A synthesis of published vegetation mercury (Hg) data across 11 contiguous states in the western United States showed that aboveground biomass concentrations followed the order: leaves (26μgkg(-1))~branches (26μgkg(-1))>bark (16μgkg(-1))>bole wood (1μgkg(-1)). No spatial trends of Hg in aboveground biomass distribution were detected, which likely is due to very sparse data coverage and different sampling protocols. Vegetation data are largely lacking for important functional vegetation types such as shrubs, herbaceous species, and grasses. Soil concentrations collected from the published literature were high in the western United States, with 12% of observations exceeding 100μgkg(-1), reflecting a bias toward investigations in Hg-enriched sites. In contrast, soil Hg concentrations from a randomly distributed data set (1911 sampling points; Smith et al., 2013a) averaged 24μgkg(-1) (A-horizon) and 22μgkg(-1) (C-horizon), and only 2.6% of data exceeded 100μgkg(-1). Soil Hg concentrations significantly differed among land covers, following the order: forested upland>planted/cultivated>herbaceous upland/shrubland>barren soils. Concentrations in forests were on average 2.5 times higher than in barren locations. Principal component analyses showed that soil Hg concentrations were not or weakly related to modeled dry and wet Hg deposition and proximity to mining, geothermal areas, and coal-fired power plants. Soil Hg distribution also was not closely related to other trace metals, but strongly associated with organic carbon, precipitation, canopy greenness, and foliar Hg pools of overlying vegetation. These patterns indicate that soil Hg concentrations are related to atmospheric deposition and reflect an overwhelming influence of plant productivity - driven by water availability - with productive landscapes showing high soil Hg accumulation and unproductive barren soils and shrublands showing low soil Hg values. Large expanses of low-productivity, arid ecosystems across the western U.S. result in some of the lowest soil Hg concentrations observed worldwide.

Understanding the emissions of mercury (Hg) from wildfires is important for quantifying the global atmospheric Hg sources. Emissions of Hg from soils resulting from wildfires in the Western United States was estimated for the 2000 to 2013 period, and the potential emission of Hg from forest soils was assessed as a function of forest type and soil-heating. Wildfire released an annual average of 3100±1900kg-Hgy(-1) for the years spanning 2000-2013 in the 11 states within the study area. This estimate is nearly 5-fold lower than previous estimates for the study region. Lower emission estimates are attributed to an inclusion of fire severity within burn perimeters. Within reported wildfire perimeters, the average distribution of low, moderate, and high severity burns was 52, 29, and 19% of the total area, respectively. Review of literature data suggests that that low severity burning does not result in soil heating, moderate severity fire results in shallow soil heating, and high severity fire results in relatively deep soil heating (<5cm). Using this approach, emission factors for high severity burns ranged from 58 to 640μg-Hgkg-fuel(-1). In contrast, low severity burns have emission factors that are estimated to be only 18-34μg-Hgkg-fuel(-1). In this estimate, wildfire is predicted to release 1-30gHgha(-1) from Western United States forest soils while above ground fuels are projected to contribute an additional 0.9 to 7.8gHgha(-1). Land cover types with low biomass (desert scrub) are projected to release less than 1gHgha(-1). Following soil sources, fuel source contributions to total Hg emissions generally followed the order of duff>wood>foliage>litter>branches.

This paper investigates the early-age dynamics of mineral formation in metakaolin-based alkali-activated cements. The effects of silica availability and alkali content on mineral formation were investigated via X-ray diffraction and solid-state 29 Si magic-angle spinning nuclear magnetic resonance at 2, 7, 14, and 28 days. Silica availability was controlled by using either liquid-(immediate) or solid-based (gradual) sodium silicate supplements. Mineral (zeolitic) and amorphous microstructural characteristics were correlated with observed changes in bulk physical properties, namely shrinkage, density, and porosity. Results demonstrate that, while alkali content controls the mineralization in immediately available silica systems, alkali content controls the silica availability in gradually available silica systems. Immediate silica availability generally leads to a more favorable mineral formation as demonstrated by correlated improvements in bulk physical properties.

Relatively few discharging playas in western United States extensional basins have high concentrations of lithium (Li) and calcium (Ca) in the basin-center brines. However, the source of both these ions is not well understood, and it is not clear why basins in close proximity within the same extensional trough have notably different concentrations of Li and Ca. In the Barstow-Bristol Trough, California, USA, three playas in separate topographically closed basins vary in Li and Ca concentrations from northwest to southeast: 71–110 mg/L Li and 17–65 g/L Ca at Bristol Dry Lake, 20–80 mg/L Li and 7.5–40 g/L Ca at Cadiz Dry Lake, and <5 mg/L Li and <0.5 g/L Ca at Danby Dry Lake. Using new and historic data from recently drilled wells (2017–2018), it has been determined that there is minimal variation of temperature, Li, and major ion concentrations with depth (down to 500 m), suggesting that the brines are well mixed and likely to circulate slowly due to density driven flow. Although it has been postulated that geothermal fluids supply the Li and Ca to Bristol and Cadiz closed basins, there is little to no surface evidence for geothermal fluids, except for a young (80,000-year-old) volcanic crater in Bristol Dry Lake. However, major-ion chemistry of fluid inclusions in bedded halite deposits show no change in brine chemistry over the last 3 million years in Bristol Dry Lake indicating that the source of lithium is not related to these recent basaltic eruptions. Mg–Li geothermometry of basin-center brines indicates that Bristol and Cadiz brines have possibly been heated to near 160 °C at some time and Danby brine water has been heated to less than 100 °C, although Cadiz and Danby lakes have no known surface geothermal features. The difference in Li concentrations between the different basins is likely caused by variable sources of both ions and the hydrology of the playas, including differences in how open or closed the basins are, recharge rates, evaporative concentration, permeability of basin-center sediments, and the possible amount of geothermal heating. The differences in Ca concentrations are more difficult to determine. However, historic groundwater data in the basins indicate that less saline groundwater on the north side of the basins has molar Ca:HCO3 and Ca:SO4 ratios greater than one, which indicates a non-saline groundwater source for at least some of the Ca. The similar Li and Ca concentrations in Bristol and Cadiz lakes may be because they are separated only by a low topographic divide and may have been connected at times in the past three million years. All three basins are at least Miocene in age, as all three basins have been interpreted to contain Bouse Formation sediments at various depths or in outcrop. The age of the basins indicates that there is ample time for concentration of Li and Ca in the basins even at low evaporation rates or low geothermal inputs. The source of Li for brines in Bristol and Cadiz basins is postulated to be from ancient geothermal fluids that no longer exist in the basin. The source of Li to the sediment may be either geothermal fluids or dissolution and concentration of Li from tephra layers and detrital micas or clays that are present in the sediments, or a combination of both. The source of Ca must at least partially come from groundwater in the alluvial fans, as some wells have Ca:HCO3 ratios that are greater than one. The source of Ca could be from the dissolution of Ca-bearing igneous rocks in the surrounding catchments with limited HCO3 contribution, or dilute geothermal water migrating up through faults in the basin margin. The relatively low concentration of Li and Ca in Danby playa is likely caused by a lack of sources in the basin and because the basin was (or is) partially hydrologically open to the south, reducing the effectiveness of evaporative concentration of solutes. Bristol Dry Lake is likely the only hydrologically closed basin of the three because although Cadiz has a similar brine chemistry and salinity, there is almost no halite deposition in the basin. It is only Bristol Dry Lake that contains 40% halite in its basin center.

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