Recharge in northern clime calcareous sandy soils: soil water chemical and carbon-14 evolution

Chemical analyses were performed on soil water extracted from two cores taken from a sandy calcareous soil near Delhi, Ontario. Calcite saturation is attained within the unsaturated zone over short distances and short periods of time, whereas dolomite undersaturation persists to the groundwater table. The progressive dissolution of dolomite by soil water, within the unsaturated zone, after calcite saturation is reached results in calcite supersaturation.Deposition of iron and manganese oxyhydroxide phases occurs at the carbonate leached/unleached zone boundary. This is a result of soil water neutralization due to carbonate dissolution during infiltration but may also reflect the increased rate of oxidation of dissolved ferrous and manganous ions at higher pH's. The role of bacteria in this process has not been investigated.The depth of the carbonate leached/unleached zone boundary in a calcareous soil has important implications for ¹⁴C groundwater dating. The depth of this interface at the study site (?2 m) does not appear to limit ¹⁴C diffusion from the root zone to the depth at which carbonate dissolution occurs. Thus, soil water achieves open system isotopic equilibrium with the soil CO₂ gas phase. It is calculated that in soils with similar physical properties to the study soil but with depths of leaching of 5 m or more, complete ¹⁴C isotopic equilibration of soil water with soil gas would not occur. Soil water, under these conditions would recharge to the groundwater exhibiting some degree of closed system ¹⁴C isotopic evolution.     

Dissociation constants for alkali earth and sodium borate ion pairs from 10 to 50°C

Potentiometric measurements in dilute sodium borate solutions with added alkali earth chlordie salts yield the following expressions for the dissociation constants of alkali earth borate ion pairs from 10 to 50°C:

$\text{p}\text{K(}\text{MgH}{}_2\text{BO}{}_3{}^{\mathrm{+}}\text{=1.266+0.001204 T}$

$\text{p}\text{K(}\text{CaH}{}_2\text{BO}{}_3{}^{\mathrm{+}}\text{=1.154+0.002170 T}$

$\text{p}\text{K(}\text{SrH}{}_2\text{BO}{}_3{}^{\mathrm{+}}\text{=1.033+0.001738 T}$

$\text{p}\text{K(}\text{BaH}{}_2\text{BO}{}_3{}^{\mathrm{+}}\text{=1.942+0.001850 T}$

where T is in °K. Enthalpies for the dissociation reactions at 25°C are less than 1 kcal./mole for all the alkali earth borate ion pairs.Values for pK(NaH₂BO₃°) from 5 to 55°C computed from the experimental data of Owen and King are in good agreement with those determined potentiometrically. The average value from both methods is 0.22 ± 0.1 at 25°C.Application to seawater of computed pK's for MgH₂BO₃⁺, CaH₂BO₃⁺ and NaH₂BO₃⁰ yields an apparent dissociation constant for boric acid of 8.73 vs. 8.70 measured by Lyman, 8.68 by Buch and 8.73 by Byrne and Kester.     

Groundwater Governance in a Water‐Starved Country: Public Policy,Farmers' Perceptions,and Drivers of Tubewell Adoption in Balochistan,Pakistan

Pakistan faces the challenge of developing sustainable groundwater policies with the main focus on groundwater management rather than groundwater development and with appropriate governance arrangement to ensure benefits continue into the future. This article investigates groundwater policy, farmers' perceptions, and drivers of tubewell (groundwater bore) adoption and proposes possible pathways for improved groundwater management for Balochistan, Pakistan. Historical groundwater policies were mainly aimed at increasing agricultural production and reducing poverty, without consideration of adverse impact on groundwater availability. These groundwater policies and governance arrangements have resulted in a massive decline in groundwater tables. Tubewell owners' rankings of the drivers of groundwater decline suggest that rapid and widespread installation of tubewells, together with uncontrolled extraction due to lack of property rights, electricity subsidy policies, and ineffective governance, are key causes of groundwater decline in Balochistan. An empirical “tubewell adoption” model confirmed that the electricity subsidy significantly influenced tubewell adoption decisions. The article proposes a more rational electricity subsidy policy for sustaining groundwater levels in the short‐run. However, in the long run a more comprehensive sustainable groundwater management policy, with strong institutional support and involvement of all stakeholders, is needed.     

Experimental observations on carbon isotope exchange in carbonate-water systems

This study presents data from experiments investigating carbon isotope exchange between carbonate solution and solid calcite using carbon-13 as a tracer. All experiments were done with calcite saturated solutions and results show that a two-step adsorption-recrystallization reaction takes place. Isotope effects are caused by exchange by carbonate on the solid surface with carbon in the aqueous phase. Adsorption reactions are characterized by a maximum isotopic exchange capacity (IEC) on crystal surfaces of about 10¹¹ reaction sites per cm², following a second order rate law with respect to ¹³C concentration in solution ($\text{constant}\text{kex ? 10}{}^6\text{cm}{}^5\text{mole}{}^{\mathrm{?1}}\text{s}{}^{\mathrm{?1}}$ and $\text{half-life}\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 700}\text{s}$). The adsorption reaction was followed by a first order recrystallization which is characterized by a rate constant of the order of 10^?8 s^?1 and a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 10⁷ s. Negative isotopic gradient experiments and runs with calcite crystals in Mg²⁺ spiked solutions provided the preliminary basis for the characterization of the mechanisms of both proposed reactions.     

Electrochemical interaction of Shewanella oneidensis MR-1 and its outer membrane cytochromes OmcA and MtrC with hematite electrodes

Bacterial metal reduction is an important biogeochemical process in anaerobic environments. An understanding of electron transfer pathways from dissimilatory metal-reducing bacteria (DMRB) to solid phase metal (hydr)oxides is important for understanding metal redox cycling in soils and sediments, for utilizing DMRB in bioremedation, and for developing technologies such as microbial fuel cells. Here we hypothesize that the outer membrane cytochromes OmcA and MtrC from Shewanella oneidensis MR-1 are the only terminal reductases capable of direct electron transfer to a hematite working electrode. Cyclic voltammetry (CV) was used to study electron transfer between hematite electrodes and protein films, S. oneidensis MR-1 wild-type cell suspensions, and cytochrome deletion mutants. After controlling for hematite electrode dissolution at negative potential, the midpoint potentials of adsorbed OmcA and MtrC were measured (−201 mV and −163 mV vs. Ag/AgCl, respectively). Cell suspensions of wild-type MR-1, deletion mutants deficient in OmcA (ΔomcA), MtrC (ΔmtrC), and both OmcA and MtrC (ΔmtrC–ΔomcA) were also studied; voltammograms for ΔmtrC–ΔomcA were indistinguishable from the control. When the control was subtracted from the single deletion mutant voltammograms, redox peaks were consistent with the present cytochrome (i.e., ΔomcA consistent with MtrC and ΔmtrC consistent with OmcA). The results indicate that OmcA and MtrC are capable of direct electron exchange with hematite electrodes, consistent with a role as terminal reductases in the S. oneidensis MR-1 anaerobic respiratory pathway involving ferric minerals. There was no evidence for other terminal reductases operating under the conditions investigated. A Marcus-based approach to electron transfer kinetics indicated that the rate constant for electron transfer k_et varies from 0.025 s⁻¹ in the absence of a barrier to 63.5 s⁻¹ with a 0.2 eV barrier.     

Solar dynamics imaging system a back-end instrument for the proposed NLST

The Solar Dynamics Imaging System (SDIS) will be one of the focal plane instruments operated at the National Large Solar Telescope (NLST). The prime objective of the instrument is to obtain high spatial and temporal resolution images of the region of interest on the Sun in the wavelength range from 390 nm to 900 nm. The SDIS provides filtergrams using broad-band filters while preserving the Strehl ratio provided by the telescope. Furthermore, the SDIS is expected to provide observations that allow image reconstruction to extract wave front information and achieve a homogenous image quality over the entire FOV.     

Complexing of silica by iron(III) in natural waters

Determination of amorphous silica solubility in acidified ferric nitrate solutions confirms the presence of ferric silicate complexing. A dissociation constant for the reaction:

$\text{FeH}{}_3\text{SiO}{}_4{}^{\mathrm{2+}}\text{→}\text{Fe}{}^{\mathrm{3+}}\text{+}\text{H}{}_3\text{SiO}{}_4{}^{\mathrm{?}}$

of 10^?9.8 ± 0.3 pK units at room temperature (22 ± 3°C) is obtained, in close agreement with reported values at 25°C corrected to zero ionic strength of 10^?9.9 by Weber and Stumm and 10^?9.5 by Olson and O'Melia. Iron-silicate complexing may be of significance to the mobilization of silica in acid waters associated with oxidizing sulphide deposits and coal strip mining and the precipitation of secondary silicate mineral phases.     

Activity coefficients of MgCOo3 and CaSOo4 ion pairs as a function of ionic strength

Based on potentiometric data and gypsum solubility in mixed salt solutions, respectively, the activity coefficients of MgCO^o₃ and CaSO^o₄ ion pairs decrease with ionic strength (I) at 25°C. Computed γ's for the ion pairs fit the empirical equation log γ_i = ? BI. B coefficients of 0.63 ±0.10 for MgCO^o₃ and 0.45 ± 0.15 for CaSO^o₄ are obtained from linear regression of log γ_i values vs I between 0.04 and 0.6 molal. Assumptions that the activity coefficients of these neutral ion pairs equal unity or are approximated by the Setchenow expression (log γ_i = kI) are therefore invalid at moderate ionic strengths. Log γ_i = ? BI is the same general form as the equation of Kirkwood (Chem. Rev.24, 233–251, 1939) for neutral dipoles.