Characteristics of Oxygen-18 and Deuterium Composition in Waters from the Pecos River in American Southwest

Abstract The Pecos River, situated in eastern New Mexico and western Texas, receives snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The isotopic composition of the two water sources differs from each other due to their different geographical origins in the Pacific North and the Gulf of Mexico. To better assess the physical features of the Pecos River, oxygen and hydrogen isotopic compositions (δ 18 O and δD), major ion concentrations, and other physical variables (e.g., water temperature and electrical conductivity) were measured on water samples collected from the main stem and its selected tributaries during a growing season (March, May, and July) in 2005. The results of this work indicate that stream water from the Pecos River contains a relatively large magnitude of variations in δ 18 O and δD, with δ 18 O ranging from − 8.9‰ to 3.6‰ and δD from − 64.5‰ to 1.6‰. The average value of δ 18 O is around − 3‰, which is significantly larger than that of the snowmelt but almost identical to that of the Mexican monsoonal rainfall. On the other hand, the average value of δD is around − 30‰, which is significantly larger than that of the snowmelt but lower than that of the Mexican monsoonal rainfall. Application of a dual isotope index, deuterium excess (d-excess), allows us to assess the relative contribution of various hydrologic components and processes that shape the stream hydrology of the Pecos River. The river water from the upper valley is characterized by relatively low values of δ 18 O and δD and relatively high values of d-excess ( d = 10‰), documenting the isotopic fingerprint of the snowmelt. The middle basin is topographically gentle and its water quality has been severely affected by anthropogenic disturbances (e.g., water impoundments and diversions). As a result, chemical and isotopic composition of the water from the middle basin is highly variable, depending on its time, location, and degree of disturbances. Both δ 18 O and δD increase significantly from upstream to downstream and from cold to warm seasons because of evaporation-induced isotopic enrichments. The average δ 18 O value of the heavy-isotope-enriched waters from the middle basin is identical to that of the waters from the lower valley. In contrast, d-excess of the waters from the middle basin usually is negative, and substantially lower than that of the waters from the lower valley. Using a simple d-excess based hydrologic model, we estimated that there was up to 85% of stream flow which was derived from local freshwater sources (mainly from the Mexican monsoonal rainfall) in the lower valley and that there was up to 33% of stream water which was lost through evaporation occurring in stream channels and fields of the middle basin. Additionally, the correlation of d-excess and electrical conductivity further highlights the role of evaporative enrichments in regulating stream chemistry and isotope hydrology. This study demonstrates the usefulness of combined isotopic and geochemical data, especially the applicability of d-excess, for watershed baseline assessments.