WATER QUALITY SAMPLING PROGRAM FOR THE LITTLE VENICE SEWAGE COLLECTION AND TREATMENT PROJECT IN MARATHON, FLORIDA
Joseph N. Boyer
Southeast Environmental Research Center
Florida International University
Miami, FL 33099
The ocean side area of Vaca key from Vaca Cut (east) to 94th Street (west), Marathon, Florida has a large percentage of houses and trailers that are currently serviced by inadequate septic tank systems or cesspit disposal. This are has been collectively called the “Little Venice” Service Area, whereas in fact, Little Venice Subdivision is located on the westernmost portion of the service area. The Little Venice Service Area includes approximately 540 residences (Figure 1).
The Little Venice Service Area was selected as the first phase of wastewater improvements for the Marathon Service Area because of the large number of homes on cesspits, the small average size of lots, the density of homes, and known water quality problems in the canals that occur in the area. Water quality of the 89th – 91st Street canals was thoroughly studied in 1984-1985 as part of the Florida Department of Environmental Regulation’s Monitoring Study (FDER, 1987). That study demonstrated significant nutrient enrichment of the canals, high chlorophyll a content, and high coprostanol concentrations in sediments. Coprostanol is a break-down product of cholesterol and is an indicator of fecal contamination.
The Little Venice Service Area will receive a low-pressure, vacuum wastewater collection system that will transmit wastewater to a central treatment plant. The treatment plant will produce effluent that meets or exceeds the current advanced wastewater treatment (AWT) standards of 5:5:3:1 (BOD5, TSS, TN, TP) and will use a Class V injection well for disposal of treated wastewater. Central collection and treatment of wastewater will remove a substantial portion of nutrient loading into the canals by removing the sources of wastewater (poorly functioning septic tanks and cesspits).
The purpose of this water quality sampling program is to document water quality improvements in the canals of the Little Venice Service Area. The sampling program consists of two phases. Phase 1 will be conducted for two years prior to the initiation of operation of the central sewage treatment system. Phase 1 will establish existing conditions in the canals within the service area. Phase 2 will be conducted for two years after initiation of the central sewage treatment system and will document changes in water quality and sediment chemistry of the canals.
Four canals within the Little Venice Service Area will be selected for sampling (Figure 1). Canal 1 and 2 are a connected “U-shaped” canal system located at 112th Street. These canals may receive better tidal flushing than other canals within the Service Area because of their flow-through design and their relatively short length. Canals 1 and 2 are lined with single-family residences that were constructed prior to 1970 and a high percentage of those residences are thought to have no sewage treatment systems (cesspits). Canal 3 is located adjacent to 100th Street and Canal 4 is located adjacent to 97th Street. Both Canal 3 and 4 are dead-end canals that are lined with single-family houses and mobile homes. Many of these residences are thought to have poorly functional septic systems or cesspits. The 91st Street canal has been selected as a reference canal and is located outside the Little Venice Service Area. Historic water quality and sediment data exist for this canal (FDER 1987).
Field Sampling Regime
Nine sampling stations were chosen for this project: two per canal with an extra in the U-shaped 112th Street canal (Fig. 1). Stations were located at the mouth of the canal and at the dead-end with the exception of the 112th Street canal where there is only one station in the U end and two at each mouth. Each of the 9 stations will be visited weekly via small boat. Surface and bottom measurements of temperature, salinity, and dissolved oxygen (DO) will be performed at each station. Duplicate water samples will be collected in mid-channel at 1 meter below surface. Water samples will also be collected just below the surface for bacteriological analysis. To ensure that we capture the greatest potential terrestrial inputs, sampling will be performed on the low, low tide whenever possible. Localized data from a ongoing study by FIU of the Florida Keys National Marine Sanctuary will be used as the background ambient water quality in the nearshore waters for comparative purposes.
Water samples will be analyzed for total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chla) by the SERC laboratory using standard methodology outlined in the Quality Assurance Project Plan (QAPP). Once a month the samples will be analyzed for the full suite of nutrients including ammonium (NH4+), nitrate + nitrite (NOx-), nitrite (NO2-), silicate (Si(OH)4), soluble reactive phosphate (SRP), and total organic carbon (TOC). Some parameters will not be measured directly, but calculated by difference. Nitrate (NO3-) will be calculated as NOx- - NO2-. Dissolved inorganic nitrogen (DIN) will be calculated as NOx- + NH4+. Total organic nitrogen (TON) will be defined as TN - DIN.
In addition to the weekly sampling program, each month we will deploy 2 ISCO autosamplers at rotating sites which will be programmed to collect 12 samples per day over a 2 day period. Water samples will be analyzed for TN and TP. Hydrolab datasondes will accompany the autosamplers and will measure and log temperature, salinity, DO, and pH on an hourly basis. This will result in diurnal profiles of physical and chemical variables associated with tidal cycles and precipitation events.
Water samples will be collected as above and transported to Davis Labs?? for enumeration of enterococci.
FDEP and EPA personnel at Marathon (Gus Rios and Bill Kruczyinski) are very interested in the initiation of this project. FDEP will supply a boat and office space for a technician for this project. The Nature Conservancy has also expressed interest in supplying volunteers for sample collection.
Reporting will include production of a geo-referenced station map, quarterly data reports, and an annual interpretive report. The principal investigator will be responsible for ensuring the results are compiled and the complete data set is submitted in a timely fashion to the contractor .
Upon completion of the analysis of samples from each quarterly period, the principal investigator will produce a statistical summary of the data in a logical format based on the station design. The statistical summary will include calculated averages, sample variances, ranges, and number of samples. When appropriate, the principal investigator will provide the summaries in a graphical format. The principal investigator will submit a data and narrative report documenting the results of each quarterly survey. The data report will include the raw data in STORET format and statistical summaries in hard copy and on disk. The investigator will evaluate the data in accordance with the data quality objectives developed in the QAPP.
After completion of analysis of samples from the fourth quarterly survey, the principal investigator will produce statistical summaries of the data collected at each station to be incorporated into an annual report. All data will be evaluated in relation to the data quality objectives developed in the QAPP. The data will be analyzed using appropriate statistical tests of significance to meet the specific objectives of the monitoring program.