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Results of chemical analyses are provided in Table IV. Note in Table IV that data from sources other than the wells drilled for this study are also included. MO-171, MO-173, MO-175 and MO-176 are the Department of Environmental Protections (DEP) onshore monitoring wells adjacent to the RV camp on Saddlebunch Keys. 2307-SW and 2315-SW are surface-water samples from a canal adjacent to the RV camp, and 2315-EFF is a sample of the effluent collected from the sewage treatment plant at the RV camp. This is the effluent that enters the ground via the two 90-ft-deep (27.4 m) disposal wells. The disposal wells are cased to 60 ft (18.3 m). SCF is a sample from a privately owned 160-ft-deep (49 m) well on Key Largo north of Garden Cove. SCF was collected during sampling round 4 in November 1993, whereas the DEP wells were sampled during the first sampling round on February 22, 1993. Data from these samples may serve as useful background data for any future studies.
Wells were sampled four times during the one-year study period except for wells KL-5 and OR-3, which were sampled only three times. During the first sampling run, well KL-5 was not located due to weather and OR-3 could not be located during the last sampling run due to murky water. KLI-1A&B, the wells on high ground at Key Largo, were located on private property and after the first sampling round, the owner (on advice from his attorney) sealed the wells with cement.
This study was initiated on the hypothesis that nutrients entrained in the fresh water that enters Class V disposal wells would be trapped beneath the Q3 unconformity, would form a freshwater "bubble" and would migrate laterally. None of the monitoring wells, however, contained water fresher than sea water except in the shallow onshore wells and MO-173, one of the DEP monitoring wells at the Saddlebunch Keys location. At ORO-1B, salinities ranged from 5 to 8 ppt, undoubtedly in response to rainfall, and during the initial sampling of KLI-1B (the well destroyed by the owner), the salinity was 22.4 ppt. Water from the deep well (KLI-1A) at the same time had a salinity of 38.9 ppt. Water from MO-173, screened from 26 to 36 ft (8-11 m) depth, had a salinity of 33.9 ppt. The salinity of typical reef tract water-column water ranges from 35 to 36 ppt. The shallow well at the NOAA facility (KLI-2B) is greatly influenced by the nearby canal. Salinity in this well ranged between 36 and 41.5 ppt.
Waters from all the onshore deep wells, except MO-173, and offshore wells were either the same salinity as sea water or higher. For the most part, water from wells near shore had higher salinities than those farther from shore. Salinities in OR-5 and KL-5 water, for example, were similar to that of the overlying sea water, whereas the deeper nearshore well SB-1A had salinities higher than shallow well SB-1B. Similar observations were made at OR-1A and OR-1B. Except for MO-173, all of the onshore MO wells installed by DEP at Saddlebunch Keys adjacent to two disposal wells contained hypersaline water. These salinity analyses suggest that onshore disposal wells have not significantly reduced ground water salinity.
A plot of dissolved solids (ROE in Table IV) against chloride concentration for all groundwater points is shown in Figure 8. Average sea water contains 36,000 mg/L dissolved solids and 19,800 mg/L chloride concentration. This point of intersection is plotted in Figure 8. Note that the data for most wells fall below the average seawater line. Plots of dissolved solids concentration and chloride concentration against depth demonstrate the tendency for salinity to increase with depth (Figs. 9 and 10).
The ratio of dissolved solids to chloride concentrations for the average seawater analysis was used to calculate the change of dissolved solids of a sample above or below that expected from the concentration or dilution of sea water, assuming that chloride is a conservative solute. The equation used is as follows:
DELTADS=DS - (1.818CL) (1)
where DS and CL are the dissolved solids concentration and chloride concentration, respectively, of a sample, 1.818 is the ratio of DS to CL of the seawater analysis, and DELTADS is the elevation (or depression) of the dissolved solids concentration over what would be expected. A plot of DS against DELTADS shows that most of the samples with high DS have a positive DELTADS (Fig. 11). The elevation of dissolved solids is as high as 4,000 to 5,000 mg/L. Wells with DELTADS over 3,000 mg/L are SBB-1, SBB-2, SBB-3, SB-1A, SB-1B, SB-2, KL-1, OR-3, and OR-5. Excessive elevation in dissolved solids either results from mineralization of pore water or addition from other sources. Mineralization of pore water indicates residence time sufficiently long for dissolution of host rock or sediment to occur.
Hypersalinity of the ground water could have two sources: 1) evaporation through the thin vadose zone and possible dissolution of limestone by acidic rainfall as it passes through the vadose zone, is washed down to the groundwater table and is mixed by tidal pumping; or 2) during times of increased evaporation, the salinities in bays rise, especially in the shallow bays of the Lower Keys and upper Florida Bay, and because of increased density, hypersaline water moves downward into the groundwater system. Salinities as high as 70 ppt have recently been reported in Florida Bay (Mike Robblee, pers. commun., 1994) and salinities up to 60 ppt were reported during the 1950s (Ginsburg, 1956; McCallum and Stockman, 1964).
Three box plots were made to compare surface water with ground water (Figs. 12, 13, 14 and 15). An "S" has been added to each of the parameters plotted to distinguish surface waters. Salinity data are shown in Figure 12, including specific conductance, dissolved solids concentration, and chloride concentration. These parameters are very comparable between the two types of samples in terms of position and spread of the distribution. However, surface-water samples, not surprisingly, show more positive outliers than groundwater samples.
Data for three different nitrogen parameters are shown in Figure 13, including ammonia (NH4), dissolved ammonia+organic nitrogen (NH4+ORG-N, dissolved), and total ammonia+organic nitrogen (NH4+ORG-N, total). All of these parameters are in units of mg/L as N, for easy comparability. Groundwater samples for these parameters plot higher and are more variable than the surface-water samples. Also, the groundwater samples have more positive outliers. The differences for NH4 are the most striking.
Data for the three phosphorous parameters are shown in Figure 14. They are orthophosphate, dissolved phosphorus and total phosphorus, all in units of mg/L as P. These comparisons show tendencies similar to those for the nitrogen parameters shown in Figure 13.
Dissolved and total organic carbon (DOC and TOC) in mg/L as C are shown in Figure 15. Although the medians are similar for both parameters, in both cases the groundwater data are more positively skewed.
The odor of hydrogen sulfide (H2S) was prevalent in waters from most wells. H2S blackened the drill rods and silver filters used to filter samples collected for dissolved organic carbon analysis. Thus, during the last sampling round, H2S was measured in the field to confirm earlier observations. The field method is described in the methods section.
Measurements of dissolved H2S were divided into four levels, none, low, medium and high. The values for these levels approximate (0) for 0 mg/L, (1) for less than 1 mg/L, (2) for 1 to 4 mg/L, and (3) for greater than 5 mg/L. These data are plotted against well depth in Figure 16. This plot shows a tendency for an increase in H2S concentration with well depth. However, several shallow wells also have high H2S. Presence of H2S is a reliable indicator of anoxic conditions and indicates poor exchange of oxygen from the surface. Whether the organics required for the sulfate reduction, which produces H2S, result from sewage or natural inputs cannot be determined at this time. Only a small amount of organic matter can result in consumption of all the dissolved O2 in ground water. The O2 is easily depleted because the solubility of O2 in water is low (Freeze and Cherry, 1979). Two natural sources of organic matter are likely: 1) hypersaline water formed at the surface during dry periods moves downward into the ground water, carrying with it dissolved and particulate organic material, and 2) daily introduction into the ground water during tidal pumping, although this mechanism should introduce O2 as well.
U.S. Department of the Interior, U.S. Geological Survey
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