RECENT ENHANCEMENTS OF THE OTIS MODEL TO SIMULATE MULTI-SPECIES REACTIVE TRANSPORT IN STREAM-AQUIFER SYSTEMS

The study of solute fate and transport in stream systems often is assisted by the use of surface water solute transport models. These models typically simulate the spatial and temporal variation in concentration of one or more solutes due to advection, dispersion, chemical reactions, and sources and sinks such as lateral inflow/outflow. In addition, some models, including the United States Geological Survey’s OTIS (One-Dimensional Transport with Inflow and Storage) model, also incorporate transient storage mechanisms wherein eddies and stagnant water pools can temporarily detain solutes. This presentation summarizes recent modifications to the OTIS model that extend its applicability to more diverse and complex hydro-chemical systems. These modifications include: extension of the model to simulate solute transport in a network of connected streams; the inclusion of an ordinary differential equation solver to simulate interactions between multiple chemical species; the inclusion of dissolved oxygen and nutrient cycling processes from the QUAL2E in-stream water quality model; the inclusion of selenium cycling, transformation, and speciation processes to simulate the reactive transport of selenium species in stream networks; and the coupling of OTIS with a groundwater flow and reactive transport model to simulate solute reactive transport in a coupled stream-aquifer system. Results are shown for using the modified OTIS model within a sensitivity analysis framework to assess the environmental factors governing in-stream concentrations of dissolved oxygen and nitrogen within the regional-scale stream network of the Lower Arkansas River Valley in southeastern Colorado. Preliminary results also are presented for using the enhanced OTIS capabilities to assess the fate and transport of dissolved oxygen, nitrate, and selenium in a coupled stream-aquifer system, also within the Lower Arkansas River Valley.