fwrj rising tides and sinking brines: managing the threat ... the threat of s… · rising tides...

32 August 2017 • Florida Water Resources Journal

The Florida Keys Aqueduct Authority (au-thority) has adapted its Biscayne Aquifermonitoring plan to track changes to the

saline water interface and to address a growingthreat from a localized hypersaline plume insoutheast Miami-Dade County. The authority’sBiscayne Aquifer wellfield in Florida City is lo-cated about 10 mi west of Biscayne Bay andabout 5 mi east of Everglades National Park. Theauthority currently monitors a network of 16Biscayne Aquifer wells within an area of approx-imately 30 sq mi south and east of its FloridaCity wellfield as part of a saline water intrusionmonitoring (SWIM) program. Continued in-land movement of the saline water interface to-wards the authority’s wellfield has necessitatedchanges in its monitoring program to include sixnew wells inland of the saline water interface andto cease monitoring at eight existing wells.

At least two distinct sources of saline waterhave been identified as impacting the authority’sSWIM network: 1) hypersaline density drivenseepage from the Florida Power and Light (FPL)cooling canal system at Turkey Point east of itswellfield, and 2) seawater encroachment via seep-age from the C-111 and other nearby canals southof the wellfield. The FPL salinity source has beenconfirmed by its tritium signature within the sur-rounding groundwater monitoring network.

The leakage of seawater from tidally influ-enced canals has been indicated by correlationof salinity variation with canal stage data. RecentComprehensive Everglades Recovery Plan(CERP) projects involving improvements tocanal control structures and the plugging ofcanals have begun to have a measurable impactin the form of decreasing chlorides at some wellsadjacent to canals in the western half of theSWIM network. All of the monitor wells locatedin the eastern half of the network exhibit in-creasing chloride level trends and also have thehighest levels measured in the network.

In order to facilitate optimization of theSWIM network, the authority’s historical recordsof chloride concentrations at multiple well sitesand multiple depths within the Biscayne Aquiferwere graphed and mapped using a geographicinformation system (GIS). The data were evalu-ated to determine trends in existing wells thathad previously experienced saline water invasionand to identify target areas where monitoring

coverage was lacking. New well sites were cho-sen within the target areas based on estimates ofthe rate of intrusion and logistical factors, suchas accessibility, land ownership, utilities, andright of way concerns. The depths for samplingpoints in each new well were chosen to fill gapsin data from neighboring wells. The new wellconstruction design generally matches that ofexisting SWIM wells and includes provisions forcontinuous salinity profiling across the verticalextent of the Biscayne Aquifer, as well as discreetsampling at separate and distinct intervals withinthe well. The design facilitates continuity of sam-pling and monitoring procedures within the net-work and provides for a variety of approaches tocharacterize the movement of saline waterwithin the aquifer.

Time series plots of chloride levels at anumber of the SWIM wells have exhibited vari-able degrees of saline water influence, rangingfrom highly fresh and stable to exponentially in-creasing salinity. Several wells with multiple dis-creet sample depths have demonstratedpronounced density-controlled stratification,which indicates that the saline water interfacehas moved preferentially along the bottom of theaquifer, while water in the upper portion of theaquifer has remained relatively fresh.

This stratification has been occasionally re-versed with wells showing episodic increases insalinity within the upper sampling interval, whiledeeper samples remained fresh. This inversion ofthe normal salinity distribution has been transientand indicates transient encroachment of salinesurface water. The accuracy and integrity of theSWIM program is critical to the management andsustainability of the authority’s Biscayne Aquiferwellfield. Continued improvements and opti-mization of the monitoring program ensureslong-term viability of the critical Biscayne Aquifergroundwater resource.

Historical Overview

The 1935 Labor Day Hurricane destroyedthe Florida Overseas Railroad and halted thetransport of large quantities of fresh water to theFlorida Keys via railroad tank cars. The lack of adependable water supply prompted the FloridaLegislature to create the Florida Keys AqueductCommission (FKAC) in 1937, which was the

predecessor of the authority. The U.S. Navy re-opened its naval base in Key West in 1939 andentered into an agreement with FKAC in 1941 tobuild a water supply pipeline from the mainlandto Key West, with the federal government payingtwo-thirds of the cost.

The Navy acquired 353 acres of land inFlorida City in 1941 and contractors begandrilling wells, building pump stations, and lay-ing approximately 128 mi of 18-in. ductile steelpipe to bring water to Key West. Water pumpedfrom three 10-in. Biscayne Aquifer wells at theFlorida City Navy wellfield first reached KeyWest on Sept. 22, 1942. Due to the high level ofcalcium hardness in the water, which caused ex-cessive buildup of lime scale in pipes, a filtrationand lime softening water treatment plant wasadded to the Florida City facility in 1944.

In the late 1980s, a new lime softening/fil-tration water treatment plant was built and thenexpanded to meet the demands of growth andtourism in the Florida Keys and Monroe County.Although alternative and emergency water sup-ply systems have been developed, including re-verse osmosis treatment plants at Florida City,Stock Island, and Marathon, the primary sourceof fresh water for the Florida Keys remainsgroundwater from the Biscayne Aquifer pumpedfrom the Navy wellfield in Florida City.

Saline Water Intrusion Monitoring Program Background

The Central and Southern Flood ControlDistrict, later to become the South Florida WaterManagement District (SFWMD), originally issuedthe authority’s Water Use Permit Number 13-00005-W in 1974 for 13.5 mil gal per day (mgd)of Biscayne Aquifer water. Under two SFMWDprograms, entitled Saline Water Intrusion Moni-toring and Management (SWIMM) and Multi-

Rising Tides and Sinking Brines:Managing the Threat of Salt Water Intrusion

Andrew W. McThenia, W. Kirk Martin, and Jolynn Reynolds

Andrew W. McThenia is seniorhydrogeologist and W. Kirk Martin ispresident and principal scientist with WaterScience Associates in Fort Myers. JolynnReynolds is compliance and planningmanager with Florida Keys AqueductAuthority in Key West.

F W R J

Florida Water Resources Journal • August 2017 33

depth Potentiometric Head Monitoring Program(MUD-POHMP), the original permit requiredthat the permittee collect water samples fromcanals and wells for analyses of dissolved chlorideconcentrations and other parameters.

The permit required the construction of oneonsite and two offsite monitor wells to beequipped with monitoring devices, and recordsto be submitted monthly. In 1974 the U.S. Geo-logical Survey (USGS) entered into a cooperativeagreement with the U.S. Navy to assess the ade-quacy of the water supply and to evaluate poten-tial impacts of seawater intrusion in response toU.S. Navy and authority concerns that increasedwithdrawals from the wells could cause seawaterto move inland and contaminate the wellfield. Anearly USGS saline water intrusion map as it re-lates to the Navy wellfield is excerpted as Figure 1.

In 1980, the authority entered into a jointfunding agreement with USGS to perform waterlevel monitoring and dissolved chloride analysis ofwells owned by the authority, as well as other USGSwells in established monitor well networks. Severalof these wells remain in the SWIM network andcontinue to be maintained by USGS. Under coop-erative funding between the authority and USGS,two of these wells provide real-time water level datathat are available on the USGS website.

During the water use permit renewalprocess in 1990, SFMWD required an update ofthe SWIM monitoring program due to increasesin salinity observed in several wells. The author-ity implemented changes to the monitoring pro-gram, namely by addition of six new monitorwells, designated as FKS-1 through FKS-6, andby conductivity profile monitoring in additionto the previously required chloride and waterlevel data. In 2001, SFWMD again required up-dates to the monitoring plan and the authorityresponded by the addition of three more moni-tor wells, FKS-7 through FKS-9, by 2004.

As part of a wellfield protection study con-ducted in 2010, the authority’s consulting engi-neer recommended a number of changes to theprogram, including the addition of seven newmonitor wells and ceasing monitoring at fourexisting wells. In 2015, the authority requestedthat Water Science Associates (WSA) reevaluatethe previous consultant’s recommendations,provide an updated analysis, and design and im-plement modifications to the SWIM program.

Geographic Information Systemsand Data Evaluation

As part of the initial evaluation of the SWIMprogram, WSA assembled the files of monitoreddata from the authority for the 16 monitor wellsin the network. These data encompassed monthly

Figure 1. Inland Extent of Seawater Intrusion for 1946, 1970, and 1971 (Meyer, F.W., 1974)

Figure 2. Map Showing Saline Water Intrusion Monitoring Wells and Chloride Levels Over Time

Figure 3. Chloride Levels in Upper and Lower Zones of Monitor Wells FKS-2 and FKS-5Continued on page 34


sampling events from 1991 through 2016. Thechloride levels in all of the wells were plotted astime series and positioned on a GIS map of thenetwork, as shown in Figure 2. Additionalmapped information included a 2011 representa-tion of the approximate extent of the 1000 mg/lisochlor based on work conducted by USGS (Pri-nos 2011) and a GIS contour map showing thebase of the Biscayne Aquifer (Ramirez, J., 1990).

A review was conducted by WSA of CERPwork that included SFWMD and U.S. ArmyCorps of Engineers (USACE) projects as theypertain to the effectiveness and spatial distribu-

tion of the SWIM network. Projects that wereevaluated included the SFWMD’s C-111Spreader Canal Western Project, the USACE’s C-111 South Dade Project, and the USACE’s Bis-cayne Bay Coastal Wetlands Project.

Perhaps the most significant CERP projectimpacting the SWIM program is the plugging ofseveral canals that previously have served as con-duits for direct inland movement of seawater dur-ing dry periods and have allowed rapid drainageand reduced groundwater recharge during wet pe-riods; specifically, wells FKS-2 and FKS-5 are lo-cated adjacent to a recently plugged section of theC-110 canal. As shown in Figure 3, over a period of

several months in 2014, FKS-2 and FKS-5 experi-enced elevated chlorides in their upper zones,while the lower zones did not show correspondingincreases. This pattern of salinity indicates that thecanal and upper zones of the wells were intrudedby surface water rather than groundwater.

The lowering of the water table elevation inthe immediate vicinity of canals also producedlocalized areas of reduced hydraulic head withinthe aquifer that facilitate saline water intrusionin the groundwater system. This effect is believedto have caused transient intrusion of saline waterinto FKS-1 and adjacent G-1603 during two low-stage episodes that occurred in April 2009 andMay 2011. During drought conditions, the canalstage on the C-111 adjacent to the two wells waslowered to about 1-ft National Geodetic VerticalDatum (NGVD), and chloride levels in FKS-1and G-1603 experienced corresponding dra-matic increases on the order of 3000 to 4000mg/l, as shown in Figure 4.

This correlation of wellfield water levels andchlorides with the canal stage had previouslybeen established by a detailed statistical analysisin 2013 by one of the authors (Martin, 2013).The study concluded that canal-stage levels hada much more significant impact on observedgroundwater levels and chlorides than with-drawals from the wellfield by the authority orother nearby users. This conclusion was furtherbolstered by the current evaluation and was sub-sequently used as a basis for a permit modifica-tion linking dry-season Biscayne Aquifer wateruse restrictions to actual groundwater levels inUSGS monitor well G-613, rather than the cal-endar-based criteria previously imposed. The G-613 is located about 2 mi southeast of theauthority’s wellfield.

Florida Power and Light Hypersaline Plume

The FPL maintains a cooling canal system(CCS) for operation of power generation units atits Turkey Point power generation facility insoutheast Miami-Dade County. The westernboundary of the CCS is located about 9.5 mi eastof the authority’s wellfield and about 5.5 mi eastof FKS-9. The CCS consists of some 6,000 acresof canals through which water is circulated fordissipation of heat created by the power genera-tion units. The CCS is characterized as a “closed-loop” cooling system, in that the same water iscirculated through the extensive canal networkwithout direct input of new water to the system;however, the CCS does not function as a closed-loop system hydrologically in that, as the warmedwater is circulated, evaporation losses to the at-mosphere remove freshwater from the canal sys-

Figure 4. Canal Stage Correlation With Salinity Increases at FKS-1 and G-1603

Figure 5. Contour Map of Salinity in Deep Bicayne Aquifer Groundwater:Average December 2014 Practical Salinity Units (PSU) Values Continued on page 36

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tem, causing a concentration of salinity that ex-ceeds typical ocean salinities by a factor of two ormore. This increased salinity is accompanied by acorresponding increase in water density thatcauses hypersaline water to migrate downwardinto the underlying groundwater system and ra-dially outward beneath the CCS (Figure 5).

Tritium and total dissolved solids concen-trations in groundwater samples show that hy-persaline water emanating from the CCS hasmoved westward of the L-31E Canal more than2 mi and is influencing movement of the salinewater interface within the Biscayne Aquifer morethan 4 mi inland. Currently, FPL is under a con-sent order to remove the hypersaline plume andhas begun operation of a recovery and injection

system designed to remove hypersaline waterfrom the Biscayne Aquifer and inject it into theBoulder Zone via a deep injection well. TheSFWMD has permitted FPL to capture 5.475 bilgal per year (15 mgd) of hypersaline water fromthe Biscayne Aquifer and the Florida Depart-ment of Environmental Protection (FDEP) hasgranted FPL an underground injection control(UIC) permit to inject up to 15.59 mgd of waterinto a deep injection well.

Monitoring Network Upgrades

Discontinuation of monitoring at eight ex-isting wells was recommended by WSA, based onseveral factors, including proximity of the saline

front, existing monitoring by others, and previ-ous chloride trends. The locations of existingSWIM wells, including those where monitoringis proposed to cease and the proposed new wells,are shown in Figure 6.

Construction of six new SWIM wells, des-ignated as FKS-10 through FKS-15, was recom-mended based on perceived gaps in the spatialdistribution of the network, particularly in areaswhere the saline water had advanced inland be-yond an existing site. Because the saline water in-terface has moved northward of FKS-2 at depth,but has not yet reached FKS-5, an intermediatewell FKS-13 was proposed to monitor the move-ment of the salinity front between those twowells. The FKS-10, 11, and 12 are positionedalong US Highway 1 to intercept the front thathas previously moved inland of FKS-4, 7, and 8.Because FKS-7 and 8 have upper monitor inter-vals that are fresh, these wells remain useful fordetermining the vertical extent and inferred mo-tion of the front, as is evidenced by chlorides atdifferent depths and conductivity profiles.

The area to the east of Card Sound Roadand south of the Florida City Canal, historicallyreferred to as Model Land, required additionalwells due to the rapid encroachment of the hy-persaline plume in this area. Prolonged elevatedsalinity has rendered G-1264 not useful and wellG-3164 was lost in 2009 (Figures 7a and 7b). Be-cause G-3164 was relatively fresh when it was lastmonitored in 2009, a well situated between G-3164 and G-1264 was initially proposed; how-ever, a lack of road access and pre-existinggroundwater monitoring by the operator of anearby mine dictated that the proposed locationfor FKS-14 be shifted southwest of the originallocation.

Salinity monitoring data in the vicinity ofFKS-9 indicates that the salinity front is ap-proaching from the east in this area and poten-tially intruding the upper portion of theBiscayne Aquifer via surface flow in the FloridaCity Canal or from an abandoned rock pit adja-cent to the well. Data from G-1264 indicate thatchlorides began to rapidly increase at this site be-ginning in June 2001. Chloride levels at an FPLmonitor well TPGW-7 (Figure 8) located about1.3 mi west of G-1264, started to increase in De-cember 2013. Based on the time span of about12.5 years between salinity level surges at G-1264and TPGW-7, the saline front is estimated tohave moved at a rate of approximately 475 ft peryear in this particular area.

Well Design

The basis of design for the new monitorwells included consideration of many factors,including cost, effective life span, functionality,

Figure 6. Map of Existing and Proposed Authority Saline Water Intrusion Monitoring Wells and Florida Power and Light Monitor Well TPGW-7

Figure 7a. Chloride Data from G-1264 Figure 7b. Chloride Data from G-3166



nearby well coverage (both vertical and hori-zontal), accessibility, security, property owner-ship, easement availability, and consistency withhistoric data collection. Based on the authority’slong-term success with sampling and recordkeeping of its existing multizone FKS wells, thechoice was simple: keep the well design consis-tent with that of the pre-existing wells. Althoughmore complicated alternative well designs in-volving multiple discrete interval wells at eachlocation were considered, the theoretical bene-fits of discrete sampling from isolated wells donot outweigh the additional costs that would beincurred by the authority.

Consideration was also given to a well de-sign using individual screened sampling tubesgravel-packed and grouted over discreet verti-cal intervals to be identified by geophysical log-ging; however, the risks of sand and grout loss tocreate such “nested” wells in a single boreholeand time and costs for logs and tailoring eachwell were deemed excessive for the intendedpurpose of detecting the saline front. The in-ferred risk of upward interborehole flow is in-significant at most of the SWIM wells given thedensity stratification inherent to the saline waterinterface, especially in a highly transmissiveaquifer setting.

Currently there are no significant with-drawals from the Biscayne Aquifer in the im-mediate vicinity of any of the wells that mightdisrupt the static density equilibrium betweenseparate and highly permeable zones within theaquifer; however, a salinity barrier system pro-posed by the nearby mine operator that involvespumping up to 7 mil gal per day of water froma single large shallow well into a north-southlinear array of deeper injection wells has the po-tential to impact FKS-14, which is within 0.5 miof the barrier wells. Increased hydraulic head ina lower flow zone and simultaneous reductionof head in the upper zone could potentially in-duce upward flow through a connected pene-tration, such as a borehole or mine pitexcavation.

The chosen monitor well design includesmultiple small diameter (1- and 2-in.) polyvinylchloride (PVC) tubes secured from the top bymeans of a compressive well seal inside of alarger diameter (6-in.) PVC surface casing withan open borehole beneath the surface casing tototal depth (Figure 9).

This design allows discrete sample collec-tion from upper and lower depths via the 1-in.open-ended pipes and for continuous verticalconductivity profiling in the 2-in. screen over theentire open-hole portion of the well. A 1-ft-longscreened interval at the base of each sample tubewas included in case plugging of the bottom ofthe tube was to occur. Protective measures at the

wellheads include 12-in. diameter-schedule 40steel casings, locking well caps, steel reinforcedconcrete well pads, and cement-filled 4-in. steelpipe corner bollards at each pad. There is a highrisk of vandalism in the Model Lands area andthe protective features are likely to be challenged.

Permitting

The various permits and agreements ob-tained for modification of the SWIM program

and construction of the new wells include thefollowing: S SFWMD Water Use Permit No. 13-00005-W,

Application No. 160317S Florida Dept. of Health Well Construction

Permits 13-59-13036 to 13-59-13040S Miami Dade Department of Transportation

(DOT) and Public Works Permits2016005628, 2016005629, 2016005630

S SFWMD Right of Way Occupancy

Figure 8. Chloride Level in Florida Power and Light TPGW-7 Well Nest (D) = Deep = Screened from -77 to -81 ft (NGVD 1929)

Figure 9. Schematic of Typical Saline Water Intrusion Monitoring Well Profile

Continued on page 38


Permit/General Permit No. 14687S SFWMD Revocable Right of Entry/License

AgreementS Memorandum of Agreement between Florida

DOT and FKAA finalized in April 2017

Conclusions

Salinity data reviewed in this study suggestthat the authority’s Biscayne Aquifer wellfield inFlorida City has not been currently impacted bysaline water intrusion, nor have wellfield opera-tions contributed to regional saltwater en-croachment. The recent modifications to theSWIM program are a proactive response toman-made groundwater problems, namelydrainage canals lowering groundwater heads andproviding direct conduits and the hypersalineplume originating at Turkey Point.

Although control structures and plugs nowexist on most of the canals that have acted in thepast as conduits for salt water intrusion, canalscontinue to pose risk for saline intrusion in theregion due to their high potential to conveysaline water rapidly inland toward the wellfieldduring storm surges, king high-tide events, and

extreme drought and low water conditions.Eleven of the 16 chloride monitoring wells in thecurrent SWIM network are positioned within100 ft of a major canal and are the most likely toexhibit salinity impacts. These canals include theC-111, C-110, Card Sound Canal, and theFlorida City Canal. The SWIM wells FKS-7 andG-3342 are also located on a likely flow path forsaline water, which is the abandoned Florida EastCoast Railway borrow ditch extending southfrom the wells to the C-111 canal. The closeproximity of these wells to the main pathways ofintrusion ensures early detection and allowsrapid response if any water management actionscan be taken to arrest the problem.

Apart from naturally occurring seawater,the single most damaging source of groundwa-ter pollution threatening the authority’s wellfieldis the FPL cooling canal system at Turkey Point.Because of the intense public and regulatoryscrutiny focused on the hypersaline plume andremediation efforts being undertaken by FPLand others to halt and reverse the intrusion, fu-ture data from FKS-9, FKS-14, and FKS-15 willprovide ever more valuable intelligence for thecommunity of professionals involved in thisissue.

References

• Martin, W.M. “Florida Keys Aqueduct Author-ity Water Supply Protection Program: Phase IIDetailed Groundwater Data Analysis.”

• Meyer, F.W., 1974. USGS Open File Report FL74014. “Availability of Groundwater for theU.S. Navy Well Field Near Florida City, DadeCounty, Florida.”

• Prinos, S., 2011. “Approximate Inland Extent ofthe Saltwater Interface in the Biscayne Aquiferin 2011, Miami-Dade County, Florida.”https://www.envirobase.usgs.gov/FlGisData/FLWSC/metadata/SaltlineBiscayne2011_arc.htm.

• Prinos, S.; Wacker, M.A.; Cunningham, K.J.;and Fitterman, D.V. USGS Scientific Investiga-tions Report 2014-5025. “Origins and Delin-eation of Saltwater Intrusion in the BiscayneAquifer and Changes in the Distribution ofSaltwater in Miami-Dade County, Florida.”101 pp.

• Ramirez, Jorge, unpublished material. “Con-figuration of the Base of the Biscayne Aquiferin Dade County.” WRIR 90-4108, figure 16.https://www.envirobase.usgs.gov/FlGisData/FLWSC/descriptions/dade_config_base_bis-cayne_arc.htm. SS


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