Horizontal Hydraulic Gradients

Average directions of ground-water flow were toward the northwest in the vicinity of the ENR, and toward the southwest in north-central WCA-2A (table 13). At ENR, there was relatively little change between wet and dry seasons in the direction of ground-water flow. Ground-water flow at ENR was toward the northwest during both wet and dry periods, toward 296 and 297 degrees (clockwise from north) at intermediate depths in the Surficial aquifer (table 13). The direction of ground-water flow in the deeper part of the aquifer also was stable over time but was more northerly in direction (310 degrees). It is possible that the estimated differences in flow direction at different levels in the aquifer are erroneous, the result of having fewer deep wells to use in the calculation of flow direction.

The magnitude of horizontal hydraulic gradients at ENR increased by 15 to 20 percent during wet conditions compared with dry conditions (table 13). Increased hydraulic gradients during wet conditions reflect the greater wet-season differences in water level between WCA-1 and the agricultural area.

At the WCA-2A study site, ground-water flow was toward the south-southwest (toward WCA-3A). During the wet season hydraulic gradients varied only a few degrees between intermediate and deeper wells (flow directions of 224 degrees and 219 degrees, respectively). Between wet and dry seasons, WCA-2A horizontal hydraulic gradients shifted considerably. The direction of flow in the shallow layer was toward 224 degrees during the wet season compared with 173 degrees during the dry season. The more easterly component of flow in dry conditions reflects the effect of seasonally low water levels to the east where ground-water pumping is from a well field between the Everglades and the Atlantic Coastal Ridge (Miller, 1988).

The magnitude of horizontal hydraulic gradients at WCA-2A increased by approximately a factor of 2 during wet conditions. Increased ground-water hydraulic gradients during wet conditions likely are the result of opening spillways between WCA-1 and WCA-2A, and between WCA-2A and WCA-3A, which reduces ponding in WCA-2A and increases the surface-water slope.

Figure 10. Water levels and horizontal hydraulic gradients along research transects in Everglades Nutrient Removal (ENR) project (top, A) and Water Conservation Area 2A (WCA-2A) (bottom, B), north-central Everglades, south Florida. Wet conditions (August 1997) and dry conditions (June 1998) are compared. A negative gradient indicates flow towards the northwest (ENR) or southwest (WCA-2A). See figures 2 and 3 for transect locations. [larger version]

The importance of land and water-surface slopes in affecting ground-water flow is apparent on plots of water-surface slope and hydraulic gradient along each research transect (fig. 10). The largest horizontal hydraulic gradients in ground water are near levees in ENR and WCA-2A. Hydraulic gradients near levees were approximately 4 times greater at ENR compared with WCA-2A. That difference largely is the result of the geographic position of the ENR on the boundary between WCA-1 and the EAA, where the land-surface and water-surface slopes are relatively steep because of subsidence in the EAA.

Greater seasonal variability in horizontal hydraulic gradients beneath WCA-2A, compared with ENR, results in large part from greater variation in the difference between surface-water levels across the levee that borders WCA-1 (Harvey and others, 2000). As reported earlier, temporal variability in water-level differences across levees was greater at WCA-2A (coefficient of variation of 50 percent compared with 18 percent for ENR). More variable horizontal gradients at WCA-2A also result from larger fluctuations in surface-water level in WCA-2A compared with ENR. For example, over a period of approximately a year and a half (June 1997 to October 1998), water levels in the interior wetlands of WCA-2A varied over 5 ft compared with only 2 ft at ENR (Harvey and others, 2000).