PARENT LITHOLOGY, SURFACE-GROUNDWATER EXCHANGE, AND NITRATE RETENTIONIN HEADWATER STREAMS

We address the ecological ramifications of variation in hydrologic int eraction between streams and alluvial aquifers in catchments with allu vium derived from parent materials of contrasting geologic composition . We present a conceptual model in which solute retention in streams r esults from hydrologic retention (increased water residence time resul ting from surface-groundwater exchange), biological nutrient cycling, and chemical processes. Solute injection experiments were done in stud y catchments comprised of sandstone-siltstone (site 1), volcanic tuff (site 2), and granite-gneiss (site 3). Distribution of an injected con servative tracer (Br) illustrated that rate and extent of surface-wate r penetration into the alluvial aquifer increased across study catchme nts as was predicted from increasing alluvial hydraulic conductivity. Concurrently, groundwater inputs at baseflow represented between 13 an d 57% of aboveground discharge at upstream transects, indicating bidir ectional hydrologic exchange along the study reaches. N:P ratios in su rface water ranged from 4 to 16, suggesting strong biotic demand for i norganic N. Coinjection of NaBr and NaNO3 revealed longest nitrate upt ake length (S-w) at site 1, intermediate S-w at site 2, and shortest u ptake length at site 3. Modeling of transient hydrologic solute storag e revealed that S-w correlated with hydraulic storage, suggesting an i mportant role for subsurface processes in total nitrate retention.