Groundwater flow model investigation of the vulnerability of water resources at Chaco Culture National Historical Park related to oil and natural gas drilling

Situated in a remote corner of northwestern New Mexico, Chaco Culture National Historical Park (CHCU) was once the center of a sophisticated social, political, and architectural civilization. With a 50,000-square mile sphere of influence, the CHCU protects the greatest concentration of Chacoan historical sites in the American Southwest. The parks 60,000 annual visitors, the park staff, and neighboring Navajo families all rely on a single vital resourcegroundwater from the Gallup sandstone aquifer. The main CHCU well was drilled 3,100 feet deep to gain access to the 100-foot thick Gallup sandstone, which is sandwiched between thick confining sequences of lower-permeability Manocs shale. Both the Mancos shale and the Gallup sandstone are part of the San Juan Basin and, critically, both units have long been targeted for oil and natural gas development. As part of a larger study to investigate the vulnerability of water resources at CHCU related to oil and natural gas drilling, a MODFLOW groundwater model of the Mancos shale and Gallup sandstone units was created and calibrated to a recent (2019) potentiometric surface in the Gallup sandstone in order to determine groundwater flow paths through these units and to delineate the capture zone to the main supply well for CHCU. After successful model calibration and delineation of the baseline capture zone, model scenarios were run to see how the capture zone changed following potential drilling development, simulated primarily as changes in hydraulic conductivity in the Mancos shale at various horizontal and vertical distances from the main CHCU supply well in the Gallup sandstone. In addition, advective groundwater travel times through the model domain were assessed. These spatial and temporal analyses of groundwater flow paths through the model extent help provide an understanding of how oil and natural gas drilling operations may affect the water resources at CHCU.