David M. Borrok

Zn and Cu Isotopes as Tracers of Anthropogenic Contamination in a Sediment Core from an Urban Lake

In this work, we use stable Zn and Cu isotopes to identify the sources and timing of the deposition of these metals in a sediment core from Lake Ballinger near Seattle, Washington, USA. The base of the Lake Ballinger core predates settlement in the region, while the upper sections record the effects of atmospheric emissions from a nearby smelter and rapid urbanization of the watershed. delta(66)Zn and delta(65)Cu varied by 0.50 per thousand and 0.29 per thousand, respectively, over the 500 year core record. Isotopic changes were correlated with the presmelter period ( approximately 1450 to 1900 with delta(66)Zn = +0.39 per thousand +/- 0.09 per thousand and delta(65)Cu = +0.77 per thousand +/- 0.06 per thousand), period of smelter operation (1900 to 1985 with delta(66)Zn = +0.14 +/- 0.06 per thousand and delta(65)Cu = +0.94 +/- 0.10 per thousand), and postsmelting/stable urban land use period (post 1985 with delta(66)Zn = 0.00 +/- 0.10 per thousand and delta(65)Cu = +0.82 per thousand +/- 0.12 per thousand). Rapid early urbanization during the post World War II era increased metal loading to the lake but did not significantly alter the delta(66)Zn and delta(65)Cu, suggesting that increased metal loads during this time were derived mainly from mobilization of historically contaminated soils. Urban sources of Cu and Zn were dominant since the smelter closed in the 1980s, and the delta(66)Zn measured in tire samples suggests tire wear is a likely source of Zn.

Zinc Isotopic Composition of Particulate Matter Generated during the Combustion of Coal and Coal + Tire-Derived Fuels

Atmospheric Zn emissions from the burning of coal and tire-derived fuel (TDF) for power generation can be considerable. In an effort to lay the foundation for tracking these contributions, we evaluated the Zn isotopes of coal, a mixture of 95 wt % coal + 5 wt % TDF, and the particulate matter (PM) derived from their combustion in a power-generating plant. The average Zn concentrations and δ(66)Zn were 36 mg/kg and 183 mg/kg and +0.24‰ and +0.13‰ for the coal and coal + TDF, respectively. The δ(66)Zn of the PM sequestered in the cyclone-type mechanical separator was the lightest measured, -0.48‰ for coal and -0.81‰ for coal+TDF. The δ(66)Zn of the PM from the electrostatic precipitator showed a slight enrichment in the heavier Zn isotopes relative to the starting material. PM collected from the stack had the heaviest δ(66)Zn in the system, +0.63‰ and +0.50‰ for the coal and coal + TDF, respectively. Initial fractionation during the generation of a Zn-rich vapor is followed by temperature-dependent fractionation as Zn condenses onto the PM. The isotopic changes of the two fuel types are similar, suggesting that their inherent chemical differences have only a secondary impact on the isotopic fractionation process.

Zinc isotope investigation of surface and pore waters in a mountain watershed impacted by acid rock drainage

The pollution of natural waters with metals derived from the oxidation of sulfide minerals like pyrite is a global environmental problem. However, the metal loading pathways and transport mechanisms associated with acid rock drainage reactions are often difficult to characterize using bulk chemical data alone. In this study, we evaluated the use of zinc (Zn) isotopes to complement traditional geochemical tools in the investigation of contaminated waters at the former Waldorf mining site in the Rocky Mountains, Colorado, U.S.A. Geochemical signatures and statistical analysis helped in identifying two primary metal loading pathways at the Waldorf site. The first was characterized by a circumneutral pH, high alkalinity, and high Zn/Cd ratios. The second was characterized by acidic pHs and low Zn/Cd ratios. Zinc isotope signatures in surface water samples collected across the site were remarkably similar (the δ(66)Zn, relative to JMC 3-0749-L, for most samples ranged from 0.20 to 0.30‰±0.09‰ 2σ). This probably suggests that the ultimate source of Zn is consistent across the Waldorf site, regardless of the metal loading pathway. The δ(66)Zn of pore water samples collected within a nearby metal-impacted wetland area, however, were more variable, ranging from 0.20 to 0.80‰±0.09‰ 2σ. Here the Zn isotopes seemed to reflect differences in groundwater flow pathways. However, a host of secondary processes might also have impacted Zn isotopes, including adsorption of Zn onto soil components, complexation of Zn with dissolved organic matter, uptake of Zn into plants, and the precipitation of Zn during the formation of reduced sulfur species. Zinc isotope analysis proved useful in this study; however, the utility of this isotopic tool would improve considerably with the addition of a comprehensive experimental foundation for interpreting the complex isotopic relationships found in soil pore waters.

Heavy metal distribution and bioaccumulation in Chihuahuan Desert Rough Harvester ant (Pogonomyrmex rugosus) populations.

Heavy metal contamination can negatively impact arid ecosystems; however a thorough examination of bioaccumulation patterns has not been completed. We analyzed the distribution of As, Cd, Cu, Pb and Zn in soils, seeds and ant (Pogonomyrmex rugosus) populations of the Chihuahuan Desert near El Paso, TX, USA. Concentrations of As, Cd, Cu, and Pb in soils, seeds and ants declined as a function of distance from a now inactive Cu and Pb smelter and all five metals bioaccumulated in the granivorous ants. The average bioaccumulation factors for the metals from seeds to ants ranged from 1.04x (As) to 8.12x (Cd). The findings show bioaccumulation trends in linked trophic levels in an arid ecosystem and further investigation should focus on the impacts of heavy metal contamination at the community level.

Zinc isotopic signatures in eight lake sediment cores from across the United States.

Zinc is an important trace element pollutant in urban environments; however, the extent of Zn contamination and the sources of urban Zn pollution are often unclear. We measured Zn concentrations and isotopes in sediment cores collected from eight lakes or reservoirs across the United States. We paired these data with historical records of land use within each watershed to calculate a mean Zn concentration and δ(66)Zn for natural (less than 5% urban land use; 123 ± 21.7 mg/kg; +0.33 ± 0.08‰, n = 6 lakes) and urban (greater than 70% urban land use; 389 ± 200 mg/kg; +0.14 ± 0.07‰, n = 3 lakes) lake sediments. The relation between Zn concentration data and Zn isotope data allows us to create a mixing model between two end members: natural and urban. The δ(66)Zn of the urban end-member is consistent with Zn pollution from vehicle-related sources (tire wear and emissions). Application of this model to samples collected from lakes recording periods ranging from 5 to 70% urban land use in their surrounding watersheds indicates that the lakes and reservoirs were affected by large amounts of Zn from urban sources.

Assessment of Corrosion Potential of Coarse Backfill Aggregates for Mechanically Stabilized Earth Walls

The service life of mechanically stabilized earth walls depends on the rate of corrosion of the metallic reinforcements used in their construction. The assessment of corrosion potential requires an accurate evaluation of pH, resistivity, and sulfate and chloride concentrations of aqueous solutions in contact with the surrounding aggregate. Highway agencies tend to use larger aggregates that contain only a small amount of fine material (passing the Number 40 sieve) in the backfill. Evaluation of the electrochemical parameters of coarse aggregates is challenging because traditional evaluation methods call for the use of fine material. In this study, the suitability of traditional soil characterization techniques for use with coarse aggregates was assessed through leaching experiments performed on coarse limestone and dolomite aggregates from six quarries in Texas. Chemical differences were isolated from size-related kinetic leaching effects by comparing the results from same-sized material collected in the field with material derived from the crushing of larger (≥⅜ in.) aggregates in the laboratory. The testing demonstrated that the fines collected from the field were enriched in chemicals that, when exposed to water, decreased pH and resistivity and increased sulfate concentrations compared with the bulk rock. This was likely the result of sulfur compounds in the atmosphere reacting with carbonate rocks to produce reactive surface layers that were mechanically abraded into the fines. This phenomenon could bias traditional soil testing results and, therefore, the assessment of corrosion potential. This study demonstrated that a more accurate assessment of the electrochemical parameters can be obtained by crushing the coarse material to meet testing size specifications.

Water, climate, and social change in a fragile landscape

We present here and in the companion papers an analysis of sustainability in the Middle Rio Grande region of the U.S.-Mexico border and propose an interdisciplinary research agenda focused on the coupled human and natural dimensions of water resources sustainability in the face of climate and social change in an international border region. Key threats to water sustainability in the Middle Rio Grande River region include: (1) increasing salinization of surface and ground water, (2) increasing water demand from a growing population in the El Paso/Ciudad Juarez area on top of an already high base demand from irrigated agriculture, (3) water quality impacts from agricultural, municipal, and industrial discharges to the river, (4) changing regional climate that portends increased frequency and intensity of droughts interspersed with more intensive rainfall and flooding events, and (5) disparate water planning and management systems between different states in the U.S. and between the U.S. and Mexico. In addition to these challenges, there is an increasing demand from a significant regional population who is (and has been historically) underserved in terms of access to affordable potable water. To address these challenges to water resources sustainability, we have focused on: (1) the determinants of resilience and transformability in an ecological/social setting on an international border and how they can be measured and predicted; and (2) the drivers of change … what are they (climate, social, etc.) and how are they impacting the coupled human and natural dimensions of water sustainability on the border? To tackle these challenges, we propose a research agenda based on a complex systems approach that focuses on the linkages and feedbacks of the natural, built/managed, and social dimensions of the surface and groundwater budget of the region. The approach that we propose incorporates elements of systems analysis, complexity science, and the use of modeling tools such as scenario planning and back-casting to link the quantitative with the qualitative. This approach is unique for our region, as are our bi-national focus and our conceptualization of “water capital”. In particular, the concept of water capital provides the basis for a new interdisciplinary paradigm that integrates social, economic, and natural sectors within a systems framework in order to understand and characterize water resources sustainability. This proposed approach would not only provide a framework for water sustainability decision making for our bi-national region at the local, state, and federal levels, but could serve as a model for similar border regions and/or international rivers in arid and semi-arid regions in the Middle East, Africa, Asia, and Latin America.

Satellite-Based Estimates of Groundwater Storage Changes at the Najd Aquifers in Oman

The Najd aquifers in Oman, are located in one of the most arid zones in the world. In such regions, there is a shortage in the water resources where groundwater is a very critical component for human life. The main aim of this contribution is to use the satellite remote sensing data of the Gravity Recovery and Climate Experiment (GRACE) along with the Global Land Data Assimilation System (GLDAS), to estimate the groundwater storage changes at the Najd aquifers. Groundwater storage changes were calculated from both GRACE/GLDAS data and from the groundwater level measurements. It was found that the estimated groundwater storage changes from GRACE and water levels coincide in their trends showing a noticeable depletion within the time period from Oct. 2002 to Sept. 2014. The spatial distribution maps of the groundwater storage changes show slightly changes from Oct. 2003 to Sept. 2010, but a significant decreasing were observed from 2010 to 2014. The groundwater storage over Najd aquifers was decreased by about 0.44 and 0.46 km3/year as calculated from GRACE data and groundwater levels, respectively. We also found that groundwater storage was affected by the strong storm events as observed in 2007 and 2010. This contribution could be helpful for the long term sustainable groundwater management in the study area and other arid regions.