ῐ
J. Jpn. Soc. Soil Phys.
῏῍ῌῐῑ῎
No. 22, p. ,1ῌ-/ ῍,**+῎Application of Electromagnetic Technique to
Identify Recharge and Discharge Areas for
Reforestation in Northeastern Thailand
Chaiyanam DISSATAPORN*, Hajime NARIOKA**, Pramot YAMCLEE* and Somsri ARUNIN** Land Development Department, Bangkok, Thailand** Okayama University, Okayama, Japan
Abstract
Areas a#ected by salinization in northeastern Thailand were studied using data obtained fromground geophysics methods, multi frequency electromagnetic induction terrain conductivity
meter, and hydrological data from the routine piezometer method. The electromagnetic technique is not only a useful tool for identifying the spatial distribution of salt content in landscapes,
but it may also be applicable for delineating local recharge and discharge areas by comparing
results with hydrological data. It could also aid in reforestation planning in large-scale areas forrehabilitation of salt-a#ected soils.
Keywords : Electromagnetic induction terrain conductivity meter (EITCM), Salinization, Recharge and discharge areas, Reforestation, Northeastern Thailand

Introduction
Secondary salinization of salt-a#ected soils
in northeastern Thailand is mainly due to therising of shallow saline groundwater in discharge areas after clearing of native Dipterocarpus forest in recharge areas. The source ofadditional recharge to groundwater discharge
aquifers is rainfall, which percolates throughthe root zone of annual, short-rooted and lowwater-use cash crops in cleared lands. Salt isthen transported and accumulated on soil surfaces by evapotranspiration and capillaryaction from rising shallow saline groundwaterin discharge areas. Halite, of the Mahasarakram Formation founded at the depth of 0* to+** m from soil surface, is widely accepted as asource of salt in the process of soil salinization.
A remedial measure for controlling shallow
saline groundwater in discharge areas is toutilize recharge water before it reaches groundwater aquifers (Williamson et al., +323).
Reforestation in recharge areas is recognized

as one land management technique for minimizing groundwater recharge. Perennial, deeprooted, fast-growing and high-water-use trees
are planted in recharge areas where groundwater tables rapidly respond to rainfall. The
goal of this technique is to intercept and utilize
precipitation before it percolates through theroot zone to groundwater aquifers (Sedgley etal. +32+).
Regarding this method, it is necessary todeﬁne signiﬁcant recharge areas before reforestation schemes are begun (Bullock andWilliams, +321). They can be roughly delineated from topographic, soil and land surface
properties but the most accurate, althoughcostly and time-consuming technique, is to
study groundwater characteristics (Freeze andCherry, +313).
The electromagnetic terrain conductivity
technique is a simple and rapid method fordelineating the distribution of salt on surface
and subsurface soils at depth of 1./, +/ and -* mfrom the soil surface (McNeil, +32*). The low῏28  22

conductivity of upland soils indicates recharge
areas while high conductivity indicates a salinity hazard as well as discharge areas (Bullock
and Williams, +321, and Engle et al.,+321).
Williams and Arunin (+33*) employed the
EITCM for predicting recharge and discharge
areas from the empirical average ratio of deep
to shallow depths of sounding at Nakorn
Ratchasima in northeast Thailand. Ratios ofgreater than or less than one indicate recharge
and discharge areas, respectively. Area where
terrain conductivity increases with reading
depth indicates recharge areas due to adequate
leaching of salt and nutrients. In the discharge
areas, where salt moves from shallow saline
groundwater to soil surfaces, readings decrease
with depth of transverse.
The objective of this research was to study
the application of the EITCM for delineating
recharge areas to be reforested for salinity control.
Study area and methodology
Salt a#ected soils in Ubonrachathani province were selected for studying the applicationof electromagnetic terrain conductivity formapping recharge areas.
The study area
covers -0* square km. Annual average rainfall
and evaporation is +,/11 and ,,*/. mm, respectively. Soil proﬁle description and type ofrocks from soil surface to -* m depth wererecorded during the installation of piezometer.
Soils in high terrace were silt and sand while
low elevated area soils were silt with increased
in clay content. It was founded that soils were
underlain with shale, sandstone and siltstoneof the Mahasarakram Formation but no salt
layers were detected.
The hydraulic head of groundwater fromnested piezometers at depths of /, +*, +/, ,* and-* meters from soil surface (U+U3*) was used
for mapping recharge and discharge areas.
The horizontal hydraulic gradients of groundwater at a depth of / meter from soil surfaces
were contoured and ﬂow lines of groundwater
were drawn (Surfer for Windows, +330). The,**+

vertical hydraulic gradient between each
depth of nested piezometer was calculated.
The downward and upward vertical hydraulicgradient was identiﬁed as recharge and discharge areas, respectively (Freeze and Cherry,+313).
The EITCM measures the bulk apparent electromagnetic terrain conductivity (Eca) of soluble salt concentrations in soils (mS/m) to
depths of 1./, +/ and -* m from the soil surface.
The instrument consists of transmitter and receiver coil with diameter of 0- cm and weightof - and 1., kg, respectively. They are connected to transmitter and receiver meter, respectively. The coil conﬁgurations are either vertical or horizontal and are +*, ,* and .* meterapart (Fig. +). The transmitter coil generates
primary electromagnetic currents in the soil
proﬁle, which induce the secondary magnetic
ﬁeld from soluble salts in the soil proﬁle. The
magnitude of secondary magnetic ﬁeld depends on the concentration of soluble salts,
moisture and clay contents. The receiver coildetermines the primary and secondary magnetic ﬁeld and calculates the ratio between the
two. This ratio yields the apparent electricalconductivity of the soil proﬁle. The depth ofsoil measured depends on the current frequency, distance between the transmitter and receiver coils and the coil arrangement relative
to the soil surface. The coils in the horizontal
dipole mode provide the depth of sounding of
1./, +/ and -* m from the soil surface (McNeil,+32-).
The EITCM survey was carried out at grid of+ km and at each nested piezometer using a +*,,* and .* m coil separation and horizontal
dipoles. The apparent electromagnetic conductivity at depths of 1./, +/ and -* m was obtained. Surfer for Windows was used for contouring the apparent electromagnetic conductivity of the soil proﬁle at depths of 1./, +/ and-* m from soil surface (Surfer for Windows,+330). Salinity is classed into non-salt-a#ectedsoils (*2* mS/m), slightly salt-a#ected soils(2*+,* mS/m), moderately salt-a#ected soils

ῒῑ : Application of Electromagnetic Technique to Identify Recharge and Discharge Areas for Reforestation in Northeastern Thailand 29

Results

Fig. +

Electromagnetic induction terrain conductivity meter (EITCM), in horizontal
and vertical dipole mode.

(+,*ῌ+0* mS/m), severely salt-a#ected soils(+0*ῌ,** mS/m) and very severely salta#ected soil (ῐ,** mS/m).
Williams and Arunin (+33*) proposed thatsalinity at di#erent depths in the soil proﬁledetermined from EITCM measurements could
predict recharge and discharge areas. Areas inwhich Em values increase or decrease with thedepth of the soil proﬁle can be identiﬁed asrecharge and discharge areas, respectively.
Thus, the EM slope can be derived as
EM slope῎((EM-*/EM+/)῍(EM-*/EM1./)
῍(EM+//1./))/ῌ
Recharge area EM slope ῐ +῍Discharge area EM slope ῏ +῎EM1./ : Apparent electrical conductivity
from soil surface to 1./ m depth
EM+/ : Apparent electrical conductivity
from soil surface to +/ m depth
EM-* : Apparent electrical conductivity
from soil surface to -* m depth
The EM slope formulated from this formula
was then contoured using Surfer for Windows.
The contour of the EM slope was overlain on
the recharge and discharge area, which delineated the vertical and horizontal hydraulicgradient of groundwater from the same area.

Groundwater hydrology and the electromagnetic technique was used for investigation of
the development of salt-a#ected soil in northeastern Thailand
Groundwater hydrology
Data from nested piezometers was collectedfrom April +330 to March +332. The distribution of horizontal hydraulic head and groundwater ﬂow lines at a depth / meter from soil
surfaces were drawn using data gathered in
April +331. Groundwater moved from the elevated areas (+-*ῌ+.* m above mean sea level)to the lower areas of the study area. The
pattern of groundwater ﬂows were the same
during the period of study. The vertical hydraulic gradients between nested piezometer
were calculated.
Nested piezometers withdownward and upward vertical hydraulic gradients were identiﬁed as recharge and discharge areas, respectively. It was found thatmost of the recharge areas were situated primarily over the horizontal hydraulic head of+,, meter. The depth of ground water ranged
from ,.. to /.+* m from the soil surface in therecharge areas, while that of the discharge
areas was in the range of +.3/ to -.2 m from soil
surface (Fig. ,).
Electromagnetic technique
Soil salinity maps of the study area wereprepared and are shown in Fig. -. The measured apparent electromagnetic terrain conductivity from soil surface to 1./ m depth ranged
from 0 mS/m in the recharge areas to +3/ mS/
m in the discharge areas. All recharge areas
identiﬁed by the groundwater hydrology wereclassiﬁed as non-salt a#ected soils, while thedischarge areas were classiﬁed as slightly toseverely salt-a#ected soils. The severely salta#ected soil occurred in small areas in thevicinity of U-3 and U+/ and U+0. The bulk
apparent electromagnetic terrain conductivityof soils located +/ and -* m from soil surface
ranged from +*ῌ+3/ and 2ῌ,,* mS/m, respectively. The boundary of salt a#ected area atΐῐ῎῔῕ῑ ῒ 22 ῏ ῌ,**+῍

30

Fig. ,

Distribution of horizontal hydraulic head (m), ﬂow lines of groundwater measured inpiezometers at a depth of / m from soil surface in April +331 and recharge [R] anddischarge area [D]. Discharge area identiﬁed from vertical hydraulic gradient (map scale isin meter).

῍ῌ : Application of Electromagnetic Technique to Identify Recharge and Discharge Areas for Reforestation in Northeastern Thailand 31

Fig. - a, b, c

The apparent electromagnetic terrain conductivity (mS/m) from soil surface to 1./ m(a), +/ (b) and -* m (c) depth (map scale is in meter).

the depth of +/ and -* m from soil surface
increased with the depth of investigation. The
boundary of severely salt a#ected soil was
found at the depth of -* m.
Recharge and discharge evaluation
The frequency distribution of EM slopes
ranged from *.1/ to -./*. The EM slope wasdistributed around +.,2. The relationship between the recharge areas and EM slope is il-lustrated in Fig. .. The result of this overlay
shows that the EM slopes in the recharge areas
were more than + and could be classiﬁed asrecharge areas. The EM slopes which werelower than + were situated in the salt-a#ectedareas near U.+, U., and U/+. But in the rest of
the salt-a#ected soils or discharge areas, EM
slopes were greater than +.
High values of EM slopes were found in veryΐῐ῎῔῕ῑ ῒ 22 ῏ ῌ,**+῍

32

Fig. .

The distribution of EM slope (map scale is in meter).

low apparent electromagnetic terrain conductivity upland areas which were classiﬁed as
non-salt-a#ected soils, while the EM slopes insalt-a#ected soils ranged from *.1, to +./2. The
frequency distribution of EM slopes in the discharge areas showed that only +*ῌ were lessthan +.
Discussion
Salinization process
The salinization process is a#ected by waterand salt sources, as well as the mechanism of
salinization. The major source of water in local
recharge areas is rainfall. Recharge occurredin upland areas where coarse-texture-soil, sandand silt exist in the soil proﬁle. The distribution of vertical hydraulic gradient shows thedownward movement for the period of time,
which is longer than the upward vertical hydraulic gradient. The electrical conductivity ofgroundwater in the recharge area is low.
Groundwater salinity increases along the path-῍ῌ : Application of Electromagnetic Technique to Identify Recharge and Discharge Areas for Reforestation in Northeastern Thailand 33

way of groundwater ﬂow. The ionic composition of salty groundwater is dominated by Na
and Cl. The ratio of Na and Cl toward + withincreasing the total dissolved solid (TDS) indicates that halite (NaCl mineral) dissolution is
the major source of salt (Williamson et al.,+323).
Groundwater in the discharge area rises towards the soil surface to the depth where capillary rise can occur because of the upward
hydraulic gradient. The evaporation of saltwater leaves salt in the soil proﬁle. The horizontal
hydraulic gradient of salty groundwater insalt-a#ected soil was very low, which allowed
time for the salinization process. The di#erence in the salinity hazard of salt-a#ected soil
is due to the magnitude of the vertical andhorizontal hydraulic gradient in the recharge
and discharge areas, the period of upward anddownward hydraulic gradient, and the depthof groundwater from the soil surface. Excesswater in discharge areas needs to be manipulated by minimizing water input in the recharge areas and water output in the discharge
areas.
Salinity level
The EITCM was adopted for delineating saltstorage in the landscape. Low Eca readings
were found in the upland areas indicating thatrecharge is dominant in these areas. High apparent electrical conductivity was found in thelower slopes and alluvial plains, indicating thatsalt was discharged and accumulated after
evaporation in the surface or near-surface soil.
Three depths of sounding provided the distribution of salt content in the soil proﬁle. The
increasing Eca reading with the depth of sounding in the upland recharge area indicates leaching of soluble salts or nutrients in the soil
proﬁle. The reverse of this phenomenon is seenin discharge areas where shallow salty groundwater is discharged to soil surface (Williams
and Arunin, +33*). From this study it wasdetermined that salinity in discharge areas increased with depth of sounding. This is probably because these areas are underlain withhighly electrically conductive material which
is being mobilized from the depth deeper than-* m to the soil surface by groundwater.
The empirical EM slope is developed forpractical determination of salt content in recharge and discharge areas. All of the identiﬁed local recharge areas had EM slope values
greater than + and apparent electromagnetic
terrain conductivity of non-salt-a#ected soil.
The EM slope in the salt-a#ected soils ranged
from less than + to more than + due to a highsalt concentration in the deeper depths. These
areas may be interpreted as recharge areas.
From the results presented here, it can be seenthat EM slope is not the proper tool for identifying recharge areas because the value of the
EM slope, which is used to divide recharge anddischarge areas, varies from place to place.
The distribution of the EM slope and the salt
concentration in the soil proﬁle should be examined and rejected when the EM slope isgreater than + in salt-a#ected soil. In addition,
one should consider the relationship of highEM slopes and the amount of recharge togroundwater aquifers. Even though the electromagnetic technique is not successful for
identifying recharge areas properly, salt distribution in the soil proﬁle is important data forreforestation program.
Conclusion
The process of salinization can be determined from routine piezometer method thatrecharge from rainfall occurred in the uplandarea and discharged to the alluvium plain
where upward hydraulic gradient was dominated phenomenon. The source of salt mightcome from the Mahasarakram formation
which underlies salt-a#ected areas.
The geophysics technique was adopted toinvestigate hydrosalinity in salt-a#ected soils.
The mapping of soil salinity by EITCM wasproven to be very useful in detecting anddelineating salinity in landscape where it can
not be noticed on the surface soil.
In addition, the empirical approach, usingΐῐ῎῔῕ῑ ῒ 22 ῏ ῌ,**+῍34ratios of deep to shallow sounding for identifying recharge and discharge areas did not accurately handle the assumption due to the increasing of Eca value with increasing depth ofsounding in the discharge area. As the accumulation of soluble salt in the deeper soil
proﬁle. Such situation lead to interpret as arecharge area instead of discharge area. This
exception can be solved by rejecting the highEM slope data which is in the high salt content
proﬁle.
Acknowledgement
The authors wish to thank Prof. Makoto
ANASE and Prof. Rokuro YASUTOMI at TUA
for their guidance and advice as well as Dr.
Mastur Koeshadi of Watershed Management
Technology Center, Indonesia for his conﬁrmation of this paper.
Reference
Bullock, P.R. and Williams B.G. (+321) : Electromagnetic induction as an aid for tree-planting
strategies in salt-a#ected land. Editor R.S.
Rana. A#orestation of salt a#ected soils +0ῌ,*
Feb +321. Central Soil Salinity Research Institute, Karnal, India.
Engle, R., McFarlane, D. J. and street, G. (+321) : The
inﬂuence of dolerite dykes on saline seep insouth western Australia. Aust. J. Soil Res. ,/ :,.
Freeze, R.A. and Cherry, J.A. +313. Groundwater.
Englewood Cli#s, New Jersey, Prentice-Hall
Inc. Lo%er, E. and Kubiniok, J. +322. Soilsalinization in north-east Thailand. Erdkunde., (,), 23ῌ+33. et al.,+321.
McNeil, J.D. (+32*) : Electromagnetic terrain conductivity measurement at low induction numbers. Tech. Note TN 0. Geonics Ltd.
Sedgley, R.H., Smith, R.E. and Tennant, D. (+32+) :Management of soil water budget of recharge
area for control of salinity in south western
Australia. Agric. Water Mange. . : -+-ῌ--..
Sinanuwong S. and Takaya, Y. (+31.) : Saline soils
in Northeast Thailand. Southeast AsianStudies. +, : +*/ῌ+,*.
Surfer for Windows. +330. Surfer Mapping System.
Golden Software, Inc.
Williamson, D.R., Peck, A. J., Turner, J.V. andArunin, S. (+323) : Groundwater hydrology andsalinity in a valley in Northeast Thailand. In‘Groundwater Contamination’. IAHS Publ. no.
+2/.
Williams, B.G. and Arunin, S. (+33*) : Inferring recharge/discharge areas from multi frequency
electromagnetic induction method. Technical
Memorandum 3*/++, CSIRO.
Williams B.G. and Baker, G.C. (+32,) : An electromagnetic induction technique for reconnaissance survey for salinity hazards. Aust. J. Soil
Res. ,* : +*1ῌ+2.

p : Application of Electromagnetic Technique to Identify Recharge and Discharge Areas for Reforestation in Northeastern Thailand 35



  !"#$% &'(Chaiyanam Dissataporn*ῌ **ῌPramot Yamclee*ῌSomsri Arunin** 
**  

)*ῌ !"#$%&' ()*+, -./0)1234

56789:;<=> EITCM ?multi frequency Electromagnetic Induction Terrain Conductivity Meter@ )ABCDE#FGH4 *AIJKLMNN:)ABCDE#OFHPQRSTUE#V WX67Y
Z[:\4('X$]^_X`abcdeZ)fg;,#h)^i<jklmBn<o4 .p

ZqNrs;,W!)At3 (Z< -u.Xvw (!xy (Xdz{,W!Pn|,^}<[:n~,V n\4 WX[:P$ X "#<XST)^}n~,W!"4 EITCM )A, -.Xvwῌxybc"#V+,-,. : m+X 5694 $%&4 vw (ῌxy (4  4  : ,***  ++  ,.  : ,**+  -  -* 