Christopher Amrhein

Wood-ash composition and soil pH following intense burning

Thousands of hectares of wildlands are burned annually in the western United States. The composition and mineralogy of wood-ash produced by severe burning, and the changes in pH of soils underlying the ash, were examined at five sites in Califor-nia. Soil pH increased by as much as 3 pH units (to pH 10.5) immediately after burn-ing compared with unburned soil. Approx-imately 1 to 2% of each burn area was affected to a maximum observed depth of 20 cm. The major component of fresh, white wood-ash is calcite, while K and Na carbonates are present in minor amounts. The initial very high pH values of wood-ash and surface soil are caused by K and Na oxides, hydroxides, and carbonates. These compounds are very soluble and do not persist through the wet season. The calcite is much less soluble and was present in soils 3 years after burning, maintaining moderately alkaline pH in surface soils that are normally neutral to strongly acid.

Some factors affecting the dissolution kinetics of anorthite at 25°C

Abstract Batch dissolution experiments were conducted at 25°C to determine the effects of agitation, particle size, suspension density, wetting and drying cycles, drying temperature, sequential rinses, ionic strength, and the addition and removal of products on the rates of anorthite (An93) dissolution. In general, the dissolution kinetics at constant pH were not zero order with respect to products in solution, and this nonlinear release persisted beyond the time when Ca and Si stoichiometric dissolution was reached. The failure to establish zero-order kinetics could not be attributed to the weathering of damaged surfaces or fine, broken particles. Leached layer depths, calculated from solution composition, ranged from 2.6–3.5 nm; but a Ca-depleted surface layer was observed by energy dispersive X-ray analysis only on the reaction fines. Agitation rate had a marked effect on dissolution rate, contrary to expectations based on a surface reaction control mechanism. Anorthite dissolution in the presence of cation- and anion-exchange resins produced zero-order kinetics at sustained high rates. We hypothesize that these linear rates were due to the continuous removal of Al from solution by the resins. Consistent with these results, there was no effect of added Ca or Si on the rate of reaction; but the addition of Al slowed the initial rate of reaction at pH 3.6 and 6.0 but not at pH 3.0. Long-term dissolution studies (up to 4.5 ys) resulted in final reaction rates over two hundred times slower than previously reported for feldspar dissolution. These data are consistent with the idea that the presence of Al in solution and the incorporation of Al into the hydrous silanol surface slow the rate of anorthite dissolution and are important factors affecting the rate over all time periods. The addition of KCl slowed the dissolution rate either through competitive exchange with structural Ca or adsorbed H, or by blocking the polymerization reactions at the surface.

Pedogenesis in a wetland meadow and surrounding serpentinitic landslide terrain, northern California, USA

Landslide deposits in serpentinitic terrain of northern California are common, and often support wet meadows, which are strikingly different from the surrounding xeric landscape. These landslide deposits provide an opportunity to observe pedogenic processes across differing moisture conditions and depositional environments in ultramafic terrain. The objective of this study was to assess predominant pedogenic processes by landscape position for a stabilized landslide bench and the surrounding area in serpentinitic terrain. Distributions of dithionite extractable metals and exchangeable cations were compared to interpret processes involved in element redistribution. Landscape positions were defined as the scarp, flanks, foot, and bulge. The foot, which supports a wet meadow, is inundated during a significant part of the year due to water ponding behind the bulge. The forested scarp and flank positions are subject to episodes of erosion, probably following wildfires, thereby delivering sediments to the lower landscape positions. The major pedogenic processes on the slopes of the scarp and flanks include mineral weathering, oxidation, and illuviation of clay, as evidenced by argillic horizons in which Cr- and Al-bearing Fe-oxyhydroxides are concentrated. Serpentine-weathering products are transported from the scarp and flanks to the poorly drained foot where fine sediments and neoformed smectite yield clayey soils. On the foot, cycles of vegetation growth and deposition of sediments, coupled with reducing conditions, result in deep, organic-rich soils.

Factors influencing uranium reduction and solubility in evaporation pond sediments

Evaporation ponds in the San Joaquin Valley (SJV), CA, USA that are used for the disposal of irrigation drainage waters, contain elevated levels of U that may be a threat to pond wildlife. The ponds support euryhaline algae, which become incorporated in the sediments as depositional organic matter (OM) — facilitating reducing conditions. Our earlier studies have shown that U in one SJV sediment was primarily present as the highly soluble U(VI) species (as opposed to the less soluble U(IV) species), despite the presence of volatile sulfides. In this research, we investigated the effects of native pond algae (Chlorella) and potential reducing agents on U redox chemistry of SJV pond sediments. San Joaquin Valley pond sediments were equilibrated with natural and synthetic pond inlet waters containing approximately 10 mg U(VI) L−1 to which reducing agents (acetate, sucrose, and alfalfa shoot) were added. The equilibrations were done under oxic (Chlorella only) and O2-limiting conditions (remaining treatments). Sediments were examined for changes in average U oxidation state by X-ray near-edge absorption structure (XANES) spectroscopy and U concentration by ICP-MS.For the alfalfa treatments, a 95 percent loss of U(VI) from solution, the presence of sulfides, and results from the XANES studies suggest U(VI) was reduced to U(IV). Upon exposure to air, the precipitated U was readily oxidized, suggesting the reduced U is susceptible to oxidation. Much less reduction of U(VI) was observed in the other 3 treatments and the solid phase was dominated by U(VI) as in the natural pond sediments. A second study was conducted with pond sediment-water suspensions to determine the effects of controlled PCO2 and low redox potential (Eh) on U solubility. These suspensions were equilibrated at 0.22 and 5.26 kPa PCO2 and allowed to “free-drift” from an oxidized to a reduced state. At high Eh and high PCO2, dissolved U concentrations were higher than in the low PCO2 systems due to greater complexation with CO3. Dissolved U concentrations decreased only under intense sulfate reducing conditions, even at low Eh conditions. It appears that U reduction occurred by chemical reduction via sulfide ion. Comparing the XANES data of the pond sediments with the laboratory-produced solids we conclude that biosorption by algae and bacteria is the dominant mechanism depositing U in the sediments. Even though there are organisms that can use U(VI) as a terminal electron acceptor, we found that sulfate reduction was preferred in these high-SO4 waters. Mixed oxidation state U-solids were preferentially formed in the pond sediments and in the lab except under intense SO4 reducing conditions.

Toxicity and repellency of borate-sucrose water baits to Argentine ants (Hymenoptera: Formicidae).

Abstract The oral toxicity of boron compounds to the Argentine ant, Linepithema humile (Mayr), was evaluated in laboratory tests. The ants were provided 25% sucrose water containing 0.5 and 1% boric acid, disodium octaborate tetrahydrate, and borax. Lethal times of these solutions were a function of the concentration of boron. In field tests, the ants showed no discrimination between disodium octaborate tetrahydrate and boric acid. There was a significant reduction in consumption of sucrose water with >1% boric acid.

Comparison of Gypsum and Sulfuric Acid for Sodic Soil Reclamation

In some field and laboratory studies , H2SO4 has shown better reclamation efficiency than gypsum, but the explanation for this has been debated. We tested the hypothesis that significant amounts of HCO might be formed during the reaction of HSO3 with CaCO3 leading to additional Ca2+ in solution and enhanced displacement of exchangeble Na +. To determine if H2SO4 is more efficient than gypsum , we compared four equivalent treatments of H2SO4 and gypsum on three soils and two clay minerals. Gypsum and H2SO4 were reacted with calcareous sodic soils and clay minerals in closed-system batch studies, with CO2 pressures typical of soil root zones. Column studies comparing two equivalent treatments of H2SO4 and gypsum were conducted on one of the soils and produced results comparable to the batch studies. Soluble ions, electrical conductivities (EC), and mineral saturation indices were determined and compared between treatments . Clay dispersion and average dispersed particle size were compared between amendments...

Large near-surface nitrate pools in soils capped by desert pavement in the Mojave Desert, California

We found exceptionally high nitrate levels (up to 12,750 kg ha−1) at shallow depths (≤1 m) in soils mantled by desert pavement, a common land-surface feature in arid regions. Nearby soils without desert pavement had nitrate contents that were one to two orders of magnitude lower. The soil conditions coincident with desert pavement (i.e., stability, antiquity, and virtually no leaching) favor the retention and accumulation of nitrate delivered by atmospheric deposition or in situ fixation. The nitrate stored in soils under desert pavement is a previously unrecognized pool of nitrogen that has the potential to increase the global nitrogen inventory for near-surface desert soils to five times previous estimates. Its near-surface occurrence makes this labile nitrogen pool particularly susceptible to mobilization by climate change or human disturbance, risking contamination of surface and groundwaters.

Nature of uranium contamination in the agricultural drainage water evaporation ponds of the San Joaquin Valley, California, USA

Evaporation ponds used for agricultural subsurface drainage water disposal in the Tulare Lake Bed (TLB) of the San Joaquin Valley, California, USA, have elevated levels of U. Waterfowl which inhabit and forage the ponds and surrounding areas are threatened by exposure to U. The ponds, which receive irrigation drainage waters and seasonal rain, are subject to wetting and drying periods. The periods result in the accumulation of decaying algae and other organic material in surface sediments. Sediment and waters in the ponds were sampled to determine what factors control U solubility and sediment U concentrations. Data from a 1990 study conducted by Chilcott et al. in 1989 on the TLB ponds were used to help identify what factors may control U solubility. Pond sediment U concentrations decreased abruptly with depth and surface sediment U concentrations were related to dissolved Ca:HCO3 ratios. Pond algal U bioaccumulation was favored in waters with high Ca:HCO3 ratios, which had lower pH values and carbonate ...

Effect of zero-valent iron and a redox mediator on removal of selenium in agricultural drainage water

Effective and economical removal of selenium (Se) in agricultural drainage water is very important in Se bioremediation. Zero-valent iron (ZVI) and a redox mediator [anthraquinone-2,6-disulfonate (AQDS)] were assessed for their ability to enhance the removal of Se(VI) or Se(IV) (500 microg/L) in synthetic drainage water by Enterobacter taylorae. The results showed that E. taylorae was capable of using inexpensive sucrose to remove Se from the drainage water. During a 7-day experiment, Se(VI) was almost entirely reduced to Se(0) and transformed to organic Se in the drainage water with sucrose levels of 500 to 1000 mg/L. Addition of ZVI to the drainage water increased the removal of total soluble Se to 94.5-96.5% and limited the production of organic Se. Addition of AQDS to the drainage water with or without ZVI decreased Se(VI) removal, but enhanced the removal of Se(IV), suggesting that E. taylorae only can use anthrahydroquinone-2,6-disulfonate (AHQDS, a reduced form of AQDS) to respire Se(IV), and not Se(VI). These results show that ZVI has promising application potential in the bioremediation of Se in Se-contaminated water.

Geochemistry of iron in the Salton Sea, California

The Salton Sea is a large, saline, closed-basin lake in southern California. The Sea receives agricultural runoff and, to a lesser extent, municipal wastewater that is high in nutrients, salt, and suspended solids. High sulfate concentrations (4× higher than that of the ocean), coupled with warm temperatures and low-redox potentials present during much of the year, result in extensive sulfate reduction and hydrogen sulfide production. Hydrogen sulfide formation may have a dramatic effect on the iron (Fe) geochemistry in the Sea. We hypothesized that the Fe(II)-sulfide minerals should dominate the iron mineralogy of the sediments, and plans to increase hypolimnetic aeration would increase the amount of Fe(III)-oxides, which are strong adsorbers of phosphate. Sequential chemical extractions were used to differentiate iron mineralogy in the lake sediments and suspended solids from the tributary rivers. Iron in the river-borne suspended solids was mainly associated with structural iron within silicate clays (70%) and ferric oxides (30%). The iron in the bottom sediments of the lake was associated with silicate minerals (71% of the total iron in the sediments), framboidal pyrite (10%), greigite (11%), and amorphous FeS (5%). The ferric oxide fraction was <4% of the total iron in these anaerobic sediments. The morphological characteristics of the framboidal pyrite as determined using SEM suggest that it formed within the water column and experiences some changes in local redox conditions, probably associated with alternating summer anoxia and the well-mixed and generally well-aerated conditions found during the winter. The prevalence of Fe(II)-sulfide minerals in the sediments and the lack of Fe(III)-oxide minerals suggest that the classic model of P-retention by Fe(III)-oxides would not be operating in this lake, at least during anoxic summer conditions. Aeration of the hypolimnion could affect the internal loading of P by changing the relative amounts of Fe(II)-sulfides and Fe(III)-oxides at the sediment/water interface.