Successful management of constructed wetlands for water treatment in the Everglades requires a better understanding of the interactions between surface water and ground water. These interactions affect the water budget of constructed wetlands as well as the transport and fate of environmental contaminants. In order to identify and quantify the key relations, surface-water and ground-water interactions were investigated in the Everglades Nutrient Removal project, a 1,544-hectare constructed wetland that was built as a prototype to evaluate the effectiveness of constructed wetlands in removing nutrients from agricultural drainage (fig. 1).

Ground-water recharge (from the ENR to the under-lying aquifer) and discharge (from the aquifer to ENR) were determined by using a combined water and solute mass-balance approach (Choi and Harvey, 2000). Over a 4-year period (1994-98), ground-water recharge averaged 0.9 cm/day, which is approximately 31 percent of the surface water pumped into the ENR for treatment (fig. 2). In contrast, ground-water discharge was much smaller (0.09 cm/day or 2.8 percent of water input to ENR for treatment) (fig. 2). Using a water-balance approach alone only allowed net ground-water exchange (discharge-recharge) to be estimated (-0.78 ± 0.16 cm/day). Discharge and recharge were individually determined by combining a chloride mass balance with the water balance.

Figure 2. Comparison of estimated ground-water recharge with (a) inflow rate of surface water from supply canal into ENR (G-250); and (b) returned flow from seepage canal (G-250_S) and water level in ENR (ENR202d). [larger image]

For a variety of reasons, the ground-water discharge estimated by the combined mass balance approach was not reliable (0.09 ± 2.4 cm/day). As a result, ground-water interactions could only be reliably estimated by comparing the mass-balance results with other independent approaches, including direct seepage-meter measurements (Harvey and others, 2000) and previous estimates using ground-water modeling (Hutcheon Engineers, 1996; Guardo and Prymas, 1998). All three independent approaches provided similar estimates of average ground-water recharge, ranging from 0.84 to 0.9 cm/day (table 1). There was also relatively good agreement between ground-water discharge estimates for the mass balance and seepage meter methods, 0.09 and 0.06 cm/day, respectively. However, ground-water-flow modeling provided an average discharge estimate that was approximately a factor of four higher (0.35 cm/day) than the other two methods (table 1).

To determine the control that managers have over the extent of ground-water recharge, our estimate of ground-water recharge was compared with the rate of surface-water inflow to the ENR by pumping from the supply canal. Recharge was positively correlated with the pumping rate of surface water from the supply canal into ENR (fig. 3-a). This demonstrates that the relatively high surface-water inputs to this constructed wetland have the unintended effect of increasing recharge of surface water. A considerable part of the recharged ground water (73 percent) was collected and returned to the ENR by a seepage collection canal. Additional recharge that was not captured by the seepage canal only occurred when pumped inflow rates to ENR (and ENR water levels) were relatively high (fig. 3-b).

In conclusion, we have shown how management of surface water in the northern Everglades increases interactions with ground water. The increased ground-water recharge causes environmental contaminants in surface water to migrate to ground water and, possibly, discharged back to surface water outside of the treatment wetland. Consequently, a detailed understanding of wetland and ground-water interactions will be necessary to improve the operational efficiency of these treatment wetlands.

Back to Project Homepage