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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:
Chloride data from the 1940s, indicate that fresh water was present more than 40 feet below the production zone near the Pompano Canal and 1961 data show fresh water present at 33 feet below the production zone near a finger canal east of the wellfield. The database contains no concentrations greater than 20 mg/L prior to 1972, so early isosurfaces cannot be mapped.
A substantial drop in groundwater levels in the wellfield in 1971 from an average of 3.7 feet above sea level in the 1960s to an average of 0.9 feet below sea level in 1971 correlates with reduced rainfall. Wellfield pumpage did not change abruptly in 1971, although it increased gradually from 1972 through 1980.
Low groundwater levels from 1971 to 1990 coincide with documented saltwater intrusion. Water levels in a monitoring well in the wellfield averaged 0.1 feet below sea level during this period. Water levels in a monitoring well located about a mile east of the wellfield averaged 1.8 feet above sea level during the period 1974 (first data) to mid-1990.
Saltwater intrusion was faster in the more permeable Biscayne Aquifer than in the subjacent less permeable part of the Surficial Aquifer System. This produced a perched wedge of salty water.
Higher groundwater levels from mid-1990 to 1999 coincide with large declines in chloride concentrations (recovery from intrusion) in the Biscayne Aquifer in the Pompano area. Water levels rose abruptly in mid-1990 and averaged 4.1 feet above sea level in the monitoring well in the wellfield and in the eastern monitoring well.
Planning implications of the results are the following:
Water management activities that maintain average groundwater levels higher than four feet above sea level between the Pompano wellfield and tidal water are likely to prevent salty water from reaching the wells via rapid density-driven intrusion in the Biscayne Aquifer in the Pompano area.
Water management activities that maintain average groundwater levels higher than four feet above sea level in the Pompano wellfield are likely to prevent salty water from reaching the wells via slow density driven intrusion through the lower part of the Surficial Aquifer System.
THREE-DIMENSIONAL ANALYSIS OF SALTWATER INTRUSION
POMPANO BEACH AREA, BROWARD COUNTY, FLORIDA
INTRODUCTION
Saltwater intrusion has long been recognized as a threat to public drinking water supplies in coastal Broward County, Florida. Several high capacity wellfields are close to the Atlantic Ocean, and other wellfields are close to tidal canals that contain salty water. Certain monitor wells that once yielded freshwater have shown increases in chloride concentration. In the past, information from such wells has been used to produce lines on maps showing interpretations of the landward extent of groundwater with chloride concentrations exceeding a specified value at a selected level in the Surficial Aquifer System, the base of the Biscayne Aquifer for example. When such maps showed lines corresponding to different times, they provided information on temporal change in the landward extent of the chloride concentration at the selected subsurface level.
In this report, water containing more than 250 mg/L chloride will be called salty, because this is the taste threshold. It is also a secondary water quality standard. Water containing less than 250 mg/L chloride will be called fresh, although it is recognized that water containing more than 100 mg/L is likely to have been contaminated by mixing with salty water. Seawater intrusion produces a wedge of salty water extending inland from the coast. The wedge is three-dimensional. Chloride concentrations within the wedge grade from about 100 mg/L for slightly contaminated water near the interface with fresh water to concentrations approaching seawater (approximately 19,350 mg/L). There is a spatially varying continuum of chloride concentration within the wedge. The concentration continuum may be represented by mapping isosurfaces. 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. For example, a 250 mg/L chloride isosurface shows the position of this concentration in the subsurface. It is a curved, continuous, irregular surface. The intersection of an isosurface with a plane is a line. Consequently, an isosurface can be represented by contouring its elevation. The contours represent the intersection of regularly spaced horizontal planes with the isosurface. Three-dimensional visualization can also be aided by representing chloride concentrations as contours on vertical section. Three-dimensional visualization software can be used for viewing chloride isosurfaces and variously oriented sections through the wedge.
This report describes a three-dimensional analysis of saltwater intrusion in the vicinity of the City of Pompano Beach (Pompano) coastal wellfield. This area was chosen because it would provide a comprehensive test of the method of investigation. It contains a large wellfield that is close to the coast and tidal canals. It has changed its monitoring well system from cased wells sampling short subsurface intervals to one dominated by wells consisting of long screens; and previous investigations of the area have shown active saltwater intrusion.
PREVIOUS WORK
Several early reports contain information relevant to saltwater intrusion in the Pompano area. Parker et al (1955) stated that prior to canal construction saltwater was probably no more than one mile inland from natural brackish surface water bodies. Schroeder et al (1958) produced an early delineation of the approximate areas of saltwater intrusion in Broward County. Tarver (1964) showed a well about 1/10th mile from the Intracoastal Waterway in Pompano with a chloride concentration of 17,000 mg/L for the years 1960-1961 at a depth of 40 feet. He also showed a correlation between chloride concentration and water-level in a 70 foot deep monitor well. Sherwood and Grantham (1965) investigated the mechanics of saltwater intrusion and the effects of salinity control measures in Broward County. Benham-Blair & Associates (1970) stated that water from the Pompano wellfield showed an increase in chlorides of about 30 to 40 percent along the southeast edge of the field in 1961 when rainfall was low.
Klein and Hull (1978) presented a map showing the area in which water at the base of the Biscayne Aquifer had an average chloride concentration of 1000 mg/L or higher in 1975. Koszalka (1995) produced a similar map showing a 1000 mg/L line in 1990, and he also showed chloride concentrations in certain wells in 1980 compared to 1990. This comparison showed large increases in two monitor wells located just west of Federal Highway 1, and small increases (less than 20 percent) in some other monitor wells that yielded salty water both years. He also provided a graph of chloride concentration versus time for selected monitor wells. One of the wells was G-2055 (Figure 2), located southeast of the Pompano wellfield, which yielded freshwater until 1982 when the chloride concentration began increasing and reached 5100 mg/L in 1991.
Howie (1987) showed chloride concentrations in monitor wells constructed in 1981 and 1982 throughout Broward County. One of the wells was in the southern part of the Pompano wellfield and encountered freshwater to its deepest sampling depth of 409 feet, where the chloride concentration was 31 mg/L. Monitor wells to the west of the study area showed higher concentrations at shallower depths. Monitor wells in the Everglades showed salty water at depths less than 200 feet. Howie considered this salty water to be diluted residual seawater. Fish (1988 ) discussed the occurrence of deep freshwater beneath the Atlantic Coastal Ridge. He introduced the possibility that the deep freshwater was caused by higher water table during pre-development time, and that reduction of the water table elevation by tidal canals and wellfields has produced an unstable situation wherein the deep freshwater may eventually be replaced by seawater. The South Florida Water Management District (1991, Appendix E) produced a series of small scale maps that show chloride concentration at various depths in coastal Broward County.
PURPOSE AND SCOPE
The purpose of the present investigation was to use isosurface mapping to describe and interpret the spatial and temporal variation of groundwater chloride concentration in the vicinity of the Pompano wellfield. This purpose supports the ultimate goal of improving analytical tools for making water resources management decisions. A conceptual understanding of the three-dimensional behavior of the variable density system in response to such hydrologic stresses as drought and water well pumping should be helpful. An understanding (conceptual model) of the three-dimensional system is also a good starting point for developing a predictive mathematical model of the system.
The investigation utilizes chloride concentration data found in the electronic databases of the U.S. Geological Survey and the South Florida Water Management District, plus some data provided by the City of Pompano Beach. The analysis is limited to three-dimensional interpretation and interpolation of the chloride data, and does not include variable density numerical modeling.
METHOD
The boundaries of the study area are shown in Figure 1. The northern boundary is an east-west line intersecting monitor well G-2895 (Figure 2), which was constructed in October, 1997, using the dual tube reverse air rotary technique. During construction, the specific conductance of the groundwater was measured at five-foot intervals to provide a salinity profile (Hydrologic Associates, 1998). This profile provides salinity data at the monitoring well's location on the north boundary. Likewise, the south boundary was placed to intersect G-2896, which was also constructed in October, 1997, and provides a specific conductance profile. The east boundary intersects G-2063, which samples high chloride concentrations at shallow depth near the Intracoastal Waterway. No chloride data were found east of this boundary. The west boundary was placed far enough west to provide a fairly square area for three-dimensional viewing.
Once the study area was defined, the literature was reviewed, and selected monitor well data for the study area and the surrounding region were examined so that interpretations of isosurfaces could be made consistent with known concentrations outside of the study area. For example, Howie (1987) showed chloride concentrations increasing westward from the Pompano area, and a westward increase is shown on maps in the present report. Deeper monitor wells in the western part of the study area would be required to test this interpretation.
Vertical salinity gradients in the transition zone between fresh and very salty water have been observed in a number of monitor wells drilled in Broward County by the dual tube reverse air rotary method. Specific conductance data from these wells were converted to chloride concentration via the formula:
CL = -149.02 + 0.3400 * SC + 5.878E-7 * SC2
where
CL is chloride concentration (mg/L), and
SC is specific conductance (µs/cm).
This is a regression curve for U.S. Geological Survey and South Florida Water Management District data for Broward County (Broward County, 1996). Concentrations less than 100 mg/L generally show no increase with depth and probably represent water uncontaminated by chloride. It appears that 100 mg/L is usually the top of a transition zone wherein chloride concentrations increase relatively rapidly until they reach a high value. For concentrations greater than 100 mg/L, the concentration tends to increase with depth at rates that vary from less than 20 mg/L per foot to about 400 mg/L per foot. An example of a well with a low rate of increase in the transition zone is G-2901 which increases at about 18 mg/L per foot (Hydrologic Associates, 1998). G-2901 is located about 6.5 miles south of the Pompano study area. An example of a well with a high rate of increase is HMW-2D reported by the City of Hollywood (1997), which increases at about 395 mg/L per foot. For the present investigation a provisional vertical concentration gradient of 100 mg/L per foot was imposed for concentrations greater than 100 mg/L at monitor wells where data were insufficient to establish the true vertical gradient. This gradient was used to calculate vertical concentration profiles from monitor well data that only provided one concentration at a known sampling elevation, which was the elevation of a well screen or the bottom end of open casing. Vertical concentration profiles calculated in this way were supplied as input to the three-dimensional viewing software.
There are no deep monitor wells to show whether the concentrations actually increase with depth to 19,000 mg/L in the study area. However G-2063, near the Intracoastal Canal, has produced water at 17,000 mg/L from about 70 feet below mean sea level. In the absence of local data to the contrary, the concentrations were projected to increase to 19,000 mg/L at depth in vertical sections. This may not actually be the case. In the Hollywood area, fifteen miles to the south, chloride concentrations have been found to stop increasing with depth once 11,000 mg/L to 15,000 mg/L has been reached (City of Hollywood, 1997, 1998). Deep monitor wells would be needed in the Pompano area to determine true concentrations deeper than the existing monitor points.
GEOLOGY
The Pompano wellfield draws water from the Biscayne Aquifer at 73 to 140 feet below sea level. The Biscayne comprises the highly permeable part of the Surficial Aquifer System. It is composed of very permeable limestone with solution channels and very permeable, poorly consolidated sandstone. Since the Biscayne Aquifer is defined on the basis of hydraulic conductivity, insufficient data are available to establish the elevation of the upper and lower boundaries. Fish(1988, Figure 37) gives the base of the Biscayne Aquifer in the Pompano area as about 320 feet below sea level. Restrepo et al (1992, Figure A-11) mapped the base of the Biscayne Aquifer at about 110 feet and the top at about 40 feet below sea level. The part of the Surficial Aquifer System between the Biscayne Aquifer and the land surface is less permeable limestone, sandstone, and sand. Similar material is present in the part of the Surficial Aquifer System below the Biscayne Aquifer. In this report the parts of the Surficial Aquifer System above and below the Biscayne are called the upper and lower surficial aquifer, respectively. The Surficial Aquifer System extends to about 370 feet below sea level in the study area (Fish, 1988, Figure 35). It is underlain by low permeable clay, silt, limestone, and fine sand, which acts as an aquitard between it and the Floridan Aquifer. In Broward County, the top of the Floridan Aquifer is about 950 to 1,000 feet below sea level (Fish, 1988, p.14).
The northern part of the Pompano wellfield is on the Atlantic Coastal Ridge, which is a low sandy ridge parallel to Atlantic coast. It rises to an elevation of about 18 feet near the north end of the wellfield, and is interrupted by a natural drainage-way near the south end of the wellfield.
HYDROLOGY
In general, the Pompano area is on the east flank of a large water table mound that extends southward from the Hillsboro Canal into the north-central part of eastern Broward County (Restrepo et al, 1992, Figure D-1). Consequently, the general condition is flow toward the wellfield from the west with interception of the water by the wellfield. The Pompano Canal and wellfield drawdown has depressed the hydraulic head in the area close to sea level. In detail, the groundwater flow system is transient, and hydraulic head, wellfield drawdown, flow direction, and flow velocity change with time.
POMPANO WELLFIELD HISTORY
The first well in the Pompano wellfield (Well 1) was completed in 1927 at a location about 800 feet south of the southernmost well in the present wellfield. No additional wells were constructed until 1950. Wells 2 and 3 were constructed in 1950 and 1952. Wells 4, 5, and 6 were constructed in the period 1955 through 1959. Wells 7 through 11 were constructed from 1960 through 1965. Wells 12, 13, and 14 were constructed from 1967 through 1969. Wells 15 and 16 were constructed in 1972. Well 1 was abandoned in the mid-1980s. The locations of the existing wells are shown in Figure 2. The City of Pompano Beach constructed a western wellfield in 1984, called the Palm Aire wellfield (Figure 2). This wellfield is not in the present study area.
POMPANO CANAL HISTORY
The Pompano Canal passes about 1600 feet south of the southernmost well (Well 8). The water level in the canal is tidal south of the wellfield. The tidal water extends upstream to control structure G-57 located about 3600 feet west-southwest of Well 8 (Figure 2). The original Pompano Canal was a secondary canal in the Everglades Drainage District project (Sherwood et al, 1973) which was started in 1906 and completed in 1928. The Pompano Canal predated Well 1. Control structure G-57, which maintains upstream freshwater levels above sea level was not built until the 1950's by the Corps of Engineers for the Central and Southern Florida Flood Control Project. Originally there were two control structures on the Pompano Canal, the Market Spillway and the City Spillway (G-57). The Market Spillway has been removed. Its location was not found in the literature reviewed for this report. Upstream canal stage measurements for G-57 date back to 1962. Upstream water levels have been maintained between 1 and 6 feet above mean sea level, with an average level of 4.1 feet. Since 1975 mean monthly water levels have generally been maintained between 3 and 5 feet above sea level.
HISTORICAL SALINITY CONDITIONS
This section describes an interpretation of the evolution of subsurface chloride concentrations beginning with the earliest monitoring well data. After 1974, 250 mg/L chloride isosurface maps were constructed at five-year intervals. The data on the maps include representative chloride concentrations, elevations of sampling points, and depths of production wells. The isosurfaces are interpretive, based on the 100 mg/L per foot vertical chloride concentration gradient and relevant data outside of the study area.
Pre- 1972
Little pre-1972 data were found, and all chloride concentrations found were less than 20 mg/L. Data from two monitor wells are noteworthy. Well S-341, near the future location of the G-57 control structure on the Pompano Canal (Figure 2) yielded a chloride concentration of 14 mg/L in 1940 from 180 feet below sea level, indicating that salty water was at considerable depth beneath the canal at that time. Well S-340, located a little farther from the canal also sampled fresh water from 180 feet below sea level in 1940. This well was sampled until 1951, and continued to yield fresh water. Well G-1559, located at the end of a long finger canal extending westward from the Intracoastal Waterway, sampled 12 mg/L from 173 feet below sea level in 1961. These three wells provide an early record of freshwater at depths below the producing interval of the Pompano wellfield. The producing interval extends from 73 to 140 feet below sea level. Tarver (1964) showed a well about a tenth mile from the Intracoastal Canal sampling 17,000 mg/L from a depth of 40 feet. A saltwater wedge must have existed close to the ocean at that time. This high concentration at a shallow depth suggests the absence of significant discharge of fresh water to the ocean even this early.
Rainfall was variable, but normal, from 1960 through 1970 (Figure 3); and water levels measured in monitor well G-853 (near Well 3) were relatively high, averaging 3.7 feet above sea level from 1960 through 1970 (Figure 4). Canal water levels above the G-57 control structure averaged 3.4 feet above sea level from 1962 to 1969 (Figure 5). Precipitation was very low in 1971 and this year was the first in a sequence of dry years (Figure 3) associated with low groundwater level elevations (Figure 4). Note that a water level measured in a cased monitoring well, such as G-853, that is open to the aquifer at some depth below the water table is approximately the same as the water table elevation, but not necessarily exactly the same.
It seems likely that reduction of the elevation of the water table near the Pompano Canal had caused movement of salty water into that area long before 1972. However, data are insufficient to document early intrusion.
1972
The earliest time when sufficient data existed to interpret the configuration of the 250 mg/L chloride isosurface was about 1972. The configuration is presented in Figure 6 along with the data used for the interpretation. Figure 7 is an east-west vertical slice through the middle of the Pompano wellfield. The influence of the ocean and the Intracoastal Waterway can be seen in the data from wells G-2064 and G-2063, where the western well (G-2064) yielded freshwater from 191 feet below sea level, whereas the eastern well (G-2063) yielded salty water (5900 mg/L) from 75 feet below sea level. These data indicate that the 250 mg/L isosurface was sloping downward away from the ocean. Likewise, data from wells G-2062 and G-2149 show that the 250 mg/L isosurface was sloping down northward from the Pompano Canal. The gradient on the isosurface south of G-2054 is interpreted as being about the same as the gradient between G-2064 and G-2063 to the north where the data provide a better indication of the gradient near tidal canals. This gradient was extrapolated to the area south of G-2062 to produce the configuration shown. The interpretation is that the 19,000 mg/L (seawater) isosurface had not risen to the level of the Pompano Canal in this area. This interpretation is supported by data from G-2063 which is oceanward from the end a long finger-canal and shows that concentrations approaching that of seawater had not necessarily occurred at all canals in the early 1970's.
1972 through 1974
Water levels dropped abruptly in late 1970 and early 1971 (a very dry year) and remained very low until mid-1981 (Figure 4). This decline in water levels correlated with reduced precipitation. Wellfield withdrawals did not increase significantly in 1971 (Parada and Sanchez, 1986, Figure 4-37), so should not have significantly contributed to a change in water levels. However, wellfield drawdown had already depressed water levels near the wellfield, as evidenced by the lower waterlevels in G-853 (Figure 4) located in the wellfield compared to G-2147 (Figure 8), located east of the wellfield. During the period 1971 to mid-1981, water levels in G-853 averaged 0.3 feet below sea level. During the period 1974 (first data) through mid-1981 water levels in G-2147 averaged 1.1 feet above sea level.
Headwater canal stages at G-57 and S-37B, are shown in Figures 5 and 9. No data is available to show how the canals were managed in the 1970-1974 period.
The low groundwater levels may have resulted in the development of a perched wedge of salty water in the production zone. This perched wedge is indicated by data from G-2055A and G-2055 (Figure 10). These two wells were at the same location. G-2055A monitored the production zone and G-2055 monitored beneath the production zone. Monitoring began in G-2055A in October,1974; and the chloride concentration was 580 mg/L at that time. A sample collected from G- 2055 in October, 1974 was fresh. Thus, salty water was present above fresh water.
Monitor well G-2062 had chloride concentrations similar to G-2055A. It was first sampled in 1973, when it yielded a chloride concentration of 620 mg/L, that remained relatively unchanged by October, 1974, when the chloride concentration was 530 mg/L. Since monitor well G-2055A and G-2062 both sampled from the production zone and exhibited about the same concentration, the perched wedge may have extended throughout this area southeast of the wellfield. If so, the wedge did not extend as far north as G-2149, which sampled fresh water from the production zone near the south end of the Pompano wellfield.
The earliest chloride concentration data found for G-2063, located east of the wellfield, was 5900 mg/L in October, 1973. This concentration had increased to 9000 mg/L by October, 1974 (Figure 10), indicating intrusion in this area east of the wellfield. G-2063 monitored from the upper part of the production zone. Whether the intrusion involved a perched wedge, like that to the south, is not documented. Well G-2064 was located about a third of a mile west and monitored from below the production zone. It yielded freshwater.
1975 through 1979
Saltwater intrusion associated with reduced precipitation and low water levels continued through the 1975-1979 period, and the perched wedge of salty water in the production zone continued to develop. Well G-2055A yielded water with steadily rising chloride concentrations during this period, which reached 10,000 mg/L (Figure 11), although G-2055 continued to yield freshwater. Well G-2054 located southeast of G-2055A, and sampling from about the same elevation, also yielded water with rising chloride concentrations. A vertical section through the perched saltwater wedge is depicted in Figure 12. The base of the perched wedge was not contoured in Figure 11 to simplify the presentation. Only contours on the top of the wedge are shown. The contours on the 250 mg/L isosurface in the vicinity of G-2055A are on the top of the wedge. The -150 contour to the west is on the main isosurface beyond the terminus of the perched wedge.
To the west, the behavior of chloride concentrations in G-2062 was similar to G-2055A, suggesting that a perched wedge of salty water may have extended to G-2062. No deeper well was associated with G-2062 to test this hypothesis. To the north, chloride concentrations were also rising in G-2063 and G-2064. The increase in chloride concentrations in this area was probably also caused by the low water levels that began in 1971.
The effect of the increase in chloride concentrations was to cause the interpreted 250 mg/L isosurface to rise in the area to the east and south of the well field. G-2149, south of Well 8 continued to yield fresh water from the production zone, so the isosurface remained beneath the production zone at this location. The concentration of 32 mg/L at 379 feet below sea level measured in G-2344 in 1981 shows that fresh water extended to the base of the Surficial Aquifer System beneath the wellfield at least until that time.
Canal water levels above the G-57 control structure averaged 4.26 feet during this period, which was 0.9 feet higher than during the 1962 to 1969 period when the average headwater elevation was 3.4 feet (Figure 5). The higher canal water levels were not sufficient to counter the effect of low rainfall on groundwater levels.
1980 through 1984
Water levels were very low in 1980 and the first half of 1981, and continued to be low, though rising, through the remainder of the period. Chloride concentrations in most monitor wells continued to rise during this period (Figure 13). However, the chloride concentrations in G-2055A and G-2062 decreased. The decrease of concentrations in G-2055A may have been caused by the perched wedge of salty water being more sensitive to water level fluctuations than wells closer to the ocean or beyond the landward terminus of the wedge. Precipitation was temporarily above normal in 1982 through 1984, and groundwater levels also rose slightly (Figures 4 and 8). Since the wedge contained denser water overlying less dense water, it was unstable and may have been especially sensitive to a temporary rise in water levels. Since G-2062 behaved like G-2055A, there is additional reason to suspect that it was also sampling a perched wedge. Concentrations in G-2055, which was monitoring concentrations beneath the perched wedge, began to rise in 1982, and continued to rise through the period, reaching 840 mg/L (Figure 13). Some of the chloride reaching this depth may have been percolating downward from the overlying perched wedge.
The concentration in G-2064 to the east of the wellfield, rose from about 350 mg/L to 12,000 mg/L during this period. Most of the rise occurred early in the period, and the chloride concentration had reached 11,000 mg/L in 1981. This well monitored from a depth that is below the production zone. The increased concentration at this location caused a bulge to appear in the interpreted 250 mg/L isosurface (Figure 13). This bulge persisted through all subsequent periods. It is associated with a finger-canal that extends farther westward from the Intracoastal Waterway than others in the area. The bulge may be related to reduction in water table near the canal.
The concentrations recorded in G-2063 and nearby wells suggest no regional discharge of fresh groundwater to the ocean, because applying the 100 mg/L per foot concentration gradient to G-2063 projects salty water at sea level. Furthermore, the concentration of 160 mg/L from G-2001 at a depth of 44 feet indicates salt contaminated water at shallow depth above the production zone. G-2063 and G-2001 were directly east of the wellfield and about a mile from the ocean. The lack of uncontaminated fresh water as high as 44 feet below sea level in this area suggests no regional discharge to the ocean. Furthermore the water level was higher in G-2147 (located at G-2064) than in G-853 located in the wellfield, so the regional hydraulic gradient east of the wellfield was not toward the ocean. However, the water level in G-2147 was above sea level; so some discharge of local recharge to the finger-canal and other tidal water may have been occurring.
Average headwater levels at the G-57 and S-37B control structures declined slightly during the period (Figures 5 and 9), while groundwater levels in the G-853 and G-2147 were low, but rising, and annual precipitation was increasing. Canal water levels do not appear to have been an important factor in changing groundwater levels.
1985 through 1989
Chloride concentrations in most of the monitor wells did not change much from 1985 through 1989 (Figure 14). The general stability of chloride concentrations correlated with water levels that were similar to water levels in the preceding period (1980-1984). Precipitation declined with each successive year during the period (Figure 3), but water levels declined only slightly to a low during 1989 (Figures 4 and 8), which was a very dry year.
As before, G-2055A, G-2055, and G-2062 did not behave like the other wells in the study area. The concentration in G-2055A rose to over 10,000 mg/L early in the period and then began to decline in 1989 (Figure 15). The rise corresponded to very low water levels in the first half of 1985 (Figures 4 and 8). Subsequently, the water levels may not have been low enough to sustain the perched condition. Concentrations in G-2055 continued to rise, and reached 3000 mg/L by the end of the period (Figure 15). The perched wedge still existed, but the difference in concentrations within and below the production zone were reduced. Again, G-2062 behaved somewhat like G-2055A. It fluctuated during the period with rises in 1985 and 1989 that correlate with low water level conditions shown on Figures 5 and 8. The concentration near the end of 1989 (a very dry year) was about 11,000 mg/L.
Average headwater levels at the G-57 and S-37B control structures continued to decline slightly during the period (Figures 5 and 9). Canal water levels above control structure G-57 were generally less than four feet above sea level in 1989 and were not raised until the drought of 1989 was over (Figure 5).
1990 through 1994
Chloride concentrations in most of the monitor wells rose slightly during this period but were not significantly different from concentrations in the 1985-89 period (Figure 16). This behavior shows a lack of immediate response of chloride concentration to water levels that had risen substantially. Water levels rose abruptly in mid-1990 and remained high (Figures 4 and 8). The average water level was 4.1 feet above sea level in both G-853 and G-2147. The higher water levels correlate with an increase in precipitation above normal annual rates (Figure 3). Canal water levels above the G-57 control structure were maintained close to 4.6 feet during this period and through 1998.
Again, G-2055A, G-2055, and G-2062 did not behave like the other wells. Concentrations in G-2055A continued to decline as concentrations in G-2055 continued to rise through the period. The most recent data found for these wells were from 1993 when concentrations in both wells had approached 5000 mg/L (Figure 15). The perched wedge had disappeared and left a uniform vertical concentration in the production zone and the subjacent part of the Surficial Aquifer System. The disappearance of the wedge is consistent with higher water levels causing the salty water of the perched wedge in the production zone to move downward into the subjacent part of the Surficial Aquifer System. G-2062 still behaved somewhat like G-2055A. Its concentrations declined from 11,000 mg/L to about 4360 mg/L at the end of the period.
Figures 17, 18, and 19 contain vertical sections that depict the interpretation of 250, 1000,10,000, and 19,000 mg/L chloride isosurfaces at the end of 1994, before most chloride concentrations began to decline in association with high water table conditions. Comparison of Figures 12 and 19 illustrates how a perched wedge might evolve and contribute to a rise in the main wedge.
1995 into 1999
Available data indicate a decrease in chloride concentrations during this period (Figure 20). Large decreases occurred in G-2062 south of the wellfield and G-2054 southeast of the wellfield. A small decrease was observed in G-2063 located on the eastern boundary of the study area, and a small decrease occurred in G-2149 located near the south end of the wellfield. Data were no longer available for some previously monitored wells, including G-2001, G-2055, G- 2055A, G-2064, and G-2445, which were used to interpret the 250 mg/L isosurface at the end of 1994.
Eight new monitoring wells were constructed in the vicinity of the Pompano wellfield. The new wells were screened from near the land surface to a depth of 200 feet. These long-screen wells are the MW series shown on Figure 2. No interpretation of the depth to the 250 mg/L isosurface could be made for MW-1, MW-2, and MW-6. MW-5 contained freshwater to total depth. Data from MW-7 compared to previous data from nearby G-2001 suggest that chloride concentrations decreased in that area. This widespread decrease in chloride concentrations may have been caused by the relatively high water table conditions that began in 1990.
Figure 20 interprets the new position of the 250 mg/L isosurface. The isosurface is shown as declining considerably in the eastern and southeastern part of the study area where the isosurface was above the base of the production zone. Little change is interpreted near the south end of the wellfield due to the modest decline in chloride concentration in G-2149. Concentrations in this well had been a little less than 250 mg/L for many years. It is possible that the 250 mg/L isosurface is at a lower elevation here than the interpretation indicates. The well may be reflecting a low vertical concentration gradient between the 100 and 250 mg/L points on the profile. Although the data from the nearby MW-2 long-screen well could not be reliably interpreted, the data were consistent with a low elevation of the 250 mg/L isosurface.
Well G-2149 provides the best information about chloride concentrations close to the wellfield. It samples from the production zone. Concentrations rose slowly from about 30 mg/L in 1972 to about 210 mg/L in 1989 and 1994, then declined slightly to 191 mg/L in 1999. Since data from G-2055A and G-2055 suggest that salty water moves into the portion of the aquifer below the production zone more slowly than into the production zone, it seems likely that the 250 mg/L isosurface has not risen much near the wellfield yet, and that concentrations beneath the wellfield have not increased much since 1981 when G-2344 sampled water at 32 mg/L at about 379 feet below sea level. However, no chloride monitoring data were found that provides information on the movement of the 250 mg/L isosurface where it is beneath the base of the production zone near the wellfield. Monitoring beneath the production zone near the wellfield would be needed to evaluate the interpretation on Figure 20. The interpretation implies very slow movement of salty water in the lower surficial aquifer.
SUMMARY OF SALINITY CONDITIONS
The observed salinity conditions are consistent with a density-driven rise in salinity in the subsurface associated with a decline in water table elevation. The decline in water table elevation has been associated with drainage of the Everglades, which extended to the Atlantic Coastal Ridge before development began in the mid-1800's. Effective drainage of the eastern part of Broward County began in the early 1900's and accelerated in the mid-1900's. Operation of public and private water supply wells have also contributed locally to the water table decline.
During a 20-year period of unusually low water table elevation in the study area (1971 through 1989), a perched wedge of salty water moved into the production zone. This caused a rise in the interpreted 250 mg/L isosurface some distance to the east and south of the wellfield. The perched wedge did not reach the wellfield. The water table rose abruptly in 1990, and concentrations in the perched wedge had declined substantially by 1999. The decline in concentrations in the perched wedge was associated with a gradual increase in chloride concentrations in a well monitoring the lower surficial aquifer below the production zone.
Since the perched wedge of salty water did not reach the wellfield and the movement of salty water beneath the production zone appears to have been delayed, the water in the Surficial Aquifer System beneath the wellfield may still be fresh, as it was when it was sampled in 1981.
The maps of the estimated elevation of the 250 mg/L chloride isosurface show an interpretation of the evolution of the coastal saltwater wedge. The maps provide estimates of the distance of the 250 mg/L isosurface from the producing interval of any well. The shortest horizontal distance at any time is the shortest distance from the well to the contour that is equivalent to the depth of the well (bottom of the producing interval). This distance may be measured on each map to see how it changed with time. The vertical distance is the difference in elevation of the bottom of the well and the elevation of the isosurface at the well. As an example, Figure 21 shows the change in horizontal and vertical separation of Well 8 and the 250 mg/L isosurface, as measured on the maps.
CONCLUSIONS
In the absence of adequate protective measures, salty water could eventually reach the wells in the Pompano wellfield by either of the two following mechanisms:
Lateral intrusion of a perched wedge in the production zone.
Lateral intrusion of a main wedge resting on the base of the Surficial Aquifer System and rise beneath the wellfield high enough to reach the production zone.
Both of these mechanisms are driven by density of the salty water that is connected to the ocean and by the water table elevation. Intrusion of a perched wedge during low water table conditions is a relatively fast process compared to intrusion into the lower surficial aquifer. The difference in intrusion rates may be related to the difference in hydraulic conductivity. The two mechanisms are interrelated. A perched wedge is an appendage of the main wedge, and a perched wedge may contribute to the growth of the main wedge by downward percolation of salty water from the perched wedge to the main wedge. If either of these mechanisms brought salty water into the wellfield capture zone, or the capture zone expanded into salty water, the movement of salty water toward the wells would be accelerated.
The response of isosurfaces lags change in groundwater level. An example of this phenomenon is shown in Figure 22, where standardized values of chloride concentration from G-2054 are plotted with standardized water level elevations from G-853. Standardization facilitates comparison by making the range of values similar. These wells were chosen for this illustration because they have long periods of record. Water levels dropped in the area at the end of 1970, but the chloride concentration in G-2054 was at about 199 mg/L (average of three values) on May 15, 1972. The next measurement of chloride concentration in G-2054 was in October, 1974, when it was 5300 mg/L. The lag time for the beginning of the rise in chloride was one and a half to three and a half years. Water levels rose in mid-1990, but chloride concentrations in G-2054 did not fall until the end of 1994. The lag time for the fall in chloride concentrations was about four and a half years. G-2054 monitors the production zone.
The lag-time appears to be greater below the production zone, where hydraulic conductivity is relatively low, and less within the production zone, where hydraulic conductivity is great. The relationship of lag-time to hydraulic conductivity is indicated by the difference in arrival times of isosurfaces in G-2055A and G-2055 (Figure 15). The lag-time also appears to be related to horizontal distance that an isosurface has moved. Evidence suggesting this relationship in the lower surficial aquifer is that salty water had not reached G-2344 by 1981 at any depth, whereas salty water had reached G-2064, which was monitoring about 50 feet below the production zone. Evidence suggesting this relationship in the production zone is that salty water had not reached G-2149 by 1974, whereas it had reached G-2062, which samples from about the same depth in the production zone. A similar relationship may be seen for G-2055A and G-2054.
The data suggest that a water table elevation of about four feet above sea level is sufficient to stop intrusion of a perched wedge of fresh water into the production zone and reverse saltwater contamination that has occurred. A period of low water levels from 1971 through 1989 appears to have caused a perched wedge of salty water to enter the production zone. In 1990 water levels in G-2147 rose abruptly and averaged about 4.1 feet above sea level into 1999. During this time, chloride concentrations declined considerably in three wells monitoring the production zone (G-2055A, G-2054, and G-2062). Concentrations in Wells G-2054 and G-2062, which were still being monitored, were approaching 250 mg/L. This occurrence suggests that maintaining the water table at greater than four feet above sea level will tend to depress the 250 mg/L isosurface below the production zone. Therefore water resource planning options that maintain water levels higher than 4 feet above sea level between the wellfield and tidal water would help protect the wellfield from relatively rapid salinization, regardless of water level in the wellfield; because the inland movement of salty water would be through the less permeable material below the production zone. However, if the average water level in the wellfield were less than 4 feet above sea level, salty water might eventually reach the wells through the lower surficial aquifer. Maintaining a water level of four feet in the wellfield would probably keep the 250 mg/L isosurface below the base of the production zone there, as it seems to do farther east. However, in the long-term, salty water might rise to near the base of the production zone. To maintain freshwater to the base of the Surficial Aquifer System beneath the wellfield might require maintaining the water table near its elevation prior to drainage and urban development, which does not seem practical.
The long-term performance of the Pompano wellfield appears to depend on management of the water table elevation. A practical strategy might be to maintain water table elevations above four feet throughout as much of the area as possible. Water table elevations can be influenced by artificial recharge, canal control structure operation, and well-field operation. Artificial recharge might be accomplished by over-irrigation, recharge impoundments, and recharge ditches. The strategy could be continuously evaluated by monitoring the water table throughout the area, and monitoring the 250 mg/L isosurface with a comprehensive coverage of saltwater intrusion monitor wells.
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 select the best one in terms of maximizing long-term withdrawal of fresh water. The present study provides a conceptual model that might 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.
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(Revised 11/8/2013)