CSI MEETS GROUNDWATER: “AUTOPSYING" THE EFFECT OF A “500-YEAR” FLOOD ON AN ALLUVIAL/BURIED VALLEY AQUIFER IN AMES, IOWA

Flooding causes major property damage each year; however, relatively little attention has been given to its impact on aquifers. A “500-year” flood event in Ames, Iowa on August 11, 2010 provided a unique opportunity to investigate the impact of flooding on an alluvial/buried valley aquifer – the Ames aquifer – that supplies 2.2 billion gallons/year of drinking water to about 56,000 residents. Instead of ballistics tests, this study used hydraulic head, groundwater temperature, stable isotope, and anion data to demonstrate that flood water (the perpetrator) affected only the upper part of the aquifer (the victim) and did not compromise drinking water. The 2010 flood event resulted from the South Skunk River and Squaw Creek watersheds receiving upwards of 10 inches of rain in the 24 hours prior to August 11. The South Skunk River crested at 19.55 ft at 1:00 pm at the USGS gage – the 4^th highest stage on record – and discharge was ~13,800 cfs. In anticipation of a flood event, piezometers outfitted with pressure transducers were installed in 2008 at depths between 8 and 97 ft in the sand and gravel aquifer adjacent to the river. Post-flood water samples were collected from the river and groundwater within two days of the flood peak. Samples from municipal wells were collected within two weeks of the flood. Results suggest that precipitation, characterized by isotopically enriched δ¹⁸O (-4.6‰) and Cl concentrations similar to rainfall (about 10x less than ambient stream values), dominated post-flood river water and shallow groundwater. Hydraulic head and temperature data showed that flood water temporarily increased the saturated thickness of the aquifer by about 12 ft. Isotope and Cl data also suggest that isotopically depleted (older?) groundwater was apparently “pushed” by the flood water to a depth of 14 ft (δ¹⁸O = -10.4‰) within two weeks of the event. Groundwater below that depth maintained a constant isotope and anion composition and samples from municipal wells downstream showed no change in isotope or anion composition reflective of flood water. In summary, the “autopsy” results suggest that the “500-year” flood primarily drove river water into the “new” part of the aquifer, moved “older” water out of the upper part of the aquifer, and left drinking water from municipal wells unscathed.