Contributions to predicting contaminant leaching from secondary materials used in roads

Slags, coal ashes, and other secondary materials can be used in road construction. Both traditional and secondary materials used in roads may contain contaminants that may leach and pollute the groundwater. The goal of this research was to further the understanding of leaching and transport of contaminants from pavement materials. Towards this goal, a new probabilistic framework was introduced which provided a structured guidance for selecting the appropriate model, incorporating uncertainty, variability, and expert opinion, and interpreting results for decision making. In addition to the framework, specific contributions were made in pavement and embankment hydrology and reactive transport, Bayesian statistics, and aqueous geochemistry of leaching. Contributions on water movement and reactive transport in highways included probabilistic prediction of leaching in an embankment, and scenario analyses of leaching and transport in pavements using HYDRUS2D, a contaminant fate and transport model. Water flow in a Minnesota highway embankment was replicated by Bayesian calibration of hydrological parameters against water content data. Extent of leaching of Cd from a coal fly ash was estimated. Two dimensional simulations of various scenarios showed that salts in the base layer of pavements are depleted within the first year whereas the metals may never reach the groundwater if the pavement is built on adsorbing soils. Aqueous concentrations immediately above the groundwater estimated for intact and damaged pavements can be used for regulators to determine the acceptability of various recycled materials. Contributions in the aqueous geochemistry of leaching included a new modeling approach for leaching of anions and cations from complex matrices such as weathered steel slag. The novelty of the method was its simultaneous inclusion of sorption and solubility controls for multiple analytes. The developed model showed that leaching of SO4, Cr, As, Si, Ca, Mg, and V were controlled by corresponding soluble solids. Leaching of Pb was controlled by Pb(VO4)3 solubility at low pHs and by surface precipitation reactions at high pHs. Leaching of Cd and Zn were controlled by surface complexation and surface precipitation, respectively

Probabilistic modeling of one dimensional water movement and leaching from highway embankments containing secondary materials

Predictive methods for contaminant release from virgin and secondary road construction materials are important for evaluating potential long-term soil and groundwater contamination from highways. The objective of this research was to describe the field hydrology in a highway embankment and to investigate leaching under unsaturated conditions by use of a contaminant fate and transport model. The HYDRUS2D code was used to solve the Richards equation and the advection–dispersion equation with retardation. Water flow in a Minnesota highway embankment was successfully modeled in one dimension for several rain events after Bayesian calibration of the hydraulic parameters against water content data at a point 0.32 m from the surface of the embankment. The hypothetical leaching of Cadmium from coal fly ash was probabilistically simulated in a scenario where the top 0.50 m of the embankment was replaced by coal fly ash. Simulation results were compared to the percolation equation method where the solubility is multiplied by the liquid-to-solid ratio to estimate total release. If a low solubility value is used for Cadmium, the release estimates obtained using the percolation/equilibrium model are close to those predicted from HYDRUS2D simulations (10–4–10–2 mg Cd/kg ash). If high solubility is used, the percolation equation over predicts the actual release (0.1–1.0 mg Cd/kg ash). At the 90th percentile of uncertainty, the 10-year liquid-to-solid ratio for the coal fly ash embankment was 9.48 L/kg, and the fraction of precipitation that infiltrated the coal fly ash embankment was 92%. Probabilistic modeling with HYDRUS2D appears to be a promising realistic approach to predicting field hydrology and subsequent leaching in embankments

Navigating the Academic Job Search for Environmental Engineers: Guidance for Job Seekers and Mentors

Students and postdoctoral associates interested in tenure-track environmental engineering positions have limited resources to aid them. The Student Services Committee (SSC) of the Association of Environmental Engineering and Science Professors (AEESP) organized a workshop to fill some of the need. Newly hired faculty and experienced search committee and department chairs shared information on how to create competitive applications. Students and postdoctoral associates were also provided with individualized feedback on their faculty application packages. Survey data were collected for all participants, and these data are presented in this paper along with tips shared during the workshop and information collected from the literature. The objective of this paper is to share that information (1) to inform applicants (students and postdoctoral associates) about the academic job search process, and (2) to inform mentors about how to mentor applicants. Survey responses revealed that participants felt they learned valuable information about the job search process. They found the personalized feedback on application packages to be the most helpful activity; other mentors are encouraged to provide similar personalized feedback. A wiki website component included in the workshop was effective at engaging the participants and helped broaden the impact beyond the workshop attendees. Key lessons learned by comparing workshop results with literature were the importance of fit, the unique diversity of environmental engineering, mentoring and networking, Ph.D. planning sessions, being informed, and becoming a more skilled candidate. The recommendations presented in this paper can be used by students, postdoctoral associates, and their mentors to facilitate the candidate’s path toward academia

From Cascade to Bottom-Up Ecosystem Services Model: How Does Social Cohesion Emerge from Urban Agriculture?

Given the expansion of urban agriculture (UA), we need to understand how this system provides ecosystem services, including foundational societal needs such as social cohesion, i.e., people’s willingness to cooperate with one another. Although social cohesion in UA has been documented, there is no framework for its emergence and how it can be modeled within a sustainability framework. In this study, we address this literature gap by showing how the popular cascade ecosystem services model can be modified to include social structures. We then transform the cascade model into a bottom-up causal framework for UA. In this bottom-up framework, basic biophysical (e.g., land availability) and social (e.g., leadership) ecosystem structures and processes lead to human activities (e.g., learning) that can foster specific human attitudes and feelings (e.g., trust). These attitudes and feelings, when aggregated (e.g., social network), generate an ecosystem value of social cohesion. These cause-effect relationships can support the development of causality pathways in social life cycle assessment (S-LCA) and further our understanding of the mechanisms behind social impacts and benefits. The framework also supports UA studies by showing the sustainability of UA as an emergent food supplier in cities

Private and Externality Costs and Benefits of Recycling Crystalline Silicon (c-Si) Photovoltaic Panels

With solar photovoltaics (PV) playing an increasing role in our global energy market, it is now timely and critical to understand the end of life management of the solar panels. Recycling the panels can be an important pathway, possibly recovering a considerable amount of materials and adding economic benefits from currently installed solar panels. Yet, to date, the costs and benefits of recycling, especially when externality costs resulting from environmental pollution are considered, are largely unknown. In this study, we quantified the private and externality costs and benefits of recycling crystalline silicon (c-Si) PV panels. We found that the private cost of end-of-life (EoL) management of the c-Si PV module is USD 6.7/m2 and much of this cost is from transporting (USD 3.3/m2) and landfilling (USD 3.1/m2), while the actual recycling process (the cost of consumed materials, electricity or the investment for the recycling facilities) is very small (USD 0.3/m2). We found that the external cost of PV EoL management is very similar to the private cost (USD 5.2/m2). Unlike the breakdown of the private costs, much of the externality costs (USD 4.08/m2) come from the recycling process, which suggests that more environmentally friendly methods (e.g., recycling methods that involve fewer toxic chemicals, acids, etc.) should be preferred. We estimated that the total economic value of the recycled materials from c-Si PV waste is USD 13.6/m2. This means that when externality costs are not considered, the net benefit of recycling is USD 6.7; when the externality cost of recycling is considered, there is still a net benefit of USD 1.19 per m2

Community respiration rates in Lake Superior

Phytoplankton photosynthesis and community respiration are two key components of the carbon cycle that determine the magnitude of net ecosystem production and the balance between oxygen production and oxygen consumption in lakes. As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, rates of community respiration were measured in 1998 and 1999 in near-and offshore waters along the Keweenaw Peninsula in Lake Superior. Because of the difficulties in measuring low rates of respiration, three methods were employed: bottle incubations, measurements of changes in hypolimnetic oxygen inventories, and rates of CO2 evolution from the lake surface. All three techniques yielded similar rates of CO2 production. Rates of community respiration (bottle incubations) ranged from 2 to 166 μg C/L/d; rates of hypolimnetic oxygen consumption ranged from 3 to 12 μg C/Ld; and rates of CO2 evasion from the lake (positive flux is out of lake) ranged from \u3c 0 to 270 mg C/m2/d corresponding to volumetric rates of \u3c 0 to 11 μg C/L/d. Little change in respiration rate with water depth was noted, but respiration rates near-shore were significantly higher than rates in offshore waters. Higher rates of respiration were measured in the El Niño year of 1998 as compared to 1999, but higher temperatures are not thought to be the direct cause. Rates of respiration were higher than simultaneously measured rates of photosynthesis, and there was a net evolution of CO2 from the lake; the lake appears to be net heterotrophic

Modeling Hydrology and Reactive Transport in Roads: The Effect of Cracks, the Edge, and Contaminant properties

The goal of this research was to provide a tool for regulators to evaluate the groundwater contamination from the use of virgin and secondary materials in road construction. A finite element model, HYDRUS2D, was used to evaluate generic scenarios for secondary material use in base layers. Use of generic model results for particular applications was demonstrated through a steel slag example. The hydrology and reactive transport of contaminants were modeled in a two-dimensional cross section of a road. Model simulations showed that in an intact pavement, lateral velocities from the edge towards the centerline may transport contaminants in the base layer. The dominant transport mechanisms are advection closer to the edge and diffusion closer to the centerline. A shoulder joint in the pavement allows 0.03 to 0.45 m3/day of infiltration per meter of joint length as a function of the base and subgrade hydrology and the rain intensity. Scenario simulations showed that salts in the base layer of pavements are depleted by 99% in the first 20 years, whereas the metals may not reach the groundwater in 20 years at any significant concentrations if the pavement is built on adsorbing soils