Peter G. Cook

Environmental Tracers in Subsurface Hydrology

List of Contributors. Preface. Acknowledgements. 1. Determining Timescales for Groundwater Flow and Solute Transport P.G. Cook, J.-K. Bohlke. 2. Inorganic Ions as Tracers A.L. Herczeg, W.M. Edmunds. 3. Isotope Engineering - Using stable isotopes of the water to solve practical problems T.B. Coplen, et al. 4. Radiocarbon Dating of Groundwater Systems R.M. Kalin. 5. Uranium-Series Nuclides as Tracers in Groundwater Hydrology J.K. Osmond, J.B. Cowart. 6. Radon-222 L. DeWayne Cecil, J.R. Green. 7. Sulphur and Oxygen Isotopes in Sulphate R. Krouse, B. Mayer. 8. Strontium Isotopes R.H. McNutt. 9. Nitrate Isotopes in Groundwater Systems C. Kendall, R. Aravena. 10. Chlorine-36 F.M. Phillips. 11. Atmospheric Noble Gases M. Stute, P. Schlosser. 12. Noble Gas Radioisotopes: 37Ar, 85Kr, 39Ar, 81Kr H.H. Loosli, et al. 13. 3H and 3He D.K. Solomon, P.G. Cook. 14. 4He in Groundwater D.K. Solomon. 15. Chlorofluorocarbons L.N. Plummer, E. Busenberg. 16. delta11B, Rare Earth Elements, delta37Cl, 32Si, 35S, 129I A. Vengosh, et al. Appendices. Index.

Determining natural groundwater influx to a tropical river using radon, chlorofluorocarbons and ionic environmental tracers

Measurements of 222Rn, CFC-11, CFC-12, major ions and temperature of river water and springs are used to quantify rates of groundwater inflow to a tropical lowland river in the Northern Territory of Australia. Groundwater inflow results in increases in 222Rn concentrations within the river, but decreases in concentrations of CFC-11 and CFC-12, because the inflowing groundwater is relatively old. 222Rn, CFC-11 and CFC-12 concentrations are affected by gas exchange with the atmosphere, while ion concentrations are not. Additionally, CFC concentrations in the river appear to have been increased by air entrapment and dissolution during turbulent flow at river rapids. Because the regional groundwater is old, CFC concentrations in groundwater inflow are zero. In contrast, 222Rn and ion concentrations in the river are very sensitive to concentrations of these tracers in groundwater inflow. Numerical simulation of 222Rn, CFC-11 and CFC-12 stream concentrations allows the groundwater inflow rate, gas transfer velocity and air entrapment coefficient to be reasonably accurately constrained.

Determining Timescales for Groundwater Flow and Solute Transport

One of the principal uses of environmental tracers is for determining the ages of soil waters and groundwaters. (We may refer to this as ‘hydrochronology’by analogy with the dating of solid materials known as geochronology.) Information on soil water and groundwater age enables timescales for a range of subsurface processes to be determined. For example, ‘groundwater stratigraphy’is used increasingly to decipher past recharge rates and conditions in unconfined aquifers, in much the same way that sedimentary stratigraphy yields information about past depositional environments. The use of environmental tracers to determine water ages allows groundwater recharge rates and flow velocities to be determined independently, and commonly more accurately, than with traditional hydraulic methods where hydraulic properties of aquifers are poorly known or spatially variable. Studies of groundwater residence times in association with groundwater contamination studies can enable historic release rates of contaminants and contaminant transport rates to be determined. Where input rates are known, measurements of groundwater contaminant concentrations, together with groundwater dating, can sometimes be used for estimating chemical reaction rates. The combination of these dating methods with stable isotope measurements has sometimes allowed changes in contaminant sources over time to be determined.

Transport of Atmospheric Trace Gases to the Water Table: Implications for Groundwater Dating with Chlorofluorocarbons and Krypton 85

Chlorofluorocarbons and krypton 85 are trace gases whose atmospheric concentrations have been increasing over the past few decades. As they are soluble in water, they have been used as groundwater age indicators over timescales ranging from a few years to a few decades. In this paper we show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones. The apparent time lag is largely dependent on the gas solubility, the gas diffusion coefficient, and the soil water content. In coarse-grained soils the lag time will typically range between 1 and 2 years for a water table depth of 10 m to between 8 and 15 years for a water table depth of 30 m. For the shallower water tables ( 10 m), if this effect is not considered, the use of these gaseous tracers will overestimate the age of the groundwater. In very fine-grained soils where the soil water content in the unsaturated zone may be close to saturation, the effect will be much more pronounced.

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.

Chlorofluorocarbons as Tracers of Groundwater Transport Processes in a Shallow, Silty Sand Aquifer

Detailed depth profiles of Chlorofluorocarbons CFC-11(CFCl3(, CFC-12 (CF2Cl2) and CFC-113 (C2F3Cl3) have been obtained from a well-characterized field site in central Ontario. Aquifer materials comprise predominantly silty sands, with a mean organic carbon content of 0.03%. Nearly one-dimensional flow exists at this site, and the vertical migration of a well-defined 3H peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with 3H profiles enables estimation of chlorofluorocarbon transport parameters. CFC-12 appears to be the most conservative of the CFCs measured. Sorption at this site is low (Kd < 0.03), and degradation does not appear to be important. CFC- 113 is retarded both with respect to CFC-12 and with respect to 3H (Kd = 0.09−0.14). CFC-11 appears to be degraded both in the highly organic unsaturated zone and below 3.5 m depth in the aquifer, where dissolved oxygen concentrations decrease to below 0.5 mg L−1. The half-life for CFC-11 degradation below 3.5 m depth is less than 2 years. While apparent CFC-12 ages match hydraulic ages to within 20% (up to 30 years), apparent CFC-11 and CFC-113 ages significantly overestimate hydraulic ages at our field site.

Quantifying groundwater discharge to Cockburn River, southeastern Australia, using dissolved gas tracers 222Rn and SF6

Groundwater discharge to the Cockburn River, southeast Australia, has been estimated from comparison of natural 222Rn activities in groundwater and river water, interpreted using a numerical flow model that simulates longitudinal radon activities as a function of groundwater inflow, hyporheic exchange, evaporation, gas exchange with the atmosphere, and radioactive decay. An injection of SF6 into the river to estimate the gas transfer velocity assisted in constraining the model. Previous estimates of groundwater inflow using 222Rn activities have not considered possible input of radon due to exchange between river water and water in the hyporheic zone beneath the streambed. In this paper, radon input due to hyporheic exchange is estimated from measurements of radon production by hyporheic zone sediments and rates of water exchange between the river and the hyporheic zone. Total groundwater inflow to the Cockburn River is estimated to be 18500 m 3/d, although failure to consider hyporheic exchange would cause overestimation of the volume of groundwater inflow by approximately 70%. Copyright 2006 by the American Geophysical Union.

Hydrogeologic controls on disconnection between surface water and groundwater

[1] Understanding how changes in the groundwater table affect surface water resources is of fundamental importance in quantitative hydrology. If the groundwater table below a stream is sufficiently deep, changes in the groundwater table position effectively do not alter the infiltration rate. This is referred to as a disconnected system. Previous authors noted that a low-conductivity layer below the surface water body is a necessary but not sufficient criterion for disconnection to occur. We develop a precise criterion that allows an assessment of whether surface water-groundwater systems can disconnect or not. We further demonstrate that a disconnected system can be conceptualized by a saturated groundwater mound and the development of a capillary zone above this mound. This conceptualization is used to determine the critical water table position at the point where full disconnection is reached. A comparison of this calculated critical water table position with a measurement of the water table depth in a borehole allows the assessment of the disconnection status. A sensitivity analysis of this critical water table showed that for a given aquifer thickness and river width, the depth to groundwater where the system disconnects is approximately proportional to the stream depth and the hydraulic conductivity of the streambed sediments and inversely proportional to the thickness of these sediments and the hydraulic conductivity of the aquifer. The conceptualization also allows the disconnection problem to be analyzed using both variably saturated and fully saturated groundwater models and provides guidance for numerical and analytical approaches.

Water balance of a tropical woodland ecosystem, Northern Australia: A combination of micro-meteorological, soil physical and groundwater chemical approaches

A combination of micro-meteorological, soil physical and groundwater chemical methods enabled the water balance of a tropical eucalypt savanna ecosystem in Northern Australia to be estimated. Heat pulse and eddy correlation were used to determine overstory and total evapotranspiration, respectively. Measurements of soil water content, matric suction and water table variations were used to determine changes in soil moisture storage throughout the year. Groundwater dating with chlorofluorocarbons was used to estimate net groundwater recharge rates, and stream gauging was used to determine surface runoff. The wet season rainfall of 1585 mm is distributed as: evapotranspiration 810 mm, surface runoff (and shallow subsurface flow) into the river 410 mm, groundwater recharge 200 mm and increase in soil store 165 mm. Of the groundwater recharge, 160 mm enters the stream as baseflow in the wet season, 20 mm enters as baseflow in the dry season, and the balance (20 mm) is distributed to and used by minor vegetation types within the catchment or discharges to the sea. In the dry season, an evapotranspiration of 300 mm comprises 135 mm rainfall and 165 mm from the soil store. Because of the inherent errors of the different techniques, the water balance surplus (estimated at 20 mm) cannot be clearly distinguished from zero. It may also be as much as 140 mm. To our knowledge, this is the first time that such diverse methods have been combined to estimate all components of a catchments water balance.

Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers

The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured rocks of various lithologies that have low primary permeability and porosity, and (3) a shallow residuum of medium permeability and high total porosity. Conceptual models of shallow groundwater flow and solute transport in this system have been developed but are difficult to evaluate using physical characterization or short-term tracer methods due to extreme spatial variability in hydraulic properties. In this paper we describe how chlorofluorocarbon 12, 3H, and 3He were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are <0.05 m, suggesting that concentrations of these tracers in fractures and in the matrix have time to diffusionally equilibrate. The relatively smooth nature of tracer concentrations with depth in the residuum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr−1. In contrast, groundwater flow within the unweathered rock appears to be controlled by fractures with spacings of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes in the unweathered rock cannot be estimated from the tracer data.