Florida has experienced rapid growth in population during the last 50 years with a 5.5-fold population increase from 1950 to 2000 (Perry and Mackum, 2001; Smith, 2005; U.S. Census Bureau, 1997). Groundwater withdrawal for domestic and irrigation use has increased by 15.5 and 20.7 times, respectively, during the same period (Marella and Berndt, 2005). Likewise, the amount of wastewater generated by cities in Florida has increased more than fivefold since 1950. Environmental concerns about degradation of surface waters by treated effluent water have caused many communities to consider advanced secondary-treated wastewater (reclaimed water) reuse. Currently, there are 440 reclaimed water reuse systems in Florida irrigating 92,345 ha with 2385 million liters of reclaimed water per day (Florida Department of Environmental Protection, 2005). The majority of these systems irrigate golf courses, public right-of-ways, and home landscapes. However, 6144 ha of production agriculture is currently irrigated with reclaimed water with citrus (Citrus spp. L.) orchards accounting for all but 364 ha.
Florida citrus production benefits from irrigation because the average annual rainfall of more than 1200 mm is unevenly distributed throughout the year with ≈75% of annual rainfall occurring from June to September (Koo, 1963). Furthermore, Florida citrus trees are grown on sandy soils with very low water-holding capacity, particularly orchards in the central “ridge” portion of the state. Typical available water content values for central Florida ridge citrus soils range from 0.05 to 0.08 cm3·cm−3 (Obreza and Collins, 2003). Increased water use by the growing population and localized water shortages during low rainfall years have resulted in the development of water use restrictions and decreases in permitted water use for agriculture. Increased use of reclaimed water for agricultural irrigation would not only reduce the wastewater disposal problem for urban areas, but could also reduce the amount of water withdrawn from surficial and Floridan aquifers for irrigation.
Before 1986, the city of Orlando and Orange county were discharging treated effluent into Shingle Creek that leads into Lake Tohopekaliga in central Florida. As a result of concerns that nutrient loading from this effluent would reduce lake water quality, Orlando and Orange county, along with the U.S. Environmental Protection Agency, developed a plan to use the wastewater normally disposed of in Shingle Creek for agricultural irrigation instead. Initial funding of $180,000,000 established the project, which is called Water Conserv II (Parsons et al., 2001a). The project currently delivers ≈133,000 m3 of reclaimed water per day (cmd) (275,000 cmd maximum flow) to ≈1750 ha of citrus (Phil Cross, pers. comm., 2006). Other users of reclaimed water from the Water Conserv II project are eight foliage greenhouse operations, four tree farms, two ferneries, and three golf courses. The reclaimed water is distributed though 80 km of pipelines maintained by the project. Excess reclaimed water is disposed of in 71 ha of rapid infiltration basins that recharge surficial and Floridan aquifers. Water Conserv II is the largest reclaimed water agricultural irrigation project of its type in the world and was the first project in Florida to be permitted to irrigate crops for human consumption with reclaimed water (McMahon et al., 1989).
The reclaimed water distributed by the Water Conserv II project and used for irrigation of orchards in this study was treated with advanced secondary treatment with high-level disinfection, coagulation, filtration, and chlorination (Parsons et al., 2001b). Water quality standards were negotiated among Water Conserv II, Univ. of Florida researchers, and local growers.
To receive reclaimed water for irrigation at no cost, citrus growers were required to sign a contract with the city of Orlando and Orange county to accept 1270 mm of water per year for a period of at least 20 years. Initially, there was grower resistance because of concerns that use of the reclaimed water might damage citrus trees or make the fruit unmarketable. As part of the contract, the growers requested that the Univ. of Florida study the long-term effects of reclaimed water on citrus tree health and fruit quality. Dr. R.C.J. Koo of the Univ. of Florida (IFAS) Citrus Research and Education Center initiated a study to determine tree appearance (i.e., foliage density and color), weed growth, leaf nutrient status, and soil nutrient retention in citrus orchards irrigated with either groundwater or reclaimed water in 1987. No adverse affects of reclaimed water use on tree health and productivity were noted in the initial phase of the orchard survey; however, continued monitoring was suggested to determine long-term effects (i.e., metal accumulation in soil, leaves, or fruit). Orchards are not now required to accept the full 1270 mm of water per year under the contract because rapid infiltration basins (RIBs) were installed in the early 1990s. As a result of the highly porous nature of the soils, the RIBs function as alternate disposal sites (particularly during the normally wet summer rainy season) where the reclaimed water is applied at high rates and allowed to percolate to the groundwater. Still questions persisted regarding the effect of long-term use of wastewater on tree productivity.
This orchard monitoring project continued through the 1990s and was ended in 2004. This article reports data collected since the preliminary reporting of results in 1993 (Zekri and Koo, 1993) to the conclusion of the study in 2004. The monitoring of citrus orchards was continued to determine if adverse effects on citrus tree health and production were associated with irrigation using reclaimed water. Therefore, the objective of this project was to determine whether long-term irrigation with treated municipal wastewater 1) reduces tree health (i.e., canopy appearance and leaf nutrient content), 2) decreases visual fruit loads, 3) impacts internal fruit quality (i.e., Brix, titratable acid, Brix:acid ratio), or 4) increases in soil contaminant concentrations.
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