Groundwater analysis using a Geographic Information System following finite-difference and -element techniques

Abstract

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 same way, groundwater can be represented as a finite-element analysis problem. This document describes the approach in which a Geographic Information System (GIS) was used to evaluate changes in groundwater levels resulting from a dam impoundment. This study focuses on areas of landuse impact caused by changes in groundwater conditions. A finite-element-type model created potentiometric steady-state surfaces. The computer software converted the finite elements to finite differences for raster based GIS multiconditional analysis. The raster GIs divided the study 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 high water levels. Once effects of the dam to the groundwater regime were understood, grid cells were compared against landuse critical threshold values to determine areas of probable impact. Graphic results included a series of potentiometric maps, pre-dam and post-dam depth to groundwater maps, cross valley groundwater profiles, effected areas maps, and possible impact maps. The GIS provided a management tool to select areas for future site specific assessment.