Assessment of the Environment Risks of Reuse of Untreated Wastewater in Urban and Peri Urban Agriculture: A Case Study of Nairobi in Kenya

Abstract

A study was carried out to determine pollution levels in soil and crop as a result ofwastewater reuse for irrigation in Nairobi and establish the benefits and risksassociated with this. Irrigation waters (raw sewage), soil and crop samples werecollected from Kibera and Mailisaba wastewater irrigation farms during the dry andwet season. Irrigation water was analyzed for both physical and chemicalparameters. Soil and crop samples were analyzed for heavy metals: lead, cadmiumand chromium. Heavy metals in waters, soils and crops were determined by AtomicAbsorption Spectrometry (AAS) method. Crops sampled were maize, kales, blacknightshade and arrowroots that represented grain, exotic leafy vegetable, indigenousleafy vegetable and root crop respectively. Samples for analysis were obtained fromroots, stems, leaves and grains. Soils were sampled from plots containing maize andkales and black nightshade over depths of 0-30 cm and 30-60 cm. Householdquestionnaires were also administered to collect data on farmers’ perspective onwastewater use for irrigation.The results showed that wastewater is reused in agriculture in many countriesworldwide mostly because of inadequate water supply. Wastewater is also usedbecause it has nutrients and is available all year round. Kibera and Mailisaba farmershowever complained of some crops being adversely affected by the quality of thewastewater. Mailisaba farmers were more aware of the health risks than Kiberafarmers with 7.7% of respondents at Kibera compared to 37.9% at Mailisaba. Cropselection is one of the risk mitigation strategies in using wastewater for irrigation asmost of the crops grown including kales, maize, amaranth, black nightshade,cowpeas, spinach, arrowroots, are cooked before consumption. Another mitigationstrategy, as cited by farmers, is wearing of protective clothing. Nevertheless, manyof the farmers confessed to not using any protective clothing.Most of the farmers produce crops for sale at the local markets with some of theproduce being consumed at the household level. From the farmers’ perspective, themain benefits of wastewater farming are: food security and nutrition (35.8% of therespondents); source of income (33.7%) and employment (15.1%).In both sites, pH of the water was within the permissible range while ElectricalConductivity (EC) at Mailisaba was higher than the recommended level forirrigation. EC of Mailisaba irrigation water was in the range slight to moderatedegree of restriction (0.7-3.0 mg/l), an indication that treatment would be required toavoid salinization of soils. Dry season average values for nitrates (NO3) were 97.32mg/l at Kibera and 126.46 mg/l at Mailisaba while wet season values were 16.45mg/l and 25.38 mg/l respectively. The average nitrate values placed the wastewaterat “slight to moderate” (5-30 mg/l) restriction for both sites during the wet seasonand “severe” (>30 mg/l) restriction during the dry season. Given that farmers usuallyirrigate during the dry season, these results indicate that the wastewater may not besuitable for irrigation as it poses a threat to the environment. Farmers at both siteschose to grow leafy vegetables such as kales, spinach, black nightshade andcowpeas, which give high yields probably due to excessive nitrogen in the irrigationwater.Lead and Cadmium in irrigation water were within the safe concentrations for cropproduction (<5.0 and <0.01 mg/l respectively). These metals pose no risk to cropgrowth. They may however pose a risk to human health if they accumulate in thesoils to levels where they become bioavailable and accumulate in the edible parts ofthe crops. Chromium values exceeded the standards, indicating that extended use ofwastewater for irrigation has the potential for accumulation of chromium in soils andcould be a threat against public health. Farmers at both sites indicated that theywould rather die a slow death from heavy metal toxicity than die today of starvation.The Nairobi wastewater has quality that may be termed as acceptable for cropproduction. It therefore has a potential for being used in agricultural production.This use should be encouraged as a disposal method for wastewater. However, someform of treatment may be necessary to reduce the concentrations of parameters suchas nitrates, EC and heavy metals that were found to be excessive.Cadmium was not detected in irrigation waters at both sites but its presence in soiland crops was noted, indicating the possibility of accumulation in both soil andcrops. Accumulation of the three metals in soil was found to be in both 0-30 cm and30-60 cm layers with the levels ranging as follows: 0.40 – 98.66 ppm for Lead, 0.01– 9.69 ppm for Cadmium and 0.06 - 74.30 ppm for Chromium. Lead levels pose norisk as they were within the allowable limits (50 – 300 ppm) for agricultural soils.Cadmium was above the allowable limits (1 - 3 ppm) posing a major risk to humanhealth. The three heavy metals were found in the different crop parts (roots, stems,leaves and grains) for the four crops tested. During the dry season, the concentrationof Lead in crops ranged from 16.17 to 74.83 ppm, Cadmium from 3.33 to 13.98 andChromium from 0.63 to 47.17 ppm. These ranges indicate a definite accumulationfrom wastewater to soil and from soil to crops (bioaccumulation).The results showed decreased concentrations of all the metals during the wet seasonat both sites regardless of depth and cropping system. The highest concentration ofheavy metals in soils was that of lead in the two cropping systems and at both sites.Although the heavy metal levels in soils were found to be within the allowablelimits, the levels may pose threat to human and animal if wastewater farming isallowed to continue without anything being done to reduce the levels of thepollutants in the wastewater.The results point to the recommendation that some form of treatment be consideredto make the wastewaters safe for reuse in irrigation of food crops. In addition, there isneed for awareness creation among farmers and consumers on the risks associated withwastewater reuse for irrigation.