Avaliação dos compostos nitrogenados no aquífero livre em agroecossistemas de citros no alto do rio Capitão Pocinho, Amazônia oriental

Abstract

The objective of this research was to define the vulnerability to pollution of the unconfined aquifer Barreiras and to characterize the hydrochemistry and study the temporal dispersion and nitrogen compounds in shallow groundwater in an area of intensive citrus cultivation, located in the headwaters of the river Capitão Pocinho, of the municipality Capitão Poço, in northeastern Pará State. Material and methods used to develop this work consisted of: a literature review, surveying, drilling of 16 monitoring wells, one well located in the reference area of environmental protection, used to define hydrochemical background of regional; sampling of unconsolidated sediments for particle size analysis as well as for undisturbed sediments in order calculate the porosity in the laboratory; infiltration test and slug-test, to estimate hydraulic conductivity of unsaturated and saturated zones of unconfined aquifers; monitoring static level, by systematically analyzinge physical and chemical samples of shallow groundwater during the months of January, March, May, July, September and November 2009. Laboratory physicochemical tests (pH, electrical conductivity, Ca2+, Na+, K+, Mg2+, Cl-, SO4 2-, CO3-, HCO3- , NH4+ e NO3-) were performed at the Instituto Evandro Chagas and the physical test (grain size and porosity ) in the Soils Department of the Federal Rural University of Amazonia. In this study, we used the following technical software: ArcGIS 9.0, SURFER 8, QUALYGRAF and TAL Windows. The results show that: 1) the potentiometric level oscillations are around 2 meters, 2) the direction of shallow groundwater flow follows the direction of the town of Capitão Pocinho, 3) the vulnerability of shallow groundwater DRASTIC and Pesticide DRASTIC pollution was classified as high to very high, 4) hydrochemical values obtained in the pit of reference (background) are compatible with those of the aquifer of Tertiary age Barreiras, 5) shallow groundwater were classified as calcic bicarbonate and mixed , 6) the waters of the unconfined aquifers showed indicators of nitrate contamination and; 7) the process of nitrification occurred in 60% of wells monitored during the hydrological cycle in the year 2009. The results derived from statistical analysis confirmed the change in content of Na+, K+, Ca2+, Mg2+, HCO3- and NO3-. These ions showed a statistically significant difference, whereas for the remaining ions multivariate analysis the relationship was not statistically significant, for intervals of 95% for the population mean of each variable. This indicates a trend of increasing concentrations of Na+, K+, Ca2+, Mg2+, HCO3- and NO3-, with the passage of time. It was observed that in 60% of wells (PM02 PM05, PM06, PM07, PM08, PM10, PM11, PM13 and PM14) there is an increase of the latter constituent from one time to another, while occurring decrease of NH4 +, which may suggest the oxidation of NH4+, resulting in NO-3 (nitrification). Therefore, the shallow groundwater used for human consumption in the village of Capitão Pocinho are of dubious potability due to the presence of nitrate.