Relação entre a precipitação pluviométrica e a geologia na hidrodinâmica do aquífero Alter do Chão no Estado do Pará

Abstract

The Alter do Chão Aquifer System occurs in the center-north of the Brazilian Amazon in an estimated area of 9,780 km2 in the State of Pará, a region where December to June is the rainy season and July to November is the dry season and the annual rainfall between 1,500 mm and 3,000 mm is one of the key terms for balancing the water level of the aquifers. Time and space are important factors to understand the relationship between rainfall and the Alter do Chão Aquifer. In this way, the present work focus investigation on the relationship of the hydrostatic levels of the Alter do Chão aquifer with rainfall during the years of 2013-2020, based on the analysis of free online access database of the 6 tubular wells available by the Integrated Network of Groundwater Monitoring (RIMAS/CPRM) and of the 5 rainfall stations of the National Water Agency (ANA) in the region covering the municipalities of Almeirim, Prainha, Monte Alegre, Oriximiná, Óbidos and Santarém. So, the proposed analysis has been based on the mathematical tools as descriptive statistics and modeling, using Pearson's Correlation and Cross-Correlation Function, in addition to the evaluation of thematic maps and the lithostratigraphy each well. The results of the correlations between the time series of rainfall and the variation of the hydrostatic level of the wells showed a late rise in the water level of the wells, since it is natural during the rainy season rainwater infiltration reaches the underground reservoir. Because of this, it was possible to identify that the water levels of the monitoring wells increase between 2 and 5 months after the rainfall events. The wells that raised their water levels in less time, present sandstone lithological sections located in flat relief, and those with predominantly clayey lithological sections in flat relief are susceptible to slower elevation of levels, as well as the sandstone section located in wavy relief. On the other hand, it was observed that the soil type where the wells are located, unlike their lithostratigraphic section type, does not influence the response time of the well water levels to precipitation events. The obtained results made it possible to understand that the relationship between rainfall and the Alter do Chão Aquifer is not one-to-one, as water levels rise only after the precipitation events have occurred, and the prediction of the elevation of water levels can be associated with the geomorphological and geological composition of the terrain where the monitoring wells are located. This study was necessary to reinforce the idea and importance of the integrated use of statistical tools, physical groundwater monitoring data,constructive data from wells and the use of geoprocessing to identify the influence of the natural physical environment, and its relationships, in the process of recharge the Alter do Chão Aquifer System.