Avaliação dos efeitos da mata ciliar sobres os fluxos hidrogeoquímicos em área de pastagem: caso das bacias hidrográficas dos igarapés do Sete e Pajeú - Paragominas (PA)

Abstract

It was studied the waters of uncofined aquifer and of portions of superficial drainage in the wartersheds of the Sete and the Pajeú rivers, in the municipal district of Paragominas, in the northeast of Pará. With total areas of around 150 and 45 Km2, respectively, those basins are mainly occupied by cattle areas. They are inserted in the sedimentary basin of Grajaú, occurring from the base to the top the Itapecuru formation, Ipixuna formation, Debris-Lateritic Paleogenic covering, Pleistocenic Sedimentary Covering and Alluvial Deposits. The chemistry of the water was evaluated in pasture areas, with and without preservation of the riparian forest that, although protected by law, is usually removed for several land uses. The waters of 9 monitoring wells were studied, installed in uncofined aquifer, and of 13 portions of superficial drainage placed in pasture areas and in riparian forest. The evaluated aquifer is constituted by sediments of the Ipixuna Formation, with kaolinite sandstones and mudstone kaulinic. Eight sampling campaigns were accomplished, during the dry and rainy periods in 2005. It was determined the contents of sodium, potassium, calcium, magnesium, nitrate, ammonium, sulfate, phosphate, bicarbonate and chloride, and in the Field pH, electrical conductivity, and temperature. In the waters of superficial drainage, the results indicate significant leaching of soils, evidenced by the higher the contents of nutrients. In the waters of superficial drainage concentrations of sulfate, potassium, magnesium and chloride were higher in the pasture areas. On the other hand, the contents of ammonium, nitrate, phosphate, calcium, sodium and bicarbonate were, in most case higher in an area of riparian forest, in which the main source of nutrients for the waters is the organic matter of the soil. I pastures these elements come from the either the limestone or the fertilizers used for pH correction and increasing of the soil fertility. Another source comes from the contribution of ashes, resulting from burning used to convert the original vegetation in pasture. In the freatic water, the contents of the cations and anions evaluated were, in general, higher in the pasture areas, except for the concentrations of sodium and chloride that were larger in the wells in riparian forest. Among the constituents analyzed, stood out, mainly, the concentrations of ammonium and bicarbonate, whose maximum values obtained in wells in riparian forest and pasture were, respectively, 0,81 and 17,6 mg.L-1 (ammonium) and 15,98 and 109,78 mg.L-1 (bicarbonate). When compared to the water of superficial drainage, it was verified that in the freatic waters most of the substances showed an increase in their contents in pasture areas. That 4 result is attributed to the fixation of nutrients by the riparian forest when the percolating water crosses the insaturated zone, while in the pastures the nutrients can reach the water table, once in those areas a significant retention of those substances does not happen. Most of the cations and anions analyzed showed higher concentrations in the wells of the point ST6, in the Sete river, where the area is of pasture and without riparian forest. The ions with higher contents were nitrate, ammonium and bicarbonate obtained in the dry period. For the alkalinity, pH and CE values were higher in pasture areas in the freatic waters. On the other hand, in the waters of superficial drainage, except for the pH, those parameters showed the highest values in the areas of riparian forest. Acid waters prevailed with pH between 5 and 6, although values close and even superior to 7 were obtained, attributed to the limestone used in the area and to the ashes, rich in bases, coming from the burning. In relation to the temperature, it was verified that it reflects the local average, 26,33oC. In the Pajeú watershed, the cation and anion contents were, in general, higher during the rainy period, while in the point ST6 in the basin of the Sete there was a relative increase in the contents of most of the ions analyzed during the dry period, due to the lowering of the water table and the low recharge of the aquifer. In the areas with riparian forest that effect was neglectable due to the small variations in the hydrological conditions along the year, resulting into a smaller annual variation in the depth of the water table.