Parent lithology, surface–groundwater exchange, and nitrate retention in headwater streams

We address the ecological ramifications of variation in hydrologic interaction between streams and alluvial aquifers in catchments with alluvium derived from parent materials of contrasting geologic composition. We present a conceptual model in which solute retention in streams results from hydrologic retention (increased water residence time resulting from surface‐groundwater exchange), biological nutrient cycling, and chemical processes. Solute injection experiments were done in study catchments comprised of sandstone‐siltstone (site 1), volcanic tuff (site 2), and granite‐gneiss (site 3). Distribution of an injected conservative tracer (Br) illustrated that rate and extent of surface‐water penetration into the alluvial aquifer increased across study catchments as was predicted from increasing alluvial hydraulic conductivity. Concurrently, groundwater inputs at baseflow represented between 13 and 57% of aboveground discharge at upstream transects, indicating bidirectional hydrologic exchange along the study reaches. N: P ratios in surface water ranged from 4 to 16, suggesting strong biotic demand for inorganic N. Coinjection of NaBr and NaNO 3 revealed longest nitrate uptake length (S w ) at site 1, intermediate S w at site 2, and shortest uptake length at site 3. Modeling of transient hydrologic solute storage revealed that S w correlated with hydraulic storage, suggesting an important role for subsurface processes in total nitrate retention.