Eurybiades Busenberg

The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O

Calculations based on approximately 350 new measurements (CaT-PCO2) of the solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C indicate the following values for the log of the equilibrium constants KC, KA, and KV respectively, for the reaction CaCO3(s) = Ca2+ + CO2−3: Log KC = −171.9065 − 0.077993T + 2839.319T + 71.595 log TLog KA = −171.9773 − 0.077993T + 2903.293T +71.595 log TLog KV = −172.1295 − 0.077993T + 3074.688T + 71.595 log T where T is in oK. At 25°C the logarithms of the equilibrium constants are −8.480 ± 0.020, −8.336 ± 0.020 and −7.913 ± 0.020 for calcite, aragonite and vaterite, respectively. The equilibrium constants are internally consistent with an aqueous model that includes the CaHCO+3 and CaCO03 ion pairs, revised analytical expressions for CO2-H2O equilibria, and extended Debye-Huckel individual ion activity coefficients. Using this aqueous model, the equilibrium constant of aragonite shows no PCO2-dependence if the CaHCO+3 association constant is Log KCahco+3 = 1209.120 + 0.31294T — 34765.05T − 478.782 log T between 0 and 90°C, corresponding to the value logKCahco+3 = 1.11 ± 0.07 at 25°C. The CaCO03 association constant was measured potentiometrically to be log KCaCO03 = −1228.732 − 0.299444T + 35512.75T + 485.818 log T between 5 and 80°C, yielding logKCaCO03 = 3.22 ± 0.14 at 25°C. The CO2-H2O equilibria have been critically evaluated and new empirical expressions for the temperature dependence of KH, K1 and K2 are log KH = 108.3865 + 0.01985076T − 6919.53T − 40.45154 log T + 669365.T2, log K1 = −356.3094 − 0.06091964T + 21834.37T + 126.8339 log T — 1684915.T2 and logK2 = −107.8871 − 0.03252849T + 5151.79/T + 38.92561 logT − 563713.9/T2 which may be used to at least 250°C. These expressions hold for 1 atm. total pressure between 0 and 100°C and follow the vapor pressure curve of water at higher temperatures. Extensive measurements of the pH of Ca-HCO3 solutions at 25°C and 0.956 atm PCO2 using different compositions of the reference electrode filling solution show that measured differences in pH are closely approximated by differences in liquid-junction potential as calculated by the Henderson equation. Liquid-junction corrected pH measurements agree with the calculated pH within 0.003-0.011 pH. Earlier arguments suggesting that the CaHCO+3 ion pair should not be included in the CaCO3-CO2-H2O aqueous model were based on less accurate calcite solubility data. The CaHCO+3 ion pair must be included in the aqueous model to account for the observed PCO2-dependence of aragonite solubility between 317 ppm CO2 and 100% CO2. Previous literature on the solubility of CaCO3 polymorphs have been critically evaluated using the aqueous model and the results are compared.

Use of chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age‐dating tools: The alluvium and terrace system of central Oklahoma

The use of Chlorofluorocarbons (CFCs) as an age-dating tool and tracer in shallow groundwaters has been investigated. New methodology for field sampling and preserving groundwaters containing parts per trillion concentrations of the CFCs, F-1l and F-12, is presented. Samples are analyzed by purge-and-trap gas chromatography with an electron capture detector. Physical and chemical processes that can alter natural concentrations (air-water equilibrium) of CFCs were investigated to assess dating uncertainties. CFC model recharge ages appear to be defined within 2 years under optimum conditions. The method was applied to central Oklahoma to demonstrate the usefulness of CFCs as (1) an age-dating tool of shallow groundwaters, (2) a tracer of sewage effluent in surface and shallow groundwaters, and (3) a tracer of shallow groundwater. Results of dating indicate two primary recharge periods in central Oklahoma over the past 45 years that correspond to the wet periods 1945–1960 and 1967–1975.

Dating young groundwater with sulfur hexafluoride: natural and anthropogenic sources of sulfur hexafluoride

Sulfur hexafluoride (SF6) is primarily of anthropogenic origin but also occurs naturally. The troposphere concentration of SF6 has increased from a steady state value of 0.054±0.009 to more than 4 parts per trillion volume during the past 40 years. An analytical procedure was developed for measuring concentrations of SF6 to less than 0.01 fmol/L in water. Groundwater can be dated with SF6 if it is in equilibrium with atmospheric SF6 at the time of recharge and does not contain significant SF6 from other sources. The dating range of SF6 is currently 0 to 30 years. The tracer was successfully used to date shallow groundwater of the Atlantic Coastal Plain sand aquifers of the United States and springs issuing near the top of the Blue Ridge Mountains of Virginia. Significant concentrations of naturally occurring SF6 were found in some igneous, volcanic, and sedimentary rocks and in some hydrothermal fluids.

Kinetic and thermodynamic factors controlling the distribution of SO32− and Na+ in calcites and selected aragonites

Abstract Significant amounts of SO42−, Na+, and OH− are incorporated in marine biogenic calcites. Biogenic high Mg-calcites average about 1 mole percent SO42−. Aragonites and most biogenic low Mg-calcites contain significant amounts of Na+, but very low concentrations of SO42−. The SO42− content of non-biogenic calcites and aragonites investigated was below 100 ppm. The presence of Na+ and SO42− increases the unit cell size of calcites. The solid-solutions show a solubility minimum at about 0.5 mole percent SO42− beyond which the solubility rapidly increases. The solubility product of calcites containing 3 mole percent SO42− is the same as that of aragonite. Na+ appears to have very little effect on the solubility product of calcites. The amounts of Na+ and SO42− incorporated in calcites vary as a function of the rate of crystal growth. The variation of the distribution coefficient ( D ) of SO42− in calcite at 25.0°C and 0.50 molal NaCl is described by the equation D = k 0 + k 1 R where k 0 and k 1 are constants equal to 6.16 × 10 −6 and 3.941 × 10 −6 , respectively, and R is the rate of crystal growth of calcite in mg·min−1·g−1 of seed. The data on Na+ are consistent with the hypothesis that a significant amount of Na+ occupies interstitial positions in the calcite structure. The distribution of Na+ follows a Freundlich isotherm and not the Berthelot-Nernst distribution law. The numerical value of the Na+ distribution coefficient in calcite is probably dependent on the number of defects in the calcite structure. The Na+ contents of calcites are not very accurate indicators of environmental salinities.

Dating of shallow groundwater: Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr

This paper describes a direct comparison of apparent ages derived from 3H/3He, chlorofluorocarbons (CCl3F and CCl2F2), and 85Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficial aquifers in late Cenozoic Atlantic Coastal Plain sediments of the Delmarva Peninsula, on the east coast of the United States. Most of the apparent tracer ages agree within 2 years of each other for recharge dates between 1965 and 1990. Discrepancies in apparent tracer ages usually can be explained by hydrological processes such as mixing in a discharge area. Recharge rate calculations based on apparent tracer age gradients at multilevel well locations agree with previous recharge estimates. High recharge rates on the Delmarva Peninsula result in nearly complete dissolved-gas confinement in the groundwater. The remarkable agreement between the different tracer ages indicates negligible mixing of waters of different ages, insignificant dispersion, minimal gas loss to the atmosphere, and insignificant sorption-desorption processes at this location.

The use of simulation and multiple environmental tracers to quantify groundwater flow in a shallow aquifer

Measurements of the concentrations of chlorofluorocarbons (CFCs), tritium, and other environmental tracers can be used to calculate recharge ages of shallow groundwater and estimate rates of groundwater movement. Numerical simulation also provides quantitative estimates of flow rates, flow paths, and mixing properties of the groundwater system. The environmental tracer techniques and the hydraulic analyses each contribute to the understanding and quantification of the flow of shallow groundwater. However, when combined, the two methods provide feedback that improves the quantification of the flow system and provides insight into the processes that are the most uncertain. A case study near Locust Grove, Maryland, is used to investigate the utility of combining groundwater age dating, based on CFCs and tritium, and hydraulic analyses using numerical simulation techniques. The results of the feedback between an advective transport model and the estimates of groundwater ages determined by the CFCs improve a quantitative description of the system by refining the system conceptualization and estimating system parameters. The plausible system developed with this feedback between the advective flow model and the CFC ages is further tested using a solute transport simulation to reproduce the observed tritium distribution in the groundwater. The solute transport simulation corroborates the plausible system developed and also indicates that, for the system under investigation with the data obtained from 0.9-m-long (3-foot-long) well screens, the hydrodynamic dispersion is negligible. Together the two methods enable a coherent explanation of the flow paths and rates of movement while indicating weaknesses in the understanding of the system that will require future data collection and conceptual refinement of the groundwater system.

Chlorofluorocarbons (CCl3F and CCl2F2) as dating tools and hydrologic tracers in shallow groundwater of the Delmarva Peninsula, Atlantic Coastal Plain, United States

Concentrations of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC-modeled ages were calculated independently for CFC-11 and CFC-12, and agreed to within 2-3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from + or - 2 degrees C over most of the peninsula to 14 + or - 2 degrees C at the southernmost tip of the peninsula in Virginia. The CFC-modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC-modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus [sup 3]He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of this study strongly support the use of CFCs for dating shallow, aerobic groundwater.

Age dating of shallow groundwater with chlorofluorocarbons, tritium/helium: 3, and flow path analysis, southern New Jersey coastal plain

Groundwater age dating through the combination of transient tracer methods (chlorofluorocarbons (CFCs) and tritium/helium 3 (3H/3He)) and groundwater flow path analysis is useful for investigating groundwater travel times, flow patterns, and recharge rates, as demonstrated by this study of the homogeneous shallow, unconfined Kirkwood-Cohansey aquifer system in the southern New Jersey coastal plain. Water samples for age dating were collected from three sets of nested observation wells (10 wells) with 1.5-m-long screens located near groundwater divides. Three steady state finite difference groundwater flow models were calibrated by adjusting horizontal and vertical hydraulic conductivities to match measured heads and head differences (range, 0.002–0.23 m) among the nested wells, with a uniform recharge rate of 0.46 m per year and porosities of 0.35 (sand) and 0.45 (silt) that were assumed constant for all model simulations and travel time calculations. The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1.5 to 6.5 years for the shallow wells (screen bottoms 3–4 m below the water table), from about 10 to 25 years for the medium-depth wells (screen bottoms 8–19 m below the water table), and from about 30 to more than 40 years for the deep wells (screen bottoms 24–26 m below the water table). Apparent groundwater ages based on CFC- and 3H/3He-dating techniques and model-based travel times could not be statistically differentiated, and all were strongly correlated with depth. Confinement of 3He was high because of the rapid vertical flow velocity (of the order of 1 m/yr), resulting in clear delineation of groundwater travel times based on the 3H/3He-dating technique. The correspondence between the 3H/3He and CFC ages indicates that dispersion has had a minimal effect on the tracer-based ages of water in this aquifer. Differences between the tracer-based apparent ages for seven of the 10 samples were smaller than the error values. A slight bias toward older apparent ages, found not to be statistically significant, was noted for the 3H/3He-dating technique relative to the CFC-dating technique. This result may be caused by enrichment of local air in CFC-Il and CFC-12 from urban and industrial sources in the northeastern United States and minor contamination from sampling equipment. The demonstrated validity of the combined tracer-dating techniques to determine the age of water in the Kirkwood-Cohansey aquifer system indicates that groundwater flow models can be refined when apparent ages based on 3H/3He- and CFC- dating are used as calibration targets.

Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: A multi-tracer approach

Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August–September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 (3H/3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), sulfur-35 (35S), and stable isotopes (δ18O and δ2H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF6 and 3H/3He, with most ages in the range of 0–3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35S (>1.5 years) and seasonal variation of δ18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35S and δ18O data reflect travel times through the unsaturated zone and, in the case of 35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3H/3He data (median age of 0.2 years) are lower than those obtained from SF6 (median age of 4.3 years), and in contrast to median ages from 3H/3He (0.3 years) and SF6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF6 apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF6 data. Water from springs has low excess air (0–1 cm3 kg−1) and N2–Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N2–Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm3 kg−1 of excess air with ages in the range of 0–25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions.

Thermodynamics of magnesian calcite solid-solutions at 25°C and 1 atm total pressure

The stability of magnesian calcites was reexamined, and new results are presented for 28 natural inorganic, 12 biogenic, and 32 synthetic magnesian calcites. The magnesian calcite solid-solutions were separated into two groups on the basis of differences in stoichiometric solubility and other physical and chemical properties. Group I consists of solids of mainly metamorphic and hydrothermal origin, synthetic calcites prepared at high temperatures and pressures, and synthetic solids prepared at low temperature and very low calcite supersaturations (Ωcalcite ≤ 1.5) from artificial sea water or NaClMgCl2CaCl2 solutions. Group I solids are essentially binary s of CaCO2 and MgCO2, and are thought to be relatively free of structural defects. Group II solid-solutions are of either biogenic origin or are synthetic magnesian calcites and protodolomites (0–20 and ∼ 45 mole percent MgCO3) prepared at high calcite supersaturations (Ωcalcite≥ 3) from NaClNa2SO4MgCl2CaCl2 or NaClMgCl2CaCl2 solutions. Group II solid-solutions are treated as massively defective solids. The defects include substitution foreign ions (Na+ and SO42−) in the magnesian calcite lattice (point defects) and dislocations (~2 · 109 cm−2). Within each group, the excess free energy of mixing, GE, is described by the mixing model ge = X(1− x)[A0 + A1(2x − 1)] , where x is the mole fraction of the end-member Ca0.5Mg0.5CO3 in the solid-solution. The values of A0 and A1 for Group I and II solids were evaluated at 25°C. The equilibrium constants of all the solids are closely described by the equation ln Kx = x(1−x)RT[A0 + A1(2x− 1)]+ (1 − x) ln [KC(1− x)]+ x ln (KDx) , where KC and KD are the equilibrium constants of calcite and Ca0.5Mg0.5CO3. Group I magnesian calcites were modeled as sub-regular solid-solutions between calcite and dolomite, and between calcite and “disordered dolomite”. Both models yield almost identical equilibrium constants for these magnesian calcites. The Group II magnesian calcites were modeled as sub-regular solid-solutions between defective calcite and protodolomite. Group I and II solid-solutions differ significantly in stability. The rate of crystal growth and the chemical composition of the aqueous solutions from which the solids were formed are the main factors controlling stoichiometric solubility of the magnesian calcites and the density of crystal defects. The literature on the occurrence and behavior of magnesian calcites in sea water and other aqueous solutions is also examined.