Calculations of horizontal mixing rates using222Rn and the controls on hypoxia in western Long Island Sound, 1991

Late summer hypoxia (<3 ppm oxygen) in western Long Island Sound (WLIS) is a persistent environmental and management issue whose controlling processes are poorly understood. Measured rates of sediment and water-column oxygen consumption in the bottom water suggest that a condition of no oxygen should be attained on the time scale of 13–30 d. Observations, however, indicate the onset of hypoxia is of the order 150 d. Therefore, horizontal and/or vertical transport of oxygen into the area of hypoxia must play an important role. Hypoxia decreases benthic activity and the sediment flux of²²²Rn. The resulting horizontal gradient in bottom water²²²Rn was measured and used to estimate the effective horizontal transport rate (>5–50 m² s^?1), which is considerably slower than previous estimates. Scale analysis of the hypoxia process indicates that horizontal transport rates alone can explain the slow progression of hypoxia in XLIS but that vertical processes may also be capable of delaying the onset of hypoxia especially under conditions of weak stratification or weak intermediate layer oxygen consumption. This scale analysis indicates a delicately balanced process that is sensitive to both climatologically-driven variability in the rates of horizontal and vertical transport as well as the biologically-driven rates of oxygen consumption. An improved ability to predict and/or control hypoxia must be based on a better understanding of temporal and spacial variations in circulation, mixing, and stratification as well as the biological processes in the water column and the sediments.     

Controls on pore-fluid concentration of He and Rn and the calculation of He/Rn ages

Controls governing the production of ⁴He and ²²²Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. ²²²Rn activity is used to quantify the uranium sources for ⁴He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GL_Rn//GL_He, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated ⁴He. It was found that /GL_Rn//GL_He can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for ²²²Rn decay, r_e.⁴He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of ⁴He/ ²²²Rn measurements to continental freshwater springs indicates that the weathering release of accumulated ⁴He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the ⁴He/²²²Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested.     

DNA tracers with information capacity and high detection sensitivity tested in groundwater studies

 In order to investigate the usefulness of unique synthetic DNA tracers in groundwater, a field experiment was conducted in Norway. DNA tracers and a sodium-chloride tracer were injected into an aquifer. The transport of DNA molecules was interpreted by comparing with the plume of chloride ions under forced-gradient steady-state flow conditions. Spatial concentration moments described the migration of conservative tracers. Mobility and migration of DNA in groundwater demonstrate that DNA tracers can be detected by using the polymerase chain reaction (PCR) and DNA sequence analysis. The results indicate that DNA tracers can be valuable tools as tracers in groundwater investigations. Received, June 1997 / Revised, July 1998, December 1998 / Accepted, January 1999     

Helium isotopes in geothermal systems: Iceland,The Geysers,Raft River and Steamboat Springs

Helium isotope ratios have been measured in geothermal fluids from Iceland, The Geysers, Raft River, Steamboat Springs and Hawaii. These ratios have been interpreted in terms of the processes which supply He in distinct isotopic ratios (i.e. magmatic He, ~10 R_a; atmospheric He, R_a; and crustal He, ~0.1 R_a) and in terms of the processes which can alter the isotopic ratio (hydrologic mixing, U-Th series alpha production and weathering release of crustal He, magma aging and tritiugenic addition of ³He). Using this interpretational scheme, Iceland is found to be an area of hot-spot magmatic He implying an active volcanic source although the data are suggestive of high-temperature weathering release of crustal He incorporated in the geothermal fluids. By comparison to fumarolic gases from Hawaii and Juan De Fuca and Cayman Trench basaltic glass samples, The Geysers contains MOR type magmatic He again implying an active volcanic source possibly a “leaky” transform related to the San Andreas Fault System. Raft River contains only crustal He indicating no active volcanic sources. Steamboat Springs He isotope ratios are distinctly less than typical plate margin volcanics but must still have a magmatic source. A preliminary assessment of the cause for this low ratio is made assuming an “aging” magma source.     

Helium accumulation in groundwater,I: An evaluation of sources and the continental flux of crustal 4He in the Great Artesian Basin,Australia

The rate of accumulation of ⁴He in the groundwaters of the J-aquifer of the Great Artesian Basin, Australia has been determined using ¹⁴C and hydrologic ages. For groundwaters less than 50 Kyr in age, the ⁴He accumulation rate is 4.6 × 10^?12 HeAU (where ) in close agreement with in-situ production rate of ⁴He (3.95 × 10^?12 HeAU) based on U and Th concentrations of 1.7 and 6.1, respectively, of the sandstone. For groundwaters older than 100 Kyr, the rate of ⁴He accumulation is 2.91 × 10^?10 HeAU based on hydrologic ages; or 74 × the rate of in-situ production. The rate of ⁴He “production” due to weathering of the aquifer rock is calculated to be ~10^?16 HeAU, indicating that the weathering input of ⁴He is insignificant. If the groundwater of the GAB can be considered as a trap for the total crustal production of ⁴He, the rate of ⁴He accumulation under a steady-state flux is calculated to be 3.02 × 10^?10 HeAU, in agreement with the measured accumulation rate. It is concluded that over long times the ⁴He accumulation rate in groundwater aquifers may be controlled by the whole crust flux of ⁴He.     

Helium accumulation in groundwater, III. Limits on helium transfer across the mantle-crust boundary beneath Australia and the magnitude of mantle degassing

The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (³He/⁴He 1.6 × 10⁻⁸) is isotopically distinct from the crustal degassing helium flux (³He/⁴He 6.6 × 10⁻⁸). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (³He/⁴He= 3.5 × 10⁻⁸) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 10⁶⁴He atoms mW⁻¹ s⁻¹ which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 10⁸⁴He atoms mW⁻¹ s⁻¹ measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of⁴He and⁴⁰Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of⁴He and⁴⁰Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of⁴He and⁴⁰Ar to the atmosphere must come from the continental crust.     

Dating ultra-deep mine waters with noble gases and Cl, Witwatersrand Basin, South Africa

Concentrations and isotopic ratios of dissolved noble gases, ³⁶Cl, δD and δ¹⁸O in water samples from the ultra-deep gold mines (0.718 to 3.3 km below the surface) in the Witwatersrand Basin, South Africa, were investigated to quantify the dynamics of these ultra deep crustal fluids. The mining activity has a significant impact on the concentrations of dissolved gases, as the associated pressure release causes the degassing of the fissure water. The observed under saturation of the atmospheric noble gases in the fissure water samples (70-98%, normalized to ASW at 20°C and 1013 mbar) is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature. Corrections for degassing result in ⁴He concentrations as high as 1.55 · 10⁻¹cm³STP⁴He g⁻¹, ⁴⁰Ar/³⁶Ar ranging between 806 and 10331, and ¹³⁴Xe/¹³²Xe and ¹³⁶Xe/¹³²Xe ratios above 0.46 and 0.44, respectively. Corrected ^134(136)Xe/¹³²Xe and ^134(136)Xe/⁴He-ratios are consistent with their production ratios, whereas the nucleogenic ⁴He/⁴⁰Ar, and ^134(136)Xe/⁴⁰Ar ratios generally indicate that these gases are produced in an environment with an average [U + Th]/K-content 2-3 times above that of crustal average. In two scenarios, one considering only accumulation of in situ produced noble gases, the other additionally crustal flux components, the model ages for 14 individual water samples range from 13 to 168 Ma and from 1 to 23 Ma, respectively.The low ³⁶Cl-ratios of (4-37) · 10⁻¹⁵ and comparatively high ³⁶Cl-concentrations of (8-350) · 10⁻¹⁵ atoms ³⁶Cl l⁻¹ reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 times the half-life of ³⁶Cl.In combination, the results suggest residence times of the fluids in fissures in this region (up to 3.3 km depth) are of the order of 1-100 Ma. We cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk.     

Inclined K–Ar illite age spectra in brittle fault gouges: effects of fault reactivation and wall‐rock contamination

K–Ar clay fraction ages of brittle faults often vary with grain size, decreasing in the finer size fractions, producing an inclined age–grain‐size spectrum. K–Ar ages and mineralogical characterization of gouges from two normal faults in the Kongsberg silver mines, southern Norway, suggest that inclined spectra derived from brittle fault rocks reflect the mixing of inherited components with authigenic mineral phases. The ages of the coarsest and finest fractions constrain faulting at c. 260–270 Ma and reactivation around 200–210 Ma, respectively. This study demonstrates how wall‐rock contamination influences the K–Ar age of the coarsest size fractions and that authigenic illite and K‐feldspar can crystallize synkinematically under equivalent conditions and thus yield the same K–Ar ages.