To characterize the lithology, the deepest borehole at each levee site was sampled continuously from top to bottom. Split-spoon sampling was used for unconsolidated sediments, and conventional or wireline coring was used for consolidated sediments. Detailed sampling methods are described in Harvey and others (2000). Visual observations of split-spoon and core samples indicate that the aquifer generally is composed of sand overlying limestone with interbedded sand stringers in the top third. The bottom two-thirds of the aquifer is composed of sands of varying grain size. A detailed lithologic description of these core and sand samples and the grain-size analyses are given in Harvey and others (2000). Lithostratigraphic interpretations for two sections (locations shown in figure 1) are provided in figures 4 and 5.

Figure 4. Lithostratigraphic cross section with formation names, Everglades Nutrient Removal (ENR) project, north-central Everglades, south Florida. [larger version]

The relative percentage of sampling by split spoon or coring provides an indication of site-to-site lithologic variation. Sites MP1, MP3, and MOP1 in the northern part of ENR (fig. 2) were sampled by coring for more than 80 percent of their lengths. The high percentage of coring indicates that sediments are mostly consolidated in the upper 100 ft at those sites. In contrast, two boreholes south of ENR, sites MOP2 and S10C, were sampled by standard penetration tests (SPT) for at least 75 percent of their lengths, indicating mostly unconsolidated sediments (figs. 2 and 3). Site MP2 in central ENR was sampled with roughly an equal combination of SPT and coring.

Figure 5. Lithostratigraphic cross section with formation names, Water Conservation Area 2A (WCA-2A), north-central Everglades, south Florida. Note that this section shows only the top half (approximately 100 feet) of the Surficial aquifer in this part of the study area. [larger image]

The results in table 3 indicate that the unconsolidated sediments at WCA-2A are less dense than those at ENR. Blow counts for sites S10C (in WCA-2A) and MOP2 (in ENR) are plotted with sample elevation in figure 6. Gaps indicate elevations where SPT sampling was not possible. Whereas the overall range of N values at the two boreholes is similar, variations are apparent. Near the top of the aquifer (5 ft NGVD) N is equal to 3 at S10C compared with 50 at MOP2. At —46 to —56 ft NGVD, the N values at MOP2 also are higher than those of S10C. The denser sands at ENR could be a result of differences in grain-size sorting or greater sediment compaction.

Figure 6. Standard penetration test N values from the Surficial aquifer at sites S10C (WCA-2A) and MOP2 (ENR), north-central Everglades, south Florida. [larger image]

Grain-size analysis is a tool that is used for interpretation of depositional environments (Boggs, 1995) and for determination of the sediment hydraulic conductivity. To quantify variations in sample grain size, sorting, and sediment classification, samples were sieved according to ASTM Standard No. D-422 (American Society of Testing and Materials, 1991). The results were analyzed to determine the d₁₀, d₁₇, d₂₀ and d₆₀; that is, the grain size for which the indicated fraction of sample (10, 17, 20, 60) is smaller. This information then was used to estimate hydraulic conductivity (Vukovic and Soro, 1992) and to determine the degree of sorting of the sand grains. The uniformity coefficient (d₆₀/d₁₀) and the gradation coefficient [(d₃₀)²/(d_10*d₆₀)] were the selected measures of sorting (Kasenow, 1994). Whereas only selected data are given as part of this discussion, all sieve analysis data from this investigation are available in Harvey and others (2000).

The sorting of sediment grains may reflect depositional conditions and processes (Boggs, 1995). Sediments underlying the ENR generally were well sorted with some poorly sorted layers near the top of the Surficial aquifer (Harvey and others, 2000). Two ENR sites, MP1 and MP2, demonstrate well-sorted sediments throughout the depth of the boreholes (fig. 7). Several boreholes at the northern edges of the ENR study area had more poorly sorted sands in the upper portions of the Surficial aquifer. For example, poorly sorted sediments were found in the upper layers at MP3 (+10 to +2 ft NGVD), grading to mostly well sorted sediments below a limestone layer. MOP1 had poorly sorted sands at 6 to —4 ft NGVD. Well sorted sediments with a poorly sorted layer found at —18 to —22 ft NGVD were the primary sediments found at MOP2, located just south of ENR.

Sediment analyses of WCA-2A sediments are limited to the S10C borehole. The other sites were drilled using a modified wireline core barrel system that does not collect unconsolidated sediments (MacIntyre, 1975). Sands at S10C primarily were poorly sorted and coarser in grain size than the sands at ENR (table 4). The exception to poor sorting of sediments at S10C sediments is the interval between —50 to —60 ft NGVD, where the sediments are well sorted (fig. 7). These variations are reflective of the changing depositional processes in a marginal-marine environment.