Enhanced Surface Water System Studies

Studies

In December 1995, the Tampa Bay Water Board of Directors approved the Master Water Plan. As we continued our dialogue with the public, we heard many concerns about the possible effects of these projects on the environment.

The primary concern we heard was that additional freshwater withdrawals would reduce flows in the river systems to the point that recreation and the ecology of these systems would be adversely affected. There was also concern about increased salinity in Tampa Bay as a result of the harvest of surface waters.

To address these concerns, flow-based withdrawal schedules were developed for the Tampa Bypass Canal and Hillsborough and Alafia River sources. These schedules insure that only a small percentage of flow in the system is harvested, and that harvest only occurs when flow reaches a certain point. This ensures that we will not take water that is necessary to sustain the environment. In addition, sophisticated environmental studies were conducted so the Tampa Bay Water Board could better understand the potential impacts of all proposed Master Water Plan projects on Tampa Bay. The studies were designed and completed by some of the foremost experts in the field of environmental science, professionals who have extensive experience studying the environment in our region.

The studies were performed to identify "fatal flaws," or major impacts, that could result from Master Water Plan projects. The team evaluated various project combinations, and estimated potential environmental impacts, including those under the most intensive and unlikely scenarios: "worst-case scenarios".

For Tampa Bay, the worst-case scenario was defined as the simultaneous implementation of all proposed Master Water Plan projects, at the highest proposed levels of production.

The studies revealed no significant adverse environmental effects from implementing the Master Water Plan projects, even under the most intensive, or "worst case" scenarios.

Studies included the Assessment of the Potential Impacts of the Tampa Bay Water Surface Water Source Projects and the Cumulative Impact Analysis for Master Water Plan Projects, which are detailed below.

A Numerical Modeling Investigation of a Proposed Desalination Facility at Big Bend.

A University of South Florida College of Marine Science study that looked at potential cumulative effects of the desalination and surface water source projects has confirmed earlier findings that the change in salinity from the projects would be much less than the normal salinity variations observed in Tampa Bay. Researchers also found that the salinity would remain well within Tampa Bay's normal salinity range and that the projects would not be harmful to the environment.

This was the fifth scientific study by noted scientists meant to determine how the water projects would affect the Bay.

USF's College of Marine Science completed the latest modeling study, the most detailed and integrated look to date at the potential cumulative effects of the projects. The study took a 'worst case scenario' approach, using rainfall and withdrawal conditions that tend to over-predict the average impacts from these projects.

The study showed that the largest salinity changes associated with desalination operations alone would occur immediately adjacent to the cooling water outfall in the discharge canal at Big Bend, and these changes would be minor (well within the normal variation currently observed). Adjacent to the concentrate discharge, the salinity increase dissipates rapidly to normal levels.

Salinity changes associated with the harvest of water from the Alafia and Hillsborough rivers and the Tampa Bypass Canal were largest in the summer months (when both flow and withdrawals would typically be highest), and lowest in the dry season when little or no water would be withdrawn from these systems. Even during the summer, when withdrawals would normally be at their highest levels, salinity changes were small compared with the natural variations currently observed in the system.

Assessment of the Potential Impacts of the Tampa Bay Water Surface Water Source Projects on Tampa Bay and its Tributaries -- November 1998

Study Objectives

To estimate small-scale, local modifications to salinity and tidal circulation patterns in Tampa Bay that could result from surface water withdrawals from the Alafia River and the Hillsborough River/Tampa Bypass Canal system. To estimate small-scale salinity increases and potential impacts to living resources in McKay Bay that could be caused by withdrawals from the Hillsborough River/Tampa Bypass Canal system.
To estimate small-scale salinity increases and potential impacts to living resources in the tidal Alafia River that could be caused by withdrawals from the Alafia River.

Study Findings

In Hillsborough Bay, the cumulative impacts of the two surface water withdrawal projects on surface water salinities are negligible, typically one part per thousand (ppt) or less. The greatest increases in salinity were predicted for July and August. In lower Hillsborough Bay, the predicted salinity increases for these months were 2 to 2.5 ppt; in McKay Bay they were 2 to 3 ppt; in the tidal portion of the Tampa Bypass Canal they were 3 to 4 ppt. These increases are well within the tolerance level for the dominant estuarine plants and animals that inhabit Hillsborough Bay. Predicted changes in tidal circulation or flushing are less than 5% for most of Tampa Bay with a few isolated areas showing changes of 10% to 20% and no areas with changes larger than 30%. The greatest predicted changes in tidal circulation occur in northwestern Hillsborough Bay, south of the interbay peninsula and in portions of the main ship channel. Because these changes are so small, the flushing time of the Bay will not be adversely affected by the proposed river withdrawals. Withdrawals from the Tampa Bypass Canal will slightly alter freshwater inflows to McKay Bay; however, predicted changes in salinity throughout most of the year are approximately 1 to 1.5 ppt. The greatest changes in salinity, 3 to 4 ppt, occur only during one or two months of the wet season. Comparisons of the salinity tolerances and preferences of the predominant vegetation, shellfish and fish in McKay Bay with the predicted salinity changes indicate that there will be no significant effect on the living resources of McKay Bay, including the extensive wading bird populations that frequent the area.
The greatest predicted change in average monthly salinity caused by withdrawals from the Alafia River is 1 ppt during October. In all other months, predicted salinity increases are less than 1 ppt. Because these changes are so minor, and are predicted to occur only at the river surface, no significant impacts on the living resources of the lower Alafia River are expected.

Cumulative Impact Analysis for Master Water Plan Projects -- April 1998

Study Objective

The objective of the Cumulative Impact Analysis was to determine the extent and magnitude of any large-scale environmental effects on Tampa Bay that might be caused by the concurrent implementation of all proposed Master Water Plan projects, including desalination.

Study Findings

Implementation of the Alafia River surface water withdrawal project alone would not significantly increase salinity (i.e., the concentration of salt) in the tidal river under low or high flow conditions. Implementation of the Hillsborough River/Tampa Bypass Canal surface water withdrawal project alone would not significantly affect salinity, nor would there be significant impacts to living resources of interest in the tidal Hillsborough River and McKay Bay.
Implementation of all Master Water Plan projects concurrently, an unlikely scenario, would not significantly increase salinity in Tampa Bay beyond the normal range of variation, and is not expected to result in any adverse impacts to living resources of interest.