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

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

A Probabilistic Source Assessment Framework for Leaching from Secondary Materials in Highway Applications

Recovered materials from the transportation sector or secondary or by-product materials from the industrial, municipal, or mining sector can be used as substitutes for natural materials in the construction of highway infrastructure. The environmental impact of traditional and newer secondary materials needs to be determined for the conditions of their expected use. The purpose of this paper is to introduce a probabilistic framework for evaluating the environmental acceptability of candidate secondary materials based on the risk of soil and groundwater contamination from leached metals and organics from the pavement. The proposed framework provides a structured guidance for selecting the appropriate model, incorporating uncertainty, variability, and expert opinion, and interpreting results for decision making. This new approach is illustrated by a probabilistic analysis of arsenic leaching from Portland cement concrete and asphalt concrete materials that were constructed using virgin and secondary products

A review of roadway water movement for beneficial use of recycled materials

The purpose of this chapter is to provide a comprehensive review of water movement in roadways so that this knowledge may be used in environmental impact studies of traditional and recycled pavement materials. Long term leaching of contaminants is dictated in part by the hydrology of the roadway environment. To determine the hydraulic regimes in the field, ingress and egress routes and the hydraulic conductivity of the materials need to be known. This paper demonstrates that the major water ingress routes are along cracks, joints, and shoulders. It is shown that both saturated and unsaturated conditions in the field occur, suggesting that the contaminant leaching studies that consider saturated conditions only may overlook the effects of unsaturated conditions and the effects of wetting and drying. Furthermore, moisture content and unsaturated conditions have significant spatial and temporal variations in pavement systems. The hydraulic conductivity of pavement materials presented in the literature vary significantly due to various pavement designs, however, the hydraulic conductivity of pavement is less significant in influencing pavement system hydraulic regime than are cracks, joints, shoulders, and drainage systems

Sediment trap studies in Lake Superior: Insights into resuspension, cross-margin transport, and carbon cycling

As part of the Keweenaw Interdisciplinary Transport Experiment in Lake Superior (KITES) project, sediment traps were deployed at multiple locations along the northern coast of the Keweenaw Peninsula in Lake Superior. Traps were deployed at multiple depths (25 m below water surface to 5 m above sediments) at varying distances from shore (0.5-21 km) during the months May-October over a 3-year period. Material captured in the sediment traps was analyzed for total carbon (C), total nitrogen (N), total phosphorus (P), total copper (Cu), 210Pb, and stable isotope ratios of C and N. Copper in sediment trap material, a tracer for mine residues, indicated transport of material in both directions along the peninsula. Cross-margin transport of material occurred at all trap locations along the coast and in all seasons. The most significant finding was that sediment traps suspended just below the thermocline collected large amounts of resuspended sediments even when the trap was moored in 120-220 m of water (9-21 km from shore). Estimates of the contribution to settling fluxes of organic carbon from resuspended sediments ranged from 10-30% in offshore traps; estimates based on 210Pb agreed well with estimates based on a carbon mixing model. Element ratios suggest that resuspended material in offshore traps originated in near-shore regions. Despite the strong influence of resuspension on the composition of sedimenting material, stable isotope ratios were controlled by processes occurring in the water column. Seasonal variations in isotope ratios may reflect seasonal shifts between predominance by autotrophic and heterotrophic processes in the water column