[This is an html version of the following report, which was published in paper copy. Click link to see a non-technical discussion of groundwater quality.]

BROWARD COUNTY GROUNDWATER QUALITY - AN OVERVIEW

By

Darrel Dunn, Ph.D.

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.