Avaliação dos impactos da ocupação urbana sobre as águas da bacia hidrográfica do igarapé Mata Fome, Belém-PA

Abstract

The watershed of the Mata Fome stream, with 6 km2, is located to the north of the Metropolitan Region of Belém. It is the pilot area for the implementation of the UN's Urban Management Program (PGU), which has among its objectives the environmental recovery of this area. The present study, inserted in the above mentioned program, evaluates alterations related to the anthropic occupation of the basin, through hydrogeological and water quality data from the stream and the free aquifer. In the analyzes of the waters carried out in the dry (November) and rainy (April) periods of 2000, the nitrogen components (NH4+, NO2-, NO3-), dissolved oxygen (OD), total dissolved solids (STD), fecal coliforms and total values, pH and electrical conductivity (EC). The hydrological regime of the stream was evaluated through discharge measurements and readings of limnimetric rulers, which were installed in an intensely occupied area, at the mouth of the stream, under the influence of the tides (station 1), and at its source, relatively preserved and without tidal influence (station 2). The stream showed a regime characterized by 9 hours of low tide and 4 hours of high tide, reaching higher water level heights in the rainy season (January-June), in the high tide, with a maximum height of about 3 meters, and a minimum of 0.4 meters at low tide. In the dry season (July-December) the maximum height, 1.99 m, also occurred at high tide, while the minimum height, 0.40 m, occurred at low tide. the net discharge from the stream was measured only in the dry period and presented minimum and maximum values ​​of 0.03 and 0.201 m3/s, respectively, with the minimum value occurring in the transition from the ebb to flood regime. Double cylinder infiltration tests, carried out in the occupied area and at the source, revealed a rapid stabilization of infiltration in the first area, in relation to the most preserved area. The water from the stream showed pH values ​​close to or greater than 7, the highest being obtained in the dry period. The electrical conductivity was also higher in this period, with an average of 260 µS/cm. The DO contents, quite low, varied between 1.0 and 3.5 mg/L, with the highest values ​​obtained in the rainy season, possibly resulting from a greater oxygenation of the water in this period. The presence of domestic waste and sewage in the stream is mainly portrayed by the high amount of fecal coliforms that, in station 1, during the rainy season, reach a maximum of 92,000 CF/100 mL, at high tide. In the spring area, although relatively preserved, the CF values ​​were also high, reaching a maximum of 65,000 CF/100 mL in the dry period. Among the nitrogen components analyzed, the NH4+ contents stand out, reaching values ​​above 3 mg/L, reaching 12 mg/L in station 2, during the dry period, reflecting a small "invasion" that was beginning to settle in that area. area. The load carried by the stream evaluated at station 1, only for the dry period, presented higher values ​​in the ebb tide, due to higher discharges in this tidal regime. Nitrate discharge was the highest, reaching a maximum of 0.44 µg/sec in the ebb tide regime. The ionic balance (t+km-2+year-1) showed positive values ​​for all analyzed parameters, indicating a greater outflow of substances to the Guajará Bay, in relation to the solutes brought from this bay, during the flood tide. According to CONAMA Resolution No. 20/86, the water in the creek is unsuitable for bathing (primary contact recreation). The free aquifer studied, the main source of supply for the residents of the area, has a static level with an average depth ranging from 2.26 m to 1.21 m, between the dry and rainy seasons, respectively. Hydraulic potential maps, prepared from static level measurements carried out in 30 excavated wells, in these two periods, indicate that the underground flow converges to the stream. The regulatory reserve, calculated from the natural flow rate (VEN), presented a value of 1,050,000 m3, with a restitution for the stream of 175,000 m3/Km2. Among the water quality indicators evaluated, the ammonium levels are worth mentioning, reaching 3.54 mg/L, well above the potability limit (0.06 mg/L) established by USEPA. The nitrate content reached 30 mg/L, still below the potability limit (45 mg/L), but already deserving attention, due to its conservative character. The presence of fecal coliforms in the water of some analyzed wells also indicates that their water is unfit for human consumption.