MULTI-YEAR DISTRIBUTION OF OXYGEN AND HYDROGEN STABLE ISOTOPE DATA FOR METEORIC PRECIPITATION AND GROUNDWATER RECHARGE IN A MOUNTAIN WATERSHED, JEFFERSON COUNTY, COLORADO, USA

An ongoing study using the stable isotopes of oxygen and hydrogen continues to show extreme variability in isotopic composition of meteoric precipitation, with a concomitant attenuated response by the groundwater. The watershed in question is the Turkey Creek Basin (TCB), located about 30 kilometers west of Denver in the foothills and mountains of the Colorado Front Range. Groundwater is present within fractured, heterogeneous Proterozoic crystalline basement rocks. Individual water wells and sewage disposal systems support a population of approximately 12,000 people living within the basin.

We are in the third year of study that examines controls on the stable isotopic composition of meteoric precipitation and its relation to groundwater isotope values. Eleven collection stations located in TCB are complemented by four sites in surrounding locations. We have recorded extreme variability in the isotopic composition of precipitation, with a total range in δD exceeding 290‰. Individual storm events can vary by as much as 70‰ δD within and around the basin. On a basin-wide scale, an annual cycle in precipitation isotope values, likely representing a first-order control by air temperature, is repetitive by season, with summer isotope values strongly higher than winter values. Although limited, temperature data from collection stations correlate grossly with the isotopic composition of precipitation at those sites. Other potential controls on precipitation values include moisture source composition, transport distance, elevation, station air temperature, precipitation intensity (amount effect), slope angle and aspect, among many others.

As previously recognized, groundwater isotope compositions do not directly reflect short-term variability of the precipitation values. Whereas annual precipitation values for δD span well over 200‰, groundwater variability does not generally exceed about 40‰. Isotopic composition of groundwater falls approximately in the mid-range of that recorded for precipitation, implying a time- and volume-averaged source for aquifer recharge.