Four hydrostratigraphic units or aquifers are present on the Picuris Pueblo. Quaternary alluvial deposits (Qa) form thin, shallow aquifers beneath the active floodplains of major streams and are at greatest risk for degradation from land-use and waste management activities. The Dixon Member of the Tesuque Formation (Ttd) forms the primary aquifer along the southern edge of the Pueblo, near Chamisal and south and east of Peñasco. The upper volcaniclastic member of the Picuris Formation (Tpu) forms a major aquifer in and between the Rio Pueblo and Rio Santa Barbara valleys, but productivity is variable and greater well depths may be required. The middle tuffaceous member of the Picuris Formation (Tpm) forms a relatively minor aquifer in the Rio Pueblo Valley, and contains fine-grained, volcanic-rich sediments with low permeability. The Peñasco horst, an up-thrown block of Precambrian crystalline rock within the Picuris-Pecos fault system, contains heavily mineralized and uranium-bearing rocks and influences both groundwater and surface water flow and groundwater quality.
Groundwater in thin alluvial aquifers exists under unconfined conditions and is in direct hydraulic connection with deeper aquifers. Vertically downward hydraulic gradients drive circulation of oxygen-rich shallow groundwater down to deeper aquifers over much of the Pueblo. Vertically upward hydraulic gradients adjacent to the Peñasco horst provide a mechanism for localized upward movement of deep circulating groundwater that degrades water quality in shallow aquifers adjacent to and downstream of the horst. Regionally high concentrations of dissolved solids, chloride, and silica are observed adjacent to the horst.
Perennial streams on the Pueblo are generally gaining streams, collecting a portion of their flow from the shallow alluvial aquifers. However, flows in the Rio Santa Barbara and Chamizal Creek appear to change from gaining to losing as they cross the downstream edge of the Peñasco horst, where thickness and transmissivity of the aquifer increase.
Infiltration of oxygen-rich surface water near Chamisal may contribute to chemical conditions favorable for mobilizing naturally occurring uranium to concentrations reaching the healthbased drinking water standard (30 μg/L). Elevated concentrations of naturally occurring arsenic and fluoride are attributed to groundwater originating deep within mineralized crystalline rocks of the Peñasco horst or circulating through volcanic-rich sediments in the Picuris Formation. Both arsenic and fluoride exceed maximum contaminant levels for drinking water, and together with uranium present a significant public health concern.
Shallow aquifer contamination from waste related contaminants does not presently pose a significant health concern. Excess iron and manganese in one well (PW-65) accompanied by an extremely low nitrate concentration indicate conditions that may be associated with septic tank effluent or merely reflects natural, local chemical conditions in the aquifer. Chemical testing for additional constituents would be required to further clarify the source. Based on observations of naturally occurring contaminants in excess of EPA drinking water standards, several recommendations are presented:
- Additional Testing. The sporadic occurrence of elevated levels of uranium, fluoride, and arsenic pose a significant health concern. Sampling of additional wells is recommended to further define groundwater quality and identify problems within the Pueblo boundary.
- Long-Term Monitoring. A long-term monitoring program is recommended to track water-quality trends and contaminant migration in response to pumping of large community supply wells.
- Public Education. Rural residents, particularly in the communities of Chamisal and Vadito, should be advised of potential health concerns and methods of addressing water quality problems.
- Water Treatment. In instances of significant drinking water impairment, installation of on-site water treatment units or alternative water sources should be considered.
Results
- Johnson, Peggy, Bauer, Paul, and Timmons, Stacy, 2018, Hydrogeology and Water Resource Assessment of the Pueblo of Picuris, Taos County, New Mexico, New Mexico Bureau of Geology & Mineral Resources, Open-File Report 596, 27 p. 6 plates.