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Best management practices for water quality improvement in the Lake Okeechobee watershed
ECOLOGICAL ENGINEERING ELSEVIER
Ecological Engineering 5 (1995) 341-356
Best management practices for water quality improvement in the Lake Okeechobee Watershed * A.B. B o t t c h e r a, *, T e r r y K. T r e m w e l b, K e n n e t h L. C a m p b e l l c a Soil and Water Engineering Technology, Inc. 3443 N W 12th Aue., GainesciUe, FL 32605, USA b Applied Technology and Management Gainescille, Florida USA ¢ Agricultural Engineering Dept., Unicersity o f Florida Gainescille, FL 32611, USA
Abstract A significant amount of work has been done in the Okeechobee/Everglades Dept., in south central Florida to understand and control the nutrient inputs to Lake Okeechobee, the Everglades, and surrounding estuaries. Phosphorus (P) control has been the primary focus of these studies because P has been found to be the limiting nutrient for the accelerated eutrophication of these aquatic systems. The watersheds of these systems are dominated by beef and dairy operations north of Lake Okeechobee and vegetables and sugarcane south of the Lake. As a result, most of the research activities have focused on agricultural transport processes and control practices (known as best management practices or BMPs) for P discharge. This paper focuses on the BMPs that have been developed for agricultural water quality control in south Florida. BMPs are discussed from the perspectives of conceptual operational theory and field evaluation data, as well as practical in-field design constraints. An itemized listing of the proposed BMPs is also presented, as well as a discussion of implementation strategies.
Keywords: BMPs; Okeechobe Basin; Phosphorus
1. Background Fishermen's complaints of excessive aquatic weeds in Lake Okeechobee during the late 1960s and early 1970s prompted eutrophication studies of the Lake. These
Paper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the South Florida Water Management District and the University of Florida, Institute of Food and Agricultural Sciences. " Corresponding author. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 5 - 8 5 7 4 ( 9 5 ) 0 0 0 3 i - 3
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studies, using the Vollenweider model (Brezonik and Federico, 1975 and Davis and Marshall, 1975), found the lake to be eutrophic and potentially on the verge of becoming hypereutrophic due to excessive phosphorus loading. These results prompted extensive monitoring of the lake and its nutrient inputs. Several other studies were also initiated including additional modelling efforts of the lake itself as well as land-based nutrient transport process studies (many of which are presented in this special issue of Ecological Engineering). In general, these studies concluded that the primary source of P to the lake was the dairies and beef operations north of the Lake and the sugarcane/vegetable lands (Everglades Agricultural Area, EAA) south of the Lake. Since the EAA was being backpumped into the Lake (note that historic flow in the EAA was to the south from Lake Okeechobee to the Everglades), these pumps became an easy P input control point to the Lake and were turned off (Interim Action Plan, SFWMD, 1978). This, of course, forced the additional EA.A drainage water into the Everglades, which has caused problems there. The dairies (approximately 35000 head) were found to be the single most intensive source of P in the Lake's watershed (Federico et al., 1981) and have since come under a Dairy Rule regulation (FDER, 1992) to implement specific BMPs, which will be discussed in greater detail. Though dairies were considered the "hottest" source of P, beef operations occupy the majority of the land and have also come under regulation (SFWMD Regulatory Program, Chapter 40E-61.020) through the implementation of the Okeechobee SWIM Plan (SFWMD, 1993). As dairy P loads are being reduced, beefland's relative contribution of P has become relatively higher, thereby attracting greater attention. The SWIM water quality regulation does not specify the BMPs to be used as did the Dairy Rule, it simply requires a water quality discharge standard to be met (0.35 mg/l P). All other agricultural operations also have to meet specific P discharge standards. In order to meet the above mentioned regulations, land-use-specific BMPs have been and are still being developed by IFAS (Institute of Food and Agricultural Sciences, University of Florida) and the South Florida Water Management District. This research and specialist expertise have resulted in the recommended BMPs presented in this paper.
2. Basin description
The Okeechobee/Everglades basin's physical system is well described in several other references (Bottcher and Izuno, 1993). Historically, Lake Okeechobee (2000 km z) received drainage water from the north via Kissimmee River (3500 km2), Taylor Creek/Nubbin Slough/Lettuce Creek (560 km2), and Fisheating Creek/other (1100 km 2) basins. The Lake would periodically overflow its southern shore and drain south through the Everglades or would flow westerly down the Caloosahatchee River. Note that approximately 50% of the water input to and output from the Lake is from direct rainfall and evaporation, respectively.
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Significant physical modifications have been made to the Lake and its drainage basins as a result of channelization, diking, and pump station installations. These structures have allowed the lake stage to be regulated for water storage and flood protection with regulated discharge now being possible through up to six separate structures. The surrounding basins' channelization has allowed much quicker flow to and from the Lake and in non-historic directions. Therefore, inputs and outputs of water and nutrients have been significantly altered and are blamed for much of the degradation of Lake Okeechobee and the Everglades (SFWMD, 1989 and SFWMD, 1992). The lands north of the lake are predominantly sandy spodosols, while the soils to the south are Histosols. Land uses in the sand lands north of the lake are 65% beef, 15% native, 3% dairy, 5% citrus, 4% urban, and 8% misc (Allen et al., 1982). The Dairy Rule and recreational growth have changed the land use patterns in the area with citrus and urban acreage increasing while dairy and beef acreage is decreasing. Land uses on the Histosols are 80% sugarcane, 12% vegetables, and 8% other (rice, sod, nurseries, urban, etc.)
3. Definition of a B M P
Best Management Practices (BMPs) are those on-farm activities designed to reduce nutrient losses in drainage waters to an environmentally acceptable level, while simultaneously maintaining an economically viable farming operation for the grower. Practices which have a high potential for negatively impacting the financial profitability of a farm should not, therefore, be considered BMPs. In cases where the economic cost of implementing certain BMPs puts an excessive financial burden on the farmer, such practices should be considered BMPs only if external funds are available to return an acceptable level of profitability to the farm. In this paper, only BMPs for P control will be discussed because they are most critical for environmental protection of the region.
3.1. Setting of discharge standards BMPs are designed to bring the discharge levels of a contaminant into compliance with downstream water quality standards, which can either be concentration a n d / o r total load based. In the case of Lake Okeechobee the discharge standard for P is concentration-based, whereas the standard for the EAA is load-based. These standards were set based on the needs of the receiving water body, feasibility of obtaining water reductions, and the relative efficiency of serial components of multiple treatment systems, such as the agricultural BMPs and stormwater treatment areas (STAs) in the EA.A. Phosphorus load reduction regulations are more difficult to implement because they require both water flow and P concentrations to be monitored. Dual monitor-
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ing requirements result in potentially higher measurement errors and added cost as compared to concentration standards alone. However, some questions concerning the flow proportionality of samples collected for concentration measurements alone come into play, if flow is not measured. Therefore, some flow measurement may be needed to supplement concentration-based standards. BMPs to reduce P loadings must either reduce P concentration or water volume. In the sand lands, water reductions are difficult and costly, so P-standards in these areas are concentration based. However, in the EAA significant on-farm storage is feasible and most farms have monitorable pumped discharges, allowing the more effective use of a load-based standard. 3.2. Methods to reduce P concentrations
Conceptually, there are only three ways to reduce P concentrations in the runoff water from agricultural lands. They are: 1. Reduce the amount of phosphorus on the farm by minimizing P inputs to the farm and maximizing non-runoff P outputs from the farm. 2. Reduce the hydraulic mobility of the P that is on the farm by limiting water contact a n d / o r reducing the solubility or erodibility of phosphatic materials. 3. Edge of field/farm pre-discharge treatment using uptake, adsorption, deposition, or precipitation technologies, such as wetlands a n d / o r chemical additives. 3.3, Methods to reduce discharge ~'olume
Reducing runoff volume is straight-forward conceptually, but can be difficult to achieve in practice. Runoff volume reduction can only be obtained by two methods. They are: 1. Increase the evapotranspiration (ET) from the farm. 2. Decrease off-farm or groundwater irrigation water inputs to the farm by improved irrigation efficiency or by using storage runoff as a substitute irrigation supply. The sand lands have limited irrigation, have very few water control structures, exhibit soil wetness (high water tables), and have limited water storage areas, all of which make hydraulic reductions impractical. However, in the EAA the irrigation inlet and pumped outlet structures are ideal for optimizing on-farm water use and thereby reducing discharges. Getting the farmers to understand the principles behind BMPs will make the design and acceptance of a farm-level BMP program much easier while significantly improving the farmers' operation and maintenance of the practices. The source of P for potential runoff is the starting place of understanding the process and therefore will be presented in general terms before the specific BMPs are described. Presentation of the biogeochemical processes affecting P transport and the impact of BMPs on P retention is beyond of the scope of this paper, but are well covered by Campbell et al. (1992, 1993) and Graetz and Nair (1995) in two major project reports for the SFWMD.
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4. Sources of P to be controlled
Phosphorus in agricultural drainage waters originates from one of four sources: fertilizers, animal manures, mineralization of native organic materials, or atmospheric deposition. These sources of P must be available if agriculture is to exist in south Florida, and the relative importance of each source depends on the plant/soil system. Atmospheric deposition (1-2 kg ha -t yr - t ) is important only in native areas. The other three sources can vary significantly in amount, but are controllable by proper management or BMPs. As in most situations, problems occur when things are done in excess. Phosphorus management is no exception. If plant available P exists in a soil/plant system beyond what the plants can utilize, then the excess P readily moves by water in south Florida soils. Therefore, the conceptual management approach is to keep the P sources in balance within the soil/plant environment. Control of P sources is particularly critical in south Florida as compared to other locations because of the low P sorption properties of the native soils. The sandy soils in the area have low clay, iron and aluminum contents in the surface horizons which greatly limits P sorption. The organic soils south of the Lake also have relatively low P retention properties. In many of the regional soils, P is a mobile nutrient, similar to nitrate.
5. Phosphorus BMPs for the Okeechobee/Everglades basin
The following BMPs are the result of numerous studies in the Okeechobee/Everglades basin as well as generally accepted practices from other parts of the country. Where possible, the specific study or basis for the BMP is referenced. The BMPs are presented by P source and land use.
5.1. Fertility BMPs The following fertility BMPs are applicable to all crops in the basin with a few noted exceptions. Specific fertilizer recommendations are crop-dependent and are available from the local Florida Cooperative Agricultural Extension Service. Fertility BMPs will be most effective on rowcrop and other high value crops which presently represents only a small percentage of basin. However, significant vegetable and sugar cane acreage in the organic soil basins south of Lake Okeechobee and the renewed interest in vegetable farming on the sand lands north of the lake warrant their discussion. Calibrated soil testing (CST). Calibrated soil testing is the procedure by which the actual crop response to supplemental fertilizer is determined as a function of the P level measured in the soil prior to fertilization. Laboratories providing CSTs can then provide fertilization recommendations. Refinement of the CSTs has been a high research priority. For example, the recent study by Rechcigl and Bottcher
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(Rechcigl et al., 1992) resulted in a 50% reduction in P recommendations for pasture grasses with no expected yield reductions. Banding of fertilizer. The banding of P, limiting application to the rooted area of a crop, can reduce fertility needs significantly. Sanchez et al. (1990, 1991) found that banding in some vegetable crops can reduce fertilizer application rates by 50%. lzuno and Bottcher (1991) further found that banding reduced P losses to water by as much as 20-30%. However, banding is effective only for those crops with a limited root area, such as vegetables, citrus, and other row crops. Preuention of misplaced fertilizer. Prevention of spills and the direct spreading of fertilizer into open drainage ditches can reduce P losses significantly. Because it takes as little as 0.4 kg P/hectare in drainage waters to exceed standards, it is critical to prevent any concentrated spills which can be washed into nearby streams. Also, areas of intensive ditching, as found in the EAA and citrus areas, are most vulnerable to direct fertilizer applications into the ditches due to their frequency and close proximity to the crops. Once P is in surface waters there are less effective processes for removing it than in the soil/plant system. Use of side-throw fertilizer spreaders along drainage ditches or appropriate spacing of drive lanes to prevent spread fertilizer from reaching ditches are necessary practices. Turning off the spreader at the end of the field is also important to prevent overlapping of fertilizer during turns. Aerial applications are more difficult to control but can also be better controlled by proper flagging and pilot training. Split applications. Split application (multiple applications of P during the growing season) of P fertilizers and the use of slow release forms of P fertilizers have limited application for field crops. Only under special conditions such as intensive vegetable production or sod production would split applications of phosphorus need to be considered, and these conditions would normally only require a single split application. Slow release forms of phosphorus, such as rock phosphate are normally inefficient for providing plant needs. Therefore, split application and slow release phosphorus forms would have limited applicability, except for the special cases mentioned above. For these special cases, phosphorus losses could be reduced anywhere from 0 to 5%. Split application and slow release techniques are much more applicable to nitrogen fertilization on mineral soils. For a general discussion of nitrogen and other fertility topics, please read IFAS Circulars 816 (Bottcher and Rhue, 1983) and 817 (Hanlon et al., 1990).
5.2. Animal manure BMPs Animal manure BMPs, as indicated earlier, are based on the concept of spreading the manure on crops at a rate that is in nutrient balance as determined by the CST procedure also described earlier. Therefore, good manure management requires a farm to have sufficient cropland to handle the manure load it generates. In cases where the animals are confined, a physical collection and distribution system is needed to deliver the manure to cropland evenly. If over-
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grazing, uneven grazing, or a poorly designed collection/distribution system exists, then "hot spots" having excessive manure loadings will occur. These "hot spots" are areas needing special attention, because, for example, 10 m 2 of a high intensity area (HIA) (animal density sufficient to cause bare ground conditions) can have a P load equivalent to that generated from 1 hectare of improved pasture, which is about 1000 times greater on a per unit area basis. Note that a well-managed improved pasture will just meet the current P discharge standard as found by Rechcigl (1992). To effectively manage any manure utilization system, knowledge of the actual P content is essential. This allows for the balancing of nutrients in the receiving soil/plant system. Manure characteristic data are available (ASAE, 1993), however collection and storage techniques can significantly vary nutrient contents. Therefore, a good manure testing program is needed. Such testing programs in north Florida and Lancaster, Pennsylvania have proven very effective in better balancing nutrient applications to crops. The following specific manure BMPs apply primarily to dairy and to a lesser extent to beef cattle operations. The dairy related BMPs are currently required under Florida Department of Environmental Protection "Dairy Rule", Chapter 670 of DEP Administrative Code. Note that these rules apply only in the Okeechobee basin. Dairy HIA drainage control. The high water table soil conditions in the region prevent deep percolation, therefore 100% of the surface and ground water from the HIA can be captured by constructing a perimeter drainage ditch around the HIA near the dairy barn. The collected water can then be pumped to a storage pond for later delivery to cropland, via an irrigation system. This is essentially a recycling system if the crop is feedstuff for the cows. Gunsalus et al. (1992) reported that this BMP has brought over fifty percent of the dairies in the Okeechobee basin within compliance and the remaining dairies, with only a couple of exceptions, exhibiting a trend toward compliance. The dairies where this BMP appears not to be working, appear to have historical accumulations of manures not directly addressed by the BMP due to not recognition of old HIAs. Collection and distribution of barn manure. Manure that is deposited on impervious surfaces, such as concrete, in and around barns must be collected, stored, and delivered to cropland in a controlled fashion. Deposited manure can be either flushed to storage ponds or anaerobic lagoons or scraped onto concrete storage pads. The stored manure must be spread on crops by either a mechanical spreader (scraped manure) or an irrigation system (flushed manure). The appropriate rate of P application to the crop would best be determined by the CST procedure as if manure was a fertilizer source of P. However, the need to size manure application areas for P loads without knowledge of future soil test information the USDA Soil Conservation Service in association with the University of Florida developed manure P loading rates based on estimated long-term crop uptake of P. This assessment resulted in loading rates of 50 and 67 kg P ha- t yr- ~ for pastures and manure sprayfields (forage crops), respectively, being approved. Watering, feed, and shade facilities placement. These facilities will normally cause
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small HIAs or "hot spots" to form around them. These areas can be controlled by the following procedures which are listed in order of greatest control: 1. Move all watering, feed, and shade facilities into an area which has a manure collection and distribution system, such as barn or dairy HIAs. 2. Move these facilities frequently enough to prevent the formation of a HIA. 3. Place these facilities in upland locations where maximized overland flow distances can significantly reduce P in runoff (see a later section entitled Maximizing flow distances for P control). Feed ration control. Minimize the P in the animal feed. Work by Morse et al. (1992) has shown that P concentrations in feed rations may be reduced by as much as 30% in some cases while maintaining good milk production. Fencing animals from ditches and streams. Soil and plant assimilation of P is obviously limited when direct application of manure is allowed to a flowing stream or areas that hydraulically flush periodically. This possibility should be eliminated by appropriate fencing. Also, manure deposited near or on stream banks has a higher potential for transport to the stream, especially when the animals disturb bank vegetation and stability. Therefore, fences should be placed at least fifteen feet from the top of the ditch/stream bank, because banks are often ill-defined and storm flows occasionally spread to the floodplain. Also, there must be sufficient space to get harvesting and maintenance equipment along the bank. Harvesting of vegetation within the buffer will improve its P removal efficiency. Grazing management. A critical factor in reducing P in runoff is to keep animal densities on pastures at levels that maintain appropriate nutrient balance. Table 1 shows the relative effects of grazing densities on runoff P concentrations as estimated by the authors based the review of water quality data (various sources) from various tributaries of known land use and unpublished modeling work using CREAMS. Also, grazing patterns or field rotation can be used to maximize P uptake by the animals and minimize HIA formation. Rotational grazing has been shown to be effective for improving forage quality and uptake (Adjei et al., 1987; Mislevy et al., 1991) for beef cattle in south Florida. Select high P uptake crops for manure application areas. Crops vary substantially in their P uptake potential, which means higher P uptake crops will require less
Table 1 Estimated runoff P concentrations by animal density on land Land area Native Pastures Native range Semi-improved Improved Intensive Holding areas HIAs
Animal density (cows/ha) 0 0.025-0.50 0.50-1.2 1.2-2.5 2.5-5.0 5.0-25.0 25.0-250.0
Conc. (mg/l) 0.04-0.20 0.08-0.25 0.10-0.30 0.25-0.50 0.35-1.20 0.50-30.0 30.0-900.0
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acreage for spreading the manure (SCS, 1992). A balance between usability of the crop and cost saving of a smaller land application area will be required. Composting. Composting is an alternative manure management approach which converts the manure to a marketable soil amendment (SCS, 1992). If markets are available, then composting is an excellent choice for operations with limited land for spreading the manure on-site.
5.3. General BMPs Crop management. Profitability and nutrient runoff control are both dependent on a healthy/productive cropping system to assure maximum P uptake. Therefore, proper crop management programs, including cultural, water, nutrient, and pest (insects and weeds) management, are needed on every farm. Irrigation and drainage management. Irrigation and drainage system design and management control the water status of the crop root zone, as well as the rate and quantity of water leaving a field. Proper irrigation provides optimal P uptake conditions, while excessive irrigation significantly increases leaching/runoff and related P losses. Proper drainage is needed to assure optimal moisture conditions during wet periods, while excessive drainage can cause drought conditions and increase aerobic organic matter decomposition with its associated P release. In the basin drainage and irrigation are typically achieved by controlling the shallow water table by using ditches and canals to move water to and from the field. Therefore, appropriate water management is achieved by a well designed and managed system of water conveyance and control structures, such as ditches, canals, weirs, culverts, and pump stations. Maximize flow distances for P control. Table 2 shows the relative potential assimilatory capacity of various flow conditions, going from overland to channelized flow. The presented values are rough estimates made by the authors based
Table 2 Estimates of % phosphorus removal efficiency in flow systems over 150 m Area
High P source (100 ppm)
Low P source (1 ppm)
Overland Woods Grass Native Improved pasture
40
5
80 80
l0 3
Field ditches Bare Grass Native Improved pasture
5
1
10 10
2 0.5
Streams /canals
0.5
0.1
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upon related overland flow studies for municipal waste treatment and observed stream reach data. Therefore, Table 2 is not intended to provide absolute assimilatory capacities, but to show the relative advantages of different types of overland flow. The relative impact of vegetation is also presented in Table 2. As can be seen, the further upland a high P source exists, the greater the P reduction. This illustrates the benefits of buffer strips between high source areas and stream systems. Phosphorus removal efficiencies are affected both by the P concentration of the source runoff and the characteristics of the land it passes through as also indicated in Table 2. Higher P concentrations typically afford a higher percent reduction, while more intensively managed land use areas offer less treatment as water passes through it due to higher in situ P conditions. Flow-way buffer strips. The off-site effects of a high-P-source are intensified by its proximity to a flow-way. A vegetated buffer area between this P-source and the flow-way may partially mitigate these effects. Though P assimilation will occur in the buffer as water passes through it, the primary benefit of the buffer strip is to assure that no high-P-source area occur near the flow-way. The effectiveness of removing upslope P is limited by the width of the strip as seen in Table 2. Also, for continuous treatment of upslope sources, buffer strips must be maintained and harvested to be fully effective. Limit Drainage of Organic and/or Wetland Soils. As indicated earlier, drainage (aeration of soil) can increase mineralization of the organic matter. Because wetlands typically have high organic contents, their drainage can lead to excessive P release during aerobic mineralization. The organic soils south of Lake Okeechobee can mineralize between 20-80 kg h a - t yr-~. The management goal for organic soils is, therefore, to maintain the highest possible water table (soil moisture condition) while keeping a productive crop, if cultivated. Alternatiue Land Use. In limited situations it may be necessary to change the land use in order to meet regulatory constraints. This could involve a minor or a complete modification of the farming operation. For example, converting a dairy to a cattle ranch or vegetable production to sugar cane in the EAA. If there is an economic advantage (which was the case for many dairies in the Okeechobee basin because a government dairy buy-out program), then it should be considered.
5.4. Edge-of-field/farm treatment Edge-of-field/farm treatments have a strong appeal in that they allow farming operations to be less constrained. However, these can be more expensive, so trade-offs between flexibility and cost should be evaluated before considering these systems. The principal edge-of-field treatment technologies that should be considered are presented below. Runoff Retention/Detention System. The storage of runoff water in a retention/detention area can reduce net runoff by either increased evaporation rates or by recycling the stored water to the cropland to reduce other irrigation water supplies and increase crop ET. The storage ponds can also assimilate P by vegetative uptake and sediment deposition and adsorption, however long-term P
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assimilation would be limited in a size optimized retention/detention ponds (Taylor, 1987; Burleson, 1988). However, if these ponds are managed as wetlands (discussed in next section), which would typically require the ponds to be much shallower and wider to achieve effective plant to water volume ratios, then higher P assimilation could be achieved. Also, if storage ponds are allowed to de-water through soil seepage, then much higher P adsorption to soils can be expected. Use of Wetlands. Wetlands which are managed for high vegetative growth and organic debris accumulation can remove significant amounts of P if the following conditions are met (Hammer, 1989): 1. Retention times and flow rates are managed to allow sufficient time for plant uptake and no storm flushing to occur. 2. Keep the wetland wet, i.e. prevent dry-oUt that can release accumulated P. Chemical Treatment. These systems involve the addition of chemicals that will precipitate a n d / o r flocculate P compounds so that they can be easily settled from the water. Chemical treatment would include chemical injection and deposition (pond, wetlands, etc.) components. Chemical treatment can be very effective, but expensive.
5.5. EAA-Specific BMPs The above fertility and water management BMPs also apply directly to the EAA, but the unique nature of the fiat organic system requires site-specific considerations in BMP design. EAA-specific BMPs are discussed in detail by Bottcher and Izuno (1994).
5.6. Summary of BMPs Table 3 is a summary table of the BMPs discussed. The table has additional information relating to the crops that specific BMPs apply to and the relative reduction potential of the BMPs. However, in using these reduction ranges, it is necessary to understand both what they represent and their uncertainty. First, only a few of the listed BMPs have been field tested and even those were tested for only a limited set of conditions. Therefore, most of the stated phosphorus reduction ranges are based on corollary data and the authors' basic knowledge of the physical and chemical processes of the region. The presented BMP effectiveness (% phosphorus reduction) ranges include this uncertainty. They also reflect the variabilities of existing conditions among farms. That is, farms implementing a BMP for the first time can expect to experience the full benefit of that BMP, whereas those farms already practicing a specific BMP should expect no additional phosphorus reduction due to continued implementation of that BMP. Again these ranges should be considered only as a guide as to relative beneficial impact with little wait given to their absolute values. Additional research will be needed to further narrow these very broad reduction ranges.
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Table 3 Summary of Okeechobee Basins BMPs BMP code/name
P reduction range ~ (%)
Crop b
Fertility BMPs Calibrated soil testing Banding of fertilizer Misplaced fertilizer Split appl. of fertilizer
0-50 0-40 0-15 0-10
SC, V,C.P,F, RC V,C, RC All All
60-95 10-90 5-80
D.F D,F D,P
Manure BMPs Dairy HIA drainage control Barn manure handling Watering, feed, and shade facilities placement Feed ration control Fencing ditches and streams Grazing management Select high P uptake crops Composting General BMPs Crop management Irrig. and drain, management Buffer strips Limit drainage of wetlands Use of wetlands Chemical treatment Alternative land use E,AA specific BMPs Minimizing water-table Fluctuations retention of drainage on-farm Retention of vegetable field Drainage water in sugarcane or fallow lands Use of aquatic cover crops Coordinated farm cropping patterns
0-15 0-40 0-20 0-25 0-100
D,P P P D,P D
0-40 0-30 5-40 0-20 0-50 50-90 0-60
All All D,P,RC.F,V W All D,V D,V,RC,P
0-50 15-60 20-90
All Sugarcane Vegetables
5-20 n /a
All All
a Ranges are for individual farms after considering uncertainty and the variability of farm management unless otherwise noted. b SC = sugarcane, V= vegetable, C = citrus, P = pasture, D = dairy, RC = row crop, S = sod. W= wetland, and F = forage.
6. Implementation strategies for BMPs T h e d e v e l o p m e n t o f B M P s for P c o n t r o l in the O k e e c h o b e e / E v e r g l a d e s basin d o e s not in itself get B M P s i m p l e m e n t e d o r i m p r o v e w a t e r quality. I m p l e m e n t a tion r e q u i r e s f a r m e r s to clearly see th e b e n e f i t s o f B M P s for t h em . F a r m e r s must a c c e p t the B M P s as a n e c e s s a r y p a r t o f t h e ir o p e r a t i o n . A c c e p t a n c e is not always easy to obtain, but t h e r e are t h r e e g e n e r a l ways to a c h i e v e a c c e p t a n c e / a w a r e n e s s in t h e a g r i c u l t u r a l c o m m u n i t y . T h e y are: 1. P r o v i d e the f a r m e r s with sufficient, s u p p o r t a b l e e v i d e n c e that the B M P will i m p r o v e t h ei r f a r m i n g o p e r a t i o n . ( V o l u n t a r y )
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2. Provide the farmers funds to entice them to implement the BMPs. (Incentive) 3. Make it the law that they must implement BMPs. (Enforcement) These are listed in order of palatability to farmers. However, each approach may achieve the desired water quality goals, but at varying rates and cost-effectiveness. Each of these strategies have been tried in various parts of the country and in Florida with varying degrees of success. Typically, the voluntary approach has been accused of not working as well as the incentive or regulatory approaches, but this was likely due to poor knowledge bases at the startup of these programs to gain acceptance to the BMPs. On the other side, once better knowledge bases were developed governmental agencies felt more confident in developing incentive or regulatory programs. Choosing the appropriate BMP implementation approach requires an understanding of the true cost-effectiveness of individual BMPs and their on-farm operational impacts, so that the actual economic/operational impacts to the farmer can be assessed. If this assessment shows that certain BMPs are economically beneficial and operationally compatible with current conditions, then the first approach is all that is needed. However, if the assessment shows the BMPs are operationally compatible but potentially cost prohibitive, then the incentive approach is needed. Finally, if the BMPs are found to be economical, but not operationally compatible, then the regulatory approach is usually best. In all of the above cases, both the farmers and government agency staffs must be fully educated as to BMP responses before acceptance of any approach can be achieved. The bottomline is that we must articulate to ("educate") all parties, as much as possible, the costs/benefits of a BMP program. A problem often faced is that the BMPs' effectiveness and costs are not well known and will vary significantly from farm to farm. For this situation, agreement can only occur if all parties have similar, detailed understandings of the state-of-the-art of BMPs and have a willingness to compromise so that the programs match our knowledge base. The real question becomes, "How can we best educate all parties?". Often our inability to educate is because we do not know exactly what a BMP will do. This clearly points out the need for more research and always avoiding the situation of demanding (regulating) BMPs which scientific knowledge can not fully justify. The complexity and variability of P transport processes and their relationship to BMPs will require innovative research and educational approaches to gain mutual understanding. Obviously, research to describe the basic processes and science is essential, but getting this information in a coherent form that farmers can understand is also essential. Farmers must be able to say "Yes, I see what is going to happen to me!" so that they can make reasonable decisions. If appropriate knowledge is not available, then none of the above approaches will work. Standard extension workshops and publications describing BMP programs are helpful. However, getting complex information to the farm-level will require more sophisticated approaches. Farmer-level education programs must answer more specific questions such as actual equipment purchases, degree of earth work, labor needs, maintenance cost, farm profitability, and expected water quality improvement.
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Computerized BMP expert systems have a high potential to provide individual farmers with the information they need to make decisions. These systems will need to expertly ask the farmer site-specific information, so that this information can be combined with other data sources to simulate the total economic/operational impact of various BMP scenarios. Optimization for individual farmer needs can then occur. The WQFARM (Bottcher, 1987), LOADSS (Negahban et al., 1993), and Interactive Dairy Model (Negahban et al., 1993) provide a good framework for evaluating the water quality impact of various BMPs in the region, but will need to be expanded significantly to become true expert systems.
6.1. Monitoring It important to note that any effective BMP program will require monitoring of both the implementation of BMPs, as well as their impact on water quality. As mentioned earlier, monitoring costs are a function of the type of BMP implementation program, where the regulatory programs have the highest monitoring cost and the voluntary programs have the lowest cost. The authors estimate that for a discharge standard based regulatory program, the annual water quality monitoring cost for individual farms will be between 5 to 50 US$ ha-1 yr-t depending on the size and type of operation. This would not include any permit cost and additional record keeping and reporting costs. A performance based standard (proof of implementation and maintenance of BMPs, only) on the other hand would eliminate water quality monitoring costs, but could increase record keeping costs. Currently, much of the monitoring cost in the basin has been paid by the SFWMD through its compliance program in association with the Dairy Rule. This is effectively a cost-share program. SFWMD's monitoring program, in addition to the state's cost-share program for BMP implement on the dairies has allowed the discharge standard based regulatory approach to be workable. However, without these cost-share programs, the success of the Dairy Rule would have been unlikely.
7. Conclusions
Our current knowledge of BMPs is sufficient to develop effective basin-wide P control programs for working toward meeting the water quality standards set by the State of Florida as show by the success of the Okeechobee Dairy Rule. However, additional research is needed to better refine actual cost and how much additional water quality improvement can be achieved. The cost of effective BMP programs for individual farms will vary greatly, leaving some farmers short of resources to fully implement the required BMPs. In these cases, an incentive or cost share program will be necessary. Acceptance of the benefits of a BMP program by all involved parties (farmers, government staffs, environmentalists) is critical to their success. Approaches to achieve agreement of the parties was presented. Also, a technique to address the specific educational/informational needs of the individual farmer was discussed.
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In the final analysis, phosphorus control in south Florida is technologically feasible, if the social and political obstacles are overcome. Good scientific data and mutual education are the best ways to overcome these obstacles.
References Adjei, M.B., P. Mislevy, K.H. Quesenberry and W.R. Ocumpaugh, 1987. Grazing-frequency effects on forage production, quality, persistence and crown total non-structural carbohydrate reserves of limpograsses. Soil Crop Sci. SOc. Fla. Proc., 47: 233-236. Allen, L.H., Jr., J.M. Rluddell, G.H. Ritter, F.E. Davis and P. Yates, 1982. Land use effects on Taylor Creek water quality. In: Environmentally Sound Water and Soil Management. Am. Soc. of Civil Eng., New York, NY, pp. 67-77. Bottcher, A.B., 1987. WQFARM, Water quality assessment model for an individual farm. Department of Agricultural Engineering, University of Florida. Gainesville, Florida. Bottcher, A.B. and F.T. Izuno, 1993. Procedural Guide for the Development of Farm-Level Best Management Practices Plans for Phosphorus Control in the Everglades Agricultural Area. IFAS Bulletin, University of Florida, Gainesviile, Florida. Bottcher, A.B. and F.T. Izuno (Ed.), 1994. Everglades Agricultural Area (EAA): Water, Soil. Crop, and Environmental Management. University Press of Florida, Gainesville, Florida. Brezonik, P.L. and A.C. Federico, 1975. Effects of backpumping from agricultural drainage canals on water quality in Lake Okeechobee. Technical Report Series 1 (1). Florida Department of Environmental Regulation, Tallahassee, Florida. Burleson, R.W., 1988. Development of a continuous stormwater management model for agriculture in Florida's flatwoods resource area. Masters Thesis, Department of Agricultural Engineering, University of Florida, Gainesville, Florida. Campbell, K.L., T.K. Tremwel, J.C. Capece and T.B. Cera, 1992. Biogeochemical Behavior and Transport of Phosphorus in the Lake Okeechobee Basin: Area II Final Summary Report. Deliverable 2.4.7. South Florida Water Management District, West Palm Beach, Florida, 15 pp. Campbell, K.L., T.K. Tremwel, A.B. Bottcher and D.A. Graetz, 1993. Performance of Selected BMPs for Phosphorus Reduction in High Phosphorus Source Areas. South Florida Water Management District, West Palm Beach, Florida. Davis, F.E. and M.L. Marshall, 1975. Chemical and biological investigations of Lake Okeechobee, January 1973-June 1974. Interim Report. Central and Southern Florida Flood Control District, Technical Publication No. 75-1, West Palm Beach, Florida. FDER, 1992. Feedlot and dairy waste water treatment and management requirement. Fla. Dept. of Environmental Regulation Administrative Code, Chapter 17-670.500-540. Federico, A.C., F.E. Davis, K.G. Dickson and C.R. Kratzer, 1981. Lake Okeechobee water quality studies and eutrophication assessment. South Florida Water Management District, Technical Publication 81-2, West Palm Beach, Florida. Graetz, D.A. and V.D. Nair, 1995. Fate of phosphorus in Florida Spodosols contaminated with cattle manure. Ecol. Eng., this issue. Gunsalus, B., E.G. Flaig and G. Ritter, 1992. Effectiveness of Agricultural Best Management Practices Implemented in the Taylor Creek/Nubbin Slough Watershed and the Lower Kissimmee River Basin. Proceedings of National RCWP Symposium - 10 Years of Controlling Nonpoint Source Pollution: The RCWP Experience. EPA/625/R-92/006, pp 161-171. Hammer, D.A. (Ed.), 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural. Lewis Publishers. Chelsea, MI. Izuno, F.T. and A.B. Bottcher, 1991. The Effects of On-Farm Agricultural Practices in the Organic Soils of the EAA on Phosphorus and Nitrogen Transport-Screening BMPs for Phosphorus Loadings and Concentration Reductions. Final Report to South Florida Water Management District, West Palm Beach, Florida, May 1991.
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Misle,,3~, P.. G.W. Burton and P. Busey, 1991. Bahiagrass response to grazing frequency. Soil Crop Sci. Soc. Fla., Proc.. 50: 58-64. Morse, D., H.H. Head. C.J. Wilcox, H.H. Van Horn. C.D. Hissem and B. Harris, 1992. Effects of concentration of dieta~ phosphorus on amount and route of excretion. J. Dairy Sci.. 75: 3039-3049. Negahban. B.. C. Fonyo. W. Boggess, J. Jones. K. Campbell, G. Kiker. E. Hamouda. E. Flaig and H. Lak 1993. LOADSS: A GIS-based decision support system for regional environmental planning. Conference proceedings of Application of Advanced Information Technologies: Effective Management of Natural Resources. ASAE. St. Joseph, Michigan. Rechcigl, J.E.. G.G. Payne, A.B. Bottcher and P.S. Porter, 1992. Reduced phosphorus application on bahia grass and water quality. Agron. J., 84: 463-468. Sanchez, C.A., S. Swanson and P.S. Porter, 1990. Banding P to improve fertilizer use efficiency of lettuce. J. Am. Soc. Horticult. Sci., 15: 581-584. Sanchez, C.A., P.S. Porter and M.F. Ulloa, 1991. Relative efficiency of broadcast and banded phosphorus for sweet corn produced on histosols. Soil Sci. Soc. Am, J., 55: 871-875. SCS. 1992. Agricultural Waste Management Field Handbook. USDA Soil Conservation Service, Washington, D.C. SFWMD, 1989. Interim Surface Water Improvement and Management (SWIM) Plan for Lake Okeechobee. Part I: Water Quality and Part VII: Public Information. South Florida Water Management District, West Palm Beach, Florida. SFWMD, 1992. Everglades SWIM Plan. South Florida Water Management District, West Palm Beach, Florida. SFWMD, 1993. Update of Okeechobee SWIM Plan. South Florida Water Management District, West Palm Beach, Florida. January, 1993. Taylor, R.B., Jr., 1987. The use of retention/detention for controlling nitrogen and phosphorus in surface waters. Masters Thesis, Department of Agricultural Engineering. University of Florida. Gainesville, Florida.
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Best management practices for water quality improvement in the Lake Okeechobee Watershed * A.B. B o t t c h e r a, *, T e r r y K. T r e m w e l b, K e n n e t h L. C a m p b e l l c a Soil and Water Engineering Technology, Inc. 3443 N W 12th Aue., GainesciUe, FL 32605, USA b Applied Technology and Management Gainescille, Florida USA ¢ Agricultural Engineering Dept., Unicersity o f Florida Gainescille, FL 32611, USA
Abstract A significant amount of work has been done in the Okeechobee/Everglades Dept., in south central Florida to understand and control the nutrient inputs to Lake Okeechobee, the Everglades, and surrounding estuaries. Phosphorus (P) control has been the primary focus of these studies because P has been found to be the limiting nutrient for the accelerated eutrophication of these aquatic systems. The watersheds of these systems are dominated by beef and dairy operations north of Lake Okeechobee and vegetables and sugarcane south of the Lake. As a result, most of the research activities have focused on agricultural transport processes and control practices (known as best management practices or BMPs) for P discharge. This paper focuses on the BMPs that have been developed for agricultural water quality control in south Florida. BMPs are discussed from the perspectives of conceptual operational theory and field evaluation data, as well as practical in-field design constraints. An itemized listing of the proposed BMPs is also presented, as well as a discussion of implementation strategies.
Keywords: BMPs; Okeechobe Basin; Phosphorus
1. Background Fishermen's complaints of excessive aquatic weeds in Lake Okeechobee during the late 1960s and early 1970s prompted eutrophication studies of the Lake. These
Paper presented at the workshop on Phosphorus Behavior in the Okeechobee Basin, sponsored by the South Florida Water Management District and the University of Florida, Institute of Food and Agricultural Sciences. " Corresponding author. 0925-8574/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 5 - 8 5 7 4 ( 9 5 ) 0 0 0 3 i - 3
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studies, using the Vollenweider model (Brezonik and Federico, 1975 and Davis and Marshall, 1975), found the lake to be eutrophic and potentially on the verge of becoming hypereutrophic due to excessive phosphorus loading. These results prompted extensive monitoring of the lake and its nutrient inputs. Several other studies were also initiated including additional modelling efforts of the lake itself as well as land-based nutrient transport process studies (many of which are presented in this special issue of Ecological Engineering). In general, these studies concluded that the primary source of P to the lake was the dairies and beef operations north of the Lake and the sugarcane/vegetable lands (Everglades Agricultural Area, EAA) south of the Lake. Since the EAA was being backpumped into the Lake (note that historic flow in the EAA was to the south from Lake Okeechobee to the Everglades), these pumps became an easy P input control point to the Lake and were turned off (Interim Action Plan, SFWMD, 1978). This, of course, forced the additional EA.A drainage water into the Everglades, which has caused problems there. The dairies (approximately 35000 head) were found to be the single most intensive source of P in the Lake's watershed (Federico et al., 1981) and have since come under a Dairy Rule regulation (FDER, 1992) to implement specific BMPs, which will be discussed in greater detail. Though dairies were considered the "hottest" source of P, beef operations occupy the majority of the land and have also come under regulation (SFWMD Regulatory Program, Chapter 40E-61.020) through the implementation of the Okeechobee SWIM Plan (SFWMD, 1993). As dairy P loads are being reduced, beefland's relative contribution of P has become relatively higher, thereby attracting greater attention. The SWIM water quality regulation does not specify the BMPs to be used as did the Dairy Rule, it simply requires a water quality discharge standard to be met (0.35 mg/l P). All other agricultural operations also have to meet specific P discharge standards. In order to meet the above mentioned regulations, land-use-specific BMPs have been and are still being developed by IFAS (Institute of Food and Agricultural Sciences, University of Florida) and the South Florida Water Management District. This research and specialist expertise have resulted in the recommended BMPs presented in this paper.
2. Basin description
The Okeechobee/Everglades basin's physical system is well described in several other references (Bottcher and Izuno, 1993). Historically, Lake Okeechobee (2000 km z) received drainage water from the north via Kissimmee River (3500 km2), Taylor Creek/Nubbin Slough/Lettuce Creek (560 km2), and Fisheating Creek/other (1100 km 2) basins. The Lake would periodically overflow its southern shore and drain south through the Everglades or would flow westerly down the Caloosahatchee River. Note that approximately 50% of the water input to and output from the Lake is from direct rainfall and evaporation, respectively.
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Significant physical modifications have been made to the Lake and its drainage basins as a result of channelization, diking, and pump station installations. These structures have allowed the lake stage to be regulated for water storage and flood protection with regulated discharge now being possible through up to six separate structures. The surrounding basins' channelization has allowed much quicker flow to and from the Lake and in non-historic directions. Therefore, inputs and outputs of water and nutrients have been significantly altered and are blamed for much of the degradation of Lake Okeechobee and the Everglades (SFWMD, 1989 and SFWMD, 1992). The lands north of the lake are predominantly sandy spodosols, while the soils to the south are Histosols. Land uses in the sand lands north of the lake are 65% beef, 15% native, 3% dairy, 5% citrus, 4% urban, and 8% misc (Allen et al., 1982). The Dairy Rule and recreational growth have changed the land use patterns in the area with citrus and urban acreage increasing while dairy and beef acreage is decreasing. Land uses on the Histosols are 80% sugarcane, 12% vegetables, and 8% other (rice, sod, nurseries, urban, etc.)
3. Definition of a B M P
Best Management Practices (BMPs) are those on-farm activities designed to reduce nutrient losses in drainage waters to an environmentally acceptable level, while simultaneously maintaining an economically viable farming operation for the grower. Practices which have a high potential for negatively impacting the financial profitability of a farm should not, therefore, be considered BMPs. In cases where the economic cost of implementing certain BMPs puts an excessive financial burden on the farmer, such practices should be considered BMPs only if external funds are available to return an acceptable level of profitability to the farm. In this paper, only BMPs for P control will be discussed because they are most critical for environmental protection of the region.
3.1. Setting of discharge standards BMPs are designed to bring the discharge levels of a contaminant into compliance with downstream water quality standards, which can either be concentration a n d / o r total load based. In the case of Lake Okeechobee the discharge standard for P is concentration-based, whereas the standard for the EAA is load-based. These standards were set based on the needs of the receiving water body, feasibility of obtaining water reductions, and the relative efficiency of serial components of multiple treatment systems, such as the agricultural BMPs and stormwater treatment areas (STAs) in the EA.A. Phosphorus load reduction regulations are more difficult to implement because they require both water flow and P concentrations to be monitored. Dual monitor-
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ing requirements result in potentially higher measurement errors and added cost as compared to concentration standards alone. However, some questions concerning the flow proportionality of samples collected for concentration measurements alone come into play, if flow is not measured. Therefore, some flow measurement may be needed to supplement concentration-based standards. BMPs to reduce P loadings must either reduce P concentration or water volume. In the sand lands, water reductions are difficult and costly, so P-standards in these areas are concentration based. However, in the EAA significant on-farm storage is feasible and most farms have monitorable pumped discharges, allowing the more effective use of a load-based standard. 3.2. Methods to reduce P concentrations
Conceptually, there are only three ways to reduce P concentrations in the runoff water from agricultural lands. They are: 1. Reduce the amount of phosphorus on the farm by minimizing P inputs to the farm and maximizing non-runoff P outputs from the farm. 2. Reduce the hydraulic mobility of the P that is on the farm by limiting water contact a n d / o r reducing the solubility or erodibility of phosphatic materials. 3. Edge of field/farm pre-discharge treatment using uptake, adsorption, deposition, or precipitation technologies, such as wetlands a n d / o r chemical additives. 3.3, Methods to reduce discharge ~'olume
Reducing runoff volume is straight-forward conceptually, but can be difficult to achieve in practice. Runoff volume reduction can only be obtained by two methods. They are: 1. Increase the evapotranspiration (ET) from the farm. 2. Decrease off-farm or groundwater irrigation water inputs to the farm by improved irrigation efficiency or by using storage runoff as a substitute irrigation supply. The sand lands have limited irrigation, have very few water control structures, exhibit soil wetness (high water tables), and have limited water storage areas, all of which make hydraulic reductions impractical. However, in the EAA the irrigation inlet and pumped outlet structures are ideal for optimizing on-farm water use and thereby reducing discharges. Getting the farmers to understand the principles behind BMPs will make the design and acceptance of a farm-level BMP program much easier while significantly improving the farmers' operation and maintenance of the practices. The source of P for potential runoff is the starting place of understanding the process and therefore will be presented in general terms before the specific BMPs are described. Presentation of the biogeochemical processes affecting P transport and the impact of BMPs on P retention is beyond of the scope of this paper, but are well covered by Campbell et al. (1992, 1993) and Graetz and Nair (1995) in two major project reports for the SFWMD.
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4. Sources of P to be controlled
Phosphorus in agricultural drainage waters originates from one of four sources: fertilizers, animal manures, mineralization of native organic materials, or atmospheric deposition. These sources of P must be available if agriculture is to exist in south Florida, and the relative importance of each source depends on the plant/soil system. Atmospheric deposition (1-2 kg ha -t yr - t ) is important only in native areas. The other three sources can vary significantly in amount, but are controllable by proper management or BMPs. As in most situations, problems occur when things are done in excess. Phosphorus management is no exception. If plant available P exists in a soil/plant system beyond what the plants can utilize, then the excess P readily moves by water in south Florida soils. Therefore, the conceptual management approach is to keep the P sources in balance within the soil/plant environment. Control of P sources is particularly critical in south Florida as compared to other locations because of the low P sorption properties of the native soils. The sandy soils in the area have low clay, iron and aluminum contents in the surface horizons which greatly limits P sorption. The organic soils south of the Lake also have relatively low P retention properties. In many of the regional soils, P is a mobile nutrient, similar to nitrate.
5. Phosphorus BMPs for the Okeechobee/Everglades basin
The following BMPs are the result of numerous studies in the Okeechobee/Everglades basin as well as generally accepted practices from other parts of the country. Where possible, the specific study or basis for the BMP is referenced. The BMPs are presented by P source and land use.
5.1. Fertility BMPs The following fertility BMPs are applicable to all crops in the basin with a few noted exceptions. Specific fertilizer recommendations are crop-dependent and are available from the local Florida Cooperative Agricultural Extension Service. Fertility BMPs will be most effective on rowcrop and other high value crops which presently represents only a small percentage of basin. However, significant vegetable and sugar cane acreage in the organic soil basins south of Lake Okeechobee and the renewed interest in vegetable farming on the sand lands north of the lake warrant their discussion. Calibrated soil testing (CST). Calibrated soil testing is the procedure by which the actual crop response to supplemental fertilizer is determined as a function of the P level measured in the soil prior to fertilization. Laboratories providing CSTs can then provide fertilization recommendations. Refinement of the CSTs has been a high research priority. For example, the recent study by Rechcigl and Bottcher
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(Rechcigl et al., 1992) resulted in a 50% reduction in P recommendations for pasture grasses with no expected yield reductions. Banding of fertilizer. The banding of P, limiting application to the rooted area of a crop, can reduce fertility needs significantly. Sanchez et al. (1990, 1991) found that banding in some vegetable crops can reduce fertilizer application rates by 50%. lzuno and Bottcher (1991) further found that banding reduced P losses to water by as much as 20-30%. However, banding is effective only for those crops with a limited root area, such as vegetables, citrus, and other row crops. Preuention of misplaced fertilizer. Prevention of spills and the direct spreading of fertilizer into open drainage ditches can reduce P losses significantly. Because it takes as little as 0.4 kg P/hectare in drainage waters to exceed standards, it is critical to prevent any concentrated spills which can be washed into nearby streams. Also, areas of intensive ditching, as found in the EAA and citrus areas, are most vulnerable to direct fertilizer applications into the ditches due to their frequency and close proximity to the crops. Once P is in surface waters there are less effective processes for removing it than in the soil/plant system. Use of side-throw fertilizer spreaders along drainage ditches or appropriate spacing of drive lanes to prevent spread fertilizer from reaching ditches are necessary practices. Turning off the spreader at the end of the field is also important to prevent overlapping of fertilizer during turns. Aerial applications are more difficult to control but can also be better controlled by proper flagging and pilot training. Split applications. Split application (multiple applications of P during the growing season) of P fertilizers and the use of slow release forms of P fertilizers have limited application for field crops. Only under special conditions such as intensive vegetable production or sod production would split applications of phosphorus need to be considered, and these conditions would normally only require a single split application. Slow release forms of phosphorus, such as rock phosphate are normally inefficient for providing plant needs. Therefore, split application and slow release phosphorus forms would have limited applicability, except for the special cases mentioned above. For these special cases, phosphorus losses could be reduced anywhere from 0 to 5%. Split application and slow release techniques are much more applicable to nitrogen fertilization on mineral soils. For a general discussion of nitrogen and other fertility topics, please read IFAS Circulars 816 (Bottcher and Rhue, 1983) and 817 (Hanlon et al., 1990).
5.2. Animal manure BMPs Animal manure BMPs, as indicated earlier, are based on the concept of spreading the manure on crops at a rate that is in nutrient balance as determined by the CST procedure also described earlier. Therefore, good manure management requires a farm to have sufficient cropland to handle the manure load it generates. In cases where the animals are confined, a physical collection and distribution system is needed to deliver the manure to cropland evenly. If over-
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grazing, uneven grazing, or a poorly designed collection/distribution system exists, then "hot spots" having excessive manure loadings will occur. These "hot spots" are areas needing special attention, because, for example, 10 m 2 of a high intensity area (HIA) (animal density sufficient to cause bare ground conditions) can have a P load equivalent to that generated from 1 hectare of improved pasture, which is about 1000 times greater on a per unit area basis. Note that a well-managed improved pasture will just meet the current P discharge standard as found by Rechcigl (1992). To effectively manage any manure utilization system, knowledge of the actual P content is essential. This allows for the balancing of nutrients in the receiving soil/plant system. Manure characteristic data are available (ASAE, 1993), however collection and storage techniques can significantly vary nutrient contents. Therefore, a good manure testing program is needed. Such testing programs in north Florida and Lancaster, Pennsylvania have proven very effective in better balancing nutrient applications to crops. The following specific manure BMPs apply primarily to dairy and to a lesser extent to beef cattle operations. The dairy related BMPs are currently required under Florida Department of Environmental Protection "Dairy Rule", Chapter 670 of DEP Administrative Code. Note that these rules apply only in the Okeechobee basin. Dairy HIA drainage control. The high water table soil conditions in the region prevent deep percolation, therefore 100% of the surface and ground water from the HIA can be captured by constructing a perimeter drainage ditch around the HIA near the dairy barn. The collected water can then be pumped to a storage pond for later delivery to cropland, via an irrigation system. This is essentially a recycling system if the crop is feedstuff for the cows. Gunsalus et al. (1992) reported that this BMP has brought over fifty percent of the dairies in the Okeechobee basin within compliance and the remaining dairies, with only a couple of exceptions, exhibiting a trend toward compliance. The dairies where this BMP appears not to be working, appear to have historical accumulations of manures not directly addressed by the BMP due to not recognition of old HIAs. Collection and distribution of barn manure. Manure that is deposited on impervious surfaces, such as concrete, in and around barns must be collected, stored, and delivered to cropland in a controlled fashion. Deposited manure can be either flushed to storage ponds or anaerobic lagoons or scraped onto concrete storage pads. The stored manure must be spread on crops by either a mechanical spreader (scraped manure) or an irrigation system (flushed manure). The appropriate rate of P application to the crop would best be determined by the CST procedure as if manure was a fertilizer source of P. However, the need to size manure application areas for P loads without knowledge of future soil test information the USDA Soil Conservation Service in association with the University of Florida developed manure P loading rates based on estimated long-term crop uptake of P. This assessment resulted in loading rates of 50 and 67 kg P ha- t yr- ~ for pastures and manure sprayfields (forage crops), respectively, being approved. Watering, feed, and shade facilities placement. These facilities will normally cause
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small HIAs or "hot spots" to form around them. These areas can be controlled by the following procedures which are listed in order of greatest control: 1. Move all watering, feed, and shade facilities into an area which has a manure collection and distribution system, such as barn or dairy HIAs. 2. Move these facilities frequently enough to prevent the formation of a HIA. 3. Place these facilities in upland locations where maximized overland flow distances can significantly reduce P in runoff (see a later section entitled Maximizing flow distances for P control). Feed ration control. Minimize the P in the animal feed. Work by Morse et al. (1992) has shown that P concentrations in feed rations may be reduced by as much as 30% in some cases while maintaining good milk production. Fencing animals from ditches and streams. Soil and plant assimilation of P is obviously limited when direct application of manure is allowed to a flowing stream or areas that hydraulically flush periodically. This possibility should be eliminated by appropriate fencing. Also, manure deposited near or on stream banks has a higher potential for transport to the stream, especially when the animals disturb bank vegetation and stability. Therefore, fences should be placed at least fifteen feet from the top of the ditch/stream bank, because banks are often ill-defined and storm flows occasionally spread to the floodplain. Also, there must be sufficient space to get harvesting and maintenance equipment along the bank. Harvesting of vegetation within the buffer will improve its P removal efficiency. Grazing management. A critical factor in reducing P in runoff is to keep animal densities on pastures at levels that maintain appropriate nutrient balance. Table 1 shows the relative effects of grazing densities on runoff P concentrations as estimated by the authors based the review of water quality data (various sources) from various tributaries of known land use and unpublished modeling work using CREAMS. Also, grazing patterns or field rotation can be used to maximize P uptake by the animals and minimize HIA formation. Rotational grazing has been shown to be effective for improving forage quality and uptake (Adjei et al., 1987; Mislevy et al., 1991) for beef cattle in south Florida. Select high P uptake crops for manure application areas. Crops vary substantially in their P uptake potential, which means higher P uptake crops will require less
Table 1 Estimated runoff P concentrations by animal density on land Land area Native Pastures Native range Semi-improved Improved Intensive Holding areas HIAs
Animal density (cows/ha) 0 0.025-0.50 0.50-1.2 1.2-2.5 2.5-5.0 5.0-25.0 25.0-250.0
Conc. (mg/l) 0.04-0.20 0.08-0.25 0.10-0.30 0.25-0.50 0.35-1.20 0.50-30.0 30.0-900.0
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acreage for spreading the manure (SCS, 1992). A balance between usability of the crop and cost saving of a smaller land application area will be required. Composting. Composting is an alternative manure management approach which converts the manure to a marketable soil amendment (SCS, 1992). If markets are available, then composting is an excellent choice for operations with limited land for spreading the manure on-site.
5.3. General BMPs Crop management. Profitability and nutrient runoff control are both dependent on a healthy/productive cropping system to assure maximum P uptake. Therefore, proper crop management programs, including cultural, water, nutrient, and pest (insects and weeds) management, are needed on every farm. Irrigation and drainage management. Irrigation and drainage system design and management control the water status of the crop root zone, as well as the rate and quantity of water leaving a field. Proper irrigation provides optimal P uptake conditions, while excessive irrigation significantly increases leaching/runoff and related P losses. Proper drainage is needed to assure optimal moisture conditions during wet periods, while excessive drainage can cause drought conditions and increase aerobic organic matter decomposition with its associated P release. In the basin drainage and irrigation are typically achieved by controlling the shallow water table by using ditches and canals to move water to and from the field. Therefore, appropriate water management is achieved by a well designed and managed system of water conveyance and control structures, such as ditches, canals, weirs, culverts, and pump stations. Maximize flow distances for P control. Table 2 shows the relative potential assimilatory capacity of various flow conditions, going from overland to channelized flow. The presented values are rough estimates made by the authors based
Table 2 Estimates of % phosphorus removal efficiency in flow systems over 150 m Area
High P source (100 ppm)
Low P source (1 ppm)
Overland Woods Grass Native Improved pasture
40
5
80 80
l0 3
Field ditches Bare Grass Native Improved pasture
5
1
10 10
2 0.5
Streams /canals
0.5
0.1
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upon related overland flow studies for municipal waste treatment and observed stream reach data. Therefore, Table 2 is not intended to provide absolute assimilatory capacities, but to show the relative advantages of different types of overland flow. The relative impact of vegetation is also presented in Table 2. As can be seen, the further upland a high P source exists, the greater the P reduction. This illustrates the benefits of buffer strips between high source areas and stream systems. Phosphorus removal efficiencies are affected both by the P concentration of the source runoff and the characteristics of the land it passes through as also indicated in Table 2. Higher P concentrations typically afford a higher percent reduction, while more intensively managed land use areas offer less treatment as water passes through it due to higher in situ P conditions. Flow-way buffer strips. The off-site effects of a high-P-source are intensified by its proximity to a flow-way. A vegetated buffer area between this P-source and the flow-way may partially mitigate these effects. Though P assimilation will occur in the buffer as water passes through it, the primary benefit of the buffer strip is to assure that no high-P-source area occur near the flow-way. The effectiveness of removing upslope P is limited by the width of the strip as seen in Table 2. Also, for continuous treatment of upslope sources, buffer strips must be maintained and harvested to be fully effective. Limit Drainage of Organic and/or Wetland Soils. As indicated earlier, drainage (aeration of soil) can increase mineralization of the organic matter. Because wetlands typically have high organic contents, their drainage can lead to excessive P release during aerobic mineralization. The organic soils south of Lake Okeechobee can mineralize between 20-80 kg h a - t yr-~. The management goal for organic soils is, therefore, to maintain the highest possible water table (soil moisture condition) while keeping a productive crop, if cultivated. Alternatiue Land Use. In limited situations it may be necessary to change the land use in order to meet regulatory constraints. This could involve a minor or a complete modification of the farming operation. For example, converting a dairy to a cattle ranch or vegetable production to sugar cane in the EAA. If there is an economic advantage (which was the case for many dairies in the Okeechobee basin because a government dairy buy-out program), then it should be considered.
5.4. Edge-of-field/farm treatment Edge-of-field/farm treatments have a strong appeal in that they allow farming operations to be less constrained. However, these can be more expensive, so trade-offs between flexibility and cost should be evaluated before considering these systems. The principal edge-of-field treatment technologies that should be considered are presented below. Runoff Retention/Detention System. The storage of runoff water in a retention/detention area can reduce net runoff by either increased evaporation rates or by recycling the stored water to the cropland to reduce other irrigation water supplies and increase crop ET. The storage ponds can also assimilate P by vegetative uptake and sediment deposition and adsorption, however long-term P
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assimilation would be limited in a size optimized retention/detention ponds (Taylor, 1987; Burleson, 1988). However, if these ponds are managed as wetlands (discussed in next section), which would typically require the ponds to be much shallower and wider to achieve effective plant to water volume ratios, then higher P assimilation could be achieved. Also, if storage ponds are allowed to de-water through soil seepage, then much higher P adsorption to soils can be expected. Use of Wetlands. Wetlands which are managed for high vegetative growth and organic debris accumulation can remove significant amounts of P if the following conditions are met (Hammer, 1989): 1. Retention times and flow rates are managed to allow sufficient time for plant uptake and no storm flushing to occur. 2. Keep the wetland wet, i.e. prevent dry-oUt that can release accumulated P. Chemical Treatment. These systems involve the addition of chemicals that will precipitate a n d / o r flocculate P compounds so that they can be easily settled from the water. Chemical treatment would include chemical injection and deposition (pond, wetlands, etc.) components. Chemical treatment can be very effective, but expensive.
5.5. EAA-Specific BMPs The above fertility and water management BMPs also apply directly to the EAA, but the unique nature of the fiat organic system requires site-specific considerations in BMP design. EAA-specific BMPs are discussed in detail by Bottcher and Izuno (1994).
5.6. Summary of BMPs Table 3 is a summary table of the BMPs discussed. The table has additional information relating to the crops that specific BMPs apply to and the relative reduction potential of the BMPs. However, in using these reduction ranges, it is necessary to understand both what they represent and their uncertainty. First, only a few of the listed BMPs have been field tested and even those were tested for only a limited set of conditions. Therefore, most of the stated phosphorus reduction ranges are based on corollary data and the authors' basic knowledge of the physical and chemical processes of the region. The presented BMP effectiveness (% phosphorus reduction) ranges include this uncertainty. They also reflect the variabilities of existing conditions among farms. That is, farms implementing a BMP for the first time can expect to experience the full benefit of that BMP, whereas those farms already practicing a specific BMP should expect no additional phosphorus reduction due to continued implementation of that BMP. Again these ranges should be considered only as a guide as to relative beneficial impact with little wait given to their absolute values. Additional research will be needed to further narrow these very broad reduction ranges.
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Table 3 Summary of Okeechobee Basins BMPs BMP code/name
P reduction range ~ (%)
Crop b
Fertility BMPs Calibrated soil testing Banding of fertilizer Misplaced fertilizer Split appl. of fertilizer
0-50 0-40 0-15 0-10
SC, V,C.P,F, RC V,C, RC All All
60-95 10-90 5-80
D.F D,F D,P
Manure BMPs Dairy HIA drainage control Barn manure handling Watering, feed, and shade facilities placement Feed ration control Fencing ditches and streams Grazing management Select high P uptake crops Composting General BMPs Crop management Irrig. and drain, management Buffer strips Limit drainage of wetlands Use of wetlands Chemical treatment Alternative land use E,AA specific BMPs Minimizing water-table Fluctuations retention of drainage on-farm Retention of vegetable field Drainage water in sugarcane or fallow lands Use of aquatic cover crops Coordinated farm cropping patterns
0-15 0-40 0-20 0-25 0-100
D,P P P D,P D
0-40 0-30 5-40 0-20 0-50 50-90 0-60
All All D,P,RC.F,V W All D,V D,V,RC,P
0-50 15-60 20-90
All Sugarcane Vegetables
5-20 n /a
All All
a Ranges are for individual farms after considering uncertainty and the variability of farm management unless otherwise noted. b SC = sugarcane, V= vegetable, C = citrus, P = pasture, D = dairy, RC = row crop, S = sod. W= wetland, and F = forage.
6. Implementation strategies for BMPs T h e d e v e l o p m e n t o f B M P s for P c o n t r o l in the O k e e c h o b e e / E v e r g l a d e s basin d o e s not in itself get B M P s i m p l e m e n t e d o r i m p r o v e w a t e r quality. I m p l e m e n t a tion r e q u i r e s f a r m e r s to clearly see th e b e n e f i t s o f B M P s for t h em . F a r m e r s must a c c e p t the B M P s as a n e c e s s a r y p a r t o f t h e ir o p e r a t i o n . A c c e p t a n c e is not always easy to obtain, but t h e r e are t h r e e g e n e r a l ways to a c h i e v e a c c e p t a n c e / a w a r e n e s s in t h e a g r i c u l t u r a l c o m m u n i t y . T h e y are: 1. P r o v i d e the f a r m e r s with sufficient, s u p p o r t a b l e e v i d e n c e that the B M P will i m p r o v e t h ei r f a r m i n g o p e r a t i o n . ( V o l u n t a r y )
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2. Provide the farmers funds to entice them to implement the BMPs. (Incentive) 3. Make it the law that they must implement BMPs. (Enforcement) These are listed in order of palatability to farmers. However, each approach may achieve the desired water quality goals, but at varying rates and cost-effectiveness. Each of these strategies have been tried in various parts of the country and in Florida with varying degrees of success. Typically, the voluntary approach has been accused of not working as well as the incentive or regulatory approaches, but this was likely due to poor knowledge bases at the startup of these programs to gain acceptance to the BMPs. On the other side, once better knowledge bases were developed governmental agencies felt more confident in developing incentive or regulatory programs. Choosing the appropriate BMP implementation approach requires an understanding of the true cost-effectiveness of individual BMPs and their on-farm operational impacts, so that the actual economic/operational impacts to the farmer can be assessed. If this assessment shows that certain BMPs are economically beneficial and operationally compatible with current conditions, then the first approach is all that is needed. However, if the assessment shows the BMPs are operationally compatible but potentially cost prohibitive, then the incentive approach is needed. Finally, if the BMPs are found to be economical, but not operationally compatible, then the regulatory approach is usually best. In all of the above cases, both the farmers and government agency staffs must be fully educated as to BMP responses before acceptance of any approach can be achieved. The bottomline is that we must articulate to ("educate") all parties, as much as possible, the costs/benefits of a BMP program. A problem often faced is that the BMPs' effectiveness and costs are not well known and will vary significantly from farm to farm. For this situation, agreement can only occur if all parties have similar, detailed understandings of the state-of-the-art of BMPs and have a willingness to compromise so that the programs match our knowledge base. The real question becomes, "How can we best educate all parties?". Often our inability to educate is because we do not know exactly what a BMP will do. This clearly points out the need for more research and always avoiding the situation of demanding (regulating) BMPs which scientific knowledge can not fully justify. The complexity and variability of P transport processes and their relationship to BMPs will require innovative research and educational approaches to gain mutual understanding. Obviously, research to describe the basic processes and science is essential, but getting this information in a coherent form that farmers can understand is also essential. Farmers must be able to say "Yes, I see what is going to happen to me!" so that they can make reasonable decisions. If appropriate knowledge is not available, then none of the above approaches will work. Standard extension workshops and publications describing BMP programs are helpful. However, getting complex information to the farm-level will require more sophisticated approaches. Farmer-level education programs must answer more specific questions such as actual equipment purchases, degree of earth work, labor needs, maintenance cost, farm profitability, and expected water quality improvement.
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Computerized BMP expert systems have a high potential to provide individual farmers with the information they need to make decisions. These systems will need to expertly ask the farmer site-specific information, so that this information can be combined with other data sources to simulate the total economic/operational impact of various BMP scenarios. Optimization for individual farmer needs can then occur. The WQFARM (Bottcher, 1987), LOADSS (Negahban et al., 1993), and Interactive Dairy Model (Negahban et al., 1993) provide a good framework for evaluating the water quality impact of various BMPs in the region, but will need to be expanded significantly to become true expert systems.
6.1. Monitoring It important to note that any effective BMP program will require monitoring of both the implementation of BMPs, as well as their impact on water quality. As mentioned earlier, monitoring costs are a function of the type of BMP implementation program, where the regulatory programs have the highest monitoring cost and the voluntary programs have the lowest cost. The authors estimate that for a discharge standard based regulatory program, the annual water quality monitoring cost for individual farms will be between 5 to 50 US$ ha-1 yr-t depending on the size and type of operation. This would not include any permit cost and additional record keeping and reporting costs. A performance based standard (proof of implementation and maintenance of BMPs, only) on the other hand would eliminate water quality monitoring costs, but could increase record keeping costs. Currently, much of the monitoring cost in the basin has been paid by the SFWMD through its compliance program in association with the Dairy Rule. This is effectively a cost-share program. SFWMD's monitoring program, in addition to the state's cost-share program for BMP implement on the dairies has allowed the discharge standard based regulatory approach to be workable. However, without these cost-share programs, the success of the Dairy Rule would have been unlikely.
7. Conclusions
Our current knowledge of BMPs is sufficient to develop effective basin-wide P control programs for working toward meeting the water quality standards set by the State of Florida as show by the success of the Okeechobee Dairy Rule. However, additional research is needed to better refine actual cost and how much additional water quality improvement can be achieved. The cost of effective BMP programs for individual farms will vary greatly, leaving some farmers short of resources to fully implement the required BMPs. In these cases, an incentive or cost share program will be necessary. Acceptance of the benefits of a BMP program by all involved parties (farmers, government staffs, environmentalists) is critical to their success. Approaches to achieve agreement of the parties was presented. Also, a technique to address the specific educational/informational needs of the individual farmer was discussed.
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In the final analysis, phosphorus control in south Florida is technologically feasible, if the social and political obstacles are overcome. Good scientific data and mutual education are the best ways to overcome these obstacles.
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Misle,,3~, P.. G.W. Burton and P. Busey, 1991. Bahiagrass response to grazing frequency. Soil Crop Sci. Soc. Fla., Proc.. 50: 58-64. Morse, D., H.H. Head. C.J. Wilcox, H.H. Van Horn. C.D. Hissem and B. Harris, 1992. Effects of concentration of dieta~ phosphorus on amount and route of excretion. J. Dairy Sci.. 75: 3039-3049. Negahban. B.. C. Fonyo. W. Boggess, J. Jones. K. Campbell, G. Kiker. E. Hamouda. E. Flaig and H. Lak 1993. LOADSS: A GIS-based decision support system for regional environmental planning. Conference proceedings of Application of Advanced Information Technologies: Effective Management of Natural Resources. ASAE. St. Joseph, Michigan. Rechcigl, J.E.. G.G. Payne, A.B. Bottcher and P.S. Porter, 1992. Reduced phosphorus application on bahia grass and water quality. Agron. J., 84: 463-468. Sanchez, C.A., S. Swanson and P.S. Porter, 1990. Banding P to improve fertilizer use efficiency of lettuce. J. Am. Soc. Horticult. Sci., 15: 581-584. Sanchez, C.A., P.S. Porter and M.F. Ulloa, 1991. Relative efficiency of broadcast and banded phosphorus for sweet corn produced on histosols. Soil Sci. Soc. Am, J., 55: 871-875. SCS. 1992. Agricultural Waste Management Field Handbook. USDA Soil Conservation Service, Washington, D.C. SFWMD, 1989. Interim Surface Water Improvement and Management (SWIM) Plan for Lake Okeechobee. Part I: Water Quality and Part VII: Public Information. South Florida Water Management District, West Palm Beach, Florida. SFWMD, 1992. Everglades SWIM Plan. South Florida Water Management District, West Palm Beach, Florida. SFWMD, 1993. Update of Okeechobee SWIM Plan. South Florida Water Management District, West Palm Beach, Florida. January, 1993. Taylor, R.B., Jr., 1987. The use of retention/detention for controlling nitrogen and phosphorus in surface waters. Masters Thesis, Department of Agricultural Engineering. University of Florida. Gainesville, Florida.