Application of Environmental Tritium in the Measurement of Recharge and Aquifer Parameters in a Semi-Arid Limestone Terrain

This project is part of a continuing inquiry into regional patterns of recharge and groundwater flow in a large and complex artesian basin plagued by problems of overdraft, declining water quality, and saltwater intrusion.

Starting from the Hydrologic Model established in a previous investigation, the present phase of the work has concentrated on determinations of environmental tritium in the basin’s Recharge Belt. Sampling density, frequency, and areal coverage in the Discharge Belt were also increased. Special attention was given to the southern part of the study area (Artesia) where tritium peaks predicted by the Model apparently had failed to materialize.

For the sampled wells, all available information on depth and geologic character of water-bearing horizons, as well as construction and history of each well have been collected and summarized. The areal precipitation over the basin has been recomputed for the years 1955-1974, and statistics on surface runoff have been assembled. All of the relevant data are included as appendices to the present report.

Tritium concentrations in groundwater from the Recharge Belt are, on the whole, lower than might be expected from the Model. Also, in considerable parts of the Recharge Belt the groundwater is confined. It is tentatively concluded that a slow recharge component is more important than had been assumed by the Model. Rapid recharge seems to occur along present drainage systems, especially that of Rio Hondo. This requires a re-examination of the sources of recharge, the loci where recharge is preferentially fed into the groundwater system, and the details of the recharge process. In addition to tritium determinations, other methods of physical measurements and of mathematical analysis will have to be used, such as: Oxygen-18/Oxygen-16 isotope ratio measurements; analysis of observation well water level records, and their correlation with precipitation and runoff; recharge computations. Some of these studies are presently in progress.

The distribution of environmental tritium within the basin itself, both in time and space, suggests that interaquifer leakage is an important factor in these patterns, not considered in the previous study.

Project No. B-041-NMEX