Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART D - ANNEXES
Groundwater Assesment Technologies
This section provides an overview of some techniques that have been
found useful in assessing the extent and sustainable yield of freshwater
lenses on small coral islands. More detailed discussion of this topic is
available in Dale et al. (1986) and UNESCO (1991).
To estimate the sustainable yield of a freshwater lens it is necessary
to know the extent (location, width, depth) and behaviour (response to
external influences) of the lens; and, the rate of recharge to the lens.
These parameters provide information about the storage characteristics of
the groundwater and the input (from rainfall) to the groundwater. There
are a number of techniques which can be used to assess the location and
size of freshwater lenses. Some are relatively inexpensive methods and can
be used for preliminary reconnaissance, while more intensive (and
expensive) methods are suited to more detailed investigations and for
On low-lying coral atolls, an initial ground survey combined, where
possible, with aerial photography will provide a reasonable idea of
potential freshwater lens areas and areas where freshwater is unlikely to
occur. The latter areas can be identified by signs of seawater inundation
or by elevations that are close to the high tide level. Exposed water
surfaces (e.g., wells and ponds) can also be identified by these means,
and, if tested with an electrical conductivity (EC) meter, can form the
basis of a surface-salinity map which will give a reasonable indication of
the extent of a freshwater lens on a small island. In addition,
measurements of water table movements can be useful in determining the
mean height of the water table above mean sea level. These data are useful
in setting the levels the mid-depth of the transition zone for
infiltration gallery conduits and pumping facilities (they cannot be used
to determine the thickness of the freshwater lens using the
Ghyben-Herzberg ratio, approximately 40:1, as the sharp interface
assumption of this model does not apply to small islands). Water table
measurements taken close to pumping facilities can be used to identify the
relative influences of natural phenomena on pumping regimes (e.g., the
cyclical response to the tidal signal, hydrograph response to major
recharge events, etc.). They can also be used to determine tidal
efficiencies and lags within the freshwater lenses. Depending on
availability of resources, either manual or automatic recordings can be
made. The advent of relatively cheap electronic data loggers makes this an
effective method of recording, particularly since the data is in digital
format. Electronic data loggers have been successfully used in the Cocos
(Keeling) Islands (Falkland, 1988) and more recently on Aitutaki in the
Cook Islands (Falkland, 1995a).
In the absence of other data, the likely thickness of a freshwater lens
can be estimated using empirical methods. Oberdorfer and Buddemeier (1988)
developed a relationship between freshwater lens thickness (to mid-point
of the transition zone), annual rainfall, and island width:
H/P = 6.94 log a B 14.38 (3)
H = depth from watertable to mid-point of transition zone
P = annual rainfall (m); and
a = island width (m).
Equation (3) is based on observations from nine coral islands (atolls
and raised limestone islands) and indicates that no permanent freshwater
lens can occur regardless of rainfall where the island width is less than
about 120 m. For instance, using the mean annual rainfall of 1 950 mm of
the Cocos (Keeling) Islands, the minimum island width for a small
freshwater lens (say 5 m thick) to occur is about 280 m. This prediction
is in reasonable agreement with observed freshwater lenses. Nevertheless,
it should be noted that other factors which are not accounted for in
equation (3) may also have an effect on the occurrence of freshwater
lenses (e.g., the permeability of the coral sediments, the density of
vegetation, etc.). Recent salinity monitoring studies in the Cocos
(Keeling) Islands (Falkland, 1992, 1995b) showed that a freshwater lens on
West Island, with a thickness of 11 m to the mid-point of the transition
zone, occurs in a location where the island is only 270 m wide. Thus,
empirical relationships should be used only as a preliminary means of
estimation. Further, given the availability of additional and more
detailed data, the development of an updated equation should be
considered. Should such a model be developed, it would be preferable to
develop one for atolls and one for limestone islands, owing to major
differences in permeability and, hence, freshwater lens thickness between
the two island types.
More recently, Underwood et al. (1992) carried out modeling simulations
using the SUTRA groundwater model (Voss, 1984) to derive diagrammatic
relationships for atoll islands between freshwater lens thickness (depth
from mean sea level), annual recharge, and island width (Figure 50). These
relationships are more realistic, predicting thicker freshwater lenses for
a given width and rate of recharge (assumed to be approximately 0.3 x
rainfall) than equation (3), but should still be treated as preliminary
estimates as they do not always match with observed behaviour.
Geophysical methods (mainly, electrical resistivity (ER) and
electromagnetics (EM)) are useful for providing relatively quick and cheap
assessments of freshwater lens locations and thicknesses. However, the
results from ER and EM surveys are subject to differing interpretations
and require independent calibration to be used with confidence. This can
best be provided by vertical salinity profiles obtained from appropriately
drilled and constructed boreholes (see below). Combined with a drilling
program, geophysical techniques offer a particularly suitable means of
assessing freshwater lenses.
Figure 50. Relationship between island width and
simulated depth of portable water at the centre of an atoll island for
four steady recharge rates (after Underwood et al, 1992)
ER surveys are normally conducted, in conjunction with a drilling
program, to assess freshwater lens locations and thicknesses on atolls and
sand islands, including Tarawa (Jacobson and Taylor, 1981) and Kiritimati
(Falkland; 1983a, 1983b, 1984); Pingelap, Federated States of Micronesia
(Ayers and Vacher, 1986); the Cocos (Keeling) Islands (Falkland, 1988;
1992); and, Warraber and Yorke Islands in Torres Strait, Australia
(Falkland, 1990). Careful site selection is required to avoid known metal
cables and pipes, and to orient soundings parallel to the coastline to
minimise violation of the horizontal layering principle on which the ER
method is based (Mooney, 1980).
EM surveys (see Stewart, 1988) have been used successfully on Majuro
Atoll, Marshall Islands (Kauahikaua, 1987); the atolls of Mwoakkilloa,
Pingelap and Sapwuahfik of the Pohnpei State, Federated States of
Micronesia (Anthony, 1992); and, the limestone islands of Tonga (Furness
and Helu, 1993). In general, EM surveys are more rapid than ER surveys but
provide less information. The choice of method is dependent on the
availability of time and funds and the degree of accuracy required.
Freshwater lens thicknesses, based on salinity profiles, and ER
soundings at borehole sites generally indicate close correlation.
Exceptions are found in areas where buried objects are present or where
freshwater lenses are relatively thin. For example, on Kiritimati Island,
Kiribati, the correlation was not good where the lens was thinner than
about 5 m. This was partly due to the relatively thick, unsaturated zone
(typically about 2.5 m to 3 m) which tended to mask the freshwater zone
(i.e., the principle of suppression; Mooney, 1980). In contrast, on South
Keeling, where one lens was found to be relatively thin (maximum thickness
of 8 m), the correlation was generally good for thicknesses less than 5 m.
This was mainly due to a thin unsaturated zone, typically 1 m to 1.5 m
thick, which reduced the suppression effect. Thus, an important feature of
ER soundings is not to rely on any one result, but, rather, to build up an
overall picture from the combined results since individual readings are
susceptible to anomalies. The ratio of ER soundings to the total number of
boreholes should be maximised to reduce the cost of a freshwater lens
assessment programme, and have ranged from 4:1 on South Keeling Atoll
(Home and West Islands), to 10:1 on Tarawa, Kiribati, to 16:1 on South
Keeling Atoll (South Island), to 18:1 on Kiritimati Island, Kiribati. The
combined results of ER soundings and salinity profiles, and, if available,
surface conductivity measurements can be used to derive freshwater lens
boundaries and contour plans of their thicknesses.
Seismic surveys have been used to analyse the subsurface geology of a
number of atolls including Pingelap (Ayers and Vacher, 1986) and Nukuoro
(Ayers, 1990), Micronesia; and, the Cocos (Keeling) Islands (Creighton,
1988). Depths to water table and the unconformity between the upper and
lower sediments can be determined by such surveys. However, they do not
provide estimates of freshwater lens thickness, and, therefore, are not as
useful as ER and EM methods in the conduct of groundwater studies.
The lower surface of the freshwater zone can only be accurately
determined by establishing a recognisable salinity limit for freshwater.
To do this, it is necessary to drill through the lens to find where the
limit occurs. Drilling through to seawater will provide additional data on
the thickness of the transition zone. Apart from providing accurate data
about the variation of salinity with depth (salinity profiles) in
freshwater lenses, a drilling and testing programme can provide control
data for geophysical soundings; and, information about permeability,
porosity, and depth to major hydrogeological features such as solution
channels and unconformities. Such programmes can also facilitate the
installation of permanent monitoring systems. If drilling is done in
conjunction with a geophysical survey, the holes should ideally be drilled
after some idea of the configuration of the freshwater lens is known. Some
holes should be placed adjacent to pumping installations to determine the
effects of pumping on lens configuration.
Drilling of 75 mm diameter holes up to 30 m below ground surface for
groundwater investigations has been successfully undertaken with rotary
rigs on Tarawa (Murphy, 1981), Kiritimati (Murphy, 1982), Cocos (Keeling)
Islands (Murphy, 1988; Murphy and Falkland, 1992), and the islands in the
Torres Strait (Falkland, 1990). An alternative to drilling holes is the
driving of suitable pipes. Steel pipes have been successfully driven to 15
m below ground surface on coral atolls. On Kwajalein, 80 mm diameter pipes
were driven to depths of up to 15 m using an air hammer (Hunt and
Peterson, 1980). Additional monitoring holes have more recently been
drilled on Kwajalein using rotary methods (Hunt et al., 1995). On Majuro,
30 mm diameter pipes were driven with a manually operated drop hammer to
similar depths (Hamlin and Anthony, 1987). Boreholes should be sited to
provide a wide coverage of known and likely freshwater lens areas. A line
of holes from the ocean side to lagoon side of an island enables cross
sectional information on freshwater and transition zones to be determined
(cf., Figure 43). In particularly vulnerable lenses, such as that on Home
Island, Cocos (Keeling) Islands, additional data may be required to
effectively map and monitor the behaviour of the lens.
Coring or inspection of drill cuttings provides useful information about
the subsurface geology including the depth to the unconformity between
upper and lower sediments. Permeability estimates at selected depths in a
number of bores can be effectively obtained using either in-situ falling
head or constant head tests (Hvorslev, 1951). This information is
essential for groundwater modelling work and, hence, these tests or
similar should be conducted if detailed investigations are required.
Typical permeability values obtained for the upper sediments on a number
of coral atolls are 5 to 10 m/day, while the lower sediments have yielded
values from about 50 to greater than 1 000 m/day. Permeability tests on
core samples are not considered suitable as cores are not of sufficient
size to be representative of the aquifer. Also, during drilling, water
samples should be obtained from the base of the hole at depth intervals of
typically 1 m to 3 m, and tested for salinity. Vertical salinity profiles
(salinity versus depth) can then be plotted.
Figure 51. Permanent salinity monitoring system used for
measuring water salinity in freshwater lenses on Cocos (Keeling)
Islands, Warraber and Yoke Islands in Torre
Strait. Similar types have
been installed on Tarawa and Kirimati.
Salinity-monitoring systems enable long term data on the behaviour of
freshwater lenses to be collected and used for the calibration of
numerical models. On coral atolls, open holes or continuously perforated
casings in holes are not considered suitable for the accurate
determination of salinity profiles since the mixing of freshwater and
seawater can easily occur in the hole. This problem is known in mainland
aquifers (Rushton, 1980; Kohout, 1980) and has been demonstrated in coral
atolls (e.g., Enewetak in the Marshall Islands; Buddemeier and Holladay,
1977). Also, contamination of the freshwater zone by underlying saline
water can be introduced if this approach is used. Hence, two types of
salinity-monitoring systems for coral atolls are commonly used, including
the use of multiple holes at each location, terminated at different depths
with the bottom of each hole left open, and the use of single boreholes
with multiple tubes or pipes terminated at a number of pre-determined
depths, between which bentonite (sealing) layers are inserted and gravel
is used to fill the spaces (Figure 51). The multiple hole techniques has
been used on the atolls of Kwajalein (Hunt and Peterson, 1980) and Majuro
(Hamlin and Anthony, 1987), while the single hole technique has been used
on Tarawa and Kiritimati in Kiribati (Murphy, 1981, 1982); Cocos (Keeling)
Islands, Warraber and Yorke Islands in Torres Strait (Falkland, 1990);
and, Diego Garcia in the Chagos Archipelago, Indian Ocean (Surface and
Lau, 1986). Samples can be obtained by bailing or pumping from the bottom
of each hole, or, alternatively, a salinity probe can be lowered to the
base of each hole to obtain measurements; water samples may be pumped to
the surface using a small electric or hand-operated diaphragm pump.
Testing is done, as during drilling, with a conductivity meter and
repeated until readings stabilise. Vertical salinity profiles obtained at
different times can be plotted on the same graph to observe trends (Figure
52). For example, the differences in salinity profiles between Home and
West Islands are evident by a simple comparison of these graphs.
Figure 52. Variation of salinity with depth over the
period from late
1987 to early 1995 at borehole H11, Home Island,
Islands. The limit of the freshwater zone is shown by
the 2500 uS/cm
line and the mid-point of the transition zone by the
25,000 uS/cm line.
Reliable estimates of recharge are required as input to groundwater
models in order to estimate sustainable yields of groundwater resources.
Preliminary estimates can be obtained using the relationship between mean
annual rainfall and mean annual recharge developed by Chapman (1985) based
upon the results of a number of studies of recharge to small low-lying
islands. The graphic form of this relationship has since been further
extended using the results of other studies and is shown in Figure 53.
This Figure shows that a non-linear relationship best fits the data
available, which suggests that, as the mean annual rainfall increases, the
percentage recharge also increases. In contrast, the influence of
vegetation is inversely related to recharge rate on South Keeling Atoll,
Cocos (Keeling) Islands, and Kiritimati (Christmas Island), Kiribati, with
thinner vegetation resulting in a greater rate of recharge. The influence
of climate is also shown in Figure 52. For example, Warraber and Yorke
Islands, located within the Torres Strait between Cape York peninsula,
Australia and Papua New Guinea, have very seasonal rainfall, with about
90% falling during the six-month monsoonal or wet season between the
months of December and May (Falkland, 1990). Hence, these two islands
receive a greater recharge for a given annual rainfall than would be
forecast from the curve because the rainfall effectively occurs for only
one-half of a year (the data for Warraber and Yorke Islands was not used
in developing the curve). This seasonal rainfall effect suggests that two
curves should be developed; one relevant to those islands with marked
seasonal rainfall, and another, as shown, for those islands where rainfall
is more evenly distributed.
Results of work by Nullet (1987), using the Priestley-Taylor method of
determining actual evapotranspiration and a water balance procedure, are
shown on an areal basis in Figure 54. The analysis assumed vegetated areas
on atolls with a rooting depth for coconut trees of 1 m. It is not known
whether similar analyses have been done for other islands outside the
Preliminary estimates of recharge can also be made using a solute
balance approach (e.g., chloride ion). Recharge from rainfall contains a
small concentration of chloride due to airborne sprays from the ocean.
Evapotranspiration subsequently removes some of the water leaving the
chlorides behind in the groundwater. The ratio of the chloride ion
concentration in rainwater to the chloride ion concentration in shallow
groundwater provides an approximation of the ratio of recharge to
rainfall. Ayers (1981) used this approach for estimating recharge on Guam.
However, due to the facts that sea spray may affect samples of rainwater
used for chloride balances, and the groundwater may be more saline than
the recharging water due to saltwater intrusion where there is a thick
transition zone, this method should be considered very approximate and may
not be valid for use on small islands. Notwithstanding, Chapman (1985)
suggests that samples for use in chloride ion balances be obtained from
just below the land surface and just above the water table; although, a
further problem with this method is that, in low-lying areas, the top
phreatic water, which can more easily be sampled than water from the
unsaturated zone, may not have originated locally but may have originated
from the island's interior, where there is generally lower salinity than
the local recharge.
For a more detailed assessment of recharge rates, continuous groundwater
level records can be analysed in conjunction with rainfall, sea level, and
barometric pressure records. This method was used by Furness and Gingerich
(1993) on the island of Tongatapu, Tonga. If the effects of sea level and
pressure changes are removed from the water level hydrograph, recharge can
be estimated from the residual trace and knowledge of the aquifer specific
yield. The water balance method of recharge estimation has often been used
for estimating recharge on small islands. For example, Falkland (1992)
used this method to determine recharge for the island of Bonriki in the
atoll of Tarawa, Kiribati (Figures 35 and 43). This island is the location
of the international airport and is partly cleared, while the rest of the
island is covered by coconut trees and other vegetation. There is a
relatively large and robust freshwater lens underlying the island.
Extensive infiltration galleries have been constructed to enable
groundwater extraction to occur (Figure 43).
The water-balance equation for the surface of a small island, used for
estimating groundwater recharge, can be expressed generally as
P = ETa + SR + R + dV (4)
P = precipitation (most commonly rainfall),
ETa = actual
evapotranspiration (including interception),
SR = surface runoff,
= recharge to groundwater, and
dV = change in soil moisture storage.
by vegetation and other surfaces can be treated as a separate term in the
water balance, but here it has been included with ETa since the
intercepted water is evaporated.
On low islands with permeable soils and subsurface geology (typically,
coral atolls and small limestone islands), there is no surface runoff, and
equation (4) reduces to:
R = P B ETa + dV (5)
Figure 54 shows a water balance model used for estimating
recharge on a typical low coral island with a shallow water table.
As shown in Figure 54, actual evapotranspiration (ETa) is
comprised of three terms:
ETa = EI + ES + TL (6)
EI = evaporation from interception storage,
= evapotranspiration from soil moisture storage, and
transpiration by deep-rooted vegetation directly from groundwater.
Figure 54. Water-balance model to estimate recharge for a
typical coral island.
From equations (5) and (6), the water-balance equation can
be expressed in terms of groundwater recharge as follows:
R = P B (EI + ES + TL) + dV (7)
Normally, it is assumed that rainfall first fills the
interception storage (to a maximum value ISMAX), with the residual (or
overflow) entering the soil moisture zone. Typical values of ISMAX are 1
mm for predominantly grassed areas and 3 mm for areas consisting
predominantly of trees (particularly coconut trees). Evaporation is
assumed to occur from this zone at the potential rate.
Roots of shallow-rooted vegetation (grasses, bushes) and
the shallow roots of trees can obtain water from within the soil moisture
zone (SMZ), which is typically 0.3 m to 0.5 m thick. Water requirements of
vegetation from this zone are assumed to be met before any excess drains
to the water table. A maximum soil moisture limit (SMCMAX, which occurs at
field capacity, FC) and a minimum soil moisture limit (SMCMIN, which
occurs at wilting point, WP) are set. Above the field capacity, water is
assumed to drain to the water table; below the wilting point, no further
evaporation is assumed to occur and the shallow rooted vegetation wilts
and possibly dies. The amount of evaporation from the SMZ is normally
assumed to be linearly related to the available soil moisture content. At
WP, evaporation is assumed to be nil from the SMZ. Potential evaporation
is assumed to reach full potential when the SMZ is at FC. Thus, the
evaporation rate is half that of the potential rate at a soil moisture
content midway between FC and WP.
Vegetation types are assigned "crop factors"
(Doorenbos and Pruitt, 1977). Crop factors of 1.0 and 0.8 were assumed in
water-balance studies for shallow-rooted vegetation and coconut trees
respectively (Falkland, 1983a, 1988a, 1992b). Thus, ETp for coconut trees
is taken as 80% of that for shallow-rooted vegetation which is in turn
assumed to be equal to ETp of a reference crop (Doorenbos and Pruitt,
1977). The proportions of freshwater lens areas covered by deep rooted
vegetation can be estimated from ground observations or aerial
photographs. On islands with a thick coverage of coconut trees, this
proportion is 100%; on islands where limited clearing for houses and
possibly airstrips, the proportion is generally between about 70% and 90%;
while in urban areas with few trees, the proportion is typically less than
30% and may, in extreme cases, be zero.
Excess water remaining after evaporative losses, EI and ES,
enters the water table. This water is assumed in the model to be gross
recharge to the freshwater lens. A further evaporative loss (TL) is
experienced due to transpiration by trees whose roots penetrate to the
water table. Net recharge is that water remaining after TL is deducted
from gross recharge. Observations in pits on a number of atolls reveal
that a considerable number of roots penetrate to the capillary fringe just
above the water table at typical depths of about 1 m to 2 m below ground
level. It is estimated that between 25% and 50% of the roots of mature
coconut trees penetrate to the water table. Because the movement of the
water table is relatively small, even during drought periods, these roots
enable transpiration when the soil moisture storage has been depleted.
Thus, coconut trees are able to survive prolonged drought periods on coral
atolls where other shallow rooted vegetation has wilted and died.
The water-balance procedure is more complex for raised
atolls and limestone islands. Typical depths to water table on these
islands are 10 m to 100 m, and extensive karstic formations often occur.
These islands may have soils of variable thicknesses, unlike low coral
islands, and roots of trees may penetrate through fissures and reach
pockets of water at different levels. Flow paths from the surface to the
water table may have major horizontal components due to karstic formations
(solution channels), in contrast to the essentially vertical flow paths
with low coral islands.
The time step for the surface water balance should not
exceed one day, because the turnover time in the soil zone is measurable
on this time scale (Chapman, 1985). Thus, the use of either mean or actual
monthly rainfall data, rather than actual daily rainfall data, will
underestimate recharge. Two coral atoll studies (Kwajalein, Hunt and
Peterson, 1980; Cocos (Keeling) Islands, Falkland, 1988a) have shown that
the assessed recharge is decreased by between 6% and 10% of the rainfall
volume if actual monthly (rather than actual daily) rainfall data are
used. The latter study showed that the use of mean monthly evaporation
estimates rather than daily evaporation data, however, was acceptable. In
this study, a computer program was written to simulate the water balance
and derive a monthly time series of recharge rates using recorded daily
rainfall and estimated monthly potential evapotranspiration (ETp) volumes.
Computations were made using a daily time interval.
Water Balance for Tarawa
The ETp estimates were derived from mean
monthly pan evaporation data for the period 1981 to 1991, using a pan
coefficient of 0.8. Water balance simulations for the Cocos (Keeling)
Islands showed that similar recharge estimates were obtained from data
sets using either actual monthly values of ETp or mean monthly
values of ETp (Falkland, 1994). This result is consistent with
the generally low interannual variation in potential evaporation for
particular months in humid tropical environments, such as that of Tarawa.
For simplicity, therefore, mean monthly ETp estimates were used for the
Tarawa water-balance studies. The SMZ is typically 300 mm to 500 mm thick,
based on observations from shallow pits and wells. FC was assumed to be
0.15 based on observations of local soil type and typical values for this
type of soil, and WP was assumed to be 0.05 based on typical values for
sand-type soils (e.g., Linsley and Franzini, 1973). The operating range of
soil moisture was assumed to be from 25 mm to 75 mm. The amount of
evaporation from the SMZ was assumed to be linearly related to the
available soil moisture content. The predominant vegetation types on the
island are coconut trees and a variety of grasses and other shallow-rooted
vegetation. Crop factors of 1.0 and 0.8 were assumed for grasses and
shallow-rooted vegetation, and coconut trees, respectively, based upon
values in Doorenbos and Pruitt (1977). Thus, the potential evaporation
rate for coconut trees is taken to be 80% of ETp while that
for grasses or other shallow rooted vegetation is assumed to be equal to
Table 15 shows sample water balance calculations using
daily rainfall data for the island of Bonriki. The results are summarised
in monthly segments over the two years, 1989 and 1990, and are part of a
44-year water balance simulation starting in 1948 and ending in 1991.
Input data were: ISMAX, 3 mm; SMZ, 500 mm; FC, 0.15; WP, 0.05; ratio of
deep-rooted vegetation to shallow-rooted vegetation, 0.8 (i.e., 80% tree
cover); proportion of roots of deep-rooted vegetation reaching water
table, 0.5; proportion of roots reaching the water table, 50%; crop factor
for shallow-rooted vegetation, 1.0; and, crop factor for trees
(predominantly coconut trees): 0.8. Initial values (in 1948) of water in
interception storage and soil-moisture storage were assumed to be 1 mm and
50 mm (these initial values make very little difference to the results,
particularly for long sequences of data such as the 44-year period used in
this example and summarized in Figure 55 for an estimated tree coverage of
80 percent). The results in Table 19 show the effects of a dry year (1989)
and a wet year (1990) on recharge. The total rainfall in 1989 (916 mm) was
only 45% of the long-term mean annual rainfall; by comparison, the
rainfall in 1990 (3 605 mm) was about 80% greater than the mean.
As shown in Table 20 and Figure 55, the recharge in 1989
was negative, since there was a net loss of groundwater of about 170 mm.
Heavy rainfall in November and December of that year prevented this loss
from being even greater. In 1990, the recharge was estimated to be 2 130
mm or 59% of the total rainfall. Over the period from 1948 to 1991, the
mean annual recharge was calculated to be 735 mm or 36% of mean annual
Figure 55. Annual rainfall and recharge for Tarawa,
Kiribati, 1948-1991. Recharge is for the condition of 80% tree cover,
which approximates current tree desity.
From Figure 55, years with high-annual rainfall generally
result in high-annual recharge and vice versa. The relationship is not
linear as recharge is dependent not only on total rainfall but on the
distribution of daily rainfall. The most critical times for freshwater
lenses are where there are a succession of low-recharge years. Recharge is
even more variable at a monthly scale than at an annual scale. Annual
values of the three evapotranspiration components and corresponding
recharge values are shown in Figure 56 for the same period. The relative
proportions of the four sets of annual components, which when summed equal
the annual rainfall values, can be clearly seen. The effect of deep-rooted
vegetation, primarily coconut trees, on the water balance is very
significant. This effect can be analysed with the model. The percentage
area covered by deep-rooted trees as a proportion of the total freshwater
lens area was varied from 80% to 0%, and the water balance model was run
for the full period of 44 years. The effects of different percentages of
trees are summarised in Figure 57. If all trees were removed, the mean
annual recharge would increase to about 1 020 mm or about 50% of mean
annual rainfall. Evapotranspiration losses would be restricted to the soil
moisture zone and average about 1 000 mm. Assuming Bonriki had been
totally cleared of trees, the recharge estimates for 1989 and 1990 were
268 and 2 264 mm, respectively, which are equivalent to 29% and 63% of the
respective annual rainfalls. The differences between rainfall and recharge
in the two years (71% and 27%, respectively) represent evapotranspiration
from the soil moisture zone.
Once the location and volume of a freshwater lens is known,
and the recharge conditions have been estimated, the flow through the lens
and the sustainable yield of the lens can be estimated. Sustainable (or
safe) yield estimates are required to enable groundwater extraction
systems to be properly planned and managed. Flow through freshwater lenses
is influenced by hydrological (variable recharge) and geological (variable
permeabilities with depth and distance) factors, as well as by tidal
movements and human influences such as water abstraction. The sustainable
yield of an aquifer is the rate at which water can be extracted without
causing adverse effects. For non-coastal mainland aquifers, the
sustainable yield can be approximated by the long-term recharge rate. For
freshwater lenses on small island and some coastal-mainland aquifers, such
an approximation is not valid as only a small portion of the recharge is
available as sustainable yield; most of the recharge is required for
maintenance of the lens by literally flushing away salt at the top of the
transition zone. To avoid adverse effects from extraction (i.e., to avoid
an increase in the salinity of extracted water), the pumping rate from the
lens should not exceed the sustainable yield. An additional requirement is
that pumping be distributed over the surface of the lens to avoid local
upconing of saline water. Methods for estimating sustainable yield range
from simple empirical approaches to complex numerical models. Whatever
method is chosen for a particular situation, it is important that the
effects on the lens of any recommended pumping scheme be monitored.
Ideally, this should involve regular monitoring of salinity profiles in
56. Variations in annual evapotranspiration components (EI,
ES, and TI) and annual
Bonriki, Tarawa for the period 1948-1991. YEAR: 1990.
Empirical methods generally assume that a proportion of
average recharge can be safely extracted. They also assume that there is
sufficient freshwater stored in the lens to allow for dry or drought
periods when recharge is low or nil.
Figure 57. Relationship between recharge and coconut-tree
the Bonriki freshwater lens, Tarawa.
Mather Procedure: Mather (1975) developed a
procedure which enabled an estimate of yield to be made by reducing
recharge and analysing the effect on the thickness of the lens. His
analysis is based on an equation by Henry (1964) that relates the
thickness of the lens (assuming a sharp interface) to permeability,
distance from shore, and recharge. Mather's method includes consideration
of extended drought periods. As the method assumes a sharp interface, it
can only be used for thick freshwater lenses wherein the transition zone
tends to be thin. This method was used by Jacobson (1976) in a study of
the freshwater lens on Home Island, Cocos (Keeling) Islands. Based on an
estimated freshwater lens thickness of 15 m (based on head measurements),
he concluded that the sustainable yield was 200 m3/day. Subsequent
investigations (Falkland, 1988, 1992) showed that, although the freshwater
lens was much thinner (approximately 2 m to 6 m), the aquifer has been
able to sustain a pumping rate of about 115 m3/day. More recent monitoring
indicates that this rate may be able to be increased to about 150 m3/day
Maximum Unit Pumping Rate: Another approach is to
use an estimate of the maximum allowable pumping rate per unit area, or
per unit length for a freshwater lens, derived from detailed studies for
similar hydrogeological environments. The water balance within a small
island groundwater system can be expressed in its simplest form as:
R = O + Q + dV (8)
R = recharge into the lens after all
evapotranspiration losses have been taken into account, including
transpiration directly from the lens by deep-rooted vegetation,
flow through the lens (or 'flux') which either flows out at the edge of
the lens or mixes with the transition zone at the base of the lens,
= total amount of water pumped from the lens, and
dV = change in
Equation (8) is particularly applicable to
relatively simple groundwater systems such as freshwater lenses on small
coral islands and atolls. In equation (8), the value of R is estimated
from either a water balance at the surface or other method. Further, due
to a longer turnover time within the groundwater system, a monthly time
step for the groundwater balance is acceptable; often the turnover time of
the fresh groundwater on a small island is a number of years (even for
relatively small freshwater lenses on a coral atoll, the turnover time is
generally greater than 12 months). Thus, in the long term, dV tends to be
negligible and equation (8) can be rewritten as:
R = O + Q (9)
Mink (1976), in a study of a freshwater lens on Guam,
suggested an extraction rate equal to 25% of flux (or flow) through the
lens as a good first approximation of the sustainable yield. This approach
was used as a guide in estimating sustainable yields on the atolls of
Kwajalein (Hunt and Peterson, 1980) and Kiritimati (Falkland, 1983). In
the latter study, this preliminary estimate was used in conjunction with a
sharp interface model to analyse the effects on a number of freshwater
lenses on that island.
Equation (8) implies that pumping, Q, should be less than
20% of the recharge based on the condition that pumping should be less
than 25% of flow through the lens. Given that recharge on many small
islands is in the order of 25% to 50% of the rainfall volume in the long
term, the allowable pumping rate (or sustainable yield) translates to
about 5% to 10% of mean rainfall. However, in relatively large and stable
lenses, the sustainable yields may approximate 30% or even 40% of the
average recharge, while, for thin and fragile lenses as on Home Island,
Cocos (Keeling) Islands, it is considered prudent to maintain the pumping
at less than 20% (currently 17%) of average recharge, at least until
further monitoring data has been collected.
Modelling: Modelling of freshwater lenses can
provide more exact estimations of sustainable yields provided that there
are sufficient data to justify their use. Early conceptual models and
solution techniques for freshwater lens flow assumed a sharp interface
between freshwater and seawater based on the Ghyben-Herzberg theory (Badon
Ghijben, 1889; Herzberg, 1901) and the Dupuit assumption of horizontal
flow. Observations have shown that this is not the case on atolls, where
wide transition zones are the norm, and where these types of models tend
to overestimate the freshwater resource. A more realistic, conceptual
freshwater lens flow model has evolved (Buddemeier and Holladay, 1977;
Wheatcraft and Buddemeier, 1981; Oberdorfer et al., 1990;
Underwood et al., 1992) based upon detailed observations on
atolls. The conceptual model accounts for vertical and horizontal tidal
propagation through a dual aquifer system consisting of the upper
(Holocene) coral sediments and lower (Pleistocene) limestone layer
(Figures 58 and 59). This conceptual model is supported by observations on
a number of atolls in the Pacific (Hunt and Peterson, 1980; Ayers and
Vacher, 1986; Anthony et al., 1989) and on the Cocos (Keeling)
Islands (Falkland, 1988). These studies have shown that tidal lags and
efficiencies at water level monitoring locations within atolls are largely
independent of their horizontal distance from the shore. Tidal lags and
efficiencies (or the time difference between, and amplitude ratio of,
water table movement to tidal movement) are, in fact, greatly influenced
by the depth of the holes used for water level monitoring. Vertical
propagation of tidal signals tends to be dominant in the middle of the
island whereas both horizontal and vertical propagation are significant
near the edges.
Figure 58. Cross section through a small coral island
showing main features of a
freshwater lens (exaggerated vertical
scale) and location of an infiltration gallery.
Using this conceptual model, the numerical solution of
freshwater lens flow problems can more realistically be computed using
dispersion (variable-density) models rather than sharp interface models.
Dispersion models are available which account for a two-layered
hydrogeologic system with flows of variable density water, and mixing of
freshwater and seawater. Dispersion models are inherently more complex,
requiring additional parameters to be evaluated or estimated, than sharp
interface models. However, both types of models can be run on personal
computers, although dispersion models take much longer to run.
Figure 59. Conceptual flow model of a freshwater lens
with a two-layer (dual aquifer) system (after Underwood et al., 1992)
One such dispersion model, SUTRA, developed by the United
States Geological Survey (Voss, 1984), was applied to the study of
freshwater lenses and coastal aquifers on a number of islands. This model
uses finite elements to solve the equations, rather than the finite
difference method used in the sharp interface models. Case studies of
atolls and small carbonate islands using the SUTRA model include Enewetak
Atoll, Marshall Islands (Oberdorfer and Buddemeier, 1988; Oberdorfer et
al., 1990), Majuro Atoll, Marshall Islands (Griggs and Peterson; 1989,
1993), and Nauru, a raised atoll (Ghassemi et al., 1990). A
generic atoll was analysed for different conditions by Underwood et al.
(1992). Griggs and Peterson (1989, 1993) modelled the freshwater lens on
Laura, an island of the Majuro Atoll in the Marshall Islands, Pacific
Ocean, using the SUTRA model. They showed that the effects of pumping at
20% of mean annual recharge were minor, while pumping at 40% and 60% of
mean annual recharge caused major upconing and destruction of the lens,
Other methods and examples of recharge estimation for small
islands are described in UNESCO (1991).
Tony Falkland, ACTEW Corporation, Australia
Anthony, S.S. 1992. Electromagnetic Methods for Mapping
Freshwater Lenses on Micronesian Atoll Islands. Journal of Hydrology,
Anthony, S.S., F.L. Peterson, F.T. MacKenzie, and S.N.
Hamlin 1989. Geohydrology of the Laura Fresh-water Lens, Majuro Atoll, A
Hydrogeochemical Approach. Geological Society of America Bulletin,
Ayers, J.F. 1981. Estimate of Recharge to the
Freshwater Lens of Northern Guam. Water Resources Research Center
Technical Report No. 21, University of Guam, Guam. 20 pp.
Ayers, J.F. 1990. Shallow Seismic Refraction Applied to a
Ground Water Investigation on Nukuoro Atoll, Micronesia. Journal of
Ayers, J.F. and H.L. Vacher 1986. Hydrogeology of an Atoll
Island, A Conceptual Model from a Detailed Study of a Micronesian Example.
Badon Ghijben (Ghyben), W. 1889. Nota in Verband Met De
Voorgenomen Putboring Nabij Amsterdam. Tijdschrift het Koninklijk
Instituut voor Ingenieurs, 8-22.
Buddemeier, R.W. and G.L. Holladay 1977. Atoll Hydrology,
Island Ground-water Characteristics and Their Relationship to Diagenesis.
In: Proceedings of the 3rd International Coral Reef Symposium,
Chapman, T.G. 1985. The Use of Water Balances for Water
Resource Estimation with Special Reference to Small Islands. Pacific
Regional Team Bulletin No. 4, Australian Development Assistance Bureau
Creighton, A. 1988. Geophysical Report. Cocos (Keeling)
Islands Water Resources and Management Study. Scientific Services
Branch Report No. 88/G8, Australian Construction Services, Canberra.
Dale, W.R., B.C. Waterhouse, G.F. Risk, and D.R. Petty
1986. Coral Island Hydrology, A Training Guide for Field Practice.
Commonwealth Science Council Technical Publication Series No. 214.
Doorenbos, J. and W.O. Pruitt 1977. Crop Water
Requirements. FAO Irrigation and Drainage Paper 24 (revised). Food and
Agriculture Organisation of the United Nations, Rome. 144 pp.
Falkland, A.C. 1983a. Christmas Island (Kiritimati)
Water Resources Study. Australian Department of Housing and
Construction Report No. HWR83/03, Australian Development Assistance
Bureau, Canberra. 425 pp. + appendices.
Falkland, A.C. 1983b. Groundwater Resources Study of
Christmas Island, Republic of Kiribati. In: Proceeding of the
International Conference on Groundwater and Man, Australian Water
Resources Council Conference Series 8, Part 3, 47-56.
Falkland, A.C. 1984. Assessment of Groundwater Resources on
Coral Atolls: Case Studies of Tarawa and Christmas Island, Republic of
Kiribati. In: Proceedings of the Regional Workshop on Water Resources
of Small Islands. Commonwealth Science Council Technical Publications
Series No. 154, Part 2, 261-276.
Falkland, A.C. 1988. Cocos (Keeling) Island Water
Resources and Management Study. ACT Electricity and Water Report No.
HWR88/12, ACT Electricity and Water, Canberra. 211 pp. + appendices.
Falkland, A.C. 1990. Warraber and Yorke Islands, Torres
Strait, Groundwater Resources Study. ACT Electricity and Water Report
No. HWR90/1, ACT Electricity and Water, Canberra. 98 pp. + appendices.
Falkland, A.C. 1992a. Review of Tarawa Freshwater
Lenses, Republic of Kiribati. ACT Electricity and Water Report No.
HWR92/681, ACT Electricity and Water, Canberra.
Falkland, A.C. 1992b. Review of Groundwater Resources
on Home and West Islands, Cocos (Keeling) Islands. ACT Electricity and
Water Report No. HWR92/1, ACT Electricity and Water, Canberra.
Falkland, A.C. 1992c. Further Investigations of the
Northern Freshwater Lens, West Island, Cocos (Keeling) Islands. ACT
Electricity and Water Report No. HWR92/002, ACT Electricity and Water,
Falkland, A.C. 1994a. Review of Vaipeka Gallery
Extension and Water Supply Needs. Australian International Development
Assistance Agency, Canberra.
Falkland, A.C. 1994c. Management of Freshwater Lenses on
Small Coral Islands. In: Proceedings of. Water Down Under '94
Conference, University of Adelaide, Adelaide. 1:417-422.
Falkland, A.C. 1995a. Vaipeka Water Gallery Extension
Project, Aitutaki, Cook Islands. Australian Agency for International
Development, Canberra. 62 pp. + appendices.
Falkland, A.C. 1995b. Water Monitoring Annual Report.
ACT Electricity and Water Report No. HWR95/04, ACT Electricity and Water,
Furness, L.J. and S. Gingerich 1993. Estimation of Recharge
to the Fresh Water Lens of Tongatapu, Kingdom of Tonga. In: Hydrology
of Warm Humid Regions, IAHS Publication 216, 317-322.
Furness, L.J. and S.P. Helu 1993. The Hydrogeology and
Water Supply of the Kingdom of Tonga. Ministry of Lands, Survey and
Natural Resources, Tonga. 143 pp.
Ghassemi, F., A.J. Jakeman, and G.J. Jacobson 1990.
Mathematical Modelling of Sea Water Intrusion, Nauru Island. Hydrological
Griggs J.E. and F.L. Peterson 1989. Groundwater Flow
and Development Alternatives: a Numerical Simulation of Laura, Majuro
Atoll, Marshall Islands. Water Resources Research Centre Technical
Report No.183, University of Hawaii.
Griggs, J.E. and F.L. Peterson 1993. Ground-Water Flow
Dynamics and Development Strategies at the Atoll Scale. Ground Water,
Hamlin, S.N. and S.S. Anthony 1987. Ground-water
Resources of the Laura Area, Majuro Atoll, Marshall Islands. Water
Resources Investigations Report No. 87-4047, U.S. Geological Survey,
Hawaii. 69 pp.
Hunt, C.D. and F.L. Peterson 1980. Groundwater
Resources of Kwajalein Island, Marshall Islands. Water Resources
Research Centre Technical Report No. 126, University of Hawaii. 91 pp.
Hunt, C.D., S.R. Spengler, and S.B. Gingerich 1995.
Lithologic Influences on Freshwater Lens Geometry and Aquifer Tidal
Response at Kwajalein Atoll. In: Proceedings of the American Water
Resources Association Symposium, AWRA, Bethesda. 10 pp.
Jacobson, G. 1976. Preliminary Investigation of
Groundwater Resources, Cocos (Keeling) Islands, Indian Ocean, 1975.
Bureau of Mineral Resources Record No. 1976/64, Canberra.
Jacobson, G. and F.J. Taylor 1981. Hydrogeology of
Tarawa Atoll, Kiribati. Bureau of Mineral Resources Record No.
Kauahikaua, J. 1987. Description of a Fresh Water Lens
at Laura Island, Majuro Atoll, Republic of the Marshall Islands, Using
Electromagnetic Profiling. U.S. Geological Survey Open-File Report No.
87-0582, USGS, Reston.
Kohout, F.A. 1980. Differing Positions of Saline Interfaces
in Aquifers and Observation Boreholes - Comments. Journal of Hydrology,
Mather, J.D. 1975. Development of the Groundwater Resources
of Small Limestone Islands. Quarterly Journal of Engineering Geology,
Mooney, H.M. 1980. Handbook of Engineering Geophysics,
Vol. 2, Electrical Resistivity. Bison Instruments Inc., Minnesota.
Murphy, P. 1981. Tarawa Water Supply Investigation
(Drilling Report). Central Investigations and Research Laboratory
Report No. 232 (81/G3), Australian Government Department of Housing and
Murphy, P. 1982. Kiritimati (Christmas Is.) Water
Resources Investigation, Drilling and Testing. Scientific Services
Branch Report No. 82/G34, Australian Government Department of Housing and
Murphy, P. 1988. Drilling Report. Cocos (Keeling)
Islands Water Resources and Management Study. Scientific Services
Branch Report No. 88/G6, Australian Construction Services, Canberra.
Murphy, P. and A.C. Falkland 1992. Review of
Groundwater Resources on Home and West Islands. Volume 2, Drilling Report
and Appendices to Volume 1. Australian Construction Services,
Department of Administrative Services, Canberra.
Nullet, D. 1987 Water Balance of Pacific Atolls. Water
Resources Bulletin, 23(6):1125-1132.
Oberdorfer, J.A. and R.W. Buddemeier 1988. Climate Change:
Effects on Reef Island Resources. In: Proceedings of the Sixth
International Coral Reef Symposium, Townsville, 3:523-527.
Oberdorfer, J.A., P.J. Hogan, and R.W. Buddemeier 1990.
Atoll Island Hydrogeology: Flow and Fresh Water Occurrence in a Tidally
Dominated System. Journal of Hydrology, 120:327-340.
Rushton, K.R. 1980. Differing Positions of Saline
Interfaces in Aquifers and Observation Boreholes. Journal of Hydrology,
Stewart, M. 1988. Electromagnetic Mapping of Fresh-water
Lenses on Small Oceanic Islands. Groundwater, 26(2):187-191.
Surface, S.W. and E.F. Lau 1986. Fresh Water Supply System
Developed on Diego Garcia. The Navy Civil Engineer, XXV(3):2-6.
UNESCO [United Nations Education Scientific and Cultural
Organization] 1991. Hydrology and Water Resources of Small Islands, A
Practical Guide. Studies and Reports on Hydrology No. 49, UNESCO,
Paris. 435 pp.
Underwood, M.R., F.L. Peterson, and C.I. Voss 1992.
Groundwater Lens Dynamics of Atoll Islands. Water Resources Research,
United Nations 1986. Water Resources Legislation and
Administration in Selected Caribbean Countries. United Nations Natural
Resources/Water Series No. 16, 163 pp.
Voss, C.I. 1984. SUTRA: A Finite-element Simulation
Model for Saturated-unsaturated, Fluid-density-dependent Ground-water Flow
with Energy Transport or Chemically-reactive Single- Species Solute
Transport. U.S. Geological Survey Water- Resources Investigation
Report No. 84-4389, USGS, Reston. 409 pp.
Wheatcraft, S.W. and R.W. Buddemeier 1981. Atoll Island
Hydrology. Groundwater, 19(3): 311-320.