D. Kip Solomon

On the isotopic composition of carbon in soil carbon dioxide

Abstract In this study it is shown that the isotopic composition of carbon in soil CO2 differs from the isotopic composition of carbon in soil-respired CO2. Soil CO2 collected from a montane soil has an endmember δ13C value of −23.3%. whereas soil-respired CO2 in this system has a δ 13C value of −27.5%. This difference is very close to the theoretical difference of 4.4%. which is predicted by the difference in the diffusion coefficients for 12CO2 and 13CO2. Because of the measured and the theoretical difference between the isotopic composition of carbon in soil CO2 and soil-respired CO2 it is suggested that, in the future, a distinction should be made between them.

3H and 3He

Tritium (3H) is the only radioactive isotope of hydrogen, and has a half-life of 12.43 years (Unterweger et al., 1980). Large quantities of tritium were introduced into the hydrological cycle by atmospheric thermonuclear testing in the 1950s and 1960s, providing a useful environmental tracer for water originating from this period. Tritium decays by beta-emission to 3He, the rare, stable isotope of helium. Under favourable conditions, measurements of both 3H and 3He in groundwater allow the reconstruction of tritium concentrations in precipitation and the determination of water flow paths. Ratios of 3H to 3He can be applied to quantify the extent of radioactive decay, and hence determine subsurface water residence times up to 40 years.

Using noble gases to investigate mountain-front recharge

Mountain-front recharge is a major component of recharge to inter-mountain basin-fill aquifers. The two components of mountain-front recharge are (1) subsurface inflow from the mountain block (subsurface inflow), and (2) infiltration from perennial and ephemeral streams near the mountain front (stream seepage). The magnitude of subsurface inflow is of central importance in source protection planning for basin-fill aquifers and in some water rights disputes, yet existing estimates carry large uncertainties. Stable isotope ratios can indicate the magnitude of mountain-front recharge relative to other components, but are generally incapable of distinguishing subsurface inflow from stream seepage. Noble gases provide an effective tool for determining the relative significance of subsurface inflow, specifically. Dissolved noble gas concentrations allow for the determination of recharge temperature, which is correlated with recharge elevation. The nature of this correlation cannot be assumed, however, and must be derived for the study area. The method is applied to the Salt Lake Valley Principal Aquifer in northern Utah to demonstrate its utility. Samples from 16 springs and mine tunnels in the adjacent Wasatch Mountains indicate that recharge temperature decreases with elevation at about the same rate as the mean annual air temperature, but is on average about 2 8C cooler. Samples from 27 valley production wells yield recharge elevations ranging from the valley elevation (about 1500 m) to mid-mountain elevation (about 2500 m). Only six of the wells have recharge elevations less than 1800 m. Recharge elevations consistently greater than 2000 m in the southeastern part of the basin indicate that subsurface inflow constitutes most of the total recharge in this area. q 2003 Published by Elsevier Science B.V.

Dissolved gas tracers in groundwater: Simplified injection, sampling, and analysis

Simplified injection, sampling, and analytical procedures using dissolved gases as groundwater tracers are presented for use in saturated conditions at both the laboratory and field scales. The injection of gases into the groundwater is accomplished by allowing the gas to diffuse through semipermeable tubing, minimizing the formation of bubbles that could modify the hydraulic properties around the well. We have simplified the collection of dissolved gases by developing a passive in situ headspace sampler the employs a semipermeable membrane and copper tubing equipped with a schrader valve. The headspace within the sampler equilibrates with the dissolved gases in the groundwater in around 24 hours, and no groundwater is collected, which is of great advantage for use in contaminated sites. The design parameters and the time to equilibrium of the headspace sampler can be adjusted for investigation requirements using the analytical equation presented. The analysis of the gases for tracer content is performed by using a common gas Chromatograph fitted with a thermal conductivity detector. Examples of the use of these methods at both the laboratory and field scales are presented.

4He in Groundwater

Helium-4 is produced within the Earth by the decay of 238U, 235U and 232Th. Shortly after the discovery of the radioactivity of U and Th, the idea of using the accumulation of He in minerals as a dating tool was proposed by Ernest Rutherford (Hurley, 1954). The U-He dating method for rocks is based on the assumption that U- and Th- bearing minerals quantitatively retain the He produced within them. However, comparison of U-He dates with other methods (e.g., K-Ar) has shown that U-He ages frequently underestimate the true age of the sample as a result of incomplete He retention.

Noble Gas Thermometry in Groundwater Hydrology

Concentrations of dissolved atmospheric noble gases in water constitute a thermometer, whose application to the groundwater archive provides a method of paleoclimate reconstruction. In addition, noble gases have found wide application as tracers in hydrogeology. This chapter reviews the historical development, the theoretical foundations, the sampling and analytical techniques, as well as the spectrum of applications of this important tool of tracer hydrology. A detailed account of currently available sampling techniques is given, as this information is of great practical importance but not fully available in the scientific literature. The analytical methods are better documented in the literature, although the many lab-specific details and constant development make it hard to provide an authoritative overview, so that this part is kept comparatively short. The focus of the chapter lies on the methods for data reduction and interpretation, which have undergone rapid and important development in the recent past. Nevertheless, in this respect still substantial research needs exist. Finally, this chapter provides an overview of applications of noble gases in groundwater hydrology, which range from the classical paleothermometry and the determination of other paleoclimate parameters such as humidity to various hydrological investigations, such as groundwater dating or the study of water origin and recharge conditions in hydrothermal, glaciated, alluvial, coastal, managed, and mountainous aquifer systems.

Dating of 'young' groundwaters using environmental tracers: advantages, applications, and research needs.

Many problems related to groundwater supply and quality, as well as groundwater-dependent ecosystems require some understanding of the timescales of flow and transport. For example, increased concern about the vulnerabilities of ‘young’ groundwaters (less than ∼ 1000 years) to overexploitation, contamination, and land use/climate change effects are driving the need to understand flow and transport processes that occur over decadal, annual, or shorter timescales. Over the last few decades, a powerful suite of environmental tracers has emerged that can be used to interrogate a wide variety of young groundwater systems and provide information about groundwater ages/residence times appropriate to the timescales over which these systems respond. These tracer methods have distinct advantages over traditional approaches providing information about groundwater systems that would likely not be obtainable otherwise. The objective of this paper is to discuss how environmental tracers are used to characterise young groundwater systems so that more researchers, water managers, and policy-makers are aware of the value of environmental tracer approaches and can apply them in appropriate ways. We also discuss areas where additional research is required to improve ease of use and extend quantitative interpretations of tracer results.

Stream Measurements Locate Thermogenic Methane Fluxes in Groundwater Discharge in an Area of Shale-Gas Development

The environmental impacts of shale-gas development on water resources, including methane migration to shallow groundwater, have been difficult to assess. Monitoring around gas wells is generally limited to domestic water-supply wells, which often are not situated along predominant groundwater flow paths. A new concept is tested here: combining stream hydrocarbon and noble-gas measurements with reach mass-balance modeling to estimate thermogenic methane concentrations and fluxes in groundwater discharging to streams and to constrain methane sources. In the Marcellus Formation shale-gas play of northern Pennsylvania (U.S.A.), we sampled methane in 15 streams as a reconnaissance tool to locate methane-laden groundwater discharge: concentrations up to 69 μg L(-1) were observed, with four streams ≥ 5 μg L(-1). Geochemical analyses of water from one stream with high methane (Sugar Run, Lycoming County) were consistent with Middle Devonian gases. After sampling was completed, we learned of a state regulator investigation of stray-gas migration from a nearby Marcellus Formation gas well. Modeling indicates a groundwater thermogenic methane flux of about 0.5 kg d(-1) discharging into Sugar Run, possibly from this fugitive gas source. Since flow paths often coalesce into gaining streams, stream methane monitoring provides the first watershed-scale method to assess groundwater contamination from shale-gas development.

Groundwater transit time distribution and mean from streambed sampling in an agricultural coastal plain watershed, North Carolina, USA

We measured groundwater apparent age (τ) and seepage rate (v) in a sandy streambed using point-scale sampling and seepage blankets (a novel seepage meter). We found very similar MTT estimates from streambed point sampling in a 58 m reach (29 years) and a 2.5 km reach (31 years). The TTD for groundwater discharging to the stream was best fit by a gamma distribution model and was very similar for streambed point sampling in both reaches. Between adjacent point-scale and seepage blanket samples, water from the seepage blankets was generally younger, largely because blanket samples contained a fraction of “young” stream water. Correcting blanket data for the stream water fraction brought τ estimates for most blanket samples closer to those for adjacent point samples. The MTT estimates from corrected blanket data were in good agreement with those from sampling streambed points adjacent to the blankets. Collectively, agreement among age-dating tracers, general accord between tracer data and piston-flow model curves, and large groundwater age gradients in the streambed, suggested that the piston flow apparent ages were reasonable estimates of the groundwater transit times for most samples. Overall, our results from two field campaigns suggest that groundwater collected in the streambed can provide reasonable estimates of apparent age of groundwater discharge, and that MTT can be determined from different age-dating tracers and by sampling with different groundwater collection devices. Coupled streambed point measurements of groundwater age and groundwater seepage rate represent a novel, reproducible, and effective approach to estimating aquifer TTD and MTT.

Dissolved Gases in Subsurface Hydrology

Publisher Summary This chapter provides an overview of dissolved gases. It has been used as tracers of water residence times, recharge temperatures and elevations, ground-water/surface-water interactions, and injected directly to evaluate mass transport processes. Many dissolved gases are essentially geochemically conservative making them ideal tracers of physical transport in the subsurface. Dissolved gases in groundwater can be of either atmospheric or subsurface origin. It transports through fully saturated porous media is similar to ionic species as far as advection, dispersion, and diffusion are concerned. However, when both liquid and gas phases exist, dissolved gases having low solubilities will tend to partition into the gas phase that will strongly influence dissolved gas transport. Gases that are of atmospheric origin are typically undersaturated in shallow groundwater systems below the annual low water table. Dissolved gases also have been used to evaluate interactions between groundwater and surface water systems.