Today, observation wells are important tools in investigating groundwater pollution and its mechanisms. Particularly, geogenic groundwater pollution associated with water-rock interactions could originate from distinct layers and different depths. The use of production wells in monitoring groundwater pollution often results in determining a depth-averaged mixed water quality. Thus, such wells do not permit the determination of actual depth and layer of pollution. Since the primary objective of production wells is to maximize water extracted from underground, filter zones are used in all possible formations from which water could be obtained. Consequently, water samples taken from such wells represents a mixed quality of water that inflows to the borehole from all depths and it is not possible to make a clear identification of the actual depth where pollution originates. However, the important point in groundwater monitoring activities is to clarify exactly where the pollution originates by using scientifically consistent methods, which necessitates the use of specifically equipped observation wells. The correct approach to installing these wells is to take core samples during well installation, to determine the rock/sediment lithology of the well cross-section, to select the groundwater sampling and filter depth by using this lithological characterization and to properly isolate other sections of the well. In this regard, 22 boreholes with a total depth of 846 m were drilled in 15 different locations in the surface aquifer of Simav Plain to investigate arsenic pollution. Of these 22 boreholes, 21 of them were converted to observation wells. The 15 locations were homogeneously distributed within the plain while dual wells that goes down to two different depths were installed in four locales, and a triple well that goes down to three different depths was installed in one location. Based on the lithological characterization of core samples from all locations, different sampling depths and filter separations were determined for each borehole. Upon completion of the wells, well heads are covered and protected. Periodic water samples are being taken from each one of these wells by special sampling equipments that are lowered to the associated filter depth. Furthermore, groundwater level measurements are taken from all wells in selected intervals and from some wells in a continuous manner. Based on the results obtained from these studies, it could be concluded that groundwater samples obtained from such specifically designed monitoring wells are more informative in identifying the problem and provide much clearer information regarding the depth from which arsenic pollution originates.