57

Invasion of Seawater and Marine Sludge into Arable Land
Satoshi MATSUMOTO
Japanese Soil Association
1-58 Kandajinboh-cho, Chiyoda, Tokyo, 101-0051 Japane-mail: dqchs219@ybb.ne.jp

Abstract
The tsunami that followed the Great East Japan Earthquake on March 11, 2011 flooded arable lands withseawater, leaving farmland invaded by marine sediment over a total area of approximately 24,000 hectares insix prefectures along the Pacific coast. Even now, after three years have passed since the earthquake, somearable lands remain flooded with seawater due to land subsidence. In other cases, however, where land subsidence was not observed, salt removal has fully advanced owing to rainfall after the earthquake, and the land has
recovered as quickly as for normal farmlands. On the other hand, in farmlands that were invaded by marine
sediments along with the seawater, big differences have been observed in the quantity of marine sediment andrubble carried by the tsunami into the farmlands depending on the presence or absence of sequences of strong
artificial structures lying perpendicular to the direction of tsunami invasion. In Japan, land has been reclaimed
mainly by using marine sediments as parent materials for the soil. The essential difference between soil
obtained from traditional Japanese reclaimed lands and soil deposited in farmland by a tsunami is that thelatter contains various kinds of heavy metals while the former contains little of them. This paper discusses
changes in soil conditions when marine sediment accumulates on open land, and the behavior of heavy metals
in sediments.
Key words: acid sulfate soil, arable lands, desalinization, heavy metals, marine sediments, tsunami

1. Introduction
Japan is surrounded by the sea and suffers damage
from typhoons almost every year, so it is not unusual forarable land to be damaged by high tides and large
amounts of salt carried in by winds (Yoneda, 1961).
However, there have been few cases where salinity
damaging to arable land persisted for a prolonged periodof time, making the land unusable for agriculture. This isbecause Japan is located in the Asian monsoon region, so
it is in a natural environment where it receives significant
precipitation from spring through fall, meaning that salt
is typically washed away, mitigating salinity hazards(Japanese Study Group for Climate Impact and Application, 2004). The Great East Japan Earthquake of March
11, 2011, caused tremendous damage to arable land,
resulting in the complete loss of its functionality (Fig. 1).
Damage from the disaster included seawater invasion

Fig. 1 Agricultural fields covered with huge amounts of rubble and marine sludge.
(Wakabayashi, Sendai, June, 2011)
Global Environmental Research
18/2014: 57-63
printed in Japan©2014 AIRIES

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S. MATSUMOTO

into arable land, causing salinity hazards; accumulationsof marine sediments (Miura, 2012), debris (Ministry of
the Environment, 2011) and oil balls (Matsumoto, 2011)
in agricultural fields left by the huge tsunami; and land
subsidence, such that sea water remained in the fields(Japanese Geotechnical Society, 2011). The Ministry ofAgriculture, Forestry and Fisheries reported earthquake
damage to a total of about 24,000 hectares of arable landin six prefectures along the Pacific coast (Aomori, Iwate,
Miyagi, Fukushima, Ibaraki, and Chiba) (Statistic
Department and Rural Development Department, 2011).
Miyagi Prefecture was the most severely affected, with
about 15,000 hectares, or 11% of the arable land in theprefecture, being damaged. Fukushima Prefecture was
the second most severely affected, with damage to about
6,000 hectares, or 4% of its arable land. The degree ofdamage to arable land differed significantly from one
prefecture to another, due partially to differences intsunami effects, and especially to topographical differences, such as the amount of flat arable land along thecoastline (Seibido Publisher, 2012).
The author collected field survey reports on damage
to arable land, mainly on the Sendai Plain (an area of
about 14,000 hectares, including the cities of Sendai,
Natori and Iwanuma, and the town of Watari), the largest
arable land area damaged in the earthquake (National
Research Institute for Earth Science and Disaster Prevention, 2011; Ibaragi University, 2011; Tohoku University, 2011; Kanto Regional Agricultural Administration
Office, 2011; Ishinomaki Agricultural Development andExtension Center, 2011).
This paper presents the results of the author’s literature and field surveys, analyzes causes of delay inrestoration of the damaged arable lands, and describes theoutlook for their complete restoration.

2. Description of the Earthquake Damage to
Arable Land
Since the damage to arable land varied by region, wemust be wary of any general description; however, thedamage may be roughly divided into two types: 1) damage to agricultural soil, and 2) damage to agricultural
facilities (irrigation canals, irrigation and drainage facilities, crop-growing facilities, farm roads, etc.). Each will
be described in detail.
2.1 Damage to the soil of arable land
2.1.1 Accumulation of debris
Agricultural soil is, of course, soil for growing crops.
Therefore, materials that can interfere with crop root
growth, harmful materials, substantial and hard debris
that can interfere with the operation of agricultural
machinery, and strong string-like materials that can windaround the rotating components of cultivators are strictly
excluded from agricultural fields. In the earthquake,
however, an enormous amount of debris was introduced
into agricultural fields near the coastline. Mineral matter,such as rocks and concrete fragments and blocks, andother debris, such as construction debris, vehicles, smallboats, electrical and office equipment, household goods,
fallen trees, and fishing nets accumulated in these fields,
often completely covering the surface of the agriculturalsoil, and remaining there for some time after the earthquake. The fields were hardly recognizable as such, thedevastation was so profound.
The total amount of debris deposited in Iwate, Miyagi
and Fukushima Prefectures was estimated to be
28,000,000 tons <http://kouikishori.env.go.jp/diasater_
waste/progress/#progress02>. It is unknown how muchof this was deposited on arable land. The debris has since
been rapidly removed. Over 62% of the debris depositedon the arable land in Miyagi Prefecture was removed asof November 2011, and the debris on the arable land in
the above three prefectures is scheduled to be almost
completely removed by the end of 2012 (see abovementioned website).
However, complete debris removal does not necessarily mean complete restoration of the agricultural fields.
To ensure the restoration of these fields, it must be
demonstrated that (after the debris is removed) over several crop seasons, crops are planted and grow as they had
before the earthquake, and their quality is satisfactory.
Assessment of the agricultural fields will be performed,
including demonstration of the growth and quality ofcrops, and the remaining agricultural fields will be completely restored in 2011 (see above-mentioned website).
Details on the debris are provided in another article inthis special issue.
2.1.2 Marine sediment accumulations
Older marine sediments on the seabed are tightly
consolidated into rocks and difficult to separate(Heibonsya, 1984). These sediments are often found atsites bordering the open sea. New marine sediments
brought by the tsunami accumulate on the land, and they
consist of a non-consolidated, dark-grey mud. Due to its
form, the combined sediment is commonly referred to as
marine sludge and this latter term will be employedhereafter.
Figure 2 shows a rice field with a thick accumulationof marine sludge.

Fig. 2 Rice fields with a thick accumulation of marine sludge.
Big rifts in the marine sludge accumulation occur when itdries. (Watari, Sendai, August, 2011)

Invasion of Seawater and Marine Sludge into Arable Land

Arable land covered by accumulations of marine
sludge is estimated to total about 12,000 hectares in
Miyagi Prefecture and 4,000 hectares in Fukushima
Prefecture (Moori, 2011). The depth of such marine
sludge varies significantly with location. In locations, forexample, where a highway embankment ran perpendicular to the direction of tsunami propagation, the amount ofmarine sludge accumulating on the interior side of the
highway was less than in other locations. In addition, theamount of tsunami marine sludge introduced throughhighway underpasses was further reduced by farm roads.
Figure 3 shows the relationship between the direction oftsunami propagation introduced through highway underpasses and the difference in agricultural field levels anddepth of marine sludge.
Obviously, the marine sludge depth depends, among
other things, on the presence of farm roads runningroughly perpendicularly to the direction of tsunami
propagation with deeper (darker) deposits on arable land
to the left (ocean-ward) of the farm road in the figure,than to the right (lighter) (Matsumoto, 2011). In addition
to debris removal, the removal of such marine sludge
must be considered. The composition of the sludge isbriefly described below (Viruthagin & Ponnarasi, 2011;
Tray et al., 2004).
Marine sludge is a mixture of mineral particles
formed when rock is weathered and fragmented on land,soil particles, and organic matter from plant and animal
remains. In other words, it is a kind of soil. When theFig. 3 A comparison of surface soil color and depth of marine
sludge between rice fields separated by the presence offarm roads running roughly perpendicular to the directionof tsunami propagation.
(Wakabayashi, Sendai, August, 2011)59

sludge is suspended in rain water and enters a river, thesoil particles are dispersed in water (in a colloidal state)
and transported to the ocean in the form of a turbid
suspension. Upon contact with sea water in the rivermouth, the particles in the colloidal state precipitate andsettle on the river bed. The sediment in the river mouth
forms shoals over time, and some is transported by waves
down to the seabed, forming marine sludge. After settlingon the seabed, the marine sludge continues to be used as afeeding area or habitat for marine life. It is important tomarine life because it contains various land-basedminerals, and is particularly rich in iron, in which marine
life tends to be deficient; it contains different types ofland-based organic matter from animals and plants in theform of humic substances, and is an important source ofnutrients for marine life (Botton, 1979; Duedall et al.,1975).
In itself, then, we might think that such marine sludge
deposited on arable land would be beneficial; not something difficult to handle, which should be removed likedebris. And indeed, if the salt is removed from suchsludge, it can be beneficially employed as crop-growing
soil for a long time. The fact that polders in Netherlands
and reclaimed agricultural land in Japan are composed ofmarine sludge shows that it was once used as a valuable
source of soil (Clarijs, 2008). However, in order for suchsludge to be used for reclaimed agricultural land, theconcentrations of hazardous materials contained in it
must be less than the limits specified by law. Somemarine sludge samples gathered from across the country
contain cadmium (Cd), copper (Cu) and arsenic (As),which are toxic substances specified in the AgriculturalLand Soil Pollution Prevention Law. Table 1 presents an
example of marine sludge taken from Oga Bay, Akita
Prefecture. This marine sludge consists of sediments bothfrom the mouth of the Omono River, where a metal factory is in operation, and from reclaimed land in Ogata
Village, Akita Prefecture.
The latter consists of colloidal materials from rice
field drainage. Table 1 shows that cadmium, copper, andarsenic are contained in the sludge, and the copper content is higher than the limit of 125 ppm set forth in theabove law.
Below, the paper will discuss the levels of suchspecified toxic substances in the marine sludge introduced into the agricultural fields by the tsunami, andwhether such sludge should be removed.

Table 1 An example of physicochemical properties of marine sludge taken from the mouth of the Omono
River. (Matsumoto, 2011)
Concentration of nutrients
Electrical conductivity
Permeability(weight % / dry matter)pH-6(dS/M)(x10 cm/sec)
T-C
T-N
T-P
T-K
7.2
0.2
9.2
3.2
0.4
12.2
10.5
T-C: Total carbon, T-N: Total nitrogen, T-P: Total phosphorus, T-K: Total potassium

Cd
3.6

As
3.0
(15)

Heavy metal content (mg/kg, dry matter)
Hg
Cu
Zn
Pb
1.1
160
1050
40.1
(125)
(120)

( ): standard value for arable lands

Cr
38.3

Ni
29.1

(Matsumoto, 2011)

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S. MATSUMOTO

2.1.3 Presence of oil balls
When crude or waste oil spills into the ocean anddrifts, it loses its volatile components over time, typicallycongealing into dark-brown spherical solids generally
referred to as oil balls (Fig. 4).
When waste oil from homes and businesses adheres tothe inner surface of sewage pipes, scum (a muddy solid)
is formed, and this scum, when discharged into anddrifting in the ocean, also forms such oil balls. Oil balls
do not decompose easily, and they are highly adhesive.
When they get into agricultural fields, they significantlyinterfere with the growth and oxygenation of crops,
causing the crops to wither and die. However, there havebeen very few cases of such oil balls being depositedin agricultural fields. As shown in Fig. 4, our survey
revealed that, in many cases, oil balls are not present inan easily identifiable form, and/or are mixed with marine
sludge. Komai found that after the earthquake, an oily
film had formed in many locations where the sludge wasdeposited, suggesting the possibility that in such locations small oil balls were mixed in the sludge (Komai,2011). Oil balls are difficult to visually identify in suchsludge, due to color similarity between the balls andsludge. However, they can be identified by the presenceof an oily smell or oily film, or by dry-state differences incolor (when dry, the grayish black sludge alone has asignificantly different color, but changes color only
slightly when mixed with oil balls).
At the moment, it is impossible to estimate theamount of arable land damaged by oil balls, and it willremain very difficult. The only way to estimate the area is
to plant a crop in agricultural soil, adding desalinatedmarine sludge mixed with oil balls, and identify featuresindicating damage from the oil balls by evaluating the
growth response of the crop. The amount of damaged
arable land can then be estimated by measuring the landarea exhibiting these features.
2.2 Damage to agricultural facilities
A public relations brochure from the Ministry ofAgriculture, Forestry and Fisheries (2011) notes that in
16 prefectures, including the six prefectures along the
Pacific coast, along with Niigata, Shizuoka, Mie andFig. 4 Oil balls washed up on a seashore and arable lands by thetsunami.
(Ishinomaki, Miyagi, July, 2011)

other prefectures, 106,546 agricultural facilities (irrigation pump stations, drainage pump stations, water gates,
irrigation canals, drainage canals, farm roads, plastic
greenhouses, etc.) were damaged in the earthquake (theMinistry of Agriculture, Forestry and Fisheries (2011)),and the damage was estimated at 305.1 billion yen(Ichinose, 2011). This amount does not include damage
from the withering of greenhouse vegetables, due to lowtemperatures as a result of the blackout after theearthquake(Fig. 5).
Figures 6 and 7 are photos of damage to major agricultural facilities. Irrigation and drainage control isextremely important in agriculture in the Tohoku district,which has been growing in size, and a large control centerwith a dedicated electronic control system is required.
However, even if the present control center is returned toservice, it will be of no use as long as the irrigation canals
under its control are damaged. More time and labor will
be required to repair the irrigation and drainage canals
under its control. The earthquake and tsunami affected
elevation differences in the canals, and caused leakage inmany locations. An enormous amount of marine sludge
was deposited in irrigation and drainage canals for
agricultural land near the coastline. Removal of suchsludge from these canals, and its subsequent disposal,
will be a major issue. In addition, for low-lying agricultural land near swamps, underground drains have typically been installed in the soil to facilitate drainage.
Non-flexible tiles are no longer commonly used in suchdrains, having largely been replaced by flexible plastic orrice husks (Senda et al., 2008). It is necessary to determine whether such underground drains have become
blocked due to the intense ground perturbation during theearthquake.
Assessing damage to agricultural facilities includes
an assessment of whether the crop-growing functionalityof arable land has been restored, in addition to assessing
facility damage. An effective way to perform this functional assessment is to plant crops in the respective agricultural fields, examine their growth, and identify
damage that cannot be visually detected.

Fig. 5 Withered tomato plants in a green house due to low temperatures as a result of the blackout after the earthquake.
(Ishinomaki, Miyagi, April, 2011)

Invasion of Seawater and Marine Sludge into Arable Land

Fig. 6 A destroyed agricultural water gate.
(Shinchi, Fukushima, July, 2011)

3. Factors Delaying the Restoration of Arable
Land
3.1 Removal of debris
Theoretically, debris should be completely removed
from arable land, but in reality this is impossible. The
three major types of debris that should be removed fromarable land are described below. Only those types thatcan be physically removed (those with appreciable volume and weight) are described here. Soil debris present
in liquid or powder form, and difficult to remove byphysical means, is excluded. The presence of such substances should be evaluated according to the allowable
limits for toxic substances specified by law or ministerial
ordinance, after physical removal of debris.
1) Materials that can damage or impair the operating
efficiency of agricultural machinery (tractors, cultivators, combines, rice planting machines, etc.)
2) Materials that can contaminate arable land or interfere with the growth of crops
3) Materials that can be hazardous to the health of agricultural workers
The materials in (1) include construction debris, suchas hard concrete, slate and roofing tile fragments with adiameter over 0.5 cm; as well as rocks, glass and metal
debris, fishing nets, synthetic fiber cords over 5 cm inlength, plastic products, synthetic bark, light bulbs,
fluorescent tubes, and fallen trees.
The materials in (2) include chemicals; oil-based
products, such as paints, detergents, inks, and toners;
petrochemicals; batteries; and large quantities of agrochemicals and chemical fertilizers stored in containers,
which can be physically removed provided that the damage, if any, to the containers is minor.
The materials in (3) include volatile organic solvents,
coarse organic matter, and malodorous organic matterthat can be a source of pathogenic microbes.
3.2 Issues relating to marine sludge
As noted in the previous section, marine sludge can bebeneficially employed as crop-growing soil, but for this it
must satisfy the limitation criteria for specified toxic
substances. The treatment of marine sludge introduced

61

Fig. 7 A drainage canal filled with rubble and marine sludge.
(Minamisoma, Fukushima, August, 2011)by the tsunami can be divided into the following threelevels, according to difficulty.
Level 1 is treatment of marine sludge that can be
relatively easily treated. The sludge contains no hazardous materials other than salt, and desalination is the primary purpose of the treatment.
Level 2 is treatment of marine sludge containing
specified hazardous substances exceeding the prescribed
limits, but not containing oil balls.
Level 3 is treatment of marine sludge containing both
specified hazardous substances exceeding the limits, andoil balls.
With regard to the treatment of marine sludge deposited on arable land, the organizations involved conducted
extensive discussions on the possibility of complete removal of the sludge in the early stages of the arable land
restoration project after the earthquake. However, even in
the Arahama area of the Sendai Plain, where the largest
amount of marine sludge was deposited and flatland area
was available for temporary storage of the sludge.
Thereafter, almost of all the marine sludge deposited on
the land was utilized as materials for the land development. None of the marine sludge, however, was returned
to arable lands. This is probably due to the concern that if
the sludge remained on the ground in any quantity, andwas exposed to the air, various substances would leachout from the oxidized sludge and degrade the environment (Tsuji et al., 1994). The likely mechanism behindsuch environmental pollution from the accumulated
sludge is the following.
Pyrite (FeS2) which is commonly contained in marine
sludge undergoes chemical and microbial reactions in the
air and turns into a strong form of sulfuric acid (Horisaka
et al., 1995). Figure 8 illustrates the process of transformation of marine sludge including FeS and FeS2 into acidsulfate soils.
When strongly acidified, pyritic soil is transformed
into what is commonly called acid sulfate soil, fromwhich various substances leach out in rainwater and pollute the environment. For this reason, marine sludge
deposited on arable land should be treated in situ. A suitable in-situ treatment is discussed below for each treatment level.

62

S. MATSUMOTO

Fig. 8 Process of transformation from marine sludge including FeS and FeS2 to acid sulfate soils.
(Matsumoto, 2013)

For Level 1 sludge, the desalination process is slow,
due to the sludge’s low water permeability. Adding gypsum (calcium sulfate, CaSO4) to the sludge is known tobe an effective solution to this problem. Calcium sulphatehemihydrate (CaSO4 · 1/2H2O) is reported to be more
effective than calcium sulphate dehydrate (CaSO4 · 2H2O)(Schippers & Jorgensen, 2002). The addition of gypsum
granulizes the muddy sludge, increasing its permeability
and thereby promoting the desalination process(Matsumoto, 2002; 2013).
Level 2 sludge introduced by the tsunami contains
arsenic, a specified toxic substance. Sludge samples were
found to contain arsenic exceeding the leaching limit of
0.01mg/l for soil in Miyagi Prefecture (highest concentration: 8 mg/l), Iwate Prefecture (highest concentration:
5 mg/l), Aomori Prefecture (highest concentration:
1 mg/l), and Chiba Prefecture (highest concentration:
1 mg/l). Komai gathered 130 samples of marine sludge
introduced by the tsunami from the six prefectures along
the Pacific coast and analyzed them to determine theheavy metal content, including arsenic, of the sludge.
Table 2 shows the results of his analysis. Although
Komai states that the number of samples analyzed isinsufficient for determining the distribution of heavy
metals, he attempts to explain, from a geological perspective, why the sludge deposited near the coastline in
Miyagi and Iwate Prefectures contains a particularly
large amount of arsenic.
Techniques for insolubilizing toxic heavy metals, including arsenic, to reduce their content to below theleaching limits have been developed and put into practical use (Matsumoto, 1987). The chief constituents of theinsolubilizing agent used in these techniques are naturally occurring materials, and only a small weight percentage of the agent is added to the soil. Therefore, its use
in arable land is unlikely to cause many difficulties.
Level 3 sludge contains arsenic and other heavy
metals exceeding the leaching limits, as well as oil balls.
A significant amount of time and money would be re-Table 2 An example of a chemical analysis of marine sludge
brought in by the tsunami from the Great East Japan
Earthquake. (Komai, 2011)
items
prefecture
namecollected
Aomori
Iwate
Miyagi
Fukushima
Ibaragi
Chiba

Maximum value among samples collected ineach prefecture (mg/l)( ): Limit under soil contamination
Countermeasures Law in Japan (mg/l)
B
Pb
Cd
As
Cr
Se
(1) (0.01) (0.01) (0.01) (0.05) (0.01)
0
1
0
1
0
0
0
2
0
5
0
0
0
1
0
8
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
0
(Komai, 2011)quired to restore such soil to the point where it could
support safe crop growth without difficulties, because,though the heavy metals can be removed using theinsolubilization techniques above, subsequent different
action is required to reduce the oil balls.
In one case, where a bioremediation process was
employed for the remediation of soil contaminated by
crude oil, the soil remediation took at least three years.
Moreover, the sludge is very likely to turn into soil withhigh levels of sulfuric acid during the oil ball treatment
process, and efforts will be required to prevent this. For
these reasons, the author believes that if the sludge requires Level 3 treatment, its removal should be a toppriority for early restoration of arable land.

4. Concluding Remarks
A high-quality agricultural field, high in bioproductivity and producing delicious products, cannot be
achieved in a day. Many of the fertile fields across thecountry began as agricultural fields with low productivity
and many problems. However, the continued efforts ofagricultural producers, combined with investment ofInvasion of Seawater and Marine Sludge into Arable Landsocial capital, have improved them over time; and suchhigh-quality fields are important cultural assets.
The Great East Japan Earthquake caused damage tomany fertile fields and other cultural assets. Ironically,
this damage illustrates modern civilization’s negative
legacy with regard to arable land. The only way to mitigate the consequences of this negative legacy is toimprove the land by using currently available advanced
land-improvement processes. The plan developed by thegovernment, to restore the damaged arable land in three
years, is undoubtedly based on such techniques. The plan,
however, will ultimately achieve only a superficial
recovery of the arable land; it will not necessarily restore
the fields to their original fertility. For such restoration, it
will be necessary to grow crops in the restored agricultural fields and make appropriate and continuous adjustments as needed, while monitoring the growth of thecrops in a detailed manner. It must be understood that thisprocess will take a considerable amount of time.
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Satoshi MATSUMOTO
In addition to serving as president of the Japan
Soil Association in several official activities, Prof.
Matsumoto currently also studies the behavior ofradioactive cesium in soils contaminated withfallout originating from the accidents at the
Fukushima nuclear power plants. He has
published some papers already in the reports of
the Science Council of Japan.

(Received 16 May 2014, Accepted 2 July 2014)