Hydrogeology and Groundwater Quality of Highlands ... - USGS

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58 Hydrogeology and Groundwater Quality of Highlands County, Florida 27°30’ 27°15’ 27°00’ 19 HARDEE COUNTY 220 385 140 DESOTO COUNTY 75 482 13 204 6.9 274 381 198 115 CHARLOTTE COUNTY Figure 46. Generalized distribution of sulfate concentrations in water from the Upper Floridan aquifer. Hardness in water sampled from the Upper Floridan aquifer in Highlands County ranged from 64 to 410 mg/L (fig. 48, app. 2), with a median value of 130 mg/L (table 4). Lowest concentrations (less than 120 mg/L) occurred in the recharge area, primarily along or adjacent to the Lake Wales Ridge. The highest concentrations (greater than 180 mg/L) occurred primarily in the eastern and southwestern parts 81°30’ 81°15’ 81°00’ 18 Avon Park 27 98 6.7 9.2 58 37 POLK COUNTY 7.8 2.4 130 0.57 Kuhlman 36 HIGHLANDS COUNTY Sebring 0.20 7.2 10 4.8 17 26 14 27 Lake Placid 6.0 14 7.0 Archbold Biological Station Base modified from U.S. Geological Survey digital data, 1:100,000, 1985 Albers NAD projection 1983 SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER Dashed where approximate < 10 10 - 29 30 - 250 > 250 ? 0 0 102 14 26 Venus 1.7 27 43 19 36
Nitrate Concentration Unlike the surficial aquifer, nitrate concentrations in groundwater from the Upper Floridan aquifer in Highlands County generally were very low. Nitrate concentrations in water ranged from less than 0.04 to 0.22 mg/L (app. 3), and were less than the detection limit in 21 of 22 samples from the Upper Floridan aquifer. Processes that may be responsible for the low nitrate concentrations in groundwater in the Upper Floridan aquifer underlying Highlands County are denitrification and dilution. Where sinkholes are absent, nitrate concentrations typically decrease with depth below land surface. Denitrification is the primary process likely responsible for the reduction in nitrate concentration with depth. Nitrate accumulates in groundwater under aerobic conditions (nitrification) and is converted to reduced species of nitrogen under anaerobic conditions (denitrification). Geochemical factors required for denitrification to occur include: (1) the presence of nitrogen oxides as terminal electron acceptors; (2) the presence of bacteria capable of reducing nitrate; (3) suitable electron donors, such as organic carbon and iron; and (4) anaerobic conditions or restricted availability of dissolved oxygen (Tihansky and Sacks, 1997). Low nitrate concentrations in the Upper Floridan aquifer can also result from the dilution of high nitrate water from the surficial aquifer with low nitrate water naturally present in the Upper Floridan aquifer. However, the occurrence of somewhat elevated chloride and sulfate concentrations (two indicators of agricultural activity) and elevated specific conductance above background levels from wells completed in the Upper Floridan aquifer indicate that dilution is probably not the dominant process responsible for the decrease in nitrate with depth. Where sinkholes are present, breaches in the IAS/ICU SULFATE TO CHLORIDE EQUIVALENT RATIO 1,000 100 10 1 0.10 EXPLANATION UPPER FLORIDAN AQUIFER Well 68 GYPSUM DISSOLUTION LINE SEAWATER MIXING TREND 0.01 1 10 100 1,000 10,000 SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER Assessment of Groundwater Quality 59 can allow nitrate from the surficial aquifer to directly enter the Upper Floridan aquifer, which may explain locally elevated nitrate concentrations in groundwater from the aquifer. Vertical Distribution of Chloride and Sulfate Concentrations Data from deep wells drilled throughout Florida indicate that mineralized water underlies the entire State. The depth to the mineralized water varies considerably and is controlled by the subsurface stratigraphy, land-surface altitude, and the altitude of the freshwater head (Franks, 1982). Several monitoring wells have been drilled into the middle confining units in Highlands and adjacent counties to acquire information about variations in water quality within the intermediate aquifer system and Upper Floridan aquifer. Profiles of chloride and sulfate concentrations of four monitoring wells drilled to nearly 2,000 ft are illustrated in figure 49. Water samples from these wells were either collected through the drill stem as the wells were drilled or collected using bailer or packer test methods. In general, limited well data indicate that in much of the study area, groundwater having chloride and sulfate concentrations of less than 100 mg/L extends to considerable depths in the Upper Floridan aquifer, and chloride and sulfate concentrations generally increase with depth. Sulfate concentrations increase rapidly with depth just above the top of middle confining unit II at ROMP 13, 14, and 29A. Chloride concentrations increase in a similar manner in ROMP 13 and ROMP 14. At ROMP 13 (figs. 1 and 49), chloride concentrations are less than 100 mg/L in the intermediate aquifer system and the upper part of the Upper Floridan aquifer (Baldini and Rappuhn, 1998). Chloride concentrations in the intermediate aquifer system ranged from about 30 to 90 mg/L. Chloride concentrations in much of the Upper Floridan aquifer were slightly higher, ranging from about 50 to 100 mg/L between 699 and 1,579 ft below land surface. Chloride concentrations increased rapidly from 107 to 1,981 mg/L between 1,579 ft and 1,610 ft below land surface. Chloride concentrations further increased from 1,981 to 4,804 mg/L at depths of between 1,610 and 2,075 ft below land surface. Sulfate concentrations in ROMP 13 increased slightly with depth in the intermediate aquifer system, ranging from about 2 to 64 mg/L. In the Upper Floridan aquifer, sulfate concentrations changed little with depth, ranging from 42 to 91 mg/L in the interval from 699 and 1,579 ft. Sulfate concentrations increased sharply to 745 mg/L at about 1,610 ft and eventually leveled off at about 1,700 mg/L. This sharp increase in the mineralization of the groundwater, which was also observed at ROMP 14 and to a lesser degree at ROMP 28, indicates a less Figure 47. Relation between sulfate-chloride equivalent ratio and sulfate concentration in water from the Upper Floridan aquifer.
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Prepared in cooperation with Highla
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Acknowledgments The author grateful
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viii Conversion Factors Multiply By
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Hydrogeology and Groundwater Quality of Highlands ... - USGS

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