Cadmium in soils and groundwater: A review

Abstract

Cadmium (Cd) is a non-essential trace element that is widely distributed in the environment. Both geogenic and anthropogenic sources can elevate Cd concentrations in soils and groundwater, which are important for maintaining healthy supplies of food and safe drinking water. Elevated Cd doses are carcinogenic to humans. The WHO Guidelines for Drinking-Water Quality recommend a guideline value for Cd of 3 μg/L. Important anthropogenic Cd sources include mining, atmospheric deposition of combustion emissions, and the use of Cd-containing fertilizers. We document several cases of Cd pollution in soil and groundwater based on worldwide accounts. Besides anthropogenic Cd sources, Cd is also incorporated into sulfides, carbonates, and phosphorites resulting in elevated Cd concentrations in associated rock types. The crustal median Cd content is 0.2 mg/kg. In soils, Cd occurs at concentrations of 0.01 to 1 mg/kg with a worldwide mean of 0.36 mg/kg. Weathering can lead to Cd concentrations up to 5 μg/L in soil water and up to 1 μg/L in groundwater. In aqueous solutions, Cd generally occurs as the divalent Cd²⁺ and it is mobilized mainly in oxic, acidic conditions. Cadmium sorption is enhanced by the presence of high amounts of hydrous oxides, clay minerals, and organic matter, and its mobility is further influenced by pH, the redox state, and ionic strength of the solution. However, Cd can remain in solution as water-soluble complexes with anions, such as CdCl⁺ and Cd(SO₄)₂ ^2-, and dissolved organic matter while sorption and precipitation decrease the aqueous concentration of most other heavy metals. As a consequence, Cd is one of the most mobile heavy metals in the environment. The elevated mobilization potential, e.g., through competition and ligand induced desorption, is the reason for faster Cd release from soil into groundwater than other heavy metals. The goal of this study was to present a broad overview of the origin and concentration of Cd in groundwater, and its reaction pathways in aquatic environments. To gain an overview of the hydrochemical behavior of Cd, cases of Cd pollution in soil and groundwater, studies investigating Cd release, and information about the legal framework were compiled.

Keywords: Cadmium; Contaminant; Geogenic; Groundwater; Heavy metals; Natural; Soil; Toxic.

Fig. 1.

Cadmium concentrations in groundwater related to stratigraphic and petrographic aquifer features (BGR, 2014). Bars represent 50th percentile (lower edge) and 90th percentile (upper edge) of Cd in groundwater. Red bars show limestone dominated aquifers, blue bars show miscellaneous aquifer material. Aquifer's order indicate median pH in groundwater from acidic (left) to alkaline (right). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2.

Cadmium contents in raw phosphates by origin (Dittrich and Klose, 2008; Kharikov and Smetana, 2000; Mar and Okazaki, 2012; Oosterhuis et al., 2000; Roberts, 2014).

Fig. 3.

Eh vs. pH diagram for Cd. The concentrations of dissolved species are [Cd²⁺] = 10⁻⁸, [HCO₃⁻] = 10⁻³, [SO₄²⁻] = 10⁻³.

Fig. 4.

Concentration versus pH diagrams showing a) Cd speciation from pH 4 to 7 for the composition Cd (10^−6.4 M), Cl (10⁻³ M), ΣCO₂ (10⁻³ M), and SO₄ (10⁻³ M). b) Otavite solubility as a function of pH and Σ CO₂ from 10⁻³ to 10⁻² M. C) Concentration versus dissolved sulfide diagram showing the solubility of CdS at pH 6. Reference concentrations for drinking water standards are shown (3 to 5 μg/L).

Fig. 5.

Cadmium sources and influences on Cd release into groundwater.