| Summary: | A method was developed for assessing the vulnerability
of groundwater to contamination from contaminant sources
at the soil surface, using a numerical groundwater flow
model linked to a digital map database. The method was
applied using the pcARC/INFO Geographic Information System
(GIS) to input, store, and manipulate base maps, resulting
in a database of digital maps for the alluvial aquifer
system in the Willamette Valley of western Oregon. Digital
elevation maps were created by digitizing topographic
maps of land surface (1:250,000 scale), water surface, and
the base of the Tertiary-Quaternary sedimentary deposits
(1:500,000 scales). Soil association and aquifer unit
maps digitized from 1:500,000 scale map sheets were also
used. Data were extracted from ARC/INFO to the SURFER
software package to create a 3-D surface model for each of
the digital elevation maps. An ARC/INFO point coverage
was then used to store and overlay these surfaces, allowing
the creation of maps of depth to water, saturated
thickness, and water table gradient. These data became
the input to a numerical finite element groundwater flow
model. The model solves a dual formulation problem for
the potential function and the stream function to calculate
the time-of-travel for water to flow from the surface
to the water table and laterally for 100 meters as an
index of groundwater vulnerability. A cluster analysis is
used to condense the data and form a training data set for
a multiple regression model. The regression model is fit
to the results of the finite element model with an
R-squared of greater than 0.96. The simpler regression
model is then used for mapping travel times for the entire
study area. When properly calibrated against the finite
element model and when combined with the digital map database
and Geographic Information System (GIS) procedures
described, the regression model can be conveniently used
to assess the vulnerability of groundwater to contamination
over large areas. === Graduation date: 1989
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