EBTAG Annual Workshop and Field Trip
May 20-21, 2013

¹New Mexico Bureau of Geology & Mineral Resources, 801 Leroy Place, Socorro, NM, 87801, USA, peggy@nmbg.nmt.edu

Groundwater-fed wetlands that form large cienega complexes in semi-arid landscapes 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, lower 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 and inflow 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 the 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 of wetlands east and west of Cienega Creek verify that groundwater sources for the cienegas include mixtures of deep regional flow through the Tesuque Formation and local flow along unique pathways in the shallow Ancha aquifer.

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. These changes correspond to transitions between growing and dormant seasons and illustrate 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 for the 8-year period in aquifers east (up-gradient) of the wetlands. Forty-year hydrographs from area wells also show a persistent trend of declining levels, with rates of change from -0.11 to -0.25 ft/yr. Wells with high-frequency measurements 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. Our results strongly suggest that long-term reduction in aquifer storage in the Ancha Formation poses a significant threat to the existence and viability of wetlands.