Trichlorofluoromethane as a ground-water tracer for finite-state models

by Thomas Robert. Schultz

Institution: University of Arizona
Year: 1979
Keywords: Hydrology.; Hydrologic models.; Tracers (Chemistry)
Record ID: 1544723
Full text PDF: http://hdl.handle.net/10150/191056


The use of trichlorofluoromethane (C1₃CF) as a refrigerant, aerosol-can propellant, and foam-blowing agent leads to the subsequent release of C1₃CF into the atmosphere. During the mid-1970's, the build-up of C1₃CF in the stratosphere resulted in much atmospheric research investigating the destruction of the earth's ozone layer by C1₃CF. C1₃CF enters the hydrologic cycle during precipitation. Hydrologic investigations led to the detection of C1₃CF in ground water and postulation of C1₃CF as a tracer and age dating technique. The Edwards Aquifer in the vicinity of San Antonio, Texas, was chosen as a test case for demonstration of C1₃CF as an environmental tracer. The aquifer has been modeled for mass transport using extensive tritium measurements to calibrate and verify a discrete-state model (DSM). The DSM was the first successful attempt at modeling both flow and mass transport in that aquifer. Without changing the calibration, the DSM was used to model the input-output and predict the concentration of C1₃CF in the aquifer. Field sampling of surface water, well discharge, springs, and the atmosphere was done in 1977 in order to compare actual with predicted results. C1₃CF was measured using a custom field operable gas chromatograph (FOGC). The FOGC has a solute-stripping bottle, a backflushing column, a pulsed electron capture detector, and an automatic peak-window integrator. The theoretical detection limit is 10⁻⁴ grams per second. The FOGC was calibrated for a dynamic range of 0.25 to 300 picograms, using a permeation tube and dynamic gas sampler. The FOGC can measure C1₃CF in ground waterat concentrations of 0.01 picograms per milliliter (pg/ml) within 2 percent. Analysis and interpretation of the ground-water data indicated two distinct associations  – regional data points and plume data points. The regional data points fell into two separate groups, those above and those below the lower calibration limit. The data points were high in the recharge areas, intermediate in the center of the aquifer, lower at the discharge points (springs), and below the calibration limit in areas of low circulation in the aquifer. Comparison of the regional data with other investigations indicates that the C1₃CF technique works. The data points in the plume area indicate artificial introduction of C1₃CF into the aquifer near San Antonio. The distribution of the concentrations follows the flow paths toward the springs and confirms the movement of ground water as determined from previous investigations utilizing other techniques. The artificial introduction of C1₃CF may have been accidental or intentional. It appears that about 15 liters C1₃CF would be required to produce the concentrations detected. Air sample concentrations of C1₃CF (0.42 pg/ml) were about half those reported for recent global measurements. Surface water concentrations of C1₃CF (0.12 pg/ml) were very close to recent global measurements (0.13 pg/ml), but below the input to the model (0.34 pg/ml). The data indicate that the C1₃CF technique is workable in the Edwards…