DETECTION OF EVENT-DRIVEN STABLE HYDROGEN AND OXYGEN ISOTOPIC SIGNATURES IN WATER SAMPLES FROM A LARGE RIVER

Through the U.S. Geological Survey (USGS) National Stream Quality Accounting Network (NASQAN) program to characterize the water quality of large rivers of the U.S., we have opportunistically obtained quasi-monthly water samples from seven stations in the Rio Grande system since 1997 and analyzed them for d²H and d¹⁸O. It was expected that isotopic variation would reflect mainly interannual or seasonal patterns, but we have found that even individual meteorological events can be detected in this large river system.

Surface water samples from 7 Rio Grande stations do become more enriched in both ²H and ¹⁸O between stations in a downstream order toward the mouth at the Gulf of Mexico. For example, the multi-year average and standard deviation of d²H values increased from -66.6 ± 3.1 ‰ to -14.9 ± 10.0 ‰ from El Paso to Brownsville, TX, respectively. However, at the Rio Grande below Laredo, a series of 3 samples within 1 month (8/26/1998, 9/11/1998, and 9/22/1998) had d²H values of -65.3 ‰, -28.4 ‰, and -64.6 ‰, respectively, the first and last of which are more than 3 standard deviations away from the long-term average d²H value at this station of -24.5 ‰. The d¹⁸O values behaved accordingly. Discharge records show that the samples depleted in ²H and ¹⁸O came on days of high flow, suggesting that these samples are indicative of river response to high precipitation events. Samples from adjacent stations, when taken on comparable dates, were similarly depleted in ²H and ¹⁸O, and also occurred during high river flow, indicative of regional precipitation events. Southern Texas was subject to unusually heavy precipitation in 1998: the August sample corresponds to flooding following Tropical Storm Charley and the mid-September sample to that following Tropical Storm Frances. The low d¹⁸O values observed for these Rio Grande samples are consistent with the low d¹⁸O values in precipitation reported by Lawrence and Gedzelman (1996) for 5 tropical cyclone rains in the western Gulf of Mexico between 1988 and 1993, which had an average d¹⁸O value of -9.4 ‰.

This work shows that an isotopic signal is not necessarily homogenized even in a large river system and that apparently anomalous isotopic values cannot be dismissed as outliers, but rather may be indicative of relatively short duration extreme hydrologic and meteorlogic conditions.