Suitability of urban river water for irrigation: the case of the Houet river in Burkina Faso

Abstract

This study focused on the suitability of using an urban river for the irrigation of vegetable crops. The relevant urban river is Houet river located in the city of Bobo-Dioulasso (Burkina Faso) receives wastewater released by a wastewater treatment and purification unit (WWTP). To do this, a diagnosis of the functioning of WWTP and an assessment of the quality of the effluents released in the river were indeed carried out. The WWTP treats wastewater from industries, households and the percolating water from the treatment of the city's sewage sludge. Water released by the WWTP has a poor physico-chemical quality and high microbiological pollution. The urban river water suitability for irrigation was carried out by taking 8 water samples from the river sections at upstream and downstream parts of the WWTP junction. The physico-chemical (pH, EC, STD, Ca2+ , Mg2+ , K+ , Na+ , Cl-, HCO3- , PO43- , NO2- , NO3- , NH4+ ), microbiological (total coliform, fecal coliform, Escherichia coli and fecal Streptococci), trace metallic elements (Ni, Cr, Zn, Cu, Al, Fe and Mn) and indices such as sodium adsorption rate (SAR), percentage of soluble sodium (%Na), permeability index (PI), magnesium adsorption ratio (RAM) and Kelly ratio (RK) were analyzed and the results compared with WHO, FAO and USSL guidelines for irrigation water. The results show that the physico-chemical parameters are in line with the WHO and FAO standards for irrigation. However, the river water is reported to have medium salinity with a slight to moderate degree of restriction for EC and STD. The concentrations of ammonium (NH4+), manganese (Mn) and nickel (Ni) in the river’s water have values above standard reference for irrigation water. This suggests (i) overuse of nitrogenous fertilizers, (ii) industrial and anthropogenic discharges directly into the river, (iii) and an incomplete treatment process of water released by the WWTP. The presence of Total Coliforms, Fecal Coliforms, Escherichia coli and Fecal Streptococci at levels of around 104 to 109 CFU/100ml, above the FAO standard (2000 CFU/100ml), indicate that the river water is probably contaminated by viruses, parasites and other pathogenic bacteria of fecal origin. Houet river water is in excellent range for irrigation according to Wilcox diagram and in the C2 S1 range of medium to good quality according to the Riverside diagram. The SAR, %Na, PI, RAM and RK indices showed that the infiltration problem issue of the river water on irrigated soils is low before its junction with the WWTP and moderate after its junction. Overall, this river water is suitable for irrigation, but the presence of bacteria, ammonium and trace metallic elements require for the regional water and agricultural authorities to (i) pay particular attention to human activities in the vicinity of the river, (ii) improve the efficiency of the wastewater treatment process at the WWTP and (iii) more strictly control fertilizers’ uses by market gardeners. This will make it possible to preserve the quality of the river’ water for the irrigation of 175 hectares of market gardens products (lettuce, parsley, pepper, mint…) using a commercial urban outland to feed an urban population estimated of one million people.