Steady-state two- and three-dimensional ground-water-flow models coupled with particle tracking (fluid-particle pathline analysis) have been evaluated to determine their relative effectiveness in delineating contributing areas and particle traveltimes to public-supply wells in two contrasting stratified-drift aquifers of Cape Cod, Massachusetts. Several contributing areas delineated by the three-dimensional pathline analysis do not conform to simple ellipsoidal shapes that are typically delineated by use of two-dimensional analytical and numerical modeling techniques. They also include discontinuous areas of the water table and do not surround the supply wells. Because two-dimensional areal models do not account for vertical flow, they cannot adequately represent many of the hydrogeologic and well-design variables that complicate the delineation of contributing areas in three-dimensional flow systems on Cape Cod, including the presence and continuity of discrete zones of low hydraulic conductivity, large anisotropic ratios of horizontal to vertical hydraulic conductivity, partially penetrating supply wells, shallow streams and lakes, and low (less than about 0.1 million gallons per day) pumping rates. Particle traveltimes calculated by the two-dimensional models are longer than those calculated by the three-dimensional models, and time-related capture zones determined by the two-dimensional models underpredict the size of the land area contributing water to simulated wells. It appears that the two-dimensional models do not accurately represent shallow, partially penetrating wells, or heterogeneous and anisotropic sediments for the purposes of simulating contributing areas and traveltimes in complex ground-water systems.
Additional Publication Details
Two- and three-dimensional pathline analysis of contributing areas to public-supply wells of Cape Cod, Massachusetts