The effect of long-term irrigation using wastewater on heavy metal contents of soils under vegetables in Harare, Zimbabwe

https://doi.org/10.1016/j.agee.2004.11.005Get rights and content

Abstract

The magnitude of contamination, regulatory compliance and annual loadings of soils with copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni), chromium (Cr) and lead (Pb) were determined at three sites in Harare where wastewater was used to irrigate vegetable gardens for at least 10 years. Heavy metal total concentrations (mg kg−1) in sandy and sandy–clay soils of pH 5.1–8.1 from all sites ranged from 7.0 to 145 for Cu, 14 to 228 for Zn, 0.5 to 3.4 for Cd, <0.01 to 21 for Ni, 33 to 225 for Cr and 4 to 59 for Pb in the 0–20 cm soil depths. The concentrations had increased significantly in the gardens compared with control soils and subsoil. Annual heavy metal loading rates showed that within 5–60 years, all studied heavy metals would have exceeded their permitted limits in soils, depending on site. It was concluded that the use of wastewater in urban horticulture enriched soils with heavy metals to concentrations that may pose potential environmental and health risks in the long-term.

Introduction

Horticultural production has contributed to rapid economic growth in Zimbabwe, and demand for horticultural products has continued to increase with increasing population (Jackson, 1997). This has resulted in more horticultural enterprises, one of which is vegetable production at municipal farms and along riverbanks in Harare using wastewater for irrigation. Wastewater use occurs either indirectly, when partially and untreated effluent is discharged into rivers that supply water for agriculture, or directly, at municipal farms when partially treated sewage effluent is conveyed into some gardens.

Past experience had shown that these developmental projects, created with the aim of producing socio-economic benefits, have also produced adverse environmental impacts (FAO, 2000) such as land degradation. Earlier studies (Zaranyika et al., 1993, Mangwayana, 1995, Oloya and Tagwira, 1996, Nyamangara and Mzezewa, 1999) showed that the concentrations of heavy metals in wastewater that is used for irrigation at Pension, Crowborough and Mukuvisi vegetable production sites in Harare were several-fold (3 to <13-fold) higher than the recommended limits in wastewater in Zimbabwe. The studies also implicated land disposal of wastewater as the chief source of Cu, Zn, Cd and Pb enrichment of pasturelands. However, it has not been clear whether vegetable production sites irrigated with wastewater have also been enriched with heavy metals in the same magnitude or not. Although no cases of heavy metal poisoning due to the ingestion of vegetables irrigated with wastewater have been reported in humans in Harare, heavy metals remain important cumulative poisons (Kitagishi and Yamane, 1981).

Soils, as filters of toxic chemicals, may adsorb and retain heavy metals from wastewater. But when the capacity of soils to retain toxic metals is reduced due to continuous loading of pollutants or changes in pH, soils can release heavy metals into groundwater or soil solution available for plant uptake. The amount of heavy metals mobilized in a soil environment is a function of pH, clay content, organic matter content, cation exchange capacity and other soil properties making each soil unique in terms of pollution management (Kimberly and William, 1999). With the exception of Mo, Se and As, heavy metal mobility decreases with increasing soil pH due to precipitation of hydroxides, carbonates or formation of insoluble organic complexes (Smith, 1996). Heavy metals are capable of forming insoluble complex compounds with soil organic matter and according to Sauve et al. (2000) solid-solution partitioning of Cd, Cu, Ni, Pb and Zn is dependent on soil solution pH, total metal content and soil organic matter.

Heavy metals contribute to environmental pollution because of their unique properties, mainly that they are non-biodegradable, non-thermo-degradable and generally do not leach from the topsoil. Unlike petroleum hydrocarbons and litter that visibly build-up on soils, heavy metals can accumulate unnoticed to toxic concentrations (Bohn et al., 1985) that affect plant and animal life. The duration of contamination by heavy metals may be for hundreds or thousands of years, even after their addition to soils had been stopped. The time taken for Cd, Cu and Pb to reach half their concentrations (half lives) in soil were found to be 15–1100, 310–1500 and 740–5900 years, respectively, depending on soil type and physiochemical parameters (Alloway and Ayres, 1993).

Metals added in small concentrations find specific adsorption sites in soil where they are retained very strongly, either on inorganic or organic colloids (Sauve et al., 2000). Following addition to soil, organic loading of wastewater undergoes decomposition to CO2, low molecular weight soluble organic acids, residual organic matter and inorganic constituents (Boyd et al., 1980). Decomposition can also release heavy metals into soil solution. But, because of their low solubility and limited uptake by plants, heavy metals tend to accumulate in surface soil and become part of the soil matrix (McGrath et al., 1994). With repeated wastewater applications, heavy metals can accumulate in soil to toxic concentrations for plant growth (Chang et al., 1992).

Not all heavy metals in soil are results of human activity. Trace metals in soil originally arose from the net effects of geological and soil-forming processes of the elements (Kabata-Pendias and Adriano, 1995) and the concentration in soil is governed by the parent material, climate, topography and human activities, factors which are responsible for soil formation. Sandy soils from granite rocks normally contain lower concentrations of heavy metals than clay soils derived from mafic rocks (Ross, 1994a). According to Alloway and Ayres (1993) heavy metals may enter the soil from agricultural related sources such as pesticides, fertilizers, composts and manure, and sewage sludge.

There are currently no locally derived permissible limits of heavy metals in soils amended with wastewater in Zimbabwe. However, there are limits for heavy metal concentrations in wastewater applied on agricultural land, which are presented in the ‘Zimbabwean Waste Discharge and Disposal (Water Pollution) Regulations’ of 1998. These limits were derived primarily from international sources of information like the United States Environmental Protection Agency (USEPA), World Health Organization (WHO) and the European Union (EU), with special considerations to local water resources and utilization in Zimbabwe (Mtetwa, 1996). Under the Zimbabwean Public Health Protection Act (1972), the use of wastewater is prohibited for irrigation of vegetable crops, salad crops to be eaten raw and berry fruits in order to protect consumers from exposure to pathogens and toxic chemicals found in some wastewaters. However, the history of urban cultivation in Harare shows that institutional responses to wastewater use have not been very prohibitive (Mbiba, 1994).

Within each country in the EU, the concentrations of heavy metal contaminants are controlled under an EC Directive (86/278) on the protection of the environment and in particular, of the soil, when wastewater and sewage sludge are used in agriculture. The UK limits for heavy metals in agricultural soils (MAFF, 1993) are based on the EC Directive, which insists that soils should be monitored before and after wastewater or sludge is applied. They are more adaptable to Zimbabwean environment than the stricter Dutch policy, which states that all soils should not be contaminated and heavy metal concentrations in effluent should be very low (Mtetwa, 1996). Furthermore, UK maximum permissible limits were set in relation to soil pH, accounting for the varying availability of heavy metals for crop uptake with soil pH. In this study, the limits of heavy metals in soils and their annual loading rate limits refers to the UK permissible limits.

The objectives of the paper were to determine the total concentrations of Cu, Zn, Cd, Ni, Pb and Cr, and estimate their annual loading rates in soils at the Mukuvisi, Pension and Crowborough vegetable production sites where wastewater has been applied for at least 10 years. This would provide knowledge that guides future research into the protection of the environment and people from exposure to heavy metals with potential to cause health problems. Although total concentrations of heavy metals in soil poorly indicate their availability for plant uptake (Kimberly and William, 1999), existing permissible limits of heavy metals in soils are based on total concentrations. Thus, the information would be useful from a policy point of view. In addition, the total concentration also indicates the potential risks from other contamination pathways such as soil ingestion by children (and some adults, commonly pregnant women), inhalation of dust from the sites, soil adsorption on edible leaves and other potential risks associated with handling the soil.

Section snippets

Site description and management practices

Three vegetable production sites, Pension, Crowborough and Mukuvisi, were selected from Harare (Fig. 1) where many pollution problems as well as high commercial activities related to horticulture are found. Harare has cold–dry winters and hot–wet summers (subtropical). Average annual rainfall is about 850 mm and average annual temperature is 18–20 °C. Crowborough Farm extends from 17°49′S to 17°52′S and lies between 30°52′E and 30°58′E, while Pension Farm extends from 17°52′S to 17°55′S and lies

Annual loading rates

The estimated annual heavy metal loading rates at Pension and Mukuvisi sites (Table 2) showed that all selected heavy metals, except Cd, had annual loading rates below the maximum permissible limits at the Mukuvisi site. On the contrary, the annual loading rates of all selected heavy metals, except Zn, were above their maximum permissible limits at the Pension site. Cadmium loading rate had the greatest magnitude of deviation from the maximum permissible limit (0.15 kg ha−1 year−1), being over

Discussion

The application of wastewater at the Pension and Mukuvisi sites increased soil pH by 0.5–3 units comparing the wastewater-irrigated sites to the non-irrigated soils. Past research (Zaranyika et al., 1993, Oloya and Tagwira, 1996) has indicated that the wastewaters applied for irrigation at the Pension, Crowborough and Mukuvisi sites have in most cases neutral to alkaline pH (6.5–8.0) in addition to the high concentrations of basic cations such as Ca, Mg and K. Oloya and Tagwira (1996) also

Conclusions

Given the prevailing conditions of generally low pH, light textured soils and significantly high concentrations of Cu, Zn, Cd, Cr, Pb and Ni compared with control sites, it was concluded that soil contamination by wastewater use present long-term environmental and health risks. Some heavy metals, notably Cu, Zn and Cd, have begun to exceed their maximum permitted limits, especially at Pension and Crowborough gardens, and had high annual loading rates. Contamination at Mukuvisi gardens was

Acknowledgements

This publication is an output from research project R7519 funded by the United Kingdom Department for International Development (DFID) for the benefit of developing countries. The Crop Post-Harvest Research Programme operates in Zimbabwe under a Memorandum of Understanding with the Department of Agricultural Research and Extension (AREX). The views expressed are not necessarily those of DFID nor of AREX.

References (35)

  • D.W. Connell et al.

    Chemistry and Ecotoxicology of Pollution

    (1984)
  • FAO

    Food for the Cities

    (2000)
  • G.W. Gee et al.

    Particle size analysis

  • V.J.G. Houba et al.

    Soil and Plant Analysis, Part 5

    (1989)
  • Hranova, R.K., 2002. Water reuse in Zimbabwe—an overview of present practice and future trends,...
  • J.E. Jackson

    Vegetable crops production in communal area in Mash. East and Mash. West

  • A. Kabata-Pendias et al.

    Trace metals

  • Cited by (505)

    View all citing articles on Scopus
    View full text