Metals occur naturally in soils primarily as amorphous oxides and hydroxides, and to a lesser extent carbonates, phosphates, sulfates, and sulfides, which are relatively insoluble. The same is generally true for metal contaminated soils, because metals quickly undergo precipitation and coprecipitation reactions forming relatively insoluble solid phases, and/or are strongly complexed by soil minerals or organic matter. Thus, many elements are not very bioavailable or toxic to animals even when element-rich soils are ingested. Toxicity testing on the other hand usually employs very soluble metals not commonly found in any appreciable amounts in soils relative to total metal concentrations. Most of the 23 contaminants considered for Eco-SSLs are metals that typically exist as cationic species (aluminum, antimony, barium, beryllium, cadmium, cobalt, copper, iron, lead, manganese, nickel, silver and zinc). These metals can complex with inorganic soil constituents, e.g., carbonates, sulfates, hydroxides, sulfides, to form either precipitates or positively charged complexes. Both complexation and precipitation reactions are pH dependant. Therefore, although these metals can form complexes with a net negative charge, under most environmentally relevant scenarios (pH = 4 to 8.5), these metals either precipitate or exist as cations.

Arsenic, chromium, selenium, and vanadium complex with oxygen and typically exist as anionic species under most environmentally relevant scenarios. The most common forms of arsenic are arsenate (arsenic V) and arsenite (arsenic III), which are present in soil solution in the form of ASO43- and ASO2-, respectively. The chemistry of arsenic resembles that of phosphate. Chromium can exist as chromate (chromium VI or CrO42-), which is usually considered more soluble, mobile and bioavailable than the sparingly soluble chromite (Cr(III)), which is normally present in soil as the precipitate Cr(OH)3. Similarly, selenium can be present as selenates (SeO42-) and selenites (SeO32-). For vanadium, vanadate (VO43-) is the most common form. Metals in their various forms can exist in the pore-water as charged species, as soluble complexes, or precipitate out of solution. Retention by soil is usually electrostatic with cationic species and anionic species being associated with negatively and positively charged sites on the soil, respectively. For most soils in the United States, negatively charged sites are more plentiful with less than 5% of the total available charge on the soil surface being positively charged. Therefore, metals existing as cationic species have a greater propensity to associate with the soil and are less bioavailable, whereas, distribution of anionic metals is generally more towards the pore-water for most soil/water systems. The soil pH and availability of charged sites on soil surfaces are the primary soil factors controlling their release to the pore-water, and subsequently, bioavailability.

For additional information, see the Ecological Soil Screening Level site.