GROUND-WATER FLOW

The highest water levels in Miami-Dade County are maintained in Water Conservation Areas (WCA) 3A and 3B (fig. 1). In a regional sense, ground water moves from WCA 3A and 3B eastward and southward to the ocean (Fish and Stewart, 1991). Canals, control structures, or large well fields cause local variations in the flow pattern. During the wet season, ground-water seepage from WCA 3A and 3B is partly captured by peripheral canals, but large quantities pass under the canals or across the canals (Fish and Stewart, 1991). Ground-water flow directions interpreted by Fish and Stewart (1991) generally show that ground-water flows toward the study area from the west and northwest during the wet and dry seasons.

Figure 5. Hydrogeologic section showing ground-water flow classes in the Biscayne aquifer along Levee 31N (section B-B'). Line of section is shown in figure 1. [larger image]

Data collected for use in delineating ground-water flow beneath Levee 31N included water-level data and borehole-flowmeter, fluid-conductivity, and fluid-temperature data. The water-level data were obtained as continuous ground- and surface-water-level readings. Borehole flowmeters can be used to measure vertical flow within a single well and to use these data to identify water-producing zones in open-hole wells. For this study, flowmeter data were used to quantify vertical ground-water flow under existing hydraulic conditions in wells along Levee 31N in order to examine differences in the vertical hydraulic gradient between hydrostratigraphic zones of the Biscayne aquifer (fig. 5). Stationary heat-pulse flowmeter measurements within the limestone of the Biscayne aquifer were obtained from wells G-3671, G-3778, G-3782, G-3783, G-3784, G-3788, and G-3789.

Logs of fluid conductivity, the reciprocal of fluid resistivity, were used to assess changes in borehole-fluid column salinity. Fluid-temperature logs were used in combination with flowmeter data to define movement of water through wells, including delineation of intervals that produce or accept water; thus, these logs can provide information about permeability. Fluid-conductivity and fluid-temperature logs were collected from wells G-3671, G-3778, G-3782, G-3783, G-3784, G-3788, and G-3789 (fig. 1).

In the subsequent sections, water-level data are reported for a period from February 12 to May 4, 2004, during the dry season; flowmeter measurements are reported for a period from August 8 to September 16, 2003; and borehole-fluid conductivity and fluid temperature are reported for a period from July 7 to September 16, 2003. Both summer time periods were during the wet season. Water-level, flowmeter, conductivity, and temperature data were not collected during synoptic conditions and were recorded in a hydrologic system subject to transient changes in hydraulic gradient. A comparative analysis between ground-water levels, surface-water levels, flowmeter, conductivity, and temperature data is considered to be inappropriate. Additionally, flowmeter, conductivity, and temperature data were collected quasi-synoptically and should be treated with caution.

Water-Level Data

Ground-water-level data were collected from the northern and southern monitoring well clusters, each composed of four monitoring wells completed in the surficial aquifer system, and wetland surface-water stage was obtained from a recorder (NESRS2) in ENP about 4 miles southwest of the northern well cluster (fig. 1 and 6). Rainfall was monitored at a station (K8652) located at structure S-334 (figs. 1 and 6). At both monitoring well clusters, each well was completed with a single 2-foot screened interval, but at different depths to isolate separate hydrogeologic zones. The deepest well is screened in the semiconfining unit that includes the upper clastic unit of the Tamiami Formation (fig. 4). The other three wells are completed in separate highly permeable flow zones contained in the lower, middle, and upper parts of the Biscayne aquifer (figs. 4 and 5).

Figure 6. Water level and rainfall for selected sites in the Levee 31N study area during a dry-season period, February 12 to May 4, 2004. All sites are shown in figure 1. VLS is vertical lithofacies succession, HFC is high-frequence cycle, and ENP is Everglades National Park. Click on graphs to view larger versions.

Two well-construction strategies influenced selection of the screened interval depths in the two wells completed in the semiconfining unit that includes the upper confining unit of the Tamiami Formation and the six wells screened in the Biscayne aquifer (figs. 4 and 5). The depths of the screened intervals in the upper semiconfining unit of the Tamiami Formation were selected on the basis of three criteria: porosity, depth, and log signature. The depths correspond to a: (1) hydrogeologic unit characterized by relatively high porosity and assumed relatively high permeability based on low SPT blow counts and geologic evaluation of SPT samples; (2) depth of about 90 feet below NAVD 88; and (3) geologic interval with correlativity using gamma-ray log signatures (fig. 4). Depth selection of screened intervals in the six monitoring wells (figs. 4 and 5) completed within the Biscayne aquifer was based on different criteria. The depths correspond to a: (1) hydrologic zone interpreted as representing the horizontal conduit ground-water flow class, and (2) zone of high permeability assumed to have lateral continuity between the two monitoring well clusters. Both conditions were met with the screened intervals constructed within the upper Biscayne aquifer or HFC5 (figs. 4 and 5). However, inaccurate well depth accounting procedures employed during monitoring well construction resulted in a partially screened middle Biscayne conduit flow zone (VLS3a in well G-3785), and mismatched VLS screened intervals within the lower Biscayne aquifer (VLS2c in well G-3779 and VLS2d in well G-3785) (figs. 4 and 5). It is not known whether well-construction screen-completion errors that result in “miscorrelated” flow units at wells G-3779 and G-3785 significantly impact comparison of lower Biscayne aquifer vertical hydraulic gradients.

Comparison of hydrographs from February 12 to May 4, 2004, at the northern and southern monitoring well clusters and surface-water stage in ENP indicates there was good hydraulic connection between the wetlands and the aquifer, as demonstrated by changes in surface-water stage reflected in water-level trends within the aquifer (fig. 6). Precipitation-driven changes in surface-water stage produced a rapid increase in ground-water levels. At both monitoring well clusters, vertical head differences between the upper semiconfining unit of the Tamiami Formation and various flow zones of the Biscayne aquifer were generally less than 0.4 foot (fig. 6A-B). Additionally the vertical gradients at both monitoring well clusters generally had a similar response to rainfall events and periods of ground-water decline (fig. 6). The vertical hydraulic gradient of the semiconfining unit (Tamiami Formation) was upward at the northern monitoring well cluster, whereas the overlying lower Biscayne was downward; an upward vertical gradient characterized both the middle and upper Biscayne aquifer relative to the lower Biscayne aquifer (fig. 6A). Vertical gradients at the southern monitoring well cluster were temporally variable. Between middle February to late March 2004, water levels in the upper and middle Biscayne aquifer and in the semiconfining unit were approximately equivalent; the lower Biscayne aquifer exhibited a downward vertical gradient (fig. 6B). In late March to early May 2004, the vertical hydraulic gradient of the semi-confining unit (Tamiami Formation) was upward, whereas nearly equivalent water levels within the Biscayne aquifer were downward (fig. 6B).

Flowmeter Data

The direction of vertical borehole flow in test coreholes and monitoring wells between August 8 and September 16, 2003, varied within the Biscayne aquifer (figs. 7 and 8). In general, the direction of vertical borehole flow in test coreholes G-3782, G-3783, and G-3788 and monitoring well G-3784 was downward within the Biscayne aquifer, whereas in test coreholes G-3671 and G-3789, the direction was upward. The vertical borehole flow direction in monitoring well G-3678 was mixed, with upward flow in much of the middle and most of the lower Biscayne aquifer, except near the lower part of the middle. The varying flow directions in G-3778, G-3782, G-3783, and G-3788 suggest possible confinement or semiconfinement between the more permeable flow zones of the Biscayne aquifer. Semiconfinement is defined as a leaky confining unit or units (Wilson and Moore, 1998).

Figure 7. Hydrogeologic section showing wet-season borehole-fluid conductivity and borehole flow for the Biscayne aquifer along Levee 31N (sectionB-B'). Flowmeter measurements were obtained from August 8 to September 16, 2003, and fluid conductivities were obtained from July 7 to September 16, 2003. Line of section is shown figure 1. [larger image]

Borehole-Fluid Conductivity and Temperature Data

Borehole-fluid conductivity and temperature data were collected between July 7 and September 16, 2003, from the wells (test coreholes and monitoring wells) in the Biscayne aquifer with varying results (figs. 7 and 8). Borehole-fluid conductivity ranged from about 280 to 725 µS/cm (microsiemens per centimeter) throughout the Biscayne aquifer along the Levee 31N canal reach between wells G-3671 and G-3789. The conductivity of the surface water in the Levee 31N canal was 855µS/cm near the southern monitoring well cluster on June 22, 2004, which is about 130 µS/cm higher than any measurements of borehole fluid in any of the wells. The lowest borehole-fluid conductivity was in the shallow subsurface and was highest in the lower part of the Biscayne aquifer.

Figure 8. Hydrogeologic section showing wet-season borehole-fluid temperature and borehole flow for the Biscayne aquifer along Levee 31N (sectionB-B'). Flowmeter measurements were obtained from August 8 to September 16, 2003, and fluid conductivities were obtained from July 7 to September 16, 2003. Line of section is shown figure 1. [larger image]

Fluid conductivity is directly related to salinity, with low and high values of conductivity corresponding to low and high values of salinity, respectively. The fluid conductivity measured along the canal reach indicates that the salinity of borehole fluid mostly decreased upward within the Biscayne aquifer in each of the seven wells shown in figure 7. Relatively low conductivities were observed in wells G-3782, G-3783, G-3784, and G-3788 and correspond to the same area where there was mostly downward movement of borehole fluid. The conductivity data suggest that the Biscayne aquifer may be recharged mostly by lower salinity surface water from the ENP wetlands in the reach of the Levee 31N canal spanning wells G-3782 and G-3788. Surface water in the Levee 31N canal was probably excluded as a source of recharge of the Biscayne aquifer by the markedly higher conductivity (855 µS/cm) than measured in the boreholes. Additionally, the conductivity pattern shown in figure 7 suggests that higher salinity Biscayne aquifer ground water may have dominated the ground-water flow field west of Levee 31N, and there was more limited surface-water recharge in the southern and northern parts of the study area.

Between wells G-3671 and G-3789, the measured borehole-fluid temperature in the Biscayne aquifer between July 7 and September 16, 2003, ranged from about 75.2 to 87.8 degrees Fahrenheit (fig. 8). Thick sections of relatively high temperatures were observed in wells G-3782, G-3783, G-3784, and G-3788 and correspond to the same area where, on the basis of heat-pulse flowmeter measurements, there was downward movement of borehole fluid. Therefore, the temperature data suggest that the Biscayne aquifer may have been recharged mostly by warmer surface water from the ENP wetlands in the reach of the Levee 31N canal spanning wells G-3782 and G-3788. Borehole-fluid temperature diminished with depth in most wells; temperatures were lowest at the southernmost and northernmost parts of the study area as shown in figure 8. This latter temperature pattern suggests that cooler Biscayne aquifer ground water may have dominated the flow field west of Levee 31N, and there was more limited surface-water recharge in the southern and northern parts of the study area.