Well Locations

Six well-cluster sites have been established in a 25-km-long transect leading from onshore to offshore (Figure 1 and Table 1). The near shore site 1 (Black Point Inshore) is a single well located south of Black Point. The well head is approximately 2 ft below sea level, and the well penetrates to a depth of 17 ft below seafloor (fbsf), terminating in a quartz-sand zone of the Miami Limestone (Fish and Stewart, 1991). Site 2 (Mid-Bay) is located in the middle of Biscayne Bay approximately 9 ft below sea level and consists of three monitoring wells to depths of 15, 33, and 42 fbsf. Sites 3 and 4 are located on opposite sides of Elliott Key. Site 3 (Billy's Point), the bayside site, consists of two wells at 6 and 22 fbsf. Site 4 (Petrel Point), the seaward site, consists of two wells at 20 and 45 fbsf. Site 5 (Alina's Reef) is located on a patch reef where diverse reef research and monitoring is continuing and is a site where BNP staff have recorded low conductivity (salinity) on a moored instrument (Porter and Porter, 2002, p. 12-13). Three wells installed at Alina's Reef provide sampling access to 12, 32, and 60 fbsf. Site 6, located south of the Pacific Reef light structure, consists of two monitoring wells to depths of 10 and 41 fbsf. Procedures used to complete all monitoring wells are described below. For comparison, a pre-existing shallow (80 ft, below land surface) onshore well in the Biscayne Aquifer was sampled, as well as an additional well (BkP, 20 fbsf) located just offshore of the Black Point site.

Well Installation

Well installation was accomplished by SCUBA divers with surface support. A USGS work boat, hydraulic-powered drill, and standard 5-ft NQ-2 wire-line core barrels and drill rods were used for core drilling. SCUBA divers drilled most of the offshore wells. Wells can range in depth from 10 to 60 ft (3-20 m) and can be installed both on land and offshore in water depths up to 20 ft (6 m) (Figure 2). Rock cores obtained during drilling are 2 in. (50 mm) in diameter. Each hole drilled was completed as a water-quality monitoring well (see Shinn and others, 1994, for diagrams of well completion). A flush-threaded 5-ft long, 2-in.-ID PVC well screen with 0.01-in. slots was attached to enough PVC casing (flush threads) such that between 1 and 2 ft of casing protruded from the open hole. Two well sites, at Alina's Reef and Pacific Reef, were completed using 1-in.-ID PVC screen and casing due to caving of the borehole. Coarse quartz sand (20-40 silica sand was poured into the annulus of the borehole to fill the space between the screen and formation. Too coarse to clog well-screen slots, the sand allows unrestricted passage of fluid from the porous limestone to the screen.

Figure 2. Drilling on Alina's Reef using SCUBA. [larger image]

A slurry of Portland cement was then poured into the annulus to fill voids and irregularities in the rock. The cement prevents water in the annulus, higher in the well, from entering the screened zone. Quick-setting hydraulic cement, composed of 1 part molding plaster (plaster of Paris) and 7 parts type II Portland cement, was mixed with water to form a stiff ball. The ball of cement was quickly taken to the bottom and hand-molded into the annulus around the PVC pipe. The plug of cement in the top of the hole creates a barrier between the borehole and surface water. Hydraulic cement sets in approximately 5 min and is very hard in a few hours. Next, the excess PVC pipe was sawed off with a hacksaw, leaving 15 to 30 cm protruding above the surface. A tight-fitting PVC end cap sealed the wells. Once the cement had hardened, the wells were developed by pumping until the water ran clear. Purging was accomplished by fitting a PVC end cap (equipped with 3/4-in. by 50- ft-long, 15-m, Tygon hose) over the 2-in.-diameter PVC wellhead. The other end of the hose was attached to a small 12-VDC-rubber impeller pump aboard the boat. The water pump, with a discharge rate of approximately 5 gal/min, was run for 5 to 10 min or until the water ran clear. The completed wells were allowed to equilibrate for 90 days before sampling commenced.

Water Sampling

Ground- and surface-water samples were collected using USGS water-quality sampling protocols that follow clean procedures for all constituents, whether constituents are nutrients, trace elements, wastewater compounds, or pesticides (Wilde and others, 1998). The following sections describe preparation, collection, preservation, and cleanup procedures.

Preparation

The bottles for each constituent went through a four-step cleaning process. The bottles (except baked-glass bottles) were first washed in Liquinox, then rinsed in tap water, followed by soaking in a 10% HCl solution for 30 min, and finally rinsed in de- ionized (DI) water. The same procedure was followed for all tubing, fittings, and equipment (the acid rinse was not used on metallic equipment). Bottles were capped, and labels placed on the bottles. Prior to field collection, bottles were pre-rinsed twice with de-ionized (DI) water to save time in the field. Bottles were sorted for each well site and placed in double zipper bags. The same doublebagging method was used for tubing and other equipment and supplies that would come in contact with water samples. Three or four days prior to field sampling, Gelman capsule filters (0.45-µm) were pre-conditioned with DI water. As long as pre-conditioned filters are kept on ice or refrigerated, the shelf life is up to 2 weeks.

Collection

Once on site, a diver was sent to connect a fitting to the wellhead. The fitting provided a tight seal so that surface water could not enter when pumping commenced. The fitting was attached to Polytetrafluoroethylene (PTFE) tubing that reached from the wellhead to the boat. The PTFE tubing was connected to peristaltic tubing (C-flex), which passed through a peristaltic pump and was then split, with one tube leading to a multi-probe (temperature, pH, oxygen-reduction potential (ORP), salinity, and dissolved oxygen) and the other to the sampling chamber. Several well volumes of water were pumped from the well. After readings on the probe stabilized, values were recorded in a notebook. The tubing to the probe was clamped and flow to the chamber commenced. Throughout water collection, 'clean hands/dirty hands' procedures were followed.

Figure 3. Equipment and collection-chamber layout on the R/V Halimeda in Biscayne National Park. Collection chamber, peristaltic pump, and flow-through multi-parameter probe can be seen on the table. [larger image]

A collection chamber was assembled, which was constructed of a PVC frame with a clear Polyethylene bag clipped to the frame (Figure 3). The chamber created an enclosure where samples were collected in bottles and helped assure that atmospheric deposition or other possible sources of contamination did not enter the sample. The person designated 'dirty hands' opened the outer zipper bag and the person designated 'clean hands' pulled the inner zipper bag out and placed it in the chamber. Only the 'clean-hands' person touched the bottles and tubing inside the chamber. Bottles were rinsed once and then filled to the appropriate level. This procedure was conducted for all bottles for each well. Finally, the bottles were removed from the chamber for preservation (acidification).

Preservation and Cleanup

Some studies require a second chamber called a preservation chamber for acidification of samples. After each well site was sampled and before anchor is pulled to move to next well site, the tubing was rinsed with a 0.1% Liquinox solution and followed by a DI rinse until Liquinox soap residual was unnoticeable.

Sample Analyses

Salinity (specific conductance), temperature, dissolved oxygen (DO), oxidation- reduction potential (ORP or Redox), and pH were measured in the field using a multi- parameter probe (YSI model 556MP). Hydrochemistry for 64 trace elements (Table 2) were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at Actlabs- Skyline in Tucson, Arizona.

Three elements (arsenic, nickel and bromine), typically determined in fresh water by this method, had serious interferences from the high concentrations of calcium and magnesium in seawater and had to be excluded from the results. Groundwater and surface-water nutrients (ammonium, nitrates, nitrites, total soluble nitrogen, total soluble phosphorus, and soluble reactive phosphorus) were analyzed on a nutrient auto-analyzer at the University of Florida. Dissolved organic carbon (DOC) was analyzed at the USGS Water Quality Laboratory in Ocala, FL, on a Shimadzu TOC-5050A analyzer with an ASI-5000A auto sampler. Determination of 66 wastewater compounds in ground- and surface-water samples were conducted at the U.S Geological Survey National Water Quality Lab in Denver, CO. USGS analytical procedures for wastewater compounds (USGS schedule 1433) were by solid-phase extraction (SPE) and subsequent gas-chromatograph mass spectrometry (GC-MS) analyses (Zaugg and others, 2002). Radium and radon samples were analyzed at the USGS Center for Coastal and Watershed Studies (CCWS) office in St. Petersburg. The St. Petersburg lab used an alpha-scintillation counter for measuring the four isotopes of radium (223, 224, 226, and 228). Strontium-isotope ratios (⁸⁷Sr to ⁸⁶Sr) were determined for selected samples by the University of Florida in Gainesville (August 2002) and Geochron Laboratories in Cambridge, MA (March 2004) using thermal ionization mass spectrometry (TIMS).

All samples were shipped immediately (via FedEx) upon return to the CCWS office in St. Petersburg. Holding times for nutrients were < 28 days per USGS protocols when kept frozen; ²²³Ra and ²²⁴Ra were run in house as soon as possible due to their short half-life (11.4 days and 3.7 days, respectively); trace elements were shipped to Actlabs and run within 4 to 6 weeks; and wastewater compounds were run in the order in which they were received at the USGS National Water Quality Laboratory (Denver, CO). Turn-around time ranged from 6 to 8 weeks.

Potentiometric Measurements

Our fourth task was to investigate the hydrology of the region by installing pressure transducers in many, if not all, of the wells. The transducers were started, placed in the wells, and left to collect data on pressure variations within the wells. A transducer was also mounted to the outside of the well to collect data on surface water-level changes (tides). Well- and surface-pressure data were compared to determine if potentiometric gradients occurred between subsurface and surface that would indicate either positive vertical flow (discharge) or negative vertical flow (recharge). This part of the study is ongoing, funded by the USGS Eastern Region, and is not reported here. The information will be useful for calculating nutrient or other chemical-enrichment loading of surface water by groundwater.