Richard A. Wildman

Burning of forest materials under late Paleozoic high atmospheric oxygen levels

Theoretical models suggest that atmospheric oxygen reached concentrations as high as 35% O2 during the past 550 m.y. Previous burning experiments using strips of paper have challenged this idea, concluding that ancient wildfires would have decimated plant life if O2 significantly exceeded its present level of 21%. New thermochemistry and flame-spread experiments using natural fuels contradict these results and indicate that sustained burning of forest fuels at moisture contents common to living plants does not occur between 21% and 35% O2. Therefore, the fires under atmospheres with high oxygen concentrations would not have prevented the persistence of plant communities. Times of high O2 also agree with observations of concurrent fire-resistant plant morphology, large insects, and high concentrations of fossil charcoal.

Physical, Chemical, and Mineralogical Characteristics of a Reservoir Sediment Delta (Lake Powell, USA) and Implications for Water Quality during Low Water Level

Lake Powell is a large reservoir in Utah and Arizona that has experienced large changes in water level during a recent drought. As a first step in assessing the connection between hydrologic and chemical changes at Lake Powell, we characterized the particle size and solid-phase bulk concentrations for 31 elements and 25 minerals in sediment from the inflow region and some shoreline locations by using laser diffractometry, X-ray fluorescence, elemental analysis, and X-ray diffraction Our results are consistent with previous results that show a negative correlation between particle size and concentrations of most elements and most minerals other than quartz and some feldspars. In our samples, however, solid-phase iron, rather than particle size or organic carbon, is the best predictor variable for the solid-phase concentrations of elements and minerals. Sediment characteristics vary on a scale of tens of kilometers, with fine sediment that is enriched in trace elementsnearer to the dam. These trends allow formulation of an algorithm for determining a water-level threshold below which sediment resuspension may alter water chemistry in a generic reservoir with a long and narrow sediment delta.

Hydrous manganese oxide doped gel probe sampler for measuring in situ reductive dissolution rates. 2. Field deployment.

In situ rates of reductive dissolution in submerged shoreline sediments at Lake Tegel (Berlin, Germany) were measured with a novel hydrous manganese (Mn) oxide-doped gel probe sampler in concert with equilibrium gel probe and sequential extraction measurements. Rates were low in the top 8 cm, then showed a peak from 8 to 14 cm, with a maximum at 12 cm depth. This rate corresponded with a peak in dissolved porewater iron (Fe) at 11 cm depth. Below 14 cm, the reductive dissolution rate reached an intermediate steady value. Lower rates at depth corresponded with increases in operationally defined fractions of carbonate-bound and organic- and sulfide-bound Mn and Fe as detected by sequential extraction. Observed rates of reductive dissolution, which reflect a capacity for Mn reduction rather than actual rates under ambient conditions, appear to correlate with porewater chemistry and sequential extraction fractions as expected in early sediment diagenesis, and are consistent with previous measurements of in situ reductive dissolution rates. Significant downward advection in this bank filtration setting depletes the Mn and Fe oxides in the sediments and enhances the transport of dissolved Fe and Mn into the infiltrating water.

Mercury and methylmercury in a reservoir during seasonal variation in hydrology and circulation

Abstract This study explored the effect of seasonally varying inflow hydrology and circulation on total mercury (Hg) and methylmercury (MeHg) in the water column of a warm, temperate reservoir. The study site in Grand Lake, Oklahoma, is >80 km long and usually 36 m deep at its dam. During this study, aside from 2 large, storm-driven springtime events, drought caused low inflows. During maximal summer stratification, the surface mixed layer was 8–11 m thick, water below was anoxic, and inflows entered as an interflow. Total Hg in filtered samples was <4 ng/L with an interquartile range of 0.27–0.96 ng/L. Highest concentrations occurred in spring and summer, and lowest concentrations followed overturn. In filtered samples, MeHg was ≤0.62 ng/L with an interquartile range of 0.01–0.08 ng/L. Highest concentrations of MeHg occurred in some large inflows and in anoxic summertime bottom water in specific locations where total suspended sediment was also elevated. During this study, Hg concentrations in Grand Lake were driven by inflow hydrology when inflows were high. During low inflows, biogeochemistry controlled the enrichment of MeHg in specific locations of the anoxic bottom water and sequestered both Hg species in autumn. This sequestration suggests that Grand Lake decreases watershed-derived Hg pollution downstream. Occurrence of elevated Hg and MeHg concentrations primarily in large inflows and anoxic bottom water suggests that exposure of biota to Hg species is greatest when floods enter the reservoir because Hg enrichment in deep, anoxic water does not coincide with the habitat of most fauna.

Hydrologic and biogeochemical controls of river subsurface solutes under agriculturally enhanced ground water flow

The relative influences of hydrologic processes and biogeochemistry on the transport and retention of minor solutes were compared in the riverbed of the lower Merced River (California, USA). The subsurface of this reach receives ground water discharge and surface water infiltration due to an altered hydraulic setting resulting from agricultural irrigation. Filtered ground water samples were collected from 30 drive point locations in March, June, and October 2004. Hydrologic processes, described previously, were verified by observations of bromine concentrations; manganese was used to indicate redox conditions. The separate responses of the minor solutes strontium, barium, uranium, and phosphorus to these influences were examined. Correlation and principal component analyses indicate that hydrologic processes dominate the distribution of trace elements in the ground water. Redox conditions appear to be independent of hydrologic processes and account for most of the remaining data variability. With some variability, major processes are consistent in two sampling transects separated by 100 m.

Long-Term and Seasonal Trends of Wastewater Chemicals in Lake Mead: An Introduction to Time Series Decomposition

ABSTRACT A recent paper published time series of concentrations of chemicals in drinking water collected from the bottom of Lake Mead, a major American water supply reservoir. Data were compared to water level using only linear regression. This creates an opportunity for students to analyze these data further. This article presents a structured introduction to time series decomposition that compares long-term and seasonal components of a time series of a single chemical (meprobamate) with those of two supporting datasets (reservoir volume and specific conductance). For the chemical data, this must be preceded by estimation of missing datum points. Results show that linear regression analyses of time series data obscure meaningful detail and that specific conductance is the important predictor of seasonal chemical variations. To learn this, students must execute a linear regression, estimate missing data using local regression, decompose time series, and compare time series using cross-correlation. Complete R code for these exercises appears in the supplementary information. This article uses real data and requires that students make and justify key decisions about the analysis. It can be a guided or an individual project. It is scalable to instructor needs and student interests in ways that are identified clearly in this article.