Balancing Water Needs and Sources in Lake Tarpon, Florida
Fluctuating lake levels create opportunities for storing and reusing fresh water.
By
Douglas E Robison,
Thomas Farkas,
Andrew P Squires,
Thomas Farkas,
David Slonena
Lake Tarpon is located in Pinellas County, FL, immediately northwest of Tampa Bay (Figure 1). With a surface area of 4 mi.2, Lake Tarpon is one of the largest natural lakes in west-central Florida.
To relieve periodic flooding in this historically closed basin, the United States Army Corps of Engineers completed construction of the Lake Tarpon Outfall Canal control structure in 1972. Subsequent reduction in lake fluctuations and watershed urbanization resulted in the overgrowth of shoreline vegetation, a decline in sport fisheries, and the degradation of water quality.
Managing Lake Levels
To restore degraded aquatic habitats and improve water quality, Pinellas County has been evaluating the adoption of an enhanced lake-level fluctuation schedule (Figure 2) to modify the current operations of the Lake Tarpon Outfall Canal control structure. This would reestablish a more natural pattern of seasonal and interannual variations to be repeated every four years.
High water elevations are proposed for the winter and summer months to flood shoreline vegetation and control the expansion and proliferation of superabundant species, predominantly cattails, willows, and hydrilla. Low water elevations are proposed for the spring and fall months to flush and dilute in-lake nutrient concentrations before and after the summer growing season, when algae growth is greatest.
Balancing Consequences
Although the Lake Tarpon Outfall Canal control structure provides a flexible and cost-effective means of fluctuating lake levels, the discharge of lake water into the saline waters of Old Tampa Bay results in an irrevocable loss of a substantial volume of potentially reusable fresh water. If, however, this water could be harvested, stored, and later recovered for various beneficial reuses, it would constitute a significant, new source of water for Pinellas County and the entire Tampa Bay region, an area that has experienced significant water-supply shortages during the past decade.
An estimated 1.1 to 1.8 billion gallons of fresh water would be discharged annually over the structure through the operation of the enhanced lake-level fluctuation schedule. This conservative estimate does not take into account the additional surface water that would flow over the Lake Tarpon Outfall Canal control structure after target lake levels are attained. Based on historical gauge data, the estimated annual average volume of water discharged through the Lake Tarpon Outfall Canal will almost always exceed 3.4 billion gallons, even during the driest years. Harvesting, storing, and later recovering this volume of water would yield a daily freshwater flow of approximately 9.3 million gallons per day available for beneficial reuse. An effective means of storing, recovering, and distributing the water, however, will be needed.
Storage Options Studied
Two alternatives were considered: surface-water storage in one or more reservoirs and aquifer storage and recovery (ASR). Surface-water storage was determined to be unfeasible in the Lake Tarpon watershed because of the lack of adequate vacant uplands. Pinellas County therefore looked to ASR.
ASR is the storage of available excess water in suitable subsurface storage zones and the recovery of stored water when needed. Typically storage is accomplished by injecting water into an appropriate geologic storage zone through a well and later by using the same well for recovery of the stored water. If properly implemented, ASR is a viable, cost-effective water-resource management tool that can provide more significant advantages than conventional surface-water storage facilities. The use of ASR also protects the stored waters from surface contamination and maximizes water conservation by eliminating evaporative loss.
Two ASR test-well sites were identified during a feasibility study conducted for Pinellas County: Chestnut Park on the southeastern shore of Lake Tarpon and the land near the Lake Tarpon Outfall Canal control structure, approximately 9,000 ft. south of Lake Tarpon. Based on an analysis of existing data, development of ASR wells at both test sites was considered feasible. Subsequent exploratory testing at the outfall canal site revealed very poor ambient groundwater quality and marginal confinement of the storage zone. Exploratory testing at the Chestnut Park site, however, indicated favorable conditions for future development of ASR wells.
Beneficial Reuse Alternatives
After ASR was determined to be a potentially viable storage alternative, the following beneficial reuse alternatives for surface waters discharged from Lake Tarpon were considered:
- Augmentation of the Pinellas County reclaimed water supply
- Augmentation of the regional potable water supply
- Rehydration of adversely impacted wetlands
- Restoration of tributary baseflows
To evaluate the relative merits of these reuse alternatives, a weighted decision matrix was developed and applied and the following criteria were used:
- Demonstrated need or demand
- Public acceptance
- Environmental or public health impacts
- Regulatory permittability
- Construction cost
- Operation and maintenance cost
Based on this analysis, only lake management and augmentation of the Pinellas County reclaimed water supply were considered viable alternatives.
Lake Management
Under the more or less static lake levels that have existed since the construction of the Lake Tarpon Outfall Canal control structure, weeds and exotic aquatic plant species have proliferated and overrun desirable native species. A potential benefit of the enhanced lake-level fluctuation schedule is the reduction of these vegetational nuisances with the proliferation of native aquatics, such as eelgrass and bulrush.
Another anticipated benefit of the enhanced lake-level fluctuation schedule is in-lake water-quality improvement through increased dilution and flushing. Using water recovered from an ASR well increases lake flushing by adding a source of inflow water that does not currently exist, and using recovered water to refill the lake following lake-level drawdowns dilutes in-lake concentrations of nitrogen and phosphorus.
Dilution of nutrient concentrations will occur if, prior to recovery, the stored discharge water is exposed to pretreatment before injection. Natural treatment processes in the aquifer during the storage period might further reduce nutrient concentrations. Based on model analyses, lake flushing and dilution could result in a 3.5% reduction in the annual nitrogen load to the lake, which would be sufficient to reduce periodic algae blooms.
Augmenting Reclaimed Water Supply
Pinellas County provides reclaimed water (enhanced, secondary, treated wastewater) to users in the northern portion of the county to offset partially the use of potable water supplies for residential irrigation. In 1999, the annual deficit in reclaimed water was approximately 580 million gallons, which—on an average daily basis—equated to approximately 1.6 million gallons per day. Deficits are greatest during the dry season, March through May, when regional rainfall is sparse. Demand for reclaimed water in the county's northern area has continued to increase; therefore, deficits are expected to increase without a reliable augmentation source.
The use of Lake Tarpon surface water in conjunction with an ASR system was determined to be a viable option for supplementing the county's reclaimed water supply. Lake surface waters could be captured and stored in ASR wells during implementation of the enhanced lake-level fluctuation schedule and possibly also during wet-weather discharges. During the dry season when the demand for reclaimed water is greatest, stored water recovered from the ASR wells could be used to augment the reclaimed water supply and lake levels. Approximately five ASR wells, each sized at 1 million gallons per day, would be needed at build-out to reliably supplement the reclaimed water system to meet both current and future reclaimed water demands.
Operational Scenarios
Four ASR operational scenarios were developed to illustrate the potential uses and benefits of stored/recovered surface water (Figures 3A , 3B, 3C, and 3D).
- Scenario A: The captured water is stored within an ASR well system and later recovered from the well(s) for Lake Tarpon water-level augmentation during drought periods and for increased dilution and flushing.
- Scenario B: The captured water is stored within an ASR well system and later recovered from the well(s) to supplement the Pinellas County reclaimed water system. In addition, during periods when lake water is pumped to the ASR wells—which results in the wells not being available to supplement the reclaimed supply—some of the lake water can be diverted directly from Lake Tarpon to the reclaimed water system.
- Scenario C: The operational components of Scenarios A and B are combined to maximize the single ASR well concept with storage and recovery of lake water to augment lake water levels and supplement the reclaimed water system. Seasonal and yearly variations in the supply and demand of the reclaimed water system combined with the naturally occurring lake-level recovery will determine largely how the ASR well system is managed.
- Scenario D: All—or at least a significant portion—of the water discharged annually through the Lake Tarpon Outfall Canal, including all lake discharge water and a significant portion of the wet-weather overflows, is captured. Because of the potentially large volume of available water, a network of ASR wells would be necessary. Only a small portion of the captured water would be needed to augment the reclaimed water supply and manage the lake level during periodic droughts. The stored water potentially could provide a future potable water supply with the appropriate treatment.
Based on current needs and demands, Scenario C appears to be the most flexible and cost-effective ASR operational scenario. This option would support the augmentation of both the reclaimed water supply and lake-level management with a minimal investment in infrastructure. Scenario D represents a more aggressive approach to the same multiuse concept. It would, however, only be pursued after sufficient hydrogeologic and water-quality data have been collected from a single ASR system operating under Scenario C and if no adverse environmental effects are observed.
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Looking to the Future
Pinellas County is currently in the process of obtaining a state permit for the construction and testing of an ASR production well at the Chestnut Park site. The first ASR production well is expected to be completed by 2005.
All surface waters harvested from Lake Tarpon and stored in the ASR well will be recovered and initially used to augment the Pinellas County reclaimed water supply. The use of recovered water for lake management purposes is likely to be delayed for several years until Pinellas County completes several stormwater treatment retrofit projects for priority sub-basins contributing excessive pollutant loads to the lake. Once nonpoint-source pollutant loads from surface-water inflows are reduced, the county again will evaluate the potential for using stored lake water to improve lake water quality through dilution and flushing.
Author's Bio: Ecological Sciences Program Manager Douglas E. Robison, P.W.S, is with PBS&J in Tampa, FL.
Author's Bio: Senior Hydrogeologist Thomas Farkas, P.G., is with PBS&J in Tampa, FL.
Author's Bio: Andrew P. Squires is division administrator of environmental resources management for the Pinellas County (FL) Department of Environmental Management.
Author's Bio: Senior Hydrogeologist Thomas Farkas, P.G., is with PBS&J in Tampa, FL.
Author's Bio: David Slonena, P.G., is hydrogeology manager for the Pinellas County Department of Utilities.
March-April 2004
Balancing Water Needs and Sources in Lake Tarpon, Florida
Fluctuating lake levels create opportunities for storing and reusing fresh water.
By
Douglas E Robison,
Thomas Farkas,
Andrew P Squires,
Thomas Farkas,
David Slonena
Lake Tarpon is located in Pinellas County, FL, immediately northwest of Tampa Bay (Figure 1). With a surface area of 4 mi.2, Lake Tarpon is one of the largest natural lakes in west-central Florida.
To relieve periodic flooding in this historically closed basin, the United States Army Corps of Engineers completed construction of the Lake Tarpon Outfall Canal control structure in 1972. Subsequent reduction in lake fluctuations and watershed urbanization resulted in the overgrowth of shoreline vegetation, a decline in sport fisheries, and the degradation of water quality.
Managing Lake Levels
To restore degraded aquatic habitats and improve water quality, Pinellas County has been evaluating the adoption of an enhanced lake-level fluctuation schedule (Figure 2) to modify the current operations of the Lake Tarpon Outfall Canal control structure. This would reestablish a more natural pattern of seasonal and interannual variations to be repeated every four years.
High water elevations are proposed for the winter and summer months to flood shoreline vegetation and control the expansion and proliferation of superabundant species, predominantly cattails, willows, and hydrilla. Low water elevations are proposed for the spring and fall months to flush and dilute in-lake nutrient concentrations before and after the summer growing season, when algae growth is greatest.
Balancing Consequences
Although the Lake Tarpon Outfall Canal control structure provides a flexible and cost-effective means of fluctuating lake levels, the discharge of lake water into the saline waters of Old Tampa Bay results in an irrevocable loss of a substantial volume of potentially reusable fresh water. If, however, this water could be harvested, stored, and later recovered for various beneficial reuses, it would constitute a significant, new source of water for Pinellas County and the entire Tampa Bay region, an area that has experienced significant water-supply shortages during the past decade.
An estimated 1.1 to 1.8 billion gallons of fresh water would be discharged annually over the structure through the operation of the enhanced lake-level fluctuation schedule. This conservative estimate does not take into account the additional surface water that would flow over the Lake Tarpon Outfall Canal control structure after target lake levels are attained. Based on historical gauge data, the estimated annual average volume of water discharged through the Lake Tarpon Outfall Canal will almost always exceed 3.4 billion gallons, even during the driest years. Harvesting, storing, and later recovering this volume of water would yield a daily freshwater flow of approximately 9.3 million gallons per day available for beneficial reuse. An effective means of storing, recovering, and distributing the water, however, will be needed.
Storage Options Studied
Two alternatives were considered: surface-water storage in one or more reservoirs and aquifer storage and recovery (ASR). Surface-water storage was determined to be unfeasible in the Lake Tarpon watershed because of the lack of adequate vacant uplands. Pinellas County therefore looked to ASR.
ASR is the storage of available excess water in suitable subsurface storage zones and the recovery of stored water when needed. Typically storage is accomplished by injecting water into an appropriate geologic storage zone through a well and later by using the same well for recovery of the stored water. If properly implemented, ASR is a viable, cost-effective water-resource management tool that can provide more significant advantages than conventional surface-water storage facilities. The use of ASR also protects the stored waters from surface contamination and maximizes water conservation by eliminating evaporative loss.
Two ASR test-well sites were identified during a feasibility study conducted for Pinellas County: Chestnut Park on the southeastern shore of Lake Tarpon and the land near the Lake Tarpon Outfall Canal control structure, approximately 9,000 ft. south of Lake Tarpon. Based on an analysis of existing data, development of ASR wells at both test sites was considered feasible. Subsequent exploratory testing at the outfall canal site revealed very poor ambient groundwater quality and marginal confinement of the storage zone. Exploratory testing at the Chestnut Park site, however, indicated favorable conditions for future development of ASR wells.
Beneficial Reuse Alternatives
After ASR was determined to be a potentially viable storage alternative, the following beneficial reuse alternatives for surface waters discharged from Lake Tarpon were considered:
- Augmentation of the Pinellas County reclaimed water supply
- Augmentation of the regional potable water supply
- Rehydration of adversely impacted wetlands
- Restoration of tributary baseflows
To evaluate the relative merits of these reuse alternatives, a weighted decision matrix was developed and applied and the following criteria were used:
- Demonstrated need or demand
- Public acceptance
- Environmental or public health impacts
- Regulatory permittability
- Construction cost
- Operation and maintenance cost
Based on this analysis, only lake management and augmentation of the Pinellas County reclaimed water supply were considered viable alternatives.
Lake Management
Under the more or less static lake levels that have existed since the construction of the Lake Tarpon Outfall Canal control structure, weeds and exotic aquatic plant species have proliferated and overrun desirable native species. A potential benefit of the enhanced lake-level fluctuation schedule is the reduction of these vegetational nuisances with the proliferation of native aquatics, such as eelgrass and bulrush.
Another anticipated benefit of the enhanced lake-level fluctuation schedule is in-lake water-quality improvement through increased dilution and flushing. Using water recovered from an ASR well increases lake flushing by adding a source of inflow water that does not currently exist, and using recovered water to refill the lake following lake-level drawdowns dilutes in-lake concentrations of nitrogen and phosphorus.
Dilution of nutrient concentrations will occur if, prior to recovery, the stored discharge water is exposed to pretreatment before injection. Natural treatment processes in the aquifer during the storage period might further reduce nutrient concentrations. Based on model analyses, lake flushing and dilution could result in a 3.5% reduction in the annual nitrogen load to the lake, which would be sufficient to reduce periodic algae blooms.
Augmenting Reclaimed Water Supply
Pinellas County provides reclaimed water (enhanced, secondary, treated wastewater) to users in the northern portion of the county to offset partially the use of potable water supplies for residential irrigation. In 1999, the annual deficit in reclaimed water was approximately 580 million gallons, which—on an average daily basis—equated to approximately 1.6 million gallons per day. Deficits are greatest during the dry season, March through May, when regional rainfall is sparse. Demand for reclaimed water in the county's northern area has continued to increase; therefore, deficits are expected to increase without a reliable augmentation source.
The use of Lake Tarpon surface water in conjunction with an ASR system was determined to be a viable option for supplementing the county's reclaimed water supply. Lake surface waters could be captured and stored in ASR wells during implementation of the enhanced lake-level fluctuation schedule and possibly also during wet-weather discharges. During the dry season when the demand for reclaimed water is greatest, stored water recovered from the ASR wells could be used to augment the reclaimed water supply and lake levels. Approximately five ASR wells, each sized at 1 million gallons per day, would be needed at build-out to reliably supplement the reclaimed water system to meet both current and future reclaimed water demands.
Operational Scenarios
Four ASR operational scenarios were developed to illustrate the potential uses and benefits of stored/recovered surface water (Figures 3A , 3B, 3C, and 3D).
- Scenario A: The captured water is stored within an ASR well system and later recovered from the well(s) for Lake Tarpon water-level augmentation during drought periods and for increased dilution and flushing.
- Scenario B: The captured water is stored within an ASR well system and later recovered from the well(s) to supplement the Pinellas County reclaimed water system. In addition, during periods when lake water is pumped to the ASR wells—which results in the wells not being available to supplement the reclaimed supply—some of the lake water can be diverted directly from Lake Tarpon to the reclaimed water system.
- Scenario C: The operational components of Scenarios A and B are combined to maximize the single ASR well concept with storage and recovery of lake water to augment lake water levels and supplement the reclaimed water system. Seasonal and yearly variations in the supply and demand of the reclaimed water system combined with the naturally occurring lake-level recovery will determine largely how the ASR well system is managed.
- Scenario D: All—or at least a significant portion—of the water discharged annually through the Lake Tarpon Outfall Canal, including all lake discharge water and a significant portion of the wet-weather overflows, is captured. Because of the potentially large volume of available water, a network of ASR wells would be necessary. Only a small portion of the captured water would be needed to augment the reclaimed water supply and manage the lake level during periodic droughts. The stored water potentially could provide a future potable water supply with the appropriate treatment.
Based on current needs and demands, Scenario C appears to be the most flexible and cost-effective ASR operational scenario. This option would support the augmentation of both the reclaimed water supply and lake-level management with a minimal investment in infrastructure. Scenario D represents a more aggressive approach to the same multiuse concept. It would, however, only be pursued after sufficient hydrogeologic and water-quality data have been collected from a single ASR system operating under Scenario C and if no adverse environmental effects are observed.
Looking to the Future
Pinellas County is currently in the process of obtaining a state permit for the construction and testing of an ASR production well at the Chestnut Park site. The first ASR production well is expected to be completed by 2005.
All surface waters harvested from Lake Tarpon and stored in the ASR well will be recovered and initially used to augment the Pinellas County reclaimed water supply. The use of recovered water for lake management purposes is likely to be delayed for several years until Pinellas County completes several stormwater treatment retrofit projects for priority sub-basins contributing excessive pollutant loads to the lake. Once nonpoint-source pollutant loads from surface-water inflows are reduced, the county again will evaluate the potential for using stored lake water to improve lake water quality through dilution and flushing.