Agricultural, as well as other, areas in the C-111 basin in Dade County have experienced problems with flooding in recent years, especially the 1994-95 water year in which major crop damages resulted from high water tables (Jonathan Crane, UF/IFAS Tropical Research and Education Center, personal communication). One component of this Initial Assessment was to examine and evaluate existing geographic and monitoring databases, and provide a preliminary overview of historic events in the study area and recent trends in hydrologic conditions. Hydrologic conditions are influenced both by climatic variability and by water management strategies of the South Florida Water Management District and U.S. Army Corps of Engineers, which have changed in recent years in order to provide more water to the ENP.
The objective of this section is to present and discuss hydrologic data from various agencies to assist in evaluating recent changes in the high water tables in the agricultural sector of Dade County. Geographical databases stored using ARC/INFO software are described, and graphs are presented based on data from selected monitoring stations. Annual graphs were made based on a water year that extends from November 1st to October 31st (MacVicar et al., 1983). Data were obtained for all the structures and data stations that were identified within the general study area, but only stations with superior records within the specific study area were analyzed. Maps and graphs of all these stations are presented in Appendices A through F. Representative maps and graphs are presented and discussed in this section for illustrative purposes. At present, this is a preliminary assessment, and further modifications will be needed to make adjustments for changes in gauge datums, correct for questionable survey elevations at some gauges, evaluate other discontinuities (reasons for separate station coding and data overlap at selected locations), and integrate land surface elevations with gauge data.
Existing areal coverages created using Geographical Information Systems (GIS) software (ARC/INFO) were compiled for the study area. This was conducted primarily through contacts with government agencies. The primary sources were the South Florida Water Management District (Contact: Lisa Smith), U.S. Army Corps of Engineers, South Atlantic Division, Jacksonville District (Contact: Susan Bullock and Larry Stanislawski), and the University of Florida, Department of Urban and Regional Planning, GEOPLAN Center (Contact: Stanley Latimer). Existing areal coverages and their sources are shown in Table 5-1, and those that have been modified are shown in Table 5-2. These coverages have been checked and are currently stored on a UNIX-based SUN-SPARC computer workstation with an ARC/INFO site license in the Department of Geography at the University of Florida. A well layer was not available, so one was created consisting only of the wells with long-term data which were examined in this report.
Although the areal coverages are extensive, some additions were made to the data bases and several additions are still recommended for mapping and evaluation of the problem of high water tables in Dade County. One problem observed with many layers was a lack of "meta-data". Meta-data describes the attributes and characteristics of points, lines, and polygons in the layers. More meta-data were needed for the canal layer, which did not have names or labels associated with the canals. Also, it would be useful to add more points to the well layer to include the most recent wells. For all variables it would be useful to add meta-data regarding data characteristics (period of record, variables measured). In addition, a more detailed and current land use layer would be desirable, especially one characterizing the type of agriculture and management practices in the study area.
Past flood experiences and crop damages should also be compiled and mapped. Also, more detail concerning topography would be desirable since the area has low relief, and the small variations are important to understand regional flow directions as well as flooding and high water tables in agricultural fields. Subsurface geologic layers would also be desirable, to show the variation and thickness of the Biscayne aquifer and the potentiometric surface. Fieldwork, questionnaires, and analysis of other published and unpublished sources would be needed to compile much of this recommended information.
With any computer-generated map, it is too important to select an accurate and consistent projection. The Florida State Plane East (FSPE) was chosen because it includes all of Dade County and has slightly greater accuracy than the alternatives (UTM) for plotting monitoring stations, etc. All other layers were projected into FSPE, if they had not been previously. The next decision dealt with establishing the boundaries of the study area. It was decided that a "general area" and a "project area" (the latter closely following the USACOE C-111 project boundary) would be generated. ARCEdit was used to make two clipping areas, the general and specific project areas. Flow gauges, rain gauges, stage gauges, and structures were clipped to create a general and project subset (Appendix A).
Maps included in Appendix A show monitoring station locations, while Tables 5-3 and 5-4 show the length and period of record of various stations examined in this study. Time series plots were made to examine trends in variables, including rainfall (Appendix B), groundwater levels (Appendix C), canal flow (Appendix D), and canal stages (Appendix E). The data were plotted in twenty-year intervals, using the statistical package SigmaPlot. In general, there are few data collection stations, and there is a limited period of record for hydrologic data in the agricultural area. Three paired sets of stations, comprised of well and stage monitoring sites located close to one another, were differenced to determine the local drainage directions (Appendix F). Both time series plots and frequency plots of well minus stage elevations were made for these three paired sites.
Graphs in the Appendices B, C, D and E show hydrologic variations for rainfall, groundwater levels, canal flows, and canal stages, respectively, over twenty-year intervals across the top. For the graphs from 1960 through 1979, the bottom portion shows detailed annual graphs of wet, dry and average years that were used in prior hydrologic modeling studies. For the C-111 project the wet year was from November 1968 to October 1969, the dry year was from November 1973 to October 1974, and the average year was from November 1976 to October 1977. On the graphs from 1980 through present, two wet years were shown. The first wet year extends from November 1980 to October 1981, which includes the period when Tropical Storm Dennis dropped record rainfall in the study area. The second wet year extends from November 1994 to October 1995, a recent period in which much of the agricultural area in Dade County experienced severe problems associated with high water tables in the study area. The dry year used, November 1989 to October 1990, appeared to be much drier than other years in this area although it was not the subject of a South Florida Water Management District report.
Although further work is necessary to evaluate and make adjustments for possible changes in gauge elevations, and the land surface elevations are missing or inadequate in some circumstances, many graphs show that minimum and dry season water levels in wells and canals in both agricultural and nearby non-agricultural areas have increased over the past decade (see for examples Figures 5-1 through 5-4).
A comparison was made of all available groundwater level time series in the C-111 basin east of L-31N and C-111 that had data for the three high water years: 1968-1969, 1980-1981, and 1994-1995. All of these wet years contain events with rainfall depths which roughly coincide with the 1 to 20 day -10 year return period storm for the C-111 basin (see Table 5-5 and Figures 5-5 through 5-7).
Time series plots and frequency plots of well minus nearby canal water level elevations suggest that local drainage is dominantly from the canals to the agricultural land (see Appendix F). Comparing data at USGS Well G789 with data from Structure 176 on Canal C-111 from 1969 to 1996, it was found that well values were typically higher than stage in the early 1970s, but more recently canal stages are higher than elevations in wells. Frequency plots of well elevations minus stage elevations show that the canal stage elevation was higher than the well elevation for about 90% of the observations made during this period. Similarly, in a comparison of S-196A USGS Observation Well minus stage elevation at the S-196 Culvert on Canal C-103 for 1980 to 1996, the canal stage elevation was higher than the well elevation for about 95% of the observations made during this period (Appendix F). Also, comparisons of the G-757A USGS Observation Well and stage elevation at the S-194 Culvert on Canal C-102 for 1980 to 1996, suggest that the canal stage elevation was higher than the well elevation for about 50% of the observations made during this period (Appendix F). Thus, groundwater flow is generally in a direction from the canals to the agricultural areas east of the canals, with the elevation of the groundwater table controlled by the height of water in the canals. Only for brief time periods during flooding events is the flow gradient reversed allowing water to flow from the agricultural lands east of L-31N and C-111 into the canals.
Threats to the water quality in south Florida come from agricultural, industrial, and urban areas. Agriculture involves the use of many chemicals such as fertilizers, herbicides, insecticides, and fungicides (USGS, 1996a). Among the agricultural chemicals of concern in the Everglades are phosphorus, nitrate, insecticides, and herbicides. Table 5-6 shows water quality criteria established for the ENP in the 1979 Memorandum of Agreement (MOA) between the ENP, the SFWMD, and the Army Corps of Engineers (USACOE, 1995a). The main piece of legislation currently affecting water quality issues in the Everglades is the Everglades Forever Act. The Act outlines a program that requires all water reaching the Everglades by December 31, 2006 not upset its natural balance (SFWMD, 1995a).
The Everglades Forever Act states that while the Florida Department of Environmental Protection (FDEP) and the SFWMD have developed plans and programs for the management and improvement of the Everglades Protection Area, implementation has been insufficient to restore and protect the flora and fauna of the Everglades (Florida Statutes, 1995). Also, the Act states that the Legislature has determined that waters entering the Everglades Protection Area contain excessive levels of phosphorus and that reducing these levels would benefit the ecology of the Everglades (Florida Statutes, 1995; SFWMD, 1995b). Finally, the Act states that the Legislature recognizes that the Everglades Agricultural Area (EAA) and adjacent areas provide a basis for an important agricultural industry. The Legislature intends to maintain the quality of life for all residents of south Florida, including those in agriculture, and minimize the impact on south Florida jobs, including agricultural, tourism, and natural resource-related jobs. However, the Legislature intends to preserve natural values in the Everglades, and to promote its restoration and protection through legislative action (Florida Statutes, 1995).
The Act required the SFWMD and the FDEP to evaluate water quality standards in EAA canals and the Everglades Protection Area by December 31, 1998 (Florida Statutes, 1995). Phosphorous limits for inflows into Everglades National Park were set in July 1991 as part of a settlement agreement that ended a federal lawsuit brought by the U.S. Department of Justice against the Florida Department of Environmental Regulation and the South Florida Water Management District. The settlement agreement prescribes long term discharge limits which apply to the combined inflow to Taylor Slough (i.e. from S-332 and S-175) and Coastal (S-18C) basins. The long-term limit for Taylor Slough and the Coastal basins is 0.011 mg/l (as P) beginning July 1, 2002.
5.3.1 Phosphorus Data
The Everglades Forever Act also requires monitoring to evaluate efforts in improving water quality and other aspects of ecosystem health (SFWMD, 1995a). The SFWMD has been collecting phosphorus data in the vicinity of the C-111 basin for several years. Data were acquired from the SFWMDâs DBHYDRO database and graphed to show possible trends (see Appendix G). Analyses of the water quality data times series plots in Appendix G show that total phosphorus concentrations within the C-111 basin (both in the agricultural areas and in the ENP) often exceed the 0.011 mg/l (as P) inflow limit for Taylor Slough and the Coastal basins that will become effective July 1, 2002. There is insufficient long-term data, however, to determine whether water quality has been affected as a result of the Experimental Program of Water Deliveries to ENP.
A comparison of the data from station S-332 (Figure 5-11) and station TSB (Figure 5-12) are indicative of the nutrient problem that the Everglades has been experiencing. Station S-332 pumps water from the agricultural areas westward into the ENP via the headwaters of Taylor Slough. Station TSB is about two miles southwest of S-332 near the bridge crossing Taylor Slough on the main road going through the ENP (see Figure 3-1). Much of the water passing through S-332 presumably reaches TSB. Figures 5-11 and 5-12 indicate that phosphorus is consistently high at both locations. Research into potential cause-and-effect mechanisms for the high levels at both stations is needed
5.3.2 Nitrogen, Insecticide and Herbicide Data
Since phosphorus is the growth-limiting nutrient in the Everglades, most nutrient research concentrates on phosphorus. However, the USGS is currently attempting to quantify historical nutrient levels including nitrogen-based compounds such as ammonium (USGS, 1996b). Pesticides have been monitored quarterly at 27 sites in surface water and sediment throughout the SFWMD's jurisdiction for over a decade. From the 60 pesticides monitored in 1995, greater than 99% of the residues were below measurable limits. The majority of detections in water are herbicides such as atrazine, ametryn, bromacil, and simazine. The insecticide endosulfan and its metabolite, endosulfan sulfate, were found on two of four sampling events at structures near ENP in 1995; however, endosulfan has not been detected in water actually entering the ENP.
The flow of groundwater in the C-111 basin is predominantly from the northwest to the southeast. Thus, groundwater flow is generally in a direction from the canals to the agricultural areas east of the canals, with the elevation of the groundwater table controlled by the height of water in the canals. Only for brief time periods during flooding events is this gradient temporarily reversed to allow drainage from the agricultural lands east of L-31N and C-111 to the canals.
Although further work is necessary to evaluate and make adjustments for possible changes in gage elevations, and the land surface elevations are missing or inadequate in some circumstances, hydrologic data show that both minimum and dry season water levels in wells and canals in both agricultural and nearby non-agricultural areas have increased in the past few decades. This shift in water levels appears to be more influenced by water management strategies, determined by the South Florida Water Management District and U.S. Army Corps of Engineers, rather than climatic variability. Wet season importation of water into the C-111 basin has also increased markedly since the Experimental Program of Water Deliveries to ENP began in 1983.
Further work is necessary to improve the hydrologic and geographic data bases. With the hydrologic data bases, necessary work includes making adjustments and corrections to data, especially determining any changes in datums and elevations of monitoring sites, many of which may occur in conjunction with discontinuities. Then, more detailed statistical analyses and interpretation can be performed. In terms of GIS coverages, more meta-data is needed for a number of hydrologic layers. A more detailed (increased spatial resolution) and current land use layer would be desirable, especially one characterizing the type of agriculture and management practices in the study area, as well as past flood experiences and crop damages. Also, more detail concerning topography would be desirable since the area has low relief, and the small variations are important to understand regional flow directions as well as flooding and high water tables in agricultural fields. Subsurface geologic layers would also be desirable, to show the variation and thickness of the Biscayne aquifer and the potentiometric surface. Field work, questionnaires, and analysis of other published and unpublished studies would be needed to compile much of this recommended information.
The agricultural water quality issues faced by the C-111 study basin are consistent with those faced by the entire Everglades Protection Area. The focus of the water quality problem from a regulatory standpoint has been phosphorus, but the legislative action and scientific research have taken place north of the C-111 basin. If increased quantities of water are transferred from agricultural areas into the ENP as a result of the MWDP and the C-111 Project, phosphorus concentrations may rise in the ENP, creating an impetus for new regulation of agricultural management practices in the C-111 basin. Thus, the impact of these projects on the quality of water entering the ENP must be more thoroughly examined.
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