Nutrient data were collected upstream of 15 coastal control structures in Miami-Dade County. Samples were collected over a typical hydrologic period during various flow conditions. Sampling began at 5 sites in May 1996 and at 10 sites in October 1996. Constituents collected included ammonia, nitrite, nitrate, orthophosphate, and total phosphorus.
Nutrient data have been collected from the east coast canals for many years by various government agencies. Much of the data collected have been from grab samples at 0.5 or 1.0 meter below the stream surface near the centroid of flow. The degree to which these samples adequately represent nitrogen and phosphorus concentrations within the water column of the canals of south Florida is presently unknown and limits confidence in loading estimates. Furthermore, the relation between discharge and nutrient concentration that occurs in natural uncontrolled streams in other parts of the Nation may not apply to the artificially controlled canals of south Florida. Both of these issues need to be addressed to develop nutrient budgets and to plan effective restoration strategy now and in the future.
Suite 70, Royal Palm Square
Dixon, Joann; Byrne, Michael
Depth-integrated and point (grab) samples for nutrients were collected from the gated control structures at the east coast canal sites in Miami-Dade County during periods of flow. Most of the samples were collected near low tide when the gates at the sites usually are open. Depth-integrated samples were collected by means of the equal-width-increment method, which commonly is used when a discharge measurement is not made before sampling. In this method, the width of the stream is subdivided into equal-width intervals with a sampling vertical associated with each interval.
In each cross section for the study, 8 to 10 verticals were used. The first vertical selected was half the distance of the first interval from the edge of the stream bank, and the other verticals were equally spaced apart across the stream from the first vertical. Because flow velocities of the east coast canals in Miami-Dade County are nearly always less than 2 ft/s (feet per second), the weighted-bottle method was used to collect each sample. The weighted bottle does not sample isokinetically (nozzle velocity equal to stream velocity); however, it can be used during low flows and when differences in water quality across the stream are believed to be insignificant. In the initial step, the vertical with the greatest velocity is selected; the weighted bottle is then lowered and raised at a constant rate so that it is not overfilled when returned to the surface. For each specific site, this transit rate was maintained throughout for all the verticals in the cross section, and a sample from each vertical was composited in a churn splitter for processing. The transit rate as well as the nozzle size was varied for each specific site in order to prevent overfilling of the bottle.
During sample processing, the water in the churn was stirred by the churn disc at a rate of about 9 in/s (inches per second) to minimize error, with care being exercised to prevent the churn disc from breaking the surface. A total of 125 mL (milliliters) of this composite sample was siphoned off into an amber polyethylene bottle, chilled immediately, and within 48 hours was shipped to the USGS Water Quality Service Unit in Ocala, Fla., for analysis. The constituents determined were total organic nitrogen, ammonia, nitrite, nitrate, nitrite plus nitrate nitrogen, total phosphorus, and orthophosphate. A point (grab) sample was collected concurrently with each depth-integrated sample. Point (grab) samples, depending on the site, were collected at 0.5 or 1.0 m below the surface near the centroid of flow by using a Niskin bottle. This bottle is spring loaded so that a messenger can trip it shut at the appropriate depth. After collection of the point (grab) sample, 125 mL of the sample was transferred from the Niskin bottle into an amber polyethylene bottle, chilled, and shipped for analysis according to protocol.
Before each sampling trip, the Niskin bottle, weighted bottle, and churn splitter were cleaned with a dilute nonphosphate detergent and then rinsed with tap water followed by deionized water. Two rinses with native water were required for the samplers before sample collection at each east coast canal site. Between sampling sites, the samplers were rinsed with deionized water before being rinsed with native water.
To ensure the integrity of the field data collected for the study, quality assurance samples (equipment blanks, field blanks, and split samples) were used extensively in the data-collection phase. Blank solutions essentially are samples free of the analytes being determined in the environmental samples. An equipment blank is a blank solution that is processed through all of the equipment used in the collection and processing of the environmental samples. Field blanks are actually equipment blanks done in the field and are subject to all aspects of the data-collection efforts as the environmental samples, including processing, preservation, transport, and laboratory handling. Two types of field and equipment blanks were used: one for the Niskin bottle and the other for the weighted bottle and churn splitter. These two field blank solutions consisted of inorganic blank water prepared at the USGS Water Quality Service Unit and were processed and analyzed according to protocol. Analytical results from the equipment blanks and field blanks indicated that most of the concentrations for the individual nitrogen and phosphorus species were below the detection limits for the analytical methods.
Split samples were collected concurrently by the USGS and DERM to verify interlaboratory accuracy. The sampling data consisted of point (grab) samples that were collected from the same Niskin bottle by both agencies. The samples were sent to the USGS Water Quality Service Unit and to the DERM laboratory for analysis.
Suite 70, Royal Palm Square
Data collected for water-quality include nitrogen organic total, nitrogen ammonia total, nitrogen AMM+ORG total, nitrogen nitrite total, nitrogen nitrate total, nitrogen nitrite+nitrate total, phosphorus total, and orthophosphorus total..
U.S. Department of the Interior, U.S. Geological Survey, Center for
Coastal Geology
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