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This study was carried out in the occidental part of the alluvial plain of Guilan
(Foumenat plain) to determine the potentials of the ground water resources of this area and to examine their qualitative variations. Work was started with collecting and examining the basic general data on the area: inventories concerting climatological as well as geological information. Then factors affecting water balance were determined. Numerous maps relating to area hydrogeology: piezometric cards, water table contours below the natural terrain were drown, and then attempt was made to measure the annual aquifer oscillations. With data from pumping tests, the characteristic hydrodynamics of the aquifers, that is, the transmissibility (T), storage coefficient(S), linear loss ratios of load (B) and non linear ones of load (C), representative line equation for some wells was determined. Hydrochimics and the qualitative variations were studied. The concentration of the ions Ca, Mg, Na, K, Cl, SO4, CO3, HCO3, P, N-NH4, N-NO2, NO3, Fe and certain heavy elements like Pb, Cd and Hg, as well as other parameters such as EC, pH, BOD, COD and TDS were determined. The study has shown that Foumenat plain is of high groundwater potential. The aquifers in this area consist of one free aquifer with a weak discharge and many confined or artesian ones with appreciable discharge. However due to lack of proper consideration in drilling techniques along with unsuitable equipment, exploitation discharge of the wells is not satisfactory. The chemical composition of surface as well as underground water show that the water resources often have a good quality for agriculture as well as human consumption. Interpretation of the obtained results also shows a gradual increase in electrical conductivity, Na and Cl in the general direction of water flow, but this condition does not apply to other ions. The concentration of Fe is appreciable in the southern aquifers, but it is not appreciable in others not an important quantity of nitrate is observed in the aquifers. The results of analyses do not show the existence of heavy elements (Pb, Cd, and Hg), thus the base of the international standards for clean water is established. The quantities of BOD and COD do not indicate any problems of pollution.