evaluation of groundwater quality in urban area, using geostatistical methods in drought and wet periods (case study: mashhad city)

in many urban areas, groundwater has been exploited neglecting to wet and dry periods owing to of surface water deficit. in this study, using the percent of normal rainfall index, 36 year (1971-2008) were studied and a drought event and a wet event were determined. the reason to choose two consecutive years is the effect of early wet period on groundwater quality. physicochemical characteristics include: na+, tds, th, cl¯and so4¯, meanwhile 38 deep wells in the study area were sampled. after the normalization of data, variograme was drawn and for selecting suitable model for fitness on experimental variograme, less rss value was used. through deterministic methods (idw, gpi, lpi and rbf) and geostatistical estimation (ok, sk, uk, dk, dck, ock, sck and uck) using cross-validation and rmse, the best method for interpolation was selected. results showed that spatial correlation in wet periods had increased considerably and geostatistical estimation methods compared to deterministic methods were superior. water quality survey results showed that the average areas during wet periods with reduced water quality were lower and improve the quality of groundwater in the region. water quality survey results showed drought periods in a short time had affected water quality of the ground water and caused low-quality groundwater in the region.