Peña Blanca Study
The New Mexico Bureau of Geology and Mineral Resources serves as the state’s geologic survey, and has been working in Peña Blanca since March 2016 at the request of the NM Environment Department (NMED). The goal was to understand the local hydrogeology of Peña Blanca in order to make a recommendation for an area to place a new well. The need for a new well was highlighted by the discovery of solid waste, a hydraulic fluid tank and a diesel tank immediately adjacent to but on different property than the current municipal well. We met our goal by reviewing the existing literature on the local and regional geology, historical studies of groundwater levels, and regional groundwater chemistry studies. We also collected measurements of groundwater levels and sampled water chemistry in 25 municipal and domestic wells in and around Peña Blanca.
Peña Blanca is a small municipality in the Rio Grande valley surrounded by Cochiti and Santo Domingo Pueblos in north-central New Mexico. The village is built on the eastern margin of the Rio Grande floodplain and river terraces. Peña Blanca stretches west from the valley edge across agricultural fields to the East Drain east of the Rio Grande channel. The fields are flood irrigated using Rio Grande surface water. The village is bounded on the north by the Santa Fe River and to the south by the Galisteo River. The Santa Fe River is impounded by Cochiti Dam to the east of Peña Blanca. Cochiti Dam also impounds the Rio Grande, forming the Cochiti Reservoir north of Peña Blanca. Much of Peña Blanca uses domestic water wells. Both these wells and the larger public water supply wells are generally screened in either recent Rio Grande sands or in older-than-11,500 yrs (Pleistocene-aged), deeper Rio Grande sands and gravels. The older Rio Grande sands and gravels form a continuous aquifer to the east underneath the uplands.
We measured water levels and sampled water chemistry in domestic and public supply wells in and around Peña Blaca. A summary of the hydrogeology of the Peña Blanca follows.
• The wells sampled all had good water quality based on the major ions and trace metals in the water (below recommendations by U.S. Environmental Protection Agency (USEPA) or NMED for drinking water limits). One well had high iron concentrations. We did not test for any biological contaminants.
• The primary aquifer is formed from old deposits of the Rio Grande, mainly sand and gravel.
• This reach of the Rio Grande is gaining water from groundwater, leading to shallow groundwater levels near the river.
• In Peña Blanca, groundwater is moving west-southwest from the uplands in the east to the East Drain and the Rio Grande.
• Water chemistry, isotopes and water levels indicate the groundwater is regional groundwater from the nearby uplands, is from local recharge from Rio Grande-sourced irrigation and the ditches, or a combination of both.
Based on the groundwater data available, we recommend siting a new well in the shaded zone shown in Figure 1 on the southeast edge of Peña Blanca between the existing municipal well and the arroyo flowing east-west along the southern boundary of Peña Blanca. This region is up-gradient of known land surface contamination (arrows show current groundwater flow direction and known contamination sites are labeled). Also, this region is outside of the populated valley, decreasing the potential for groundwater contamination both because of fewer potential sources and having deeper depths-to-water than in the valley.
Funding for this project is by the New Mexico Environment Department-Source Water Protection program under a memorandum of understanding between NMED and the Aquifer Mapping Program of the New Mexico Bureau of Geology and Mineral Resources.
For more information, please contact:
Dr. Alex Rinehart, Hydrogeologist