A mixed exchange lake suggests that the lake system is dominated by groundwater flowing into the lake; however, lake water flows through the bottom into the underlying groundwater system. Mixed exchange lakes usually occur where variations in lake bottom sediment properties and the presence of lower head values in earth material result in the loss of water from the lake. The mixed term is used here to suggest that exchange directions at the lake perimeter and lake bottom can be different (Figure 32e and Figure 37). This condition is presented here to alert investigators to consider the possibility of complex exchanges in some settings.
Figure 37 – Cross section and map view of a mixed lake exchange. Equipotential lines and relative head values are shown in black. Groundwater flow is in the direction indicated by blue arrows. Aquifer conditions are assumed to be isotropic and homogeneous. The stippled pattern represents a zone of higher hydraulic conductivity. Monitoring wells are open at the bottom. a) Cross sectional representation showing an upward groundwater gradient near the shore (effluent conditions), and a downward gradient beneath the lake. This causes leakage from the lake bottom in this setting. Lake stage is shown as a water level on the vertical rectangle. b) A map view showing equipotential lines and groundwater flow for a mixed lake exchange. The dashed arrow represents the loss of water from the lake bottom to the underlying groundwater flow system (Woessner, 2020).
