| Abstract | This paper aims to assess the present groundwater use and quality and its contribution to agricultural productivity and potential for groundwater development for domestic purpose in the lower Chao Phraya River Basin in Thailand. The groundwater use assessment is carried out using different techniques depending on a combination of available data/information and surveys supplemented by indirect estimation methods where survey data are unavailable. The estimated abstraction influences the potential yield of the aquifer and differentiates use to yield ratio as per district-wise. The presence of GIS mapping identifies the areas of critical abstraction to recharge zones. Changes of physical and chemical parameters are evaluated in spatial and temporal scale as per drinking water standards. Susceptibility to nitrate contaminant is assessed by logistic model as a baseline measure of risk with well depth and land use pattern around the wells and depicted as probability of elevated nitrate concentrations. Variation in groundwater quality in the upper most and extensively used three layers of the Bangkok aquifer, known as Phra Pradaeng, Nakhon Luang and Nonthaburi aquifers, is assessed using Piper diagram and multi-rectangular diagram (MRD). Samples of analyzed data from monitoring wells within ±5% error of electrical neutrality balance are selected and plotted with the two diagrams. Salinity invasion is interpreted and compared with presence of critical abstraction to recharge zones. In assessment of groundwater contribution to agricultural productivity, field measurements of water use are conducted to evaluate water use efficiency and productivity of four types of agricultural farms, namely rainwater harvesting farms (RW), groundwater irrigated farms (GW), canal-water irrigated farms (CW), and canal-water with groundwater irrigated farms (CGW). Primary survey is conducted for a total of 63 farms representing the four farm types for the wet season and intensive field investigations are carried out on 16 selected GW, CW and CGW farms during 2004-2005 dry season. In order to assess economic performance, economic analysis of four farm models is carried out based on selected samples of the four agricultural farms through primary survey. The total groundwater use in 2002 is estimated as 3.624 MCM/day with 26.0, 48.8, and 25.2% of it for domestic, agricultural, and industrial purposes, respectively. A map depicting the groundwater use in the study area is prepared using the district administrative boundary. It is observed that groundwater serves 87% of rural population in the study areas for domestic water supply and about one half of the total groundwater abstraction is used for agricultural purpose. Spatially, groundwater is extensively used for agricultural purpose in the upper part of the study area while the domestic and industrial use of groundwater is concentrated in the lower part of the study area. While groundwater potential (yield) of the study area is estimated as 4.825 MCM/day or abQut 7.9% of the average annual rainfall and the map depicting the current groundwater use to yield ratio (UTY) is prepared. Results indicate that several districts are in the critical stage of groundwater development (UTY > 0.9) with a high number of agro-wells and industrial wells located in these areas. Groundwater quality is deteriorated from the past 30 years. The probability of exceeding nitrate concentration (≥3 mg/L) with well depth, percentage of surrounding priory 2-year land use is significant correlated. Piper diagrams show that the water type is in the domain of Na-K-S0₄-CI, Na-K-HC0₃-C0₃ and Ca-Mg-S0₄-CI groups in upper three aquifer layers. The seawater was observed about 40 km toward inland where the ratio of abstraction to recharge is critical. This indicates that chloride may occur from connate water present in the area. Potential for groundwater development about 25% of the districts have very good potential for further groundwater development. Managerial strategies to reduce overexploitation were the most challenge involving technical and social aspects. Results show that groundwater usage farms, GW and CGW, are the best in application efficiency (Ea), relative water supply (RWS) and water productivity. The average Ea is 85.07 and 76.81% for GW(d) and CGW(d) farms respectively. The calculated RWS is 1.18 1.48 and 1.26 for GW, CW and CGW farms in the dry season respectively. Water productivity (process depletion or productivity per unit ET) varies from 0.95, 1.02, 1.14 and 1.10 kg per m³ for RW(w), GW(w), CW(w) and CGW(w) farms respectively, while for dry seasonal crop, it ranges 1.17,0.99 and 1.02 kg per m³ for GW(d), CW(d) and CGW(d) farms respectively. The large variation occurs to term of water productivity (inflow), the lowest is 0.41 kg/m³ in CW(w) wet season which means that presence of high water applied, seepage, percolation and rainfall. The economic analysis of the four types of agricultural farm models resulted in B/C ratio of 1.20,2.63,8.21 and 3.12 for RW, GW, CW and CGW respectively |