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Both vertical and lateral flows of rock and water occur within eroding hills. Specifically, when considered over geological timeframes, rock advects vertically upward under hilltops in landscapes experiencing uplift and erosion. Once rock particles reach the land surface, they move laterally and down the hillslope because of erosion. At much shorter timescales, meteoric water moves vertically downward until it reaches the regional water table and then moves laterally as groundwater flow. Water can also flow laterally in the shallow subsurface as interflow in zones of permeability contrast. Interflow can be perched or can occur during periods of a high regional water table. The depths of these deep and shallow water tables in hills fluctuate over time. The fluctuations drive biogeochemical reactions between water, CO 2 , O 2 , and minerals and these in turn drive fracturing. The depth intervals of water table fluctuation for interflow and groundwater flow are thus reaction fronts characterized by changes in composition, fracture density, porosity, and permeability. The shallow and deep reaction zones can separate over meters in felsic rocks. The zones act like valves that reorient downward unsaturated water flow into lateral saturated flow. The valves also reorient the upward advection of rock into lateral flow through solubilization. In particular, groundwater removes highly soluble, and interflow removes moderately soluble minerals. As rock and water moves through the system, hills may evolve toward a condition where the weathering advance rate, W, approaches the erosion rate, E. If W = E, the slopes of the deep and shallow reaction zones and the hillsides must allow removal of the most soluble, moderately soluble, and least soluble minerals respectively. A permeability architecture thus emerges to partition each evolving hill into dissolved and particulate material fluxes as it approaches steady state.

The electrical conductivities of aqueous solutions of Na(2)SO(4), H(2)SO(4), and their mixtures have been measured at 373-673 K at 12-28 MPa in dilute solutions for molalities up to 10(-2) mol kg(-1). These conductivities have been fit to the conductance equation of Turq et al.(1) with a consensus mixing rule and mean spherical approximation activity coefficients. Provided the concentration is not too high, all of the data can be fitted by a solution model that includes ion association to form NaSO(4)(-), Na(2)SO(4)(0), HSO(4)(-), H(2)SO(4)(0), and NaHSO(4)(0). The adjustable parameters of this model are the dissociation constants of the SO(4)(-) species and the H(+), SO(4)(-2), and HSO(4)(-) conductances (ion mobilities) at infinite dilution. For the 673 K and 230 kg m(-3) state point with the lowest dielectric constant, epsilon = 3.5, where the Coulomb interactions are the strongest, this model does not fit the experimental data above a solution molality of 0.016. Including the species H(9)(SO(4))(5)(-) gave satisfactory fits to the conductance data at the higher concentrations.

The electrical conductivities of aqueous solutions of NaCl have been measured at 651 and 670 K at 28 MPa for molalities up to 1.0 mol kg -1 . These conductivities as well as the results of Hwang et al. (High Temp. High Pres. 1970, 2, 651-669), Ritzert and Franck (Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 798-808), andMangold and Franck (Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 21-27) for aqueous KCl have been fit to the conductance equation of Turq et al. (J. Phys.Chem. 1995, 99, 822-827) with a consensus mixing rule and either mean spherical approximation or Debye-Hückel activity coefficients. Except at one state point (NaCl at 670 K and 28 MPa), where the interactions are the strongest ( * ) 17.8), the simplest model that fits the experimental results reasonably well (( 2-3%) at molalities up to 4.5 mol kg -1 is one with only the limiting equivalent conductance and a pair association constant adjusted. Activity coefficients calculated with either the MSA and ionic diameters or with the Debye-Hückel equation of Oelkers and Helgeson (Geochim. Cosmochim. Acta 1990, 54, 727-738) (with no salting out) can be used with similar accuracy. At high concentrations this model predicts strong redissociation of the ion pairs that form at low concentrations. Oelkers and Helgeson (Geochim. Cosmochim. Acta 1993a, 57, 2673-2697) proposed a model with substantial multi-ion association (triplets and quartets, etc.). This model has two more adjustable parameters and does not fit the data without the physically unrealistic salting out coefficient used by Oelkers and Helgeson (Geochim. Cosmochim. Acta 1991, 55, 1235-1251), so this model is not recommended. For the point with the highest *, more complex models are needed at concentrations above 0.05 mol kg -1 . Good fits to the data were found for multi-ion association models (five adjustable parameters) and reasonable fits for more complex activity models (four adjustable parameters) with only pair association, so that the models are about equally accurate at equal complexity. The cluster model of Laria et al. (J. Chem. Soc., Faraday Trans. 1990, 86, 1051-1056 for the restricted primitive model is consistent with the qualitative predictions of our preferred model with only pair association.

The Cu-Cl thermochemical cycle is among the most attractive technologies proposed for hydrogen production due to moderate temperature requirements and high efficiency. In this study, the key step of the cycle, H 2 gas evolution via oxidation of CuCl͑s͒ dissolved in high concentrated HCl͑aq͒, was experimentally investigated. The electrolysis parameters and system performance were studied by linear sweep voltammetry and electrochemical impedance spectroscopy at ambient temperature. Promising performance of the electrolyzer was obtained when pure water was used as catholyte. A thermodynamic model previously developed for speciation of the CuCl-CuCl 2 -HCl aqueous solutions was used to speculate on the effects of reagent concentration, flow rate, and temperature on electrolysis kinetics. The experimental decomposition potential necessary to initiate the hydrogen evolution reaction was more than 3 times lower than the potential necessary for water electrolysis at the same conditions. Close correspondence of the hydrogen production rate to Faraday's law of electrolysis indicated the current efficiency of about 98%, while the voltage efficiency was estimated at 80% at 0.5 V and 0.1 A/cm 2 .

in Pennsylvania, about 4000 Marcellus wells were drilled horizontally and hydraulically fractured for natural gas. During the flowback period after hydrofracturing, 2 to 4 × 10 3 m 3 (7 to 14 × 10 4 ft 3 ) of brine returned to the surface from each horizontal well. This Na-Ca-Cl brine also contains minor radioactive elements, organic compounds, and metals such as Ba and Sr, and cannot by law be discharged untreated into surface waters. The salts increase in concentration to ∼270 kg∕m 3 (∼16.9 lb∕ft 3 ) in later flowback. To develop economic methods of brine disposal, the provenance of brine salts must be understood. Flowback volume generally corresponds to ∼10% to 20% of the injected water. Apparently, the remaining water imbibes into the shale. A mass balance calculation can explain all the salt in the flowback if 2% by volume of the shale initially contains water as capillary-bound or free Appalachian brine. In that case, only 0.1%-0.2% of the brine salt in the shale accessed by one well need be mobilized. Changing salt concentration in flowback can be explained using a model that describes diffusion of salt from brine into millimeter-wide hydrofractures spaced 1 per m (0.3 per ft) that are initially filled by dilute injection water. Although the production lifetimes of Marcellus wells remain unknown, the model predicts that brines will be produced and reach 80% of concentration of initial brines after ∼1 yr. Better understanding of this diffusion could (1) provide better long-term planning for brine disposal; and (2) constrain how the hydrofractures interact with the low-permeability shale matrix.

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