SOIL WATER DYNAMICS IN THE SACRAMENTO MOUNTAINS OF NEW MEXICO AND IMPLICATIONS TO GROUNDWATER RECHARGE

The Sacramento Mountains serve as the primary recharge area to adjacent regional aquifers, including the Roswell Artesian Basin and the Salt Basin. Under pressures of population growth and climate change, land and water managers are interested in identifying land management and forest restoration methods that may increase local and regional groundwater recharge in the high mountains. Because most precipitation enters the subsurface through soils, understanding soil water dynamics can give valuable insight to recharge mechanisms in the Sacramento Mountains. For this study, the stable isotopes of oxygen and hydrogen were used to trace precipitation as it moves through the soil profile.

The study area is in the southern Sacramento Mountains where the San Andres limestone overlies the Yeso Formation, which is mainly composed of limestone and siltstones and is the main aquifer in the region. A previous study used stable isotopes to show that regional groundwater recharge occurs as infiltration from losing mountain streams and infiltration of local precipitation through fractured bedrock. The area has thin soil profiles covering shallow epikarst features. Some of the fractures within the epikarst zone provide direct conduits to the larger groundwater system while others are isolated. Once in the epikarst zone, water can percolate through fractured bedrock and potentially recharge aquifers.

Bulk soil water was collected by cryogenic distillation of soil samples and mobile soil water was sampled with passive capillary samplers. We have found that the isotopic composition of bulk soil water is controlled by evaporation of snow melt within the soil matrix, while the composition of mobile soil water reflects mixing of non-evaporated rainfall with evaporated bulk soil water. As the 2011 monsoon season progressed, the isotopic composition of mobile soil water evolved towards that of local rainfall. Short duration, high intensity monsoon rains exceed infiltration capacity, exploit preferential flow paths and quickly flush through profiles to reach the epikarst zone. Understanding how the isotopic composition of epikarst water evolves with time is important for understanding it as a potential contribution to groundwater recharge.