Os Lagos do entorno da cidade de Boa Vista - Roraima: aspectos fisiográficos, granulométricos, mineralógicos e químicos dos sedimentos e físicoquímico das águas

Abstract

The landscape of northeast of Roraima state is constituted of a flat and low plain, cover for savannahs, where many lakes and wetlands occur. Generally these are found at interfluve’s area of the rivers Cauamé and Mucajaí, close to Boa Vista city, which has a rapid and disordered urbanization, causing the degradation and disappearance of several lakes. In this context the present work has as main objective the knowledge of these environments, in order to understand its functional dynamics in the savannah formations, and then to evaluate the possible anthropogenic impacts. To reach these objectives we carried out physiographic, granulometrical, mineralogical and chemical analyses of sediments as well as measures of the physic-chemical parameters of lake waters. These lakes exhibit several shapes, but the most frequent ones are drop-shape, circular, ellipsoid and twin shapes, related to small water courses (igarapés) in its majority, constituting its springs. The lakes are small (< 5 hectares) and shallow (0,8 and 1,8 m). Due to the low depth the water level are strongly influenced by the seasonal changing, being intermittent (over 65 % of mapped lakes) during the long dry period. The lakes are colonized by dense communities of aquatic macrophytes, mainly for cyperaceae, which can be distributed along all over of the lake basin. This distribution is controlled by the depth of lake water. The lakes’ substratum sediments are mostly sandy and the grains show no roundness until to low roundness as well as low sphericity. They are essentially constituted of quartz (87,8 %), being less frequent kaolinit (8,8 %). The chemical composition displays high concentrations of SiO2 (91,92 %) and low values of Al2O3 (3,48 %) and Fe2O3, (0,24 %), which together with the mineralogy characterizes a typical Podzol. The temperature of the lake waters changes from 30 to 34 °C and reflect the thermal conditions of the regional climate (Awi in the Koeppen classification). The pH values indicate a domain of acid waters (pH values from 5,0 to 6,0). However, some urban lakes presents pH values close to neutral and basic (6,57 and 8,20), this can be attributed to anthropogenic activities (clothes laundering, end destination of domestic effluent, etc). The electric conductivity range is between 4 and 14 µS/cm, reflecting the low levels of solid total dissolved, which oscillate between 2,0 and 7,0 mg/L. The dissolved oxygen reaches 4,0 to 5,7 mg/L. excepting waters of the Nova Cidade lake that presented value as low as 2,2 mg/L, due to its high concentration of organic matter under decomposition. The lakes shapes and small length and their occurrence on flat low plain covered by open savannah vegetation suggest that lakes are related to near surface groundwater, that mostly during the rainy season intercept the shallow land surface depressions, in the form of springs forming the lakes and wetlands. The absence of typical lacustrine sediments reinforces this origin. The lakes are formed over material of grain size pattern, mineralogy and chemistry that can be well compared to those ones of the Boa Vista Formation and are completely distinct from those ones deposited by Branco river in its floodplain. The whole landscape of Boa Vista lakes studied here presents a singular lake system in the Amazon region, whose importance goes beyond its scenic beauty, being important for groundwater and regional climate control. Unfortunately, several lakes are under strong anthropogenic impact and can extinguishing in short time, much more shorter than the natural ones, if no urgent public politics were carry out.