GROUNDWATER ANALYSIS USING A GEOGRAPHIC-INFORMATION-SYSTEM FOLLOWING FINITE-DIFFERENCE AND FINITE-ELEMENT TECHNIQUES

Groundwater analysis involves understanding both spatial and temporal changes. Changes in many different types of media have been modeled in the past with finite-difference and finite-element meshes. In the sam e way, groundwater can be represented as a finite-element analysis pro blem. This document describes the approach in which a Geographic Infor mation System (GIS) was used to evaluate changes in groundwater levels resulting from a dam impoundment. This study focuses on areas of land use impact caused by changes in groundwater conditions. A finite-eleme nt-type model created potentiometric steady-state surfaces. The comput er software converted the finite elements to finite differences for ra ster based GIS multiconditional analysis. The raster GIS divided the s tudy area into 50 m grid cells, approximately equal to the area of one acre. The study was limited to the alluvial valley boundary. Pre-dam and post-dam data sets were compared to evaluate causes and effects of the dam, and on landuse. Data sets were obtained for record lows and highs both before and after the dam was constructed. Analysis revealed the dam caused changes in the low groundwater regime but not in the h igh water levels. Once effects of the dam to the groundwater regime we re understood, grid cells were compared against landuse critical thres hold values to determine areas of probable impact. Graphic results inc luded a series of potentiometric maps, pre-dam and post-dam depth to g roundwater maps, cross valley groundwater profiles, effected areas map s, and possible impact maps. The GIS provided a management tool to sel ect areas for future site specific assessment.