Dynamics of water table level and groundwater salinity at an irrigated field

ABSTRACT

Physical and hydro chemical analysis were carried out in shallow wells with the objective of identifying the impact of irrigation and of the climatic seasonality in the process of salinization and in dynamic of the groundwater level of the Distrito Irrigado do Baixo Acaraú - DIBAU, Ceará. The samples were taken in two periods: Dec/2003 to May/2007 and Aug/2010 to July/2011 in five shallow wells (P1, P2, P3, P4, P5), three being inserted into irrigated fields. In identifying the similarities or dissimilarities of the sodium (Na⁺⁾, chlorine (Cl^-) and nitrate (NO₃ ^-) ions between the two studied time periods and between the sampled wells, statistical multivariate analysis of hierarchical cluster analysis was applied. The average data were submitted to the average test and confronted through the t-test. The waters of the unirrigated fields presented the highest average concentrations of Na⁺ and Cl^-, 123 mg L^-1 and 253.2 mg L^-1, respectively, while the highest average values of NO₃ ^- (16 mg L^-1) were found in the irrigated area. An elevation of the level of the water table was identified in the wells inserted in the irrigated area (P3, P4 and P5) of 6.13; 5.25 and 3.40 m, respectively. The smallest seasonal fluctuations of the water table occurred in the irrigated area wells, showing an excess of irrigation water, which upper the groundwater level during the dry season. The concentrations of Na⁺ and Cl^- in the groundwaters are determined by natural factors, differently from the NO₃.

Key words:
irrigation; contamination; groundwater; nitrate