Arsenic (As) occurs at elevated concentrations in many waters and soils worldwide. Elevated concentrations of As result from both geogenic and anthropogenic sources. Naturally occurring As is ubiquitous in many geothermal areas and in groundwater that is in contact with As-bearing rocks. Human activity has also resulted in innumerable As-contaminated sites. Arsenic is used as a timber preservative in a copper-chromium-arsenic (CCA) cocktail. Consequently, soils and waterways near timber processing plants can be contaminated with As. There is evidence from several studies that CCA leaches out of treated timber into surrounding soil and may possibly leach into groundwater (see sustainable land use). Arsenic is present in many pesticides, herbicides and fertilizers. Historically, extensive use of the horticultural pesticides, lead arsenate (PbAsO4), calcium arsenate (CaAsO4), magnesium arsenate (MgAsO4), zinc arsenate (ZnAsO4), zinc arsenite [Zn(AsO2)2] and Paris Green [Cu(CH3COO)2.3Cu(AsO2)2] in orchards has contributed to soil arsenic contamination The use of sodium arsenite (NaAsO2) to control aquatic weeds has also contributed to the arsenic burden of some lakes.

Some species of aquatic plants, namely Myriophyllum propinquum, Elodea canadensis, Ceratophyllum demersum, Egeria densa, Lagarosiphon major and Rorippa nasturtium-aquaticum (watercress) accumulate As to concentrations in excess of 1000 mg/kg on a dry matter basis, the threshold for hyperaccumulation. By contrast, aside from a few As-hyperaccumulating ferns, terrestrial plants generally take up negligible amounts of this element, even when grown in soils laden with As. Our experiments have indicated that aquatic plants probably accumulate As via physicochemical adsorption, rather than physiological uptake. There is limited translocation of As within the plant.

The accumulation of As and other elements by aquatic plants may facilitate the entry of this toxic element into the food chain. Plants are the primary producers of most food chains. Humans may be affected directly, if plants such as watercress and mint are consumed, or indirectly when humans consume species that have high As levels due to contamination of the food-chain.

In a lake or river system, the amount of plant-bound As at any one time may be a significant portion of the total amount of As in the river. Therefore if, for example, drought or herbicides kill the plants, then there may be a large pulse of As released into the water as the plants decay.

The effect on humans of consuming As-rich watercress is unclear. Any As toxicity from consuming watercress will depend on the amount eaten, the frequency, how the watercress is prepared, what it is consumed with, and the chemical form of As in the plant. Inorganic As is considerably more toxic than As in organic compounds.

Toxic salad. Watercress can accumulate As to over 1000 mg/kg on a dry matter basis. Submerged portions have a higher As concentration than the aerial portions.

Arsenic-related publications

Robinson BH, Kim N, Marchetti M, Moni C, Schroeter L, van den Dijssel C, Milne G, Clothier BE (2006). Arsenic hyperaccumulation by aquatic macrophytes in the Taupo Volcanic Zone, New Zealand. Environmental and Experimental Botany 58(1-3), 206-215.

Mahimairaja S, Bolan NS, Adriano NC, Robinson BH (2005). Arsenic Contamination and its Risk Management in Complex Environmental Settings. Advances in Agronomy 86, 2-64.

Robinson BH, Duwig C, Bolan NS, Kannathasan M, Saravanan A (2003). Uptake of arsenic by New Zealand watercress (Lepidium sativum). The Science of the Total Environment 301(1), 67-73.

Robinson BH, Brooks RR, Outred HA, Kirkman JH (1995). The distribution and fate of arsenic in the Waikato river system, North Island, New Zealand. Chemical Speciation and Bioavailability 7(3), 89 - 96.

Robinson BH, Brooks RR, Outred HA, Kirkman JH (1995). Mercury and arsenic in trout from the Taupo Volcanic Zone and Waikato River, North Island, New Zealand. Chemical Speciation and Bioavailability 7(1), 27 – 32.

Robinson BH, Marchetti M, Moni C, Schroeter L, van den Dijssel C, Milne G, Bolan NS, Mahimairaja S (2006). Arsenic accumulation by aquatic and terrestrial plants: Taupo Volcanic Zone, New Zealand. In: Managing Arsenic in the Environment : From Soil to Human Health (Eds. R Naidu, E Smith, G Owens , P Bhattacharya , P Nadebaum). CSIRO publishing, Adelaide, Australia. pp 235 – 247.

Robinson BH (1994). Pollution of the aquatic biosphere by arsenic and other elements in the Taupo Volcanic Zone. MSc Thesis. Massey University, New Zealand.

Marchetti M, Milne G, Robinson B (2003) Arsenic Hyperaccumulators in the Taupo Volcanic Zone. WISPAS 84, 1-2.