Evaluation and use of parsimonious reactive solute transport models for prediction of contaminant behavior at an acid mine drainage site
Acid mine drainage (AMD) results from biogeochemical oxidation of iron and sulfur minerals in flooded mine shafts and waste rock piles. In the presence of water and oxygen, sulfur rich minerals are oxidized to produce sulfate ions and cations such as ferrous, copper and aluminum ions, which lower the pH. The low pH further dissolves and releases other metals in the ore resulting in a high concentration of metal ions in the water.
The solute concentration in a stream is a result of dynamic coupling between a number of chemical processes and transport processes. Creating a model encompassing all the processes occurring at an AMD affected area increases the complexity of the model multifold times. Different approaches toward modeling highly contaminated systems such as AMD sites have been employed over the past few decades. Two distinct styles of coupled reactive transport modeling have developed over time to model the contaminated systems such as AMD affected streams.
This research evaluates the use of parsimonious coupled models of both types - kinetic and equilibrium to predict the fate and transport of four major ions - iron, aluminum, zinc and sulfate in the Effluent Creek at the Davis Mine Site. Model performances are accessed in terms of the ability of the model to accurately match the observed concentration in the Creek, along with the stability of the model determined by Akaike and Bayesian information criteria. In addition to the fundamental question of comparing kinetic and equilibrium model, we model processes affecting the fate and transport and model complexity.
The simple kinetic models-precipitation, sorption and precipitation-sorption; outperform the equilibrium models for the three cations - iron, aluminum and zinc. For sulfate, both the kinetic and equilibrium models give comparable predictions. The simple kinetic model is however unable to define exact processes that causes the chemical transformations. The equilibrium sorption model predicts the expected chemical reactions at the given pH conditions. The chemical rates in the hyporheic zone are a couple orders of magnitude higher than the stream, except for the anion, sulfate, which shows comparable reactivity in the hyporheic zone and the stream.