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Evaluation of Constructed Wetlands on Phosphate Mined Lands in Florida. Volume II: Hydrology, Soils, Water Quality, & Aquatic Fauna

03-103-139-v2Final

During the process of mining phosphate matrix the landscapes of the Florida phosphate mining districts are greatly altered in terms of both the surface environment and the subsurface hydrogeology. Upon the completion of mining, it is desirable to restore the surface environment into a viable, functional, and productive condition. One of the most significant factors controlling the viability of the surface environment is the hydrologic character. There is a unique interrelationship between the type of environment and the hydrology, which dictates the distribution and function of all types of wetland and upland ecosystems.

It is a specific goal of all reclamation efforts to restore wetland areas so as to meet the national goal of no net loss of wetland habitat. In order to successfully restore or to create wetlands it is necessary to thoroughly understand the hydrology of the watersheds in which the wetlands reside and the hydrology of specific wetland types. Without some baseline information on the watershed hydrology, it is not possible to develop a viable restoration plan for the mined watershed. One very important fact is that the water balance of the restored watershed will not necessarily function in a completely similar manner to the pre-mined watershed. This water balance is fundamental in the control of both surface-water and groundwater flows. The creation of wetland environments can only be successful if the hydrology at the location of the new wetland matches the required hydrologic regime of the specific wetland type desired.

Changes in the hydrogeologic framework of the shallow aquifer system tend to cause permanent changes to the water balance in each part of the created watershed. For example, about 40% of the created watersheds (average) have clay as a shallow substrate compared to a much smaller percentage in the natural landscape. The clay occurs either within former settling ponds or as mixed overburden. Therefore, the created wetlands must be located in proper relation to the new hydrologic regime. Restoration efforts attempting to locate wetlands at their former geographic positions alone are more likely to have a high percentage of failures. Because of changes to the watersheds, it may be necessary to locate wetland environments lower in the new watersheds and it will be necessary to increase the size of the land area surrounding isolated wetlands in order to compensate for the increased amount of clay in the new environment.

Prior to mining, the Florida phosphate districts contained about 11 different types of wetlands of which about 6 of these wetland types have been successfully created. This does not mean that wetland restoration efforts have failed, because over a period of time the created wetlands will evolve into the types most suited for their new hydrologic regime and position in the watershed.

All wetland types evolve with time and sufficient time must be allowed before the restoration efforts can be evaluated. Wetland creation efforts can be greatly improved by clearly assessing the hydrology of the restored watershed in terms of the geomorphology and water balance at different locations in the basin and in terms of the regulations applied to mine reclamation. It is necessary to establish a cooperative effort between the mining companies and the regulatory agencies to enhance the reclamation efforts by incorporating the hydrologic regime into both the mining plan and the proposed wetland locations after mining. Minor adjustments in the mining plan can increase the potential success rates of reclamation without adding significant costs. Cooperation of the regulatory agencies in providing flexibility in the location and evaluation of created wetlands with specific consideration of the new watershed hydrology would greatly increase the success rate for created wetlands.