Chaco Culture National Historical Park (CCNHP) is in the San Juan Basin of northwestern New Mexico, U.S.A. Its only water supply is in Gallup Sandstone aquifer, stratigraphically surrounded by layers long targeted for oil and natural gas extraction.
To assess groundwater flow direction, age, mixing between aquifers, and whether hydrocarbons extraction may affect water quality, we completed a geochemical groundwater sampling campaign. Groundwater at 11 sites was analyzed for major ions, hydrocarbon associated volatile organic carbon (VOC) compounds, noble gases, and the isotope systems δ2H, δ18O, 87Sr/86Sr, δ13C, and 14C.
New hydrological insights for the region
Results demonstrate that all sampled groundwaters are exceedingly old and geochemically evolved, with a median 14C age of ∼41,000 years before present and a north flowing path. Three lines of evidence suggest mixing between aquifers through relatively impermeable shale units and mixing with hydrocarbons: 1) noble gases are fractionated likely through mixing with connate water expelled during hydrocarbon genesis; 2) several wells—including the park’s main supply well—contained trace amounts of hydrocarbon related VOC compounds; and 3) major ion analysis shows mixing trends between aquifers. We hypothesize that cross-aquifer mixing may be facilitated through the region’s numerous hydrocarbon related boreholes. Whether our findings are the result of oil and gas extraction or represent the natural state of the aquifers will require more research.
|Title||Water quality at Chaco Culture National Historical Park and the potential effects of hydrocarbon extraction|
|Authors||Benjamin S. Linhoff, Kimberly R. Beisner, Andrew Hunt, Zachary M. Shephard|
|Publication Subtype||Journal Article|
|Series Title||Journal of Hydrology: Regional Studies|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Geology and Geophysics Science Center; New Mexico Water Science Center|