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An Isotopic Study of Northeast Everglades National Park and Adjacent Urban Areas
Chapter II: Technical Results
II.2 Sample Collection and Isotope Analysis
Samples were collected during a period ranging from January 1996 to December 1998. The monitoring network (figure 1) was modified over time with an emphasis placed on sites within the focus area. Early during the sampling program only a few sites were tested on a non-regular basis. Beginning in February 1997, samples were collected on a regular monthly schedule. Furthermore it is important to note that as the research continued, sites were continually added until the completion of sampling. Overall, 580 samples were collected at 26 different sites, which included the two lakes. A summary of the isotope monitoring site characteristics is provided in Table 2. For a complete set of isotope data for all sites (except for the lakes), please see Appendix A.
All samples were collected in duplicate using glass scintillation vials. These vials were filled to the top with sample water and sealed with a screw-on top. A layer of parafilm was then wrapped around the vials in order to prevent evaporation. Samples were collected from groundwater, municipal pumping wells, surface water (including lakes), and rainwater. Groundwater samples were collected using a portable pump connected to a 12-volt battery. The intake end of the pump hose was lowered into the well casing while the outflow end was allowed to flow into the scintillation vial for sample collection. For shallow wells, the pump was allowed to draw water from the well for five minutes prior to sample collection to assure that a representative sample was collected. For deep groundwater sites, the well was purged for fifteen minutes. The production well samples were taken directly from a spigot attached to the pumping well. These samples were obtained from either Well 29 or Well 30 at the West Wellfield, depending upon which pump was in operation on the day of sampling. Surface water samples from the Everglades and canal sites were collected by immersing the scintillation vials below the water surface. At the lakes, a submersible pump was used to collect water from ten-foot depth intervals from the approximate center of each lake. These samples were analyzed by Herrera, 2000. Herrera found that the isotopic composition did not vary significantly with depth and therefore only the depth averaged values were utilized in this study. Rainwater collection for isotope analysis provided a somewhat unique problem, as collected rainwater must be shielded from evaporation effects. In order to accomplish this, rainwater collection bottles were filled with a two inch deep layer of mineral oil prior to use. These bottles were fitted with a collection funnel and an air release port. As rain entered the collection apparatus, the buoyant mineral oil floated on top of the collected rain, preventing rainwater interaction with the air and insuring the isotopic integrity of the sample. Once the rainwater was collected, a syringe was used to transfer the rainwater from below the mineral oil layer into the scintillation vials.
Oxygen-18 analysis included a CO2 equilibration procedure utilizing a syringe as described by Matsui, 1980. This syringe technique was compared with the more traditional CO2 equilibration procedure (Epstein and Mayeda 1953) with good results (Standard deviation 0.18). Samples for deuterium determinations were processed using one of two methods. The first method utilized a uranium furnace as outlined by Bigeleisen et al. 1952. The second method involved the use of a chromium furnace (Gehre et al. 1996). Results from the chromium and uranium furnace were found to be statisically the same. After initial processing, samples were subject to mass spectrometry (Prism, Micromass, Inc.) for δ18O and δD determinations. "Del" or "δ" values are given by
where R1/R is the ratio of the "heavier" isotope to its more abundant "lighter" form. For example, for oxygen-18, the ratio is given by 18O/16O. The reference is the ratio of 18O/16O or D/H of Vienna Standard Mean Ocean Water (VSMOW) provided by the National Bureau of Standards. Units are provided in "per mil" or "."
A considerable effort was provided for quality assurance and quality control. Details concerning these efforts is provided in the report titled, "An Isotopic Study of Two Rock Mining Lakes." This report is dated March 8, 2000 and authored by Solo-Gabriele and Herrera.
Basics Concerning Isotope Theory
Isotopes serve as conservative tracers of different water sources as long as distinct differences are observed in the isotopic composition of each source. Water bodies that have undergone extensive evaporation will be enriched in heavier isotopes (i.e. larger δ18O and δD values). Liquids formed by the condensation of gases, such as rainfall, tend to be enriched in the lighter isotope which result in lower values of δ18O and δD. Furthermore, rainfall is also characterized by a universal relationship between deuterium and oxygen-18, commonly called the meteoric water line and described by the equation: δD= 8δ18O + 10 (Craig 1961). The relationship between oxygen-18 and deuterium for water bodies that have undergone evaporation, on the other hand, are predicted by the following equation: δD= Mδ18O + I where M<8. Thus evaporative water can be identified on the basis of its δD and δ18O values and its deviation from the meteroic water line.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM(TJE)