Mining waste and wetlands

This thesis gives a structural description of wetlands and some processes that are vital for the treatment of metal-polluted waters in wetlands. A number of factors affecting the productivity and useful lifetimes of treatment wetlands are also described. The sources of acid mine drainage (AMD) and ways to minimize production of AMD such as soil covering and water covering is discussed based on previous research results. The main goal of the thesis is to describe if wetlands are effective to use as a remediation method, which processes contribute to their efficiency and what limitations may prevent treatment performance. For consideration of this problem, natural and constructed wetlands have been studied. Natural wetlands can be used as an effective remediation method which provides good water quality and prevents the development of unacceptable pollutant levels. A number of factors such as climatic conditions, wastewater pre-treatment needs, flow control, mass and hydraulic loading rates, design of conveyance and distribution systems, existing environmental resources in the wetland water, and plant species control ecological alterations in wetlands. A good design of a constructed wetland system contributes to high quality of treatment with expected results. Such parameters as site selection, hydraulic and climatic conditions, hydroperiod, hydraulic loading rate, hydraulic residence time, and plant ability play very significant roles in wetland design and construction. However, the following limitations may reduce the effectiveness of wetland treatment processes: • Temperature and flow deviations influence wetland processes and may reduce the removal rates of contaminants in wetlands. • Cold climatic conditions can slow down or inhibit plant growth, slow remediation implementation, reduce the rate of contaminant breakdown in wetlands, and increase the period of treatment. • The removal mechanisms in a wetland can be overloaded by high inflows of water. • Dry-weather periods can damage plants and significantly limit wetland processes. • Small wetland areas are not suitable for implementation of remediation. • Employing specific plant species for phytoremediation can be difficult due to adaptability problems. • High concentrations of contaminants may create plant toxicity effects. • Remediation connected with factors such as contamination depth, for example where fastgrowing, deep-rooted trees are used to remediate contaminated groundwater. • Wetland ageing may induce a decrease of contaminant removal rates over time. This thesis also presents the results from two case studies aimed at investigating metal immobilization in a natural wetland (Kristineberg – Vormbäcken in Sweden) and in a constructed wetland (Dunka Mine in northeastern Minnesota). The results from the Kristineberg site show that the immobilization of Zn, Cu, Cd, Al, Mn, As, and Fe along a mining region recipient (the river Vormbäcken) did not decrease metal concentrations more than 5% due to high pH, rapid flow rate, and climatic conditions. The results of the investigation of metal removal (Ni, Cu, Co, Zn) from the Dunka Mine drainage are regarded as successful. However, some treatment limitations are indicated. Increasing drainage flow during spring thaw until freeze-up at the end of November contributed to the overall load to the watershed. Using data collected during an investigation of the site, it was determined that metal concentrations decreased with depth, and that wetland lifetime probably ranges from 20 to several hundred years.Validerat; 20101217 (root