Evaluation of Constructed Wetlands on Phosphate Mined Lands in Florida
Volume I: Project Summary.
Volume II: Hydrology, Soils, Water Quality, & Aquatic Fauna.
Volume III: Vegetation, Wildlife, & Ecosystem and Landscape Organization.
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The primary research task of this project was to assess and analyze the available data base on constructed wetlands on phosphate mined lands in Florida, and where necessary, to supplement existing data with limited additional sampling, and computer modeling. Research goals were directed at determining current technical and operational success of created wetlands to develop as persistent, functioning, and integrated ecosystems. This was accomplished through an evaluation of design criteria and the wetland structure and function that has developed on existing sites. Specific research goals of the project are given below followed by a synopsis of major research findings addressing each goal.
1. To provide a data base from existing studies to guide operational and policy changes needed to improve design, construction, monitoring and assessment of constructed wetland projects, and to determine the adequacy of the existing data base, providing recommendations to ensure the utility of future research and monitoring efforts;
2. To determine the extent to which existing constructed wetlands are persistent, functioning ecosystems;
3. To determine whether constructed wetlands are properly located in reclaimed landscapes;
4. To determine ecosystem functions and values provided by constructed wetlands, to identify appropriate indicators of functions and values, and to develop quantitative methods of measuring those indicators;
5. To determine how success criteria should be applied in evaluating the attainment of goals and of development trends for constructed wetland projects; and
6. To identify future research needs of industry and regulatory agencies.
1. What is the adequacy of existing data?
The overall adequacy of data is poor, limited, or lacking in standardization to do quantitative evaluations of constructed wetland success. Hydrological data, including groundwater levels, hydroperiod, and watershed characteristics, is almost nonexistent . Minimal information exists on soil sediment characteristics of natural and created wetlands visited, including physical and chemical properties such as bulk density, compaction, total or available nutrients, and cation exchange capacity. The data base for aquatic fauna and water chemistry is good quality but relatively sparse. Data for analysis of vegetation development was of limited value due to a lack of standardization and limited long-term monitoring. Virtually no site specific wildlife data exists. The majority of data is derived from permit related monitoring conditions that were not generally relevant to the specific research goals of this project.
2. To what extent are constructed wetlands persistent, functioning ecosystems?
Many of the constructed wetlands observed are apparently persistent. However, those wetlands with poor design, particularly improper hydrology are not persistent, declining in overall "robustness" with age. Constructed wetlands appear to be providing ecosystem functions to greater or lesser degrees. Functions include: surface water attenuation, runoff buffering, water quality maintenance, groundwater and aquifer recharge, shoreline protection, biological integrity, food chain support, provision of wildlife habitat, support of native plant populations, soil processes, and nutrient cycling. Constructed wetlands may provide similar functions as natural wetlands but at different capacities. Most constructed wetlands provide wildlife functions but for different groups of species as typically found in similar undisturbed wetlands.
The majority of constructed wetland projects are young sites and are typically productive ecosystems. These constructed wetlands do not offer the prerequisite habitat requirements for many species and therefore many are absent. The creation of open- water habitat in constructed wetlands has increased habitat for many waterfowl and fish species which were more limited in abundance due to the limited natural occurrence of open water in the undisturbed landscape.
3. Are constructed wetlands properly located in reclaimed landscapes?
The present unmined Florida landscape within the phosphate mining region is fragmented by a variety of human activities dominated by agriculture. Reclaimed mine lands are disconnected and dominated by agriculture with numerous fragmented habitats and watersheds. About 51% wetland construction projects are within first order drainage basins (smallest) and 16% are within second order (second smallest) drainage basins. Most have direct hydrologic connections to the regional drainage network which may result in long-term hydrological problems associated with the maintenance of sufficient storage and groundwater recharge which may affect maintenance of stream base flow during the dry season.
About 50% of the surveyed constructed wetlands projects are connected, at least minimally (with >1% of border adjacent to natural area), to natural forested lands (mostly riparian), with only about 46% of the perimeter of these sites juxtaposed to natural areas. However, 74% of constructed wetland boundaries were other mined lands with only 13% of all project boundaries natural. The majority of constructed wetland projects that share borders with other reclamation sites are not ecologically connected, or integrated. Since the reclaimed landscape is often a patchwork of reclamation projects in various stages of design, implementation and successional regrowth, it continues to be a real challenge to link reclamation projects and their natural ecological communities together in a cohesive regional habitat network. However, current approaches to reclamation and reclamation planning have improved these linkages to provide a habitat network.
4. What are appropriate indicators of wetland functions? Can they be measured?
Persistent constructed wetlands appear to be providing basic wetland functions to varying degrees. Some appropriate indicators of applicable wetland functions and methods of measurement include:
* Maintenance of water quality can be measured as the assimilative capacity of nutrient uptake capacity for nitrogen and phosphorus of the wetlands expressed as percent reduction in nutrient concentration between input and output waters. Dissolved oxygen, temperature, and pH represent easily measured field parameters that are critical for most aquatic life. Water quality monitoring programs can be very expensive, time consuming and often the results are inconclusive or misleading. However, if the assessment is specifically designed to track parameters of concern, the information obtained can be used to explain biological conditions and trends.
* Support of food chains can be assessed by sampling fish. Freshwater fishes are an important component of wetland systems, filling different trophic levels in the food chain ranging from primary consumers of detritus, algae and vegetation, through intermediate trophic levels such as predators on crustacea and aquatic insects, to top-level carnivores such as piscivores. Fish sampling methodologies can be selective for different species. Dip net sampling is a qualitative method that can be used to collect Poeciliids, Cyprinodontids and Fundulids. Breder traps will yield the greatest diversity of fish species and relative densities per unit effort and they perform efficiently in both open and dense vegetation with minimal habitat disturbance. Enclosure traps (throw traps), such as the Wegener ring, effectively samples species richness and provides statistically precise density information. These sampling methods can be used at different times of the year to provide data to indicate fish utilization of wetlands and thus habitat and support of food chains.
* Provision of habitat and wildlife utilization can be measured by conducting appropriate wildlife surveys. Semi-quantitative and qualitative studies can provide a reliable measure of this function if conducted at the proper time(s) of the year. Qualitative wildlife surveys along transects should include visual and audible observations for birds. An array of Sherman mammal traps can be used to quantitatively measure small mammals.
* Support of native plant populations can be assessed by evaluating community types within a wetland. An assessment of native plant communities can range from simple visual estimation of species composition and abundance to more exhaustive quantitative methods involving the use of various sizes of quadrats and transects. For simple, less diverse plant communities, visual estimation may be sufficient. However, when many species of similar, diminutive grasses, sedges and herbs are present more quantitative methods will increase the accuracy of the assessment.
* Maintenance of biological integrity of wetlands can be measured over time by documenting the species richness and relative abundance of aquatic macroinvertebrates inhabiting the wetland study area. Comprehensive semi-quantitative and qualitative sampling of all habitat types using two or more methods within a given wetland will reduce sampling bias and seems to provide the most information while containing costs.
* Maintenance of natural hydrologic regimes is critical to successfully establishing specific types of wetlands. Hydroperiod can be assessed by evaluating the balance between inflows and outflows of water the surface contours of the landscape and subsurface soil, geology and groundwater conditions. Wetland hydroperiod, frequency of flooding, duration of flooding and water depth can all be determined by installing and monitoring staff gauges an monitoring wells within each wetland type. Stream gauges should be installed and topographic surveys conducted across wetland cross-sections at gauge and well locations.
* Support of natural soil processes and development can be assessed by evaluating soil compaction, bulk density, organic matter (carbon) and nitrogen content, C:N ratio, available and total nutrient content, and cation-exchange capacity. All these parameters card be measured with existing techniques and methodology. However, decreases in bulk density and increases in organic matter accumulation should provide necessary and important information for evaluating the functioning of a wetland with regard to soil development.
5. How should success criteria be applied in evaluating the attainment of goals and of development trends for constructed wetland projects?
Reclamation goals should establish a landscape plan for an entire watershed; types and sizes of habitats, hydrologic pathways, topography, types and levels of functions to be provided. Success criteria should be measurable criteria used to assess the degree of goal attainment. "As built" surveys of wetlands and topography, and post-construction aerial photographs can be used to document the size and configuration of landscape features. Monitoring of constructed wetlands must adequately characterize and evaluate functions given the limitations of time, budget, type of wetland, size of wetland, context, degree of alteration from original wetland, location, and expertise of the investigator. Criteria most often used by regulatory agencies are quantitative measures of individual parameters that deal more with community development and how well the constructed wetland measures up to either a specific reference or "idealized" natural wetland. The use of reference wetlands in success criteria is problematic because most undisturbed, natural wetlands are well developed communities, with attributes and functions commensurate with age.
Long-term trends at the habitat and landscape level are more appropriate than short-term judgments of success. The act of removing the phosphate matrix and the separation of the clay component together tend to alter the geology and hydrology of the landscape in a permanent fashion. Alteration of natural hydrogeology does not mean that restoration of the mined landscape functions can not be successfully accomplished, but the concept of matching the pre-mined landscape with a created environment, duplicating the distribution and geographic positions of the natural landscape, should be avoided.
6. What are future research needs?
* Baseline information on wetland community development under natural conditions for wetland types in the phosphate mining regions for comparative purposes.
* A systematic evaluation of wetland progression should be done by careful selection of sites and sampling locations within sites to correlate vegetative growth and stand establishment with:
- compaction (penetrometer measurements), bulk density and organic matter content;
- substrate type (overburden, sand tailings, clay or mixtures thereof); and
- mucking treatments
* Vegetation nutrient concentrations need to be correlated with soil parameters to establish recommendations for soil amendments (organic and inorganic) and on substrate composition during wetland construction.
* Wetland construction practices such as compaction reduction, possibly by tillage, incorporation of organic matter such as natural muck and various types of composts and a starter application of fertilizer should be evaluated. The scope of the preliminary investigation of watershed hydrogeology in both natural and reclaimed basin conducted by the U.S. Geological Survey in 1993, should be expanded to sufficiently quantify all of the necessary parameters.
* Detailed investigations should be conducted with specific goals related to prediction of the hydrologic regime in portions of the landscapes created after mining. This would require in-depth investigations of both natural and reclaimed watersheds.
* Investigate reasonable semi-quantitative macroinvertebrate sampling protocols for streams and wetlands, both natural and constructed.
* Develop long-term data to evaluate long-term viability of constructed wetlands.
* Research related to invasive plant species, their persistence within reclaimed landscapes and their impacts, if any.
* Research related to understanding food webs, trophic structure and wildlife utilization of reclaimed landscapes.
* Investigate transpiration of reclaimed landscapes.
* Develop better indicators of success.
* Develop dynamic models of habitat quality as an aid in design and decision making.
* Document species distribution and abundance of mammals, fish, reptiles (in particular turtle species) in both natural and reclaimed habitats in phosphate mining regions.
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Last Updated: 07/31/2002
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