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Water supply

The St. Johns River Water Supply Impact Study

Planning for future water needs

The St. Johns River Water Management District’s work is focused on ensuring an adequate supply of water, and protecting and improving the health of northeast and east-central Florida’s water bodies. Water supply planning efforts show that in many areas of the district, groundwater supplies have reached their sustainable limit, or will reach these limits in the near future, and the Floridan aquifer will not be able to meet all future needs. Water conservation and alternative sources of water, such as seawater, brackish (slightly salty) groundwater, reclaimed water and surface water from rivers, will be needed to meet water supply needs.

Previous district studies indicated that limited quantities of water can be withdrawn from the St. Johns and lower Ocklawaha rivers without causing harm to water resources.

In 2007, discussions were initiated by the district to further investigate the feasibility of developing additional water supplies from the St. Johns River.

What is the St. Johns River Water Supply Impact Study (WSIS)?

A district scientist uses a hydrolab device to measure water quality

A district scientist uses a hydrolab device to measure water quality.

At the direction of the district’s Governing Board, district staff began an extensive study in December 2007 to evaluate the potential environmental effects of proposed river water withdrawals on the biological and water resources of the St. Johns River. The goal of the study was to provide an objective, comprehensive and scientifically rigorous analysis of the potential environmental effects of river water withdrawals that will help guide future decision-making by the district.

More than 70 scientists and engineers — a combination of district staff and many outside experts of international standing — contributed to this unparalleled analysis of the St. Johns River. Moreover, the district engaged the nation’s most highly respected body of scientists, the National Academy of Sciences, to conduct an impartial peer review of the district’s project.

The National Academy of Sciences is among the nation’s premier scientific organizations. Its members are chosen in recognition of distinguished and continuing achievements in original research. The National Research Council (NRC) is the operating arm of the National Academies. Nine distinguished scholars from universities across the country actively participated on the peer review panel and provided advice and recommendations to further the district’s scientific and technical work.

WSIS study areas and the two-phased approach

Phase 1
A district scientist uses a probe to take a water sample in the St. Johns River

A district scientist uses a probe to take a water sample in the St. Johns River.

During the first phase of the study, from December 2007 through September 2008, district scientists and engineers, working in collaboration with 14 nationally recognized experts, improved and extended the district’s hydrodynamic models, examined the linkages between biological resources and water levels and flow rates, and identified the suite of potential environmental effects.

In addition, data deficiencies were identified and new data collection efforts were initiated for larval fish while more intensive sampling was initiated for submersed grass beds, phytoplankton (microscopic plant-like organisms that make up the basis of the food web) and salinity.

To address the diversity of potential environmental effects, work groups were organized:

  • Hydrology and hydrodynamics
  • Biogeochemistry
  • Plankton
  • Benthic macroinvertebrates
  • Littoral zone vegetation
  • Fish
  • Wetlands and floodplain wildlife

These groups covered the complete riverine ecosystem from the mouth to the headwaters, from the channel to the upland border of the floodplain, and from bottom habitats through the water column.

The St. Johns River is not ecologically uniform. Thus, work was also divided by river segments to ensure consideration of potential variation in withdrawal effects along the river’s length. Effects would not be the same throughout the river, so this segmenting of the river was a critical element of the study.

To conclude the first phase, a technical symposium was held in September 2008 to bring together the outside experts and district staff for an exchange of information and ideas. The event was open to the public and broadcast over the Internet.

Phase 2

The second phase kicked off in October 2008 with the NRC agreement to conduct the peer review of the district’s project at key intermediate points during the study. As part of the peer review, the committee evaluated the district’s interim reports and identified areas where additional emphasis could occur during the study’s second phase. These nationally renowned experts offered numerous specific recommendations on issues and priorities related to the study. The project team continued to collect and analyze data while the NRC committee simultaneously conducted its review.

A major emphasis of Phase 2 included district experts refining and updating the hydrodynamic model, which simulates water levels, velocities, discharges, salinity and flushing rates of the river. This state-of-the-art model became even more refined through this study.

The district held a second technical symposium in September 2009, which again offered an opportunity for the project team to openly exchange information and ideas in a public forum.

State-of-the-art modeling and environmental analysis

Throughout the study, district scientists and engineers and outside experts investigated a broad range of test scenarios and their modeled impacts. Using the district’s hydrodynamic model, the WSIS analyzed a range of water withdrawal scenarios that are representative of potential future water withdrawals.

Withdrawal scenarios were evaluated for their potential effects on aquatic grasses, listed species, commercial fish species and their food base, wetlands and wetland wildlife, and potential changes to the severity, frequency and duration of algal blooms. The study then categorized the potential for environmental effects on critical biological communities, species and water resources. Simply put, the modeling predicts the hydrologic effects, and environmental information is gleaned from the modeling.

A district scientist monitors the types and abundance of macroinvertebrates found in the St. Johns River.

A district scientist monitors the types and abundance of macroinvertebrates found in the St. Johns River.

Some of hydrologic drivers that were used in the various scenarios:

  • Salinity
  • Nutrients
  • Turbidity
  • Sea level rise
  • Residence time
  • Entrainment and impingement (impacts to marine and freshwater organisms and microorganisms at the intake structures)
  • Increases in the volume of water due to the Upper St. Johns River Basin restoration projects and land use changes

The study also took into account the unique characteristics of each section of the St. Johns River, and how water flows and at what velocity in each of these sections.

With eight river segments, four basic hydrologic/hydrodynamic effects (discharge, water level, retention time and salinity) and seven ecological work groups, there were at least 3,360 different combinations for evaluation. Many work groups had a diverse set of key attributes that could be affected by withdrawals, so the actual number of potential scenario combinations was even larger.



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St. Johns River Water Management District
4049 Reid Street, Palatka, FL 32177
(800) 725-5922