Tuesday, July 5, 2016
UNM Student Developing Toolbox to Study Solute Transport in Jemez River‒Rio Grande Continuum
UNM Student Developing Toolbox to Study Solute Transport in Jemez River‒Rio Grande Continuum by Catherine Ortega Klett, Program Manager The recent Animas River spill underscores the ongoing need to be able to describe contaminant concentrations in rivers from small upstream sources to large-current downstream reaches. This problem is being addressed by Betsy Summers, a Ph.D. student in UNM's Department of Civil Engineering, in collaboration with her faculty advisor, Dr. Ricardo González-Pinzón. Investigations of solute transport typically begin by injecting tracer material and following its concentration versus time and/or distance downstream. Due to the complexity of the transport of solutes, most tracer experiments are restricted to relatively small flow discharge rates and short transport lengths of half a kilometer or less. By tracking the downstream appearance of the tracer concentration as a function of time at a given location, one obtains what is called the breakthrough tracer curve (BTC). With this curve, one can infer important stream channel properties like geometry, resistance to flow, and the dispersive or mixing power of the flow, all of which provide the basis for constructing a model of solute transport in the stream. Applying tracer experiments to rivers is challenging on multiple fronts: 1) the mass of the tracer that has to be injected is a function of unknown or difficult-to-measure channel characteristics; 2) the observation of some solute transport processes such as decay and sorption may require extensive experimental lengths or multiple sampling points; 3) extreme conditions, like high turbidity, in the river can hinder the performance of instruments that emit ultraviolet light to measure reactive tracers like nitrate in real-time; and 4) streams and rivers have regions of strong secondary circulation and relatively stagnant “dead zones,” which reduce the efficiency of solute transport, giving long tails to the BTC curves. All these layers of complication make planning and carrying out tracer experiments very difficult, especially when we want to understand solute transport processes across entire watersheds or river continuums. In order to apply tracer experiments to a river continuum, Betsy Summers and Dr. González-Pinzón are using a systems engineering approach to combine various models and data in an Excel toolbox to better estimate travel times and the reactivity of ecologically important solutes. This approach integrates: 1) the Aggregated Dead Zone (ADZ) model (Beer and Young, 1983) for solute transport to account for the dead zone phenomenon; 2) a U.S. Geological Survey meta-analysis of tracer experiments conducted in more than 60 rivers of varying discharges and sizes across the U.S. (Jobson, 1997); and 3) the quantification of nutrient cycling and transport (“spiraling”) using the Tracer Addition for Spiraling Curve Characterization (TASCC) methodology (Covino et al., 2010). Summers and Dr. González-Pinzón are developing and validating this technique along the Jemez River‒Rio Grande continuum, which spans eight stream orders, corresponding to a thousand-fold increase in flow rate from the headwaters to the larger rivers. The objective of this research is to create a user-friendly computational toolbox capable of: 1) estimating injection mass needed to properly characterize a tracer BTC as a function of river discharge and longitudinal sampling distance; 2) predicting arrival time, time-to-peak concentration and mean travel time of a solute BTC as a function of longitudinal distance and discharge; 3) analyzing nutrient uptake kinetics along river reaches from short-term and plateau tracer injections; and 4) characterizing expected concentrations of contaminants in rivers as a function of observed upstream BTCs or known contaminant mass from short-term spills. Preliminary comparisons between conservative tracer BTC predictions with the experimental data show that the toolbox predicts travel times and the mass to be injected in a tracer experiment reasonably well along the continuum. Betsy Summers received a 2016 NM WRRI Student Water Research Grant and her final report on the project will be posted on the institute’s website in July 2016. To view, click here.