New Mexico Geological Society Annual Spring Meeting — Abstracts

Groundwater-fed wetlands in semi-arid landscapes that form large cienega complexes are among the most diminished and threatened ecosystems in the Southwest. Groundwater-fed wetlands on saturated slopes and adjacent to streams cover 384 acres in the La Cienega area and are threatened by dwindling, disrupted or erratic water flows. A collaborative, multi-agency (NMED, NMOSE, NM Bureau of Geology, Santa Fe County, U.S. Fish & Wildlife Service) study of the wetlands completed in December 2012 reveals that a complex interaction between geology, groundwater, and surface flows controls the location and condition of the wetlands.

Groundwater discharging from the Ancha Formation of the Santa Fe Group aquifer feeds the cienegas. As the aquifer thins and pinches out over underlying, low permeability units, groundwater is forced to the surface to discharge in spring-wetland zones and adjacent streams. Storage of groundwater in the Ancha is controlled by three factors: (1) permeability contrasts between the Ancha and sub-Ancha formations; (2) topography of the erosion surface at the base of the Ancha; and (3) recharge to the Ancha.

Paleo-valleys eroded into the surface of the Tesuque Formation and filled with coarse Ancha deposits create altitude-dependent drains that gather groundwater, concentrate flow, and direct discharge to spring-wetland zones. The El Dorado paleo-valley controls locations for large spring-wetland zones on eastern slopes and tributaries of Cienega Creek (Guicu Creek, Canorita de las Bacas, Leonora Curtin Preserve). Sunrise Springs and wetlands along the western slopes of Arroyo Hondo are controlled by the ancestral Santa Fe River paleo-valley. Unique chemistry, isotope, and age (14C and 3H) characteristics between wetland zones east and west of Cienega Creek verify that multiple groundwater sources feed the cienegas and include mixtures, in various proportions, between groundwater from deep regional flow paths through the Tesuque and groundwater from local to intermediate flow paths within the Ancha.

Seasonal water levels in 38 shallow wells show an increase in winter levels of +0.77 ft (average) in wetland zones. A hydrograph and thermograph of continuous measurements from October 2011 to October 2012 in a wetland well show inverse, synchronized changes in water level and temperature in November and April corresponding to transitions between growing and dormant seasons, illustrating the hydrologic response of shallow groundwater to seasonal changes in evapotranspiration. Water levels measured in 22 wells between 2004 and 2012 show water-level declines of  5.03 to -0.10 ft east of the La Cienega wetlands. Thirty-year hydrographs from area wells show a persistent trend of declining levels and, where the measurement frequency is sufficient, also show the cumulative effects of seasonal water-level variations (winter highs, summer lows) and recharge events (spring 2005) superimposed on the effects of pumping and long-term withdrawals. Results suggest that long-term reduction in aquifer storage in the Ancha poses a significant threat to wetland condition.