Carol D. Frost1
(1) University of Wyoming, Laramie, WY
ABSTRACT: Sr Isotopic Characterization of Coal and Sandstone Aquifers, Powder River Basin, Wyoming: Monitor of Aquifer Interactions in an Area of Active Coalbed Methane Development
The 87Sr/86Sr isotopic ratio of groundwater represents a record of water-rock interaction along the flow path. Because aquifers are composed of rocks and minerals with varying 87Sr/86Sr ratios, groundwater from different aquifers may have different 87Sr/86Sr ratios. Thus Sr isotopic ratio can fingerprint an aquifer and identify aquifer interactions.
We have obtained Sr isotopic data on groundwater samples from coal and overlying sandstone aquifers in the eastern Powder River basin, Wyoming. The groundwater samples come from the Eocene Wasatch Fm. sandstone aquifers, and the Anderson-Wyodak coal aquifer and a lower coal aquifer of the Paleocene Fort Union Fm. Recharge to individual aquifers and interactions between the sandstone and coal aquifers are of interest because both surface coal mines and coalbed methane wells are located in this area.
The Wasatch Fm. groundwaters have uniform 87Sr/86Sr of 0.7126-0.7127, along with relatively high Ca, Mg, SO4 and TDS. Sr concentration increases with decreasing dD. The coal aquifer samples can be divided into two groups. A sample from a lower coal zone has the highest 87Sr/86Sr ratio (0.7148) and most negative dD and d18O of all the samples analyzed. The samples from the Wyodak-Anderson coal zone include a large group with 87Sr/86Sr between 0.7128 and 0.7137 and stable isotopic compositions indistinguishable from those of Wasatch Fm groundwaters. Of these, samples with the lowest 87Sr/86Sr may have incorporated some water from the overlying Wasatch Fm.
Our data suggest that the Sr isotopic ratio effectively identify groundwater from different aquifers, and that this tracer has the potential to identify aquifer interactions. The results suggest that Sr isotopic data may help to assess future human impacts on basin hydrogeology.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado