Rio Arriba County Groundwater Study Report Presents Important Information about Community Groundwater Resources

​Results of a hydrogeologic study focusing on groundwater resources for several communities in Rio Arriba County are now available online in a report titled “Evaluation of Groundwater Resources for Communities in Rio Arriba County, New Mexico.” The primary goal of this study was to characterize the local and regional aquifers that currently supply communities for domestic and municipal use. This study was a response to concerns by several communities in Rio Arriba County about their future water security. Groundwater is the principal water source for municipal and domestic use in households and businesses in the region, and many small communities in Rio Arriba County have experienced problems with their groundwater supply in terms of quantity and quality. This study, funded by a state legislative appropriation sponsored by Representative Susan Herrera, focused on the communities of Chama, Abiquiu, El Rito, Medanales, and Dixon. In the fall of 2023, researchers from the NM Bureau of Geology and Mineral Resources (NMBGMR), a division of New Mexico Tech, visited residents in these communities to measure the depth-to-water in their wells and to collect water samples that were analyzed for water quality and other parameters. The much-anticipated report describes the local and regional hydrogeology for these communities based on previous studies and newly collected data. This study also identified important data gaps that NMBGMR researchers are working to fill as this study continues over the next year and a half.
 
A section of the report focuses on the Abiquiu Valley, which includes the communities of Abiquiu, El Rito, and Medanales. Most wells in this region are completed in the shallow alluvial aquifer that is located in the valley bottom adjacent to the Rio Chama and El Rito Creek. This aquifer is composed of gravel, sand, and clay sediments deposited by the Rio Chama and El Rito Creek. The lateral extent of these deposits limits the width of the aquifer. The widest portion of the shallow aquifer is in the vicinity of El Rito (~2.3 miles), and the narrowest portion is located just west of Abiquiu, where it is only 0.25 miles wide. The total thickness of this shallow alluvial aquifer ranges from a few feet to about 150 feet, which is quite thin when compared to alluvial aquifers in other areas of the state, which can be thousands of feet thick. In other words, the shallow alluvial aquifer is relatively small, limiting the amount of water that can be stored.
 
The shallow alluvial aquifer is hydrologically connected to the Rio Chama and El Rito Creek, meaning that water moves from the shallow alluvial aquifer to the river or vice versa, depending on the relative elevations of the water levels in the river and aquifer. For Abiquiu and Medanales, which are downstream of Abiquiu Reservoir, flow rates and the water level of the Rio Chama are largely controlled by releases from Abiquiu Reservoir. Water level and water chemistry data show that Rio Chama loses water to the aquifer during high flows and gains water back from the aquifer during low flows. Seepage from irrigated fields and acequias also affect groundwater levels.
 
Groundwater produced from the shallow alluvial aquifer in the Abiquiu Valley is mostly of good quality, with total dissolved solids concentrations of less than 500 mg/L. However, some of these same wells tested positive for total coliform bacteria, likely due to septic tank contamination. Water chemistry and age dating data indicate that the shallow alluvial aquifer is recharged mainly by snowmelt and rain in nearby mountains, with ages ranging from tens to a few hundred years old. Therefore, water availability in this shallow alluvial system (alluvial aquifer and river) depends on climate conditions in the mountains, where most groundwater recharge occurs. As the climate changes in the future with predicted increased temperatures and reduced recharge to aquifers, groundwater supplies in the shallow alluvial aquifer will likely be negatively impacted.

Map showing estimated saturated thickness for the shallow aquifer. The saturated thickness was not evaluated in the area between El Rito and the Rio Chama because very few wells are located in this area.

While the shallow alluvial aquifer produces good-quality water for municipal and domestic uses, the aquifer is relatively thin and has limited storage. Many wells in these communities are completed in older rocks that lie beneath the alluvial sediments that make up the shallow aquifer. Groundwater from the deep aquifer system is characterized by more variable water quality that comes with longer, deeper flow paths. These deep regional aquifer systems may be more stable in the long term, with less variability in water quantity. Future work will focus on a more detailed characterization of both shallow and deep aquifer systems using existing wells and data, along with new approaches. Geophysical surveys and information from well logs will be used to identify the aquifer bottom (lower boundary of the shallow alluvial aquifer) and the distribution of gravel, sand, and clay. Additional work will also include further characterization of the deep aquifer systems to assess their potential for supplemental water supply.
 
For more information, please feel free to contact Talon Newton ([email protected], 575-835-6668) or Laila Sturgis ([email protected], 575-835-5327). The report is available online.