenic in mammals, including humans, is metabolized and then excreted. Because of this, chronic toxicity due to low concentrations of arsenic is uncommon. Larger doses can overwhelm the excretion mechanism and cause acute or subacute toxicity. In addition, inorganic arsenic is capable of crossing the placental barrier of many mammals, including humans, and can produce death o detects in oftspnng (Eisler,
1988).
Bioconcentration factors compiled by USEPA (1 985a) for freshwater organisms are quite low for both inorganic and organic forms of arsenic, ranging from zero to 17 (API, 1998). Few studies provide marine BCFs (USEPA, 1985a); however, because arsenic is pesent in higher marine organisms in a nontoxic form (i.e., arsenobetaine), it is of less concern in the marine environment. A recent review of metal bioaccumulation by aquatic macro-invertebrates identified arsenic bioaccumulation from sediments as a data gap in the scientific literature, as only a small number of studies have addressed this topic (Goodyear and McNeill, 1999). Additional information on arsenic toxicity and bioaccumulation can be found in API (1998) Publication Number 4676: Arsenic:
Chemistry, Fate, Toxicity, and Waslewater Treatment Options.
3.2 Mercury
Mercury is found in the environment as elemental mercury vapor (Hg°), inorganic mercury salts (including Hg” and Hg”2), and organic mercury (mostly as mono- or dimeltiylmercury). Much of the mercury in the aquatic environment is from atmospheric deposition and enters the aquatic system as Hg2 (Jonnalagadda and Rao, 1993: Westcott and Kalif, 1996). Methylation of mercury occurs primarily through the action of sulfate-reducing bacteria, although other mechanisms of methylation also exist (Gilmour and Henry, 1991).
Methylmercury is much more bioaccumulative than inorganic mercury. Methylmercury bioaccumulates quickly because it becomes protein bound and cannot be efficiently eliminated. It is biomagnitied up the food chain, potentially resulting in concentrations in predatory fish that are thousands to millions of times greater than in the surrounding water (e.g.. Bloom, 1992: Jonnalagadda and Rao, 1993). Under normal exposure conditions, human exposure to methylmercury occurs almost exclusively through fish and shellfish ingestion. Because of its biomagnification potential. water quality criteria for mercury are generally calculated using bioaccumulabon factors to protect human and wildlife consumers of fish, rather than aquatic organisms (which are affected by mercury toxicity only at higher water concentrations).