2010 E3 Environmental Sustainability Grand Prize

Marco Island Aquifer Storage and Recovery System

East Naples, Florida
ENTRANT: ENTRIX, Inc.
ENGINEER IN CHARGE: Lloyd E. Horvath, P.E.

Map of General Location of Marco Island and the Project	Marco Island and the Freshwater Source Area on the Mainland	Area and features of the Project Site - The figure shows important freshwater related features of the project. The lakes shown are abandoned rock quarries beside a major regional drainage canal. There is a salinity control/discharge structure where shown, that prevents tidal water from moving up the canal during periods of low flow. The lakes serve as the point of intake for fresh water and there is an interconnect between the canal and the lakes where shown on the photo. Fresh water is pumped from the lakes and sent to the island for treatment by lime softening. During the dry season there is very little or no water flowing in the canal and, without ASR, fresh water availability from the lakes is limited. During the wet season, water enters the lakes from the canal interconnect at the north and it flows southward to the intake.
Marco ASR Facilities Aerial - The photo shows the location of the ASR wells and the other facilities at the site. The ASR wells can recharge/inject and recover water at a rate of about 10 million gallons per day. When water is recovered during the dry season it is returned to a holding tank at the site of the pretreatment facilities, where it is pumped south to the island for treatment.	The wet season flow conditions of the drainage canal at the salinity control structure. All the freshwater discharge is lost to tide. The high discharges are also not healthy for the estuary located just downstream from this location.	Dry season condition of the canal on the upstream side of the salinity control/discharge structure approaching the canal/lake interconnect (no water flow).

Entrant Profile

ENTRIX provides multidisciplinary environmental and water resource planning, engineering, and economic consulting services to public and private entities engaged in the protection, development, treatment, delivery, use, and reuse of water resources. ENTRIX also provides services for watershed and stormwater management; watershed, wetland, upland/riparian, and stream restoration; wetland treatment systems; hydroelectric project licensing and reservoir management; environmental permitting and compliance; and monitoring services.

Working as an interdisciplinary team, ENTRIX professionals apply an integrated approach to every project and situation. ENTRIX combines water resource engineering with a deep understanding of ecological and biological systems and natural systems engineering to develop sustainable and effective approaches to watershed management. ENTRIX's goal is to work with clients, regulators, resource agencies, and stakeholders to create successful solutions that balance environmental protection, societal needs, and productive use of water resources.

ENTRIX was the prime engineering firm responsible for the City of Marco Island's aquifer storage and recovery (ASR) project providing research, design, environmental permitting, and construction management services. ENTRIX also provided turn-key construction services for the pilot project and a portion of the expansion. ENTRIX was assisted in this project by AECOM, who provided engineering for mechanical and electrical components and portions of the pretreatment facilities.

Project Description

Introduction

In providing potable water to its customers, the City of Marco Island had for many years struggled with limitations on fresh water, and it was growing evermore reliant on desalination of a brackish groundwater source that was rapidly deteriorating in quality due to salt water intrusion. A limited freshwater supply was available on the mainland, but it was not reliable due to regular droughts occurring during the peak demand dry season. However, during a four month rainy season there is an abundance of fresh surface water that normally discharges to tide. ENTRIX worked with Marco Island to develop a unique aquifer storage and recovery (ASR) system that now provides effective storage of excess runoff captured during the rainy season. The water is stored in a deep aquifer that formerly contained highly brackish water. The ASR system can recover up to 1.5 billion gallons of fresh water annually during the dry season for potable use. It provides a sustainable freshwater supply that is the foundation of reliability for the City's potable water system. Desalination is no longer the basis of reliability for the system.

Judging Criteria

Integrated Approach

Developing a reliable water source involved evaluating all alternatives, which were:

Converting the brackish water plant to a seawater quality desalination plant
Developing an off stream reservoir on the mainland for storage of freshwater captured from stormwater in the wet season
Developing a freshwater wellfield on the mainland
Developing underground storage using ASR - This alternative was a conjunctive use of both surface and groundwater to optimize the seasonally variable supplies.

All alternatives were evaluated and compared on the basis of environmental, reliability, and economic factors. The evaluations are briefly summarized below:

Converting the brackish water RO plant to seawater salinity desalination meant significantly increasing energy costs because much higher pressures are required. The increasing rate of rejection of salts meant more pumpage from the wellfield, thus further accelerating the rate of deterioration of the water quality of the aquifer.
An off-stream reservoir of appropriate volume required land area greater than 600 acres. Land in the general vicinity of the freshwater drainage system was limited and extensive mitigation would be required for environmental impacts on neighboring wetlands. The reservoir would need to be lined to prevent seepage.
A new freshwater wellfield might be developed on the mainland but the fresh water aquifers are very shallow and considerable drawdown of the water table would occur during drier periods, necessitating extensive mitigation for wetland impacts.
An ASR system was an innovative approach but it was an unfamiliar process to the City since ASR had never been implemented in the region. However, ASR was the preferred option because it could be developed entirely on the City's property and, due to the small footprint required for wells, it could be implemented with very limited surficial impacts. The facilities had the lowest energy demand compared to the other alternatives. Water would be stored in deep formations that are confined from the surficial systems. ASR was the lowest cost option and caused the least environmental impacts.

Quality

The ASR system was conceived as the solution to an unreliable water supply without having to build a costly desalination system that relied on groundwater source with continually deteriorating quality. The investment in ASR was done in a conservative manner; starting first with a pilot project and followed by expansion after a few years of demonstrated success. The owner is highly satisfied with the results and has now committed to making the ASR system the fundamental water source for the municipality.

Originality and Innovation

Implementing ASR for this project meant overcoming technical obstacles for the first time. The only aquifers available in the project area contained highly brackish water. Nowhere in the world was there a record of successfully and economically implementing ASR for a potable supply system by utilizing an aquifer that contained highly brackish native water. The aquifer utilized at the project site contained native water having a TDS of approximately 6700 mg/l. The native water needed to be effectively displaced for storage and recovery of fresh water, and only about 4 percent mixing of native water could be tolerated in the recovered water.

Complexity

After extensive testing of several alternative deep storage intervals, a zone between the depths of about 750 and 800 feet was selected, based on hydrogeologic characteristics. Computer modeling evaluated the alternative zones to demonstrate that the storage interval could be flushed of native brackish water and efficient recovery could be accomplished with minimal mixing of recharge and native water. The system is now capable of recovering approximately 80 percent of the water recharged.

Success required evaluating and testing pretreatment processes to prevent plugging of the aquifer pores by particles and preventing precipitation of minerals. ASR involves injection wells and the storage aquifer is classified by the US EPA as an Underground Source of Drinking Water (USDW). Florida had never previously permitted injection of water into a USDW without full potable treatment. The utility couldn't use its potable treatment system for this site because the treatment plant is on Marco Island, 9 miles away.

Extensive permitting hurdles were overcome to build and operate the pilot ASR well. Comprehensive monitoring and reporting was required.

Due to the complexity of the regulations, designing and constructing the ASR system to the required capacity involved a testing and operation sequence that ultimately required 10 years to build a system approaching the size necessary to meet its objectives. Only in 2009 did the regulatory authorities agree to grant an operating permit to the system of 7 wells.

Contribution to Social and Economic Advancement

This project is the first in the world to efficiently store fresh water in a highly brackish aquifer and obtain high efficiency recovery. The project demonstrates how to build a low cost sustainable supply using a resource available only seasonally, and is a model that is applicable throughout the world to help solve water shortages.

The dry season conditions at the interconnect between the lakes and the canal.	Schematic of an ASR well showing the shallow freshwater lens in the area of the lakes and the brackish groundwater (shaded red). It also shows a number of alternative aquifer storage intervals and confining layers. All of the alternative aquifers were tested and evaluated during the investigation to select the subsurface storage zone between the depths of about 750 and 800 feet.	ASR well head and associated well facilities.
Pretreatment Facilities Pretreatment is needed to condition the water for injection. This system was designed to treat 10 million gallon per day (MGD). Filtration is needed to remove suspended solids to prevent plugging of the aquifer formation. Low cost pressure sand filters are used. Disinfection is required per EPA and FDEP rules for underground injection, which is accomplished by a simple chloramine system. We also found it necessary to adjust the pH of the surface water prior to injection to prevent precipitation of minerals. The pH is adjusted by adding CO2 to lower it by 0.5 to 1 pH unit.	Graph of performance of one of the ASR wells during recovery of fresh water after storage (quality vs. recovered volume). As the stored freshwater is extracted the quality becomes more brackish, but not near that of the native water. Repeated cycles of recharge, storage and recovery are needed to flush the aquifer of brackish water and build the buffer between the native and injected water. After repeated cycles the efficiency of recovery increases. This graph shows 5 percent mixing of injected water with native water at the end of recovery of Cycle 4 and 2 percent mixing at the end of recovery of for Cycle 6.	Graph of performance from the composite of ASR wells for several different cycles of injection and recovery showing increasing efficiency and increasing quality of recovered water as cycles are repeated. The percentage of recovered water is measured against the total volume injected in the previous wet season. Percentages exceeding 100 percent would indicate recovery of water injected but not recovered from earlier cycles. Water can be stored for extended periods of time. The quality of water injected obviously affects the performance of recovery and it can be seen that higher quality water (low salinity) was injected during cycle 3, as compared to most other cycles.
Graph showing comparison of water quality recovered during Cycle 4 compared to the native water having a dissolved chloride concentration of about 2800 mg/l and TDS of approximately 6800 mg/l.