WATER RESOURCES RESEARCH, VOL. 42, W05201, doi:10.1029/2006WR004876, 2006

Impacts of the 2004 tsunami on groundwater
resources in Sri Lanka
Tissa Illangasekare,1 Scott W. Tyler,2 T. Prabhakar Clement,3 Karen G. Villholth,4A. P. G. R. L. Perera,5 Jayantha Obeysekera,6 Ananda Gunatilaka,7 C. R. Panabokke,5David W. Hyndman,8 Kevin J. Cunningham,9 Jagath J. Kaluarachchi,10William W.-G. Yeh,11 Martinus T. van Genuchten,12 and Karsten Jensen13
Received 10 January 2006; revised 25 February 2006; accepted 9 March 2006; published 9 May 2006.

[1] The 26 December 2004 tsunami caused widespread destruction and contamination ofcoastal aquifers across southern Asia. Seawater filled domestic open dug wells and also
entered the aquifers via direct infiltration during the first flooding waves and later as
ponded seawater infiltrated through the permeable sands that are typical of coastal
aquifers. In Sri Lanka alone, it is estimated that over 40,000 drinking water wells were
either destroyed or contaminated. From February through September 2005, a team ofUnited States, Sri Lankan, and Danish water resource scientists and engineers surveyed
the coastal groundwater resources of Sri Lanka to develop an understanding of the impactsof the tsunami and to provide recommendations for the future of coastal water resources insouth Asia. In the tsunami-affected areas, seawater was found to have infiltrated andmixed with fresh groundwater lenses as indicated by the elevated groundwater salinity
levels. Seawater infiltrated through the shallow vadose zone as well as entered aquifers
directly through flooded open wells. Our preliminary transport analysis demonstrates
that the intruded seawater has vertically mixed in the aquifers because of both forced
and free convection. Widespread pumping of wells to remove seawater was effective insome areas, but overpumping has led to upconing of the saltwater interface and risingsalinity. We estimate that groundwater recharge from several monsoon seasons will
reduce salinity of many sandy Sri Lankan coastal aquifers. However, the continued
sustainability of these small and fragile aquifers for potable water will be difficult
because of the rapid growth of human activities that results in more intensive
groundwater pumping and increased pollution. Long-term sustainability of coastal
aquifers is also impacted by the decrease in sand replenishment of the beaches due tosand mining and erosion.
Citation: Illangasekare, T., et al. (2006), Impacts of the 2004 tsunami on groundwater resources in Sri Lanka, Water Resour. Res., 42,
W05201, doi:10.1029/2006WR004876.

1. Introduction[2] On 26 December 2004, a magnitude 9.3 earthquake
off the south coast of Sumatra generated tsunami waves thatleft over 280,000 people dead or missing in Asia and Africa.
Hundreds of thousands of homes were destroyed resulting
in a humanitarian crisis in the hardest hit countries. From
February through September 2005, a team of United States,
Sri Lankan and Danish water resource scientists and engi1
Center for Experimental Study of Subsurface Environmental Processes,
Division of Environmental Science and Engineering, Colorado School ofMines, Golden, Colorado, USA.
2
Department of Geological Sciences and Engineering and Department of
Natural Resources and Environmental Sciences, University of Nevada,Reno, Reno, Nevada, USA.
3
Department of Civil Engineering, Auburn University, Auburn,
Alabama, USA.
4
International Water Management Institute, Colombo, Sri Lanka.
5
Water Resources Board, Colombo, Sri Lanka.
6
Hydrologic and Environmental Systems Modeling Department, South
Florida Water Management District, West Palm Beach, Florida, USA.

Copyright 2006 by the American Geophysical Union.
0043-1397/06/2006WR004876$09.00

neers surveyed the impacts of the tsunami on the coastal
groundwater resources of Sri Lanka. Sri Lanka was chosen
for study as it was one of the hardest impacted by thetsunami. Further, the coastal aquifers found in Sri Lanka aresimilar to those found in parts of Indonesia, Thailand andIndia. The objectives of the survey team were (1) toinvestigate the impacts of the tsunami on coastal groundwater resources and review well cleaning methods and theirimpacts, (2) to develop a conceptual understanding of theseawater mixing phenomenon in coastal aquifers after the7

National Science Foundation of Sri Lanka, Colombo, Sri Lanka.
Department of Geological Sciences, Michigan State University, East
Lansing, Michigan, USA.
9
U.S. Geological Survey, Miami, Florida, USA.
10
Department of Civil and Environmental Engineering, Utah State
University, Logan, Utah, USA.
11
Department of Civil and Environmental Engineering, University ofCalifornia, Los Angeles, California, USA.
12
George E. Brown, Jr. Salinity Laboratory, Riverside, California, USA.
13
Geological Institute, University of Copenhagen, Copenhagen,Denmark.
8

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W05201Figure 1. Photograph of the coastal area near Maruthumunai, Sri Lanka, showing the near complete
destruction of homes along the coast. The photographer is standing about 150 m from the coastline, and it
was reported to the team that the maximum wave height at this site reached high into the palm trees
shown in the background. Note the high density of domestic open dug wells (typically short verticalcircular structures), many of which were not destroyed by the wave but were inundated and flooded byseawater and debris.
tsunami event, (3) to investigate the medium and long-term
impacts of the tsunami on coastal groundwater resources,(4) to develop a joint program to study the regional aquiferhydrology and hydrogeology of Sri Lanka, and (5) totransfer knowledge about coastal aquifer vulnerability toother south Asian nations.
[3] We begin with a review of the tsunami and its impacts
in Sri Lanka along with a brief overview of Sri Lankan
groundwater in the tsunami-impacted areas. We then discuss
the major processes that occurred in the aquifers with anemphasis on salinity changes caused by the tsunami. We
conclude with a preliminary estimate of the recovery andlong-term sustainability of coastal aquifers in densely populated countries such as Sri Lanka.
[4] The tsunami first impacted Sri Lanka along its eastern
coastline and during a period of about 90 min, swept along
the southern and southwestern and northern coasts. Inundation distances varied significantly because of the angle ofwave incidence and the local shore and coast configurations, but in places wells up to 1.5 km inland were flooded[Villholth et al., 2005]. The height of the tsunami waves in
Sri Lanka ranged from 3 – 8 m, with runup heights as muchas 15 m [Liu et al., 2005].
[5] Although Sri Lanka is approximately 1600 km fromthe epicenter, it still suffered catastrophic losses that include
over 35,000 people known dead or missing. In addition,
more than 85,000 homes were destroyed or severely damaged along the coast of Sri Lanka and over 400,000 jobs
were lost as a result of the devastation (Joint AsianDevelopment Bank (ADB), Japan Bank for International
Cooperation, and World Bank, Sri Lanka – 2005 PostTsunami Recovery Program: Preliminary Damage and
Needs Assessment, Colombo, Sri Lanka, 10– 28 January
2005, available at http://www.adb.org/Tsunami/sri-lankaassessment.asp, hereinafter referred to as ADB report,
2005). The magnitude of the tragedy continued to dominate
the social and political structure of the region at the time thispaper was written.
[6] In most areas of Sri Lanka, the availability of potabledrinking water quickly became a major concern for relief
efforts, both from the perspective of life requirements andfrom concerns over the potential spread of waterborne
diseases from contaminated water supplies. The largest
coastal urban communities of Sri Lanka are supplied bytreated and piped surface and groundwater supplies managed by the National Water Supply and Drainage Board(NWSDB). The primary damage to the treated water supply
involved breaks in pipelines which were completely
destroyed at several river and estuary crossings. Additionalleaks were caused when the force of tsunami waves sheared
off the water connections to individual dwellings. Repairs to
the distribution system began soon after the tsunami event,and as of September 2005, piped water service had been
restored to most areas previously served and plans were
underway to expand the distribution system.
[7] While the NWSDB served about 30% of the country’s
general population via 287 water supply systems (www.
waterboard.lk) using tube wells and surface water systems,

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many of the coastal areas rely on individual or community
wells for potable water. Typically, these wells are of large
diameter (1 – 2 m) and 3 –8 m deep and are designed to tap
the shallow parts of the fresh groundwater lens found in
most of the coastal aquifers. Figure 1 shows the remains ofmany of these wells following the destruction of thetsunami. They are dug manually and/or with explosives
and excavators. These shallow wells are termed as ‘‘open
dug wells’’ in this communication. The density of open dug
wells in Sri Lankan coastal villages is very high in heavily
populated or cultivated areas with well densities of approximately 90– 125 wells km2 [Nadarajah, 2001; Villholth etal., 2006]. Figure 1 shows both the devastation of thetsunami at the coastal town of Maruthumunai on the eastern
coast of Sri Lanka, and also the high density of domestic
open dug wells (concrete cylinders and open excavations)typically used along the coast of Sri Lanka. Generally, eachof the wells shown in Figure 1 may have been used byseveral individual households that were totally destroyed by
the tsunami wave. The local villagers reported that the waterlevel reached close to the top of the coconut palm trees
shown in Figure 1. Over one thousand lives were lost when
the wave completely inundated this coastal village.
[8] The tsunami immediately inundated and contaminatedmore than 40,000 of these wells in Sri Lanka [United
Nations Environment Programme, 2005; ADB report,
2005], primarily close to the coastline. Because many of
the coastal villages are no more than 10– 15 m above sea
level, an enormous area was affected by this inundation. In
most affected areas, the open dug wells were instantly filledwith the seawater and large volumes of saltwater were
injected into the freshwater lens. Many wells were also
contaminated with organic and inorganic debris from theflood waters, rendering them unusable until they werecleaned and disinfected. In addition, the seawater inundated
low-lying areas resulting in infiltration of saline water into
the aquifers.
[9] During the post-tsunami clean up, government, nongovernmental organizations (NGOs), foreign and local
voluntary groups and local citizens began pumping contaminated wells and treating them with chlorine disinfectant.
Fears of waterborne disease outbreaks were not realized in
the coastal regions of Sri Lanka affected by the tsunami
partly because there was public awareness of the need forwell disinfection in addition to personal and food hygiene.
Although emergency well cleaning advice and instructions
are now available from various sources (e.g., http://
www.who.int/water_sanitation_health/hygiene/envsan/technotes/en/, http://www.ngwa.org/presscenter/pr2005/01-0205emerwelldisinfection.pdf, and http://www.bt.cdc.gov/
disasters/wellsdisinfect.asp), no widely accepted guidelines
were available at the time for decontaminating wells affectedby tsunami-related seawater, let alone guidelines relevant
for conditions specific to coastal aquifers such as thosefound in Sri Lanka and other portions of south Asia. Fromthe perspective of removing saltwater and restoring potabledrinking water quality, the emergency procedures used may
not have been effective. The excessive pumping may havecaused more seawater intrusion from below rather than its
removal from the upper part of the saturated zone. In
addition, the purged well water was often discharged on
the land surface close to the wells, allowing the contami-

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nated water to reenter the aquifer and the wells after vadosezone infiltration.

2. Summary of the Coastal Hydrogeology of
Sri Lanka[10] In order to better understand the nature of thetsunami impacts on the coastal aquifers of Sri Lanka, themajor aquifer characteristics are summarized below fromthe work of Panabokke [2001]. While generalizations canbe made, there also exist many site-specific attributes that
produced a wide variety of aquifer responses to the tsunami.
[11] While confined Miocene age limestone aquifers exist
along the northern and northwest coast and throughout the
Jaffna Peninsula [Lawrence and Dharmagunawardena,
1983], groundwater resources across most of the Sri Lankan
coast are dominated by the ‘‘coastal sand’’ aquifers, which
consist primarily of spits and bars, coastal dunes, raisedbeaches and paleobeach deposits [Panabokke, 2001]. The
total area of these aquifers has been estimated to be
1,800 km2 [Somasiri, 2001] and are tapped by shallow
open dug wells and occasionally by deeper, wells (termed‘‘tube wells’’). The shallow open dug wells are usually
manually pumped whereas the deeper tube wells use electric
pumps and are typically managed by the NWSDB or by
private agricultural enterprises.
[12] Although the configuration and the hydrologic properties of coastal sandy aquifers varies substantially, several
broad generalizations can be made. The aquifers along theeastern coastline tend to be parallel to the coast and often
have a brackish or saltwater lagoon just behind the beachridge or rice paddy fields that extend inland from the coast.
A limited freshwater lens, ranging in thickness up to morethan 10 m [Villholth et al., 2005] and varying seasonally, ismaintained above the seawater wedge by the northeast
monsoon (‘‘maha’’) rains. Approximately 75% of the
1,000 – 1,700 mm annual coastal rainfall along this coastline
occurs from October to February. The thickness of the
freshwater lenses varies seasonally, and may range fromas much as 10 m to as little as tens of centimeters at the endof the dry season. Panabokke [2001] suggested that recharge may be as much as 50% of the net precipitation.
Along the eastern coast, precipitation may be as low as1000 mm yr1, and assuming a typical sand porosity of
30%, a preliminary estimate of the seasonal fluctuation of
the water table is between 1.5 and 2 m. The thickness of thevadose zone is poorly documented, but the study teamtypically observed water table depths of 2 – 6 m at mostfield sites visited in February and September of 2005.
[13] Data on groundwater quality from coastal sand aquifers have been collected in a number of studies, which haveshown that salinity (as measured by specific conductance) in
the coastal sand aquifers can range from a minimum of 400 mS
cm1 to as high as that of seawater [Panabokke, 2001].
However, most wells used for drinking water or agriculture
do not exceed 2000 mS cm1. While there are no strict healthbased drinking water quality standards for salinity, levels
below 1000 mS cm1 are considered to be palatable, that is,
not giving objectionable taste. Coastal aquifer wells haveshown seasonal water quality variations, with an increase insalinity following the onset of the monsoon rains, believed tobe the result of leaching of solutes from the vadose zone built
up during the dry season. Increasing salinity is generally

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W05201Figure 2. Specific conductance of groundwater from three tube wells in Kaggola, Sri Lanka. Thesewells were inundated to various degrees by the tsunami; however, it is not believed that significant
volumes of seawater entered these wells directly during overtopping. All wells show a rapid rise in
specific conductance following the tsunami and have not yet returned to background levels.
observed during the dry season and has been attributed to theevapotranspiration and possible upconing of the fresh/saline
water interface caused by pumping during this period.
Panabokke [2001] reported that nitrate concentrations
often exceed the World Health Organization standards of
10 mg L1 as nitrate-N and also showed a seasonal fluctuation, with maximum values occurring before or at the onsetof rainy seasons. The likely sources of the elevated nitrate
are domestic septic tank systems and fertilizer application.

3. Observations of Tsunami Effects on
Groundwater[14] Across much of Sri Lanka, the earthquake produced
as many as four large tsunami waves [Liu et al., 2005].
Where multiple direct waves hit the shore, the second wave
was generally the largest and most devastating. Eyewitnesses reported the appearance of gas bubbles in wells long
before the arrival of the tsunami; this may be related to thearrival of the earthquake-generated energy rather than thetsunami. The onshore extent and timing of inundation
varied substantially depending upon shoreline geometry,bathymetry, and local topography. In some areas, floodingof low-lying areas behind the shoreline and raised beaches
and associated back lagoons continued for extended periods. Over much of the area, however, inundation by theseawater did not last more than a few hours.
[15] Immediately after the tsunami waters retreated,efforts were initiated to clean and pump contaminated wells.
Verbal reports indicated that at the time of first pumping(within hours or days), many flooded wells had alreadydeveloped a sharp seawater/freshwater interface, with astable seawater configuration, that is, at the bottom of thewell. Salinity stratification in open dug wells was observed
up to one month after the tsunami as indicated by increasesof up to one order of magnitude in specific conductancefrom the water surface to the bottom of the well [Villholth etal., 2005].
[16] Several monitoring programs have documented salinity changes in wells affected by the tsunami, althoughthese programs began several months after the tsunami.
Monitoring of flooded open dug wells between March andJuly 2005 at Kallady, Kaluthavalai and Oluvil on the eastern
coast of Sri Lanka [Villholth et al., 2005] showed large
variability of impacts from the tsunami and the subsequentwell cleaning. The data showed that changes in salinity overtime were highly variable from well to well, with somewells actually showing an increase in salinity long after thetsunami. Several factors may be responsible for the observed variations including (1) the amount of seawater that
initially entered the wells directly through the well opening,(2) the amount of seawater that infiltrated through thevadose zone, (3) the volume and frequency of water thatwas pumped from each well after the tsunami, (4) wellinterference and recontamination through the vadose zone
due to pumping from closely spaced wells, and (5) variations in monsoon recharge. In this case, the mean distance
between wells in the monitoring transects was approximately 100 – 150 m. In the study, flooded wells maintained a higher average salinity for seven months after thetsunami compared to salinity in nearby wells that were notflooded [Villholth et al., 2005].
[17] While a large amount of seawater was injected
through open dug wells, there is also strong evidence that
substantial volumes of seawater infiltrated through thevadose zone. Figure 2 shows a 5 year salinity record fromthree high-capacity tube wells in the Koggala Free Trade
Zone region (maintained by the Water Resources Board of
Sri Lanka) on the southeastern coast of Sri Lanka. Thesewells were inundated by the tsunami, but it is believed thatin most cases seawater either did not directly enter the wellbores, or did so less rapidly than open wells that have less ofa surface seal. Two of these wells still showed a rapid
increase in salinity, with well P-06 showing a delayed
response but eventually increased to high specific conductance values. The gradual increase may also be due to lateralmigration of infiltrated seawater.
[18] Contamination of coastal aquifers has added a substantial burden to the relief and resettlement efforts in Sri

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Lanka. In some cases, potable water is now supplied bytanker trucks to roadside tanks; in a few other cases,
portable reverse osmosis units have been set up to produce
fresh water from contaminated wells. Neither provides asustainable system of water delivery, and an improved
understanding of the mechanism of coastal aquifer contamination is clearly needed to assess and identify long-term
solutions for drinking water. In the next section, we develop
a conceptual model of the fate and transport of tsunami
water in the permeable coastal aquifers common in Sri
Lanka and other low-lying coastal areas for the evaluationof both well cleaning efforts and water resource planning.

4. Conceptual Model of Groundwater Impacts[19] Seawater entered the coastal aquifers from threepossible sources: (1) direct introduction through the openingof the dug wells, (2) infiltration through the vadose zone
during inundation of seawater over the land, and (3) additional subsequent recharge of the groundwater by seawaterfrom back lagoons and surface depressions. Of these threemechanisms, infiltration through the vadose zone is likely tohave injected the largest volume of seawater into the coastal
aquifers because of their highly permeable sands and large
areal extent of inundation, while direct injection into theopen dug wells had the most rapid impact on the aquifer andwater supplies.
[20] The sudden surcharge of seawater into the open dug
wells is analogous to an aquifer slug test, albeit with aslightly denser, but miscible fluid. The height of thesurcharge was highly variable and depended on the initial
depth to the water table and other factors; but given thatflood levels were as high as the tops of coconut palm trees
in many places, it is probable that most inundated wells
were completely filled with seawater. Because of the density
contrast and high permeability, it is likely that the excess
head of seawater in the open dug wells dissipated throughthe aquifer rather quickly.
[21] The surcharge of hydraulic head from infiltration to
the water table could have led initially to a forced convection regime. The saltwater infiltration would also haveresulted in a density driven unstable situation in the aquifer
in which seawater (density 1025 kg m3) overlies fresh
water (density 1000 kg m3) suggesting a free convection
system. The relative importance of forced and free convection can be assessed through a convective ratio, M, asdefined by Bear [1972] and Wood et al. [2004]. WhenM  1, forced convection dominates over free convection,
while for M  1, free convection is the dominant process.
For a simple layered system, the convective ratio can be
related to the head surcharge, Dh, the vertical length scale
over which the excess head is to be dissipated, DL and thedensity excess, Dr, as
M¼

Dr=r
:
Dh=DL

ð1Þ

The convective ratio parameters related to excess head
would have varied considerably depending upon the floodheight, vadose zone thickness and aquifer thickness, but afirst-order approximation can be made at the time ofinundation. Assuming a seawater surcharge equivalent to a

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typical coastal vadose zone (2 m) on fresh water (rsea =1025 kg m3, Dr = 25 kg m3) and an aquifer thickness of
10 m, the convective ratio from (1) immediately after thetsunami would have been 0.1, indicating that forced
convection was likely the dominant mixing mechanism andthat seawater initially migrated into the aquifer because of
the fluid head gradients. As the surcharged head dissipated,that is, Dh decreasing, the convective ratio would increase
and free convection of seawater in the aquifer could thendominate the mixing process.
[22] The Rayleigh criterion provides a preliminary assessment of the likelihood that this layer of seawater willremain superimposed on the freshwater lens where it would
gradually mix by diffusion, or if it will undergo freeconvection. The onset of free convection depends on thecritical Rayleigh number, which is dependent on fluid,
media and solute properties as well as boundary and initialconditions. Lapwood [1948] theoretically showed using
linear stability analysis that under the conditions of a linear
density contrast due to temperature differences, convection
will begin when the critical Rayleigh number, Rac exceeds
4p2 (40), or
Rac ¼

kDrgH
> 4p2 ;Dm

ð2Þ

where k is the intrinsic permeability, Dr is the difference indensities of fresh water and seawater, g is the acceleration
due to gravity, D is the effective diffusion coefficient of thesolute (accounting for porosity and tortuosity) and m is thefluid dynamic viscosity. In soils, the onset of convection canoccur at Rayleigh numbers smaller than theoretical values
obtained using linear stability methods [Simmons et al.,2001] and as a result, we used the critical Rayleigh number
here only as a qualitative criterion to evaluate if mixing ofseawater and fresh water is probable.
[23] The length scale over which the density varies is
unknown, but we assigned this length to be characteristicof the vadose zone thickness of a typical coastal sand aquifer(2 m). Assuming a coastal sand aquifer permeabilityof 1011 m2 (or 104 m s1), fluid density contrast of
25 kg m3, an effective diffusion coefficient of 109 m2 s1,
and a viscosity of 103 kg ms1, the calculated Rayleighnumber is approximately 5,000 which greatly exceeds thecritical value of 40. Simmons et al. [2002] used sand tankexperiments to show that mixing would take the form offingers in the saturated zone, but would likely be a stable
wetting front through the unsaturated zone.
[24] Our preliminary stability analysis suggests that infiltrated seawater would have been driven into the aquifer by
the excess fluid head, and then quickly mixed and fingered
through the shallow freshwater lens as free convection
began to dominate. The mixing would lead to elevatedsalinity levels throughout the freshwater zone as a result of
mixing; however, much of the salinity would be transferredto the top of the seawater wedge.

5. Physical Modeling of Post-tsunami Mixing[25] Preliminary physical experiments were conducted tofurther investigate mixing of infiltrated seawater in an
unconfined aquifer and also to gain a qualitative under-

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W05201Figure 3. Physical model of tsunami injection and infiltration into a shallow coastal sandy aquifer at(a) 2, (b) 5, (c) 11, and (d) 19 min. The pre-tsunami seawater wedge is dyed green, and the pre-tsunami
fresh water flowing from right to left is light brown. Infiltrated tsunami water, dyed red, is injected acrossthe top of the aquifer. Tsunami water is also immediately injected into the aquifer through the open
borehole. Dense seawater quickly develops into plumes that reach the preexisting seawater wedge.
standing of the fate of flooded open dug wells in this unique
scenario resulting from a tsunami. Experiments were conducted in a rectangular flat tank constructed from
Plexiglask. The dimensions of the tank were 53 cm by
30 cm by 2.7 cm. The tank was filled with homogeneous
glass beads to simulate a coastal aquifer system with an
intrinsic permeability of 1.4  109 m2, measured using a
constant head permeameter. The permeability is approximately two orders of magnitude greater than those typicallyobserved in clean beach sands, but this allowed us tocomplete a rapid visualization experiment. The flow tank
was divided into three sections: a main flow cell filled withsand, and inflow and outflow reservoirs at both ends. The
sand was fully saturated, thus there was no vadose zone
present in the experiments. The reservoirs were separated
from the main cell using a fine mesh screen. A 2-cm
diameter impermeable glass tube, open at both ends, wasinserted into the model to represent an open dug well.
Seawater was prepared in 170 L drums by dissolving NaClin deionized water. The average density of saltwater used in
the experiment was 1025 kg m3. To avoid any variation indensity or chemical composition, the same batch of water
was used in a particular set of experiments to supply bothseawater and tsunami water. A small amount of foodcoloring was used to differentiate sea, fresh and tsunami
waters and to visualize their mixing patterns.
[26] The initial condition, representing the pre-tsunami
scenario was simulated by setting up a regional hydraulicgradient across the length of the model that would force
uncolored groundwater (identified here as ‘‘fresh water’’) toflow from right to left (see Figure 3a). The green-coloredsaltwater (identified as ‘‘seawater’’) was allowed to intrude
the aquifer from the left to form a steady seawater wedge.
The inundation caused by the tsunami wave was simulated
by evenly discharging a fixed amount of red-colored saltwater (identified as ‘‘tsunami’’ water) at the top surface ofthe model and also directly into the open well. Tsunami
waters invaded the unconfined aquifer model by infiltration(identified as ‘‘infiltrated tsunami water’’) from the top of
the water table boundary. Tsunami water also entered thesaturated region by injection from the open well. Freshwater continued to enter the model from the constant headboundary on the right.
[27] Figure 3 illustrates the transport patterns at 2, 5, 11
and 19 min after the simulated inundation by a tsunami
wave. The experimental results demonstrate that within ashort period of time the tsunami water that flooded the welldescended as a large finger and contaminated the deeper
aquifer (see Figure 3b). The infiltrated tsunami water
eventually merged with seawater from the flooded well.
An important observation from the experiments is thattsunami waters (both from surface infiltration and theflooded well) always remained above the regional seawaterinterface. This is expected as the density of both tsunami
water sources decreased because of dilution as theymigrated through the aquifer and mixed with fresh water.
Hence the net density of tsunami water had decreased belowthat of seawater before reaching the seawater wedge.
Therefore tsunami waters (both via infiltration and flooded
wells) would likely not penetrate the regional seawater
wedge, which has seawater density. Instead, tsunami waterremained floating above the wedge and was eventually

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advected by the regional freshwater flow along the wedge.
This process would have occurred under natural aquifer
conditions and perhaps is a factor in the slow recovery to
pre-tsunami salinity levels observed in many wells.
[28] Another major observation was that water in the
open well remained contaminated for a relatively long
period. As shown in Figures 3a – 3b, the bulk of the tsunami
water injected into the well moved downward as a blobwithin about 5 min. However, a small amount of seawaterremained trapped inside the well, and this remnant saltwater
acted as a continuous source of saline water that discharged
from the well. After about 19 min, a clinical syringe wasused to extract three storage volumes directly from the open
well and this process removed the remnant saltwater fromthe well.
[29] These simulation results provided a conceptual
framework for understanding the seawater transport processes from both surface infiltration and from direct injection into the wells. Although permeability and scales of theexperimental tank are different from those in the field, it ispossible to scale the results to approximate the transport
times expected in the field. The velocity of the downward
moving seawater plumes can be approximately scaled
using the relationship developed by Wooding et al. [1997,
equation (6)] that provides the travel distance/time relationship of a single dense plume to the Nusselt and Rayleighnumbers. Here we scale Wooding’s equation (6) by the ratioof laboratory to field permeability values while keeping thenumerically determined constant of proportionality of
0.615. The permeability of the tank material is approximately two orders of magnitude greater than that of typicalbeach sands of Sri Lanka, whereas the density contrast issimilar to that found in the field. Figures 3a – 3c show thedense fingers descended at a rate of about 2 cm min1
before they reached the seawater interface. Scaling thisvelocity by the typical field permeability for coastal sandaquifers (about 1011 m2) and assuming a maximum fresh
water lens thickness of 10 m, the scaled travel time throughthe fresh water lens is on the order of 10 days. It is
important to note that at field scales, the plume may besubstantially influenced by heterogeneity, but this simple
analysis suggests that seawater could easily have traveledthrough a typical fresh water lens in timescales of only days
to weeks. Although the experiments did not simulate thevadose zone, the time required for seawater to pass throughthe 1 to 3 m thick vadose zone would be on the order ofhours for a permeability of 1011 m2 (or 10 m d1), which
would not add significantly to the time of mixing. This
preliminary analysis indicates that the seawater that entered
the aquifer through infiltration probably reached the bottomof the freshwater lens rather quickly after the tsunami event.
[30] These simulation results also demonstrate the mixing
processes that would have occurred in flooded open dug
wells. Although the analyses presented here are simple andqualitative, but they provide insight into the basic processes
that may have occurred during this unusual event. The
behavior of the system under much more complex field
conditions will require further study than presented in thiswork and will require more sophisticated physical modeling, field studies, and mathematical modeling. Areas needing further study include estimating: the amount of seawater
infiltration at various locations, aquifer heterogeneity, well

W05201construction affects, and the effects of residual salinity inthe unsaturated zone. The laboratory-scale model results,
however, provide a preliminary understanding of the mechanisms of contamination and the probable timescales ofaquifer contamination and corresponding remediation.

6. Discussion and Long-Term Outlook[31] The short-term impacts of tsunami flood waves oncoastal groundwater resources and supplies in much of Sri
Lanka were severe. Thousands of open dug wells, a criticalwater supply source for coastal communities, were madeunusable. Although salinity in some wells declined rapidly
within a few months after the tsunami, some wells in theselocations remain unfit for drinking because of residual
salinity.
[32] The preliminary analysis and observations of thisstudy support the conclusion that the 2004 tsunami produced a surcharged head and an unstable density contrast in
the shallow coastal sandy aquifers leading to vertical mixingof salinity within the freshwater lenses. Since March 2005,
salinity levels have only declined slowly, if at all, in manyof the wells that continued to be pumped. In some wells,
there is evidence that overpumping occurred, causing seawater to be drawn up from the interface and further
contaminating the wells.
[33] Many of the coastal areas that were most affected byinundation did not receive substantial rainfall for almost ayear, thus aquifer recharge had been very limited. Sri Lanka
has received substantial monsoon rains during December of2005, yet it remains to be seen how many monsoon seasons
will be needed for aquifers to recover. The magnitude of thesalinity reduction each year will depend on the rate ofrecharge, the accumulation of salinity in the vadose zone,
the permeability of the aquifers, and the pumping intensity. If
we assume an aquifer porosity, n, of 30%, complete mixing of
the upper 2 m of a freshwater lens, an initial residual salinity,
Spre, of the freshwater lens of 2,600 mS cm1 before the rainyseason [Villholth et al., 2005], and a recharge of 0.5 m yr1
(or 500 mm yr1), the resulting salinity, Spost, of the 2 m lens
after the monsoon recharge is
Spost ¼ SpreLn
¼ 1420 mS cm1 ;ð Ln þ RÞ

ð3Þ

where L is the initial freshwater lens thickness and R is therecharge depth. This simple mixing approach ignores
residual salinity in the vadose zone and is provided only
as a hypothetical example; however, it may indicate thataquifers will recover to pretsunami levels (<1000 mS cm1)after a few rainy seasons. While this is a positive outlook,other factors must be considered by the Sri Lankan
government and water authorities for long-term use of
these aquifers for potable water. As seen in Figure 1, thedensity of pre-tsunami housing on many coastal aquifers
was high and there exists no central wastewater collection inmost areas. The combination of highly permeable sand, ashallow vadose zone, and dense developments makes theseareas susceptible to groundwater contamination by nitrates
and pathogenic organisms. Prior to the tsunami, coastal
residents seemed well aware of this risk, and chlorination ofopen dug wells was a common practice.

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ILLANGASEKARE: RAPID COMMUNICATION[34] The long-term sustainability of the coastal aquifers
for private drinking water will continue to be a major
concern. Planning efforts are currently underway to construct piped water systems in many coastal villages rather
than rebuilding the reliance upon individual, open dug
wells. Centralized sewage collection may also expand, but
it is unlikely to be practical across the whole country.
Legislation to require minimum setbacks from the sea for
housing and buffer zones near the coast have been proposed
to reduce the hazards of inundation; but the challenges offinding appropriate land for resettlement for the thousandsof people who rely on coastal proximity for their livelihood
are enormous. Sri Lanka has no formal permitting system
for the private use of groundwater; hence limiting the use ofcoastal aquifers will be difficult. Long-term sustainability ofcoastal aquifers is also impacted by the deteriorationof beaches due to lack of sand replenishment due to mining
in both coastal areas and in the upland rivers as well as
coastal erosion. The 2004 tsunami has highlighted thevulnerability of these aquifers to contamination and theneed for conjunctive use of surface water and groundwater
for sustainable water resources. New modeling tools available for integrated management of surface and groundwater
systems, geophysical technologies for characterization and
GIS tools for data analysis and interpretation should bevaluable in this task. Introduction of these tools need to becoordinated with capacity building for training of scientists
and engineers who should be versatile in the application ofthese tools.
[35] As coastal populations continue to grow, devastating
floods from sources other than tsunamis such as flood
waves/storm surges, cyclones hurricanes/typhoons, and rising sea level will continue to impact coastal areas. While
we, the hydrology community, cannot prevent such largescale natural disasters, we can certainly participate in thedevelopment of emergency planning procedures that canconsiderably reduce the unnecessary human suffering. We
should first take the necessary efforts to carefully document
and study the hydrologic problems associated with thesedisasters. These results can and must be used to developinternationally recognized emergency guidelines for treating
contaminated water supply sources (such as wells), andlong-term planning tools for managing groundwater incoastal areas affected by seawater inundation.
[36] Acknowledgments. This work was primarily sponsored by the
U.S. National Science Foundation through grant NSF-SUGR 0519409 thatprovided travel funds for the U.S. team of scientists and engineers to visit
the field sites in Sri Lanka and organize and participate in two informationexchange workshops and a conference in Sri Lanka. L. Douglas James of
NSF provided valuable input during the development of the work plan.
Some of the preliminary information used to develop the proposal was
gathered during field visits and from testing conducted by retired Navy
Commander Champaka Paulis under the sponsorship of the Sri Lanka
Academic Network. The Sri Lanka National Science Foundation and theSoil Science Society of Sri Lanka provided critical local support for theworkshops and conferences. Sirimali Fernando, the chairperson of the Sri
Lanka National Science Foundation was instrumental in organizing local
assistance and providing leadership. Ranjith Mapa of University of Peradeniya provided valuable input and assistance. Organizational support wasprovided by M. C. N. Jayasooriya and Wasantha Amaradasa of the Sri
Lankan National Science Foundation. Charles Harvey of MIT participatedin early phases of this investigation and participated in the conceptual
understanding of the groundwater impacts of tsunami. Extensive input andsupport were also provided by many key water agencies and a number ofuniversities of Sri Lanka. The manuscript benefited greatly from theW05201constructive comments of reviewers J. Hopmans, F. Molz, and C. Simmons.
Support provided by Auburn University graduate students Matthew Hogan
and Rohit Goswami for collecting the laboratory data shown in Figure 3 iskindly acknowledged. Meththika Vithanage, a Ph.D. student at the University of Copenhagen, assisted in field data collection. The field instrumentused in the well testing was donated by In-Situ Inc. of Ft. Collins, Colorado.
The authors gratefully acknowledge all the sponsors, organizations, donors,
and the individuals listed above and numerous others who assisted in thefield trips and workshops.

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T. P. Clement, Department of Civil Engineering, Auburn University,
Auburn, AL 36849-53377, USA. (clement@auburn.edu)
K. J. Cunningham, 11465 SW 110 Lane, Miami, FL 33176, USA.
(kcunning@usgs.gov)
A. Gunatilaka, National Science Foundation of Sri Lanka, 4715 Maitland
Place, Columbo 7, Sri Lanka. (gunat@asianet.lk)
D. W. Hyndman, Department of Geological Sciences, Michigan State
University, East Lansing, MI 48824-1115, USA. (hyndman@msu.edu)
T. Illangasekare, Center for Experimental Study of Subsurface Environmental Processes, Division of Environmental Science and Engineering,
Colorado School of Mines, 1500 Illinois St., Golden, CO 80504, USA.
(tillanga@mines.edu)
K. Jensen, Geological Institute, University of Copenhagen, Øster
Voldgade 10, DK-1350 Copenhagen, Denmark. (khj@geol.ku.dk)
J. J. Kaluarachchi, Department of Civil and Environmental Engineering,
Utah State University, Logan, UT 84321, USA. (jkalu@cc.usu.edu)
J. Obeysekera, Hydrologic and Environmental Systems Modeling
Department, South Florida Water Management District, West Palm Beach,FL 33416-4680, USA. (jobey@sfwmd.gov)

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ILLANGASEKARE: RAPID COMMUNICATION

C. R. Panabokke and A. P. G. R. L. Perera, Water Resources Board, 2A,
Gregory’s Avenue, Colombo-07, Sri Lanka. (lasanthaperera@hotmail.com)
S. W. Tyler, Department of Geological Sciences and Engineering and
Department of Natural Resources and Environmental Sciences, Universityof Nevada, Reno, Reno, NV 89557, USA. (tylers@unr.edu)
M. T. van Genuchten, George E. Brown, Jr. Salinity Laboratory,
Riverside, CA 92507, USA. (rvang@ussl.ars.usda.gov)

W05201K. G. Villholth, International Water Management Institute, P. O. Box
2075, 10120 Colombo, Sri Lanka. (k.villholth@.cgiar.org)
W. W.-G. Yeh, Department of Civil and Environmental Engineering,
University of California, Los Angeles, CA 90095, USA. (williamy@seas.
ucla.edu)

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