Geology topics

T. Torgersen

Publications and source records attributed to T. Torgersen.

4 recordsLinked to original sources

USGS2013

Defining groundwater age

This book investigates applications of selected chemical and isotopic substances that can be used to recognize and interpret age information pertaining to ‘old’ groundwater (defined as water that was recharged on a timescale from approximately 1000 to more than 1 000 000 a). However, as discussed below, only estimates of the ‘age’ of water extracted from wells can be inferred. These groundwater age estimates are interpreted from measured concentrations of chemical and isotopic substances in the groundwater. Even then, there are many complicating factors, as discussed in this book. In spite of these limitations, much can be learned about the physics of groundwater flow and about the temporal aspects of groundwater systems from age interpretations of measured concentrations of environmental tracers in groundwater systems. This chapter puts the concept of ‘age’ into context, including its meaning and interpretation, and attempts to provide a unifying usage for the rest of the book.

Book chapter↗

USGS1998

Controls on the distribution and isotopic composition of helium in deep ground-water flows

The distribution and isotopic composition of helium in sedimentary basins can be used to interpret the ages of very old ground waters. The piston-flow model commonly used in such interpretation, however, does not account for several important factors and as such works well only in very simple flow regimes. In this study of helium transport in a hypothetical sedimentary basin, we develop a numerical model that accounts for the magnitude and distribution of the basal helium flux, hydrodynamic dispersion, and complexities in flow regimes such as subregional flow cells. The modeling shows that these factors exert strong controls on the helium distribution and isotopic composition. The simulations may provide a basis for more accurate interpretations of observed helium concentrations and isotopic ratios in sedimentary basins.

Geology↗

USGS1995

Mantle helium in ground waters of eastern North America: Time and space constraints on sources

Mantle helium in continental environments is generally considered to be the result of active volcanism and/or active extension. The latest episodes of volcanism in northeastern North America are the track of the New England hotspot (95–190 Ma) and the closure of the Iapetus sea (before 300 Ma). Thus, the identification of mantle helium in young ground waters of central New England is counter to the conventional wisdom. On the basis of evaluation of helium evolution in emplaced magmas, we postulate an “aged” mantle source for the excess helium component in ground waters of central New England that is either (1) a local, near-surface–emplaced, gas-rich magma that has retained significant volatiles (e.g., in fluid inclusions) or (2) a deeply emplaced gas-rich magma with high initial 2 He/ 4 He (10 −5 ) and helium transport (with dispersion) through the crust over time. This gas-rich initial condition may support the concept of a volatile-enriched mantle wedge and thus explain the increased buoyancy flux of the New England hotspot as it traversed eastern North America, as has been suggested by others.

Geology↗

USGS1994

Mantle helium in the groundwater of the Mirror Lake Basin, New Hampshire, USA, 1994

Helium isotope analyses of ground waters from the Mirror Lake drainage basin in central New Hampshire (USA) show helium in excess of air-saturated water by up to 200x. The freon ages of these waters are younger than 50 years, consistent with the local hydrology. This excess helium has an isotope ratio of ^3He/^4He = 1.65 ± 0.10 x 10^(-6). It is shown that this component cannot be the result of cosmogenic production or mixing with young water containing ^3He from the decay of (bomb) tritium. Measurements of the helium isotope ratio of local rocks indicate that they cannot be the source of this excess component. This strongly suggests that the excess helium component is the result of the addition of some external source of mantle helium. The generally accepted view suggests that mantle helium in continental environments is the result of active volcanism and extensional tectonics. The latest episodes of volcanism in this region of New England are related to the New England hot spot track (95-190 Myr) and the closure of the Iapetus (> 300 Myr). Thus, either the timescale for helium transport through the crust is of the order of 100's of Myr or the signature of mantle helium can be preserved in (e.g.) fluid inclusions for significant periods of time.

Book chapter↗