|Oklahoma Water Science Center|
By Martha Scholl, Scott Christenson, Isabelle Cozzarelli,
and Jeanne Jaeschke
U.S. Geological Survey Scientific Investigations Report 2004-5238
This report is available as a pdf.
Analyses of stable isotope profiles (δ2H and δ18O) in the saturated zone, combined with water-table fluctuations, gave a comprehensive picture of recharge processes in an alluvial aquifer riparian zone. At the Norman Landfill U.S. Geological Survey Toxic Substances Hydrology research site in Norman, Oklahoma, recharge to the aquifer appears to drive biodegradation, contributing fresh supplies of electron acceptors for the attenuation of leachate compounds from the landfill. Quantifying recharge is a first step in studying this process in detail. Both chemical and physical methods were used to estimate recharge. Chemical methods included measuring the increase in recharge water in the saturated zone, as defined by isotopic signature, specific conductance or chloride measurements; and infiltration rate estimates using storm event isotopic signatures. Physical methods included measurement of water-table rise after individual rain events and on an approximately monthly time scale. Evapotranspiration rates were estimated using diurnal watertable fluctuations; outflux of water from the alluvial aquifer during the growing season had a large effect on net recharge at the site.
Evaporation and methanogenesis gave unique isotopic signatures to different sources of water at the site, allowing the distinction of recharge using the offset of the isotopic signature from the local meteoric water line. The downward movement of water from large, isotopically depleted rain events in the saturated zone yielded recharge rate estimates (2.2 - 3.3 mm/day), and rates also were determined by observing changes in thickness of the layer of infiltrated recharge water at the top of the saturated zone (1.5 - 1.6 mm/day). Recharge measured over 2 years (1998-2000) in two locations at the site averaged 37 percent of rainfall, however, part of this water had only a short residence time in the aquifer. Isotopes showed recharge water entering the ground-water system in winter and spring, then being removed during the growing season by phreatophyte transpiration. Recharge timing was variable over the course of the study; July and August were the only months that had no recharge in both years. Recharge to the aquifer from the slough (wetland pond) was estimated at one location using the isotopic signature of water affected by evaporation. Recharge was correlated with the rainfall amount over the period of estimation, suggesting that recharge from the slough to the downgradient aquifer was an episodic process, corresponding to elevated water levels in the slough after large rain events.
Description of Norman Landfill Area
Purpose and Scope
Sampling Methods for Precipitation, Ground Water, and Slough Water
Recharge and Evapotranspiration Estimates
Determination of Specific Yield
Laboratory tests on cores
Recharge Estimates Using Stable Isotopes of Water
Recharge Estimates From Rises in Water Table
Comparison of Recharge Estimates Obtained by Chemical and Physical Methods
Recharge Estimates for the Slough Bank Well
Recharge Rate Estimates from Storm Events
Recharge Rates and Seasonal Timing
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For information about water-resource studies in the State of Oklahoma, please visit our Web site at http://ok.water.usgs.gov/.
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