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BROWARD COUNTY GROUNDWATER QUALITY - AN OVERVIEW
By
Broward County Department of Planning and Environmental Protection
Water Resources Division
Water Resources Policy and Planning Section
Technical Report Series
TR-01-07
August 2001
EXECUTIVE SUMMARY
Information on the spatial and time variation of groundwater quality in Broward County is needed to support water resources planning. The quality of the groundwater affects the cost of treating it for public use. Consequently, spatial- and time-trends might affect decisions related to the location of future water supply facilities. This report summarizes and evaluates relevant published information on the general quality of groundwater in the County. Local effects of point source contamination are not included. The information provided pertains to the Surficial Aquifer System, which contains the Biscayne Aquifer.
Most of the groundwater is a calcium-bicarbonate type water, because the subsurface materials are rich in calcite (calcium carbonate). Total dissolved solids (TDS) in water includes all solid material in solution. TDS in the Biscayne aquifer is lowest in the coastal areas, where the concentration is generally less than 200 milligrams per liter (mg/L). It increases westward, and reaches values greater than the 500 mg/L federal secondary drinking water standard in some places. Secondary standards are guidelines for constituents that may cause aesthetic effects.
Water quality index parameters include hardness, pH, and color. Hardness is caused primarily by calcium. The groundwater of the County ranges from hard to very hard. In general, hardness increases westward and with depth. The value of pH in groundwater is generally well within the range of the secondary drinking water standard, 6.5 to 8.5. Color has been described as generally increasing toward the west and decreasing with depth. Color is an indicator of organic carbon, which has the potential for forming carcinogenic trihalomethane compounds when the water is chlorinated. Consequently, treatment facilities must be designed to deal with color.
Major chemical constituents generally found in groundwater are calcium, bicarbonate, sodium, chloride, sulfate, magnesium, and silica. In Broward County, calcium and bicarbonate dominate. Sodium and chloride tend to be associated with saltwater intrusion, or relic sea water present at low concentrations in the deep part of the system in much of the area. Chloride gives water a salty taste at about 1000 mg/L, when the dominant positive ion is calcium. The secondary drinking water standard for chloride is 250 mg/L, which is exceeded where sea water intrusion has occurred. Sulfate in the groundwater is probably from several sources, including coastal mixing with sea water, decomposition of organic materials, and fertilizers. Sulfate is the only constituent that has shown a decrease with time in Broward County. Low concentrations of magnesium and silica are found in the County.
Secondary constituents found at low concentrations in most groundwater are boron, carbonate, strontium, fluoride, iron, nitrate, and potassium. No information on boron, carbonate, or strontium in the County was found in the literature reviewed. The concentration of fluoride may be limited by the solubility of the mineral fluorite (calcium fluoride). Groundwater in Broward County, in general, contains less than 1.0 mg/L of fluoride and usually less than 0.5 mg/L. Groundwater in the County is normally high in iron, generally exceeding 1 mg/L, which is high enough to cause staining and impart taste. It can be removed at water treatment plants. Nitrate concentrations found in the literature ranged from 0.01 to greater than 4 mg/L nitrate as nitrogen. The primary drinking water standard is 10 mg/L. Nitrate has been reported to decrease with depth. Denitrification of nitrate to gaseous elemental nitrogen can occur by bacterial action. Potassium tends to be removed in groundwater systems by fixation to clay minerals. No quantitative information on potassium concentrations in Broward County was found in the literature reviewed. It is reported to decrease with depth.
Minor constituents that generally occur in groundwater naturally at concentrations less than 0.1 mg/L, and are also subject to water quality standards, include aluminum, antimony, arsenic, barium, cadmium, chromium, copper, lead, manganese, mercury, nickel, phosphate, selenium, and zinc. No information on concentrations of most of these constituents specific to Broward County was found in the literature reviewed. Chromium may be released by limestone dissolution and is also commonly used in industrial processes. It has been reported in County groundwater at concentrations ranging up to 26 micrograms per liter (μg/L). The primary drinking water standard is 100 μg/L. The mobility of lead in groundwater systems is low due to the low solubility of lead carbonates and other factors. Nevertheless, low concentrations of lead may come from the solution of limestone. The median concentration reported for lead in the Biscayne Aquifer is 3.25 μg/L, but one anomalous concentration was 5,500 μg/L. The primary drinking water standard for lead is 15 μg/L. The chemistry of phosphorous favors precipitation in groundwater, so concentrations tend to be low. The median concentration of phosphate (as phosphorous) in the County has been reported as 0.02 mg/L. The Broward County groundwater standard is 0.01 mg/L. No federal drinking water standard is applied to phosphate. Groundwater containing bicarbonate will tend to precipitate zinc. Reported zinc concentrations in the County range to 260 μg/L. The secondary drinking water standard is 5000 μg/L.
Concentrations of natural dissolved organic matter are low. Most is derived from decaying organic matter in soils that the water has passed through. It contributes to the color index discussed above. A plethora of synthetic organic compounds may enter the system. Fortunately, many mechanisms tend to protect the groundwater. These include precipitation, chemical degradation, volatilization, biological degradation, biological uptake, and adsorption. Description of contaminated sites is beyond the scope of this report. No general description of the distribution of synthetic organic contaminants in groundwater in Broward County was found in the literature reviewed. Published information on total organic carbon (TOC) indicates a median value of 15 mg/L in the County. No water quality standard is applied to TOC.
Hydrogen sulfide and methane gas have been noticed in wells throughout the County. These gases may be produced from organic matter in the groundwater system. Hydrogen sulfide may cause an unpleasant odor. Methane released to the atmosphere in a confined space may explode. No information on other gases in the groundwater was found. These other gases include radon.
In summary, the quality and chemistry of groundwater in Broward County vary regionally from east to west and with depth. The variations are expressed in trends, and much variation is superimposed on the trends. Constituents that increase to the west Include TDS, hardness, color, chloride (excluding coastal sea water intrusion), fluoride, iron, and TOC. Constituents reported to decrease with depth are color, nitrate, and potassium. Sulfate is reported to have decreased with time. No constituents were reported to have increased with time. These trends are consistent with geochemical theory in a system containing much calcite, organic soils to the west, a decline in agricultural activity, and proximity to the ocean.
Regarding recommendations for future groundwater quality studies, it would seem prudent to analyze time-trends from individual monitoring wells periodically to identify any tendency for water quality to decline.
BROWARD COUNTY GROUNDWATER QUALITY - AN OVERVIEW
Information on the spatial and time variation of groundwater quality in Broward County is needed to support water resources planning. The quality of the water affects the treatment costs of public water supplies, and thus might affect the placement of future public water supply wells. Information on the current quality of the water and past changes in water quality is needed to identify trends that may affect water supplies, and to provide baseline data to identify future changes. Potential sources of future changes include fertilizers, pesticides, wastewater, leaks and spills of hazardous substances, and saltwater.
PURPOSE AND SCOPE
The purpose of this report is to summarize and evaluate published information on the quality of groundwater in the urban area of Broward County, Florida, located east of the Everglades Water Conservation Areas (Figure 1). Hereafter, this urban portion of the county will simply be called the County. The documents referenced herein provide an overview of the results of documented investigations of groundwater quality. However, they do not represent all documents that contain redundant information. Local effects of point source contamination on water quality are not included in the scope of this report.
Figure 1. Location of Broward County Urban Area.
Groundwater quality parameter values are compared to drinking water standards in this report. The primary standards are the national standards that apply to public water systems. They establish legal limits on constituents that can adversely affect public health. The secondary standards are constituents that may cause cosmetic or aesthetic effects.
SURFICIAL AQUIFER SYSTEM
The information provided below pertains to the Surficial Aquifer System in the County. The Surficial Aquifer System is divided into three units, the Upper Zone, the Biscayne Aquifer, and the Lower Zone (Figure 2) as mapped by Restrepo and others (1992, Figures A-2 through A-5). The Upper Zone extends from the water table to the top of the Biscayne Aquifer. The elevation of its base ranges from about 10 feet below sea level in the southeastern and southwestern corners of the County to 100 feet below sea level in the northern part of the County. The unit consists of quartz sand, silt, limestone, and surficial peat and muck layers. It is generally moderately permeable. The Biscayne Aquifer underlies the Upper Zone. It contains much extremely porous and permeable limestone and is the water production zone for high capacity wells in the County. The Biscayne Aquifer ranges from less than 20 feet to more than 120 feet thick, being thickest in the southeastern and central parts of the County and thinning toward the Everglades. The Lower Zone is a moderate to low permeability zone composed of sand, silt, marl, and limestone. Its thickness ranges from about 30 to 250 feet, being thinnest in the central part of the County. The elevation of the base of the Surficial Aquifer System ranges from about 200 feet below sea level in the west part of the County to about 320 feet below sea level at the coast. Since the land surface in the County is near sea level, these figures also represent the approximate depth to the base of the Surficial Aquifer System.
Figure 2. Generalized hydrogeologic column of the Surficial Aquifer System.
WATER CLASSIFICATION
Radell and Katz (1991) examined analyses of major-ions from 189 wells sampling the Biscayne Aquifer and found that 84% of the analyses were calcium-bicarbonate type. The calcium bicarbonate water is considered the result of the presence of calcite and aragonite (Maddox and others, 1992). These minerals are composed of calcium carbonate. For the remaining analyses, 9% were calcium-sodium-bicarbonate type, and 7% were mixed cation-chloride type. Samples from one well near the coast were sodium-chloride type. Most of the samples that were not calcium-bicarbonate type were from wells near the coast, near canals or conservation areas, or near well fields. No significant seasonal variation is apparent in the major-ion concentrations, although recharge to the aquifer varies seasonally (Sonenshein, 1997; Radell and Katz, 1991) due to greater infiltration of rainwater to the subsurface during the wet summer season than in the dry winter season. Maddox and others (1992, Figure 56e) provide a small-scale map of water types in the surficial aquifer system in the South Florida Water Management District (SFWMD), which includes Broward County.
The water chemistry classification used by Radell and Katz (1991) is described in Davis and DeWiest (1996, p. 119). The classification is a quantitative way to express the dominant anions and cations in a water sample. It is based primarily on the concentrations of calcium, magnesium, and sodium cations; and bicarbonate, chloride, and sulfate anions. These are major constituents commonly present in concentrations greater than 1 mg/L in potable groundwater everywhere. "Mixed cation" means that all three major cations are present in about equal amounts.
TOTAL DISSOLVED SOLIDS
Total dissolved solids (TDS) in a water sample includes all solid material in solution. It does not include suspended sediment, colloids, or dissolved gases. Water for most domestic and industrial uses should be less than 1000 mg/L TDS (Davis and De Wiest, 1966). The secondary drinking water standard is 500 mg/L. Reported TDS sometimes does include suspended sediment even though it should not. If a sample was not filtered, the analysis reported contains the contribution of unfiltered suspended solids as well as dissolved solids. Some of the published documents on groundwater quality reviewed herein do not explicitly state whether they are based on filtered samples. Whether a sample is filtered, may affect concentrations reported for individual chemical constituents, as well.
TDS in the Biscayne Aquifer is lowest in the coastal areas, where the concentration is generally less than 200 mg/L (Herr and Shaw, 1989, Fig. 1-3; Benham-Blair & Associates, 1970), although values may be higher at depths where saltwater is encountered. TDS increases westward across the County, such that TDS in the Biscayne Aquifer reaches values greater than the 500 mg/L secondary water quality standard. The areas where this occurs are not clearly delineated. TDS increases with depth below land surface throughout the County (Sonenshein, 1997). Maps of TDS at depth intervals of 0-50 feet, 50-100 feet, and 100-200 feet in the Broward Water Supply Plan Appendix do not clearly show the trend of increasing TDS with depth (SFWMD, 1991, Figures F11 - F13).
Some relationship between TDS and land use may exist. Sonenshein (1997, p. 45-46) reported that the relation between TDS and land use was not clear, but lower median dissolved-solids concentrations were found at wells related to Urban Commercial/Industrial/Transportation land use. Benham, Blair & Associates (1970) reported that the mineral content of water in the Biscayne Aquifer was highest to the northwest where large irrigated farms were present.
INDEX PARAMETERS
HARDNESS
Hardness of water is a measure of its tendency to precipitate soap and reduce its cleansing action. Hard water also forms boiler scales. Hardness is caused primarily by calcium and magnesium and is normally recorded as the total concentration of these ions expressed in mg/L equivalent calcium carbonate (CaCO3). Other cations such as iron also contribute to hardness (Fetter, 1980, p. 364). A value less than 60 mg/L hardness as CaCO3, indicates soft water and a value greater than 180 indicates hard water.
The calcium-bicarbonate groundwater of the County ranges from hard to very hard. In general, the hardness increases with depth and with distance west of the Atlantic Coastal Ridge (Benham-Blair & Associates 1970; McCoy and Hardee, 1970; Herr and Shaw, 1989). Blenham-Blair & Associates (1970) stated that hardness values range from about 100 to 200 mg/L CaCO3 beneath the coastal ridge to more than 400 mg/L in the western part of the County. SFWMD (1991, Figures F-43 through F-45) presents maps of hardness at 0-50, 50-100, and 100-200 feet depth intervals. These maps show hardness as variable, but with a tendency to increase westward.
pH
The value of pH is a measure of the acidity of water. A value of 7 standard units is neutral, neither acidic nor alkaline. Values less than 7 are acidic, and values greater than 7 are alkaline. A calcium-bicarbonate water is buffered with respect to pH, so that the water has the capacity to neutralize acids and bases to some extent. Values of pH in the Surficial Aquifer System mapped my Maddox and others (1992, Figure 9e) were mostly greater than 7.0 and less than 7.5. No spatial trend is shown, although the map is very small scale. The secondary drinking water standard for pH is the range of 6.5 to 8.5.
COLOR
Color is a physical property of natural water that usually results from leaching of organic debris. It is true color after turbidity has been removed, and is measured on an intensity scale. A color below 10 units on the scale is barely noticeable to the casual observer. Color above 15 or 20 units is considered objectionable (McCoy and Hardee, 1970; Sherwood, 1959), and color above 15 units exceeds secondary drinking water standards. Color caused by iron may stain plumbing and leave stains on buildings and sidewalks when the water is used for lawn irrigation. Such staining is common where lawns are sprinkled with untreated water.
Color in groundwater of the County is thought to be caused by organic matter extracted from peat and decaying vegetation, and may also be partly due to iron in solution. Organic matter from microbial decay of leaf litter, soil organics, and soil biota waste products may color water (Maddox and others, 1992; Sherwood, 1959). Highly colored water often retains an earthy odor (Tarver, 1964). Color is important as an indicator of organic carbon, which has the potential for forming trihalomethane compounds (THMs) when the water is chlorinated (Maddox and others, 1992). Trihalomethanes are carcinogenic. Consequently, treatment facilities must be designed to remove color, maintain low levels of THMs, and disinfect the water.
Color has been described as generally low in the Atlantic Coastal Ridge and generally higher to the west (Tarver, 1964). Maps in SFWMD (1991, Figures F-23 through F-25) do not show this trend. They do show color generally decreasing with depth. The greatest color value on these maps is an anomalous 253 units measured in the central part of the County. Other values range from 48 to 110 units in the 0 to 100 feet depth interval, and from 10 to 56 units in the 100 to 200 feet depth interval. Most of the values shown on these maps are greater than 20 units.
MAJOR CONSTITUENTS
Davis and DeWiest (1966, Table 4.2) proposed a classification of groundwater constituents based on abundance. In this classification, trace constituents are those generally present at concentrations less than 0.001 mg/L. Minor constituents are generally present in the range of 0.00001 to 0.1 mg/L. Secondary constituents tend to range from 0.01 to 10 mg/L, and other chemical constituents are those commonly found in potable groundwater at concentrations of 1.0 to 1000 mg/L. Davis and DeWiest (1966) list the major cations as sodium, calcium, and magnesium. They list the major anions as bicarbonate, sulfate, and chloride. Silica is an essentially nonionized constituent that is commonly found at concentrations greater than 1.0 mg/L.
BICARBONATE
Calcite (CaCO3) is a common mineral constituent in all parts of the Surficial Aquifer System and is the primary constituent of the Biscayne Aquifer. Under the pH conditions in this aquifer system, laboratory experiments indicate that water in intimate contact with calcite will become saturated with calcium (Ca+) and bicarbonate (HCO3-) in a matter of hours or days (Freeze and Cheery, 1979, p. 246). Consequently, chemical equilibrium explains the dominance of the bicarbonate anion and the calcium cation in the Surficial Aquifer System.
Sonenshein(1997) and Maddox and others (1992) report that bicarbonate concentrations increase with depth. Sonenshein states that the concentration is areally uniform and exhibits no seasonal variation. SFWMD (1991) presents maps that show alkalinity as CaCO3. Under the ambient pH, alkalinity is produced almost exclusively by bicarbonate ions (Davis and DeWiest, 1966, P. 106-107). Consequently, the SFWMD alkalinity maps indicate the distribution of bicarbonate concentrations. These maps show no obvious alkalinity trends.
CALCIUM
The calcium concentrations, like the bicarbonate concentrations, should be controlled by chemical equilibrium with calcite. Sonenshein (1997) and Maddox and others (1992) report that calcium concentrations increase with depth, and Sonenshein states that the concentration is areally uniform and exhibits no seasonal variation. Calcium is the main cause of water hardness, which is discussed above.
The only map of calcium concentration found is a small-scale map in Maddox and others (1992), which shows calcium increasing from about 80 mg/L in the eastern part of the County to more than 100 mg/L in the western part. This westward increase is consistent with the westward increase in hardness described above.
CHLORIDE
The primary source of chloride in groundwater in the County is sea water. Chloride from ancient sea water may be entrapped in sediments, and chloride from modern sea water may penetrate aquifers in coastal areas. Entrapped sea water has been proposed as the source of some chloride found in the Surficial Aquifer System (Howie, 1987, citing Parker and others, 1955). Coastal sea water intrusion has been described by a number of investigators. Chloride from sea water also enters Florida's aquifers via precipitation that has mixed with aerosols derived from sea spray (Maddox and others, 1992). Other possible modes of entry include flow from tidal canals to bank storage during rising tides, seepage from soils fertilized with potassium chloride, and seepage of septic tank effluent containing chloride.
All chloride salts are very soluble, so chloride is not removed from water by natural chemical precipitation unless it is concentrated by evaporation or freezing. Chloride is not much affected by adsorption or biological activity (Davis and DeWiest, 1966, p. 110). Consequently, chloride is a conservative constituent in groundwater.
The secondary drinking water standard for chloride is 250 mg/L. When the major cation is sodium, water with greater than 250 mg/L chloride has a salty taste. In water where the predominant cations are calcium and magnesium, the chloride concentration may be as high as 1000 mg/L before the water tastes salty (Herr and Shaw, 1989).
Chloride concentrations reported in the Surficial Aquifer System range from less than 10 mg/L (SFWMD, 1991, Figure F-5) to 17,000 mg/L (Tarver, 1964, Figure 19; Koszalka, 1995, Figure 3). The two high concentrations are in the Pompano Beach area between Federal Highway 1 and the Intracoastal Waterway. They are slightly less than the concentration in sea water (about 19,000 mg/L). No information on chloride concentrations between the Intracoastal Waterway and the ocean were discovered in the documents examined.
Howie (1987) presented the results of drilling monitoring wells at 27 sites along four east-west transects across the County. One transect was along the northern boundary of the County, one was from the Pompano Wellfield and along the Cypress Creek Canal, one was through the Dixie Wellfield and along the North New River Canal, and one was along the south boundary of the County. All but two of the wells in these transects were drilled to the base of the Surficial Aquifer System. Only three wells reached the base of the aquifer system without encountering water with at least 250 mg/L chloride. One of these was near the western edge of the Pompano Wellfield, and another was about five miles west of the Pompano Wellfield. The third was on the south boundary of the county about 6 miles east of the Everglades. All wells drilled in the Everglades encountered 250 mg/L chloride above the base of the Surficial Aquifer System. Consequently, a 250 mg/L chloride isosurface must be present within the Surficial Aquifer System throughout much of the county. An isosurface is a surface in three-dimensional space where the concentration has a particular value. It separates concentrations greater than the particular value from concentrations less than that value. Generally concentrations above a 250 mg/L isosurface would be less than 250 mg/L and concentrations below the isosurface would be greater than 250 mg/L.
Howie (1987) presents graphs of depth versus specific conductivity in the individual monitoring wells. Most of these graphs show abrupt transitions from fresh water to salty water. Such abrupt changes of chloride concentration with depth are also seen in similar graphs presented by Hydrologic Associates (1998) and Hazen and Sawyer and Missimer (1998). Some graphs of chloride versus depth in monitoring wells show chloride concentration rising rapidly with depth through a transition zone and then becoming virtually constant with depth below the transition zone. These constant concentrations are all less than sea water concentration.
Hazen and Sawyer and Missimer (1998, Figure 3-8) show the slope of the 1,000 mg/L chloride isosurface along an east-west line of monitoring wells located from about 7,000 to 12,000 feet from the coast in the Hollywood area. The isosurface declines from about 85 feet to 210 feet in 4,300 feet of horizontal distance, a gradient of 2 percent (1.1 degrees).
Chloride concentrations may be affected by upconing beneath wellfields as suggested by Parada and Sanchez (1986, p. B-9) for the Dixie Wellfield. Chloride concentrations may also be affected by seepage from canals (Parada and Sanchez, 1986; Merritt, 1996).
Maps showing chloride concentration in depth intervals of 0-50, 50-100, and 100-200 feet are presented in SFWMD (1991). These maps do not include concentrations greater than 104 mg/L, so do not show concentrations below the 250 mg/L isosurface. No trends are obvious on these three maps. Herr and Shaw (1989, Figure 1-4) provide a map of chloride concentration in the Surficial Aquifer System excluding concentrations in the coastal area. This map shows concentrations increasing westward from about 60 mg/L in the middle part of the County to greater than 200 mg/L in the Everglades. This trend is ascribed to incomplete flushing of connate sea water in the western part of the county. Maddox and others (1992) show a small scale map of chloride concentration that shows the chloride concentration in the Surficial Aquifer System increasing westward in the County from 10 mg/L to 50 mg/L.
Several documents show lines related to the extent of elevated chloride concentrations in the coastal area. Herr and Shaw (1989, Figure 1-5) show a line that indicates the location of the 1000 mg/L isochlor at the base of the surficial aquifer system in 1982. Koszalka (1995) provides a map showing the "line of equal chloride concentration of 1,000 milligrams per liter in 1990." This line is described as the approximate landward limit of groundwater with a chloride concentration of 1,000 mg/L. Klein and Hull (1978) presented a similar chloride concentration line for 1975. Merritt (1996) presents maps of the approximate position of the saltwater front in the pumping zone in southern Broward County in 1945, 1969, and 1993. The saltwater front in this publication is a line eastward of which water with a chloride concentration greater than 250 mg/L occurs at depths generally corresponding to the pumping depths of major wellfields in the area (50 to 150 feet below land surface). This line is interpreted to have moved inland about a half mile in the Hallandale area and about a quarter of a mile in the Dania area between 1969 and 1993. SFWMD (1991, Figure IV-10) shows a series of three maps from Grantham and Sherwood (1968) depicting the progression of saltwater intrusion in the Middle River-Prospect Wellfield Area from 1941 to 1956 to 1963. SFWMD (1991) also shows a map from Sherwood et al (1973) depicting the extent of saltwater intrusion in 1970 for the entire coastal area of Broward County. Radell and Katz (1991, Figure 4) show the area in which water at the base of the Biscayne has an average chloride concentration of 1000 mg/L or higher based on WATSTORE data of 1975-90.
MAGNESIUM
Magnesium occurs in most limestone and it is brought into solution when the limestone is dissolved (Hem, 1985, p. 97), but at lower concentrations than calcium. Magnesium is not readily precipitated, so it tends to remain in solution. Dissolved magnesium in the Surficial Aquifer System is probably from dissolution of limestone except where the groundwater has mixed with sea water. The magnesium/calcium ratio of sea water is greater than that of normal groundwater. So contamination by sea water may cause enrichment in magnesium relative to calcium. Also, when clay is present, the high concentrations of sodium in sea water cause exchange of sodium for calcium and magnesium, releasing magnesium to groundwater, even though the sorption potential of sodium is low relative to calcium or magnesium.
Water in the Biscayne Aquifer is low in magnesium. Maddox and others (1992, Figure 5c) show magnesium less than 5 mg/L in the County, except in the southwest corner, where it increases to between 5 and 10 mg/L. No other maps of magnesium concentration were found in the documents examined. Magnesium contributes to hardness of water.
SILICA
Groundwater, in general, commonly contains between 5 and 40 mg/L silica. These low concentrations are due to the low solubility of minerals containing silicon (Davis and DeWiest, 1966, p. 101). Except for oxygen, silicon is the most abundant element in the earth's crust Very little information is published on silica in the Surficial Aquifer System. Water quality analyses reported by Sherwood (1959) indicate it is present, as expected. No maps of silica concentration were found in the documents examined.
SODIUM
Sodium in groundwater comes from a variety of sources, including rainfall, weathering of plagioclase feldspars and other minerals, and mixing with sea water. Sodium salts are very soluble, so sodium generally is not precipitated from groundwater. The only common mechanism for removal of large amounts of sodium ions from natural water is through ion exchange on clays, which operates only if the sodium ions are in great abundance (Davis and DeWiest, 1966). Even though the sorption potential of sodium is low relative to calcium or magnesium, the high concentrations of sodium in sea water can cause exchange with release of calcium and magnesium.
In the Surficial Aquifer System, sodium concentrations increase with depth (Sonenshein, 1997). Mixed-ion water occurs near the 250 mg/L isosurface, where the water is grading from calcium-bicarbonate type above to sodium-chloride type below. Sodium-chloride type water tends to have a salty taste when the chloride concentration is above 250 mg/L. Howie (1987) identified evidence of exchange of calcium and magnesium for sodium on clay in the western three-quarters of the County.
Sodium concentration maps for the intervals of 0-50, 50-100, and 100-200 feet presented in SFWMD (1991, Figures F-2 through F-4) show sodium concentrations from 2.5 mg/L to 64.9 mg/L. The trends identified by Sonenshein and by Howie are not obvious on these maps.
SULFATE
Sulfate in the groundwater is probably from several sources, including coastal mixing with sea water (Maddox and others, 1992, p. 14), decomposition of organic materials, weathering of trace amounts of pyrite (Maddox and others, 1992, p. 32), and fertilizers. Sea water is relatively high in dissolved sulfate.
The sulfates in the Biscayne Aquifer have shown a gradual decrease in concentration. Between 1920 and 1940 the sulfates in the water were recorded as high as 333 mg/L. By 1960 they decreased to about 180 mg/L, and in 1967 they had further decreased to a maximum of about 60 mg/L (Benham-Blair & Associates, 1970). Maddox and others (1992, Figure 32e) show sulfate less than 50 mg/L in the County, except in the southwest corner, where it exceeds this value. No other maps of sulfate concentration were found in the documents examined.
SECONDARY CONSTITUENTS
BORON
Boron is a minor constituent in most water. Normal groundwater concentrations range from about 0.01 mg/L to 1.0 mg/L (Davis and DeWiest, 1966, p. 133). Ocean water has 4.6 mg/L boron (Hem, 1985, p. 129). Boron is an element that is essential to plant growth, but citrus trees may be damaged by as little as 0.5 mg/L (Davis and DeWiest, 1966, p.113). No information on boron in Broward County groundwater was found in the literature reviewed for this report.
CARBONATE
Carbonate does not become an important anion in water unless pH values are greater than 8.0 to 8.5. In the Surficial Aquifer System, pH is generally less than 7.5, so carbonate concentrations should be small. No information on the carbonate anion in Broward County was found in the literature reviewed for this report.
FLUORIDE
Potential sources of fluoride in groundwater include dissolution of fluorine-bearing minerals, sea water, and agricultural fertilizers and pesticides. Commercial lawn fertilizers generally do not contain fluoride (McCoy and Hardee, 1970). The natural concentration of fluoride may be limited by the solubility of fluorite (CaF2), and it has been observed that waters high in calcium do not contain more than about 1 mg/L fluoride (Davis and DeWiest, 1966, p. 113). Groundwater in Broward County in general contains less than 1.0 mg/L of fluoride and usually less than 0.5 mg/L from municipal supply wells (Benham-Blair and Associates, 1970). Maddox and others (1992, Figure 41) show fluoride concentrations less than 0.2 mg/L near the coast and between 0.2 and 0.5 mg/L farther inland. The primary water quality limit for fluoride is 4.0 mg/L.
IRON
Iron in the groundwater may come from dissolution of iron-bearing minerals and decay of organic material in the soil. The concentration of dissolved iron is sensitive to pH, oxidation potential, and bacterial action. The most common form of iron in solution in groundwater is ferrous ion, Fe2+. The concentration of ferrous ions in groundwater is probably limited by the solubility of ferrous carbonate to less than 10 mg/L (Davis and DeWiest, p.101). Concentrations greater than 0.3 mg/L cause staining (clothing, plumbing fixtures, and other objects wetted by untreated water). Concentrations greater than 0.5 mg/L impart taste. Iron can be removed using conventional treatment methods. Goundwater in Broward County is normally high in iron, generally exceeding 1 mg/L. The actual concentrations vary widely both areally and with depth, and cannot be accurately predicted. Concentrations tend to be lowest near the coast and to increase to more than 3 mg/L inland (Benham-Blair and Associates, 1970). The range is from near zero to more than 4 mg/L. The secondary drinking water standard for iron is 0.3 mg/L.
Maddox and others (1992, Figure 25) posted iron concentrations on a map, which shows variability and lack of obvious areal trend. Iron concentration maps for intervals of 0-50, 50-100, and 100-200 feet presented in SFWMD (1991, Figures F-14 through F-16) show iron concentrations from less than 0.03 mg/L to 6.9 mg/L. These maps show no obvious trend, but they seem to show a slight tendency for iron concentrations to increase westward and decrease with depth.
NITRATE
Most nitrate in groundwater comes from the soil. Sources in the soil include decomposition of organic matter, fertilizers, and septic tanks. Surface water containing ammonia or nitrate may seep into the aquifer. Ammonia oxidizes to nitrate through bacterial action. Nitrate compounds are highly soluble, so nitrate is taken out of the groundwater only through the activity of organisms. Denitrification of nitrate to gaseous elemental nitrogen can occur by bacterial action under anaerobic conditions.
Maddox and others (1992, Figure 44) post nitrate concentrations on a map, which shows concentrations ranging from 0.01 to greater than 4 mg/L. No areal trend is apparent, although the two highest values are in the southwestern part of the County. Nitrate has been reported to decrease with depth in the Surficial Aquifer System (Sonenshein, 1997). The primary drinking water standard is 10 mg/L nitrate as nitrogen.
POTASSIUM
Sources of potassium in groundwater include fertilizers, sea water and dissolution of potassium-bearing minerals. Potassium concentrations in groundwater are usually low, because it is removed by fixation to clay minerals. No quantitative information on potassium concentrations in Broward County was found in the literature reviewed. Potassium is reported to decrease with well depth in the Surficial Aquifer System (Sonenshein, 1997).
STRONTIUM
Strontium generally occurs in groundwater in concentrations less than 1 mg/L. Its concentration is probably limited by ion-exchange with clays. No information on the strontium concentration in groundwater of Broward County was found in the literature reviewed.
MINOR CONSTITUENTS
Constituents that naturally occur in groundwater in concentrations ranging from 0.0001 to 0.1 mg/L, and are also subject to water quality standards, include aluminum, antimony, arsenic, barium, cadmium, chromium, copper, lead, manganese, mercury, nickel, phosphate, selenium, and zinc.
Aluminum is an abundant constituent in minerals, including feldspars and clays. It is relatively immobile in groundwater systems, but it can be mobilized if water is acidic or organic rich (Maddox and others, 1992). No information on the aluminum concentration in groundwater of Broward County was found in the literature reviewed.
Antimony is an uncommon constituent in natural minerals and rocks. The concentrations of antimony that occur in natural waters should be very small. No information on antimony concentration in groundwater of Broward County was found in the literature reviewed.
Arsenic is an uncommon constituent in natural minerals and rocks, so elevated concentrations in groundwater are likely to be caused by pesticide contamination. No information on the arsenic concentration in groundwater of Broward County was found in the literature reviewed.
Barium may be a minor constituent of limestone in the Surficial Aquifer System. No chemical analyses of the limestone were found, but Hem (1985, Table 1) shows 30 parts per million as the average concentration of barium in carbonate rocks. If the limestone does contain barium, small amounts would have been released when the limestone dissolved to form the solution porosity. Although no information specifically on barium concentration in groundwater of Broward County was found in the literature reviewed, Radell and Katz (1991) provide information on barium in southeast Florida. They reported that barium concentration increases with calcium and bicarbonate concentration and with pH.
Cadmium is used in industrial processes and may contaminate groundwater. No information on the cadmium concentration in groundwater of Broward County was found in the literature reviewed.
Chromium is used in industrial processes and may contaminate groundwater. It may also be present in small amounts in limestone. Therefore, a small amount of chromium could be released as solution features are formed in the Surficial Aquifer System. Chromium has been found at concentrations above the detection limit in southeastern Florida (Radell and Katz, 1991), but it has not been found to be associated with any particular land use, and there are no distinct areal or vertical trends in concentrations (Sonenshein, 1997). Sonenshein (1997) shows chromium concentrations ranging up to 26 μg/L. The primary drinking water standard for total chromium is 100 μg/L.
Copper, like chromium, is found in trace amounts in limestone and may be released as the limestone dissolves to form solution porosity. Copper is frequently found in concentrations above its detection limit in southeast Florida groundwater (Radell and Katz, 1991). There are no distinct areal or vertical trends in its concentrations. Actual concentrations for copper in Broward County groundwater were not reported in the literature reviewed.
Lead has many uses, so it is widespread in the environment. However, its mobility in groundwater systems is low due to low solubility of lead carbonates, adsorption on organic and inorganic sediment surfaces, and coprecipitation of lead with manganese hydroxide (Hem, 1985). Howie and Waller (1986) found that lead in highway runoff in Broward County accumulated in the upper half foot of soil. Nevertheless, lead in concentrations above detection limits has been found in the majority of groundwater analyses of the Biscayne Aquifer in southeastern Florida. Low concentrations of lead may come from the solution of the limestone in the aquifer, because limestone may contain trace quantities of lead (Hem, 1985). Radell and Katz (1991) found no distinct areal pattern or vertical distribution. However, they noted a positive correlation with total organic carbon (TOC), which has higher concentrations in the western part of the County (see below). Sonenshein (1997) reported lead concentrations ranging from less than 1.0 μg/L to 5,500 μg/L (an outlier), with median concentration of 3.25 μg/L. The outlier was from a well near a canal. The primary water quality standard for lead is 15 μg/L.
Manganese geochemistry is similar to that of iron. Small amounts commonly are present in limestone, substituting for calcium. Recharge water to the aquifer may contain elevated concentrations of iron and manganese as a result of leaching of these metals from the overlying soils. Manganese is frequently detected in groundwater in Broward County (Sonenshein, 1997). The areal and vertical distribution is not distinct (Radell and Katz, 1991). No manganese concentrations were found in the literature reviewed.
Mercury has been widely used in industry and agriculture, but its mobility in groundwater is low because it is readily adsorbed. However it can form chemical complexes with humic substances and groundwater with high color has the potential for containing mercury. No mercury concentrations were found in the literature reviewed.
Nickel is an important industrial metal. No nickel concentrations for Broward County were found in the literature reviewed.
Phosphorus is found naturally in only one common mineral, apatite (Fetter, p. 361). Its inorganic compounds have low solubilities (Hem, 1985, p. 126). Phosphorus generally occurs in groundwater as orthophosphate (species containing PO4-3). The usual sources of phosphate in groundwater are fertilizer and sewage. It is used by biota as a nutrient. Since the element is essential to metabolism, it is always present in animal metabolic waste. The chemistry of phosphorous favors its precipitation in groundwater systems, so concentrations tend to be low. Sonenshein (1997) reported orthophosphate concentrations from less than 0.01 mg/L to 1.5 mg/L as phosphorus, with a median concentration of 0.025 mg/L. Maddox and others (1992, Figure 38) presented a small scale map of orthophosphate concentration in Broward County, which shows concentrations to be around 0.02 mg/L. The Broward County groundwater quality standard for total phosphate (as phosphorous) is 0.01 mg/L. No federal drinking water standard is applied to phosphate.
Selenium may occur naturally in groundwater as a byproduct of pyrite oxidation and dissolution. It can be added to soil by use of superphosphate fertilizer (Fetter, 1965, p. 361). Geochemical controls tend to limit its mobility. No selenium concentrations for Broward County were found in the literature reviewed.
Zinc is widely used in industry. Solubility data for zinc carbonate and hydroxide suggest that water containing bicarbonate will tend to precipitate zinc (Hem, p. 142). Sonenshein (1997, Table 13) shows zinc concentrations from 42 wells in the Surficial Aquifer System ranging from less than 10 to 260 μg/L with a median concentration of 30 μg/L. The secondary drinking water standard for zinc is 5000 μg/L, because some people can taste zinc in water above that limit.
TRACE CONSTITUENTS
All elements are at least slightly soluble in water. However, natural concentrations may be so small that they are below the limits of detection. Most natural trace constituents are not known to affect the quality of water. However, a few may have adverse effects when elevated above normal concentrations. They include naturally occurring radionuclides such as Ra-226, a toxic daughter product of U-238, which is also toxic. No information on radionuclides in groundwater of Broward County was found in the literature reviewed.
Trace metals that occur naturally in groundwater and are also subject to water quality standards include beryllium, silver and thallium. Beryllium and silver are rare in the earth's crust, but beryllium is widely used in industry (Fetter, 1980, p. 353). Beryllium hydroxide is almost insoluble, so the alkalinity of groundwater in Broward County may control its concentration. The solubility of metallic silver and silver chloride or sulfide solids are likely to limit the solubility to below the secondary water quality standard (0.01 mg/L) for groundwater (Hem, 1985, p. 141). No information was found on the occurrence of thallium in natural groundwater. No information on these metals in groundwater of Broward County was found in the literature reviewed.
ORGANIC CONSTITUENTS
Concentrations of natural dissolved organic matter are generally low in groundwater compared to inorganic constituents. Most of the natural dissolved organic matter is derived from decaying organic matter in the soils that the water has passed through. It consists of components that remain after incomplete microbial decay. The organic matter may impart color to the water, and contributes to the color index discussed above.
A plethora of synthetic organic compounds may enter the groundwater system. Fortunately, there are many mechanisms that tend to protect the groundwater. These include precipitation, chemical degradation, volatilization, biological degradation, biological uptake, and adsorption. Synthetic organic chemicals and their degradation products are present at various contaminated sites in Broward County. Description of contaminated sites es beyond the scope of this report.
No general description of the distribution of synthetic organic contaminants in groundwater in Broward County was found, although Haag and others (1996, Figure 80) show that two to four monitoring wells somewhere in Broward County detected volatile organic compounds. Some information on total organic carbon was found.
TOTAL ORGANIC CARBON
Total organic carbon (TOC) represents the organically-linked carbon in samples. It does not include inorganically-linked carbon, such as carbonates and bicarbonates. TOC in the Surficial Aquifer System is dominantly naturally occurring organic substances, but could contain some mobile synthetic organic compounds as well. TOC reported by Sonenshein (1997) ranged from 2.5 mg/L to 45 mg/L, with the median being about 15 mg/L. There is a marked tendency for TOC to be highest in the western part of the County and least in the eastern part (Sonenshein, 1997, Figure 48). No water quality standard is applied to TOC.
GASES
The solubility of gases in water is directly proportional to pressure. Analyses of gases in groundwater are rarely made. Dissolved gas probably occurs in concentrations of 1.0 to 100 mg/L in most groundwater. Hydrogen sulfide and methane have been noticed in wells throughout the County (Benham-Blair & Associates, 1970); Tarver, 1964). Hydrogen sulfide may be produced by sulfate reduction by bacteria, and methane may be produced by bacterial fermentation of organic matter in the groundwater system. Low concentrations of hydrogen sulfide produce an unpleasant odor. Methane released to the atmosphere in a confined space may explode if ignited.
Other gases that are common in groundwater are nitrogen, oxygen, carbon dioxide, radon, and nitrous oxide. No information on these gases in Broward County was found in the literature reviewed.
CHEMICAL EVOLUTION
The chemical composition of groundwater in the Surficial Aquifer System of Broward County is determined by the original composition of the water before it entered the ground and the evolution of the water chemistry as it moves through subsurface materials. The original composition may be that of rainwater, canal water, lake water, wastewater, irrigation water, ocean water, or water from any other source. Water in the unsaturated part of the soil evolves by dissolution of minerals (including fertilizer), dissolution of soluble organic chemicals (including pesticides), precipitation of minerals, plant and animal metabolism and respiration, decomposition and fermentation of dead plant and animal material, decomposition of organic chemicals, adsorption of dissolved constituents onto solid surfaces, equilibrium with gases in the soil and concentration by evaporation.
Water moving through the saturated zone of the soil is still affected by most of these processes. Exchange of gases with the soil atmosphere is less. No direct evaporation occurs, and metabolic processes are primarily bacterial. Dissolved oxygen is depleted by aerobic bacteria and reaction with oxidizable organic and inorganic material. The loss of oxygen affects chemical equilibria, as does change in hydrogen ion concentration caused by various interactions as the water moves through the subsurface. In saturated materials with much organic material the conditions are anaerobic, and reactions involving anaerobic bacteria become important.
As the water moves deeper into the groundwater system its chemical composition moves toward equilibrium with the minerals in the aquifer and constituents that are readily adsorbed move slower than the water or not at all. Beneath the Biscayne Aquifer, groundwater movement is much slower than in the Biscayne, and some ancient sea water may not be completely flushed. Near the coast, the groundwater mixes with the upper part of a wedge of ocean water that extends inland due to its greater density.
The chemical composition of the groundwater in the Surficial Aquifer System has evolved in the manner described. The water is dominantly a calcium-bicarbonate type because the water is in quasi-equilibrium with limestone, a major rock-type in the Surficial Aquifer system. It has some admixture of sodium and chloride in some places possibly due to mixing with ancient sea water and with the coastal saltwater wedge. Sodium may also come from exchange of calcium for sodium on clay minerals.
The reported trend of increasing calcium, sodium and bicarbonate with depth is consistent with the chemical evolution of soil moisture and groundwater moving through a system containing calcite. A hypothesis that would explain this trend is that water moving through the soil and the shallow part of the groundwater system becomes saturated with calcium and bicarbonate. The distance traveled before the saturated condition is reached depends on the amount of calcite present (degree of contact of water with calcite) and velocity of the water. In a simple system, saturation would be reached in accordance with the following reaction:
CaCO3 + H+ ⇌ HCO3- + Ca2+ (1)
The hydrogen ion in this reaction usually reflects dissolved carbon dioxide as follows;
CO2 + H2O ⇌ H+ + HCO3- (2)
Dissolved CO2 is in equilibrium with gaseous CO2 in the soil atmosphere. Under aerobic conditions, CO2 is also supplied to the soil atmosphere by microbial decomposition of organic matter and other reactions. In waterlogged soils, the soil water becomes acid by fermentation of organic matter. Most of the soils in Broward County are no longer waterlogged so fermentation would be a minor source of acidity for the dissolution of calcite. The reaction expressed in equations (1) and (2) would produce a calcium concentration of about 150 mg/L when the partial pressure of CO2 in the soil atmosphere is 0.1 atmosphere (Hem, 1985, Figure 18), which is a relatively high value (Freeze and Cherry, 1979, p. 256). Calcium concentrations reported in the literature for potable groundwater in the County are less than this amount.
Once the water moves into the saturated part of the system dissolved CO2 can no longer be maintained by diffusion from the soil atmosphere. If the water is not saturated with respect CaCO3 dissolution will continue while dissolved CO2 is available. In addition to dissolved CO2 acquired in the soil zone, CO2 may be generated in the groundwater system by microbial sulfate reduction and subsequent fermentation of organic matter according to the following sort of processes:
2CH2O + SO42- = HS- + HCO3- + CO2 + H2O (3)
and
2CH2O + H2O = CH4 + CO2 + H2O (4)
These processes are consistent with reported hydrogen sulfide and methane in the groundwater.
HS- + H+ = H2S (5)
Sulfate reduction is also consistent with the reported gradual decrease in sulfate over time after agricultural activities diminished. The bacteria that mediate these reactions must have sufficient nutrients.
Published information indicates a tendency for calcium, hardness, color, and TOC to be greatest in the western part of the County. This tendency is also consistent with calcium solubility being greater where organic material is present to generate CO2. More organic material may have entered the groundwater system in the western part of the area, which was everglades and poorly drained sandy flatlands before construction of drainage canals (Fish, 1988; U. S. Soil Conservation Service, 1984). The soils developed in everglades contain much organic material. The soils of the sandy flatlands tend to be poorly drained with a surface layer of dark gray fine sand. These soils may tend to supply more organic material to the groundwater than the better drained sandy soils farther east.
Fluoride concentrations are also reported to be slightly greater in the western part of the area, although less than 1 mg/L everywhere. If this is the case, the reason is not clear. Possibilities include historical agricultural use of fertilizers and pesticides, and natural differences in composition of soils that developed under the different chemical environments of sandy upland versus waterlogged lowland.
Published information indicates a tendency for potassium and nitrate concentrations to decrease with depth. Decrease in potassium with depth is consistent with removal by adsorption on clay minerals as potassium-bearing water moves downward from the land surface. Denitrification, like sulfate reduction described above, requires the presence of organic compounds. The common occurrence of color in the groundwater indicates that sufficient organic matter is present. The reaction may be the following sort of process (Freeze and Cherry, 1979, p.118):
5CH2O + 4NO3- = 2N2 + 5HCO3- + H+ + H2O (6)
This reaction is mediated by anaerobic denitrifying bacteria, so sufficient nutrients must be present to sustain them. Phosphate may limit the rate of the reaction. The chemistry of phosphate in the colored groundwater may be complex, including organic phosphate compounds and phosphate adsorbed on colloidal iron and manganese particles.
It is beyond the scope of this investigation to perform quantitative analysis of the chemical evolution of the water in the Surficial Aquifer System to test these hypotheses.
SUMMARY
According to the technical literature, groundwater in the surficial aquifer System of Broward County may be classified as calcium-bicarbonate type except where it is mixed with the sea water near the ocean or at depth. The quality and chemistry varies regionally from east to west and with depth. The variation is expressed in trends and much local variation is superimposed on the trends. Constituents reported to have relatively low values near the Atlantic Coastal Ridge and increase to the west are total dissolved solids, hardness, color, chloride (excluding coastal sea water intrusion), fluoride, iron, and total organic carbon. Other constituents may also increase west of the coastal ridge, but reports of the increases were not found in the literature. Constituents reported to increase with depth are total dissolved solids, hardness, calcium, sodium, and chloride. Constituents reported to decrease with depth are color, nitrate, and potassium. Sulfate is reported to have decreased with time. No constituents were reported to have increased with time. These trends in groundwater quality and chemistry are consistent with geochemical theory in a system containing much calcite in the subsurface, organic soils to the west, a decline in agricultural activity, and proximity to the Atlantic Ocean.
The only constituent reported to exceed primary drinking water standards was lead in a few monitoring wells. Secondary drinking water standards are exceeded for total dissolved solids, color, chloride, and iron by concentrations reported in the literature reviewed. The secondary standard for sulfate has been exceeded in the past, but concentrations have declined.
Regarding recommendations for future water quality studies, the tendency for water quality to be better near the Atlantic Coastal Ridge than elsewhere is fairly well established. The likelihood that other spatial associations would be demonstrated by detailed statistical study seems low, given the local variability of the water quality parameters. It would seem prudent to analyze the time-trends from individual monitoring wells periodically to identify any tendency for water quality to decline. Time-trend graphs of water quality parameters could be produced and analyzed. The time-trend analysis might be updated at about five year intervals to watch for change in water quality at the monitoring sites.
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