Lawrence D. Lemke

Influence of hydraulic property correlation on predicted dense nonaqueous phase liquid source zone architecture, mass recovery and contaminant flux

[1] Organic liquid saturation distributions resulting from a simulated tetrachloroethene (PCE) spill were generated with alternative models of spatially varying aquifer properties for a statistically homogeneous, nonuniform sand aquifer. The distributions were analyzed to quantify DNAPL source zone characteristics and then incorporated as initial conditions for simulated PCE recovery using surfactant-enhanced aquifer remediation (SEAR). The predicted evolution of the spatial distribution of DNAPL saturations or source zone ‘‘architectures’’ and associated remediation efficiencies are strongly influenced by the spatial correlation of aquifer parameters and multiphase flow constitutive relationships. Model predictions suggest that removal of 60 to 99% of entrapped PCE can reduce dissolved contaminant concentration and mass flux under natural gradient conditions by approximately two orders of magnitude. Aqueous phase contaminant flux, however, does not vary consistently as a function of the percentage of DNAPL removed, and notable differences in flux evolution were observed for models incorporating correlated versus uncorrelated capillary entry pressure and permeability fields. Simulation results demonstrate that the application of alternative models of aquifer parameter spatial variability can influence predicted DNAPL infiltration, entrapment, and recovery, even for relatively homogeneous aquifers of the type investigated here. Results also demonstrate potential benefits, in the form of reduced mass flux, accruing from partial mass removal that may not be readily predicted from analyses relying on simplified conceptual models for DNAPL source zone architecture or aquifer flow fields. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 1831 Hydrology: Groundwater quality; KEYWORDS: contamination mass flux, DNAPL remediation, DNAPL source zone, heterogeneity, nonuniformity

Modeling dense nonaqueous phase liquid mass removal in nonuniform formations: Linking source-zone architecture and system response

Dense nonaqueous phase liquid (DNAPL) source zones comprise persistent sources of groundwater contamination that are recalcitrant to complete remediation using conventional (e.g., pump and treat) or emerging (e.g., surfactant flushing) technologies. Increased attention to the assessment of the benefits of partial mass removal from such contaminant source zones has intensified efforts to model multiphase flow and transport behavior. This paper describes the simulated recovery of a tetrachloroethene (PCE) spill in a statistically homogeneous but nonuniform aquifer, incorporating nonuniformity in both nonaqueous phase liquid saturation and pore velocities. We developed a ganglia-to-pool metric to quantify DNAPL source-zone architecture, and explored the correlation of this metric with dissolved mass flux behavior in response to partial DNAPL mass removal. Dissolution of 20%‐70% of PCE mass from models exhibiting low ganglia-to-pool ratios resulted in a larger predicted reduction of dissolved contaminant mass flux than models with high ganglia-to-pool ratios. Results of this study suggest that DNAPL source-zone characterization at field sites with homogeneous, nonuniform aquifers would benefit from inclusion of an estimate of the overall ganglia-to-pool ratio. Simulations demonstrate that flux reduction behavior depends on the source-zone architecture, which is not readily predictable using a priori assumptions about the spatial correlation of physical aquifer parameters. Model results further suggest that stochastic investigations of DNAPL source remediation at field sites should avoid reliance upon Leverett scaling of capillary entry pressures to permeability fields, which can artificially narrow the range of simulated behaviors.

Geospatial relationships of air pollution and acute asthma events across the Detroit-Windsor international border: Study design and preliminary results

The Geospatial Determinants of Health Outcomes Consortium (GeoDHOC) study investigated ambient air quality across the international border between Detroit, Michigan, USA and Windsor, Ontario, Canada and its association with acute asthma events in 5- to 89-year-old residents of these cities. NO2, SO2, and volatile organic compounds (VOCs) were measured at 100 sites, and particulate matter (PM) and polycyclic aromatic hydrocarbons (PAHs) at 50 sites during two 2-week sampling periods in 2008 and 2009. Acute asthma event rates across neighborhoods in each city were calculated using emergency room visits and hospitalizations and standardized to the overall age and gender distribution of the population in the two cities combined. Results demonstrate that intra-urban air quality variations are related to adverse respiratory events in both cities. Annual 2008 asthma rates exhibited statistically significant positive correlations with total VOCs and total benzene, toluene, ethylbenzene and xylene (BTEX) at 5-digit zip code scale spatial resolution in Detroit. In Windsor, NO2, VOCs, and PM10 concentrations correlated positively with 2008 asthma rates at a similar 3-digit postal forward sortation area scale. The study is limited by its coarse temporal resolution (comparing relatively short term air quality measurements to annual asthma health data) and interpretation of findings is complicated by contrasts in population demographics and health-care delivery systems in Detroit and Windsor.

Identification and influence of spatio-temporal outliers in urban air quality measurements.

Forty eight potential outliers in air pollution measurements taken simultaneously in Detroit, Michigan, USA and Windsor, Ontario, Canada in 2008 and 2009 were identified using four independent methods: box plots, variogram clouds, difference maps, and the Local Morans I statistic. These methods were subsequently used in combination to reduce and select a final set of 13 outliers for nitrogen dioxide (NO2), volatile organic compounds (VOCs), total benzene, toluene, ethyl benzene, and xylene (BTEX), and particulate matter in two size fractions (PM2.5 and PM10). The selected outliers were excluded from the measurement datasets and used to revise air pollution models. In addition, a set of temporally-scaled air pollution models was generated using time series measurements from community air quality monitors, with and without the selected outliers. The influence of outlier exclusion on associations with asthma exacerbation rates aggregated at a postal zone scale in both cities was evaluated. Results demonstrate that the inclusion or exclusion of outliers influences the strength of observed associations between intraurban air quality and asthma exacerbation in both cities. The box plot, variogram cloud, and difference map methods largely determined the final list of outliers, due to the high degree of conformity among their results. The Morans I approach was not useful for outlier identification in the datasets studied. Removing outliers changed the spatial distribution of modeled concentration values and derivative exposure estimates averaged over postal zones. Overall, associations between air pollution and acute asthma exacerbation rates were weaker with outliers removed, but improved with the addition of temporal information. Decreases in statistically significant associations between air pollution and asthma resulted, in part, from smaller pollutant concentration ranges used for linear regression. Nevertheless, the practice of identifying outliers through congruence among multiple methods strengthens confidence in the analysis of outlier presence and influence in environmental datasets.

A workshop on transitioning cities at the food-energy-water nexus

Metropolitan development in the USA has historically relied on systems of centralized infrastructure that assume a population density and level of economic activity that has not been consistently sustained in post-industrial urban landscapes. In many cities, this has resulted in dependence on systems that are environmentally, economically, and socially unsustainable. Reliance on this deteriorating social and physical infrastructure results in waste and decreased efficiencies. While numerous cities could exemplify this trend, the present work highlights two compelling cases: Detroit, Michigan and Baltimore, Maryland. The paper provides important feedback from a recent workshop held with experts of both practical and academic backgrounds from both cities. The workshop focused on sustainability of the food-energy-water nexus within the context of transitioning urban landscapes, economies, and governance processes associated with post-industrial cities. The pursuit of environmental, economic, and social sustainability—especially in relation to food, energy, and water—is particularly challenging in aged and deteriorating post-industrial urban settings, and the importance of such cities to the global economy demands that attention be focused on research and education to support this mission. Given their age, geographic locations, and complex social-ecological histories, the examination and comparison of the cities of Detroit and Baltimore in the workshop described here provided a unique opportunity for evaluation of research, education and outreach needs, and opportunities in food, energy, and water (FEW) sustainability.