Stable Pb isotope ratios in aerosols, precipitation, and size-fractionated particulate matter in the Gulf of Maine, Scotian Shelf, and Labrador Sea

Measurements of Pb isotopes in aerosols, precipitation, and size-fractionated particulate matter from the Gulf of Maine, Scotian Shelf and Labrador Sea are used to investigate the source of Pb. The ²⁰⁶Pb/²⁰⁷Pb ratio in aerosols and precipitation collected at New Castle, NH suggests that anthropogenic Pb is a mixture of US and Canadian sources. ²⁰⁶Pb/²⁰⁷Pb ratios in >53 μm particulate matter from the Gulf of Maine and Scotian Shelf slope waters are consistent with contaminant Pb inputs predominantly from US and Canadian sources, while in shelf waters ²⁰⁶Pb/²⁰⁷Pb ratios in >0.4 μm and >53 μm particles are consistent with a mixture of US and Canadian sources, as well as Pb associated with resuspended surface sediment. ²⁰⁶Pb/²⁰⁷Pb ratios in particulate matter (>0.4 μm, 10-53 μm, and >53 μm) from Labrador Sea surface waters range from 1.165 to 1.211 and are a mixture of Pb derived from ore compositions consistent with Broken Hill, Australia and southeast Missouri, US sources.     

A Simple Method for Simulating Groundwater Interactions with Fens to Forecast Development Effects

Protection of fens–wetlands dependent on groundwater discharge–requires characterization of groundwater sources and stresses. Because instrumentation and numerical modeling of fens is labor intensive, easy-to-apply methods that model fen distribution and their vulnerability to development are desirable. Here we demonstrate that fen areas can be simulated using existing steady-state MODFLOW models when the unsaturated zone flow (UZF) package is included. In cells where the water table is near land surface, the UZF package calculates a head difference and scaled conductance at these “seepage drain” cells to generate average rates of vertical seepage to the land. This formulation, which represents an alternative to blanketing the MODFLOW domain with drains, requires very little input from the user because unsaturated flow-routing is inactive and results are primarily driven by easily obtained topographic information. Like the drain approach, it has the advantage that the distribution of seepage areas is not predetermined by the modeler, but rather emerges from simulated heads. Beyond the drain approach, it takes account of intracell land surface variation to explicitly quantify multiple surficial flows corresponding to infiltration, rejected recharge, recharge and land-surface seepage. Application of the method to a basin in southeastern Wisconsin demonstrates how it can be used as a decision-support tool to first, reproduce fen distribution and, second, forecast drawdown and reduced seepage at fens in response to shallow pumping.     

Influence of colloidal silica grout on liquefaction potential and cyclic undrained behavior of loose sand

Cyclic triaxial tests were performed to investigate the influence of colloidal silica grout on the deformation properties of saturated loose sand. Distinctly different deformation properties were observed between grouted and ungrouted samples. Untreated samples developed very little axial strain prior to the onset of liquefaction. However, once liquefaction was triggered, large strains occurred rapidly and the samples collapsed within a few additional loading cycles. In contrast, grouted sand samples experienced very little strain during cyclic loading. Additionally, the strain accumulated uniformly throughout loading rather than rapidly prior to collapse and the samples never collapsed. Cyclic triaxial tests were done on samples stabilized with colloidal silica at concentrations of 5, 10, 15, and 20%. In general, samples stabilized with higher concentrations of colloidal silica experienced very little strain during cyclic loading. Sands stabilized with lower concentrations tolerated cyclic loading well, but experienced slightly more strain. Thus, treatment with colloidal silica grout significantly increased the deformation resistance of loose sand to cyclic loading.     

A Framework for Quantitative Assessment of Impacts Related to Energy and Mineral Resource Development

Natural resource planning at all scales demands methods for assessing the impacts of resource development and use, and in particular it requires standardized methods that yield robust and unbiased results. Building from existing probabilistic methods for assessing the volumes of energy and mineral resources, we provide an algorithm for consistent, reproducible, quantitative assessment of resource development impacts. The approach combines probabilistic input data with Monte Carlo statistical methods to determine probabilistic outputs that convey the uncertainties inherent in the data. For example, one can utilize our algorithm to combine data from a natural gas resource assessment with maps of sage grouse leks and piñon-juniper woodlands in the same area to estimate possible future habitat impacts due to possible future gas development. As another example: one could combine geochemical data and maps of lynx habitat with data from a mineral deposit assessment in the same area to determine possible future mining impacts on water resources and lynx habitat. The approach can be applied to a broad range of positive and negative resource development impacts, such as water quantity or quality, economic benefits, or air quality, limited only by the availability of necessary input data and quantified relationships among geologic resources, development alternatives, and impacts. The framework enables quantitative evaluation of the trade-offs inherent in resource management decision-making, including cumulative impacts, to address societal concerns and policy aspects of resource development.     

Dialogue, metaphors of dialogue and understandings of geography

Increasingly, human and physical geographers alike describe their research practices as involving dialogue of some kind. However, the widespread popularity of the term belies some very different understandings of its meaning and methodological implications. In this paper we reflect on these different conceptions of 'dialogical' geography as a way, first, of illustrating the broad range of understandings of research methodology now current in the discipline and, second, of identifying their implications for three long–standing controversies in the discipline: relativism and the truth of geographical knowledge, the associated dualisms between subject/object, nature/society, and realism/constructionism, and the potential for unity between human and physical geography. We argue that, while dialogue is a potentially fruitful way of understanding and practising geography, that the defence of it, like the attacks on it, is often misconceived.