10th Asian Regional Conference of IAEG (2015)

Evaluation and Prediction of Risks Associated with Groundwater Salinization and
Dilution: examples from coastal plains around the city of Tosa, Shikoku Island, Japan

Shuichi MIYAJI, Hisao SUNOUCHI and Noriaki DOI
Kouei Engineering Consultant Co. Ltd., Japan
E-mail:miyaji@koueicon.co.jp
Abstract
This paper uses a series of case studies to evaluate and predict the risks associatedwith the salinization and dilution of groundwater around the city of Tosa, Japan. These casestudies focus on the topographic, hydrogeological, and weather conditions that may affectgroundwater risk assessments. Salinization risks were considered by examining: (1) the
progressive salinization of well water caused by a rising saltwater cone induced bygroundwater drawdown during the construction and repair of levees, and (2) the intensity ofgroundwater salinization in groundwater-rich versus groundwater-poor areas. The
evaluation of groundwater salinization is critical for predicting trends in groundwater
availability. Dilution risks were considered by examining cultured flatfish mortality causedby the dilution of saline groundwater. Estimates are made of the effects of cumulative
precipitation on the viability of cultured flatfish populations. These estimates are used,alongside previously determined relationships between cumulative precipitation andchloride ion concentrations in groundwater, to develop an index that predicts the effects ofcumulative precipitation on the health of cultured flatfish populations. By applying theindex to groundwater conditions during a record rainfall event that occurred in August 2014,
it is suggested that intense precipitation events are associated with the dilution of saline
groundwater.

Keywords: coastal plain, groundwater, salinization, dilution of seawater1.

affected cultured flatfish populations. In both cases,
the analysis performed was relatively simple andbased on topography, hydrogeology, water quality,and hydrological data such as precipitation,
groundwater levels, and tidal changes.

Introduction

In coastal plain areas around the city of Tosa,
Kochi Prefecture, Shikoku Island, alluvial groundwater
is used for human consumption, agriculture, fisheries
processing, and aquaculture. The salinity of coastal
groundwater is strongly affected by dilution caused
by intense rainfall, saltwater intrusion, andconstruction and repair of river and coastal levees.
Salinity variations may severely impact groundwater
sources and availability, as highly saline waters are
unsuitable for consumption, and cause decreased
agricultural yields and fish culture populations.
This paper presents case studies that evaluate andpredict the risks associated with groundwatersalinization and dilution. Salinization risks areillustrated using a case study in which groundwatersalinization was caused by a rising salt-water cone.
This cone was associated with groundwater
drawdown during the construction and repair oflevees. Dilution (desalinization) risks are illustratedusing a case study in which groundwater salinity
decreased after an extreme precipitation event and2.

A case study illustrating salinization risks

Flood damage has occurred frequently on the
coastal plains in the Usa district, Tosa. The discharge
capacity of the Hagitani River is regularly exceeded
following intense frontal rainfall and typhoon storm
surges. Plans to repair levees along the river; i.e.,
enlarge the cross-sectional area to increase discharge
capacity, prompted concerns that groundwatersalinization may occur because of the subsequent
intrusion of salt water from the coast. A groundwater
survey was therefore conducted in April 2004, one
year prior to the planned construction. The
salinization area was predicted by analyzing
groundwater levels, precipitation, and electricalconductivity data, in addition to the topographical andhydrogeological characteristics of the region.

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10th Asian Regional Conference of IAEG (2015)× 10−4 and 1.0 × 10−5 m/s. This sand coarsens withdecreasing depth and consists of three sub-layers: theAs-1 layer mainly composed of medium to coarse
sand, the As-2 layer composed of silt mixed withmedium to fine sand, and the As-3 layer composed ofsilt mixed with fine to medium grained sand.
Gravel layers, Ag-1, Ag-2, and Ag-3 areprincipally fines mixed with sandy gravel andprobable debris flow deposits from the mountainous
area. Although the hydraulic conductivity is between
1.0 × 10−3 and 1.0 × 10−7 m/s, the layer often exhibits
low water permeability.
A widely distributed fresh water – saline waterboundary exists on the seaward side of the A–A' crosssection in Area A (30 < EC ≦ 4000 mS/m). On theother hand, the B–B' cross section of Area B shows amostly fresh water area (EC ≦ 30 mS/m), and the
fresh – saline boundary was not detected in theobservation well on the seaward side.

2.1 Geomorphology and hydrogeology of the study
The coastal terrain in the study area comprises thedune range on the seaward (east) side of the Hagitani
River, whereas the landward (west) side lies within
the floodplain (Kochi Prefecture, 1979). A delta-related
lowland lies in the southern portion of the study
region (Fig. 1). The floodplain elevation is about 2–6
m above sea level, and the surface elevation
decreases gradually towards the coast. The south area,
the mountains behind the Usa Elementary School, is
0.3 km2 (Area A in Fig. 1), whereas the area north of
the school is 1.3 km2 (Area B in Fig. 1), or 4.3 times
the size of Area A. Given the difference in spatial
extent, we expect that there will be a clear differencein groundwater supply between the two areas.
The study area consists of a clayey layer at 15–20
m below sea level, an overlying sandy layer andaquifer, and a gravel layer near the surface (Fig. 2).
The sandy layer was deposited by a coastal current
and has a hydraulic conductivity is between about 1.0

SB=1.3km2

BW-6
Floodplain

Area BB-B’cross section
BW-5
BW-4

Dune

BW-2
BW-1
A-A’cross section
Area A
BW-3

SA=0.3km2

Usa elementary
schoolDelta-related
lowland

Hagitani River(1)Salinization site

Fig. 1 Geomorphology of the study area and survey location

Fig. 2 Hydrogeological model of the study area2

Sandbar・
Sand bankBW-1(W1)
Cross section
Sea(2)Dilution site

10th Asian Regional Conference of IAEG (2015)
2.2 Salinization risk evaluation(1) Direction and gradient of the water table
In most places, the water table gradient dips fromthe mountains towards the sea. The seaward gradientin Area A is small in both the dry (–0.03° to 0.09°)
and wet seasons (–0.05° to 0.15°). In addition, after aperiod of low precipitation, the water table gradienttilts inland in the opposite direction in a few months(Fig. 3; cross section A–A'). On the other hand, thewater table gradient in Area B is stable and inclined
seaward. In Area B, this trend does not change
through the year, with dry season gradients of 0.32°
to 0.44° and wet season gradients of 0.36° to 0.47°(Fig. 3; cross section B–B').
← seaward

Fig. 4 Relationship of two-month cumulative
rainfall and monthly average water level

inland side →(3) Evaluation of salinization by the electric
conductivity integration method
Salinization at BW-1 (Area A) and BW-4 (Area
B) on December 11, 2004 were quantitatively
evaluated using the electric conductivity (EC)
integration method (Takahashi and Ohtoshi, 2008).
This method integrates EC with depth or uses a mean
EC value obtained by dividing the integrated EC by
the aquifer thickness. EC in BW-1 begins to increaseat about T.P. -5 m, with values close to those of
seawater (about 4000 mS/m) from T.P. -12 m. The
T.P. means the Tokyo Peil, the mean sea-level ofTokyo Bay. At BW-4, groundwater was fresh (about
30 mS/m) at all depths (Fig. 5). At BW-1, integrated
EC values were about 14000 mS/m × m, and theaverage EC was about 1100 mS/m. At BW-4, theintegrated EC was about 400 mS/m × m, and theaverage EC was about 30 mS/m, or about 3% ofvalues at BW-1 (Table 1).

Fig. 3 Seasonal comparison of water table
gradients in the study area (December 2004 to
September 2006)(2) Relationship between precipitation andgroundwater level
The relationship between the monthly averagewater level in observation wells and two-month
cumulative precipitation in Area A (BW-2) and Area
B (BW-5) were compared, and there is a difference inthe relationship between the precipitation and thegroundwater level in Area A and Area B. Although
there is a linear relationship between the monthly
average water level and two-month cumulative
rainfall in both areas, the amount of water leveldrawdown that occurs after a decrease in precipitationis larger in Area A than in Area B (Fig. 4).

Fig. 5 Vertical EC distribution
Table 1. Comparison of the integrated EC andmean EC
Area
A(BW-1)
B(BW-4)

The fresh-saline
boundary height※[T.P.m]-11.7-

Integrated EC

Mean EC[mS/m*m]
14,100400[mS/m]
1,050
30※For convenience boundary height is the height of EC=4000mS/m（－：Withoutboundary exists）．

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10th Asian Regional Conference of IAEG (2015)(4) Estimation of groundwater flow
Groundwater flows in both areas were estimatedfrom the shape of the saline water edge (Fig. 6).
Estimations were obtained using the following
equations based on the Ghyben-Herzberg relationshipand Darcy’s law (Domenico and Schwartz, 1990):

Table 2. Groundwater flow results(Dec11,2004,16:00)
Area(Observation hole)A（BW-1）

z※
[m]
15

L※
[m]
100

Groundwaterflow[m3/h/100m]
10.1

B（BW-4）18

70< 20.8※z and L was estimated from ground model and interface position

hf
Fresh
water

z

z

2.3 Prediction and verification of salinization risk
(1) Prediction of salinization risk
Salinization risks in areas A and B were predictedbased on the indexes referred to in Section 2.2 (Table3). The groundwater environment in Area A issusceptible to salinization because the water table isinclined landward with a gradient of 0.03–0.05°, theelevation of the water table is relatively low when
precipitation is low, EC values are high, andgroundwater flow capability is low. The groundwater
environment in Area B is not as susceptible tosalinization because the water table is inclined
seaward with a gradient of 0.32–0.36°, the elevationof the water table is relatively high, EC values near
the coast are low, and groundwater flow capability ishigh. Therefore Area A is assumed to be an area inwhich, if the groundwater table falls because of
construction, salinization may affect residential andagricultural water supplies.

Q

Salt
water
Interface

L

Fig. 6 Groundwater flow calculation method

(1)
(2)
(3)
Where,
Ｑ = volumetric flow rate of fresh water to the sea
Ｙ = the length of the shoreline (100 m)
Ｑ’ = discharge per unit length of the shoreline
Ｋ = hydraulic conductivity–4(aquifer average = 5.0 × 10 [m/s])
Ｌ = distance from the shoreline
ρ = density of freshwater (1.0)
fρ = density of sea water (1.025)sｈ = height from sea level to the water tablefｚ = depth from sea level to the interface (m)(2) Evaluation of salinization risk
When construction work started on the river inJuly 2007, the regional extent of groundwater below
T.P. 0 m decreased, and shallow groundwatersalinization progressed around the construction zone
(Fig. 7). Salinized water is unsuitable for consumption
and caused decreased agricultural yields.
In Area B, although groundwater below T.P. 0 m
decreased slightly, the extent of EC ≧ 200 mS/m (Cl−≒ 200 mg/L) was limited to coastal areas. There wasno reported impact on the residents or industry.
Therefore, predictions in the previous section of ahigh risk of salinization in Area A and a low risk in
Area B appear to be correct.

As the fresh–saline water interface was notobserved at BW-4, this was assumed to be at the baseof the well. As a result, groundwater flows were 10.1
m3/h/100 m in Area A (BW-1), and 20.8 m3/h/100 m
in Area B (BW-4). Therefore, we inferred that the
groundwater supply capability in Area B is at least
twice that of Area A (Table 2).

Table 3 Comparison of salinization risks(1) Groundwater table
Area

A
B

Minimum value of the dry
season (Upper column)
Minimum value of the wet
season (Lower column)
Direction of tilt Gradient[°]
Sea→Land
0.03
Sea→Land

0.05

Land→Sea

0.32

Land→Sea

0.36(2) Precipitation andgroundwater level(3) EC values and salinization in thevertical direction

Groundwater level of
two-month cumulative
rainfall [T.P.m](4) Groundwaterflow

The freshsaline
boundaryheight [T.P.m]

Integrated
EC[mS/m*m]

Mean EC[mS/m]

Freshwater flow
[m3/h/100m]

100mm

700mm

0.17

0.83

-11.7

14000

1050

10.1

0.37

0.85

None

400

30

< 20.84

10th Asian Regional Conference of IAEG (2015)(m)(1)Groundwater table
BW-3
BW-2

BW-1

BW-6

Area A

BW-5
BW-4

Area B：Construction zone

Unit: (T.P.m)(m)(m)(2)EC contour lines
BW-3
BW-2

BW-6
Area A

BW-5
BW-4

Area B：Construction zone

Unit: (mS/m)(m)Fig. 7 Groundwater table and EC contour lines after construction (January 2007)
3.

A case study of dilution risk prediction

3.1 Geomorphology and hydrogeology of the study
area
The study area is a narrow plain that lies between
the mountains and the coast. It is divided into sandbar
and sandbank environments (Fig. 1) (GSI, 2006). The
sandbar and sandbank are at an elevation of about 7
m, but decrease gradually in height towards thenorthwest.
The lithostratigraphy of the study area consists of
medium to coarse-grained gravel (As-1) at the surface,which is underlain by sandy gravel (Ag-1) consistingof 2–10 mm rounded gravel and medium to coarse
sand, and then a basal silty sand (Fig. 8). The layerboundaries are gently inclined at 2–3° from themountains towards the sea. The main aquifers are
made of two layers of Ag-2 and Ag-3, with athickness of 3–4 m and about 5 m, respectively.
Permeability is generally low, and the hydraulicconductivities (k) of Ag-2 and Ag-3 are 1.35 × 10−5
m/s and 4.30 × 10−6 to 7.24 × 10−6 m/s, respectively.

Land farming of flat fish has been carried out by
pumping high salinity groundwater from the coastal
plains in the Nii district of Tosa. Aquaculture is notadversely affected by the high salinity groundwaterthat develops during dry periods, as it is similar toseawater. However, groundwater dilution whenintense rainfall increases the volume of groundwater
may be problematic.
The construction of coastal dikes for earthquake
protection in this area consisted of a curtain-like wallof steel pipe. Groundwater observations were carriedout because of the possibility of groundwater dilution
after construction. An evaluation was made of thedilution of groundwater following a record precipitation
event in August 2014. Using the relationship between
groundwater salinity and precipitation, the effect of
cumulative precipitation on farmed flatfish
populations was identified.

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10th Asian Regional Conference of IAEG (2015)

Fig. 8 Hydrogeological model of the study area
3.2 Relationship between groundwater salinity andprecipitation
In the study area, the salinity of existing wells(W1) and seawater (Sea) ranges from 3200 to 16000
mg/L, and 9500 to 19000 mg/L, respectively. Thus,
salinity at W1 is 34% to 84% of the Sea values.

Salinity at W1 tends to decrease with increasing
precipitation (Fig. 9). In addition, the salinity of bothW1 and Sea was greatly reduced in August 2014,
when monthly precipitation was 1164.5 mm, and therecovery of salinity at W1 was gradual.

Fig. 9 Changes salinity at W1 and Sea, and monthly precipitation in 20146

10th Asian Regional Conference of IAEG (2015)
3.3 Prediction of dilution risk
(1) Definition of cumulative precipitation
Cumulative precipitation is defined as the sum ofany precipitation that occurred before and including
the water sampling date. For example, the previous
30 days precipitation (R30) is the cumulative amountof precipitation over the last 30 days, and includes
any precipitation falling on the sampling day.
Although data are limited and dispersed, salinity
has a strong correlation with R30 (Fig. 10). Salinity at
W1 decreases rapidly, to about 60% of seawatersalinity, when R30 reaches 200–300 mm. Salinity thencontinues to decrease with more precipitation. It is
presumed that, when precipitation is low, the groundis able to absorb most of the rainfall. However, in thecase of intense precipitation events, residual
precipitation that cannot penetrate into the groundflows over the surface directly to the sea or rivers.

(2) The effect of cumulative precipitation on
cultured flatfish
As salinity data are limited, salinity was inferredfrom the EC values. Salinity and R30 during theperiod January–September 2014 without data lessthan 100 mm, which little affect cultured flatfish,
were included in the regression analysis. The
regression equation is as follows:
W1 = 257296 R30-0.595（R2 = 0.8389)(4)

If the threshold chloride concentration that affects
cultured flatfish us set at 4000mg/L (Ochiai andTanaka, 1998), then the R30 value that affects cultured
flatfish is 1100 mm (Fig. 11).

4000[mg/L]
1100[mm]Fig. 11 30-day cumulative precipitation–salinity
relationship and threshold for aquaculture
effects(3) Evaluation of dilution risk
The 14-day period of August 18–31, 2014, wasthe only time that R30 exceeded 1100 mm over the
past 10 years (2005–2014; Fig. 12). According toaquaculture records, flatfish mortality was high at thistime, and there are few previous records of flatfish
mortality as a result of groundwater dilution. Thus,
our calculations reflect the observational data, and theR30 index appears to be a reasonable method of easily
evaluating and predicting flatfish mortality.

Fig. 10 Relationship between precipitation andsalinity (February–August 2014)7

10th Asian Regional Conference of IAEG (2015)

Fig. 12 The impact of accumulated precipitation (R30) on cultured flatfish
4.

Summary

Prefecture for offering us the opportunity to survey
the area. Some of the figures in this paper were madeusing GMT (Wessel and Smith, 1991).

To predict the risk of salinization on coastal
plains, it is important to evaluate the groundwater
recharge capacity using the topographical andhydrogeological characteristics of the area. In thecase study presented in this paper, where thegroundwater level gradient was gradual in the vicinityof T.P. 0 m, and inclined from the sea to themountains, salinization progressed quickly, andinhabitants and industry are affected by deterioratingwater quality.
The relationship between cumulative precipitationand groundwater salinity offers a simply way toevaluate the groundwater dilution risk.
To date, the evaluation of salinization–dilution
risks in this region has been based on a qualitative
assessments of topography, hydrogeology, andweather conditions. Future quantitative verification ofthese risks will require the application of numericalsimulation techniques.

References
Domenico, P.A. and Schwartz, F.W. (1990): Physicaland Chemical Hydrogeology, John Wiley & Sons,
pp.824.
Geospatial Information Authority of Japan, (2006): 1:
25,000 Land Conditions Map "Tosa Takaoka",
pp.15.
Kochi Prefecture, (1979): Land Classification Basic
Survey “Susaki” (1:50,000), pp.30.
Ochiai A. and Tanaka K. (1998): New Version
Ichthyology (last volume), Koseishakoseikaku,
pp.1139.
Takahashi K. and Otoshi K. (2008): Verification by
the numerical analysis of salt water intrusion
phenomenon in Hagitani River area, Proceedings ofthe 14th Research Meeting, the Shikoku Branch of
the Japan Society of Civil Engineers, pp.91-92.
Wessel, P. and Smith, W. H. F. (1991): Free software
helps map and display data, EOS Trans. AGU, 72,
pp. 441.

Acknowledgements
The authors are grateful to the late Professor
Kunio Ootoshi and Professor Taku Fujiwara at Kochi
University for their advice and assistance with thefield survey. We also thank members of the
Chuo-nishi Civil Engineering Office of the Kochi

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