Análises fisico-químicas e isotópicas para identificar os principais processos de salinização de águas subterrâneas em Dunas e Cristalino (Caucaia-CE)

Abstract

The main aim in this work was to find the most important groundwater salinization processes, in two hydrogeologically distinct regions (Caucaia, Ceará, NE-Brazil); one in the coast, where Dunes and Barreiras Formation predominate, and another 20 km from shoreline (inland), a crystalline basement area. Groundwater samples of 39 wells, 27 from the coast and 12 from the inland, were collected in March/14, February/15 and August/15 to analyze physicochemical and isotopes parameters. Using the average value of the three sampling campaigns, it was concluded that the different ranges of electrical conductivity values separate the groundwater from the two regions. Piper diagram reflected the different types of water, Na-Cl-type for the inland and Na-Ca-HCO3 for the coast. About potable water, 11 coastal samples had the NO− 3 concentration above that allowed for drinking-water quality, indicating anthropogenic effects, and inland, the most samples has high salinity. Statistical analysis clustered the wells according to their salinity. Gibbs diagram showed weathering dominance in both regions, as well as the evaporation process, inland. Gaillardet diagram identified: silicate weathering in both regions, but in different level; slight carbonate weathering, in the coast; and evaporite weathering, inland. Ionic ratios suggested plagioclase alteration and calcite dissolution for the coastal samples, the molar transfer was obtained by NETPATH; marine aerosols contribution and ferromagnesian minerals alteration for the inland samples, which also presented orographic influence. The relationships δ18O - δD evidenced processes of evaporation, during the recharge, in the two areas, by rainwater; d-excess showed that coastal samples suffer a greater evaporation process. Measures of δ13C showed the influence of the soil in inland samples, while in the coast, where the recharge is faster, the waters have a less interaction with the soil during the recharge. Inland, pMC data and age corrections have ensured that the waters are from recent recharges.