GAS EXCHANGE CHARACTERISTICS OF DICHLORODIFLUOROMETHANE (CFC-12) IN STREAM WATER

Gas exchange between surface-water and the atmosphere is an important aquatic ecosystem parameter. Although dichlorodifluoromethane (CFC-12) does not participate in biological activities, this gas has significance in terms of defining the mean residence time of ground water, which directly relates to the volume of water available to support perennial streams. That is because ground-water age can be defined using CFC-12.

Perennial streams are areas where ground-water flow paths of different ages converge. If CFC exchange is slow, a portion of the dissolved CFC's in ground-water will be preserved in stream water. The flow-weighted mean CFC concentration of the ground-water can be estimated by correcting for gas exchange with the atmosphere, thereby quantifying the mean residence time. This talk addresses the gas exchange characteristics of CFC-12.

Red Butte Creek, Utah is a shallow rapidly moving stream that drains a mountain catchment. Gas exchange coefficients were determined experimentally by injection of CFC-12 into the stream. Using analytical and numerical methods, the gas exchange coefficient is estimated to range from 1.75 x 10-4 to 8.7 x 10-4. To confirm those values, a selected reach of the creek was sampled for environmental concentrations of CFC-12. Resulting exchange rates were faster than those determined experimentally.

Six-Mile Creek originates from a large regional spring located in the center of Tooele Valley, Utah. Six-Mile Creek is a deeper slow moving stream. Environmental sampling of CFC-12 from Six-Mile Creek indicates gas-exchange characteristics with the atmosphere that are similar to those determined experimentally for Red Butte Creek. Gas exchange parameters will be determined experimentally for Six-Mile Creek by injection of CFC-12. Experimental and environmental results obtained for both creeks will be compared.