Mahan S.,U.S. Geological Survey |
Kay J.,Daniel B. Stephens and Associates , Inc.
Quaternary Geochronology | Year: 2012
The long term stability and reliability of the luminescence signal for gypsum has not been well documented or systematically measured until just recently. A review of the current literature for luminescence dating of gypsum is compiled here along with original efforts at dating an intact and in-situ bed of selenite gypsum at Salt Basin Playa, New Mexico and Texas. This effort differs from other documented luminescence dating efforts because the gypsum is not powdery or redistributed from its original growth patterns within the playa basin but is instead of a crystalline form. Sixteen ages from eight cores were ultimately produced with seven of the ages coming from rare detrital quartz encased in or with the gypsum crystals while the remaining ages are from the crystalline gypsum. As far as can be ascertained, the quartz was measured separately from the gypsum and no contaminants were noted in any of the aliquots. Some basic and preliminary tests of signal stability were measured and found to be mitigated by lessening of pre-heat protocols. Ages ranged from 8 ka to 10 ka in the shallow cores and 16 ka to 22 ka in the deeper cores. These ages will be useful in determining rates of gypsum growth within a sequence of evaporates which, in turn, will help to better document historic rates of evaporation and thus estimate, with more precision, the corresponding annual evaporation rates. © 2012 . Source
Schnaar G.,Daniel B. Stephens and Associates , Inc. |
Brusseau M.L.,University of Arizona
Journal of Environmental Science and Health - Part A Toxic/Hazardous Substances and Environmental Engineering | Year: 2013
The miscible-displacement method is one commonly used approach for measuring equilibrium sorption coefficients. The objective of this research was to examine the impact of experiment conditions (detection limit, input-pulse size, input concentration) on the measurement of retardation factors and sorption coefficients for a system that exhibits significant nonideal sorption behavior. A series of miscible-displacement experiments was conducted wherein effluent solute concentrations were monitored over a range of approximately seven orders of magnitude, allowing characterization of asymptotic tailing phenomenon, which was significant. The magnitude of Kd increased asymptotically with the increase in the extent of the elution tail measured. The results also showed that the fraction of the tail required to obtain close-to-maximum measures of Kd is greater for smaller input pulses. Investigating the impact of analytical detection limit (equivalent to relative concentrations of 10-3 and 10-7) revealed that the magnitude of Kd was invariant with input pulse for the 10 -7 detection limit. Conversely, the measured Kd values were significantly smaller at low input pulses for the 10-3 detection limit. © 2013 Copyright Taylor and Francis Group, LLC. Source
Brusseau M.L.,University of Arizona |
Russo A.E.,University of Arizona |
Schnaar G.,University of Arizona |
Schnaar G.,Daniel B. Stephens and Associates , Inc.
Chemosphere | Year: 2012
A series of miscible-displacement experiments was conducted to examine the impact of sorption contact time on desorption and elution of trichloroethene from a well-characterized soil. A large number of contact times were examined, spanning 1h to 4years (∼2×106h). Effluent trichloroethene concentrations were monitored over a range of greater than six orders of magnitude, allowing characterization of potential asymptotic tailing. The results of the column experiments showed that trichloroethene exhibited extensive elution tailing for all experiments. Each increase in contact time resulted in a successive increase in the extent of tailing. In total, the number of pore volumes of water flushing required to reach the analytical detection limit increased from approximately 1000 for the 1-h contact time to almost 9000 for the 4-year contact time. These results indicate that a contact time of less than 1h produced a sorbed phase that is relatively resistant to desorption, and that a progressive increase in resistance to desorption occurred with increased contact time. A mathematical model incorporating nonlinear, rate-limited sorption/desorption described by a continuous-distribution reaction function was used to successfully simulate the measured data. The nonlinear sorption, the apparent rapid development of desorption resistance, and the progressive increase in resistance with increasing contact time are consistent with behavior anticipated for sorbate interactions with hard-carbon components of the soil. © 2012 Elsevier Ltd. Source
Stephens D.B.,Daniel B. Stephens and Associates , Inc.
Groundwater | Year: 2015
The U.S. Environmental Protection Agency (EPA) was contacted by citizens of Pavillion, Wyoming 6years ago regarding taste and odor in their water wells in an area where hydraulic fracturing operations were occurring. EPA conducted a field investigation, including drilling two deep monitor wells, and concluded in a draft report that constituents associated with hydraulic fracturing had impacted the drinking water aquifer. Following extensive media coverage, pressure from state and other federal agencies, and extensive technical criticism from industry, EPA stated the draft report would not undergo peer review, that it would not rely on the conclusions, and that it had relinquished its lead role in the investigation to the State of Wyoming for further investigation without resolving the source of the taste and odor problem. Review of the events leading up to EPA's decision suggests that much of the criticism could have been avoided through improved preproject planning with clear objectives. Such planning would have identified the high national significance and potential implications of the proposed work. Expanded stakeholder involvement and technical input could have eliminated some of the difficulties that plagued the investigation. However, collecting baseline groundwater quality data prior to initiating hydraulic fracturing likely would have been an effective way to evaluate potential impacts. The Pavillion groundwater investigation provides an excellent opportunity for improving field methods, report transparency, clarity of communication, and the peer review process in future investigations of the impacts of hydraulic fracturing on groundwater. © 2014, National Ground Water Association. Source
Leyden J.,Skin Study Center |
Stephens T.J.,Daniel B. Stephens and Associates , Inc.
Journal of the American Academy of Dermatology | Year: 2012
Background: Until now, nonablative fractional treatments could only be delivered in an office setting by trained professionals. Objective: The goal of this work was to perform clinical testing of a nonablative fractional laser device designed for home-use. Methods: This multicenter trial consisted of two clinical studies with slightly varying treatment protocols in which subjects performed at-home treatments of periorbital wrinkles using a handheld nonablative fractional laser. Both studies included an active treatment phase (daily treatments) and a maintenance phase (twice-weekly treatments). In all, 36 subjects were followed up for as long as 5 months after completion of the maintenance phase and 90 subjects were followed up until the completion of the maintenance phase. Evaluations included in-person investigator assessment, independent blinded review of high-resolution images using the Fitzpatrick Wrinkle Scale, and subject self-assessment. Results: All 124 subjects who completed the study were able to use the device following written instructions for use. Treatments were well tolerated with good protocol compliance. Independent blinded evaluations by a panel of physicians showed Fitzpatrick Wrinkle Scale score improvement by one or more grades in 90% of subjects at the completion of the active phase and in 79% of subjects at the completion of the maintenance phase. The most prevalent side effect was transient posttreatment erythema. Limitations: Lack of a control group and single-blinded study groups were limitations. Conclusion: Safety testing with self-applications by users demonstrated the utility of the device for home use. Independent blinded review of clinical images confirmed the device's proficiency for improving periorbital wrinkles. © 2012 by the American Academy of Dermatology, Inc. Source