publications > scientific investigations report > surface-water and ground-water interactions > suggestions for future investigations

Due to water-management practices and agricultural runoff, surface waters in the central Everglades tend to be contaminated with excessive levels of nutrients, salts, and mercury (Harvey and others, 2002). In the past several decades the application of best-management practices on farmlands adjacent to the Everglades has helped improve the quality of water flowing into the Everglades. The retention of recharged surface water and its solutes for decades in shallow ground water could have legacy effects for the future, because contaminants that were recently recharged potentially could be returned very slowly to surface waters over a period of decades. Of particular importance could be the recharge of phosphorus over the past few decades, which potentially could be returned to surface water in the next few decades with discharging ground water even if the quality of agricultural runoff continues to improve. The likely timescale at which contaminants now stored in peat pore water and the limestone and sand aquifer are returned to surface water could be years to decades and longer. Our findings provide a reasonable hydrologic basis and the hydrologic part of a modeling framework to investigate the phenomenon of "contamination legacy effects" further. One possible result of improved modeling of contaminants would be quantification of how long the initial improvements in quality of water inflows to the Everglades will take to have the desired effect throughout the Everglades system. A second use of such models would be predicting the effects of higher "restored" flows in causing downstream propagation of contaminants from parts of the Everglades where they are currently stored to points downstream that currently have good water quality. It should be stressed that these ideas are preliminary, and that they need to be thoroughly tested by combining the hydrologic model presented here with biogeochemical data, along with further improvements in components of the coupled surface-subsurface-biogeochemical model. Only through such improvements can predictions for future water quality be made more reliably.

The present study concluded that any single method of estimating recharge and discharge will be biased, because of its limited "window of detection" for the broad timescales of interactions between surface water and ground water that occur in the interior wetlands of the Everglades. How can inherently biased measures of recharge and discharge be made useful to practical problems related to Everglades restoration? With the proper cautions on interpretation, all methods of estimating recharge and discharge contribute useful information to an overall assessment of recharge and discharge in the Everglades. From the comparison of methods in this report, for example, comes an estimate of the depth distribution of recharge and discharge fluxes showing that the majority of the recharged water is retained in shallow flow paths within peat in contrast with a much smaller amount of water that interacts with the sand and limestone aquifer. Selecting a single method for future studies requires prejudgement by an investigator of the likely spatial and temporal timescales that are considered essential to a particular investigation's goals and objectives. For example, if the focus of a given Everglades investigation is on surface-water quality, then the investigators will be interested in timescales not too far removed from the surface-water residence time in basins of the central Everglades (months). In that case, the components of surface-water and ground-water interactions that are most relevant are the short-term exchanges associated with flow of surface water into and back out of pore water in the peat. These rapid exchanges between surface water and peat pore water could have profound effects on biogeochemical reactions, and are therefore relevant to ecological simulations. Investigators interested in those processes will therefore mainly be interested in methods to estimate recharge and discharge that are based on measurements in peat (bromide tracer, radium-flux modeling, Darcy-flux calculations) rather than results from tritium modeling or longer-term averaging of SFWMM results. In contrast, longer-term estimates determined by modeling tritium and highly spatially averaged surface-water and ground-water hydraulic head measurements (SFWMM) are probably less applicable to surface-water-quality modeling, and more applicable to ground-water-quality modeling, or longer-term investigations of ground-water transport involving horizontal as well as vertical transport conditions. A good example of a long-term research investigation is determining the fate of recharged water in locations such as WCA-3B, where recharge in the Everglades is thought to be an important source of water to municipal well fields to the east of the Everglades. Some investigations may involve several timescales and thus will require multiple measurement types and updated modeling strategies.