Christopher H. Swartz

AN AEM-TEM STUDY OF NANOMETER-SCALE MINERAL ASSOCIATIONS IN AN AQUIFER SAND : IMPLICATIONS FOR COLLOID MOBILIZATION

Abstract Analytical and transmission electron microscopy (AEM-TEM) techniques were used to identify mineral juxtapositions at the nanometer-scale in the interstitial matrix of a shallow, Southeastern Coastal plain aquifer sand (Georgetown, South Carolina, USA). In doing so, we sought to infer particle-particle interaction mechanisms holding the matrix intact. The aquifer is a fine-to-medium quartz sand with approximately 12% by weight

Association between residence on Cape Cod, Massachusetts, and breast cancer ☆

PURPOSE Massachusetts cancer registry and case-control data suggest that breast cancer incidence is elevated on Cape Cod relative to other parts of the state. We examined the association between length of residence on Cape Cod and breast cancer, since residential history could be acting as a surrogate for unidentified environmental risk factors. METHODS We computed odds ratios (OR) and 95% confidence limits (CL) for 1121 cases occurring between 1988 and 1995 on Cape Cod and 992 controls, according to categories of residence time on Cape Cod, after adjusting for age, family history, parity and age at first live or stillbirth, education, body mass index, and breast cancer history. RESULTS Breast cancer risk was elevated among women living on Cape Cod 5 or more years with a peak occurring in the 25 to less than 30 year category (adjusted OR=1.72; 95% CL, 1.12, 2.64). Adjusting for confounding strengthened the associations. Odds ratios did not increase monotonically over categories of longer residence. CONCLUSIONS Our results suggest that longer residence on Cape Cod is associated with elevated breast cancer risk, however inconsistency in the pattern of association limits conclusions that might be drawn about it. Suspected environmental exposures include pesticides and drinking water contaminated by industrial, agricultural, and residential land use.

An experimental study of mixing and instability development in variable-density systems

Abstract This study presents results of flow-tank experiments on variable-density flow in a layered system. Less dense water displaced more dense water in a system layered with a lower over a higher hydraulic conductivity unit. This configuration created a potentially unstable interface between the displaced water and the displacing water. The displacing water wedge (i.e., less dense water) in the higher hydraulic conductivity layer traveled downgradient faster than the displacing water wedge moving in the layer above. Downward movement of more dense water from the upper layer into the freshwater wedge in the lower layer caused the latter to become more saline. Flow rate and density difference between displacing and displaced water, and hydraulic conductivity difference between layers, were each analyzed for their effect on mixing behavior in this system. Fluid movement and mixing processes were monitored using time sequence photography. Digital processing of black and white negatives provided a large and semi-continuous data base of concentration values to analyze the salinization of the displacing water. In many cases, the unstable stratification and density gradient also promoted the upward growth of finger-shaped instabilities into the less permeable layer. An analytical stability analysis was able to reasonably predict the wavelength of these fingers. Results from the experiments suggest that density variations can promote complex flow and mixing patterns in even the simple layered systems considered herein. This conclusion has important implications for both contaminant transport and fluid displacement processes occurring during remediation.

Modeling instability development in layered systems

Abstract This study investigates patterns of finger development and propagation in layered porous media. Fingers are created with interfacial perturbations, formed by adding a thin zone of regularly varying hydraulic conductivity along the layer. Simulation results agree qualitatively with those observed in two-dimensional laboratory experiments. In all cases, the formation of instabilities requires seeding of perturbations, even if the system is unstably stratified. A series of simulations show how the shapes of the instabilities differ according to where along the unstable interface the instabilities form and the layer in which they develop. Pathline analyses indicate how the patterns of flow in the domain can be exceedingly complex. Concentration distributions are influenced by movements of water between layers and the formation of a large convection cell in the lowermost layer. These numerical investigations reinforce inferences from the experimental studies that classical stability theory is less useful in determining whether instabilities will form and what their shape will be. Even with the relatively simple layering, patterns of flow and resulting concentrations are complex.