SOFIA - Poster - Nutrient Transport To Biscayne Bay and Water-Quality Trends At Selected Sites In Southern Florida

Miami Canal at S-26 [larger image]

During the early 1900's, water-management practices were undertaken which eventually resulted in the development of a highly managed hydrologic system which currently exists as a system of levees, pump stations, gated control structures, and water conservation areas in southern Florida. This complex hydrologic system was created for the purposes of flood control, water storage, replenishment of ground-water supplies, and retardation of saltwater intrusion. This man-made system has altered natural hydropatterns and degraded water quality in the Everglades ecosystem.

Plans for restoring the southern Florida and Everglades ecosystems to its predevelopment state requires filling canals, removing levees, and rechanneling canal discharges that currently enter Biscayne Bay. Biscayne Bay, a shallow subtropical estuary along the southeastern coast of Florida, provides an aquatic environment that is habitat to a diverse population of plant and animal species. As a result of agricultural and urban activities, increased nutrient loads in discharges from the east coast canals in southern Florida are a potential threat to the health of Biscayne Bay. An understanding of nutrient transport to Biscayne Bay is needed to assess the ecological health of the bay and to evaluate the water-quality impact of the diverted water to Everglades National Park. Restoring the southern Florida and Everglades ecosystem also requires an understanding of the changes in water quality that have occurred over time.

Samples Composited In Churn At Bridge Upstream Of S-22 [larger image]

The U. S. Geological Survey, as part of its South Florida Place-Based Studies Program, initiated a project to document and define the concentration, distribution, and transport of nutrients to the bay from the coastal canal network. Water samples were collected from east coast canals in 1996-97 (primarily during the wet season) to determine concentrations of major organic and inorganic nitrogen and phosphorus species. Study results indicate that within the Biscayne Bay watershed, median concentrations of some nitrogen and phosphorus species were highest in selected land-use categories: (1) nitrite plus nitrate in the agricultural land-use category; (2) ammonia, total phosphorus, and orthophosphate in the urban land-use category; and (3) total organic nitrogen in the wetlands category.

Depth-integrated samples were statistically different in total phosphorus concentration than 25 percent of grab samples at 1.0 meter depth and 33 percent of grab samples collected at 0.5 meter depth. No statistically significant differences were found for total nitrogen between grab and depth-integrated samples. Grab samples also were found to be biased low when compared to depth-integrated samples. A simple linear regression analysis was used to develop models for estimating total nitrogen and total phosphorus loads from the east coast canals to Biscayne Bay. Because of the large number of water samples collected over the years (1987-96) and the availability of continuous discharge data, a log-linear model employing a minimum variance unbiased estimator was used to compute total nitrogen and total phosphorus loads for site S-26 at the Miami Canal.

Outlet on Tamiami Canal in Big Cypress National Preserve [larger image]

Restoring and enhancing the natural ecosystem requires an understanding of how water quality has been affected over time by anthropogenic influences in southern Florida. Two U. S. Geological Survey daily discharge stations, one within the Big Cypress National Preserve (Tamiami Canal - 40 Mile Bend to Monroe) and one near Biscayne Bay (Miami Canal at S-26), were analyzed for long-term (1966-94) trends in water quality to characterize prerestoration water quality and to document changes in water quality over time.

The principal tool used for the water-quality analysis was the Seasonal Kendall Trend (SKT) test, a nonparametric test that compares relative ranks of data values from the same seasons to negate variation caused by seasonality. To discern the anthropogenic influences that have affected water quality over the years, variation caused by discharge also should be negated. This was accomplished by performing the SKT onflow-adjusted concentrations (residuals) from statistically significant concentration/discharge relations developed using linear regression models. Long-term trends were determined at both sites for selected major inorganic constituents and physical characteristics; pH and dissolved oxygen; suspended sediment; nitrogen, phosphorus, and carbon species; trace metals; and bacteriological and biological characteristics.

Samples Collected With Vandorn Sampler At Snapper Creek S-22 [larger image]

Statistically significant (p-value less than 0.10) temporal trends for water-quality constituents at the Miami and Tamiami Canal stations were classified as indicators of either improvement or deterioration in water quality over time. Most downward trends indicate improvement in water quality over time; however, downward trends in pH and dissolved oxygen indicate deterioration over time and the potential for harmful effects for aquatic life. At the Miami Canal station, improvement in water quality was documented by 7 trends and deterioration in water quality was documented by 14 trends. At the Tamiami Canal station, improvement and deterioration in water quality were indicated by 4 and 9 trends, respectively. Median and maximum concentrations at both sites were compared to the State of Florida freshwater standards; most concentrations were within these standards. However, the median concentrations of dissolved oxygen at the Miami and Tamiami Canal stations were 3.3 and 2.7 milligrams per liter, respectively, and did not meet the State freshwater standard of at least 5.0 milligrams per liter. Additionally, the median and maximum concentrations of total ammonia at both sites exceeded the State freshwater standard of 0.02 milligrams per liter.

A. C. Lietz, U. S. Geological Survey, 9100 N.W. 36th Street Suite 107, Miami, FL, 33178, Phone: 305-717-5838, Fax:305-717-5801, alietz@usgs.gov Oral, Water Quality and Water Treatment Technologies