Impact of Artificial Recharge on Dissolved Noble Gases in Groundwater in California

Abstract

Dissolved noble gas concentrations in groundwater can provide valuable information on recharge temperatures and enable 3H-3He age-dating with the use of physically based interpretive models. This study presents a large (905 samples) data set of dissolved noble gas concentrations from drinking water supply wells throughout California, representing a range of physiographic, climatic, and water management conditions. Three common interpretive models (unfractionated air, UA; partial re-equilibration, PR; and closed system equilibrium, CE) produce systematically different recharge temperatures or ages; however, the ability of the different models to fit measured data within measurement uncertainty indicates that goodness-of-fit is not a robust indicator for model appropriateness. Therefore caution is necessary when interpreting model results. Samples from multiple locations contained significantly higher Ne and excess air concentrations than reported in the literature, with maximum excess air tending toward 0.05 cm3 STP g(-1) (deltaNe approximately 400%). Artificial recharge is the most plausible cause of the high excess air concentrations. The ability of artificial recharge to dissolve greater amounts of atmospheric gases has important implications for oxidation-reduction dependent chemical reactions. Measured gas concentration ratios suggest that diffusive degassing may have occurred. Understanding the physical processes controlling gas dissolution during groundwater recharge is critical for optimal management of artificial recharge and for predicting changes in water quality that can occur following artificial recharge.

Tracers Reveal Recharge Elevations, Groundwater Flow Paths and Travel Times on Mount Shasta, California

“…Noble gas and helium isotope samples were collected in 10 mL crimped copper tubes and were analyzed at the LLNL noble gas mass spectrometry facility [22][23][24]. Measurement uncertainty is 2% for the helium isotope ratio and dissolved concentrations of helium, neon and argon and is 3% for krypton and xenon concentrations.…”

“…This simplest excess air model was used in order to avoid bias in derived parameters resulting from the choice of the excess air model [23,25]. Data reduction followed the methods described in [27].…”

California GAMA Special Study: A Noble Gas Membrane Inlet Mass Spectrometry (NG-MIMS) system for water and gas samples

“…Alternatively, noble gas samples are collected in diffusion samplers (Gardner and Solomon, 2009), equilibrating a gas phase with ambient dissolved gases in situ, which eliminates the need to extract the noble gases from the water samples in the lab. After the gases have been introduced in the gas extraction vacuum manifold, abundant gases are removed by reactive metal getters and the noble gases are separated cryogenically and measured on a mass spectrometer (Cey et al, 2008;Rademacher et al, 2001). Dissolved noble gas tracers at high concentrations can be measured using gas chromatography and helium leak detectors.…”

“…Noble gases are cryogenically separated on a cold finger. Noble gas abundances are measured on a static sector field mass spectrometer (He, Ne and helium isotope ratio), by pressure measurement (Ar) and by an RGA quadrupole mass spectrometer (Kr, Xe) (Cey et al, 2008). The comparison between the NG-MIMS with a traditional NGMS based on 35 copper tube samples is shown in figure 6.…”

“…Goodness of fit measures the ability of an "excess air" model to describe the observed data, and is judged on the probability that the sum of the weighted squared deviations between the modeled and measured concentrations for Ar, Kr, and Xe could arise by chance (based on a chisquare distribution). If that probability is less than 5%, the sample is judged to have bad fit (Cey et al, 2008). For this comparison, only NG-MIMS samples for which Ar, Kr, and Xe data fit an equilibrium or unfractionated excess air model were used for comparison to traditional NGMS data.…”