Scientific Investigations Report 2007-5277
Prepared as part of the
U.S. Geological Survey South Florida Greater Everglades
Ecosystem Sciences Program
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CONTENTS Abstract Introduction Purpose and Scope Methods Data Sources Data Selection and Screening Data Analysis Rainfall, Water Levels, and Flows in the Refuge Factors to Consider in Long-Term Trend Analyses Statistical Analysis for Trends Water Quality Background Water-Quality Characteristics Trends in Water Quality and Water Level Over Time Spatial Patterns in Water Quality Pesticides and other Organic Compounds in Water and Sediment Summary and Conclusions References Cited |
Water quality in the interior marsh of the Arthur R. Marshall Loxahatchee National Wildlife Refuge is characterized by low concentrations of major ions, principally sodium and chloride, and is affected primarily by natural seasonal processes, such as evapotranspiration, rainfall, and biological activity. During the dry season, evapotranspiration exceeds precipitation, and specific conductance and conservative ion concentrations at marsh background sites typically increase by 40-70 percent between the end of the rainy season in September and the end of the dry season in May.
Water enters the Refuge mainly from rainfall and perimeter canals. Water is pumped into the perimeter canals from large pumping stations, such as S-5A and S-6. In recent years, much of the water pumped into the Refuge passes through Stormwater Treatment Areas (STAs) before being released into the perimeter canals that surround the Refuge. Since 2001, water at S-6 has been diverted south toward STA-2, away from the Refuge perimeter canals. Water from S-5A and S-6 flows through agricultural lands with intense agricultural activity and typically contains relatively high concentrations of major ions, nutrients, and pesticides. Specific conductance, major-ion concentrations, and nutrient concentrations are an order of magnitude higher at S-5A and S-6 canal sites than at interior marsh sites. Water quality in the marsh bordering the canals can be affected substantially by the canal water, and these effects can extend several miles or more into the marsh depending on location in the Refuge and on the water level in the canals. As canal water flows into the marsh, processes such as uptake by periphyton and rooted vegetation and settling of particulate matter reduce the concentrations of nutrients to a greater extent than conservative ions such as chloride.
Long- and short-term trends for specific conductance, chloride ion, sulfate ion, total phosphorus, and total nitrogen at five sites were evaluated primarily using an uncensored seasonal Kendall test with a water-level adjustment to reduce the effects of long wet or dry periods. Significant long-term trends (1974-2003) for specific conductance, chloride, total phosphorus, and total nitrogen at canal sites S-5A and S-6 were generally downward. Of the five sites, S-5A had the most pronounced decline for specific conductance at about -340 microsiemens per centimeter (µS/cm), followed by S-6 with a decline of about -280 µS/cm. The two internal marsh sites, LOX8 and LOX13, had significant long-term trends in specific conductance of about +37 and -36 µS/cm, respectively. Long-term trends for other constituents at the two internal marsh sites were generally small in magnitude or not measurable between 1978 and 2003. Marsh site LOX15 near the Hillsboro Canal showed no long-term trends, although specific conductance and sulfate concentration increased about 560 µS/cm and 30 milligrams per liter, respectively, from 1998 to 2002. Site LOX15 is influenced strongly by intrusions of canal water, and increases in specific conductance and sulfate at this site coincided with increased canal-water inflows from STA-1W between 2001 and 2003. Median concentrations at LOX13 and S-5A were used to represent background and canal concentrations, respectively. Based on these values, the median chloride concentration at LOX15 indicates that the water is typically about 31 percent canal water and 69 percent “natural” background water. Using median sulfate concentrations, similarly to chloride, the fraction of water at LOX15 was estimated to be 17 percent from canals and 83 percent from “natural” background water. This finding suggests that in the low sulfate environment of the Refuge, sulfate is not conservative and only about half of the sulfate from canal water typically reaches LOX15; the rest presumably is removed by marsh plants, algae, and bottom sediments.
Concentrations of pesticides and other organic compounds were measured at inflow pumping stations S-5A and S-6. The most commonly detected pesticides in water were atrazine, metolachlor, simazine, ametryn, ethoprop, tebuthiuron, and hexazinone. The most commonly detected pesticides in bed sediment were p,p´-DDD, p,p´-DDE, ametryn, p,p´-DDT, and atrazine, with a maximum concentration of 390 micrograms per kilogram for p,p´-DDE measured at S-6. Only two water samples from the Refuge marsh had been analyzed for pesticides and neither contained detectable concentrations.
Even if the water quality of inflows to the Refuge improves overall as a result of management actions, concentrations of many constituents in canal water probably will remain substantially different from concentrations in the background water of the marsh. Changes in the timing or location of inflows associated with Everglades restoration could adversely affect water quality over greater expanses of marsh, especially if these changes result in canal-water intrusion farther into the Refuge marsh. Inflows from the STAs, even with relatively low nutrient concentrations, may adversely affect water quality in the marsh interior if the inflows contain high concentrations of pesticides or major ions. Major ions, such as calcium and bicarbonate, are not easily removed in STAs. Together with pH, these ions control periphyton species composition and can create an imbalance within the naturally low ionic-strength, soft-water marsh ecosystem.
Miller, R.L., and McPherson, B.F., 2008, Water Quality in the Arthur R. Marshall Loxahatchee National Wildlife Refuge—Trends and Spatial Characteristics of Selected Constituents, 1974-2004: U.S. Geological Survey Scientific Investigations Report 2007-5277, 34 p.
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