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.