2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 11
Presentation Time: 1:30 PM-5:30 PM


FLORY, Megan M.1, SCHULMEISTER, Marcia K.1 and LARSEN, Lisa2, (1)Earth Science Department, Emporia State University, 1200 Commercial St., Box 4030, Emporia, KS 66801, (2)Larsen & Associates, Inc, 1311 East 25th St, Suite B, Lawrence, KS 66046, mflory@emporia.edu

Large and frequent discharge fluctuations associated with reservoir releases provide an opportunity to investigate dynamic stream-aquifer interactions over a shorter time frame than is possible along unregulated rivers. This study examined short-term temporal and spatial variations in ground-water quality associated with reservoir releases to a typical regulated, mid-continent river. Ground-water elevation, temperature, specific conductance, pH, ORP, and DO were measured every 30 min. with a multi-parameter sonde in a monitoring well located 24 m from the stream channel. The 24-day sampling period spanned high (Q = 49 m3/s) and low (Q = 1.8 m3/s) discharge conditions and four reservoir releases. Temporal variations in all parameters coincided with fluctuations in stream discharge and surface water chemistry measured concomitantly at a nearby USGS gaging station. More erratic variations in these parameters during low discharge periods indicated the influence of factors other than stream flow on ground-water chemistry. Vertical chemical profiles were generated across a 3m section from multi-level sampler with a 0.3m sampling interval during high and low stream flow periods. Lower specific conductance and Cl values at all depths during high stream flow suggest ground-water dilution by infiltrating river water. DO concentrations above 0.2 mg/L were observed in the upper 1.5 m of the aquifer during both high and low stream stages with the highest values (1.0 mg/L) observed during high stream flow. DO values below 0.2 mg/L, dissolved Fe and Mn, and low ORP values in the lower 1.8m suggest persistent reducing conditions in the deeper portion of the aquifer. NO3 to NO2 ratios at these depths suggest that NO3 reduction is more extensive during low flow conditions. The fluctuations in ground-water chemistry caused by reservoir releases, and differences in chemical profiles during high and low stream discharge provide insight into the transient ground-water conditions chemistry at our site. High-resolution evaluations such as these are essential for a better understanding of nutrient cycling and the fate of contaminants in the near-stream ground-water environment.