PATTERNS AND AGE DISTRIBUTION OF GROUNDWATER-FLOW TO STREAMS

Simulations of ground-water flow in a generic aquifer system were made to characterize the topology of ground-water flow in the stream subsy stem and to evaluate its relation to deeper ground-water flow, The flo w models are patterned after hydraulic characteristics of aquifers of the Atlantic Coastal Plain and are based on numerical solutions to thr ee-dimensional, steady-state, unconfined flow, The models were used to evaluate the effects of aquifer horizontal-to-vertical hydraulic cond uctivity ratios, aquifer thickness, and areal recharge rates on flow i n the stream subsystem, A particle tracker was used to determine flow paths in a stream subsystem, to establish the relation between ground- water seepage to points along a simulated stream and its source area o f flow, and to determine ground-water residence time in stream subsyst ems. In a geometrically simple aquifer system with accretion, the sour ce area of flow to streams resembles an elongated ellipse that tapers in the downgradient direction, Increased recharge causes an expansion of the stream subsystem, The source area of flow to the stream expands predominantly toward the stream headwaters. Baseflow gain is also inc reased along the reach of the stream, A thin aquifer restricts ground- water flow and causes the source area of flow to expand near stream he adwaters and also shifts the start-of-flow to the drainage basin divid e, Increased aquifer anisotropy causes a lateral expansion of the sour ce area of flow to streams. Ground-water seepage to the stream channel originates both from near- and far-recharge locations. The range in t he lengths of flow paths that terminate at a point on a stream increas e in the downstream direction, Consequently, the age distribution of g round water that seeps into the stream is skewed progressively older w ith distance downstream. Base flow ia an integration of ground water w ith varying age and potentially different water quality, depending on the source within the drainage basin, The quantitative results present ed indicate that this integration can have a wide and complex residenc e time range and source distribution.