ALLUVIAL CHARACTERISTICS, GROUNDWATER-SURFACE WATER EXCHANGE AND HYDROLOGICAL RETENTION IN HEADWATER STREAMS

Conservative solute injections were conducted in three first-order mon tane streams of different geological composition to assess the influen ce of parent lithology and alluvial characteristics on the hydrologica l retention of nutrients. Three study sites were established: (1) Aspe n Creek, in a sandstone-siltstone catchment with a fine-grained alluvi um of low hydraulic conductivity (1.3 x 10(-4) cm/s), (2) Rio Calavera s, which flows through volcanic tuff with alluvium of intermediate gra in size and hydraulic conductivity (1.2 x 10(-3) cm/s), and (3) Gallin a Creek, located in a granite/gneiss catchment of coarse, poorly sorte d alluvium with high hydraulic conductivity (4.1 x 10(-3) cm/s). All s ites were instrumented with networks of shallow groundwater wells to m onitor interstitial solute transport. The rate and extent of groundwat er-surface water exchange, determined by the solute response in wells, increased with increasing hydraulic conductivity. The direction of su rface water-groundwater interaction within a stream was related to loc al variation in vertical and horizontal hydraulic gradients. Experimen tal tracer responses in the surface stream were simulated with a one-d imensional solute transport model with inflow and storage components ( OTIS). Model-derived measures of hydrological retention showed a corre sponding increase with increasing hydraulic conductivity. To assess th e temporal variability of hydrological retention, solute injection exp eriments were conducted in Gallina Creek under four seasonal flow regi mes during which surface discharge ranged from baseflow (0.75 l/s in O ctober) to high (75 l/s during spring snowmelt). Model-derived hydrolo gical retention decreased with increasing discharge. The results of ou r intersite comparison suggest that hydrological retention is strongly influenced by the geologic setting and alluvial characteristics of th e stream catchment. Temporal variation in hydrological retention at Ga llina Creek is related to seasonal changes in discharge, highlighting the need for temporal resolution in studies of the dynamics of surface water-groundwater interactions in stream ecosystems. (C) 1997 by John Wiley & Sons, Ltd.