Evaluation of the Impact of Chemical Pollution on the Quality of the Environment in Rice Fields in the Commune of Ankadikely Ilafy, Antananarivo Madagascar

In the northern peripheral zone of the Urban Commune of Antananarivo (Madagascar), rice fields are irrigated by wastewater in its raw state or after treatment, of a mixed and biodegradable nature, which contains nitrates N-NO3- , Cl-, H2 PO4- , and metallic trace elements with excessive concentrations of heavy metals (Pb, Cr, Cu, Zn and Mn) that can be introduced into the environment, for example, by industrial effluents and waste. The direct effects of the use of these waters in irrigation are soil acidification, enrichment in fertilising elements (NPK) mainly from domestic waste and fertilisers used by farmers and in organic matter, and then the accumulation of trace metal elements in the soil from the plant, during the years of monitoring. However, according to the field studies, we found the following: on the one hand, chromium inputs (dry season 0.45mg/L and rainy season 0.71mg/L), conductivity (dry season 2236µS/cm and rainy season 268.6µS/cm), turbidity (dry season 42.9NTU and rainy season 78.59NTU), variable in quantity and quality during these two seasons, with high and out of range levels observed in the irrigation water. These levels can only be explained by intense emissions from the various discharges into the untreated watercourses located in the study area. This shows the direct impact of chemical pollution discharged into the rice fields on the physico-chemical and metallic quality of the water and then the soil. Then, conversely, the soil of the rice fields (SRi) of our study site shows an efficient purifying power, but according to the respective seasons according to the measurement of the water at the entrance and exit of these rice fields, where during the dry season 2015, the increasing purifying order for the elimination of the different parameters is as follows: COD (24.85%) BOD5 (25.84%) EC (27.98%) NO3- (38.333%) Turbidity (44.11%) Cr (45.31%) TSS (66.44%); and during the wet season 2016, this order is: BOD5 (6.47%) COD (8.54%) EC (19.02%) NO3- (38.02%) Turbidity (40.57%) TSS (58.13%) Cr (73.53%). However, some forms of these parameters are poorly retained by the soil and are easily leached to the groundwater. Thus, it can be seen that the efficiency of soil purification depends on the element in question and the volume of irrigation.