Technical Abstract: Arsenic (As) is ubiquitous in nature and is commonly found in earth materials, water, atmosphere, plants, animals and other environmental samples. Arsenic in agroecosystems is of concern because high concentrations of As in surface soils may be biotoxic to microbes, plants and animals. High arsenic concentrations in ground waters exceeding the current drinking water MCL occur in the western United States. Presence of high levels of As in certain hydrogeochemical settings may be partly due to mobilization and evapoconcentration from agricultural irrigation practices. Arsenic has four main valence states, +5, +3, 0, and -3. Inorganic As (V) and As (III) are the major forms in soils and waters. As (III) is more toxic and soluble than As (V). Organic arsenic may only become significant in water bodies where the microbial population is very high. The processes regulating speciation and reactivity of arsenic in soils and waters include redox chemistry, sorption and exchange, biomethylation and volatilization, microbial oxidation and reduction, effects of competing ions for sorption and precipitation, and solute and particulate transport. Source control and remediation for As removal and immobilization are based on reducing its mobility. This can be realized by oxidation to As (V), adsorption on Fe hydroxides, precipitation and coprecipitation with Ca and ferric Fe. Bioremediation measures for As may be carried out through methylation and volatilization. However, arsines are extremely toxic and should be trapped and immobilized.