Residence Time in Hyporheic Bioactive Layers Explains Nitrate Uptake in Streams

The Tracer Additions for Spiraling Curve Characterization (TASCC) model has been rapidly adopted to interpret in-stream nutrient spiraling metrics over a range of concentrations from breakthrough curves (BTCs) obtained during pulse solute injection experiments. TASCC analyses often identify hysteresis in the relationship between and concentration as BTCs rises falls. mechanisms behind these patterns have yet be determined. We hypothesized that differences time is exposed bioactive environments (i.e., biophysical opportunity) rising falling limbs causes TASCCs. tested this hypothesis using nitrate data Elkhorn Creek (CO, USA) combined with process-based particle-tracking representing travel times transformations along each flow path water column hyporheic zone, which zone comprised only thin superficial layer. In-stream uptake was controlled by exchange cumulative spend This residence generally increased limb BTC, systematically generating curves. Hysteresis decreased when primarily occurred compared increasing distance sampling points. depth Our results emphasize good characterization spatial heterogeneity surface-subsurface paths hot spots within streams essential understanding uptake.