New Mexico Geological Society Annual Spring Meeting — Abstracts

Naturally occurring isotopes that exist in water in the hydrologic cycle have been used in investigations of ground water for decades. The application of environmental isotopes in hydrologic studies (Isotope Hydrology) makes use of two main properties of isotopes: 1) some decay radioactively; and 2) the relative mass difference of common to rare isotopes results in fractionation during physical, chemical, and biochemical reactions. These properties permit ground-water age estimation, "fingerprinting" of ground water from different sources, and tracing of ground water in the subsurface. By determining the distribution of environmental isotopes in ground water, one can establish ground-water residence and travel times, ground-water flow paths, leakage between aquifers, definition of recharge areas, stream-aquifer interactions, and aquifer hydraulic conductivity.

The isotopes most commonly measured in ground water are the radiogenic isotopes tritium (³H or T) and carbon-14 (¹⁴C), and the stable isotopes deuteriwn (²H or D), oxygen-18 (¹⁸O), and carbon-13 (¹³C). Tritium and ¹⁴C are used to estimate the "age" or residence time of ground water and to solve related problems of ground-water travel time, flow path definition, and rates of recharge. The apparent age (t_a) of ground water is: t_a = t_1/2 In(A/A_o), where t_1/2 is the half-life, A is the activity ofthe isotope at the time the water entered the subsurface, and Ao is the measured activity. If age and distance traveled are known, ground-water velocity can be determined. Added knowledge of ground-water gradient and estimates of effective porosity can then provide information on aquifer hydraulic conductivity. The stable isotopes ¹⁸O and D are used as indicators of ground-water source areas, and because these isotopes reflect the environmental conditions at the time they entered the subsurface, they can also distinguish modem ground water (derived directly from modem precipitation) from paleowater (recharged under conditions different from the present-day).

³H and ¹⁴C are produced naturally by interaction of cosmic rays with the upper atmosphere, and have also been artificially injected into the hydrosphere in large quantities by above-ground thermonuclear testing from 1952-1963. Because ³H becomes a constituent of the water molecule, and has a half life of 12.3 years, it is an ideal tracer for ground water recharged since 1954. However, its use is limited because atmospheric concentrations vary over space and time, and it is difficult to reconstruct ³H input concentrations for specific geographic areas. An analytical advance of the ³H method uses combined measurements of ³H and helium-3 (³He), the stable tritium decay product, to more precisely determine the "age" of the ground water.

¹⁴C is introduced into ground water through the dissolution of atmospheric CO₂ in rain and surface water, and of gaseous soil CO₂ in water percolating through soil. The ¹⁴CO₂ equilibrates with the dissolved inorganic carbonate species in ground water. ¹⁴C "ages" are calculated based on the 5730-yr half-life, and the ratio of ¹⁴C/¹²C in a barium or strontium carbonate precipitate derived from a representative groundwater sample. ¹⁴C activities are expressed in "pmc", or percent of modern carbon, where 100 pmc is the approximate mean pre-bomb atmospheric ¹⁴C activity, and about 130 pmc is the average post-bomb activity. Several natural processes occur in the subsurface that affect the ratio of ¹⁴C/¹²C, the most important of which is the dissolution of carbonates devoid of ¹⁴C (referred to as "dead" carbon). This process dilutes or reduces the natural ratio of ¹⁴C/¹²C and increases the apparent age of the ground water. After accounting for dilution by dead carbon using correction methods based on concenrations of ¹³C. modem pre-bomb ¹⁴C activities in ground water are typically 60 to 80 pmc. ¹⁴C activities greater than 80 pmc are most likely due to input of "bomb" ¹⁴C and represent water less than about 40 years old. Because ofthe complexities ofground-water flow paths, and the potential for changes in the total mass of dissolved carbon in the subsurface, ¹⁴C often provides only a semi-quantitative estimate of ground-water age.