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Topics in Sustainability: Natural and Engineered Systems
Modeling Group: Marshall Lindsey | Carla Ng
The Gray Group's efforts in understanding the structure and function of natural and engineered systems exemplify Professor Gray's interests in the multi-scale, integrative approach necessary in studies of sustainability. Research in this division of her group ranges from organic material in rivers, to denitrification in wetlands, to contaminant accumulation in stream sediments, to lake food webs, and even to urban gardens and the residential building industry. This interdisciplinary research leverages the power of a variety of system analysis tools - be they computational, laboratory, or field investigations.
Current Projects
Life Cycle Modeling of Energy Consumption and Greenhouse
Gas Emissions for Residential Buildings in
Marshall Lindsey - PhD Student
The goal of
The value of this tool will be as a vehicle for educating
everyone, not just members of the construction industry, on the importance and benefit of incorporating
green design in the built environment. Further development of this
tool could yield a web-based form, allowing for simple access by contractors,
developers, etc., for use as a guide in development decisions. Read more
about
Predictive Modeling of Bioaccumulation in Aquatic Food Webs: the Importance of Ecological Details.
Carla Ng - PhD Candidate
Carla's work seeks to determine how dynamic food web structure affects contaminant accumulation and the resulting trajectory of environmental degradation or recovery. She uses modeling in conjunction with ecological tools (e.g., stable isotope and stomach content analyses) to predict bioaccumulation, and works to quantify the uncertainty inherent in a model prediction as a function of the level of ecological detail.
In modeling PCB
bioaccumulation in Calumet Harbor, a highly altered ecosystem in Lake Michigan,
she discovered that its trophic structure and subsequent chemical transfer
could not be captured without sufficient ecological resolution in three main
areas: (1) ontogeny (changes in diet with age/size); (2) seasonality (e.g., the availability of fish eggs as a food source during spawning season);
(3) trophic
feedback loops (the recycling of nutrients and contaminants through
consumption of detritus, e.g. mussel feces/pseudofeces and fish eggs). These
properties create a complex, dynamic food web in an ecosystem that on the
surface might seem simple, as it contains only a handful of species.
The model was also illustrated how the effects of ongoing species invasions on a low-diversity food web drastically altered by contaminants and habitat disruption can be highly non-intuitive and difficult to predict. For example, colonization by two invasive species that on the surface seem very similar - zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena bugensis) can in fact have the opposite effect on contaminant accumulation in the food web. When zebras dominated, PCB accumulation in the food web was increased, while when quaggas dominated, PCB levels in forage fish dropped three-fold. These modeling results also bring to light the difficulty in reconciling, and validating, the predictions of a model with an ecosystem that is under constant flux, leading to a conviction that sampling and modeling must work together in an iterative fashion, with modeling to guide sampling to the most important parameters, and data to continuously optimize modeling, reducing uncertainty and improving predictive power. Read more about Carla's research and background here.
Past Projects
Interactions Between Chemical, Biological, and Physical Factors in Denitrification by Periphtyon in Aquatic System.
Shai Arnon - Former PostDoc (with Aaron Packman)
Shai used some surprising results from a former student's project on denitrification in wetlands and designed a series of
laboratory and field experiments to probe the interactions between chemical,
biological and physical factors that influenced denitrification in the peryphyton community.
Shai's findings are significant because of what they tell us about coupled processes in natural systems. Shai was able to show through careful experimentation that there is an optimum range of water velocity that allows nutrient transport into the biofilm, but does not create sufficiently aerobic conditions such that the anaerobic process of denitrification is suppressed. Furthermore, he was able to demonstrate that these flow conditions stimulated a particular organization among the algal and bacterial species inhabiting periphyton. Being the environmental chemistry and applied ecologist of the collaboration, I was particularly impressed with these results. Shai was able to lead a highly multidisciplinary team to discover that under certain flow scenarios particular algal diatom species predominate which in turn support the structure and function of a particular facultative bacterial community. The net result is the ability to maintain high rates of dissimilatory nitrate metabolism.
These insights into benthic processes have broad impacts to both
fundamental understanding of aquatic systems as well as to practical application. Shai's cross-cutting work has been recognized for its important contributions in both
ecological as well as hydrodynamic circles. As importantly, his work is critical to improving our ability to predict
nitrogen cycling in surface waters. Finally,
his work suggests practical strategies to reduce nitrate loading. Globally, high nutrient loading of surface
and coastal waters is a severe problem. In the
Strategies to enhance denitrification in wetland systems: Hydrology, ecology, and microbiology
Cari Ishida, former PhD student
Cari worked on two different projects during her PhD. One was with the US Army Corps of
Engineers, to study how hydroperiod influenced
benthic bacterial structure and function as measured by denitrification. She also worked on a constructed wetland
project, continuing the work of Tanita Sirivedhin, where she measured rates of denitrification in periphyton grown on an artificial mesh structure (see photo, above). Periphyton is a complex matrix of algae, heterotrophic microbes, fungi and
detrital material attached to substrata in photic zones of aquatic systems. Tanita's work demonstrated
that installment of benthic mesh increased denitrification rates in
constructed wetlands. The purpose of Cari's research
was to survey various wetlands for highly denitrifying periphytic communities and to determine ideal biogeochemical conditions to support these
communities. Periphyton, sediment and water samples were collected from wetland
systems in
Monitoring the Behavior of Organic Carbon in Surface Waters using Pyrolysis/GC/MS
Tanita Sirivedhin - former PhD Student
Tanita worked on two separate natural systems projects during her
PhD. She analyzed and compared the organic quality of
Periphyton Community Structure in Lotic Systems: The Interactions of Metals, PCBs, and Environmental Variables
Jill Kostel - former PhD student
Phytoremediation of Lead in Urban Residential
Soils: A Study of Application,
Feasibility and Effectiveness in
Molly Finster - Former PhD student
Molly researched the application of phytoremediation as a remediation approach for soil lead in
urban residential areas while utilizing a combination 
of laboratory experiments and field studies. Phytoremediation is the use of plants to clean up
contaminated soils, sediments, and water. This study, which involved the
planning, implementation, and outcomes assessment of the phytoremediation laboratory and field experiments, piloted the soil lead treatment strategy of phytoremediation across a spectrum of soil lead
concentrations.
In collaboration with the Chicago Legal Clinic, Molly worked on a project related to the implementation of phytoremediation in urban residential neighborhoods through the use of a decision tree model. This work led to an online report entitled: A Resource Guide: The Phytoremediation of Lead in Urban, Residential Areas.
Selected Recent Publications
S. Arnon, K.A. Gray, A.I. Packman, (2006) "Biophysicochemical process coupling controls nitrogen utilization by benthic biofilms," Limnology and Oceanography, accepted.
S. Arnon, A.I. Packman, C.G. Peterson, K.A. Gray (2006) "The effects of overlying velocity on periphyton structure and denitrification," Journal of Geophysical Research, in press.
C.K. Ishida, J.J. Kelly, K.A. Gray, (2006) "Effects of variable hydroperiods and water level fluctuations on the denitrification capacity, nitrate removal, and benthic microbial community structure in constructed wetlands," Ecological Engineering, 28:363-373.
T. Sirivedhin, K.A. Gray, (2006) "Factors Affecting Denitrification Rates in Experimental Wetlands: Field and Laboratory Studies," Ecological Engineering, 26:167-181.
T. Sirivedhin, K.A. Gray, (2005) "Identifying Anthropogenic Markers in Surface Waters Influenced by Treated Effluents: A Tool in Potable Water Reuse," Water Research, 39:1154-1164.
T. Sirivedhin, K.A. Gray, (2005) "Comparison of the Disinfection Byproduct Formation Potentials between a Wastewater Effluent and Surface Waters," Water Research, 39:1025-1036
H.J. Binns, K.A. Gray, T. Chen, M.E. Finster, M. Peneff, P. Schaefer, V. Ovsey, J. Fernandes, M. Brown, B. Dunlop, (2004) "Evaluation of Landscape Interventions to Reduce Potential Exposure to Lead-Contaminated soil in Urban Residential Yards; The Safer Yards Project," Environmental Research, 96:127-138.
M.E. Finster. K.A. Gray, H. Binns, (2004) "Lead Levels of Vegetables Grown in Contaminated Residential Soils: A Field Survey," Science of the Total Environment, 320, 245-257.
J.A. Kostel, KA. Gray A. St.Amand, (2003) "The Impact of Metal and Organic Contaminants on the Structure of Periphyton in Lotic Sediments: Observations at Various Scales," International Journal of Sediment Research, 18:2:214-222.
Professor, Civil and Environmental Engineering, Northwestern University
Fax: (847) 467-4011, Email: k-gray@northwestern.edu
Last modified: 12-15-06
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