RAPID: Investigating changes to metal oxide nanoparticle stability in a contaminated stream during the initial period of remediation

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There are known impacts to streams in the United States as a result of pollution from abandoned mines. Even after the impact of the acidic mine effluents are removed by treatment, the streambeds themselves will continue to release metals that are toxic to aquatic species. The metal-containing sediments must be naturally removed in order for the stream to recover and allow both restoration of aquatic life and beneficial use by society. The sediments consist primarily of very small nanoparticles of iron oxide that can contain As, Cd, Cu, Pb, and Zn. These small particles, which have stuck to the streambed, may be more quickly removed if they become more stable in water due to changes in the chemistry of their surfaces. A treatment plant that removes the metals and acid from the mine effluents has just begun operation. The treatment process also alters the composition of the stream water in such a way that the particles may become more stable in water and, thus, removed more quickly. This study will provide data that will lead to better estimates of how quickly other streams will recover if similar mine waste treatment is performed. Incidental nanoparticulate iron oxides (NP-FeOx) are formed in streams receiving acidic mine waters. NP-FeOx is capable of transporting contaminant metals such as Cu, Zn, and As. In early 2017 a lime treatment plant will go online in the North Fork of Clear Creek (NFCC), which will dramatically alter the water chemistry of the stream. The colloidal stability of NP-FeOX is dependent on solution composition (ionic strength, divalent cations, pH, DOC) and is thus likely to be affected. The project goal is to understand how the removal of metals will be influenced by changes in the stability of the NP-FeOx. In order to determine the immediate impact of the water treatment plant (WTP) on NP-FeOx stability in the water column, the dispersion of existing NP-FeOx contained in the streambed will be studied using spICP-MS, DLS, zeta potential, and ICP-OES. It is expected that preexisting NP-FeOX will disperse from the creek bed into the water during early WTP operation, which may result in a temporary increase in toxic metal concentrations. This will be of long-term benefit, as this process may accelerate the restoration of the streambed. This is the first time that nanoparticle stability under variable water chemistry conditions has been examined in an actual stream setting. Additionally, the manipulation of pH and hardness in laboratory experiments will allow us to understand the effects of isolated water chemistry variables on particle dispersion in an AMD system. A summer field session aimed at local high school students and college seniors at the Colorado School of Mines will participate in a field session at the study site. In addition, a partnership with the Upper Clear Creek Watershed Foundation will be forged to involve community volunteers on the proposed project.