JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
Vol. 43, No. 6
AMERICAN WATER RESOURCES ASSOCIATION
December 2007
IMPACTS OF RESIDENTIAL DEVELOPMENT ON HUMID SUBTROPICAL
FRESHWATER RESOURCES: SOUTHWEST FLORIDA EXPERIENCE1
Robert G. Maliva and Karl P. Hopfensperger2
ABSTRACT: The population of Collier and Lee Counties in southwestern Florida has increased 11-fold from
1960 to 2004 with a concomitant increase in freshwater demand. Water levels and salinity within the water
table aquifer over the past two to three decades have generally been stable, with more monitoring wells
showing statistically significant temporal increases in water level than decreases. Residential development
has had a neutral impact on the water table aquifer because the total annual evapotranspiration of residential communities is comparable to that of native vegetation and less than that of most agricultural land
uses. Public water supply systems and private wells also result in net recharge to the water table aquifer
with water produced from deeper aquifers. Confined freshwater aquifers have overall trends of decreasing
water levels. However, with the exception of the mid-Hawthorn aquifer, water levels in most areas recover
to near background levels each summer wet season. Freshwater resources in humid subtropical areas, such
as southwestern Florida, are relatively robust because of the great aquifer recharge potential from the
excess of rainfall over ET during the wet season. Proper management can result in sustainable water
resources.
(KEY TERMS: sustainability; ground-water management; evapotranspiration; water supply; water policy.)
Maliva, Robert G. and Karl P. Hopfensperger, 2007. Impacts of Residential Development on Humid Subtropical
Freshwater Resources: Southwest Florida Experience. Journal of the American Water Resources Association
(JAWRA) 43(6):1540-1549. DOI: 10.1111/j.1752-1688.2007.00126.x
INTRODUCTION
Rapid population growth in the sunbelt of the United States is resulting in increasing withdrawals of
fresh ground water for domestic use and the irrigation of residential, commercial, and recreation areas.
It has been generally taken for granted that
increased water use associated with residential development has adverse impacts, such as the depletion of
aquifers, saline-water intrusion, and the drying out
of wetlands. Such negative assumptions focus largely
on water withdrawals. However, residential development of land does not necessarily cause adverse
impacts because positive changes in the water balance can occur, such as a net increase in recharge to
the water table aquifer as explored in this paper.
Meaningful evaluation of the impacts of land use
changes therefore requires consideration of changes
in the overall water balance.
Collier and Lee Counties in southwestern Florida
(Figure 1) have undergone a population and residen-
1
Paper No. J06022 of the Journal of the American Water Resources Association (JAWRA). Received February 8, 2006; accepted April 12,
2007. ª 2007 American Water Resources Association. Discussions are open until June 1, 2008.
2
Respectively, Senior Hydrogeologist, Missimer Groundwater Science, a Schlumberger Company, 1567 Hayley lane, Suite 202, Fort Myers,
Florida 33907; and Hydrogeologist, CDM, 2295 Gateway Oaks Drive, Suite 240, Sacramento, California, 95833 (E-Mail ⁄ Maliva:
RMaliva@slb.com).
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FIGURE 2. Average Annual Monthly Temperature and Rainfall
in Fort Myers, Lee County (Southeast Regional Climate Center,
2006) and Estimated Average Monthly ET Potential for Coastal
Southwestern Florida in 2003-2004 (Abtew et al., 2005).
FIGURE 1. Map of the Study Area Showing the Locations
of the Studied U.S. Geological Survey Monitoring Wells.
tial development boom over the past four decades.
Concerns over the impacts of population growth on
the water supply of the region had been noted as
early as the 1960s (Boggess, 1968). The existence of a
regional ground-water monitoring network, installed
and operated by the U.S. Geological Survey (USGS),
makes these counties a natural laboratory to evaluate
the impacts of rapid residential development on fresh
ground-water resources, the primary potable and irrigation water source in the region. The objective of
this investigation is to evaluate the impacts of rapid
residential development on water levels and salinity
in the freshwater aquifers of Collier and Lee Counties. In particular, does the monitoring data show
evidence of unsustainable withdrawals. Actual information on the sensitivity of freshwater to increasing
exploitation would provide guidance for optimal management of the ground-water resources in humid subtropical areas.
average daily high temperatures of approximately
33C in June through August and 24C during
December through February (Figure 2). The region
has a pronounced seasonality in rainfall with a
summer wet season and winter and spring dry season. Approximately 66% of the 138 cm of historic
average annual rainfall in Fort Myers falls in the
months of June through September (Figure 2), during which time rainfall exceeds evapotranspiration
(ET). During the fall and winter dry season,
ET exceeds rainfall. There has been considerable
variation in annual rainfall, but there are no
overall trends that could be responsible for longterm temporal changes in aquifer water levels
(Figure 3).
SETTING
Collier and Lee Counties adjoin the Gulf of Mexico in southwestern Florida. Both counties have
very low topographic relief. The highest point in
southwestern Florida is a Calusa Indian shell
mound (Indian Hill) on Marco Island, which has a
maximum elevation of 15.5 m above sea level.
Southwestern Florida has a subtropical climate with
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FIGURE 3. Historic Annual Rainfall for Fort Myers,
Florida (Southeast Regional Climate Center, 2006).
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TABLE 1. Collier and Lee County Population Data.
Collier County
Lee County
Total
1960*
1970*
1980*
1990*
2000*
2004 (est.)*
2020 Projected**
15,753
54,539
70,292
38,040
105,216
143,256
85,971
205,266
291,237
152,099
335,113
487,212
251,377
440,888
692,265
296,678
514,295
810,973
349,200
594,300
943,400
Source: * U.S. Census Bureau, source:** South Florida Water Management District (2000).
Southwestern Florida has been experiencing rapid
population growth. The combined population of Collier and Lee Counties increased from 70,292 in 1960
to 692,265 in 2000, and is projected to reach 943,000
in the year 2020 (Table 1). The projected year 2020
population may be conservatively low in that population growth and associated home construction have
accelerated over the past several years. The bulk of
the population is concentrated along the coast in the
cities of Cape Coral, Fort Myers, Bonita Springs,
Naples, and Marco Island, and adjoining unincorporated areas. The inland areas are mostly undeveloped, low-density residential or agricultural (mostly
winter vegetables, citrus, and pasture).
Water demand in southwestern Florida is estimated in the latest Lower West Coast Water Supply
Plan (South Florida Water Management District,
2000). Total water demand in Collier and Lee Counties is projected to increase from 552.5 million cubic
meters (Mm3) per year in 1995 to 828.7 Mm3 per year
in 2020 (Table 2). Most of recent historic and projected future increase in demand is for residential
and recreational uses, which is correlated with population growth. Recreational uses include landscape
and golf course irrigation.
The hydrogeology of the freshwater aquifers of
Collier and Lee Counties has most recently been discussed by Boggess et al. (1981), Wedderburn et al.
(1982a,b,c), Peacock (1983), Knapp et al. (1986),
Weedman et al. (1997, 1999), Missimer (2001), and
Missimer et al. (2003). Four main aquifers are
locally used in Collier and Lee Counties for freshwater supply; the water table aquifer, and three underlying confined aquifers. The water table aquifer in
most places has water levels within several feet of
land surface. The semi-confined to confined aquifers
are the lower Tamiami aquifer, Sandstone aquifer,
and mid-Hawthorn aquifer (Figure 4). Although the
strata that constitute each of the confined aquifers
are all present throughout much of the study area,
the transmissivity and leakance of the strata are
highly variable. Typically only one of the confined
aquifers is an important freshwater source in any
area. The lower Tamiami aquifer is widely used in
Collier County and southwestern Lee County. The
mid-Hawthorn aquifer is used extensively for domestic water supply in northwestern Lee County,
including parts of the City of Cape Coral. The Sandstone aquifer is used for domestic, irrigation, and
TABLE 2. Estimated Water Demand in Collier and Lee Counties.
Demand (mcm per year)
Collier County
Lee County
Use Classification
1995
2020
1995
2020
Public water supplied
Domestic self supplied
Commercial and industrial
self-supplied
Total recreational
Total agricultural
Thermoelectric power
generation
Total
61.44
7.46
8.26
113.30
8.22
15.76
59.29
8.32
7.47
92.06
11.94
11.83
81.06
178.04
0.00
169.53
218.36
0.00
58.18
82.03
1.06
102.39
85.27
1.06
336.18
525.17
216.35
303.52
Source – South Florida Water Management District (2000).
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FIGURE 4. Hydrogeologic Diagram of Lee and Collier Counties.
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public water supply in central and eastern Lee
County.
considered in the study to be a manageable degradation of water quality.
METHODS
WATER LEVEL AND CHLORIDE DATA
To evaluate the effect of population growth on
the water resources of southwestern Florida, a
desktop evaluation of available ground-water level
and salinity data was performed. Historic water
level and chloride concentration data obtained from
the Water Resources of Florida on-line database
(USGS, 2005) were used in this study. Chloride
concentration was used as a proxy for salinity. The
USGS database includes water level and chloride
measurements performed monthly or at less frequent intervals. Data from a well was included in
this study using the following salinity and time
frame criteria: an initial chloride concentration
250 mg ⁄ l or less, the first measurement was from
the year 1986 or earlier, and the latest reported
measurement was from the year 2000 or later. Year
1986 was chosen as a cutoff date to exclude wells
installed less than two decades ago, and thus provide a relatively short hydrologic record. The study
database included 97 wells for water levels, 32 of
which also have chloride concentration data for the
same period of record.
For each well database, a least-square regression
analysis was performed to identify and quantify
temporal trends in the data. A t-test was used to
evaluate the probability of the null hypothesis that
the data come from a population in which the
regression coefficient (slope) is zero. A regression
coefficient of zero means that there is no change in
water level or chloride concentration over time. Significance levels of <1% are nearly always judged to
be statistically significant (Sokol and Rohlf, 1981).
Datasets with a probability of £0.01 (1%) are considered herein to have significant changes in water
level or chloride concentrations over time.
Small changes in chloride concentration may occur
over time in some wells that are statistically significant, but not hydrogeologically significant in terms of
water resources management. For example, a
10 mg ⁄ l increase or decrease in chloride concentration from a 20 mg ⁄ l initial value may be statistically
significant, but would not impact use of the water as
the Florida secondary drinking water standard for
chloride is 250 mg ⁄ l. An additional criterion is therefore necessary to evaluate the hydrogeological significance of changes in chloride concentration with
respect to resource management. Regression coefficients of <0.0055 mg ⁄ l per day (2 mg ⁄ l per year) are
Approximately 82% of the water table aquifer monitoring wells have a trend of either stable water levels or significantly increasing water levels (Table 3).
More monitoring wells have a trend of significantly
increasing (n = 11) than significantly decreasing
(n = 7) water levels. The rates of change in elevation
of the water table aquifer (regression coefficients) are
low. For six of seven wells showing a significant
decrease in water level over time, the regression coefficient is 5.2 · 10)3 cm ⁄ day or less, which corresponds
to an annual rate of 1.9 cm ⁄ year. Plots of water level
vs. time for wells with significant increasing
(Figure 5A) and decreasing (Figure 5B) water levels
have a large seasonal variation. No spatial trends are
evident in the data (Figure 6A). For example, there is
no pattern of decreasing water levels in the western
part of the study area, which has the greatest
development and water use.
No overall trend is evident in the lower Tamiami
aquifer well data (Table 3). Ground-water levels have
been stable or increasing in coastal northern Collier
County, a major aquifer use area (Figure 6B). Approximately 78% of the Sandstone aquifer and 68% of the
mid-Hawthorn aquifer monitoring wells have a significant decrease in water levels over time (Table 3; Figures 6C and 6D). Two patterns are evident in the
time-drawdown plots of the water levels from the confined aquifers. Some wells have a trend in which both
dry and wet season water levels are progressively
decreasing over time (Figure 5C). Progressive
decrease in both dry and wet season water levels commonly occurs in the mid-Hawthorn aquifer, where a
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TABLE 3. Summary of Monitoring Data.
No
Significant Significant Significant
Increase
Decrease
Change
Water level
Water table aquifer
Lower Tamiami aquifer
Sandstone aquifer
Mid-Hawthorn aquifer
Other (deeper) aquifers
Chloride Concentration
Water table aquifer
Lower Tamiami aquifer
Sandstone aquifer
Mid-Hawthorn aquifer
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11
6
1
4
1
7
5
14
13
0
20
9
3
2
1
2
2
0
3
1
0
0
1
4
6
7
6
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FIGURE 5. Water Level vs. Time Plots From USGS (2005) Monitoring Wells: (A, B) water table aquifer wells with
significantly increasing and decreasing, respectively, water levels over time; (C) trend of progressively declining both
wet and dry season water levels in a mid-Hawthorn aquifer well; and (D) wedge-shaped pattern in a sandstone aquifer well.
deep depression of the aquifer potentiometric surface
has developed in the City of Cape Coral – western
City of Fort Myers area. The depression in the midHawthorn aquifer was present as early as the 1970s,
but has grown over time (Bengtsson and Radin, 2001).
A second common water-level vs. time plot is a
wedge-shaped pattern in which dry season water levels are progressively decreasing, but water levels
recover in the wet season (Figure 5D). During the
wet season aquifer recharge exceeds withdrawals and
the aquifer potentiometric surface recovers. Wedgeshaped time-plots are more typical of the sandstone
and lower Tamiami aquifers in high use areas.
The limited long-term chloride concentration data
(Table 2; Figures 6E and 6F) show that 7 of 32
(19%) monitoring wells have a significant trend of
increasing chloride concentration over time. The
remaining 25 wells (81%) show either no significant
trend or a significant decrease in chloride concentration over time. Monitoring of chloride concentration
in the majority of the wells for which there was
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long-term water level data was discontinued in 1993
to 1999. There is some bias in the data in that the
minority of wells in which chloride monitoring was
continued tend to be clustered towards the coast
where there is a greater potential for the intrusion
of saline water.
DISCUSSION
It is often intuitively assumed that residential
developments with irrigated landscaped areas and
associated golf courses result in a large consumption
of water. The consideration of the impacts of residential development on water resources in South Florida,
and elsewhere, typically focus largely on water withdrawals. Ground water and surface water withdrawals are only one component of the water budget.
Equally important are changes in actual water loss
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A
B
C
D
E
F
FIGURE 6. Maps of Monitoring Wells Showing Water Elevation and Chloride Concentration Trends:
(A-D) water elevations in water table aquifer, lower Tamiami aquifer, mid-Hawthorn aquifer, and sandstone aquifers,
respectively; (E-F) chloride concentrations in the water table and confined aquifers, respectively.
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associated with changes in land use and cover, particularly ET, and the cycling of water within and
between aquifers.
Water Use and Land Development
A fundamental concept concerning the impact of
residential and agricultural development activities on
water resources is that the change in actual water
use at a site is predominantly a function of difference
in ET between the existing and new land cover
(Stewart and Mills, 1967), provided that there is no
significant change in stormwater runoff and recharge.
Different vegetation types (e.g., turf grass vs. forests)
may have comparable ET rates because environment
(incoming solar radiation) is the main factor controlling ET, provided there is a continuous canopy or coverage of the soil surface (Augustin, 1983). Residential
development in southwestern Florida typically results
in a change in land cover from either native vegetation or farm fields, to a combination of turf grass and
subsidiary ornamental vegetation, lakes (excavated
for fill), and impervious areas (e.g., houses, roads,
driveways). The main variables for evaluating the
change in water use resulting from residential development are the ET rate of the native vegetation and
the area and ET rates of residential landscape and
golf course vegetation and excavated lakes.
Little data are available on measured annual average ET rates for different land covers in southern
Florida, and the available data are concentrated on
Everglades wetlands environments. Residential development in Southwest Florida occurs primarily in
uplands areas. Computed ET rates for a sometime
dry sawgrass site the Everglades is 108.7 cm per year
for (German, 1999, 2000). Calculated ET rates for
sites in west-central Florida, north of the study area,
are 106.0 cm per year for pine flatwood-type vegetation and 97.0 cm per year for cypress swamp-type
vegetation (Bidlake et al., 1996). Melaleuca forests, a
widespread invasive tree in South Florida, have an
annual ET rate 160.0 cm, which includes 30.0 cm of
ET from intercepted rain water (Chin, 1998). An
average annual ET rate of 106.0 cm is thus a reasonable estimate for upland forested areas in the study
area, which have been prime targets for development.
Land infested with exotic vegetation may have
greater ET rates.
The average annual ET rates for various crop
types in Southwest Florida is estimated with the
modified Blaney-Criddle method (Soil Conservation
Service, 1970) using climatic data from the South
Florida Water Management District (2003). The main
crops grown in the study area are citrus and winter
vegetables and fruits. Estimated average annual ET
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for agricultural land uses in the study area ranges
from 127.0 to 168.0 cm depending upon the crop and
growing season.
A typical golf course community in the study area
may contain approximately 60% landscaped area,
25% impervious area, and 15% lakes (excavated for
fill). The percentage of impervious area is greater in
communities without golf courses. Undeveloped preserve areas (wetlands and uplands) are also present
in large developments, which would not experience a
significant change in ET as a result of development
activities.
Average annual lake ET rates in Lee and Collier
Counties was reported to be approximately 132.0137.2 cm (Visher and Hughes, 1969). Computed ET
rates for an open-water site in the Everglades was
141.0 cm per year (German, 1999, 2000). The ET
potential in the southern part of the South Florida
Water Management District (SFWMD) was estimated
to be 136.9 cm per year (Abtew et al., 2003).
The annual estimated ET for turf grass in Fort
Myers calculated using the modified Blaney-Criddle
method is 151.4 cm in Fort Myers and 150.9 cm in
Naples. However, microclimate conditions of a site,
particularly solar radiation level (shading) and
wind velocity can significantly affect turf grass ET
(Feldhake et al., 1983). Open golf course fairways
may have higher than average ET rates, whereas
shaded and sheltered turf grass between houses may
have lower than average rates. Calculated ET rates
for turf grass in Southwest Florida are significantly
higher than measured rates of approximately
108.7 cm from research plots in Fort Lauderdale
(Stewart and Mills, 1967; Augustin, 1983). The
measured rate for turf grass is close to the estimated
rates for dry native environments.
Impervious areas have minimal ET, which consists
mostly of the drying of surfaces (pavement, buildings)
wetted during rainfalls. Impervious areas of the residential and commercial developments act to increase
water levels in the water table by reducing ET, provided that there is not an associated increase in
stormwater runoff. Using a lake annual ET of
137.1 cm per year and a turf grass annual ET of
151.1 cm (from the above cited data sources), the estimated total average annual ET for a residential community is approximately 111.3 cm (Table 4).
Difference in ET between undeveloped uplands and
residential communities is well within the uncertainties of the ET values. On the contrary, agricultural
land uses result in a significantly higher ET than
both undeveloped land and residential communities.
Both agricultural and residential communities are
typically irrigated. The average annual irrigation
requirement for turf grass in Fort Myers and
Naples is 81.0-91.5 cm, depending upon soil type, as
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TABLE 4. Estimated ET Rates (cm per
year) of Different Land Covers and Uses.
Vegetation ET
Lake ET
Total ET
Undeveloped
Uplands
Agricultural
Residential
Community*
106.0
0
106.0
127.0 to 168.0
0
127.0 to 168.0
90.7
20.6
111.3
Note: *Based on 60% turf grass, 25% impervious, 15% lake.
calculated using the modified Blaney-Criddle method.
Additional irrigation water is typically applied to
compensate for the different efficiencies of the various
irrigation methods or because of excessive irrigation.
Irrigation is a ‘‘use’’ of water only to the extent that
it supports higher ET rates. If there are no significant changes in overall ET rates and runoff, then
much of the irrigation water necessarily must return
to the water table aquifer, and is thus not a net consumptive use of water. The SFWMD (2006) limits
off-site discharge rates to historic (i.e., predevelopment) rates. Stormwater retention areas are constructed to reduce off-site discharges and increase
recharge rates. Where then irrigation water is
obtained from underlying confined aquifers or external sources (reclaimed water), then irrigation water
is a net addition to the water table aquifer,
which would contribute to higher water levels in the
aquifer.
Water Table Aquifer
The monitoring data for Lee and Collier Counties
indicate that water levels in the water table aquifer
are for the most part either stable or increasing
despite the large increase in population and associated land development. The drying out of wetlands, a
major issue for water use permitting, is not occurring
on a regional scale. The change in land cover from
native vegetation to golf course and residential communities is approximately neutral as far as overall
impacts to the water table aquifer.
Increases in population also result in increased
domestic water use, which also impacts aquifers.
Domestic self-supply in Collier and Lee Counties
results in an overall net local addition to the water
table aquifer because water produced predominantly
from underlying confined aquifers recharges the
water table aquifer through septic systems and residential irrigation. The public water supply systems in
Lee and Collier County use a variety of water
sources, including desalination of brackish water,
confined freshwater aquifers, surface water, and the
water table aquifer. Reclaimed water is sent to reuse
systems and, when supply exceeds demand, to
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percolation ponds, river outfalls or deep injection well
systems. Some utilities (e.g., Collier County, City of
Cape Coral) send nearly all of the dry season
reclaimed water flow to their reuse systems. Overall,
the public water and wastewater utility systems also
result in net recharge to the water table aquifer.
An additional factor responsible for local increases
in the elevation of the water table is the construction
of weirs on the drainage canals in Collier County by
the Big Cypress Basin of SFWMD. The maintenance
of water levels upstream of the weirs reduces the
local dewatering in the vicinity of the canals.
Although development activities have had a neutral to favorable impact on the regional water balance
in the water table aquifer, adverse impacts may still
occur locally, where there is a local net export of
water from the water table aquifer. For example,
water table aquifer withdrawals may adversely
impact wetlands near wells and wellfields, if the
recharge of the produced water occurs elsewhere.
Fresh ground-water resources could also be adversely
impacted if the recharge water is of poor quality. Residential, commercial, and agricultural development
also increases the potential for local contamination of
the water table aquifer from both point and nonpoint
sources.
From a water management perspective, it is preferable to irrigate using typically lower quality
water from on-site water table aquifer sources, with
the water being cycled back into the aquifer, than
using better quality water from confined aquifers,
which could be reserved for greater value uses (e.g.,
potable supply). The potential for adverse local
impacts can be managed by strategic location of
withdrawal and recharge areas. For example,
impacts to wetlands can be reduced by maximizing
the separation of withdrawal points from wetlands
and having intervening recharge (i.e., irrigation
areas).
Confined Aquifers
In contrast to the water table aquifer, the confined
aquifers of Collier and Lee counties have experienced
a significant overall lowering of their potentiometric
surface over time. Trends of decreasing elevation of
the potentiometric surface over time can have two
end-member causes. Decreases could be due to mining of the aquifer, in which long-term withdrawals
exceed recharge. Alternatively, decreases may be a
dynamic response to increased pumpage, analogous
to the growth of the cone depression around a single
well as the pumping rate increases. The lowering of
the potentiometric surface increases hydraulic gradients, and thus flow into the aquifer, which balances
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withdrawals under steady-state conditions. In the
case of aquifer mining, the current rate of withdrawal
is not sustainable. Where drawdowns are a dynamic
response to pumping, current rates of use are sustainable provided that there are no associated
adverse impacts, such as lateral or vertical migration
of saline water. However, the rate of growth of withdrawals may not be sustainable.
Where the water level vs. time plots have a
wedge-shaped pattern, current aquifer use is clearly
sustainable, as water levels recover during the wet
season.
Dry season lowering of aquifer potentiometric
heads can still cause problems. There have been
scattered reports of wells ‘‘going dry,’’ which have
been due to water levels dropping below pump
intakes, use of centrifugal pumps with their limited
lift depths, or wells that are too shallow for the
lowered water levels. The lowering of the potentiometric surface can also allow for saline-water intrusion.
The mid-Hawthorn aquifer in Cape Coral is particularly susceptible to large drawdowns because of its
relatively low transmissivity (62.1-124.2 m2 ⁄ ft; Missimer, 2001) and leakance. Water levels near the
core of the potentiometric surface depression are
approaching what is judged by the SFWMD to be
the minimal acceptable level, the top of the aquifer. An on-going expansion of the public water supply
and reuse system will result in a progressive abandonment of mid-Hawthorn aquifer domestic wells,
and corresponding recovery of the potentiometric
surface.
Saline-Water Intrusion
Saline-water intrusion has occurred in Southwest
Florida and resulted in the abandonment of some of
the earlier wellfields that were located along the
coast, near the saline-water interface (Boggess,
1968). Wellfields were subsequently moved inland.
Increases in the salinity of wells have also been due
to upward leakage through defective or improperly
constructed wells (Boggess, 1968; Boggess et al.,
1977; Schmerge, 2001). Saline water in the shallow
confined aquifers of Collier and Lee counties is
locally connate water trapped during sea level highstands (Schmerge, 2001; Missimer et al., 2003).
Ground-water withdrawals have actually locally
improved water quality as some of the saline connate water was removed from the aquifers (Missimer et al., 2003). The chloride monitoring data
evaluated in this study indicate that saline-water
intrusion into freshwater aquifers has not been
widespread over the past 20-30 years, and is more of
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a local, rather than regional concern, as far as
impacts to freshwater resources.
CONCLUSIONS
A widely held view is that rapid population growth
and associated residential development, such as has
been occurring in Collier and Lee Counties, necessarily results in an overtaxing of freshwater resources
and water-related environmental problems. Monitoring data encompassing the period of rapid growth
indicate that freshwater resources in subtropical
areas, such as southwestern Florida, are more resilient to increased use than generally considered,
although they are by no means unlimited. A key
hydrologic advantage of the subtropical areas, such
as southwestern Florida, is a large excess of rainfall
over ET during the summer wet season, which
recharges the aquifers. With the exceptions of the
mid-Hawthorn aquifer and portions of the sandstone
aquifer, the freshwater aquifers in most areas are
refilled each wet season.
Water budget considerations indicate that the
actual water use (total annual ET) of residential communities is comparable to that of native vegetation
and less than that of most agricultural land uses.
The increased ET of irrigated turf grasses and ornamental vegetation is largely offset by ET savings
from impervious areas. Stormwater management in
residential communities is critical for maintaining
water levels in the water table aquifer by minimizing
runoff and maintaining recharge. The overall public
and private water supply systems in Collier and Lee
Counties also result in a net addition of water to the
water table aquifer in irrigated areas.
The results of this study emphasize the importance
of actual monitoring data in water resources management, as a priori assumptions about the impacts of
water use may not be borne out by actual data. In general, freshwater resources are currently being managed and utilized in a sustainable manner. The water
management challenge moving forward will be to optimize the use of large potential aquifer recharge afforded by the humid subtropical climate of the region,
and to perhaps augment it with artificial recharge
(aquifer storage and recovery or other means).
ACKNOWLEDGMENTS
We thank Robert Schreiber, Lee Wiseman, Shelley Day, and
Frank Winslow of CDM and two anonymous JAWRA reviewers for
their thoughtful reviews.
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IMPACTS
OF
RESIDENTIAL DEVELOPMENT
ON
HUMID SUBTROPICAL FRESHWATER RESOURCES: SOUTHWEST FLORIDA EXPERIENCE
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