APPLICATION OF ONE-DIMENSIONAL TRANSPORT WITH EQUILIBRIUM CHEMISTRY (OTEQ) IN ACID MINE DRAINAGE IMPACTED ALPINE STREAMS

This research aims to better demonstrate and understand the transport of heavy metals and rare earth elements (REEs) in the acidic, metal-rich Snake River using solute transport models, specifically OTEQ and OTIS. The Snake River is located along the Colorado Mineral Belt in Montezuma, Colorado and is of particular interest due to its hydrological and geochemical resemblances to other alpine streams impacted by acid mine drainage, and its terminus into Denver Metropolitan’s drinking water—Dillon Reservoir.

OTEQ is a reactive transport model that couples the OTIS solute transport model with a chemical equlibrium submodel based on MINTEQ. The influence of pH on heavy metal complexation, precipitation/dissolution and oxidation/reduction are simulated using parameter estimation and further validated using synoptic sampling and tracer injection experiments. Using size partitioning, the composition of colloidal and dissolved metal oxides were distinguished to determine the dominant state and subsequent interaction with REEs over a stretch of 3 km. Most REEs were found to behave conservatively. However, Neodymium and Yttrium were found to vary in concentration from 0.02-15.72 ppb and 0.02-26.12 ppb, respectively. In general, the higher concentrations of REEs were found at the headwaters containing lower pH levels with decreasing concentrations as pH increased from 3.3 to 6.1. We can provide insight into the properties and the conservative nature of rare earth elements by validating parameter estimation and correctly modeling trace metal speciation and activity. Mapping the transport of these understudied, industrial REEs can significantly improve the efficiency of acid mine drainage remediation.