SUMMER 2011 VALUES OF δ18O AND δ2H FROM SURFACE WATER, GROUNDWATER, AND PRECIPITATION IN AMES, IOWA: EVIDENCE OF RECENT CLIMATE CHANGE?

Stable isotopes in water (δ¹⁸O and δ²H) are important to environmental research because they can act as proxies for paleoclimate (atmospheric circulation, sources, temperature), identify water sources in the hydrologic system and document shifts in isotopic compositions that signal climate change. As part of an NSF-funded REU project, water from Squaw Creek and South Skunk River in Ames was sampled daily from June 3 to July 30, 2011. Precipitation samples were collected during the same period at the South Skunk River site. Groundwater was sampled at River Valley Park, Ada Hayden Lake, and the City of Ames municipal wells. All samples were analyzed using a Picarro L1102-i Isotopic Liquid Water Analyzer at Iowa State University. The objectives of the study were to: 1) produce a new Local Meteoric Water Line (LMWL); 2) characterize the δ¹⁸O and δ²H of water sources; 3) understand the dominant factors controlling isotopic composition, such as temperature and precipitation; and 4) compare the previous 1992 LMWL to new data to detect climate change. Results show that the δ¹⁸O values of precipitation, surface water and groundwater have ranges of -10.07 to -1.23‰, -7.87 to -6.32 ‰, and -8.60 to -5.08‰, respectively, and for δ²H were -70.50 to -1.02‰, -52.53 to -39.23‰, and -61.62 to -31.33‰, respectively. Weighted averages for precipitation during the sampling period were -4.40 and -21.60‰ for δ¹⁸O and δ²H, respectively, and the LMWL is δ²H= 7.43δ¹⁸O + 6.08. These results are more isotopically enriched than described in previous studies in the region - the weighted average δ¹⁸O values for precipitation in June/July 1992 was -7.16‰ and a 2005 US-wide study suggested δ¹⁸O values for the study area between -8 and -10‰. Enriched isotope signatures may signal a climate shift favoring a moisture-rich atmospheric source such as the Gulf of Mexico. Stream water time series show nearly synchronous behavior of isotopic composition, varying over a small δ¹⁸O range between -7.87 and -6.32‰. The range is similar to that in groundwater samples (-8.60 to -5.08‰ for δ¹⁸O) suggesting an exchange between these water reservoirs and the dominance of groundwater flow over precipitation in stream water composition. Results of the study will be useful for further characterization of water sources in central Iowa and detection of climate change.