Avaliação geoquímica de área selecionada da bacia hidrográfica do rio Maracanã (Nordeste do Pará).

Abstract

The basin of the Maracanã River, located in the Northeastern portion of the State of Pará, represents an important pole of regional development. For this reason, the selected area (3500 km2) was chosen, in order to obtain the geochemical formalism for the interpretation of analytic results related to the groundwater and surficial waters, in the search for relevant aspects on the sustainability of hydric resources. The selected region can be subdivided into three areas. First, an area of springs, characterized by higher topography (share above 40m) with vegetation of hiléia-like and capoeira-like sustained mainly by material of the Pós-Barreiras; estuarine area, under strong domain of the tides dynamics, resulting from the mixture of fluvial and oceanic waters being colonized by mangrove at the main river margins and in the bay area appearing Holocenic sediments; and transition area, under minimum influence or some marked influence of tides, itself in a slow and gradual way, mixturing the three types of vegetation mentioned above. The results of all studied parameters, physical, physical-chemical and chemical groundwater analyses permitted an evaluation regarding chemical composition and a possible interrelation with geological material. For the surficial waters, the same environmental parameters and analyses of chlorophyll-a were selected, trying to better characterize the environment according to the geographical location, seasonality and the dynamics of tides. The geochemical interpretations of the analytic results confirms the peculiar characteristics of each studied aquifer as poor in electrolytes and low values of pH, which are originating from much leached profile characteristical of aquifer of Pós-Barreiras and or semi-confined of the Barreiras prevailing waters sodium chloride-rich. In contrast, the aquifers of Pirabas high values of conductivity and alkalinity (434 μS.cm-1 and 200 mg.L-1 of CaCO3, respectively), ratifying the influence of the limestone in the Formation Pirabas prevailing waters calcium bicarbonate-rich. The results of diagram of stability show domain of kaolinitic’s soils, coherent fact with the Amazon geochemical areas. Geochemical interpretations and field observations suggest appreciable hydric availability, shown by abundant recharge area and chemical quality for these waters. However, it has been detected some problems related to saline intrusion (maximum observed around 33 mg.L-1 of Cl-) and to the vulnerability of aquifer from anthropogenic activities (Nova Vila 13,42 mg.L-1 NO3 - and Maracanã - well Jango, with 20; Bocal with 39; and hotel Atlanta, with 56,51 expressed in mg.L-1 NO3-). Surficial waters, hidrogeochemical characterization is discussed by sub-areas, such as spring areas, become more enriched in electrolytes during the dry season, whereas in the rainy season there is an increase in turbidity, color (maximum of 165 UC) and in organic matter (maximum in 5,8 mg.L-1) of a predominantly humic nature and a decrease of dissolved oxygen (4 mgL-1 O2) and in acidity (pH decreases to 4,8). In the transition areas changes in the values of the selected parameters are clearly observed. During the rainy season, one points out, the high acidity and low conductivity (with minimum in pH 4,8 and 21,2 μS.Cm-1, respectively) in places without any influence of tides; whereas in Peixe-Boi River the conductivity can reach 52,4 μS.cm-1. In the estuarine areas, it should be taken into account the dynamics of the tides. That effect is exemplified in the Maracanã River, under influence of oceanic waters, the electric conductivity reaches the maximum of 3130 μS.cm-1, 7,4 mg.L-1 for organic matter and chlorophyll-a evaluation maximum of 49,3 mg.m-3 in Penha (Northeastern bay). However, up stream bay (Santarém Novo) the conductivity, the organic matter (such from fluvial as from estuarine origin) and the chlorophyll-a decreases to 34,4 μS.cm-1, 6,5 mg.L-1 and 3,0 mg.m-3 respectively. The importance of the hydrologic cycle, the seasonal variations, the presence of mangroves, the cycling of nutrients, energy of the environment as a whole, permit in this context activities of sustainability, as fishing practices in different seasons, with differentiated predominance for fish, shrimps and crabs are good examples. A strategic planning for sustainability must take into account the variables discussed here.