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Summary
THE WATER SUPPLY IMPACT STUDY
The St. Johns River Water Management District is responsible for managing water
resources in the St. Johns River basin, which comprises 23 percent of Florida. Approximately
4.73 million people (one quarter of Florida’s population) live in the area served by the District,
which contains the growing cities of Jacksonville, Orlando, and Gainesville. In order to meet the
increasing water supply needs of the District’s residents and other water users, the District is
considering supplementing its historical supply of groundwater with water from the St. Johns and
Ocklawaha Rivers. To better understand the potential ecological impacts of such withdrawals, in
2008 the District began a large scientific study called the Water Supply Impact Study (WSIS).
In late 2008, the Water Science and Technology Board (WSTB) of the National
Academies was asked to review the progress of the WSIS, including such scientific aspects as
hydrologic and water quality modeling and how river withdrawals will affect wetlands,
biogeochemical processes, plankton, benthos, the littoral zone, fish, and wetlands wildlife in the
basin. For two and half years, the WSTB Committee has followed the activities of eight District
workgroups as they modeled the relevant river basins, determined the criteria to evaluate the
environmental impacts of water withdrawals, evaluated the extent of those impacts, and
coordinated with other ongoing projects. The first report of the Committee reviewed the Phase 1
hydrologic and environmental assessment tools and relevant data, and made recommendations
regarding proposed work for the second phase of the WSIS. The Committee’s second report
focused on how the District was responding to the recommendations in its first report. The third
Committee report primarily evaluated the hydrologic and hydrodynamic work being performed
by the District. This fourth and final product of the Committee focuses on the ecological impact
analyses conducted by the environmental workgroups, presents final thoughts about the
hydrologic and hydrodynamic studies, and provides some overall perspectives on the WSIS.
This report does not discuss the recommendations and content of the previous three NRC reports
in great detail.
HYDROLOGY AND HYDRODYNAMICS
Because the goal of the WSIS was to assess potential impacts of withdrawing freshwater
from the St. Johns River, understanding the basin hydrology and river hydrodynamics was vital
to analyzing and understanding possible ecological effects. The District used a suite of surface
hydrology, hydrodynamic, and groundwater models to analyze potential physical changes that
1
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2 Review of the St. Johns River Water Supply Impact Study: Final Report
would be brought about by withdrawals, including changes in river flow, stage, salinity, and
water age (i.e., the length of time a water parcel remains in the river).
The Committee found the work of the hydrology and hydrodynamics (H&H)
workgroup on building, testing, and analyzing its hydrologic and hydrodynamic models,
including efforts to quantify the propagation of data uncertainty into hydrodynamic model
uncertainty, to be state-of-the-art science. The District is building their WSIS analyses on a
hydrodynamic foundation that is well-tested, robust, and well-understood. The H&H workgroup
could further improve its efforts by comprehensively synthesizing its model results in its final
report. This would put into context the relationships between key mechanisms of the river
system and their responses to forecast conditions. In particular, the workgroup should pay
attention to two major competing effects—sea-level rise and increased runoff due to future land
use changes such as urban and suburban development—that affect water surface levels and
salinity, and the uncertainties associated with these effects should be discussed. This “big-
picture view” of the river should be directed at non-modelers and non-hydrologists so that they
can better understand the implications of the extensive modeling studies.
A previous report of the Committee noted the limitations of the surface water hydrology
modeling program HSPF and the steady-state groundwater flow models based on MODFLOW.
Because HSPF has limited value for wetlands, the District was urged to (and subsequently did)
continue developing the Hydroperiod Tool and analyzing water level data from transects used to
develop regulations on minimum flows and levels (MFLs) to determine the correspondence
between river stage and wetland hydroperiod and thus the potential responses of different
wetland types to water withdrawals. In the future, the District also should develop a
groundwater model that simulates the full interaction of the river with the surficial aquifer
system and the Upper Floridan aquifer under both steady state and transient conditions. This
should include an uncertainty analysis for groundwater discharge to the river based on hydraulic
conductivity, which may have uncertainties of an order of magnitude or more for basins the size
of the St. Johns.
ENVIRONMENTAL WORKGROUPS
Seven environmental workgroups used information from the H&H results in combination
with hydroecological models of possible effects to predict the potential impacts of water
withdrawals on (1) wetland vegetation, (2) soil biogeochemical processes, (3) plankton
communities, (4) submersed aquatic vegetation (SAV), (5) freshwater and estuarine benthos, (6)
fish, and (7) wetlands wildlife. Each workgroup was asked to characterize potential
environmental effects of water withdrawals using three criteria: persistence, strength, and
diversity. Persistence was defined in terms of recovery time relative to the return interval for
conditions causing a given effect; strength was defined in terms of both the intensity and scale
(geographic area affected); and diversity was defined in terms of the range of environmental
attributes showing effects. Based on the three criteria, the District developed five categories of
effects ranging from negligible to extreme. For each ranking, the workgroups assigned an
uncertainty level (ranging from very low to very high) defined with reference to (1) the
availability of a predictive model, (2) supporting evidence, and (3) understanding of the
mechanism for an effect. These categories for levels of effect and uncertainty were defined in an
effort to obtain consistency among the workgroups in assessing effects—a strategy condoned by
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Summary 3
the Committee and carried out effectively by the District scientists. The following sections
summarize key findings of the environmental workgroups and describe the major issues that the
Committee had regarding the approach and/or results of each workgroup. Other detailed
criticisms are found in the body of the report (primarily in Chapters 3 and 4).
Wetlands
The wetlands workgroup was tasked with assessing the potential effects of surface water
withdrawals on floodplain wetlands, specifically changes to vegetation communities that might
result from altered hydrology and/or changing salinity regimes. To accomplish this they first
identified river segments that have the highest likelihood of change. These then became the
focus of subsequent analyses. The workgroup assessed existing MFL transect data on wetland
plant community types across the elevational/hydrological gradient of the floodplain in those
river segments to determine how the communities might change with withdrawals. LiDAR data
were acquired for some areas of the watershed to create a digital elevation model (DEM), which
was then subjected to a GIS analysis to predict hydroperiod changes in wetlands. The goal of
this analysis was to determine whether water withdrawals have the potential to (1) alter the
species composition of floodplain wetland communities, (2) alter the extent of wetlands or
various wetland communities found there, and/or (3) lead to a shift in the location of boundaries
between wetland types. The workgroup focused on river segment 8, where impacts to river stage
were predicted to be greatest, and segment 2, where changes in the salinity regime were
predicted to be highest. The wetlands workgroup found “moderate” impacts to wetlands in
segments 2 and 8 under the most extreme future withdrawal scenario.
The wetlands workgroup produced a solid analysis of potential impacts of water
withdrawals to the St. Johns River. Their integration of a LiDAR-based DEM with floodplain
stage exceedence curves to assess the spatial extent of hydrological impacts is a novel approach,
resulting in a robust picture of the spatial extent of dewatering and shifting boundaries between
wetland types. The salinity analysis strategically made use of the Ortega River tributary as a
model system from which results could be translated to the larger St. Johns River. Because the
Committee is confident that the methods developed by the workgroup will be adaptable to other
river segments and be useful to analyze potential changes in river flow in the future, it
recommends expanding the analysis as more data and resources become available.
Biogeochemistry
The biogeochemistry workgroup identified several potential effects of water withdrawals
on biogeochemical processes in the St. Johns River and its drainage basin, all related to the
possibility that soil accretion would be reduced or oxidation of organic soils (histosols) would be
enhanced in riparian wetlands of the river as a consequence of changes in stage induced by water
withdrawals. The workgroup concluded that two effects of water withdrawal had potentially
high significance: reduced nutrient sequestration and increased release of colored dissolved
organic matter. The workgroup considered how much additional release of the constituents of
interest would occur from soil organic matter as a result of water withdrawals, how much of the
additional material would be exported from the wetlands to lakes, and what effects could result
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4 Review of the St. Johns River Water Supply Impact Study: Final Report
in downstream ecosystems. Using data on release rates gathered from the literature and very
limited data from their own field and laboratory studies, the workgroup assessed the changes in
DOC, TP, NH4+-N, and TKN loading, as well as changes in dissolved oxygen concentrations,
that would accompany water withdrawals and associated lowering of water levels. Only segment
8 received the full analysis, and the workgroup found that the impacts of an extreme withdrawal
scenario would be negligible.
Although most of their efforts to obtain experimental data were not successful, the
workgroup did a thoughtful and objective analysis on their few experimental results.
Studies to determine values of nutrient and DOC release rates from exposed wetland soils were
unsuccessful for the most part, and no experimental data were available on attenuation of
nutrient and DOC loads as water would flow from re-inundated wetlands into Lake Poinsett.
Nonetheless, the workgroup found a relevant compilation of attenuation rates from natural and
constructed treatment wetlands in the literature.
The multiple regression relationship used in the report to relate increases in DOC loading
to decreases in DO in Lake Poinsett was not strong (r2 = 0.42). Given the very small values of
predicted changes in DOC concentrations produced by the modeling analysis for even the “worst
case” withdrawal scenario, however, the Committee concludes that a more sophisticated analysis
could not be justified. Overall, the Committee agrees with the general levels of effect identified
by the biogeochemistry workgroup for the various river segments—conclusions that were based
largely on the H&H modeling analysis, but it regards the workgroup’s assessment of uncertainty
levels for the predicted impacts on some river segments as too low.
Plankton
The plankton workgroup was charged with determining the possible environmental
impacts of water withdrawals on plankton communities in the St. Johns River, most of which
were consequences of enhanced growth of phytoplankton. Consequently, algal bloom dynamics
was a primary focus of the work. The plankton workgroup set thresholds for adverse ecological
effects of algal blooms, modeled the relationships between bloom characteristics and hydrology,
and determined whether water withdrawals would cause or exacerbate adverse effects of algal
blooms. The group predicted changes in phytoplankton biomass, community composition, N2
fixation, cladoceran zooplankton abundance, algal toxins, and dissolved O2 concentrations as a
function of changes in water age. They used both empirical methods based on historical data
collected on the lower, middle, and upper St. Johns River since the mid-1990s, and a mechanistic
water quality model called CE-QUAL-ICM. The workgroup concluded that the range of
withdrawal scenarios likely would have little impact in excess of pre-existing algal bloom
conditions in segments 2, 3, 4, and 6 of the St. Johns River.
The overall approach of the plankton workgroup was logical and used the best
available information to derive and parameterize the models. The plankton workgroup
adopted two relatively independent approaches to assess the impacts on phytoplankton bloom
dynamics and consequent changes in water quality. For the one segment of the river where both
approaches were applied, results of the mechanistic and empirical models were similar, which
strengthens their conclusions. Many of the multiple regression equations used to quantify the
relationships between “water age” and phytoplankton characteristics were robust (r2 > 0.80),
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Summary 5
which also promotes confidence in the models. The conclusions of the plankton workgroup are
supported by the evidence presented and the predictions are based upon the best available data.
One weakness of the plankton study is that potential changes were compared to a 1995
base case scenario. As the workgroup showed, however, conditions in 1995 were not ideal, and
many sections of the river suffered from persistent algal blooms and anoxia. Water withdrawals
may not worsen conditions, but clearly they will not improve conditions in the river. Another
weakness is that historical estimates of N2 fixation were based on nitrogen mass balances for
Lake George, thus representing the net effects of several source and sink processes. In addition,
prediction of the effects of withdrawals on N2 fixation was based only on changes in water age
and assumed that nitrogen concentrations in the river would not change from the data used to
compute the historical amounts.
Littoral Zone: Submersed Aquatic Vegetation
The littoral zone work group focused their analysis in the middle and lower basin on
Vallisneria americana, which has been identified in 92 percent of the MFL transects in the basin.
Vallisneria was thus regarded an excellent indicator of the condition of the SAV community in
the littoral zone of the St. Johns estuary. The workgroup formulated and tested two main
hypotheses: (1) salinity intrusions could result in stress to Vallisneria that over extended time
periods could reduce its growth and survival in the estuarine portion of the St. Johns River, and
(2) water withdrawals that result in the lowering of water levels in the non-tidal portions of the
St. Johns River could restrict the overall area suitable for SAV growth. To address the first
issue, a salinity/time exposure relationship was developed for Vallisneria from an extensive
literature review, bolstered by experimental data on several levels. To address the latter issue,
the workgroup determined a depth/stage relationship for SAV based on anecdotal data and
information from river segments 2 and 3. They predicted that the effects of the two worst case
scenarios (for salinity in segments 2 and 3 and for stage in segments 7 and 8) were negligible.
The conclusions of the SAV workgroup regarding impacts of lowering water levels
upstream and increasing salinity levels downstream on SAV are well thought-out and arise
from careful data analysis. The assessments appear to be “state of the art” and robust with
regard to salinity and water levels. Indeed, in several instances the workgroup pushed beyond
what is normally achieved in environmental assessments, especially with regard to creating a
useful Vallisneria “salinity stress model” from the literature and experimental data based on
stress enzymes.
The workgroup was advised to keep abreast of two management issues. The first is that
future water withdrawals will be necessitated by increased population density, which will lead to
higher nutrient loadings from the watershed and thus increase the duration and intensity of
phytoplankton blooms in the St. Johns River unless strong management efforts are undertaken to
control nutrient export. This secondary effect of the proposed water withdrawals could be as
much a problem as salinity and water levels in determining the fate of SAV in the St. Johns
River ecosystem. Second, salinity will increase in the estuarine portion of the St. Johns River as
downstream dredging projects and sea-level rise progress. As discussed in greater detail in
Chapter 3, a more detailed exploration of SAV that can withstand higher salinity is warranted.
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6 Review of the St. Johns River Water Supply Impact Study: Final Report
Benthos
The benthos and fish workgroups were divided into freshwater and estuarine components,
which took different approaches to understanding the potential impacts of water withdrawals.
Freshwater Benthos
The workgroup based its analysis on the hypothesis that changes in stage prompted by
water withdrawals would have a direct impact on the density and distribution of target taxa, as
well as on community and population metrics such as diversity, density, and biomass. Stage also
was predicted to impact benthos indirectly via changes in wetlands acreage and structure. In
contrast to most other ecological workgroups, the freshwater benthos workgroup did not have
any hydroecological models to predict the magnitude of changes in benthic conditions as a
function of hydrologic changes resulting from water withdrawals. A short-term field study at
Lake Monroe, Lake Poinsett, and Yankee Lake was conducted in 2009 to observe patterns for
several community- and population-level metrics that could be related to hydrology. Also, a
number of studies on the ecology and habitat requirements of crayfish and apple snails in the
upper St. Johns River basin and the Everglades were reviewed. The workgroup combined this
information with H&H model results on water levels to make predictions, using professional
judgment and a “weight of evidence” approach, about the potential effect of water withdrawals
on benthos. Their final predictions for the extreme withdrawal scenarios ranged from negligible
to moderate. Uncertainty was high in almost every segment because of the lack of a predictive
hydroecological model, the lack of monitoring data, and the considerable variability observed in
results of the 2009 study.
The freshwater benthos workgroup’s analysis was based on little benthic data in the
St. Johns River, which greatly limits the Committee’s ability to determine the validity of
the conclusions. None of the results gleaned from the 2009 study could be attributed uniquely
to effects of hydrologic conditions as opposed to seasonal and other possible water quality
effects. The Committee’s concerns are, however, somewhat lessened by the knowledge that the
likely future withdrawal scenario will reduce water levels by much less than the extreme
scenarios evaluated by the workgroup.
The lack of quantitative hydroecological models may reflect the state of the science in
benthological research; if so, this study illustrates the need for such models to be developed. The
workgroup recognized the limitations of their analysis and proposed a future monitoring strategy,
for which the Committee offers numerous suggestions (see Chapter 3).
Estuarine Benthos
The estuarine benthic community was hypothesized to be susceptible to changes in flow
and salinity that might accompany water withdrawal. Data for the analysis were derived from a
long-term data set of the Florida Department of Environmental Protection and a short-term data
set from the U.S. EPA’s Environmental Monitoring and Assessment Program at sites along the
lower St. Johns River. Additional data on white shrimp and blue crab were obtained from the
Fisheries Independent Monitoring Service (FIMS) Program, coupled with supporting material
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Summary 7
from the literature on life history and environmental requirements of these species. The
workgroup used linear and/or nonlinear regression analyses to relate mean macro-infaunal
abundance and abundances of the most common taxa with mean salinity. Using the same three
metrics as the freshwater analysis (community changes, population changes, and changes to
target taxa), the workgroup found that the overall response of estuarine benthos to the extreme
withdrawal scenario was negligible to minor.
Overall, the estuarine benthos analyses were more sophisticated than the freshwater
benthos analyses. Nonetheless, the Committee had several concerns. First, there were no direct
statistical models for abundance and inflow such that quantitative predictions could be made (as
in other ecological parts of the WSIS). The interpretations appear to have been derived from
how salinity changes with withdrawal scenarios and how abundance relates to salinity. Second,
more work on the direct effects of salinity on epifauna needs to be completed before epifaunal
impacts can be dismissed. Third, potential impacts on infaunal and epifaunal benthic organisms
of salinity increases and coupled low DO levels, which could occur periodically in the lower
river, should receive further study.
The benthic workgroup provided several suggestions for future work on mitigation and
data collection, all of which are appropriate and important to consider. The workgroup
expressed concerns about the lack of station-specific sediment composition and associated
benthic communities. They also discussed the need to understand water withdrawal effects on
meroplankton (pelagic larvae of benthic organisms), which are important food sources for many
fishes.
Fish
Freshwater Fish
The processes of concern to the freshwater fish workgroup included how changes in
water levels, flow, floodplain inundation and frequency, and entrainment/impingement, may lead
to changes in vital fish metrics at different levels of organization. The workgroup examined
members of five freshwater habitat-use guilds relative to water withdrawals in the Upper Basin
between Lake Poinsett to Lake Woodruff. The potential degree of impacts on four guilds was
estimated using best professional judgment by examining the ecology of selected species of each
guild with reference to a specific withdrawal scenario. The fifth habitat-use guild—littoral zone,
marsh, and floodplain small fishes assemblages—used MFL transect data in a model that
generates fish densities based on flooding duration, which was developed for a similar habitat-
use guild in the Everglades. Finally, to analyze the potential impacts of entrainment/
impingement on fish, the workgroup conducted a sampling program to determine the species
composition and abundance of spawning fish in river reaches where water withdrawal structures
have been proposed. The workgroup’s predictions regarding impacts ranged from minor to
major (for entrainment/impingement) under extreme withdrawal scenarios.
Overall, the freshwater fish workgroup posed appropriate questions related to
potential impacts on fish assemblages and addressed them as much as available data would
allow. There are, however, some concerns. First, although the approach focuses mainly on
mean water level with a few comparisons of extreme levels (low and high), it does not capture
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8 Review of the St. Johns River Water Supply Impact Study: Final Report
cumulative effects of water withdrawals on fishes, such as concentration of fishes into reduced
water volumes and loss of prey for wading and fish-eating birds, snakes, and mammals.
Consecutive drought years, which likely would have considerably more negative impacts on
fishes, were not examined. The workgroup report was written as if the entire assemblage of
fishes within each habitat-guild had been examined when, in fact, only common representatives
of each assemblage were examined. Finally, the discussion of entrainment and impingement
focused on various shad species and did not fully consider all species collected.
Estuarine Fish
The effects of water withdrawals on estuarine fish assemblages (open water small
estuarine fishes, estuarine marsh fishes, estuarine benthic fishes, sciaenid fishes, and marine
fishes) relate to changes in water levels, flow, and changes in spatial distribution of salinity.
Analyses were based exclusively on FIMS data for fish distribution and abundance, and
associated statistical relationships between various measures of abundance and inflow that were
developed. The analyses were conducted for “pseudospecies” only, defined as gear-, size class-,
month-, and zone-specific designations for each species. For all groups, the workgroup predicted
moderate impacts under the worst case withdrawal scenario in river segments 1 to 3.
The estuarine fish workgroup is commended for modifying, in response to the
Committee’s input over the course of two years, their approach to the complex issue of how
fish will respond to changes in flow and salinity. As with the freshwater fish, the estuarine
fish workgroup posed appropriate questions related to potential impacts on fish assemblages and
addressed them as much as available data would allow. With respect to the use of
pseudospecies, the detailed changes noted for each pseudospecies in a given fish assemblage are
probably not as important as the total number of changes within the assemblage relative to the
modeled scenarios.
The fish workgroup predicted a “major” response to water withdrawal, but this was for an
extreme scenario that is not plausible. Because the response surrounds the potential entrainment
or impingement of larval organisms at intake sites, it is imperative that precautions are taken
when designing intake structures to avoid these impacts. The workgroup should consider when
entrainment/impingement is temporally important (such as during seasonal spawning peaks). If
protective intake structures cannot be constructed, the District may need to write conditions into
its permits that require water suppliers to reduce surface water extraction during those peak
recruitment periods.
Wetlands Wildlife
The wetlands wildlife workgroup assessed the potential effects of surface water
withdrawals on 320 species of vertebrate wildlife that depend on the St. John’s River floodplain
habitat. The workgroup used a qualitative approach to evaluate impacts because quantitative
data are lacking on responses to changing hydrologic regimes for many of the species.
Information gleaned from the literature, in combination with input from the wetlands, benthos,
and fish workgroups, was used to make best professional judgments on the effects of
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Summary 9
withdrawals on wildlife with respect to salinity in river segments 1 and 2, and with respect to
altered hydroperiod in segments 7 and 8.
To accomplish this, species were assigned to one of four “wildlife hydrologic types” and
floodplain habitats were assigned one of four hydrologic regimes for wetlands. The wildlife
hydrologic types then were combined with the floodplain hydrologic regimes in a qualitative
model that describes the predicted distribution of species along the floodplain’s hydrologic
gradient (from permanently flooded to dry). The workgroup also used results from the H&H,
wetlands, benthic invertebrates, and fish workgroups to determine the final levels of effect.
Under the extreme withdrawal scenario, major impacts were predicted for estuarine wildlife in
segment 1, moderate impacts were predicted for estuarine wildlife in segment 2, and moderate
impacts were predicted to freshwater wildlife in segments 2, 7, and 8.
The wetlands wildlife analysis was limited by the lack of quantitative, species-
specific information on the response of wildlife to altered hydrology and salinity. Thus, the
analysis is an integration of a very thorough literature review along with the results of the
H&H modeling and input from the wetlands, benthic invertebrate, and fish workgroups.
The literature synthesis was thorough and will be of benefit to future research and management
efforts in the St. Johns basin because it covers such a broad range of species.
The findings of the wildlife workgroup were obscured by the diverse ways in which
species were classified according to their hydrologic attributes. Four categories of wildlife
“hydrologic types” were introduced, but the effects of water withdrawals were shown for only
two of these categories. Establishing wildlife hydrologic types is a sound way to deal with the
diversity of habitat requirements for the species included in the analysis, but the terms used to
describe them are not fully appropriate. For wildlife species, the categories generally describe
how much water the species needs for its annual habitat requirements without consideration of
key life history stages. This is particularly troublesome for amphibians, all of which are obligate
species in the sense that they require standing water for reproduction.
GENERAL CONCLUSIONS ABOUT THE WSIS
When the Committee first became involved with the WSIS (early 2009), the study
objectives to examine the effects of surface water withdrawal on a broad range of environmental
issues seemed to be quite an undertaking given the available data and disparate paths of analysis.
However, as the study progressed and the Committee presented its comments, relevant data were
collected and the analytical work was increasingly conducted along biological “chains of
causation.” The District scientists welcomed the Committee’s recommendations and
implemented them when feasible, thus overcoming many of the limitations noted by the
Committee early in the project. The Committee commends the WSIS workgroups for their
careful and thoughtful responses to its suggestions.
Even those workgroups stymied by a lack of relevant data and information were able to
make some conclusions about the likelihood of effects from water withdrawals (albeit with high
uncertainty). Insofar as the H&H results indicate that withdrawals will produce relatively small
changes in areas and depths of inundation, the inability of these workgroups to make more
certain predictions is somewhat ameliorated. Indeed, had the WSIS benefitted from having the
results of the hydrologic/hydrodynamic analysis at an earlier date, it is likely that the District
would not have invested so much effort in determining some environmental responses to altered
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10 Review of the St. Johns River Water Supply Impact Study: Final Report
flows and levels. In general, the District did a competent job relating the predicted
environmental responses (including their magnitude and general degree of uncertainty) to
the proposed range of withdrawals. The overall strategy of the study and the way it was
implemented were appropriate and adequate to address the goals that the District
established for the WSIS.
Several critical issues that are beyond the control of the District or were outside the
boundaries of the WSIS limit the robustness of the conclusions. These issues include future sea-
level rises and increased stormwater runoff and water quality degradation of surface runoff
engendered by future population growth and increases in impervious area and pollutant
generation associated with urban development. The predicted effects of sea level rise and land
use change on water levels and flows in the river are greater in magnitude than the effects of the
proposed surface water withdrawals, but they have high uncertainties. The District should
acknowledge these limitations in its final report and, using an adaptive management strategy, it
should plan to run its models with more recent rainfall and land use records and with emphasis
on water quality as well as quantity.
In addition, the workgroups did not appear to consider the possibility of “back-to-back”
extreme events (e.g., several extreme droughts separated by only a short period of normal
rainfall) in their impact analyses. They also tended to present mean responses to changes in
driver variables with little or no consideration of the variance in response. Although mean
values are the most likely responses from a statistical perspective, ranges (or variances) of
responses also should be considered in analyzing potential environmental impacts of changes in
driver variables. Such responses may be less likely than mean values, but they may not have
negligible probabilities and could be more detrimental than the mean responses.
Insofar as the MFL regulations limit the withdrawal allowable during low flow periods,
the Committee remains concerned whether MFLs will be rigidly enforced in the future. If there
is an extended drought in the future, water suppliers might not be able to withdraw water from
the river for months or even years on end. It is not obvious that this would be socially
acceptable. Finally, now that the WSIS is nearly complete, the District should reexamine the
results from their earlier water supply study, which concluded that additional groundwater
withdrawals would lead to undesirable impacts on natural vegetation. The Committee
recommends that the District compare the levels and nature of impacts associated with
withdrawals from the two (surface and groundwater) sources of additional water supply for the
region.
