Avaliação da contaminação ambiental em um aterro sanitário de pequeno porte, sem utilização de geomembrana, na região noroeste do estado do Rio Grande do Sul

Abstract

Landfills may lead to soil and water contamination (surface and groundwater) when improperly operated, especially if there is no geomembrane waterproofing system. Thus, this study aims to analyze changes in the sanitary landfill subsoil without the use of geomembrane and it suggests changes in the technical standards which can make this practice possible. For this, it was made an assessment of the water quality information from existing monitoring wells, the soil water flow through potentiometry, the soil physical properties (saturated hydraulic conductivity and field capacity), concentrations of chemical elements by the fluorescence technique of x-rays, evaluation of the leachate generation by the hydric balance method, then it was applied the geophysical method of the DC resistivity to indentify a possible contamination zone. The results indicated a clayed soil texture, presenting low permeability (7.98 x 10-5 cm.s-1), with values close to the ones established by technical standards and with micropores predominance (52.26 %). The chemical elements Cr, Co and Cd presented concentrations above the established for the region of study. Statistical analysis indicated that these elements were the most similar among each other regardless of the sampling site and depth analyzed. Natural characteristics of the water quality parameters presented alterations when compared to the Ordinance of Consolidation 05/2017. Mainly nitrate and coliform (total and thermotolerant) concentrations, coinciding with the underground flow direction. The microbiological parameters showed a higher relative contribution in the wells PM02 and PM03, indicating no similarity amongst the studied variables. Regarding the constructive parameter, the depth of the wells indicated greater contribution in the information variability. The geophysical method indicated a possible contamination zone, mainly towards the lower topographic levels, close to the sanitary landfill cell of operation and towards the underground flow, indicating a dispersion and formation of possible contaminated zones. Resistivity contrasts occurs from several depths of the subsoil up to 20 m. The estimated leachate generation indicated a monthly average generation of 0.31 L.s-1 between the years of 2007 and 2017, which may be related to the percolation of this volume by the soil mass to the vadose zone. Applying the landfills quality index, it was identified that the conditions of operations were adequate, however having inconsistency in the presence of birds and in the absence of geotechnical monitoring. Thus, it can be concluded that in the evaluated area, it occurred changes in the groundwater quality parameters of mainly nitrates and coliforms (total and thermotolerant). The major changes were identified in the direction of the underground flow (PM03), which were also identified by the geophysical method. The leachate generation without the complementary protection may interfere in the soil and groundwater quality, even though the clayed/kaolinitic soil having presented low permeability. Thus, even the area presenting the conditions for exemption of complementary waterproofing contained in the regulations, changes occurred in the physical environment, indicating revisions in its content.