*Corresponding Author’s Email: chhuon.k@gmail.com GROUNDWATER QUALITY ASSESSMENT FOR AGRICULTURE BY USING GIS APPLICATION, BANTUL REGENCY, DIY-INDONESIA Heru Hendrayana, Chhuon Kong, Agung Setianto Faculty of Engineering, Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia Abstract In order to evaluate groundwater quality for irrigation purpose within Bantul area situated on Yogyakarta-Sleman Groundwater Basin, 77 data points of groundwater quality within the study area were used. The relative tendency of ion in epm shows Ca 2+ >K + >Na + >Mg 2+ and HCO 3 - >Cl - >SO 4 2- . Variations in groundwater composition using Mg/Ca vs. Na/Ca in molar ratio, indicates groundwater is close to silicate rock with influence of clastic carbonate rock. Higher salinity approach to the west of the Bantul, indicates that groundwater quality is controlled by clastic carbonate rock of Sentolo Hills. Groundwater fall under class I suggested that is good and suitable for irrigation based on Doneen’s classification of Permeability Index and 78.37% is in excellent category by Wilcox classification on Sodium Percentage (Na%). According to the Sodium Adsorption Ratio (SAR) plotted in the USSL diagram, the majority of the groundwater samples belong to class C2-S1 and C2-S2, indicating medium salinity and low sodium water which can be used for irrigation with little danger. Salinity is the significant problem has slight to moderate restriction on crops productivities. The ECw map indicates good groundwater quality for crops is at the middle to the east of study area close to Opak River, while at the western effects irrigation water but no reduction on rice productivity. The suitability of groundwater quality for agriculture is determined by using ArcGIS Spatial Analyst based on FAO guideline. The slight to moderate restriction on sensitive crops indicating with different yield potential in the agricultural land in which 100%, 90% and 75% of yield potential correspond to about 2727.90 ha, 735.49 ha and 208.98 ha of the study area respectively. Keywords: Irrigation Groundwater Quality, Total Dissolved Solids (TDS), Electrical Conductivity (EC), ArcGIS Spatial Analyst, FAO Guideline, Yield Potential (YP). I. Introduction Groundwater plays a major role in augmenting water supply to meet the ever - increasing domestic, agriculture, and industrial demands. The chemical composition of water is an important factor to be considered before it is used for domestic or irrigation purpose. Characteristics of water quality through physical, chemical and biological definition influence the crop production. Salts in soil or water reduce water availability to the crop to such an extent that yield is affected (Bernstein, 1975)[1]. Relatively high sodium or low calcium content of soil or water reduces the rate at which irrigation water enters soil. Certain ions (sodium, chloride, or boron) from soil or water accumulate in a sensitive crop to concentrations high enough to cause crop damage and reduce yields. Excessive nutrients reduce yield or quality; unsightly deposits on fruit or foliage reduce marketability. Availability of adequate good quality water is one of the most important inputs in successful crop production.