Arsenic is a naturally occurring metalloid element distributed across the Earth’s crust. In its inorganic forms, arsenic is highly toxic, posing a risk to ecological systems and organisms across all trophic levels. Understanding how this element is introduced, changes form, and moves through soil and water is fundamental to grasping its environmental impact. It is readily mobilized by both natural geological processes and human activities, leading to its pervasive distribution in air, soil, and water.
Sources and Initial Environmental Entry
Arsenic enters the environment through both geological phenomena and human industrial activity. Natural sources originate primarily from the weathering and erosion of arsenic-rich minerals and rocks. Volcanic eruptions and geothermal activity release arsenic compounds directly into the atmosphere and water systems. This natural background contamination is responsible for the high concentrations often found in groundwater.
Human activities (anthropogenic sources) increase arsenic mobilization, concentrating it in specific areas. Key contributors include mining and smelting operations, particularly those targeting gold, copper, and lead ores, which release arsenic as a byproduct. Historically, the use of arsenic-based pesticides and herbicides, and chromated copper arsenate (CCA) as a wood preservative, left contamination in soil. Industrial waste disposal and the burning of fossil fuels, such as coal, further contribute to the element’s release into the air and subsequent deposition.
Arsenic Speciation and Environmental Mobility
Once released, arsenic behavior, toxicity, and mobility depend entirely on its chemical form, known as speciation. The two most relevant inorganic forms are arsenate (As(V)) and arsenite (As(III)), which behave differently in soil and water. Arsenite (As(III)) is the more mobile and toxic species. Arsenate (As(V)) is structurally similar to phosphate and binds strongly to solid phases in soil.
The conversion between these forms is governed by redox potential and pH. Under high oxygen conditions (oxidized), As(V) is the dominant, less mobile species, binding tightly to iron and aluminum oxyhydroxides in soil. Conversely, in waterlogged or deep groundwater (reducing conditions), As(V) is reduced to the more soluble and mobile As(III). This reduction can dissolve the iron oxyhydroxides holding As(V), leading to a significant increase in dissolved arsenic concentration.
Soil pH also plays a role in mobility; alkaline conditions cause arsenic to desorb from soil particles, releasing it into the surrounding water. Furthermore, phosphate, a common agricultural nutrient, competes with arsenate for binding sites on soil minerals. This competition effectively displaces arsenic and increases its concentration in the water phase, determining its leaching into groundwater.
Direct Impact on Terrestrial and Aquatic Life
Arsenic contamination damages organisms in both terrestrial and aquatic ecosystems through distinct toxicological mechanisms. In plants, arsenate (As(V)) is mistakenly absorbed through the same transport channels used for phosphate. Inside the plant cell, As(V) disrupts metabolism by competing with phosphate in phosphorylation reactions, which reduces the production of cellular energy (ATP). Arsenite (As(III)) binds to sulfhydryl groups within proteins, inactivating vital enzymes.
Exposure results in phytotoxicity, manifesting as stunted growth, reduced root development, and lower crop yields. Specialized plants, such as the fern Pteris vittata, are known as hyperaccumulators because they absorb and tolerate high concentrations of arsenic, often storing it in their fronds. In the soil, arsenic disrupts microbial communities, which are essential for nutrient cycling.
In aquatic life, arsenic bioaccumulates in organs like the gills, liver, and kidney, leading to biochemical disorders, tissue damage, and cell death. Fish are more susceptible to the toxic effects of As(III) than As(V). Chronic exposure, even at low concentrations, severely impairs reproductive success in fish, reducing fecundity and fertilization rates by disrupting the hypothalamus-pituitary-gonadal (HPG) axis.
Bioaccumulation and Transfer Through the Food Web
Arsenic is subject to bioaccumulation, the process where an organism absorbs a substance faster than it can eliminate it, causing the element to build up in its tissues over time. This process begins at the base of the food web, with aquatic plants, algae, and invertebrates absorbing arsenic from the environment. The concentration then transfers to organisms at higher trophic levels when they consume contaminated prey.
Arsenic generally does not undergo true biomagnification in the food web, unlike some other persistent pollutants. Biomagnification is the process where contaminant concentration increases at each successive trophic level. Aquatic organisms often possess detoxification mechanisms that convert highly toxic inorganic arsenic into less toxic organic forms, such as arsenobetaine, which are then rapidly excreted.
As a result, arsenic concentrations are often highest in primary producers and lower-level consumers, and may even decrease in apex predators. The primary threat to humans and higher-level consumers is direct exposure from consuming contaminated water or eating crops like rice. Rice readily absorbs and retains the more toxic inorganic forms from contaminated soil and irrigation water. The concentration and form of arsenic transferred is a matter of biotransformation efficiency rather than simple magnification.