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The Seminole Tribe of Florida (the Tribe) is partnering with the U.S. Environmental Protection Agency to develop a numeric phosphorus criterion for the 52,000-acre Big Cypress Seminole Indian Reservation (BCSIR), which is located downgradient of the Everglades Agricultural Area, and of other public and private lands, in southeastern Hendry County and northwestern Broward County in southern Florida. The U.S. Geological Survey (USGS), in cooperation with the Tribe, used water-quality data collected between October 2014 and September 2016 by the Tribe and the South Florida Water Management District (SFWMD), along with data from rainfall gages, surface-water stage and discharge gages, and groundwater monitoring wells, to (1) examine the relations between local hydrology and measured total phosphorus (TP) and orthophosphorus (OP) concentrations and (2) identify explanatory variables for TP concentrations. Of particular concern were conditions when TP exceeded 10 parts per billion (ppb) (0.01 milligram per liter [mg/L]) given that the State of Florida and the Miccosukee Tribe of Indians Alligator Alley Reservation (located downstream of the BCSIR) have adopted a 10-ppb maximum TP criterion for surface waters. From October 2014 to September 2016, the Tribe collected 47–52 samples at each of nine water-quality sites for analysis of TP and OP, except at one site where 28 samples were collected. For all sites sampled, concentrations of TP (as phosphorus [P]) ranged from less than 0.002 mg/L (2 ppb) to a maximum of nearly 0.50 mg/L (500 ppb), whereas concentrations of OP (as P), the reactive form of inorganic phosphorus readily absorbed by plants and (or) abiotically absorbed, ranged from less than 0.003 mg/L (3 ppb) to a maximum of 0.24 mg/L (240 ppb). The median and interquartile ranges of concentrations of TP and OP in the samples collected in 2014–16 by the Tribe were similar to the median and interquartile ranges of concentrations in samples collected by the SFWMD at nearby sites during the same period. Differences in concentrations can likely be explained by differences in sample collection methods, sampling locations, sample collection time, and the hydrology during sampling or by the number of samples collected. A major limitation of this study was the short duration of sample collection, which covers a limited range of hydrologic conditions within the BCSIR. The effect of surface-water and groundwater hydrologic conditions on TP and OP concentrations was assessed by using rainfall data and surface-water stage and discharge records. The highest TP and OP concentrations occurred during peak surface-water flows in the canals following long dry periods. Concentrations of TP and OP increased internal to the BCSIR in the western half of the BCSIR during wet periods, but increased concentrations tended to lag behind rainfall events, likely because control structures upstream of sampling sites do not release flows until the water levels in the canals reach predetermined levels. This pattern may indicate that bed sediments in the canals contain high concentrations of phosphorus that becomes resuspended during high flows or that phosphorus salts that had accumulated on dry land during dry periods are carried into the canals by runoff. The largest TP spikes usually occurred at the beginning of highflow events, but then quickly tapered off even when flows remained high. Groundwater flows were assessed in the BCSIR by using groundwater level observations from two preexisting USGS monitoring well clusters, each characterized by a shallow well installed in the surficial aquifer system and a deeper well installed in the intermediate aquifer system. Groundwater levels were evaluated with respect to surface-water levels and discharge in the BCSIR during the period of surface-water 2 Relations Between Total Phosphorus and Orthophosphorus Concentrations and Hydrologic Variables, Florida, 2014–16 sampling. During dry conditions water levels in canals were often higher than groundwater levels in the surficial aquifer, indicating the potential for surface water to recharge the surficial aquifer. During wetter conditions, this trend reversed, and there was potential for shallow groundwater discharge into the canals. From October 2014 to September 2016, concentrations of TP tended to decrease as surface-water inflows moved across the BCSIR from north to south. In both the western and eastern halves of the reservation, the mean concentration of TP was lower in the surface-water outflows from the BCSIR than in the inflows. The mean concentration of TP in the inflows to the western reservation was 0.04 mg/L (40 ppb), whereas the mean concentration of TP in the outflows was 0.03 mg/L (30 ppb). In the eastern reservation, the mean concentration of TP in the inflows was 0.07 mg/L (70 ppb), whereas the mean concentration of TP in the outflows was 0.04 mg/L (40 ppb). TP and OP concentrations were evaluated relative to other water-quality parameters, including turbidity, suspended solids, nitrate plus nitrite, dissolved oxygen, pH, and specific conductance, to determine if any relations existed between TP and other variables. Weak relations were indicated for turbidity and suspended solids at two sites, which indicates that there may be a relation of increased TP to mobilization of sediment. Introduction The native ecosystem of The Everglades in south Florida originally evolved primarily as an oligotrophic (nutrient poor) environment (Noe and Childers, 2007), which resulted in endemic plant species that are adapted to very low nutrient conditions, such as the monoculture of Cladium jamaicense (sawgrass) that originally dominated much of the area. The Everglades is unique in that large changes in habitat can result from even slight increases in nutrients, especially phosphorus (P), which creates substantial challenges for managing nonpoint sources of nutrients (U.S Environmental Protection Agency [EPA], 2000). Although oligotrophic ecosystems require macronutrients such as nitrogen (N), P, and potassium (K), excessive nutrient concentrations, whether from natural or anthropogenic sources, can be toxic to aquatic life and can also affect surface-water quality through eutrophication (Coupe, 2002). Anthropogenic sources of nutrients, such as N-, P-, and K-based fertilizers, and substantive changes to the hydrology, including the construction of canals and control structures, have resulted in increased nutrient concentrations in The Everglades. P enrichment is one of the most damaging anthropogenic affects to The Everglades (Noe and others, 2001). High levels of P have resulted in excessive growth of nuisance plant species including algae, duckweed, and cattails (McCormick and O’Dell, 1996; Miao and DeBusk, 1999), which are not desirable wildlife habitat. In the aquatic environment, dissolved inorganic P (as salts) or as oxidized phosphate from fertilizers, animal wastes, and natural sources is taken up (immobilized) by plants and converted to organic P (EPA, 2017a). The organic P is then available to animals that eat plant tissues. When plants or animals excrete wastes or die, the organic P becomes available to bacteria that mineralize it back to the inorganic form (EPA, 2017a). Some of the converted inorganic P remains dissolved (and readily available), and some is attached to particles. The cycle starts again when the mineralized inorganic P is taken up by plants (EPA, 2017a). The Seminole Tribe of Florida, hereinafter referred to as the “Tribe,” has collected water-quality data within the Big Cypress Seminole Indian Reservation (BCSIR) since the 1980s. The Tribe was granted status similar to that of a State in 1994 for the purpose of administering the Clean Water Act as delegated by the EPA (EPA, 2017b). The EPA first approved the Tribe’s water-quality standards in 1997. The Tribe is currently (2018) working with the EPA to develop a numeric P criterion for waters entering and exiting the BCSIR in the central part of The Everglades (figs. 1 and 2). In cooperation with the Tribe, the U.S. Geological Survey (USGS) examined existing water-quality data to (1) explore the relations between local hydrology and measured total phosphorus (TP) and orthophosphorus (OP) concentrations and (2) identify explanatory variables for TP concentrations. In particular, conditions were identified when TP levels in the BCSIR were 10 parts per billion (ppb) or greater. Research has shown that flora and fauna in The Everglades are affected by P when concentrations in the soil and water exceed about 10 ppb (Rizzardi, 2001). The State of Florida and the Miccosukee Tribe of Indians Alligator Alley Reservation (fig. 2) downstream of the BCSIR have adopted 10 ppb as the criterion for P in water in The Everglades, but the State of Florida bases compliance with the criterion on a complex four-part statistical test that averages P concentrations across multiple sites and time spans (Florida Department of State, 2017). Failure of any one part of the four-part test results in noncompliance with the criterion. The Miccosukee Tribe uses a simpler criterion whereby surface-water flows shall never exceed 10 ppb of TP at any time or under any conditions of flow (Miccosukee Tribe of Indians of Florida, 2010).

u.s. geological survey open file report/open-file report

Relations between total phosphorus and orthophosphorus concentrations and rainfall, surface-water discharge, and groundwater levels in Big Cypress Seminole Indian Reservation, Florida, 2014–16