Landscape Diversity: Florida Phosphate Mine Pit Lakes1 M
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SL364 Landscape Diversity: Florida Phosphate Mine Pit Lakes1 M. Wilson and E.A. Hanlon2 Introduction and Purpose 2006). Many of the phosphate pit lakes in Florida were created before the advent of mandatory mine reclamation Phosphate is a critical nutrient for plant growth and an regulations. These lakes typically have relatively straight essential component of the fertilizer used in agriculture that shorelines with relatively steep shoreline slopes and may helps to sustain the world’s growing population. During contain islands of uncontoured mine spoil. 2010, seven mines in Florida produced approximately 10% of the world’s phosphate supply and more than 65% of As significant components of the post-mining landscape, the phosphate in the United States (USGS 2011). Florida these lakes serve multiple functions, including uses phosphate production depends on strip-mining, which for wildlife habitat, boating, fishing, water supply, and results in the excavation of thousands of acres annually. aesthetic value. A basic understanding of the limnology, Removal of the phosphate ore (matrix) from the ground water quality considerations, and ecological functions of causes a deficit in the volume of materials available for land mine pit lakes will enhance their potential for use. This reclamation. Restoring the landscape to near pre-mining document contains a synopsis of findings obtained through elevations necessitates the creation of lakes, and mine pit research funded by the Florida Institute for Phosphate lakes constitute significant components of post-phosphate Research (now the Florida Industrial and Phosphate mining landscapes. In 1975, Florida passed laws requiring Research Institute) contrasted with limnological studies mandatory reclamation of lands mined for phosphate of natural lakes in central Florida. The audience for this (FDEP 2006). All lands mined after June 30, 1975, are document includes reclamation planners, government subject to mandatory reclamation requirements to return regulators, non-governmental organizations, and managers the landscape to a beneficial use following mining. of phosphate mine pit lakes. This document will help them quickly become informed of the published research and The mine reclamation rules acknowledge the necessity of characteristics of the phosphate mine pit lakes in Florida. creating lakes and prescribe criteria for their reclamation. Reclaimed phosphate mine pit lakes must be contoured to no steeper than 4 feet horizontal to 1 foot vertical to at Natural Florida Lakes Summary least 6 feet in depth below the seasonal low water line. At Most natural Florida lakes formed from or were influenced least 20% of the lake area is required to be less than 6 feet in substantially by dissolution of limestone. Natural Florida depth to provide shallow zones for fish bedding. Reclaimed lakes are generally shallow (<10m deep), have smooth lake shores are required to be constructed such that bottoms, and are usually mixed by wind-action (Boody seasonal fluctuations in water levels result in annual drying et al. 1985a; Shannon and Brezonik 1972). Most do not and wetting of at least 25% of the lake surface area (FDEP experience stable thermal stratification and development 1. This document is SL364, one of a series of the Department of Soil and Water Sciences, UF/IFAS Extension. Original publication date May 2012. Reviewed September 2018. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication. 2. M. Wilson, graduate student; and E. A. Hanlon, professor, Department of Soil and Water Sciences, UF/IFAS Southwest Florida Research and Education Center; UF/IFAS Extension, Gainesville, FL 32611. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county’s UF/IFAS Extension office. U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension. of depressed oxygen concentrations in the bottom waters (anoxic hypolimnia), though many do (Canfield and Hoyer 1988; Shannon and Brezonik 1972). In terms of biological productivity, the majority of natural lakes in Florida could be classified as mesotrophic (intermediate level of biological productivity) to eutrophic (high level of biological produc- tivity) (Canfield and Hoyer 1988; Shannon and Brezonik 1972). The high level of biological productivity caused by eutrophic conditions can lead to water quality problems. Severe algal blooms resulting from eutrophic conditions can impact water quality and decrease the value of lakes for fishing and recreation. Because eutrophic conditions are Figure 2. Phosphate mine pit lake during reclamation. Water levels in often problematic for water quality, management of most this lake are expected to increase by 5–10 feet following reclamation. Florida lakes is focused on attaining a trophic state closer to The slopes in the foreground have been contoured to reclamation standards. The slopes in the top right have yet to be contoured to the mesotrophic range. reclamation standards. Note the flattened platform (middle left), which will be a fish bedding/littoral zone. The inundated areas are the Typically, biological productivity in freshwater systems is mine pit bottoms and will become the deeper pockets in the lake. limited by the availability of phosphorus. However, many Credits: M. Wilson, UF/IFAS Florida lakes are nitrogen-limited, indicating that sufficient phosphorus is present in the system (Canfield and Hoyer 1988). Phosphate Mine Pit Lake Morphology In contrast to natural Florida lakes, the shorelines and bottoms of phosphate pit lakes are typically more irregular. Phosphate pit lakes also generally have a smaller surface area and less “shallow zone” than most natural Florida lakes. Most phosphate mine pit lakes are similar in mean depth to natural Florida lakes (3 m–4 m), though their maximum depth is usually substantially greater (Boody et al. 1985a; Boody et al. 1985b). Figure 3. Phosphate mine pit lake (R-14) under construction in 1994 Figure 1. The future shores of a phosphate pit lake during reclamation (top) and after reclamation (bottom). The spoil rows in the 1994 photo to contemporary reclamation standards. Slopes on the lake exterior typically remain following reclamation and are a common bathymetric can be no steeper than 4H:1V (4 feet horizontal to 1 foot vertical) to at feature of phosphate mine pit lakes. The areas without spoil rows will least 6 feet below the water’s surface during seasonal lows. become deeper pockets, resulting in a highly irregular lake bottom. Credits: M. Wilson, UF/IFAS Credits: M. Wilson, UF/IFAS Landscape Diversity: Florida Phosphate Mine Pit Lakes 2 Thermal Stratification and generally lower in phosphate mine pit lakes than in natural lakes in central Florida, and the waters in phosphate mine Development of Hypoxic pit lakes are generally neutral to slightly acidic. The pH of Hypolimnia phosphate mine pit lakes appears related to DO concentra- tion, which is consistent with what occurs in natural lakes Most phosphate mine pit lakes develop at least some (Rampenthal and Ferraro 1987). Alkalinity (a measure of stratification with regard to temperature and dissolved water’s buffering capacity, or capacity to neutralize acidity) oxygen (DO) (Boody et al. 1985a; Boody et al. 1985b). is generally estimated as the concentration of calcium carbonate (CaCO ) in milligrams per liter (mg/L). Lake Dissolved oxygen is necessary for aerobic aquatic organ- 3 systems with higher alkalinity typically are more resistant isms to survive. The main source of lake dissolved oxygen to changes in pH (Cole 1994). Alkalinity in phosphate mine is from photosynthetic activity in the photic zone, though pit lakes is typically higher than the minimum standard of diffusion from the atmosphere also contributes a substan- 20 mg/L and is generally slightly higher than natural lakes tial amount of DO (Cole 1994). The lake water is mixed in central Florida (Boody et al. 1985b; Pratt et al. 1985). primarily by wind-action, which distributes the dissolved This finding means that wide pH fluctuations in phosphate oxygen throughout a portion of the lake. Seasonal stratifica- mine pit lakes should not be a concern. tion of temperature and dissolved oxygen is common for the majority of lakes and reservoirs in the southeastern Hardness (a measure of mineral content in water, gener- United States. Stable thermal stratification and develop- ally calcium and magnesium) is also estimated as the ment of depressed dissolved oxygen concentrations in the concentration of CaCO in mg/L. While it is not harmful to bottom waters are not common in natural Florida lakes 3 human health, hard water can be problematic for industrial partly because of their generally shallow nature (Shannon and residential uses. Hardness in the phosphate mine and Brezonik 1972). Stratification in phosphate mine pit pit lakes is generally higher than in most natural lakes in lakes is likely not a function of depth