SBPVP REVELATIONS/CONFIRMATIONS OF FLOW MEASURED AT THE GROUNDWATER-SURFACE WATER INTERFACE (Invited Presentation)

The groundwater-surface water interface (GWSWI) features sharp chemical and temperature gradients, active ecosystems, and variable flow. These systems are closely intertwined. Until recently, only one technology – the seepage meter – has provided direct measurements of flux at the interface. Indirect techniques, involving temperature or hydraulic gradients, have filled the void with some success, but direct measurements offer the advantages of fewer assumptions and, arguably, easier interpretations. The Streambed Point Velocity probe (SBPVP) is a relatively new technology that makes direct, real-time seepage velocity measurements. Two case studies illustrate revelations and confirmations achieved with the SBPVP at the GWSWI.

The first study occurred on a meander of the Grinsted Å, located in central Jutland, Denmark, where a chlorinated solvent plume was discharging. The SBPVP was deployed alongside a seepage meter, minipiezometers (Darcy approach), and a two-point temperature profiling spear to characterize flow at the GWSWI (grid of 112 sampling points over a 75 m long reach of width ~5-7 m). The Darcy approach, seepage meter, and the SBPVP yielded very similar discharge estimates and the former two methods and the temperature data defined two discrete discharge zones where risk was concentrated.

The second case involved the hydrogeology of lake located near Bemidji, MN, receiving groundwater contaminated with oil degradation products. A thick muck layer partially filled the lake basin but was thought to play no role in groundwater discharge. However, two SBPBP surveys conducted from the lake surface established that 1) water beneath the muck was pressurized, 2) groundwater was skirting the base of the lake boundary at elevated speeds, 3) discharge to the lake was less than expected. The muck was apparently serving as an aquitard. With this pattern of flow, much of the pollutant mass in the plume might skirt the lake and exert minimal impact on the water quality.

These experiences confirm the occurrence of discrete discharge zones and focused risk in a streambed. The SBPVP also revealed unexpected patterns of flow beneath a lake, with implications for pollutant loadings. These revelations and confirmations would be difficult to achieve with conventional methods.