New Mexico Geological Society Annual Spring Meeting — Abstracts

In May of 1989 drill core samples were collected from two sites for a stable isotopic study of meteoric recharge through the unsaturated zone. The first site (LTER1) was west of the Rio Grande river north of San Acacia on some relative highlands. The second site (LTER2) was located east of the Rio Grande river south of Turitutu (Black Butte) in relative lowlands. Both sites are located within the Sevil leta Natural Wildlife Refuge (SNWR). Both cores had soils ranging from almost pure sand with a very low water content content (less than 1%) of almost 75% clays with a water content of about 20%. The isotopic profiles for shallow rapid recharge areas in the SNWR have been relatively well defined (Knowlton,1990). Our study focused on deeper cores which hopefully contain older groundwater

A typical vadose zone isotopic profile shows an enrichment in the delta-D and delta-180 values at the evaporation front, below which the profile approaches a steady state value. Both of our data sets show the expected behavior at the top of the profile. At the surface the composition of the water approaches that of meteoric water. At about 50-100 cm the heavy isotope enrichment peak can be seen and then the values swing back to lighter values (about -5 permil). At a greater depth LTER1 also shows a shift in the steady state value, whereas LTER2 does not. The shift in LTER1 occurs at about 400-500 cm. The shift is about -3 permil in oxygen and about -10 permil in hydrogen. Based on chloride mass balance, LTER1 contains older water at depth than LTER2 does. Presumably the observed shift is related to different climatic conditions in the past.