Groundwater quality indicators were monitored over 6 years (2007-2012) from 55 drinking water supply wells in Gharbiya Governorate (Egypt). The prime objective was to characterize, for the first time, the governorate-wide significant and sustained trends in the concentrations of the groundwater pollutants. Quality indicators included turbidity, pH, total dissolved solid (TDS), electric conductivity (EC), Cl-, SO42-, Na+, total alkalinity, hardness (total, Mg, and Ca), Fe2+, Mn2+, Cu2+, Zn2+, F-, NH4+, NO2-, NO3-, PO43-, dissolved oxygen (DO), and SiO2 contents. Detection and estimation of trends and magnitude were carried out applying the non-parametric Mann-Kendall and Thiel-Sen trend statistical tests, respectively. Factor analysis was applied to identify significant sources of quality variation and their loads. Violation of groundwater quality standards clarified emergence of Mn2+ (46%), Fe2+ (35%), and NH4+ (33%). Out of the 55 wells, notable upward trends (deterioration) were significant (>95% level) for TDS (89%), NO3- (85), PO43- (75%), NH4+ (65%), total alkalinity (62%), Fe2+ (58%), NO2- (47%), Mg hardness (36%), turbidity (25%), and Mn2+ (24%). Ranges of attenuation rates (mg/l/year) varied for TDS (24.3, -0.7), Mg hardness (3.8, -0.85), total alkalinity (1.4, -1.2), NO3- (0.52, -0.066), PO43- (0.069, -0.064), NH4+ (0.038, -0.019), Mn2+ (0.015, -0.044), Fe2+ (0.006, -0.014), and NO2- (0.006, -0.00003). Highest rates marked Tanta (total alkalinity and Fe2+), Al-Mehala Al-Kubra (TDS, Mg hardness, and NO3-), Kafr Al-Zayat (NH4+), Zifta (Mn2+), Bassyun (NO2-), and Qutur (PO43-). Precision of the trend estimate varied in goodness of fit, for TDS (86%), Mg hardness (76%), total alkalinity (73%), PO43- (67.4%), NH4+ (66.8%), Mn2+ (55%), and Fe2+ (49.6%), arranged in decreasing order. Two main varimax-rotated factors counted for more than 55% of the quality variance and, in particular, significant loads of salinity (TDS, EC, Cl-, Na+, and SO42-), followed by the alkalinity, hardness, redox potentials (Mn2+ and Fe2+), and NH4+, in decreasing order were identified. The spatial-temporal variation in pollutants originated from organic matter degradation, either naturally from the aquifer peaty sediments or anthropogenic due to improper well head protection in the urban centers or from the agricultural drains in low relief areas. Considering the latest contents of indicators and their rate of increase, the time that the permissible limits would be reached can be accurately estimated and alleviative actions could be effectively set.
Keywords: Gharbiya, Egypt; Groundwater quality; Non-parametric trend test; Spatiotemporal change.