New Mexico Geological Society Annual Spring Meeting — Abstracts

The springs of La Cienega provide an important source of water for the people and wetlands in the southern portion of Santa Fe County. The groundwater that discharges at La Cienega comes from aquifers within the Santa Fe Group. The Santa Fe Group is a thick alluvial deposit, comprised of deeper 1,000 feet thick Tesuque Formation overlain by up to 300 feet of courser Ancha Formation. The Tesuque Formation is a late Oligocene to late Miocene unit composed of sand, with subordinate gravel, silt, and clay. The Ancha Formation is a late Pliocene to early Pleistocene unit composed of arkosic sand, silty-clayey sand, and gravel found in paleochannels that unconformably overlies the tilted Tesuque Formation basin fill.

A study by Johnson et al. (2016) looked at water chemistry, among other things, from several wells and springs in the La Cienega area, and noted the principle water bearing strata. Chemical analysis showed two major groups: Ca-HCO3 dominate waters in shallow wells near streams that correlated to the Ancha Formation, and Na-HCO3 or Na-Ca-HCO3 waters in deeper Tesuque wells. Shallow wells and springs not in close proximity to streams generally had a mixed chemical signature.

In 2017, as part of a citizen science and water quality awareness effort with the New Mexico Environment Department, 25 private well owners collected grab samples of their well water. These samples were analyzed by the New Mexico Bureau of Geology and Mineral Resources. It was possible to infer the water bearing formations of these 25 wells by comparing chemical signatures to those reported by Johnson et al. (2016) that had known source formations. A few wells clearly had a source of either Ancha or Tesuque, but most had a chemical signature indicative of mixing of the two sources, which is typical of this region. After comparing chemistry to infer source units, well logs for a few of the sites were found through the New Mexico Office of the State Engineer, and water bearing strata was confirmed.

Additionally, these data show that maximum contaminant levels from the U.S. EPA quality standards for arsenic were exceeded in four of the 25 wells sampled. These data were only available because the citizens of La Cienega had questions about the source and quality of their water. This shows that in regions where previous hydrogeochemistry data have been collected, such as La Cienega, citizen water sampling campaigns can provide useful scientific contributions. With help from the local community, it is possible compare current results with previous data to show potential changes to water quality.

References:

1. Johnson P.S.; Koning, D.J.; Timmons, S.S.; Felix, B., 2016, Geology and hydrology of groundwater-fed springs and wetlands at La Cienega, Santa Fe County, New Mexico, New Mexico Bureau of Geology Mineral Resources, Bulletin, v. 161, pp. 1-92.