Lubumbashi (DR Congo)—the capital of copper mining—has been considered as one of the richest mining regions of the world for more than a decade. These riches have brought along multiple mining companies responsible for soil, river water and vegetable pollution, as in many African cities. The aim of the present study was to quantify and evaluate the pollution levels and the potential sources of soil, irrigation water and vegetable contamination by the metals As, Cd, Cr, Cu, Pb, Co and Zn in the urban gardens of Lubumbashi (DR Congo). The contamination, pollution and enrichment levels of the gardens were determined based on different indices in order to rank the soils. The results show that soils, waters and vegetables present contamination levels that represent a serious concern for human health. All soils presented contamination indices ranging from low (72% of the soils) to very high (3.4% of the soils) metal (copper, lead, zinc) contamination. The Cu and Cd contents varied between 1355 mg/kg et 236 mg/kg, much higher than the World Health Organisation (WHO) thresholds (100 mg/kg for Cu and 2 mg/kg for Cd). Moreover, the water used for crop and garden irrigation presented high Pb (57% of the waters), Fe (52%), Cu (19%) and Cd (10%) contamination levels, above the Association Française de Normalisation (AFNOR) U4441 toxicity thresholds (2 mg/kg for Cu; 0.1 mg/kg for Fe and 0.01 mg/kg for Pb) for crop irrigation. Finally, the vegetables produced in these gardens and sold in the local markets had very high metal content (47% contained Cu; 100% contained copper and cobalt) above the WHO standard (10 mg/kg for Cu, 2 mg/kg for Cd and 1 mg/kg for Co) for human consumption. In the face of these issues, it would be preferable to consider cheaper, more sustainable techniques that reduce soil-to-plant metal transfer.

Anthropogenic activities have denatured aquatic, terrestrial, and aerial environments throughout the world in general, and in Lubumbashi in particular, where market garden soils have become uncultivable for many plants. Thus, bioponics could be an effective means of producing uncontaminated vegetables in soilless cultivation, not only reducing the amount of fertilizer used and limiting contamination of agricultural produce but also achieving higher yields than in open-ground cultivation. The overall objective of this study was to implement a new bioponic technique for producing liquid fertilizer from chicken manure and utilize it in the organic hydroponic cultivation of lettuce (Lactuca sativa var. Lucrecia) installed on floating raft systems. To achieve this, two types of trials were conducted. The first was aimed at determining the quantities of organic matter to be used in the formulation of nutrient solutions. The second trial aimed to determine the optimal nitrogen concentration to be provided for hydroponic plant growth. Mineralization and/or anaerobic digestion of chicken manure were conducted for 7 days in 200 L barrels. For the first trial, nutrient solutions were created from three different concentrations of chicken manure (0.35%, 3.5%, and 7% dry matter—D.M.). These solutions were then used in bioponic rafts where total ammonia nitrogen (TAN) concentrations were fixed at 150 mg/L. For the second trial, D.M. was fixed at 2.5% for each tested modality, but TAN concentrations varied among them (i.e., 60, 90, and 120 mg/L TAN concentration). Modalities with low D.M. concentration (0.35%) and those with low TAN concentration (60 mg/L) resulted in higher yields than bioponic modalities receiving high concentrations of dry matter or TAN, respectively, for trials 1 and 2. Although the reference chemical solutions generate the greatest yields, bioponic systems operating with chicken manure present a good alternative for the cultivation of vegetables in developing countries with heavily contaminated soils. Indeed, bioponics allows for the production of vegetables in large quantities from animal waste, which does not pose health risks for human consumption. Local vegetable species commonly grown in Lubumbashi should be tested under hydroponic conditions.