Ecological Significance of Benzene

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With the exception of accidental spillage of petroleum products, the routine levels of environmental benzene exposure are not generally associated with risk to fish and wildlife. Reasons for a reduced concern of the environmental risk of benzene include: (1) the lack of evidence for bioaccumulation of benzene, (2) the low potential for persistence due to its high volatility from surface waters and soil, and (3) the rapid photooxidation of airborne benzene and its biodegradation in soil and water. Studies have shown that high levels of benzene are toxic to terrestrial and aquatic life under controlled conditions.

The levels of benzene in unpolluted air and surface waters are often below the current analytical detection limits. Although benzene does occur naturally, its primary source of production is known to be petroleum products (and the exhaust from their combustion). The median benzene concentration of ambient air samples from urban areas in the US from 1984 to 1986 was 2.1 ppb (detection limit 0.007 ppb), as determined by the USEPA. Drinking water in the US typically contains less than 0.1 ppb, yet some concern is justified for exposure from consumption of contaminated water drawn from wells near landfills, gasoline storage tanks, and industrial areas. As discussed previously, benzene is rapidly disseminated and degraded in the environment. As a result, environmental benzene toxicity is generally associated with exposure from some concentrated source, such as a leaking storage tank or a petrochemical spill. In order to determine the magnitude of benzene exposures which would be tolerated by aquatic and terrestrial life, controlled laboratory experiments have been conducted with several species exposed for short durations to increasing concentrations of benzene. Oak Ridge National Laboratory (ORNL) has also established no observable adverse effect levels (NOAELs) for several

Table 1 Benzene levels considered to be safe to terrestrial wildlife (ORNL, 1994)

NOAEL

Benchmark (water)

Species

(mg/kg/day)

ppm

Short-tailed

33.135

150.613

shrew

Little brown bat

41.65

260.318

White-footed

29.201

97.336

mouse

Meadow vole

23.23

170.355

Cottontail rabbit

7.803

80.722

Mink

8.287

83.708

Red fox

5.045

59.741

Whitetail deer

2.189

33.426

terrestrial mammals, as well as benchmark water levels which should be expected to be generally regarded as safe (Table 1). Feeding experiments determined NOAELs that ranged from 2.2 mg/kg/day for whitetail deer up to 33.1 mg/kg/day for the short-tailed shrew. It appears, therefore, that larger animals may be less tolerant to the chronic toxic effects of benzene. A similar effect is seen with benzene in drinking water, benchmark water concentrations (generally regarded as safe) which ranged from c. 30 ppm for deer, up to 150 and 260 ppm for shrews and bats, respectively.

Effects ofwaterborne benzene on aquatic wildlife have also been studied, but concentrations believed to be safe are somewhat lower. This is probably due to the ability of benzene to cross respiratory membranes and enter the circulatory system of aquatic animals. A similar situation exists for airborne toxicity in mammals. Benchmark work at ORNL has also been used to determine field concentrations which are unlikely to represent an ecological risk, such that water concentrations below these levels for each species present at a given site would generally be considered safe. The ORNL benchmark for sediment concentration unlikely to present an ecological risk is 0.052 ppm (dry weight) at 1% organic carbon. ORNL estimates that chronic exposure of most freshwater fish to 8.25 ppm is tolerated with no minimal adverse effects, while Daphnia species can tolerate c. 98 ppm under chronic exposure conditions. Low bioconcentration factors have been reported for aquatic organisms such as fish, algae, plants, bacteria, and macroplankton. In fact, there is no evidence available to support biomagnification of benzene in aquatic ecosystems. This may be a result of its rapid metabolic transformation in most species, as well as its rapid volatilization from aquatic habitats. The most sensitive aquatic animal identified by short-term continuous exposure tests is the leopard frog, with an LD50 of 3.7 ppm benzene during its 9-day embryo-larval stages (Table 2). Rainbow trout and coho salmon represent two of the more sensitive fish species, with 96-h LD50s ranging from 5 to10 ppm benzene.

The growth of freshwater algae (Selenastrum capricor-nutum) was reduced by 50% following an 8-day exposure to 41 ppm benzene.

Table 2 Acute benzene LD50 values (ppm) for several representative aquatic species after continuous exposure for up to 4 days (Toxic Substances Data Base, 2006)

24 h

48 h

96 h

Grass shrimp

(Palaemonetes pugio) Crab larvae, stage 1 (Cancer magister) Shrimp

(Cragon franciscorum) Brine shrimp

(Artemia) Pacific herring

(Clupea harengus pallasi) Bluegill

(Lepomis macrochirus) Coho salmon

(Onchorhynchus kisutch) Rainbow trout

(Onchorhynchus mykiss) Fathead minnow

(Pimephales promelas) Mexican axoltl salamander (Ambystoma mexicanum) Leopard frog

(Rana pipiens) Clawed toad (Xenopus laevis)

100 542

27 1108 20

40-45

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