Evaluation of an inactive cesspit contamination in the unsaturated zone of the Adamantina Aquifer in Urânia (SP)

Author(s)

Claudia Luciana Varnier

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2007-04-24

Examining board members

Ricardo Cesar Aoki Hirata; Reginaldo Antonio Bertolo; Rodrigo Cesar de Araujo Cunha; Chang Hung Kiang; Jefferson Mortatti

• Advisor: Ricardo Cesar Aoki Hirata

Abstract

The present work studied the impact of nitrate from an inactive cesspit to the unsaturated and saturated zones of the Adamantina Aquifer (Urânia, São Paulo, Brazil), through the hydrogeochemistry of major ions (including the nitrogen series), minor ions, stable isotopes (15NNO3, 15NN2O, 18ON2O, 18ONO3, 18OO2, 13CCO2) and gases (O2, CO2, N2O and CH4). The cesspit, which has been inactive since 2002, was monitored for three years after the construction of a monitoring station. The station is comprised by a 11.20m deep dug well, in which 12 tensiometers and 12 suction lysimeters were installed from the surface down to the saturated zone (0.5-9.0 m). A monitoring well was installed at the bottom of the monitoring station. The station was used to examine the hydraulic processes and the quality of the infiltrating water. The results from the hydraulics of the water infiltration show that the unsaturated zone presents average hydraulic conductivity between 3.1x10-6 and 1.4x10-5 m/s. The capillary tension varies laterally and vertically, which is attributed to the heterogeneities of the geological material. These results allowed the identification of three different hydraulic zones: the first one, between the depths of 0.5 and 2.0 m; the second one, between 2.0 and 7.0 m and the third one, from the depth of 7.0 m down to the water level. The recharge rate of the area is 171 mm, which corresponds to 13% of the annual precipitation. The velocities of the wetting fronts indicated a travel time of 90 days for the water to move between the depths of 0.5 m and 9.0m. The chemical analyses of the water of the unsaturated and saturated zones indicated high concentration of nitrate, along the entire unsaturated zone (up to 2028,94 mg/L). The physico-chemical, isotopic and gases data vary seasonally and spatially. The concentration of gases, isotopes and nitrogen species show a ?zig-zag? behavior, with peaks of concentration at the depths 2.0; 4.0 and 6.0 m. This behavior is a consequence of the textural features of the geological material, with less permeable areas occuring at these depths, which can restrict the access of oxygen- and gases-rich waters along the unsaturated zone and the coexistence of reducing zones among oxidizing ones (microcosms). The detailed monitoring provided data to establish the main hydrogeochemical processes occuring in the unsaturated zone: i) oxidation of organic matter, ii) ammonification, iii) nitrification, iv) methanogenesis and v) denitrification. A very important fact is the simultaneous occurrence of nitrification and denitrification processes in the unsaturated zone. This fact is verified by a series of observations, which can be demonstrated by the following relationships: i) 15NN2O and 18ON2O; ii) 15NN2O and 15NNO3 and iii) 18ONO3 and15NNO3. The values obtained for ?15NNO3 are between +6.80 and +30.09? (wet season) and between +9.54 and +23.25? (dry season). The values of ?15NNO3, which are more enriched than the isotopic fingerprint for sewage (+8.1? to +13.1?), along with the results for ?18ONO3 (-3.60 to 4.50 ?), the fractionation ratio of this isotopes and the linear regression coefficient, obtained by other authors, reinforced the coexistence of denitrification and nitrification in the studied area. The results of ?15NN2O and ?18OO2 also indicated the presence of denitrification. The signatures of these two isotopes (-16.16 to -11.94? and 28.05 to 30.69?, respectivelly), compared to values reported by other authors (-37 to -11? and -21 to +57?), indicate that the N2O detected in the area is produced by this process. However, it is worth emphasizing that the use of 18OO2 and 15NN2O isotopes is still a very new technique in hydrogeological studies, and more studies are needed to establish the isotopic fingerprints of the nitrification and denitrification processes, considering the different sources of nitrate contamination. (AU)