Soil contamination with copper (Cu) is often associated with zinc (Zn), and the biological response to such mixed contamination is complex. Here, we investigated Cu and Zn mixture toxicity to Hordeum vulgare in three different soils, the premise being that the observed interactions are mainly due to effects on bioavailability. The toxic effect of Cu and Zn mixtures on seedling root elongation was more than additive (i.e., synergism) in soils with high and medium cation-exchange capacity (CEC) but less than additive (antagonism) in a low-CEC soil. This was found when we expressed the dose as the conventional total soil concentration. In contrast, antagonism was found in all soils when we expressed the dose as free-ion activities in soil solution, indicating that there is metal-ion competition for binding to the plant roots. Neither a concentration addition nor an independent action model explained mixture effects, irrespective of the dose expressions. In contrast, a multimetal BLM model and a WHAM-Ftox model successfully explained the mixture effects across all soils and showed that bioavailability factors mainly explain the interactions in soils. The WHAM-Ftox model is a promising tool for the risk assessment of mixed-metal contamination in soils.

Zinc (Zn) and arsenic (As) are typically present as mixed contaminants in mining-impacted areas; however, their joined effects have rarely been evaluated. The present study was set up to test whether the Zn and H AsO (hereafter, As) mixture toxicity to plants is additive or whether interactions occur. Barley (Hordeum vulgare) root elongation was measured in resin buffered nutrient solutions. The design included ranges of single-element concentrations and combinations at 3 different Ca concentrations (0.5 mM, 2.2 mM, and 15.0 mM) to vary the relative toxicity of Zn . Increasing Ca concentrations decreased Zn toxicity, whereas As toxicity was unaffected by Ca. Root elongation was generally more affected in Zn-As mixtures than in corresponding single-element treatments. This is merely a joint additive effect, as 96% of the root elongation data were within a factor of 1.2 from predictions using the independent action (IA) or concentration addition (CA) model. The CA and IA predictions were similar, and data did not allow identification of equal or dissimilar modes of action. Small but significant Zn-As antagonisms were only found at high effects (>50% inhibition). The present study suggests that mixture effects of Zn and As are environmentally relevant and that current risk assessment underestimates toxicity in multielement-contaminated environments. The CA model can be used as a conservative model for risk assessment; however, for soil-grown plants, soil-exposed studies are needed. Environ Toxicol Chem 2017;36:1556-1562. © 2016 SETAC.

Zinc (Zn) and arsenic (As) occur as mixed contaminants in soil and the interactions between them remain unclear. Here, we investigated a Zn2+ and H2AsO4− mixture interaction and their effects on plant growth. Three different soils were spiked with ZnCl2 and NaH2AsO4, each dosed singly or in combination. The soils were leached to remove excessive salt and were aged (>7 days), before toxicity testing using a 5-day root elongation of barley (Hordeum vulgare L.). In the single treatments, the 50% inhibitory effect concentrations in the soil (EC50, total measured concentration) were 2000–3800 mg Zn kg−1 and 96–620 mg As kg−1, depending on the soils. The mixture analyses based on the total concentrations showed overall and significant Zn–As antagonism in two soils, either based on the concentration addition (CA) or independent action (IA) model, whereas no significant interactions (either CA or IA) were found in one soil, which had the lowest content of Fe-oxides. The soil solution composition showed a decreased As concentration upon the addition of ZnCl2 at an equal soil As total concentration; however, the reverse was not found, in line with the cation–anion electrostatic interaction or formation of ternary surface complexes on Fe-oxides. The data revealed that the Zn–As antagonisms (total concentrations) are partially related to the increased Zn immobilizing As in soil.

Isotopically exchangeable metals in soil, also termed labile metals, are reversibly bound to soil surface and are a better index of the environmental risk of the metals than are their total concentrations. In this study, labile fractions of potentially toxic elements were surveyed in metal mining‐impacted soils of Mexico to test the relative importance of soil properties (pH, effective cation exchange capacity, organic matter, etc.) or attributes of the mines (ore type and lithology, metal mineralogy, etc.) on the fractions of labile elements. Mining waste‐impacted soils, corresponding uncontaminated soils and mining waste were collected around 11 metal mines in Mexico presenting contrasting ore types. Pseudo‐total concentrations and labile fractions of Cd, Ni, Zn, Pb, Cu, and As were determined by aqua regia digestion and isotope dilution, respectively. Pseudo‐total concentrations of these elements ranked: waste > contaminated soil > uncontaminated soils, and Zn and As dominated the concentrations of toxic elements. The labile fractions (% of total) in the soils ranked, with median values in brackets, Pb (22) > Cd (18) > Cu(15) > Ni∼Zn(13) > As(9). The labile fractions of waste samples were slightly higher than those of soil samples suggesting either a high weathering of mining wastes or the stabilization of heavy metals by soil. Stepwise multiple regression showed that soil properties rather than source attributes primarily explained the %E of most elements, except for Zn and As for which the ore lithology was the dominant factor. This study showed that earlier generic models explain metal lability adequately in mining waste‐impacted soils.

The validity of soil toxicity databases for predicting ecological impacts in the field is rarely explored. The present study was set up to test whether laboratory toxicity data and the combined concepts of metal availability and mixture toxicity can predict ecological impact in mining‐affected soils. Metal and As contamination gradients were sampled approximately 5 different mines in Mexico where plant cover and abundances exhibited clear dose‐related responses. Soils were analyzed for total and isotopically exchangeable (labile) concentrations of Ni, Cu, Cd, Pb, and As and for soil properties affecting the availability of these elements. Six different indices of toxic doses were compared to evaluate their accuracy in describing the field response expressed as relative abundance and cover. Each index was based on a different method to calculate the sum of toxic units (normalΣTUs) in soil, with 1 toxic unit equal to the concentration of the element in soil yielding 50% adverse effect on plants with median sensitivity as recorded in a recent database of salt‐spiked soils. Toxic concentrations in the mine‐impacted soils were dominated by Zn and As. In the field, 50% reduced cover or abundance was found at 10 to 13 normalΣTUs if these were based on total soil concentrations and thresholds derived from freshly spiked soils, indicating a largely overestimated toxic effect. If thresholds were corrected for differences in availability among freshly spiked soils and spiked and laboratory‐aged soils, the overestimation of field toxicity was 5‐ to 6‐fold, irrespective of the consideration of soil properties. Finally, the normalΣTU calculated only with labile metals and As overestimated the field toxicity by factors 1.1 to 1.6 (95% confidence interval 1–7; i.e., rather accurate and indicating some Zn–As antagonism as confirmed in experimental studies). That latter index of dose yielded a bell‐shaped response on species richness peaking at approximately 1.6 normalΣTU. Overall, the present study shows that the current toxicity databases of metals can predict the impact of metal contamination on plant communities within factor 2, expressing the dose as soil‐labile concentrations and using the concentration addition concept in these mixed polluted environments. Environ Toxicol Chem 2020;39:1826–1838. © 2020 SETAC