U.S. Geological Survey

By Stephen R. Hinkle and Daniel T. Snyder

USGS Water-Supply Paper 2483, 47 pages, 1 plate, 9 figures, 7 tables

Available from U.S. Geological Survey, Branch of Information Services, Box 25286, Denver, CO 80225 (303-202-4700).

Abstract

Chlorofluorocarbon (CFC)-age dating was used to determine the presence or absence of modern water (water containing CFCs) in samples from 54 water wells in the Portland Basin. The accuracy of the CFC-dating method was confirmed by tritium analyses of water from six wells. CFC-dating results from 51 of the 54 wells were compared with minimum travel times of the water estimated using particle-tracking-simulation techniques.

Particle tracking was accomplished using a program that combines the results of a regional, ground-water flow model of the Portland Basin with an estimated porosity field, in order to determine ground-water flow paths and to calculate particle travel times. The particle tracker can be used to delineate areas that, on the basis of short modeled travel times and known or perceived upgradient-contaminant loadings, may be affected by contaminants.

A sensitivity analysis of various particle densities used in the simulations indicated that about 500 particles per model cell provides an adequate description of the minimum time of travel for water particles tracked from the 51 wells. The CFC results and the particle-tracker results were comparable at 39 (76 percent) of the 51 wells, if water that has entered the ground-water system since 1944 (the limit of detection using CFCs) is considered modern. Furthermore, although the particle-tracker-model results and the CFC-model results were not comparable at 12 sites, the particle-tracker model appeared to err on the conservative side when compared with the CFC model. If the CFC-model results are correct, the particle-tracker results are conservative at 9 of the 12 sites. At those nine sites, the particle-tracker model indicated the presence of modern water, but the CFC-model ages were old (pre-1944). At the other three sites, the particle-tracker model failed to indicate the apparent presence of modern water. On the basis of this comparison, it appears that particle-tracking techniques can be used to identify parts of the Portland Basin likely to yield modern water to wells, and that CFC-age dating can provide a useful check on the reliability of the particle-tracker results.

Results of both particle-tracking simulations and CFC-age dating indicate that modern ground water flows to depths of hundreds of feet in the Portland Basin. Although wells with deep open intervals generally yield old water more often than wells with shallow open intervals, many wells with deep open intervals do produce modern water. Because many wells with deep open intervals in the Portland Basin produce modern water, aquifer depth alone is not a reliable indicator of the vulnerability of ground water to contamination. Other factors, such as contaminant loadings, and contaminant transport and fate, need to be considered when determining ground-water vulnerability.