HYPORHEIC FLOW, RESIDENCE TIMES AND NITROGEN REACTIONS IN A RIFFLE-POOL SEQUENCE, TRUCKEE RIVER, NV.
Excessive algal production and subsequent decomposition contributes to fine particulate organic matter that reduces riverbed porosity and permeability and promotes microbial activity that stimulates nitrogen transformations. In the hyporheic zone, physical and biogeochemical processes controlling nitrification and denitrification processes are directly linked. Thus, this dissertation investigates hyporheic flow, residence times, oxygen dynamics and nitrogen transformations of a riffle-pool sequence in the lower Truckee River. The overall objectives of this research are to examine the physical and chemical controlling variables of nitrogen cycling in the hyporheic zone. The research approach was to 1) monitor continuous stage, sediment temperature and pressure to parameterize a flow and solute transport model to examine rates, flow direction, and residence times, 2) monitor the spatial and temporal distribution of dissolved oxygen, inorganic nitrogen (nitrate and ammonium) concentrations, and 3) relate hyporheic flow and residence times to observed nitrogen concentrations. In this first of its kind application of heat as a tracer applied to a hyporheic system, a riffle-pool sequence was heavily instrumented with in-stream piezometers and bank monitoring wells to monitor thermal and physical gradients to parameterize a two-dimensional heat and solute transport model. The model was calibrated using continuous observations of temperature and pressure in a single and multi-objective calibration approach. Field data and model results indicate the presence of a discontinuous low-permeability deposit that limited the vertical penetration of seepage beneath the riffle, whereas deeper exchange existed where the low-permeability deposit was absent. Hyporheic flow paths above the low-permeability deposit have mean residence times of 10 - 28 days at shallow depths (<0.2 m). Through global parameter estimation methods, using observations of pressure and temperature, it was possible to estimate uncertainties riverbed hydraulic conductivity and thus, residence time distributions. For this study, measured sediment temperature and pressure reduced the standard deviation of the residence time error by 29%. Uncertainty of hydraulic conductivity resulted in the greatest overall uncertainty in residence time of 30-36 days at the inter-quartile range across all piezometer locations. Streambed porosity and dispersivity resulted in residence time uncertainty of 8.5 - 9.5 days and 5.0 - 7.7 days, respectively. The riverbed has distinct hot spots and hot moments of high biogeochemical activity in both the riffle and pool areas. Nitrate and dissolved oxygen were greater in the riffle transects as compared to pool transects, due to mixing among short and long flow paths. Replenishment of dissolved oxygen supports nitrification over long flow paths (tens of meters) and residence times (days). In the riffle and pool areas, time-varying river discharge, spatially-varying hyporheic flow, and the distribution and mixing of flow paths appear to control the nitrification and denitrification processes. Further, the influx of surface water in down-welling zones limits the denitrification processes by increasing the oxygen concentrations of the hyporheic zone. These results illustrate the direct hydrologic and biogeochemical controls of longitudinal nitrogen cycling in the hyporheic zone of the Truckee River.
groundwater-surface water interaction, heat transport, hyporheic zone, nitrogen cycling, streambed
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