Determination of Temporal and Spatial Variability of Hydraulic Gradients
in an Unconfined Aquifer Using Three-Point Triangulation, Cape Cod, Massachusetts
By Timothy D. McCobb, Denis R. LeBlanc, and Kathryn M. Hess
ABSTRACT
Three-point triangulation of ground-water levels for two observation-well
networks was used to estimate horizontal hydraulic gradients in an unconfined
aquifer on Cape Cod, Massachusetts. Hydraulic gradients were estimated during
times of low and high water levels. Increasing gradient magnitudes and changes
in gradient directions with rising water levels in both networks show the
strong effects of ponds and streams on the ground-water-flow system. In the
regional network, gradients shifted towards streams as water levels rose,
reflecting an increased divergence of flow at high water levels away from
the hydraulic divide between the streams. In the local network, the magnitude
and direction of gradients near a glacial kettle pond were closely correlated
to water levels and proximity to the pond. Gradient estimates at one element
for over 15 years demonstrated that hydraulic gradient can vary temporally
and is not a constant that can be determined with a single water-table snapshot.
The accuracy of gradient estimates determined by three-point triangulation
depends on network elements that are appropriately sized to capture the curvature
of the water table and accurate measurements of water levels.