THE URMIA COASTAL AQUIFER (NW OF IRAN) IS HOSTED IN THE ALLUVIAL DEPOSITS OF THE QUATERNARY SUCCESSION. THIS AQUIFER IS AN IMPORTANT CONTRIBUTOR TO THE FRESHWATER SUPPLY OF THE LOCAL AREA. BASED ON GIBBS PLOT, THE BEHAVIOR OF THE MAJOR IONS SHOWED THAT THE CHANGES IN THE CHEMICAL COMPOSITION OF THE GROUNDWATER HAVE MAINLY CONTROLLED BY THE WATER/ROCK INTERACTION ZONE AND FEW SAMPLES RELATIVELY BY EVAPORATION. THE SI CALCULATIONS SHOWED THAT ALL GROUNDWATER SAMPLES COLLECTED IN THESE PERIODS SHOW NEGATIVE SATURATION INDICES, WHICH INDICATE UNDER-SATURATION WITH RESPECT TO ANHYDRITE, GYPSUM, AND HALITE. IN ADDITION, EXCEPT IN A FEW CASES ALL OTHER SAMPLES SHOWED THE UNDER-SATURATION RESPECT TO THE CARBONATE MINERALS SUCH AS ARAGONITE, CALCITE, AND DOLOMITE. THEREFORE, THESE MINERALS ARE SUSCEPTIBLE TO DISSOLUTION. IN THE DRY SEASON, THE SI CALCULATIONS SHOW THE MORE POSITIVE VALUES RESPECT TO DOLOMITE, ESPECIALLY IN THE NORTHERN PART OF URMIA AQUIFER (UA), WHICH INDICATE THE HIGHER POTENTIAL FOR PRECIPITATION, AND DEPOSITION OF DOLOMITE. THE PERCENTAGE OF SALTWATER IN THE GROUNDWATER SAMPLES OF URMIA PLAIN IS VERY LOW, RANGING BETWEEN 0.001% AND 0.79% IN THE WET SEASON AND 0.0004% AND 0.81% IN THE DRY SEASON. EVALUATION OF CHEMICAL TYPES OF WATER SAMPLE INDICATE THAT MOST GROUNDWATER SAMPLES COLLECTED FROM THE SAME WELL DURING DIFFERENT PERIODS IN THE STUDY AREA STILL FELL INTO THE SAME TYPE, WHICH CAN INDICATE THAT THE SEASONAL VARIABILITY DID NOT AFFECT THE GROUNDWATER MUCH. THE EVIDENCES SUGGEST THAT EXCEPT FOR A FEW POINTS NEAR THE COAST, WHICH SHOW THE SALTWATER ENCROACHMENT, THERE IS A LITTLE HYDRAULIC INTERACTION BETWEEN UA AND URMIA LAKE (UL).