Caracterização biogeoquímica de águas subterrâneas da zona urbana de Porto Velho-RO

Abstract

The city of Porto Velho - RO has a population of 293,815 inhabitants (IBGE, 1996). Of these, 56% are supplied with treated water, collected from surface springs; the rest, with water from tubular wells of the "Amazon" type. Only 4.6% of the population has sewage collection services, so that wastewater is released into the open. These waters, when infiltrated, mix with those from pits, becoming potential sources of polluting the wells, drilled in a rudimentary way, without taking into account the geological and hydrogeological characteristics (lithology and structure of the land, level of the water table, etc. ), do not have a sanitary seal and the positioning of filters is done improperly. There is also the deficiency of regular household garbage collection services, especially in peripheral neighborhoods, most of them generated by "invasions". This garbage is thrown in areas close to the residences, which makes it possible to wash the mass of residues, with the consequent production of leachate, which can also constitute a potential source of pollution of groundwater. The present work evaluated the biogeochemical conditions of these waters and identified groundwater quality characteristics. In the periods from April/95 to October/95 and November/95 to February/96 (drought/95 and rainy/95), water analyzes were carried out from 86 wells in the study area, 43 of which were tubular wells and 43 excavated wells. “Amazon” type, with an average depth of 47 m and 12 m, respectively. Chemical analyzes were also carried out, in samples of gutters, of sediments from well logs drilled during the course of this research. The analytical results proved to be efficient in the assessment of the environmental impact caused by anthropogenic activities in groundwater, resulting from chemical and bacteriological contamination. Chemical contamination was manifested by the presence of nitrate and bacteria of the coliform group in the groundwater of most of the sampled wells. Bacteriological contamination results from the inappropriate use of sanitary disposal systems, domestic and livestock waste (mainly poultry and pigs) in the vicinity of wells, drilling and inappropriate location of these wells, among other causes. The results of the analyzed parameters also allowed the evaluation of the main characteristics of the water quality of the aquifers, and their probable interrelation with the local geology. There is groundwater poor in electrolytes, representative of heavily leached land, there is also groundwater richer in electrolytes. The physical-chemical composition of these waters results from the contact with the percolated rocks and the climatic conditions of the area, with a humid climate with high rainfall.