Journal of Coastal Research 413-420 West Palm Beach, Florida Summer 2002

Aquifer Salinization from Storm Overwash

William P. Anderson, Jr.

Depar tm ent of GeologyRadford UniversityCampus Box 6939Radford VA 2414 1, U.S.A.wpanderso@ra dford.edu

ABSTRACT _

.tllllllll:.~euus~~-"+ S--

ANDERSON, W.P., JR. 2002 . Aquifer salini zat ion from storm overwash. Journal of Coastal Research, 18(3 ),413-420.West Palm Beach (Florida), ISSN 0749-0208.

Overwash processes are not only an agent of change to the morphology of barrier islands; they are also a source ofinstan taneous saltwater int rusion to coastal aquifers . Hatteras Island, North Carolina , USA is particularly susceptibleto overwash processes because of its geography and the frequency with which tropical and extra-tropical storms strikethe area. Hurricane Emily inundated the island in 1993 with saline water from Pamlico Sound. The floodwatersrecharged the Buxton Woods Aquifer , raising salinity levels from approximately 40 mg/L prior to flooding to near ly280 mg/L within severa l weeks of flooding. By 1997, chloride levels still had not returned to pre-storm levels.

One-dimensional ana lytical solutions of the advection-dispersion equation are used to simulate chloride transportwithin the aquifer utilizing a pulse SOUrce implemented with linear superposition. These simulations are matchedwith chloride breakt hrough curves. Initial simulations show that a pulse duration of five days provides the best fit tothe data. Simulation of chloride breakt hrough at two str eamline locat ions demonstrates that higher gradients advectchloride further into the aquifer, causing higher chloride concentrat ions and increasing the duration ofcontamination .The Cape Hatteras region historically is susceptible to several hurricanes in a single season. In order to analyze theeffect of multip le overwash events on water quality, predictive simulations show the effect of two overwash eventsseparated by severa l time lags between storms. Simulations indicate that higher gradients and short time lags be­tween overwash events result in chloride MCL violat ions that persist for more than four months.

ADDITIO NAL INDEX WORDS: Hurr icanes, Hurricane Em ily, North Carolina, Hatteras Island, saltwater intrusion,storm surge, ooeruntsh , barrier island, groundwater modeling, analyti cal solution. coastal aqu ifers.

INTRODUCTION

Water qu ality in coas tal aq ui fers can be threaten ed notonly by upconing and saltwate r int rusion , but a lso by ove r­wash and sto rm surge durin g se vere storms. Th ese processescaus e floodin g that recharges water-table aquifers with sa ­line wa ter. For exa mple , in August and September, 1993,sound-side flooding during Hurricane Emily in unda te d Hat ­teras Island , Nor th Ca rolina (USA) with sa lt water fromPamlico Sound, a la rge estuary on the nor th side of t he isla nd(Figu re L). Flooding was most se vere in the vicinity of theisla nd's well field that exploits a shallow aquifer for dr inkingwa ter. A large qu antity of the floodwa ter eve ntually re­charge d the shallow Buxton Woods Aquifer (BWA), rais ingsa linity and in creasi ng treatment costs. Chloride levels in theBWA inc reased from ap proximately 40 mglL prior to floodin gto nea rly 280 rng/L within sev eral week s of flooding. Chloridelevels did not fall below 100 mglL unt il March , 1994, and stillhad not reached pre-floodin g concentrations by J anuary,1997 .

The Cape Hatteras Region

Hatteras Island, North Ca rolina, is the largest island of theOuter Banks , a barrier-island chain th at se pa rates the At-

01062 received 24 October 2001; accepted in revision 2 January 2002 .

lanti c Ocean from Pamlico Sound, a 3100 krn" estuary (Figurel ). Th e Ca pe Hatteras portion of the is land lies where theisl and shift s from a southe rly trend to a west-southwesterlytrend. It is a lso at thi s point that the is land widens from lessthan 1 km in wid th to over 3 km in width . Th e "reversed-L"shape of the is land acts as a dam during winter ext ra t ropicalstorms C'Nor'easte rs" ) and hurr icanes . Northerly win ds pro­duced by t hese storms cross as much as 70 km of open waterin the sound, which, coup led wit h sto rm surge formed bythese intense low-pres sure systems, pr odu ces a rise in Pam­lico Sound of as much as three m (Figure 2). The sa linity ofPamlico Sound is a pproximate ly 60% that of seawate r.

The Buxton Woods Aquifer

The Buxton Woods Aqu ifer (BWA) has histori call y been thesole source of fresh water to residents of southe rn HatterasIsland. Although water pumped from the fresh wa ter aquiferis now suppleme nted by severa l moderately-sali ne deep wells(- 70 m in dep th ), which hav e chlori de levels of a pproximate ­ly 11,000 mglL and require reverse-osm osis treatmen t , theshallow gro undwa ter of the BWA continues to satis fy a sub­stan ti al por tio n of the is la nd's freshwater dem and. HEATH(1988, 1990 ), ANDERSON (1999), and ANDERSON et al. (2000)comb in e to provid e a detail ed hydrogeologic framework of theBWA. In general, the aquifer consi sts of tw o permeable unit s ,eac h a pproxi mately 10 m in thickness , that are se pa rate d by

414 Anderson

CBpePoin to c e a n

Study Area

Pam licoSound

!N

North Carolina

Figure 1. Location of study site on the Outer Banks barrier island chainin North Carolina, U.S.A.

Figure 3. Approximate exte nt of sound-side flooding in the Cap e Hat­ter as reg ion following Hurricane Emi ly.

contribute to this phenomenon. WILDER et al. (1978) con­ducted a study of th e water resources of northeastern NorthCarolina, including th e Ca pe Hatteras region . WINNER(1978)pr esents a study of the Ca pe Lookout, North Carolina regionwhi ch sugges ts that overwash processes are a likely occur­rence on th e low-lying barrier isla nds and th at weeks tomonths would be required to sufficiently flush the sa line wa ­ter from the freshwater lens. MEW (1999) demonstrated thatchloride levels at shallow depths within the BWA on thenorth side of th e isla nd approach the chloride maximum con­tam inan t level (MCL), whi ch is th e lega l limit for chloride indrinking water. All of these studies sugge st that overwashpro cesse s play a significant role in controlling th e ground­water quality within the surficia l aquifers of North Ca rolina 'sbarrier islands.

Coastal Aquife rs

Studies of coastal aquifers have mostly been confined to th edevelopment of the freshw ater-saltwater inte rface. Manystudies focus on methods of eva luating the freshwater- salt­wa te r interface in various unconfined settings (VACHER,1988; CHENG et al. , 1998 ; TAYLOR and PERSON, 1998; BAK­KER, 1999 ), whil e othe rs focus on specific coastal settingssuch as mainland aquifers, carbonate banks, volcanic islands,atolls, and isla nds of glac ia l origin (WALLIS et al., 1991; UN­DERWOOD et al., 1992 ; VACHER and WALLIS, 1992; GRIGGSand PETERSON, 1993 ; ROBINSON and GALLAGHER 1999 'PERSON et al., 2000 ). Barrier islands have received l e~s focu~th an th e oth er island types, but several reports present casestudies of specific barrier is lands (COLLINS and EASLEY1999; ANDERSON et al., 2000; CORBETT et al., 2000 ). Som~recent work has focused on the effect th at tid al processes

Tabl e 1. Aquifer parameters of the Buxton Woods Aquifer.

21.25.6

24.5

Bulk AquiferValue

m

mJdmJd

Units

Hori zonta l hydr aulic conductivity, K;Vertical hydraulic conductivity, K"Aquifer th ickness, b

ParameterStorm track adapted from NOAA data

,N

Figure 2. Stor m tra ck of Hurricane Emi ly off of the coast of Nor th Ca r­olin a in Augu st/September 1993.

North Carolina

a semi-confining layer of variable thickness. A silty to clayeysand underlying th e lower aquifer unit acts a s a semi-confin­ing layer. BURKETT (1996) performed a 72-hour pumping testat the center of the island (note d with a * in Figure 3) anddetermined the bulk aquifer properties shown in Tabl e 1.These parameters, plus average hydraulic gradients for twotransect s of th e island that are derived from ANDERSON(1999 ) and MORGAN et al. (1994), a re used in subse quent an­alytical simula tions .

Th e BWA and oth er coastal aquifers of North Ca rolinahave been th e focus of several papers. KIMREY (1960) pre­sents an early study of the BWA which shows elevated chlo­ride levels at shallow depths within the aquifer. HARRIS a ndWILDER (1964) and HARRIS (1967) obtained simi la r resultsfor the BWA and also suggest that aquifer heterogeneities

Jo urnal of Coastal Resear ch, Vol. 18, No. 3, 2002

Overw ash in Coasta l Aquifer s 415

Overwash Processes

HURRICANE EMILY

have on mixing zone development and nea r-shore water -tableelevations (NIELSEN, 1990; TURNER et al. , 1997 ; ATAIE-AsH­TIANI et al. , 1999; LI et al., 2000a ; LI et al. , 2000b ), bu t thesestudies do not address the effects of overwas h processes onwater quality.

Studies of overw ash processes in th e literature have fo­cused mostly on the effect of sali ne water s on back barriersa ltma rs h ecosyst ems and th e effect of overwash on barrierisland sedimenta tion. For example, SALINAS et al. (1986 ) de­scribes th e effect of submergence on the barrier islands of th eGulf Coast of Louisiana , including th e effect of overwash on

Pulse Source Analytical Solution

The analytical solution for a pulse source of duration to ascalculated by lin ear superposition is

C(x t ) = CO{erfc[X - UJ] _erfc [X - u)t - to)]} (1), 2 V4Dxt V4Dx (t - to)

ANALYfICAL MODELING

One-dimen sional an alytical solutions of th e ad vection-dis­persion equ ation are used to simulate chloride tra ns portwithin the BWA. The simulation s reflect tran sport alongstreamlines that originate as aquifer recharge , which in thiscase comprises the infi ltrating water s of Pamlico Sound. Thefollowing simulations use th e bulk chloride values measu redwith water pumped by th e Frisco Wellfield as calibrationdata for th e duration of th e chloride pul se. Although chloridelevels in Pamlico Sound in the vicinity of Cape Hatteras varyfrom 17,000 to 19,000 mgIL depending on locat ion within th eestuary and the time of yea r (WILDER et al., 1978; GIESE etal. , 1979), water sa mples collected by MEW et al. (1993) in­dicate chloride levels of approximately 12,000 mgIL in pondedsurface water near th e Frisco Wellfield . Thi s lower chloridevalue, which is used as th e source concentration in the fol­lowing simulations , may be a product of the diluting effectsof th e precipitation ( ~ 17 em) that fell during the storm (MOR­GAN et al., 1994) .

where Co is the concentration of the sound water [rng/L], x isth e distan ce along the streamline [m], Ux is the average linearvelocity [mid], and D x is th e hydrodynamic disper sion [mvd] .The pulse source duration is the length of time that the ov­erwas h waters recharge the aquifer ; once th e overwash wa­ters sub side, th e chloride source is re moved. This is simula t­ed in equation (1) through the use of linea r superposition,which consid ers the pul se source to be th e difference betweentwo continuous source simulations th at are separated in timeby th e length of the pul se, to- The complementary error fun c­tion , erfc, that is also used in equa t ion (1) is common in an­alytical solutions of solute tran sport becau se it reflects theGau ssian distribution that is typ ical of solute tran sport pro­cesses involving dispe rsion. Th is pul se-source solution ofequation (1) is used in th e following simula tions: (1) th e cal­ibration of chloride tran sport along Tran sect 1 using data de­rived from th e Frisco Wellfield ; and (2) th e simula tion of chlo-

saltmarsh communiti es. GUILLEN et al. (1994) present a casestudy of an over wash event in the Mediterranean coas t ofSpain. STONE et al. (1997) ana lyze hurrican e-induced over ­wash processes in coast al Louisian a , including predictivesimulations of wave height in response to barrier-island re­duct ions. COURTEMANCHE et al. (1999) study the colonizationof overwash sands by several sa ltma rs h plants, includingSpartina patens and Spartina alterniflora. MORGAN et al.(1994) and MORGAN (1996) present ana lyses of th e impact ofoverw ash water s on inte rduna l wetland water levels in th eCape Hatter as region , noting the rap id rise in water levelswith th e arrival of the storm. Th e studies of MORGAN et al.(1994) and MORGAN (1996), however , do not address aquifersa linization.

zooo

o

300<Do

CfMCL

::i' <DO 00

lzoo ~0r!c 0 0

<D 0 ~~ 0 0co ~O0 '0' o OIllloem

~ 0

'E 0~OO

B ~ 0o 0 0

8100 'r€ 0'0~oo 0

U 0 0

i3 0 0 8:,0

Hurrican e Emily formed in the Atlantic Ocean on Augu st22, 1993. By the time it reach ed th e Cape Hatter as, NorthCarolina region on Augu st 31, it was a Category 3 hurrican ewith winds between 178 and 209 km/h and storm su rge po­tential of 2.75 to 3.65 m. The storm tracked to th e east ofCape Hatteras , which produced northerly winds that pushedwater from Pam lico Sound onto th e north side of th e island,adding to th e storm surge at Cape Hatteras (NATIONAL OCE­ANIC and ATMOSPHERIC ADMINISTRATION, 1993; Figure 2).Some of th ese waters penetrated more th an one km ont o th enorth side of th e island (Figure 3).

Damage from the hurrican e was exte nsive. The hurricanedest royed nearly one-third of the Buxton Woods can opy ofloblolly pines (MORGAN, 1996). Water quality also declined.Figure 4 shows bulk chloride levels in the Frisco Wellfieldimmediately following th e storm. Also shown in the figur e(solid line) is the chloride MCL of 250 mgIL. Within threeweeks of th e storm, chlor ide levels in th e pumped grou nd­water were in viola tion; periodic violations of th e chloridestanda rd continued for nearly two months. These chloridedata represent bulk chloride levels within th e BWA and areused as calibration data for the ana lytical simulations dis­cussed in this paper .

50 100 150TIme since Hurricane Emily [days]

Figure 4. Chloride breakthro ugh at the Fr isco Wellfield.

J ournal of Coastal Research, Vol. 18, No.3, 2002

416

Ta ble 2. Simulation parameters-Transects 1 and 2.

Parameter Units

Source CI- Concen t ra tion, Co mg/LAvera ge Linear Velocity, v, mid

Transect 1Tran sect 2

Hyd rodyn ami c Dispers ion , D, m' /dTr an sect 1Transect 2

Bulk AquiferValue

12000

0.0910.170

4.558.85

And er son

duration of high chloride concentration will be minimized be­cau se the high velocities and dispersion values enable aquicker dil ution of solute concentrations. Thus, th e simula­tions t hat are discussed below are a minimum estima te ofchloride br eakthrough. See the subsequent sensit ivity anal­yses for the effect s of variable groundwate r velocity and hy­drodynamic dispersion on model output.

Transect I Simulatio ns

ride transport a long Transect 2 to determine the degree ofimp act a t a proposed wellfield site. Tab le 2 indicates the sim­ulation parameters (Co, v'" and D) that are used in the cal ­ibrated simulati ons of transport along the two transect s .

It is important to note that these simulations uti lize pa­rameters tha t are constant t hroughout the simula tions ,which will be a source of error at la te times. Another sourceof error is the vari ab le density of the groundwater, which isdisregarded in these simula tions . Although the source con­cen tration does not vary through the length of th e pul sesource, th e average lin ear velocity along th e streamline andthe re sulting hydrodynamic dispersion would certainly varyconsid erably over time, especia lly as water levels, and thusgradients, decrease with th e passing of th e storm. The de­crease in the water-table gr ad ient and the average lin ear ve­locity is not as significant as it wou ld be at other times of theyear, however , becau se the hurricane occurred at th e begin­nin g of th e high recharge seas on. Seven years of water-leveldata from the Cape Hatteras region show th at water levelsbegin to ri se in the autumn as recharge rates rise in responseto reduced evapotranspiration rates (ANDERS ON , 1999).Thus, the water-table gradi ent would be expect ed to be hig hthroughout the duration of the simula tions .

Because high velociti es have been used in th ese simula­tions, two contradictory aspects of th e simula t ions must benoted. First, mor e ma ss of th e solute per un it time is adv ect edinto the aquifer du e to high groundwater velocities , thus rai s­ing chloride levels considerably afte r short times. Second , the

Figure 5 shows the results of several a nalytical simulationsthat use different pul se-source durations. Th e data comprisethe chloride br eakthrough curve at the center of mass of th eplume for times following the overwash event. The center ofmass of th e plum e wa s used in calibrating the pulse-sourceduration because of the uncertainty in th e calibration data .Spec ifically, the data are bul k chloride va lues obtained fromall of th e pumping well s that were operating in the wellfield .It is likely that some of the wells detect th e br eakthrough ofth e center of mass shortly after the beginning of the overwashevent; therefore, rather than picking an arbit ra ry locationalong the st reamline for th e cal ibration , it was decided thatthe pul se source calibration should be with the center of masslocations. Subsequent simula tions use the calibrated pul se­source duration for ca libration of the average wellfield loca­tion along th e st reamline. The left pa nel of Figure 5 showssimulation results for pulse-source dura tions of one to sixdays. As is indicated in th e det ail ed plot shown in the rightpanel of th e figure, the five-day pulse source has a pr acticallyidentical fit based on a linear least-square s regre ssion (r" =0.819 ) as does the six-day pulse source (r" = 0.818 ). Althoughboth pu lse lengths are equa lly reasonable, five-day pulses areutilized throughout th e rest of the study.

Figure 6 shows breakthrough curves produced by simula­t ions at distances of 25 m, 30 m, and 35 m along the stream­lin e. These locations alon g the st reamline corresp ond roughlywith the position of th e screen s of the Frisco Wellfield well s.The best fit to the calibration data occurs at a position of x= 30 m along th e st reamline, where ,.2 = 0.810 . Deviation sfrom the breakthrough curves at late times suggest that th e

500

:::1 400Cl.§.

~ 300

f!Cfl 200coo,(3 100

50 100 150

Time since Hurricane Emily (days)200

400

300

200

60 70 80 90 100 110 120 130 140 151.

Time since Hurricane Emily (days)

Figure 5. Calibr a tion of chloride breakthrough simula tions at th e center of ma ss of th e plume at Tr an sect 1.

J ournal of Coasta l Research, Vol. 18, No. 3, 2002

Overw ash in Coastal Aquife rs 417

Table 3. Sensitivity analysis parameters- Tran sect 1.

500 .------ - - - ---- - ----- - -----,

Parameter Units Ran ge of Values

4000-200000.009-0.9

0.5- 45.5

mg/Lmi dm2/d

Source c i - Conce ntra tion, CQ

Average Linear Velocity, Vy

Hydrodynamic Dispersion, D,

Discu ssion

SENS ITIVITY ANALYSES

Th e simulate d br eak th rough curves suggest that futurewater qu ality scenarios along Transect 2 are problem atic. Al­though the area ap pea rs to be more protected geograph icallyth an the highly exposed area near Tr an sect 1 and the exist­ing wellfield (Figure 3), favorable aquifer conditions nearTransect 1 ii.e., lower hydraulic gradient) advect less chlor idein to the aquifer . This sugges ts that less time will be spent inviolatio n of chloride MCLs with future storms as a resu lt ofpumping from the existing Frisco Wellfield . The followingsection discusses the sensit ivity of th e simula ted break­through curve s to cha nges in model param et ers in an effortto dem onstrat e th eir influence on model output .

200

o

50 100 150

Time since Hurrtcane Emily [days]

x=25m

O ~~~~~----~~---~-~~-a

:::;' 400"al§.l5 300

~

~ 200

8Gloo

Figure 6. Sim ulated chloride breakthrough a t thre e streamline loca tionsat Transect 1.

dilu tion th at would be expected du rin g higher winter re­cha rge rates is not ta ken into account by the model.

Transect 2 Simulati ons

Chloride breakthrough curves were also simulate d for ast rea mline parallel to Tran sect 2. The simu lations ind icateth at the best fit to the br eak through curve occurs at a dis­tan ce of 45 m a long the st reamline; thus, th e steeper gradientat Tran sect 2 advects chloride appr oxima te ly 15 m furtherinto the aquifer tha n at Tran sect 1. This is demonstra ted inFigure 7, which compares simulation result s for Tran sect s 1an d 2. At a dist an ce of 30 m from th e chlori de source, chlorideconcen trations along Tr an sect 2 rise to nearly twi ce t he levelthat they do in Transect 1. In addition , the peak concentra ­tion at 30 m arrives near ly ten days earlier along Tr an sect 2tha n it does a long Tr an sect 1. Condit ions at a position of 45m along th e streamline at Transect 2 are simila r to Tr an sect1 at a posit ion of 30 m.

Sensiti vity analyses of th e paramet ers involved in simu­lat ing chloride breakthrough were conducted to determineth e level of influence th at each of these paramet ers has onmodel output . Three paramete rs were va ried: average linearvelocity with in the aquifer (u,) , hydrodynami c dispersion (D x),

and the sour ce concentration (Co), Both u, and I) , were variedover two order s of ma gnitude . A range of two orders of mag­nitude was impractical for Co because of th e smaller ran geth at is possible. Sensitivity param eters are shown in Table 3.

Figu re 8 shows the resu lt s of the sens it ivity ana lyses at at ime of 50 days after the overwa sh event . The chloride con­centrations that are plotted in the figur e represent tho sefound at the cente r of mass of the chlorid e plum e. As is ex­pected, the sensiti vity of u, and I) , are inversely propo rti onal.Th is is reflect ive of the Peclet nu mber , Pe = u;X/Dx' At hig h

T2-x=30m500 r-- - - --- - - - --- -----

10.001.00

Parameter Ratio to Base Case

I, , ' I

, I, ,I

I---r-~·~~--I

i

I--II

I

O '----~~~-'--'-'-'--'-'--'---~--'------'------'----'--'--'---u

0.10

400

1200 .----- - - - -,--r-,-,------ - .,----,-- -,---,....,...,....,

Figure 8. Parameter sens it ivity ana lysis result s for u, (squa res ), Dy (tr i­angles), and Co (dia monds ).

::i'll000;gII) 800II­.l5 600

!~coU

(3

200o

o 50 100 150

Time since Hurrtcane Emily [days]

Figu re 7. A comparis on of simulate d chlor ide breakthrough curves atseveral loca tions a long Transect s 1 an d 2.

:::;'400

ll5 300

!ctl 200scL...U100

J ourn al of Coastal Research, Vol. 18, No. 3, 2002

418 Ander son

200

CfMCL

- ' -.:. ~ .~ ~ '::T1: :~ :.::.: ::.: .-':.'Single Storm .

50 100 150TIme since Initial Hurricane (days]

7dLag

O'--~-~~~----~-~---~~-~~~o

_ 400

'§,S-a 300

!1:8 200cooo 100

20050 100 150Time since Initial Hurricane (days]

O '-~~~~~----~---~-~~--_---J

o

500 ~----------------------,

:r 4ooa.S-g 300

!1:fl 200cooo 100

Figur e 9. Dua l hurricane scena r io simula tion results for several lagt imes -Transect 1.

Figure 10. A comparison of dual hurricane sim ulations for Transect s 1and 2.

(2)

Peclet numbers, advect ive processes dominate and model out­put is sens it ive to u, bu t inse ns it ive to D'; At low Peclet num­bers, the opposi te is true: disper sive processes domin ate andmodel output is sens it ive to D, but insen sitive to u,. Tim e isalso an importan t cons tra int in these simula tions . The sen­siti vity of each of th ese param et er s decreases significantly astime increases.

Because of the h igh degree of sens it ivity of th e model out­put to average groundwa te r velocities, it is important thatadequate inform ation is used to estima te th is param et er .Also, becau se of th e high degre e of sens it ivity to averagegroun dwa te r velocit ies , the precision of the disper sion es ti­mate is less critical in highl y advective systems . Conversely,highly disper sive sys te ms will require less precision in av­erage groun dwater velocity es tima tes . Model output is alsodirectly prop ortional to th e source concent ration, Co' As Coincreases, th e chloride concentration at the cente r of mass ofth e plu me also increases; as Codecre ases, the chloride con­centra t ion at the center of mass of the plum e also decreases.

PREDICT IVE MODELING-MULTIPLEHURRICANES AND WATER QUAL ITY

Th er e have been seve ra l in st ances in the past decade whenmore than one hurrican e has hit the coast of North Carolinain a single season. Recent exa mples include 1996 with Hur­rican es Bertha (J uly 12) and Fran (September 5), and 1999with Hu rri can es Dennis (August 30) and Floyd (September16) (Y OU N G et al., 2000). Th e ti me intervals between th esehi sto ric sto rms are 55 days in 1996 and 17 days in 1999 .Thus, it is a likely scena rio that two storms would follow atrack like th at of Hurrican e Emil y in a single season, pro­ducing simila r conditions and threats to water quality wit hinth e BWA but raisin g the threat to water quality.

Pul se So urce Analytica l So lutio n withSecond Hurricane

Th e effects on water quality of a second hurricane t ha t oc­curs short ly after a n initial hurrican e can also be ca lcula te d

with a n ana lytical solution for solute transpor t with twopul se sources t hat are simula te d usin g lin ear superposition.Th e an alyti cal solution for two pulse sources of sound waterof duration t / and t2, with hurricane occurrence tim es of twand t1l2 , is

C ( ) Col c [x - U,tlll] c [x - Ux(tll/ - tl)]x, t = - enc - er ic -:.-:r.=;~=====-2 Y 4D,tHl Y 4D) tm - t , )

[X - Uxtll2] c [x- uX (t1l2 - t2 )]}+ erfc - en c -:-:;=:::='=:;===:0Y 4D,t1l2 Y 4Dx(t/l2 - t2 )

In these simula tions, the cal ibrated effects of Hurrican e Em­ily a re coupled with a second hurricane of equal duration, t2,

lagged a tim e tm afte r the first hurrican e.

Multiple Hurricane Simulatio ns

Figure 9 shows breakthrough curv es at a position of 30 malong a st reamline parall el to Tran sect 1 for several du al hur­ricane scena rios. In these simulations, the time of th e secondhurrican e was lagged by values ran ging from 7 to 35 days.Th e figu re shows th e resu lt s of each lagged hurricane simu­la t ion in addition to the calibrated single storm simulationt ha t was shown in Figure 6. A one-week lag time betw eenstorms shows the most severe effect , with a viola tion of th echloride MCL for more th an 100 days. Th e effects diminishwith increas ing lag tim es as chloride levels within th e BWAhave a chance to dilute before rece iving th e second pul se. Alag time of 35 days between storms reduces th e duration ofthe violation by at least three weeks.

Figure 10 compares multiple hurricane breakthroughcurves for Transect s 1 and 2. Th e br eakthrough curves rep­resent th e arrival of chloride along streamlines parallel to th etransects at distances of 30 m and 45 m for Tr an sects 1 and2, respect ively . Th ese locations wer e chosen for compa risonbecau se they represent th e location of poten tial groundwaterwithdrawa ls along the streamlines . Th e results in Tr an sect2 a re simila r to Tr ansect 1, with two exceptions: the peakchloride levels within Tr an sect 2 are higher than tho se in

J ourn al of Coastal Research, Vol. 18, No.3, 2002

Overw ash in Coastal Aquifers 4HJ

Tra nsect 1, a nd t he ch loride MCL is viola te d for a pproxi­mately t wo more week s along T ransect 2 due to h igh er hy­dra u lic grad ie nts. Along Tran se ct 2, a lag between hurricanesof se ve n day s produces ch loride MCL viola tions for more thanfour months , whil e a lag of :35 days prod uces chloride MC Lviolation s for nearly f(JUI' months a t lower pea k va lue s .

DISCUSSION

Th e effect of storm overw a sh on t he wa ter quality of coa st a laq uife rs is a real, yet rel a t ive ly un studied , ph enomen on .F ield da ta fro m the Ca pe Hatter as region of No rt h Carolinashow t ha t overwash in t he form of s torm surge ha s ser iou slya ffected wate r qu al ity in t he Bu xt on Woods Aquifer. Ea r lyst u dies of t he BWA suggest that overwash ha s been a n im­portant factor in the pa st (H AIWls an d WILIl ER, 196 4 ; H AR­HI S, 196 7 ), a nd a st udy a t nearby Cape Look out, No rth Ca r­olin a sugges ts th a t overwash cou ld be a se rious wa ter qualityth reat to low-lying ba rrie r isl a nd s (W If' :-.I lm, H)78 1. Bulkgr ou ndwa te r ch lor ide levels mon it ored at the F ri sco wa tertreatment plant in 199:3- 94 dem on strate t hat ch loride level sviolated MCL s ta nda rds for more th an one month foll owingth e stor m , with pe r iodi c viol ations for two mon ths. An a ly ti calsim u la t ions presented in t h is s t udy suggest t ha t t he effectsof mu lt iple over wa sh eve nts in s uccession cou ld rai se ch lor idelevel s in t he Bux to n Wood s aqu ife r a bove t he MCL for moret han 100 days.

Th e Ca pe Hattera s region now su pp lemen ts it s freshwatersupply wi t h bra ck ish wate r pu mped from t h ree deep we lls .Th e Cou nty of Da re , No rth Ca ro lina cur ren tly mixes th efreshwate r fro m t he ex isting Frisco Wellfie ld with saline wa ­ters fr om t he deep wells , a nd t hen t rea ts th e mi xed wa terwith a reve rse-osmosi s treat ment syste m . Although t hi smakes the ove r wa sh process less of a prob lem for the Ca peHa t t er a s region than it wa s in 199 3, it contin ues to be a con ­cern for se ve ral reason s . Firs t , overwa sh wa ters ra ise t heovera ll salin ity of t he in fl uen t wa ters , leading t o in crea sedtre atme nt dem ands , t ime , a nd costs. Second , coa stal regio nsin th e U nite d Sta tes a re seeing a n in crea se in wate r dem andas population s in t he se areas in crease. The Ca pe Hattera sregion is transfor m ing from a seasona l touris t desti nation toa ye a r-rou nd tou rist destina ti on . T he in crea sed wate r de­ma nd tha t these year -ro und to u r ists brin g to t he a re a m ea nst hat more water is be ing pumped fro m t he aquifer, therebyshr inking th e freshwa ter len s a nd diminishing the aqu ife r 'sabi lity to di lu t e t h e sali ne ove rwa sh wa ters . F ina lly , with thea dvent of globa l wa r ming, ri sing se a level s a n d mor e frequentt ro pical st orm ev ents ex pected on a warmer Earth sugges tthat t he re will be a n in cr ea se in the frequency of overwasheve nts, furth e r reducin g the aquifer's wa ter quali ty .

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