Quantifying Nitrate Uptake in an Anabranching, Unsteady, Antarctic Stream

We conducted a thirty-three hour nitrate tracer injection in Huey Creek, a high-gradient, first-order stream located in the McMurdo Dry Valleys of Antarctica. Bacteria are the dominant life form in Huey Creek. A nitrate injection will allow us to quantify the rates and processes associated with bacterial activity, as well as to further constrain the location of the communities. Transient storage models (TSM's) are often used to analyze stream tracer injections, but such analyses are hindered by unsteady flow in Huey Creek. Daily flood pulses result in branching of the stream channel, and subsequent infiltration of branch waters into the subsurface as streamflow declines. Preliminary analyses suggest that subsurface residence times of infiltrating water are on the order of tens of hours, which is significantly longer than storage resulting from near-channel hyporheic exchange. Channel branching creates a second storage zone that must be added to a TSM in order to correctly quantify model parameters. Furthermore, the diel flood cycle may result in temporal variations in model parameters including subsurface storage area (As), exchange rate (α), and bacterial activity (λ). In order to avoid these complexities, the Huey Creek nutrient injection was simulated using a groundwater flow model. The system's hydrology was modeled using MODFLOW and the streamflow routing package, DAFLOW. Solute movement was quantified with MT3DMS. These models were calibrated by comparing simulated and observed solute breakthrough curves. We believe that using a groundwater flow model will more accurately describe this system's hydrology, leading to greater confidence in nutrient uptake rates.