STABLE OXYGEN AND HYDROGEN ISOTOPES OF PRECIPITATION AND GROUNDWATER IN A MOUNTAIN WATERSHED, JEFFERSON COUNTY, COLORADO

Turkey Creek Basin (TCB) is a mountain watershed, 30 kilometers from Denver, which covers approximately 122,000 square kilometers and ranges from 1800 to 3350 meters elevation. The aquifer is present within fractured, heterogeneous Proterozoic continental crustal basement rocks. Individual water wells and sewage disposal systems support a population of approximately 12,000 people living within the basin. The Colorado Front Range experiences a semi-arid climate, with a mean annual precipitation of about 50 cm.

The present study focuses on the stable oxygen and hydrogen isotopes of atmospheric precipitation and groundwater wells collected from stations within and nearby to the watershed. Samples from the TCB collected for all precipitation events and analyzed from August 2004 to May 2005 fall along a defined meteoric water line with slope of 8.2 and a deuterium excess of 1.2. Both slope and deuterium excess values vary by storm and by location in the basin (slope from 7.1 to 8.5, and deuterium excess from -14.8 to 6.6); deuterium excess does not correlate with elevation. Overall, storms occurring within this ten-month period show a range in δ¹⁸O from -2 to -24‰ VSMOW and in δD from -24 to -217‰ VSMOW. Clearly, elevation plays an important role in isotopic variability of precipitation during any storm event. However, elevation alone cannot explain the storm-to-storm or same-site variability. Extreme isotopic variability in precipitation in the TCB relates in part to multiple moisture sources delivered to the Colorado Front Range, including moisture delivered by the Polar Jet Stream, the Subtropical Jet Stream, the Gulf of Mexico, and local convective redistribution.

Groundwater isotope compositions during the same period are strongly attenuated relative to precipitation (δ¹⁸O from -15 to -9‰ VSMOW; δD from -120 to -80‰ VSMOW). The data suggest that recharge represents a volume average of waters sourced throughout the annual cycle.