Geochemistry and hydrology of perched groundwater springs: assessing elevated uranium concentrations at Pigeon Spring relative to nearby Pigeon Mine,Arizona (USA)

The processes that affect water chemistry as the water flows from recharge areas through breccia-pipe uranium deposits in the Grand Canyon region of the southwestern United States are not well understood. Pigeon Spring had elevated uranium in 1982 (44 μg/L), compared to other perched springs (2.7–18 μg/L), prior to mining operations at the nearby Pigeon Mine. Perched groundwater springs in an area around the Pigeon Mine were sampled between 2009 and 2015 and compared with material from the Pigeon Mine to better understand the geochemistry and hydrology of the area. Two general groups of perched groundwater springs were identified from this study; one group is characterized by calcium sulfate type water, low uranium activity ratio ²³⁴U/²³⁸U (UAR) values, and a mixture of water with some component of modern water, and the other group by calcium-magnesium sulfate type water, higher UAR values, and radiocarbon ages indicating recharge on the order of several thousand years ago. Multivariate statistical principal components analysis of Pigeon Mine and spring samples indicate Cu, Pb, As, Mn, and Cd concentrations distinguished mining-related leachates from perched groundwater springs. The groundwater potentiometric surface indicates that perched groundwater at Pigeon Mine would likely flow toward the northwest away from Pigeon Spring. The geochemical analysis of the water, sediment and rock samples collected from the Snake Gulch area indicate that the elevated uranium at Pigeon Spring is likely related to a natural source of uranium upgradient from the spring and not likely related to the Pigeon Mine.     

Numerical Simulation of Mesoscale Circulations in a Region of Contrasting Soil Types

Mesoscale processes that form due to changes in surface characteristics play a dominant role in the development of the planetary boundary layer structure and the formation of convection. In this study, effects of the Sandhills region of North and South Carolina on mesoscale processes are examined. Climatological analyses indicate increased convective precipitation in this location as compared to the surrounding region. This is believed to be due to enhanced convection induced by horizontal heat flux gradients caused by sharp changes in soil type and hence the heat capacity of the soil. Simulations using a non-hydrostatic mesoscale model (MM5 version 3.3) were made for a non-precipitation case with a 5-km resolution domain centered over the Carolinas from August 15, 2000 to August 18, 2000. The results showed the existence of a mesoscale circulation over the Sandhills region. Differential heating induced by contrasting soil types dividing the Coastal Plain from the central Piedmont causes this circulation. Sea-breeze circulation often combines with the Sandhills circulation to initiate convection in this region. Diurnal variations are handled well by the model indicating that the thermodynamic structure of the atmosphere is well simulated.     

The Fundamental Challenge in Measuring Sprawl: Which Land Should Be Considered?*

Lack of agreement on how to define and measure sprawl has hampered development of policy related to its causes and consequences. We question previous work for two reasons: the use of study areas that overbound or underbound sprawl landscapes, and the failure to account for land unavailable for development. We formulate “extended urban areas,” based on housing density and commuting patterns and argue that they represent a preferable geographic basis for measuring sprawl. We operationalize with satellite imagery a way for measuring land unavailable for development in these areas. We then compute five measures of urban development using the National Land Cover Data Base and decennial census data to assess the extent of sprawl in the extended urban areas of Atlanta, Baltimore, Boston, Los Angeles, and Washington. Our sensitivity analyses reveal that the measurement of sprawl critically depends on which land area forms the basis of the analysis, and, to a lesser degree, how one accounts for land unavailable for development.     

Pele's hairs and tears: Natural probe of volcanic plume

We present a detailed petrographic study of Pele's hairs and tears sampled from Masaya volcano (Nicaragua). This study provides new observations of these little-known pyroclastic objects using both secondary electron images (SEI) and back scattering electron images (BSEI). Our work shows that Pele's tears can be associated with Pele's hairs after their formation: tears can be trapped on the walls and/or in the cavities of Pele's hairs. Moreover, chemical investigations of the Pele's hairs and tears highlight the presence of a chemical zonation. The edge of these tears and hairs show a siliceous enrichment, allowing us to quantify the interaction time of the silicate glass with acid gases in the volcanic plume. This study confirms the syneruptive and post eruptive volatile exsolution from Pele's hairs and tears.     

The Montenegro,Yugoslavia, earthquake of April 15, 1979: source orientation and strength

Long-period teleseismic P, S and Rayleigh waves and geologic considerations indicate that the Montenegro earthquake involved thrust faulting on a plane striking nearly parallel to the Adriatic coast and dipping ca. 15° toward the Yugoslav mainland. There is some support from modeling of body waves recorded on long-period WWSSN instruments for a focal depth of 22 km, but the possibility of a multiple source and the difficulty of matching some of the detailed characteristics of the P- and S-wave forms reduce our confidence in the determination of the depth. Fortunately, the source orientation and moment of the event are not sensitive functions of the depth. The long-period (256 s) moment was 4.6 × 10¹⁹ Nm (4.6 × 10²⁶ dyne-cm). The moment obtained by fitting the first cycle of P and S waves recorded on WWSSN long-period instruments is about four times smaller. This increase of moment with period is consistent with spectral estimates of the moment from SH waves recorded at SRO and ASRO stations.