CHARACTERISTICS OF THE CONEJOS AQUIFER, THE MAJOR BEDROCK AQUIFER IN THE SAN JUAN SAG, EASTERN SAN JUAN MOUNTAINS, RIO GRANDE COUNTY, COLORADO – PART 2: IMPLICATIONS FOR GROUND WATER PROTECTION AND ENERGY DEVELOPMENT

In 2012 the authors conducted a reconnaissance hydrogeologic study of the Oligocene Conejos Formation aquifer in the eastern San Juan Mountains, Colorado. The study was done due to new interest in petroleum resources underlying the Conejos, and concerns of local residents about the potential impact of energy development on the Conejos aquifer. Two new oil test wells were planned: one on Old Woman Creek (drilled in early 2013), and a second well located on San Francisco Creek. Both locations are within 5 miles of Del Norte, CO. Both are in an area of low-density mountain residential development. The study was done on behalf of Rio Grande County, Colorado, to aid the County Commissioners in assessing the risk to ground water resources. Funding was provided by a grant from the Colorado Water Conservation Board’s State Water Supply Reserve Account through the Rio Grande Basin Roundtable. The geology of the Conejos and the San Juan Sag, and baseline water quality, will be covered in the preceding talk by Kirkham and others.

The majority of the estimated 500+ water wells that draw water from the Conejos Formation aquifer in the mountainous part of Rio Grande County are for domestic or livestock use. This talk focuses on analysis of hydrogeologic characteristics and water quality from water well records, DST’s, and petrophysical analysis of logs of petroleum test wells.

This study revealed substantial heterogeneity and anisotropy in the Conejos aquifer. Available data provide evidence of deep circulation of ground water to several thousand feet depth within the Conejos aquifer and possibly in older underlying formations. Oil and gas naturally occur at shallow depths locally within the aquifer. This study found no formations of low hydraulic conductivity that hydrologically separate the underlying reservoir and source rocks from the shallower fresh-water aquifer.

The study recommendations to Rio Grande County were intended to protect potable ground water, propose appropriate safeguards for energy development to go forward, inform and involve the community, and guide further hydrogeologic studies. The study and recommendations provide an example of an approach to protect potable ground water and allow energy development to proceed in other communities where energy development is perceived as a threat to ground water resources.