Groundwater contamination due to agricultural activities and fast industrialization is a major concern for human communities. Applying certain monitoring techniques and a groundwater-monitoring network can reveal the critical condition of these resources. The main purposes of this investigation are to present an overview of present groundwater quality, determine the spatial distribution of some groundwater-quality indices such as Cl-, SO4-2, EC and NO3-1, and select the best Geostatistical method for mapping groundwater quality in the Ghara-su Basin of Golestan Province, Iran. Also chemical variables were graphically interpreted using Piper, Durov and USSL diagrams to show the groundwater facies. Kriging and Cokriging methods are evaluated for mapping of groundwater quality. The analysis shows that universal Cokriging achieves better results for estimation of EC, Cl- and NO3-1 contents than other methods; for SO4-2, universal kriging is obviously more precise than other methods. Each method depends on the distribution of samples and the characteristics of the region. The groundwater-quality maps show that the highest concentrations of groundwater-quality indices are located to the North of Gorgan City, and results of graphically analysis of physico-chemical parameters indicated that Ca - Mg was the mainly water facies dominant in this area and the water type highly exhibit So4- Cl and most of the groundwater samples for agricultural purposes are found in C2S1 which are suitable for irrigation. A few samples in the north of the study area were unsuitable for irrigation. In general, groundwater quality decreases moving from the South to the North of the Ghara-su Basin.
