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open file report > OFR 94-276 > indications of contamination in ground water
Indications of contamination in ground water
The following is a discussion of measurements that suggest the possibility of contamination of ground water at the sites sampled. The kinds of contamination considered are those from injection-well effluent or septic-tank drain fields. The parameters measured are grouped and the order of discussion is nitrogen, phosphorous, organic carbon and bacteria.
A plot of NH4 by H2S concentrations indicates a relation between these two in the groundwater environment (Fig. 17). This is to be expected since NH4 is the dominant inorganic species of nitrogen present under reducing conditions (Stumm and Morgan, 1981). NH4 and NH4+ORG-N were both plotted against well depth (Figs. 18 and 19), and in both cases their concentrations tend to increase with depth. An interesting group of data with high concentrations on both plots has a well depth of from 30 to 37 ft (9.1-11.3 m). This may indicate a geologic control on the concentration of NH4 or may simply be an artifact of most wells falling in that depth range.
A background value for NH4 concentration from similar groundwater environments is needed for comparison. The average concentration from 26 USGS analyses of water from the Floridan aquifer system in Dade and Monroe Counties was 0.30 mg/L as N. These analyses came from 25 wells. The concentrations from these data, except for one analysis, are no higher than 0.48 mg/L as N. The average NH4 concentration from the Biscayne aquifer at a baseline site in southwestern Dade County was about 0.40 mg/L as N (Pitt et al., 1975). The mean value for NH4 concentration of all the groundwater samples in the present study is 0.33 mg/L as N.
High ratios of NH4 to ORG-N concentrations were found to indicate contamination from septic tanks in the Biscayne aquifer of Dade County (Pitt et al., 1975). This occurred at sites where the aquifer was relatively impermeable and conditions were reducing because recharge by rainfall was limited. At septic-tank sites where the aquifer was cavernous, rapid infiltration of rainfall carrying dissolved oxygen apparently allowed the oxidation of NH4 to NO3. Very low NH4 and high NO3 concentrations were observed at these sites. A plot of NH4 concentration by the ratio of NH4 to ORG-N concentrations is shown in Figure 20. Wells for which this ratio is greater than 2 and the NH4 concentration is more than 0.5 mg/L as N are MO-171, MO-175, KL-5 and OR-5 (Table V). Other wells with the concentration of NH4 greater than 0.5 mg/L are MO-173 and SCF-1.
Waters contaminated by human waste or fertilizers generally contain elevated levels of NO2+NO3,and NH4 (see sample 2315-EFF in Table IV). Samples with NO2+NO3 concentrations greater than 0.02 mg/L as N are given in Table V. Wells having significant concentrations of this parameter are all onshore and shallow: KLI-1B, KLI-2B, and ORO-1B. H2S was not found in these wells, indicating the environment is not reducing, and that any NH4 produced is being converted to NO3.
High concentrations of dissolved P were also found in well ORO-1B (Table V). The median concentration of dissolved P for all groundwater samples is 0.05 mg/L, and the upper quartile value is 0.06 mg/L. Wells with a concentration of 0.07 mg/L or greater, except for ORO-1B, are: MO-175, SBB-1, SB-3, KLI-1A, KL-3, KL-4, KL-5, SCF-1, OR- 1A, and OR-4 (Table V). Particulate P concentration was calculated as total P concentration minus dissolved P concentration. Wells in which particulate P was found at a level greater than 0.02 mg/L are SBB-2, SB-1A, KL-3, ORO-1B, OR-1A, OR-1B, and OR-4, (Table V). Particulate P may travel a greater distance in an aquifer than dissolved P if 1) the form of the dissolved P is mostly orthophosphate, the soluble reactive form, and 2) the size of the pores through which the water moves is large, as in a cavernous limestone. Both of these conditions could apply to this study. It should be pointed out, however, that organic peat is present on the Pleistocene rock surface just above the well screen in OR-4. This may be a local natural source of particulate P at this location.
Concentrations of TOC and DOC are shown in Figure 21. The values obtained on sample round 4 are approximately double those obtained during the first three rounds. The reason for this is unknown. The only change from previous procedures was the use of new 5/8-inch-diameter sample tubing. Attempts to correlate the organic-carbon parameters with other indicators of possible contamination were not successful. However, one relation that may be meaningful was that samples with high NH4 occurred at low values of DOC. Samples with particulate organic carbon (POC) greater than 1.5 mg/L as C are given in Table V. POC is TOC minus DOC. All of the wells in the Lower Keys area had samples that met this minimum value for POC.
NOAA/NURC nutrient analyses
Duplicate samples for the three major dissolved nutrients, NH4, PO4 and NO2+NO3, were analyzed courtesy of the NOAA/NURC laboratory. These data, reported in molar units, are provided in Appendix C for comparison and confirmation. The NOAA analytical method is more sensitive than the USGS method and shows greater variability. For example, NO2+NO3 was below the detection level (0.02 mg/L as N) of the majority of USGS analyses (Table IV), but well within the detection level of the NOAA analyses. The lower detection limit for USGS analyses for NH4 was also 0.02 mg/L as N. However, the majority of well-water samples were above the limit of detection. Seawater samples, however, were usually close to the 0.02 NH4 mg/L as N USGS detection limit but well within the detection limit for the method used by NOAA/NURC. Because of high sensitivity, there is also considerably greater variation in the NOAA data. There are also some unexplained spikes in the data that are not picked up in the USGS data. Perhaps this is due to the six to nine months the second and third rounds of samples were in the freezer, or possible thawing during transport from Key Largo to the FIU laboratory in Miami.
Fecal coliform (FC) and/or fecal streptococci (FS) were found in 14 wells (see Table V). Colonies were found on more than one sampling round at wells SB-1A, SB-1B, KLI-2B and ORO-1B. Fecal coliform were found in only seven wells: SB-1A, SB-1B, SB-2, SB-3, KLI-2B, ORO-1B, and OR-3 (only one colony per 100 ml was found in one sample from OR-3). The values given in Table V for well OR-4, round 3, are apparently for bacteria other than FC or FS based on the color of the colonies. Additional data supporting the presence of fecal bacteria have recently been found in KLI-2B, KL-1, KL-2 ORO-1B and OR-2 (J. Paul and J. Rose, pers. commun., 1994). A ratio of fecal coliform to fecal strep of less than 0.7 indicates with high probability that the wastes are of animal origin, whereas if the ratio is over 4, it is nearly certain that the wastes are of human origin (Steel and McGhee, 1979). However, it is not known if this is true for an anoxic, saltwater environment. The death rate of coliform bacteria (fecal coliform) in salt water is much higher than in fresh water. Wells from which samples clearly contained a FC/FS ratio of greater than 4 are SB-1A, SB-1B, SB-2, SB-3, and KLI-2B.
U.S. Department of the Interior, U.S. Geological Survey
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