A Baseline Study of Oxygen 18 and Deuterium in the Roswell, N.M., Groundwater Basin

The isotopic ratios of deuterium and oxygen 18 were measured in precipitation, surface, and groundwater samples from the Roswell Artesian groundwater basin in south-central New Mexico. The purpose was to determine recharge and circulation patterns. The results would supplement earlier studies based on tritium ratios and hydrogeologic surveys. The study area comprises about 8000 square miles with elevations ranging from 3,400 ft a.m.s.l. to almost 12,000 ft. Except for minor deviations due to evaporation, the samples measured for this study conform to the meteoric water line. No exchange reactions with aquifer rock seem to have taken place. While the ëD vs. ë18O values of precipitation are spread over the whole usual range of the meteoric water line, well and spring samples (reflecting groundwater) lie within a relatively narrow range ëD = – 30ø/oo to -80ø/oo and ë18O = – 60ø/oo to -10ø/oo, all with respect to Standard Mean Ocean Water). Exceptions outside of this range are minor and can be attributed to evaporation effects. Well waters of the Principal Artesian Aquifer average ëD = -51ø/oo, ë18O = – 8ø/oo. The narrow range of ëD and ë18O indicates mixing effects which are ascribed to one or more of the following factors: long groundwater flow paths; large temperature fluctuations affecting all parts of the Basin and which overwhelm the influence of elevation on precipitation, especially in summer; two sources of atmospheric moisture (Gulf of Mexico and Gulf of California); interaquifer leakage; and recharge from intermittent streams with the flowlength expanding and contracting over large distances (tens of miles) depending on climatic conditions. Groundwaters in the southern part of the Basin seem to be, on the average, heavier in oxygen 18 by one per mil as compared to those from the northern part. This may reflect a larger contribution of high-mountain runoff in the north. It is concluded that a more precise definition of circulation patterns on the basis of stable isotope differences will require a much greater sampling frequency in both space and time, as well as a more sophisticated statistical evaluation of the results than was possible in this preliminary baseline study of a very large basin.

Project Numbers: B-059-NMEX, 1423643, 1345638.