LOCAL-SCALE DISTRIBUTION OF GROUNDWATER-SURFACE-WATER EXCHANGE: SOMETIMES IT DOES GO ACCORDING TO THEORY

Spatial distribution of groundwater discharge to surface water is often highly variable and counter to the expected exponential decrease with distance from shore. There are several reasons for this, primary among them spatial heterogeneity in hydraulic conductivity of the underlying sediments. In fluvial settings, seepage is further complicated by hyporheic exchange driven by local-scale bed topography. Numerous studies have documented the often highly complex and difficult-to-resolve spatial heterogeneity in groundwater-surface-water exchange. Therefore, it was encouraging, and somewhat surprising, to have measured almost textbook distributions of groundwater discharge at two headwater reaches of midwestern river systems; the Kankakee River in Indiana and Quiver Creek in Illinois. Studies at both locations were conducted to better understand delivery of nutrients to these fluvial systems via groundwater discharge. Study sites were of the same scale, both with 5 transects distributed along 30 to 50 m of stream reach and extending from the bank 5 to 7 m into the stream. At both locations, seepage was fastest closest to the bank. At the Kankakee River, seepage decreased exponentially with distance from shore along all transects, from an average of 17 cm/d near the bank to 3 cm/d in the thalweg. At Quiver Creek, seepage decreased from an average of 46 cm/d near the bank to 31 cm/d at mid-channel. However, smallest values were recorded at middle distances along 4 of the 5 transects, indicating the presence of a relatively consistent feature or condition that reduced seepage exchange. Slowest seepage 4 m from shore at Quiver Creek is attributed to smaller vertical hydraulic conductivity at this distance from shore. Surprisingly uniform data collected at both locations may be the result of a nearly planar bed and moderate to slow surface-water velocities, reducing hyporheic exchange, or perhaps due to uniform distribution of sediment in both headwater reaches. Additional measurements at other times of year are needed to document whether this relatively simple and predictable distribution of groundwater discharge is somewhat persistent or changes seasonally.