Arsenic :: all ecosystems

Key Concerns:

Concentrations of arsenic in soil reported to be toxic to plants span a broad range (WHO 2001), and toxicity is generally higher in sandy soils than clay soils (Sheppard 1992).

Additional Comments:

The toxicity of arsenic is affected by both biological and abiotic factors including temperature, pH, redox potential, organic matter content, phosphate concentration, adsorption to solid matrices and the presence of other substances and toxicants such as iron, aluminium, calcium and phosphate. In general, arsenate is less toxic to plants than arsenite, as arsenite can penetrate the plant cuticle to a greater degree than arsenate (WHO 2001).

Arsenic is continually cycled through all environmental compartments. Arsenic can be elevated to high levels in water and soil because of the underlying geology or geothermal activity. The highest concentrations of arsenic in soil tend to be associated with mining waste (WHO 2001).

Ecosystem specific information

Arable habitats – EC50 values for the emergence of lettuce (Lactuca sativa) seedlings exposed to either arsenite or arsenate, were 6.15 and 26.1 mg/kg respectively. Corresponding EC50 values for inhibition of root elongation in lettuce exposed via nutrient solution were 0.6 and 2.3 mg/l (Vaughan and Greenslade 1998). Reductions in growth of various arable plants including barley (Hordeum vulgare), ryegrass (Lolium perenne) and blueberry (Vaccinium angustifolium) have been reported at arsenic (arsenite or arsenate) concentrations !Y 50 mg/kg (Anastasia and Kender 1973; Jiang and Singh 1994).

Bogs, wetland and heath -  many of the studies on the effects of arsenic on plants were carried out using nutrient solutions. Reduced growth of perennial marsh plants (Spartina patens and S. alterniflora) was reported following a 30 day exposure to a nutrient solution containing an arsenic concentration of 30 mg/l (Carbonell-Barrachina et al. 1998). Exposure to a nutrient solution containing 0.2 mg/l arsenic did not affect the growth of marsh grass Spartina alterniflora. (Carbonell et al. 1998).

Coastal and rocky habitats - the effects (assuming no mitigation of toxicity in the environment) likely to be found in saltwater at various arsenic (includes arsenite and arsenate) concentration ranges are as follows (WHO 2001):

• mg/l: onset of sublethal effects in the most sensitive species of algae
• mg/l: severe sublethal effects in some species of algae, and lethal effects in amphibians; onset of sublethal effects in invertebrates
• mg/l: onset of acute lethal effects in invertebrates
• 1-10 mg/l: onset of acute lethal effects in fish; population and community effects in invertebrates
• 10-100 mg/l: Severe reduction in biodiversity across taxa
• 100-1000 mg/l: Survival of only very insensitive and adapted species; very severe reduction in biodiversity

72h-EC50 values for growth of the marine diatom (Nitzschia closterium) ranged from 0.007 to >2 mg/l, for exposures to arsenite and arsenate, respectively (Florence et al. 1994). 96h- EC50 values ranging from of 0.011 to 0.028 mg/l were reported for the copepod (Tigriopus brevicornis) (Forget et al. 1988). Other acute EC50 values for immobilization of saltwater invertebrates range from 0.23 to >30 mg/l (WHO 2001). The 96h-LC50 for striped bass (Morone saxatilis) was 10.3 mg/l (Dwyer et al. 1992). Other 96h-LC50 values for freshwater fish range from 21.4 to 157 mg/l (WHO 2001).

Freshwater –  the effects (assuming no mitigation of toxicity in the environment) likely to be found in freshwater at various arsenic (includes arsenite and arsenate) concentration ranges are as follows (WHO 2001):

• 0.001-0.01 mg/l: onset of sublethal effects in the most sensitive species of algae
• 0.01-0.1 mg/l: severe sublethal effects in some species of algae, and lethal effects in amphibians; onset of sublethal effects in invertebrates
• 0.1-1 mg/l: onset of acute lethal effects in invertebrates and chronic lethal effects in fish
• 1-10 mg/l: onset of acute lethal effects in fish; population and community effects in invertebrates
• 10-100 mg/l: Severe reduction in biodiversity across taxa
• 100-1000 mg/l: Survival of only very insensitive and adapted species; very severe reduction in biodiversity

Phytoplankton are the aquatic organisms which demonstrate the greatest sensitivity to arsenic compounds. A 14-day EC₅₀ of 0.048 mg/l has been reported for growth of freshwater algae, with a corresponding LOEC of 0.005 mg/l (Vocke et al. 1980). Other EC₅₀ values for growth of freshwater algae range from 0.26 to 3.12 mg/l (WHO 2001). An EC₅₀ of 0.78 mg/l was reported for an amphipod (Gammarus pseudolimnaeus) (Lima et al. 1984). Other acute EC₅₀ values for immobilization of freshwater invertebrates range from 1.5 to 127.4 mg/l (WHO 2001). 96h-LC₅₀ values for early life stages of Arctic grayling (Thymallus arcticus) exposed to arsenic pentoxide range from 4.8 to 5.5 mg/l (Buhl and Hamilton 1990). Other 96h-LC₅₀ values for freshwater fish range from 13.3 to 91 mg/l (WHO 2001).

Grasslands – Weaver at al (1984) reported that 90 mg arsenite / kg soil prevented growth of Bermuda grass (Cynodon dactylon) in silt loam and fine sandy soil, with a significant reduction in growth on clay soil. No effects on growth were reported at 10 mg/kg for silt loam and sandy soil, or at 45 mg/kg in clay soil. Reductions in ryegrass (Lolium perenne) growth was reported at arsenic (arsenite or arsenate) concentrations > or = 50 mg/kg (Jiang and Singh 1994). Some species of grass, including Holcus lanatus, Deschampsia cespitosa and Agrostis capillaries have developed arsenate-tolerant strains via an alteration in the uptake of phosphate and arsenate (Macnair and Cumbes 1987; Meharg and Macnair 1991a; Meharg and Macnair 1991b).

Woodland and hedgerow -  the uptake of atmospheric arsenic by trees is dependant upon many factors such as the species, age and health of the tree, transport by soil fauna, local changes in groundwater (Martin et al. 2000). Concentrations of arsenic in the wood of pear, plum and cheery trees were correlated with the concentration in soil. The concentrations within different parts of the wood varied, with highest concentrations reported in the heartwood and immediately under the bark.