OBSERVATION NETWORK DESIGN FOR SELECTING LOCATIONS FOR WATER-SUPPLY WELLS

An observation network design method, which simultaneously minimizes parameter uncertainty and exploration cost, is developed for locating water supply wells. Observation networks combining wells and geophysical measurements are considered for delineating the 'best' areas for wells. An area is considered the 'best' where the sustainable pumping rate and the degree of protection for the well are the possible maximum. The level of protection is measured by the travel time for a fixed radius protection zone around a well. The 'most appropriate' measurement network is selected based on two often conflicting objectives: (1) minimization of the uncertainty in characterizing the spatial distribution of the expected yield and travel time; and (2) minimization of the observation cost. The accuracy of estimation is characterized by the average and maximum kriging variance for yields and travel times calculated from measured hydrogeologic parameters. The parameters to be measured by an observation network for estimating yields and travel times are layer thicknesses, porosity, hydraulic conductivity and volumetric water content. They may be obtained from direct measurements (e.g. well logs, specific capacity test, and pumping tests) or estimated indirectly from geoelectric measurements. The approach combines two common techniques, geostatistics and multi-criterion decision making (MCDM), to determine the 'best' measurement network. Geostatistics are used to determine the spatial distribution of the estimation variance for yield and travel time calculated from different sources of data. MCDM is utilized to evaluate observation network alternatives based on the estimation variances and the costs associated with them. The methodology is illustrated for a study area located near Ashland, Nebraska.