EFFECT OF PIEZOMETER PLACEMENT ON ESTIMATES OF GROUNDWATER EXCHANGE WITH WETLANDS

The exchange of water between a wetland and its groundwater system can be estimated using the Darcy equation and the water table gradient surrounding the wetland. However, estimates of the water table gradient, and therefore of groundwater exchange with the wetland, are sensitive to piezometer placement.

In this poster presentation, I discuss groundwater exchange with wetland P1 at Cottonwood Lake Research Site, located in the prairie pothole region of North Dakota, USA. This wetland is surrounded by relatively thick shoreline vegetation beyond which grow sparser, upland grasses. Using piezometer and pond stage data and estimates of hydraulic conductivity, I demonstrate how estimates of groundwater exchange with the wetland are sensitive to the distance between the piezometers and wetland.

Piezometers located farther from the wetland best capture groundwater exchange with the entire wetland, including littoral vegetation. Piezometers located near the shoreline of the wetland pool are able to resolve groundwater exchange directly with the wetland pool. The difference between the two estimates can be profound. Often, groundwater flows toward wetland P1, but intense evapotranspiration from the relatively thick littoral vegetation creates a ring of depression in the water table that extends below the wetland pool elevation. In this scenario, groundwater simultaneously flows from the wetland pool and surrounding uplands, toward the ring of depression. Either piezometer placement  may produce information relevant to scientific, resource management, or regulatory questions. However, understanding the effect of piezometer placement is crucial to avoid misinterpretation of data.