Assessment of Heavy Metals in Greenhouse Cultivated Soils, Northern Jordan

Jordan has recently observed a gradual shift in vegetable production from open-fields to greenhouses with mounting consumer concerns about food quality and safety. We investigated heavy metals in soil collected from greenhouse vegetable production area in northern Jordan. Sixty-one surface soil samples were collected, of which forty-seven from plastic-covered greenhouses and fourteen were sampled from the adjacent open-field land, with both designated for vegetable production. The average concentrations of Cr, Cu, Cd, Pb, Ni, Zn were 26.1, 26.8, 0.81, 53.0, 49.3, 139.1 mg/kg, and 19.1, 19.3, 0.66, 49.7, 46.7, 104.9 mg/kg for greenhouse and open-field soils, respectively. While the accumulation of heavy metals was consistently higher in greenhouse than in open-fields, both soils revealed a similar metal ranking with a few exceptions. Greenhouse soils revealed relatively lower pH values with higher variabilities. In greenhouse cultivated soils, CaCO3 content averaged 21.4% compared to 23% measured in open-field soils. Soil salinity showed greater values for greenhouse samples (averaging 1118.6 µs/cm) than those observed in open-field agricultural soils (a mean of 503.6 µs/cm). The soil organic matter (TOM) exhibited values in the range of 1.06-3.35% relative to 0.59-2.41% found in open-field area. The spatial distribution of heavy metal concentrations for greenhouse soils revealed higher levels in the northern soils, whereas the least was found in the southern sampling points. The Enrichment results showed 23.4% of sampling sites were moderately contaminated with Pb, and 38.3% were moderately contaminated with Cd, of which 8.5% indicating moderately severe contamination. The Igeo results indicate 25.5% of greenhouse soils were moderately contaminated with Pb and 38.3% were heavily polluted with Cd. The contamination factors showed 25.5% and 38.3% of greenhouse sampling soils were considerably contaminated with Pb and Cd, respectively. 2% indicate very high contamination for Cd and 2% showed considerable contamination for Zn. PLI indicates that only two sampling sites are polluted. The ecological risk assessment showed low Ei values for all heavy metals suggesting slight risks, except for Cd which indicate strong risk. Total potential ecological risks values showed low risk to the local environment. Cd accounts for most of the total risks (72.27-82.67%) followed by Pb (11.49-14.87%). Some greenhouse soils were non-compliant with soil quality standards especially for Ni, Cd, and Pb. The observed levels of heavy metals are attributable to agricultural activities including long-term application of pesticides, phosphatic and nitrogen fertilizers, sewage sludge, wastewater irrigation and chicken manure in addition to industrial dust and traffic related emissions.