AbstractAyutthaya is a popular tourist destination located about 76km north of Bangkok. The climate of the province is typical of Central Thailand, comprising three (cool, hot, and rainy) seasons, with relatively small changes in average temperatures but significantly varying rainfall patterns. In this study, we present a geographic information system (GIS) used for the assessment and monitoring of the pollution in the watershed. The ArcInfo-based system combines geographic, meteorologic and hydrologic data for multiple uses. Water quality management is based on composite water quality index and corresponding water quality classes. The chief input parameters include temperature, pH, faecal bacteria count (FCB), dissolved oxygen (DO), nitrate (NO3), total phosphates (PO4), biochemical oxygen demand (BOD), suspended solids (SS), and total solids (TS), together with population density, rainfall and land-use. The initial results of year 2008-2009 show that water quality derogation is strongly related to high population density in the central (new City) districts that also affect the historical City. In addition, increased runoff from the mainly northern agricultural districts carries significant PO4, NO3, and BOD pollution to the Pa Sak River in the rainy season. The relationship between diversity index of mollusks and BOD, DO and WQI show the some correlate results of diversity index(H) and the quantity of oxygen in the water(DO). The low of diversity index almost locate at area of low DO. For other water quality indexes, there are no significant results in relation with diversity index of mollusks.