In order to evaluate groundwater quality for irrigation purpose within Bantul area situated on Yogyakarta-Sleman Groundwater Basin, 47 existing data points were used complementary with the results of analysis from 30 groundwater samples which were collected from dug and tub wells in various locations of study area. ECw, TDS, pH and cations: Na + , Ca 2+ , Mg 2+ , K + , and anions: Cl - , HCO 3 - , SO 4 2- , NO 3 - were analyzed from groundwater samples. 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 samples fall under class I suggested that groundwater 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) values plotted in the USSL diagram, the majority of the groundwater samples belong to C2-S1 and C2-S2 class, 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 in this area. 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 thematic maps produced from 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. Key words: Irrigation groundwater quality, total dissolved solids (TDS), electrical conductivity (EC), ArcGIS Spatial Analyst, FAO guideline, yield potential (YP). Determination of Groundwater Quality for Agriculture by Using GIS Application, Bantul Regency, Yogyakarta Special Province, Indonesia Chhuon Kong 1 , Heru Hendrayana 2 , Agung Setianto 3 . Geological Engineering Department, Faculty of Engineering, Gadjah Mada University Jl. Grafika No.2 Yogyakarta 55281 Indonesia 1 chhuon.k@gmail.com, 2 heruha@yahoo.com, 3 agung.setianto@gmail.com AUN/SEED-Net (ASEAN University Network/Southeast Asia Engineering Education Development Network) Department of Geological Engineering Faculty of Engineering Gadjah Mada University Abstract Location of Research Area Figure 02: Flow Chart of Research Methodology Groundwater Quality For Agriculture 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 productivity. Availability of adequate good quality water is one of the most important inputs in successful crop production. As the anthropogenic activities also influence the role of evaporation, leading to an increase in Na + and Cl – and thus total dissolved solids (TDS). It is therefore obvious that the groundwater contribution is a significant component of water balance and should be recognized as providing part of the water needed by the crop for evapotranspiration. Geographic Information System (GIS) application is very necessary to be integrated use with hydrogeology and agriculture for facilitating the generation and use of thematic information, has been applied and analyzed for identification of groundwater quality suitable zone for domestic and irrigation purposes. It is one of the best and fastest systems for this objective (Ganapathy & Ernest, 2004). The study area is located in the Java Island and administratively covering the share of 5 districts with the total area approximately 70.743 Km 2 within Bantul Regency, Yogyakarta Special Province (DIY), Indonesia (Figure 01). Topographically, the study area is located on the Lower Slope Merapi Volcanic Zone in the almost flat area. Its lithology is mostly resulted from volcano activity; other part is Tertiary clastic sediment. It consists of limestone, young volcanic deposits, and alluvial and old andesite volcanic deposits.  Identify the features or parameters of water quality which restrict the crop yield influenced by irrigation water in the research area.  Evaluate the groundwater quality suitable for crop influenced by irrigation water referring to existing method of classification and FAO Guidelines for agriculture water quality and identify geological condition controlling groundwater quality.  Generalize the maps of interpolated features of groundwater quality by using ArcGIS Spatial Analyst Tools and determine the suitable area. Define the area in which crop yield is already being restricted by the use of groundwater for Objectives Research Methodology In order to evaluate and determine the suitable quality of groundwater for crops influenced by irrigation, water quality data is indispensably needed. Analyze the composition of groundwater and its interaction with rock and use Schoeller diagram to know the relative tendency of dissolved ion in groundwater. Plot the Na/Ca versus Mg/Ca molar ration of Han and Liu, 2004 to evaluate the groundwater and rock type association. Features that generally were considered to determine of the suitable quality of groundwater for irrigation are salinity, water infiltration rate, and specific ion toxicity. So that, EC w , major ions Ca 2+ ,Mg 2+ , Na + , K + , HCO 3 - , SO 4 2- , Cl - , SAR, TDS, pH were analyzed as the evaluation indexes of water quality for irrigation purpose. SAR, Na% and IP are calculated by the mathematical formula: Furthermore, using classification method such as Richards (1954) rating the water quality for irrigation purpose based on ECw and SAR, U.S. Salinity Laboratory diagram (USSL, 1954), Wilcox Diagram Interpretation (1948), and Doneen’s Permeability Index (PI) to evaluate the groundwater quality for irrigation purpose.     2 me/l me/l me/l 2 2      Mg Ca Na SAR   100 %       K Na Mg Ca K Na Na 100 3      Na Mg Ca HCO Na IP Background Based on Wilcox 1995 classification, 75.86% of samples equal 76.27% of the study area falls in moderately saline water while the rest 24.13% of samples with area approximately 23.64% is classified in Medium to High saline water. U.S. by Richards (1954), the majority of samples fall under the category of C2-S1 and C2-S2 class, indicating good to medium typed groundwater which is suita- ble for irrigation. According the classification of PI by Doneen’s graphic (1964), all groundwater samples fall in the class I, suggest that groundwater is good and suitable for irrigation. Wilcox 1948 also classifies groundwater by Na%; groundwater quality in all most the whole study area is classed in "excellent". Accordingly, sodium hazard and permeability in the study area did not cause the problem on crops. However, it has some restriction due to the salinity problem that indicated via ECw. The method to create the Thematic Maps of suitable groundwater quality for agri- culture is covering the four main processes of GIS works. These working processes have been modeled in the Figure 03.  This study indicates that salinity is the significant parameters re- strict the crop yield influenced by irrigation water in the study area.  However, the general evaluation groundwater quality in the study area is in good rank for irrigation purpose. Groundwater quality con- stituent are driven by the contribution of dissolved carbonate rock of Sentolo Hills which are rich in major ions.  Good groundwater quality for crops is from the middle to the east of study area close to Opak River. Despite of groundwater quality in the west of the Bantul effects on in situ irrigation water, but it has no restriction on yield potential of tolerant crops such as rice. However, this groundwater at the west has slightly to moderately restriction on sensitive crops or horticulture such as common bean and other vegeta- tive crops. Figure 03: Conceptual Model of GIS working process Figure 01: Location of Study Area Table 01: Suitability of Groundwater Quality Description Acknowledgement I would like to thank to Indonesian Government and AUN/SEED-Net project who support in this research. I would to extend my special gratitude to both of my advi- sors Dr. Heru Hendrayana, and Dr. Agung Setianto, who supervise and encourage me to complete this research works. References [1]Doneen L. D. (1964). Notes on water quality in agriculture. Published as a Water Science and Engineering Paper 4001, Department of Water Science and Engineering, University of California. [2]Ganapathy, C., & Ernest, A. (2004). Water Quality Assessment Using Web Based GIS and Distributed Database Management Systems. Arch.: Environ. Inform. [3]Han, G., & Liu, C. Q. (2004). Water geochememistry controlled by carbonate dissolution : a study of the river waters draining karst dominated terrain, Guizhou Province, China. China Chemical Geology, 204, 1-21. [4]Langmuir, D. (1997). Aqueous environmental geochemistry, McConnin RA (ed). Englewood Cliffs, NJ.: Prentice-Hall. [5]Richards, L. A. (1954). Diagnosis and improvement of saline and alkali soils. USDA Agricultural Handbook No. 60. Washington DC: US Department of Agriculture. [6]Schoeller, H. (1977). Geochemistry of groundwater, In Groundwater Studies - An International Guide for Research and Practice. Paris: UNESCO. [7]Wilcox, L. V. (1955). Classification and Use of Irrigation Waters. Washington D C: USD Circular No. 969. Conclusion Water and Rock Interaction Geochemistry of groundwater is discussed by means of it majors ions. Schoeller (1977) diagram provides a convenient way to present chemical composition of groundwater of a region. Plots of average composition, both in mg/l and epm are presented in Figure 04. The relative tendency of ion in epm shows Ca 2+ >K + >Na + >Mg 2+ and anion HCO 3 - >Cl - >SO 4 2- . Han and Liu (2004) have used the variations in the compo- sition of water (Mg/Ca versus Na/Ca in molar ratio) to dis- tinguish limestone, dolomite and silicate rock sources of ions. From 36 samples of the study area are close to silicate rock with influence of carbonate rock of area or its vicinity (Figure 04). The studies samples have Na/Cl molar ratio 0.59 to 6.35, suggesting that some Na is derived from Na- bearing rock or exchange with Ca (Langmuir, 1997). Figure 04: Schoeller diagram showing average composition in mg/l and epm of groundwater in the study area. Stiff diagram is showing in inset. Figure 05: Plots of Na/Ca vs Mg/Ca molar ration of groundwater in the study area (after Han and Liu, 2004). Degree of Restriction on Crops and Relative Yield Potential (YP) None/YP=100 % Slight/YP=90 % Moderate/YP=75 % 2727.90 ha 735.49 ha 208.98 ha Groundwater is very good in term of quality for agriculture. It has no restriction on all kind of crops which its yield can be potentially reached 100% in term of irriga- tion water quality con- cern. Groundwater quality is still in good rank for agriculture. How- ever, it has slight re- striction on sensitive crops which its yield can be obtained ap- proximately 90% in term of irrigation wa- ter quality concern. Groundwater quali- ty is fair for agri- culture. It has mod- erate restriction on sensitive crops which its yield can be obtained only approximately 75% in term of irrigation water quality con- cern. Figure 06: Suitability of groundwater quality for agriculture and An analysis groundwater quality is essential for a proper understanding of the present environmental problems and geological condition controlling. The groundwater quality thematic maps, which are prepare, based on the spatial da- ta analysis shows that the GIS assisted database system would help to apply groundwater management practices such as; proper groundwater resource management in terms of groundwater quality and quantity, integrated man- agement of water, land use and the environment. The pro- duced groundwater related database could help as infor- mation source to institutions, researchers, or other relevant sectors.