Uso de ferramentas multidisciplinares na avaliação de vulnerabilidade e risco a subsidência no meio cárstico na cidade de Castanhal, Nordeste do Pará – Brasil.

Abstract

Castanhal was chosen for this work for being one of the cities with the most demographic growth and also for having its public water supply taken from the Pirabas aquiferous system, whiose main characteristic as the fact that it is formed by carbonaceous rocks. The hypothesis suggested in this work is that this lithology explains the loss of drilling fluid that occurred during the drilling of the three wells in the Jaderlândia district, what can yield a subsurface collapse. The methodology applied in this work consisted of several techniques remote sensing, geomorphology, neotectonics, hydrogeology, geophysics and environmental analysis. The maps were prepared utilizing a Landasat -5/TM satellite images, SRTM images, aerial photos in 1: 70.000 scale and orthophotos mosaics in 1: 10.000 scale. To enhance the geological data, geophysical methods such as gravimetry, resistivity, and well logging: γ rays, spontaneous potential and resistance were applied. The hydrogeological study of Castanhal city was conducted mainly by using data from wells intended for public water supply and from private wells. Geologically, the northeast region of Pará state, where Castanhal city is located, is composed by Pós-Barreiras sediments and Barreiras Group rocks, that are underlied by Pirabas Formation, of calcareous constitution which is the focus of this work. The drainage system is structurally ordered and is represented by a stockwork pattern striking mainly NE-SW and NW-SE, withstrong asymmetry, which allowed to visualize a tectonic sectioning of the Sheet Castanhal SA.23-V-C-I including the Castanhal city, identifying in this way high and low blocks that where confirmed by lithostratigraphics profiles of the wells and by the geophysical survey. The tectonic structure is congruous with the dextral system transcorrent of Riedel, and still contains relaystructures found in all the Sheet of Castanhal, favoring to the recharge of the water-bearing systems; however, this increases its vulnerability to contamination. The hydraulical load and the underground flow present a zone of recharge in the center of the Castanhal city and another one in the Village of Apeú, and indicate effluent draining. The analysis of the quality of the waters showed high contents of total iron and turbidness in some wells and of calcium carbonate in the wells supplied by the Pirabas aquifer system. The gravity survey showed a positive anomaly inthe center of the city, indicating a less deep basement, what was corroborated by the resistivitysurvey. In Jaderlândia and Bom Jesus quarters in the city of Castanhal negative anomalies lined up, concordant with drainage alignments of NW direction indicating the construction of two4 karstics buildings between 60 m and 100 m of depth and 200 m and 400 m of extension representing a high risk of collapse in these quarters to occur. The interaction of the abovementioned information made possible the elaboration of geological sections, blocks diagrams and thematic maps that gave origin to the map of environmental risk for the city of Castanhal.