[This is an html version of the following report, which was published in paper copy.]
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THREE-DIMENSIONAL ANALYSIS OF SALTWATER INTRUSION
CITY OF POMPANO BEACH AREA
BROWARD COUNTY, FLORIDA
By
DEPARTMENT OF PLANNING
AND ENVIRONMENTAL PROTECTION
TECHNICAL REPORT SERIES
TR:2000-00
Water Resources Division
Water Resources Planning Section
April 2000
EXECUTIVE SUMMARY
Previous investigations of saltwater intrusion have shown that salinity in the Surficial Aquifer System in Broward County, Florida, has increased east of the City of Pompano Beach (Pompano) eastern wellfield and near the southeastern edge of the wellfield. However, the three-dimensional development of this salinity condition has not been thoroughly described. The purpose of the present investigation was to perform isosurface mapping to describe the three-dimensional spatial and temporal variation of groundwater salinity in the vicinity of the wellfield. This purpose supports the goal of improving analytical tools for making water resources management decisions in Broward County.
An isosurface is a three-dimensional version of a contour line. That is, a contour line connects points of constant value in a two-dimensional space, while an isosurface connects points of constant value in a three-dimensional space. A wedge of salty water extending inland from the Atlantic Ocean is a three-dimensional feature. It may be depicted by using chloride isosurfaces.
Profiles of the electrical conductance of groundwater encountered while drilling monitor wells using the dual tube reverse circulation method in Broward County show that the vertical change in salinity from freshwater to saltwater is gradual. Review of salinity profiles throughout the county resulted in the adoption of a provisional vertical chloride concentration gradient of 100 mg/L per foot through the transition zone between 100 mg/L and 19,000 mg/L. This gradient was used to estimate the depths of concentrations in the vertical profile when only one concentration at a particular depth was known. Most saltwater intrusion monitor wells measure chloride concentration at a particular depth (total depth of the well or depth of the screen). Consequently, the interpretation presented in the maps and vertical sections in this report are based on the 100 mg/L per foot vertical gradient.
Saltwater intrusion probably began near the Pompano Canal when it was constructed in the early 1900's. Such intrusion is the expected result of reducing the elevation of the water table in a coastal aquifer. No monitor well data documenting intrusion in the Pompano wellfield area were found during the present investigation for the period prior to 1972. A monitor well near the present location of the G-57 control structure south of the wellfield yielded fresh water from 180 feet below sea level through the 1940s. A monitor well located east of the wellfield near the end of a finger-canal extending westward from the Intracoastal Waterway yielded fresh water from 173 feet below sea level in 1961. For comparison, existing wells in the Pompano wellfield have depths ranging from about 73 to 140 feet below sea level. This interval, called the production zone in this report, is within the highly permeable Biscayne Aquifer.
Water levels in groundwater monitor wells in the Pompano area dropped abruptly in 1971 during a very dry year, and remained low until mid-1981. These low water levels coincided with a period of below-normal precipitation. During this period there was a gradual increase in withdrawals from the Pompano wellfield. The low water levels were associated with saltwater intrusion into the production zone that was recorded by monitor wells to the east and the south of the wellfield. Chloride concentration data show that salty water moved inland faster in the production zone than in the underlying, less permeable, part of the Surficial Aquifer System. The result was a perched wedge of salty water in the production zone over fresh water in the lower part of the Surficial Aquifer System.
Water levels rose slightly in mid-1981 but still remained relatively low until mid-1990. These higher water levels were associated with precipitation that was normal to above normal from 1982 through 1986, but which declined in subsequent years until 1989, a very dry year. Chloride concentrations increased during these years except in those monitoring wells nearest to the ocean, which remained stable at high concentrations. The interpreted 250 mg/L isosurface rose accordingly.
Canal water levels for the Pompano Canal above the G-57 control structure are available from late-1962 to mid-1969, and early 1975 through 1999. Canal water levels averaged 4.1 feet above sea level from 1975 through 1989 (when groundwater levels were low), and 3.4 feet above sea level from 1962 to 1969 (when groundwater levels were relatively high). Chloride concentrations in the groundwater rose during the 1975-1989 period. Although water levels above G-57 may affect water table elevation and saltwater intrusion in the vicinity of the Pompano wellfield, they do not control it. Canal water levels above the S37B control structure do not control water levels in the vicinity of the Pompano wellfield either. This structure is located about three miles inland from the Pompano wellfield. Headwater levels at this structure have not varied much, do not appear to correlate with groundwater levels near the wellfield, and have not prevented saltwater intrusion although maintained higher than the drought management control level of 6.5 feet above sea level.
Groundwater levels rose abruptly in mid-1990 to levels higher than in the decade before 1971. The water levels remained high through 1999. These high water levels correlated with a period of above-normal precipitation. Chloride concentrations decreased in most of the monitor wells during this period, and the interpreted 250 mg/L isosurface declined accordingly. The chloride reduction during this period may have been affected by decay of the aforementioned perched wedge in the production zone due to movement of the denser salty water downward to displace the less dense fresh water below it.
The uncertainty of the interpretation of the elevation of the 250 mg/L isosurface increased for the most recent data, because monitoring ceased at some old wells that sampled from a specific depth in the aquifer, and eight new wells were constructed with screen from the land surface to their total depth of 200 feet. These long-screen wells yield electrical conductivity profiles that are difficult to interpret with respect to the natural chloride profile in the aquifer, possibly because of vertical flow and vertical mixing within the wells. The continuity of the sampling from the old wells was lost.
The data suggest that, at present, freshwater may extend to the base of the Surficial Aquifer System (about 370 feet below sea level) beneath all but the southernmost part of the wellfield. East of the wellfield, the data show a wedge of salty water that is closer to the southern wells than to the northern ones. The shortest estimated lateral distance from a well to salty water at the base of the production interval increases from less than a thousand feet at the south end of the wellfield to about 3000 feet at the northern end. The lateral distance to salty water at the top of the production interval is greater.
Change in chloride concentration in monitor wells lagged change in water level elevation. The lag time seems to be related to permeability and distance from salty water. High permeability and short distance to salty water decreased lag time. Conversely, low permeability and long distance to salty water increased lag time. The Biscayne Aquifer is the high permeability part of the Surficial Aquifer System. Lag time probably prevented the perched wedge of salty water from reaching the wellfield when water levels there fluctuated around sea level from 1970 to 1990.
If a variable density groundwater model were developed to simulate the features of the system that affect saltwater intrusion, it could be used to simulate alternative management strategies and identify the best one for maximizing long-term withdrawal of fresh water. The present study provides a conceptual model that could be used as a starting point for a variable density model. The model would have to be capable of simulating the advance and decay of a perched wedge of salty water in a very permeable layer underlain by a less permeable layer.
Results of the investigation include the following: