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

Abstract

This paper describes a direct comparison of a•parent ages derived from 3H/3He, chlorofluorocarbons (CC!3 F and CC12F2), and ø•Kr measurements in shallow groundwater. Wells chosen for this study are completed in the unconfined surficia! 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. 1. Introduction Quantification of groundwater flow requires information on flow velocities and directions, and recharge and discharge rates. The classical approach is to calculate groundwater flows from hydraulic head gradients, hydraulic conductivity, and porosity. Standard laboratory and field methods for determining hydraulic conductivity commonly give conflicting results with large errors because of heterogeneities in most aquifers. Laboratory and field methods often provide limited resolution of these heterogeneities. Most groundwater flow models are calibrated to reproduce the measured potentiometric surface. Because measurements of hydraulic head and other field data form the foundation of groundwater flow models, the model results are limited by the uncertainty of these parameters. Therefore any tracer that provides direct age information is a valuable tool for calibrating groundwater flow models. Groundwater contamination provides an unintentional "dye" experiment that can give insight into travel times along flow paths. However, often the source function of the contaminant(s) (i.e., concentration and date(s) of spill) is not known, adding uncertainty to flow calculations. Most contaminants are chemically reactive and undergo adsorptiondesorption, cation exchange, precipitation-dissolution, oxidation-reduction, or biological reactions within an aquifer. Most of these reaction rates are currently unknown. Thus even if the source of the contamination is known, large uncertainties due to nonconservative behavior exist for travel time calculations based on position of contaminant plumes. Such "dye" experiments can only be applicable on a local scale and, therefore, are not a universal dating method for use in natural systems. Transient tracers are geochemical trace substances that have source functions and/or known (radioactive) decay rates that change with time. Transient tracers provide time information that can be used to infer flo...

Tritium–helium 3 dating under complex conditions in hydraulically stressed areas of a buried‐valley aquifer

“…Many of the samples collected from deeper wells could not be measured as described by Ekwurzel et al [1994] because concentrations of helium were too high for measurement on the automated mass spectrometric system. These samples were measured on a multipurpose noble gas mass spectrometer (MAP 215-50) capable of splitting gas samples in the inlet system.…”

Mixing of shallow and deep groundwater as indicated by the chemistry and age of karstic springs

“…Diffusive loss of 3 He trit in the unsaturated zone has been documented, and resulted in calculated 3 H/ 3 He trit ages that were lower than those for 85 Kr (krypton), which had minor diffusive losses (Weise et al 1992). Other factors, such as rainfall source, timing of recharge, and seasonal variability of 3 H in rainfall can affect 3 H concentrations in recharge water (Ekwurzel et al 1994) and may contribute to the low ( 3 H+ 3 He trit ) concentrations in Salt Springs relative to 3 H input in rainfall. During years of low recharge to the UFA (mid-1950s, early 1960s, and 1967-1968), more closely spaced isochrons could result in a greater potential for mixing and a correspondingly lower resolution in delineating the 3 H signal from atmospheric nuclear testing (Dunkle et al 1993).…”

“…6) and are affected by the mixing of waters from different parts of the aquifer system. Concentrations of 3 H+ 3 He trit usually plot below the IAEA curve around bomb-peak years (Ekwurzel et al 1994). Large gradients of tritium around the bomb peak could create mixtures that would lower the 3 H+ 3 He trit concentrations in this region of the plot.…”

Using 18 O/ 2 H, 3 H/ 3 He, 85 Kr and CFCs to determine mean residence times and water origin in the Grazer and Leibnitzer Feld groundwater bodies (Austria)

“…where whole groundwater bodies are covered with information about their MRT. Taylor et al (1992), Ekwurzel et al (1994), Michel (2004), Phillips and Castro (2005), Phillips and Castro (2005) H/ 3 He-data to demonstrate a trend reversal of agricultural pollutants based on the time of groundwater recharge.…”