Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Permeable homes: A historical political ecology of insects and pesticides in US public housing

This paper traces changes in the political ecology of insects and chemicals in US public housing since Congress founded public housing in 1937. Drawing upon the literature of critical geographies of home, urban political ecology, and medical history, it argues that the constitution of “public” and “private” space within public housing was deeply entangled with pest control practices there. Prior to 1945, reformers treated the housing as a commons, in part compelled by the mobility of bedbugs and the pesticide used to combat them, both of which were seen as serious health threats. Managers were also motivated by social welfare ideologies, while residents eagerly assisted with communal control policies in order to achieve freedom from the health insults of bedbugs. Following 1945, however, new synthetic pesticides like DDT seemed to stay safely within one apartment unit, encouraging housing managers to abandon community-oriented pest control practices. Meanwhile, curtailed budgets, particularly after the Housing Act of 1949, left the infrastructure of public housing to decay, rendering units more physically permeable even as managers neglected the communities there. The new pesticides nearly eradicated bedbugs, but tenacious populations of German cockroaches blossomed thanks to the permeable buildings and synthetic pesticides. Residents grew increasingly resistant to pesticide use as they observed that cockroach populations went unabated. The paper serves as a case for applying political ecology frameworks to domestic spaces, and also argues that housing quality and domestic pesticide use are not merely private responsibilities but should be regarded as environmental justice issues.     

Causes and consequences of errors in determining sorption capacity of coals for carbon dioxide at high pressure

Recent comparisons of CO₂ sorption by coals at high pressures have shown major differences between the results obtained by different laboratories. These need to be resolved for laboratory estimation of CO₂ sequestration in coal seams to be useful. A compilation of potential sources of error in determination of sorption characteristics and their impact on sorption measurements is provided here. A series of tests is also provided that can be used to identify and reduce such errors in measurement. For example, an error in temperature produces a characteristic distortion of the sorption curve for carbon dioxide, which can be corrected to some extent. A negative value for excess sorption at high pressure is almost certainly diagnostic of either a cell volume that has been overestimated or that some part of the substrate that is inaccessible to the gas is accessible to helium. The major source of variation between results from the different laboratories that supplied the closest sorption values was found to be variations in the assumed free space volume, which could be due to discrepancies in determined helium density or measured cell volume. Including a term in the sorption model that is proportional to gas density will markedly reduce the influence of such errors in estimating sorption capacity or heats of sorption. The influence of swelling and moisture on sorption isotherms is also quantified here. Correction for swelling of coals in carbon dioxide changes the estimated sorption capacity by less than 1%, if a term that is proportional to gas density is included as a free parameter in the model fitting the isotherm.     

Residential preferences in Sydney's inner suburbs: a study in diversity

The revitalization of inner suburban areas is attracting a good deal of attention throughout the Western world, not least because it frequently involves a process of gentrification whereby middle-class individuals move into and renovate formerly working-class dwellings. Evidence from Sydney suggests that this process of revitalization is spatially very selective and as a result it is overly simplistic to talk about the inner suburbs as a whole, because within inner city areas there exists a very considerable mix of both land use and building quality. Moreover the socio-economic characteristics of residents and their attitudes towards the issues of preservation, rebuilding, and high-rise, tend to vary between the different types of residential environment. Consequently there can exist no single planning prognosis for all inner suburban areas. What seems to be needed is a blend of preservation and redevelopment that avoids forcing middle-class aesthetic values on working-class residents, for whom considerations of accessibility to central area employment and facilities appear dominant.     

Some airborne observations of condensation nucleus concentration

Summary The measurement of condensation nucleus concentration from an aircraft is considered and preliminary observations made with a photo-electric condensation nucleus counter discussed. It is concluded that at heights up to 6000 ft concentrations of condensation nuclei vary, from 200 cm^–3 in clean maritime air to 5 × 10³ cm^–3 generally over large industrial areas and 35 × 10³ cm^–3 downwind of specific industrial sources. It appears that sea spray makes a contribution to the nucleus population.Mr. G. J.Day, B. Sc., Meteorological Research Flight, Royal Aircraft Establishment,Farnborough, Hants. (Great Britain).     

Mineral-chemical and isotopic studies of Namaqualand granulites,South Africa: A grenville analogue

The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area. U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°–650° C and ca. 7–8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.     

Rock hardness and landform development in the Gunong Mulu National Park,Sarawak, E. Malaysia

Rock hardness, measured by the Schmidt Test Hammer, is an important factor in explaining some of the sharp contrasts in relief and wide variety of landforms in The Gunong Mulu National Park. It is also useful in consideration of contemporary geomorphological processes, such as landsliding.     

The morphology and hydrology of some Jamaican karst depressions

An area of shallow karst depressions in St. Ann parish, Jamaica is examined. Shallow, ‘temperate’ forms such as these are common in tropical karst areas. Depressions are analyzed morphometrically and a considerable degree of structural control appears to be exhibited. Some morphometric parameters, in particular the depth/diameter ratio, which have been used to postulate depression origin, are examined. Depression hydrology is investigated. Depressions appear to exhibit characteristics of small drainage basins. The occurrence of sporadic flood events is suggested as an important element in the development of surface channel networks. The integration of the depressions into the regional drainage system is investigated by water tracing of ‘underground flow sections’. Chemical analysis of waters in various parts of the drainage system reveals variations which point to the importance of solutional activity at the soil-bedrock interface. The influence of a soil cover in promoting solutional activity is indicated.