SPATIAL AND TEMPORAL VARIATIONS IN GROUND-WATER CHEMISTRY ALONG A REGULATED, MID-CONTINENT RIVER

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.