Wetland-Groundwater Interactions in Subtropical Depressional Wetlands

Restoration of ditched and drained wetlands in the Lake Okeechobee basin, Florida, USA is currently under study for possible amelioration of anthropogenic phosphorus enrichment of the lake. Here we focus on the dynamic hydrology of these systems, with emphasis on understanding the interaction between wetland surface water and adjacent upland groundwater. Based on natural drawdown events observed over 2 years at four depressional wetlands, hydraulic conductivities (K) of the soils surrounding the wetlands were calculated at the wetland scale (approximately 2 ha) using the modified Dupuit equation under a constrained water budget framework. The drawdown-based average K = 6.6 m/d (range 0.9 to 21.3 m/d) was about three times greater than slug test-based values (1.9 ± 1.5 m/d), which is consistent with scale-dependent expectations. Net groundwater recharge rate at each depressional wetland, calculated based on the mean K, corresponded to approximately 40% of rainfall in the same period (10.0 m3/d). The average net groundwater recharge decreased by approximately 15% if ET was increased by 30%. Variability in estimated K and groundwater flow between the study wetlands was likely due to the relative difference of ditch bottom elevation controlling the surface outflow, as well as the spatial heterogeneity of the soils.