Abstract <p>Groundwater monitoring of an aquifer performed by an observation well network plays an important role in the management of groundwater resources in which, if the number of wells is small, the calculation of the groundwater level is less accurate, and if the number of observation wells is large, the cost of drilling and maintenance of wells and the cost of measurement will increase. This study aimed to identify observation wells’ number and optimal density in an aquifer using classical statistics and geostatistical techniques. Sarfiroozabad aquifer with 45 observation wells was selected as a case study. In this aquifer the effective range of each observation well was determined through examination of the coefficient of variation of groundwater level in parallel profiles and perpendicular profiles to the longitudinal axis of the aquifer, and examining variograms related to groundwater level variations by specifying the optimal number of observation wells. Classical statistical calculations and geostatistical computations showed the same effective range for wells in the northern aquifer. In areas where there are few samples, the classical statistics method, which was presented in this research can be used favorably and practically. To obtain the optimal number of observation wells, at least 6 observation wells shall be added to the northern aquifer, while only one well should be added to the southernmost part of the southern aquifer and six wells should be eliminated to reduce the cost of sampling. Also, by adding essential wells, the accuracy of groundwater level estimation increases which helps manage groundwater resources better.</p>