Ralph K. Davis

Distribution and variability of redox zones controlling spatial variability of arsenic in the Mississippi River Valley alluvial aquifer, southeastern Arkansas

Twenty one of 118 irrigation water wells in the shallow (25-30 m thick) Mississippi River Valley alluvial aquifer in the Bayou Bartholomew watershed, southeastern Arkansas had arsenic (As) concentrations (<0.5 to 77 microg/L) exceeding 10 microg/L. Sediment and groundwater samples were collected and analyzed from the sites of the highest, median, and lowest concentrations of As in groundwater in the alluvial aquifers located at Jefferson County, Arkansas. A traditional five-step sequential extraction was performed to differentiate the exchangeable, carbonate, amorphous Fe and Mn oxide, organic, and hot HNO(3)-leachable fraction of As and other compounds in sediments. The Chao reagent (0.25 M hydroxylamine hydrochloride in 0.25 M HCl) removes amorphous Fe and Mn oxides and oxyhydroxides (present as coatings on grains and amorphous minerals) by reductive dissolution and is a measure of reducible Fe and Mn in sediments. The hot HNO(3) extraction removes mostly crystalline metal oxides and all other labile forms of As. Significant total As (20%) is complexed with amorphous Fe and Mn oxides in sediments. Arsenic abundance is not significant in carbonates or organic matter. Significant (40-70 microg/kg) exchangeable As is only present at shallow depth (0-1 m below ground surface). Arsenic is positively correlated to Fe extracted by Chao reagent (r=0.83) and hot HNO(3) (r=0.85). Arsenic extracted by Chao reagent decreases significantly with depth as compared to As extracted by hot HNO(3). Fe (II)/Fe (the ratio of Fe concentration in the extracts of Chao reagent and hot HNO(3)) is positively correlated (r=0.76) to As extracted from Chao reagent. Although Fe (II)/Fe increases with depth, the relative abundance of reducible Fe decreases noticeably with depth. The amount of reducible Fe, as well as As complexed to amorphous Fe and Mn oxides and oxyhydroxides decreases with depth. Possible explanations for the decrease in reducible Fe and its complexed As with depth include historic flushing of As and Fe from hydrous ferric oxides (HFO) by microbially-mediated reductive dissolution and aging of HFO to crystalline phases. Hydrogeochemical data suggests that the groundwater in the area falls in the mildly reducing (suboxic) to relatively highly reducing (anoxic) zone, and points to reductive dissolution of HFO as the dominant As release mechanism. Spatial variability of gypsum solubility and simultaneous SO(4)(2-) reduction with co-precipitation of As and sulfide is an important limiting process controlling the concentration of As in groundwater in the area.

Two-Stage Exams Improve Student Learning in an Introductory Geology Course: Logistics, Attendance, and Grades

ABSTRACT Two-stage exams—where students complete part one of an exam closed book and independently and part two is completed open book and independently (two-stage independent, or TS-I) or collaboratively (two-stage collaborative, or TS-C)—provide a means to include collaborative learning in summative assessments. Collaborative learning has been shown to have positive benefits, including increased student engagement and learning. To try to improve student learning, as measured by improvement in exam scores, two sections of introductory geology were taught using two-stage exams. It was hypothesized that class scores would be higher for semesters using two-stage exams—whether part two was TS-C or TS-I—than for semesters using traditional (T) exams. The median attendance rate was approximately 67% for all testing methods and was significantly greater when exams were TS-C (69%) rather than TS-I (53%). The class score was significantly greater during semesters when exams were TS-C (81%) but was not significantly different between T and TS-I semesters. To assess individual student learning over time, part one of the first exam and part one of the comprehensive final exam were compared. Across the F and D grade ranges, improvement on individual exam scores was significantly greater for the TS-C semester than for the TS-I and T semesters. Student learning, as measured by individual exam scores, improved due to the use of TS-C exams. The improvement in class scores due to the collaborative portion of two-stage exams was independent of increased attendance rates, greater for the lower-achieving students, and not observable if part two of the exam was completed as a take-home exam (TS-I).

Are Ozark Streams Underfit? Using Gis to Re-Examine Dury's Theory of Underfit Streams

The theory of underfit streams as described by George H. Dury has often been used to explain the distinct compound valleys found throughout the Ozark Plateaus physiographic province. Dury developed mathematical relationships between the valley meander wavelength and the stream meander wavelength to define the underfit condition. He found that Osage underfit streams fail to meander within the stream valley and have wavelength ratios of approximately 3.5, and that the valley meander wavelengths are 30 times the bankfull width of the stream. This study uses modern analytical techniques and data sets to test these mathematical relationships both for watersheds used in Durys original study and for other watersheds in the region. Using GIS-based analysis of digital raster graphics, digital elevation models, and U.S. Geological Survey hydrology data, we could not replicate Durys results. Most wavelength ratios measured for all streams were less than 3.0, and valley wavelengths ranged from 0.08 to 141 times the bankfull bed width of the river. The theory of underfit streams fails to explain the great variability of data derived in this analysis. In fact, this study suggests that structural controls have a greater influence on modern valley architecture than hydrologic controls.