Stanley N. Davis

Atmospheric and subsurface sources of stable and radioactive nuclides used for groundwater dating

Theoretical calculations together with a review of data from Sweden, Switzerland and Canada indicate that given sufficient isolation time and U and Th in the aquifer matrix, deep subsurface production of 3H, 4He, 36Cl, 37Ar, 39Ar, 40Ar, 85Kr, 129I, and 222Rn can overshadow the normal atmospheric component of these nuclides in recharging groundwater. Although direct evidence is lacking, calculations suggest that small but measurable subsurface production of 3H and 14C could also take place in aquifers having high U and Th concentrations. Some ground water which has been recharged during the past 40 years will have 3H, 3He, 36Cl, 85Kr, and 129I concentrations which are dominantly anthropogenic. Dating methods with the least complications in interpretation appear to be 3H-3He for postbomb waters and 81Kr for waters in the range of 5×104 to 1×106 a.

Geochemistry of halogens in the Milk River aquifer, Alberta, Canada

Abstract Analytical data are presented for Cl, Br and I on a regional scale for the Milk River aquifer. The three halides show strikingly similar spatial distributions and are highly correlated. Concentrations are low in the freshwater portions of the aquifer but increase by as much as two orders of magnitude along the margins. However, halide ratios reach nearly constant values moving down-gradient, suggesting the dominance of a common subsurface source for these ions. Ratios of Cl/I and Cl/Br are less than those of seawater and fit an origin derived from the diagenesis of organic matter in the sediments. Halide ratios rule out leakage and/or diffusion from the underlying Colorado Group as a major influence on the chemistry; the favored hypothesis is altered connate seawater diffusing from low-permeability units within the Milk River Formation as the primary source of salts. This hypothesis of an internal source has important implications for solute sources in other aquifers affected by saline waters because it does not require the importation of a distant fluid. The129I/I ratio has a meteoric value in groundwater collected near the recharge area, but ratios for downflow waters are only 8–70% of this value. Due to the 16 Ma half-life of129I, these data indicate that most of the increase in dissolved I cannot derive from concentration of a meteoric source by ion filtration, but must have a subsurface origin. Concentrations of129I producedin situ by spontaneous fission of238U attain measurable levels only in the oldest waters sampled (ages≥ 105a), in which it may account for nearly 90% of the total dissolved129I concentration. Water ages based upon36Cl/Cl data range up to 2 Ma if uncorrected for any dilution by subsurface sources of dead Cl. If one assumes that the subsurface contributions of Cl contribute at least 90% of total Cl in the distal portion, then the36Cl-based ages are reduced to ∼ 1Ma, somewhat greater than those estimated by hydrodynamic modeling.

Applications of 129I and 36Cl in hydrology

Since the first AMS measurements of 36Cl in 1978, this cosmogenic radionuclide has proved to be a versatile tracer of hydrologic processes in over 20 field studies. Natural 129I also appears to be useful for studying hydrologie processes although incomplete understanding of its production in nature and geochemical behavior largely limits interpretation to qualitative discussions. The range of hydrologic applications demonstrated for these radionuclides covers: estimation of residence time of water in the subsurface and net infiltration in arid soils; evaluation of ion filtration, leaching of connate water, and salt dissolution as sources of ground-water salinity; estimation of lithospheric thermal-neutron fluxes; and emanation and migration characteristics of fission-product 129I in different geochemical environments.

Chlorine-36, bromide, and the origin of spring water

Natural ratios of chlorine-36 (36Cl) to stable chlorine (i.e., 36Cl/Cl×10−15) vary in shallow groundwater of the United States from about 50 in coastal areas to about 1400 in the northern Rocky Mountains. Ratios lower than these indicate the presence of chloride (Cl−) that has been isolated from the atmosphere for hundreds of thousands of years, if not longer. Higher ratios, which can exceed 5000, usually originate from fallout from testing thermonuclear devices in the western Pacific in the 1950s. Natural mass ratios of chloride to bromide (Cl−/Br−) in precipitation vary in the United States from about 250 in coastal areas to about 50 in the north-central states. Lower ratios may suggest contamination from human sources. Higher ratios, which may exceed 2000, commonly reflect the dissolution of halite. Seawater has a Cl−/Br− ratio of 290. Both 36Cl and Cl−/Br− ratios have been measured in 21 samples of spring water collected from springs in 10 different states. Brackish water from Saratoga Springs area in New York has low values for both 36Cl and Cl−/Br− ratios. This indicates that a large component of the water has a very deep origin. Brackish water from Alexander Springs in Florida has a low 36Cl ratio but a high Cl−/Br− ratio similar to seawater. This suggests the addition of ancient seawater that may be trapped in the aquifer. Big Spring in Iowa discharges water with a very high Cl−/Br− ratio but a moderate 36Cl ratio. The high ratio of Cl−/Br− may be produced by dissolution of road salt or agricultural chemicals. Of the 21 springs sampled, only 10 appeared to have potable water not significantly affected by human activity. Chlorine-36 from testing of nuclear devices is still being flushed out of four of the spring systems that were sampled. Thus, more than 45 years have passed since 36Cl was introduced into the aquifers feeding the springs and the systems, as yet, have not been purged.

Paleoclimatic inferences from an isotopic investigation of groundwater in the central San Juan Basin, New Mexico

Groundwater from the Ojo Alamo and Nacimiento aquifers in the central San Juan Basin. New Mexico, has yielded 14C ages ranging from modern to 35,000 yr B.P. The Pleistocene-age samples are characterized by a stable isotope content about 25‰ lighter in D and 3‰ lighter in 18O than modern precipitation and groundwater. We attribute this difference to a colder mean annual temperature and perhaps increased winter precipitation. Consideration of various factors controlling the stable isotope composition of the groundwater allows estimation of a 5° to 7°C temperature decrease during the late Wisconsin, accompanied by increased effective precipitation. A similar estimate of the temperature change is obtained from noble-gas paleothermometry. These data support a model of moderately cooler late Pleistocene climate in the American Southwest characterized by summers with less precipitation than today, but wetter winters.

In situ production and migration of 129I in the Stripa granite, Sweden

Abstract Concentrations of natural 129I and 36Cl in ground water from the Stripa granite, Sweden, were determined by accelerator mass spectrometry. The 129I values range from 1 × 106 to 2 × 108 atoms 1−1, orders of magnitude greater than the estimated background concentration in pre-1945 rainwater of 2 × 104atoms 1−1. Expressed as a ratio 129 I 127 I , values range from 3 × 10−12 to 2 × 10−10, compared to about 1 × 10−12 in pre-1945 rainwater. Ratios, as well as concentrations, generally increase with sample depth, which ranges from 360 to >1000 m below the surface. The accumulation of 129I in the water is attributed to subsurface production by spontaneous fission of 238U in micro-fractures in the granite matrix and subsequent transfer of this 129I by diffusion into fractures constituting the flow system of the granite. Uncertainties in the spontaneous-fission yield at mass 129 and in the assumption of steady-state mass flux prevent quantitative estimates of the rate of 129I transfer.

A case study simulation of DBCP groundwater contamination in Fresno County, California 1. Leaching through the unsaturated subsurface

Abstract This paper is the first installment of a multi-paper series concerned with simulating the potential vulnerability of groundwater in Fresno County (California) to contamination resulting from long-term, agriculture related, applications of the nematocide DBCP. In this paper our focus is on the surface and the unsaturated subsurface. Using PRZM-2, we quantitatively estimate, for a 35 year period, the potential fate and transport of DBCP between the surface and the water table for multiple non-point source applications, related to different and changing land-use, between 1960 and 1977. Our transport simulations include consideration for advection, decay, sorption, and volatilization. The results presented here indicate tremendous spatial and temporal variability in DBCP loading at the water table at the regional scale. Our simulations also suggest that the DBCP loading to the water table, resulting from the legacy of the chemicals use, is currently well below the detectable limit. The characterization of spatially variable DBCP loadings to the saturated subsurface through time, which are presented in this paper, is used as input for the 3-D groundwater models of DBCP fate and transport in Fresno County between 1960 and 1994 that are reported in the companion paper (Loague et al., 1997).

A case study simulation of DBCP groundwater contamination in Fresno County, California 2. Transport in the saturated subsurface

Abstract This paper is the second installment of a multipaper series concerned with simulating regional-scale groundwater contamination in Fresno County, CA as the result of long-term nonpoint source applications of 1,2-dibromo-3-chloropropane (DBCP). In the first part of this study [Loague, K., Lloyd, D., Nguyen, A., Davis, S.N., Abrams, R.H., 1997. A case study simulation of DBCP groundwater contamination in Fresno County, California: 1. Leaching through the unsaturated subsurface. J. Contamin. Hydrol. (this issue)], our focus was on the unsaturated zone and DBCP leaching to the water table. Our focus in this paper is the assessment of DBCP transport in the saturated subsurface. Here, the groundwater flow code MODFLOW is coupled to the solute transport code MT3D to simulate the potential fate and transport of DBCP below the water table in the Fresno County study area for a 35-yr period. The results presented here illustrate the long-term regional-scale evolution of the DBCP plume in the study area. The simulated DBCP concentrations are limited to the relatively shallow younger sediments and are generally well below the maximum contaminant level set for the chemical. The simulations presented here suggest that nonpoint source applications of DBCP are not responsible for the observed hot spots in the study area. At the end of this paper we discuss the implications and extensions of the Fresno case study which has been reported here and in the companion paper mentioned above (Loague et al., 1997).

Estimation of mountain front recharge to regional aquifers: 1. Development of an Analytical Hydroclimatic Model

This paper addresses the hydroclimatic modeling of mountain front recharge to regional aquifers. An analytical relationship between the mean seasonal precipitation and runoff is obtained based on a conceptualization of the hydrologic processes occurring in hard rock mountainous terrain and a derived-distribution approach where the input variables are considered to be stochastic and their probability distributions are transformed into the probability distribution of the output variable by using the deterministic physical process. In a first-order approximation a relationship between the seasonal values of precipitation and runoff is obtained. An analytical model of the seasonal streamflow is then developed where initial abstraction and the long-term effective subsurface outflow, or mountain front recharge, are viewed as unknown model parameters. In addition, a procedure that combines the water balance equation with a relationship provided by the so-called “vegetal equilibrium hypothesis,” and which enables the estimation of effective soil-related parameters jointly with the mean seasonal evapotranspiration and surface runoff, is introduced. This procedure is applied to a mountainous watershed in southern Arizona.

Application of isotopic methods to dating of very old groundwaters: Milk River aquifer, Alberta, Canada

The aim of this joint project was to evaluate the usefulness of available geochemical and isotope techniques for dating very old groundwater. This paper represents a synthesis of an IAEA sponsored study for which purpose the Milk River aquifer groundwaters were sampled from 16 wells during 1985 and subsequent years for the following measurements:2H,3H,13C,18O,14C,39Ar,81Kr,85Kr, noble gases including222Rn,36Cl,129I and U isotopes. The Milk River aquifer was selected for this study because several preceding investigations had established that this groundwater system contains waters whose ages range from recent to 1Ma. The present study has established that the Milk River aquifer system is very complex both in terms of groundwater origin and in terms of the evolution of its chemical and isotopic contents. However, this apparent complexity proved to be an interesting challenge with respect to the use of different and complementary approaches to interpretation of the geochemical and isotope data in terms of groundwater residence time. Thus, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater age in the Milk River aquifer (∼0.25Ma) while those based on the36Cl/Cl data uncorrected for any dilution by dead Cl should be considered as an upper limit (<2Ma). The Milk River Aquifer International Project is an excellent illustration of the variety of insight and increased confidence possible in flow-system analysis when multiple, independent dating methods are combined with detailed hydrogeological studies. The conclusion of the present study is that the potential for dating of very old groundwaters by a variety of isotope techniques is very high, provided a combination of methods is applied (never a simple method by itself).