Arsenic in freshwater and marine water
Toxicant default guideline values for protecting aquatic ecosystems
Extracted from Section 8.3.7 ‘Detailed descriptions of chemicals’ of the ANZECC & ARMCANZ (2000) guidelines.
The default guideline values (previously known as ‘trigger values’) and associated information in this technical brief should be used in accordance with the detailed guidance provided in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality.
Description of chemical
Arsenic is released into the environment naturally by weathering of arsenic-containing rocks and volcanic activity. The estimated amount of arsenic released as a result of human activities is about twice that from weathering (Ferguson & Gavis 1972). Several forms of arsenic occur in natural waters, depending upon the redox potential and pH, the two most common being arsenic (III) and arsenic (V). Both arsenic (III) and arsenic (V) form stable bonds with carbon, resulting in numerous organo-arsenic compounds, some of which are very toxic (e.g. methylarsine). The current analytical practical quantitation limit (PQL) for total arsenic is 0.03 µg/L in both fresh and marine water (NSW EPA 2000).
Summary of factors affecting arsenic toxicity
Valency state is the major factor affecting arsenic toxicity.
As (III) is the more toxic form but is less common in seawater.
Arsenic can bioaccumulate to some extent in marine organisms but secondary poisoning is unlikely. Formation of organo-arsenical compounds complicates assessment of bioaccumulation.
As (V) toxicity is not affected by salinity but increases with increasing temperature.
As (III) is removed by sulfides and As (V) by clays. Iron (III), chromium (III) and barium also reduce arsenic toxicity.
A variety of analytical methods are available for determining the speciation of arsenic in water. These include anodic stripping voltammetry, selective hydride generation, chromatography and ion exchange [see reviews by Mach et al. (1996) and Burguera & Burguera (1997)]. Geochemical speciation modelling is of limited use as arsenic (V) and arsenic (III) are rarely in true thermodynamic equilibrium in surface waters because of biologically-mediated reduction reactions and the slow kinetics of arsenic (III) oxidation. In addition, modelling cannot predict the concentration of methylated arsenic species formed by biological activity.
Bioassays are typically used to ascertain metal-organism interactions. These can be coupled with the measured speciation of As to determine the bioavailability of various arsenic species.
Phytoplankton are among the most sensitive organisms to both forms of arsenic. The Australian diatom Nitzschia closterium is highly sensitive to arsenic (III), with a 72-h EC50 for growth inhibition of 7 µg/L (Florence & Stauber 1991), compared to >2000 µg/L for arsenic (V). The toxicity of arsenobetaine and dimethylarsenic to this species was found to be intermediate between the two inorganic arsenic forms, at >1000 and >500 µg/L, respectively. For the freshwater alga Scenedesmus obliquus, the 96-hour EC50 values for arsenite, arsenate, monomethylarsenate and dimethylarsenate were 0.08, 0.16, 12.4 and 35.7 mg/L, respectively. Methylation of arsenic to non-toxic forms is a common detoxification mechanism in algae. Higher trophic levels are less sensitive to arsenic because they generally accumulate the element from food rather than the water column. Although these organisms are typically more sensitive to arsenic (III) than arsenic (V), the difference in sensitivity is not as marked as for phytoplankton. Marine fish and invertebrates accumulate organoarsenical compounds from their food and they contain tissue residues from 1 to 100 mg/kg dry weight (Neff 1997). Arsenobetaine, the most common organoarsenical compound in seafood has low toxicity to mammals so arsenic is not considered to have a high risk for secondary poisoning (Neff 1997).
Acute toxicity of arsenic (III) to freshwater invertebrates occurred at concentrations as low as 812 µg/L (Sanders & Cope 1968, Call et al. 1983, Lima et al. 1984). The lowest concentration of arsenic (V) causing acute toxicity was 850 µg/L for a cladoceran Bosmina longirostris (Passino & Novak 1984). Adult freshwater fish are generally less sensitive to arsenic. Concentrations of arsenic (III) causing an acute toxic response in fish ranged upwards from 13,300 µg/L (CCREM 1987). The lowest acute toxic concentration of arsenic (V) for freshwater fish (rainbow trout) was 10 800 µg/L (Hale 1977). The range of LC50 values for As (III) reported by Vaughan (1996) was 910 to 24,700 µg/L for crustaceans, 1500 to 7400 µg/L for annelids, 3000 to 7500 µg/L for molluscs and 3400 to 83,000 µg/L for fish.
The chronic toxicities of arsenic (III) and arsenic (V) to freshwater organisms are detailed below. Arsenic (V) seems to be more toxic to plants than arsenic (III).
USEPA (1986) reported that acute toxicity values of arsenic (III) to 12 species of marine organisms ranged between 232 and 16,030 µg/L. A single acute to chronic ratio was 1.95. The range of marine acute LC50 values for arsenic (V) was 230 to 9600 µg/L for crustaceans and 330 to 800,000 µg/L for molluscs (Vaughan 1996). In general, early life stages were more sensitive to arsenic than adults.
Salinity had no effect on the toxicity of As (V) to the clam Macoma balthica, whereas toxicity increased with increasing temperature from 5 to 10°C (Bryant et al. 1985c).
Freshwater guideline—As (III)
Screened chronic toxicity data for arsenic (III) comprised 7 taxonomic groups, to give the following figures (pH range 6.90 to 8.03):
Fish: seven species, chronic LC50 between 540 and 67,300 µg/L, converting to NOEC range of 108 to 13,460 µg/L plus a measured NOEC figure of 961 µg/L.
Amphibian: Ambystoma opacum, chronic LC50 of 4450 µg/L to give NOEC of 890 µg/L.
Crustaceans: two species, NOEC of 88 to 961 µg/L (the geometric mean was 290 µg/L for the more sensitive Gammarus sp.).
Insect: one species, Pteronarcys dorsata, NOEC of 961 µg/L.
Mollusc: two species, NOEC of 961 µg/L.
Macrophyte: two species EC50 (growth) of 3600 to 4100 to give NOEC of 720 to 820 µg/L.
Algae: two species, EC50 (population growth) of 79 to 31,200 µg/L, to give NOEC of 16 to 6240 µg/L.
A high reliability freshwater trigger value of 24 µg/L was derived for arsenic (III) using the statistical distribution method with 95% protection.
Marine guideline—As (III)
It was not possible to assess marine data from USEPA (1986) and Vaughan (1996) for the current revision. The USEPA (1986) noted that acute toxicity for As (III) to marine animals ranged between 232 and 16,030 µg/L. The ranges that Vaughan (1996) reported are also consistent.
An Environmental Concern Level (ECL, see Section 22.214.171.124 of the ANZECC & ARMCANZ (2000) guidelines) of 2.3 µg/L was derived for As (III) in marine waters, using an AF of 100. This figure could be adopted as a marine low reliability trigger value, to be used only as an indicative interim working level. Further review at a later revision may produce a more reliable trigger value.
Freshwater guideline—As (V)
Data were available for arsenic (V) for 5 taxonomic groups:
Fish: one species, 28-day NOEC, Oncorhynchus mykiss, 973 µg/L.
Crustaceans: two species, a NOEC range of 932 to 973 µg/L, and EC/LC50 of 1400 to 2850 µg/L; the overall range of corrected NOECs was 280 to 973 µg/L.
Insects: one species, Pteronarcys dorsata, 28-day NOEC, 973 µg/L.
Molluscs: two species, 28-day NOEC, 973 µg/L.
Algae: five species, mixture of NOECs, LOECs and LC50s; lowest measured NOEC was 48 mg/L but the corrected range was 32 to 30,760 µg/L.
A high reliability freshwater trigger value of 13 µg/L was calculated for As (V) using the statistical distribution method with 95% protection.
This figure is above the chronic NOEC for one of the more sensitive algal species but is considered sufficiently protective for slightly-moderately disturbed ecosystems.
Marine guideline—As (V)
Chronic data were only available for two taxonomic groups, as follows: The pH range was 6.7 to 8.2 but pH data were only available for 2 of the 10 data points:
Crustaceans: three species, 8 to 51-day mortality and reproduction LC50 and MATC, 893 to 70,000 µg/L.
Algae: two species, 6 to 9-day NOEC growth, 1000 to 10,000 µg/L.
There were insufficient data to derive a reliable marine trigger value. A low reliability marine guideline trigger value of 4.5 µg/L for As (V) was derived using an AF of 200 on the lowest NOEC (200 was used because the limited data were chronic). This should be used only as an indicative interim working level.
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