SOIL MATERIAL RICH I N PYRITE IN NON-COASTAL AREAS J. N. B. Poelman Soil Survey Institute, Wageningen I. Introduction Acid sulphate soils are normally associated with deltas and coastal areas which are, or recently have been, under the influence of seawater. Pons (14) states in his abstract: "Acid Sulphate Soils are formed during ripening of marine muds rich in pyrites when insufficient neutralizing components are present to prevent a dangerous drop in pH level." Van Beers (I), in a review of this topic, formula- tes the principal processes. His first proposition reads as follows: "In marine and brackish water sediments sulphur may be bound and accumulate under certain conditions in the form of iron sulphides and elementary sulphur as a result of the reduction of sulphates. The latter are chiefly derived from sea water". In the present paper particular attention will be given to acid sulphate soils which occur far from the coast and remote from the recent influence of sea water. 2. Examples of pyrite rich soil material in non-coastal areas Outside The Netherlands ChenerY (3), in a search of the literature, found only two examples of acid sul- phate soils which were not associated with marine or estuarine deposits. One CaSe was that of the Solfatara muds of Java (4), the other was that of deposits in a lake bed in the Taiga region of Russia (15). He himself added one case, that of "Papyrus Peat enriched with sodium sulphate from Precambrian schists and phyl- lites in the Kigezi District of Uganda at about 6,000 ft elevation". Petersen et al. (12) describe the presence of pyrite in the lignite districts of central and western Jutland where there are open-cast brown-coal mines with a depth of 15-25 m. The soil which overlies the brown coal is being dumped to form steep hills known as "tippings"; these consist of a heterogeneous mixture of sand, gravel, and clay. This so-called lignite clay, which was deposited in the Tertiary as a mud, is interlayered with peat layers (brown coal) and contains much pyrite. The latter leads to strong acidification following oxidation, so that attempts to afforest the tippings have been unsuccessful. Wind and water erode the surface of the bare hills as a result of which less-oxidized soil ma- terial becomes exposed and the process of acidification is repeated. An approximately similar situation to that in Jutland arises with the goldmine 197
Transcript
S O I L MATERIAL RICH I N PYRITE I N NON-COASTAL AREAS
J . N . B. Poelman S o i l Survey I n s t i t u t e , Wageningen
I . Introduction
Acid sulphate soils are normally associated with deltas and coastal areas which
are, or recently have been, under the influence of seawater. Pons (14) states in his abstract: "Acid Sulphate Soils are formed during ripening of marine muds
rich in pyrites when insufficient neutralizing components are present to prevent
a dangerous drop in pH level." Van Beers ( I ) , in a review of this topic, formula-
tes the principal processes. His first proposition reads as follows: "In marine
and brackish water sediments sulphur may be bound and accumulate under certain
conditions in the form of iron sulphides and elementary sulphur as a result of
the reduction of sulphates. The latter are chiefly derived from sea water".
In the present paper particular attention will be given to acid sulphate soils
which occur far from the coast and remote from the recent influence of sea water.
2. Examples of pyrite rich soil material in non-coastal areas
Outside The Netherlands
ChenerY ( 3 ) , in a search of the literature, found only two examples of acid sul-
phate soils which were not associated with marine or estuarine deposits. One
CaSe was that of the Solfatara muds of Java ( 4 ) , the other was that of deposits
in a lake bed in the Taiga region of Russia (15). He himself added one case, that
of "Papyrus Peat enriched with sodium sulphate from Precambrian schists and phyl- lites in the Kigezi District of Uganda at about 6,000 ft elevation".
Petersen et al. (12) describe the presence of pyrite in the lignite districts of
central and western Jutland where there are open-cast brown-coal mines with a
depth of 15-25 m. The soil which overlies the brown coal is being dumped to form
steep hills known as "tippings"; these consist of a heterogeneous mixture of
sand, gravel, and clay. This so-called lignite clay, which was deposited in the
Tertiary as a mud, is interlayered with peat layers (brown coal) and contains
much pyrite. The latter leads to strong acidification following oxidation, so
that attempts to afforest the tippings have been unsuccessful. Wind and water
erode the surface of the bare hills as a result of which less-oxidized soil ma-
terial becomes exposed and the process of acidification is repeated.
An approximately similar situation to that in Jutland arises with the goldmine
197
dumps in South Africa (6): "The Witwatersrand ores contain iron pyrites in vary-
ing amount and oxidation of the pyrite, often accelerated by bacteria, causes a
gradual accumulation of acidity in the surface layers of the dump".
In the course of an excursion forming part of the Congress of Northwest German
Geologists in May 1970, the present author was able to observe distinct cat-clay
phenomena at the open-cast lignite mine in Frechen (Germany), in the lowest lig-
nite seams at the transition to the clay subsoil.
During a palaeontological study of samples from the Permian Kupferschiefer of
Mansfeld, Germany, and the equivalent Marl Slate from northeast England, Love
(9) found: "...myriads of small sulphide spheres mostly up to 20 1~ in diameter".
These pyrite spheres are found in remains of microfossils, completely identical
with that which Love (8) found in "many dark shale rocks and some recent sedi-
ments" and which he named Pyritosphaera barbaria. "The principal forms are regar-
ded as the remains of microorganisms that caused the authigenic precipitation of
the primary sulfide in the sediment of the foul-bottomed early Zechstein sea of
Europe. Similar observations have been made from the Marl Slate of northeast
England, a contiguous deposit of the same sea".
Pyrite is also a not unknown phenomenon in the Carboniferous coal seams. The
theory has long been held that pyrite plays an active part in the self-combustion
of coal. This self-combustion can be a great problem, but in the mines in Lim-
burg, i n the southeast of The Netherlands, it occurs but rarely as compared with
other countries (7).
In The Netherlands
In Twente and the eastern part of Gelderland in the east of The Netherlands, Ter-
tiary marine deposits occur at, or close to, the surface; examples are the Heze
and Kuiperberg (70 m above ordnance datum), which form the highest parts of the ice-pushed ridge near Clotmarsum'). As a result of the considerable relief, large
valleys were formed by erosion and the lowest parts of these became filled with
colluvium. Later everything was covered with a thin mantle of cover sand. Peat
occurs locally in the valleys.
The grassland in the valleys is in a bad condition mainly because insufficient
drainage gives rise to poaching. An attempt was made at Ootmarsum to improve such
+) See for location of Ootmarsum Fig.1 of paper by C.J.W.Westerveld & A.F.van Ho1:;t p.256, V 0 1 . 1 1
198
a grassland parcel by loosening the 80-100 cm thick peat layer and to sand it with a layer of 20 cm of sand from the subsoil. However, the pH level of the new topsoil went down locally to 1 . 9 , so that the grass which was sown did not ger-
minate. It appeared that the sandy subsoil material contained a large quantity of
pyrite which caused acidification when spread over the surface and exposed to the
air. Wind and Steeghs ( 1 7 ) , who described these phenomena in 1964, gave these
acid sulphate soils the name of cat sand, in analogy with the term cat clay.
About hundred years earlier Van Bemelen had already mentioned similar phenomena
from the hamlet of Reutum, also close to Ootmarsum ( 1 1 , 13) .
In 1968 various other occurrences of cat sand were reported from Tubbergen, situ-
ated in the same area ( I O ) .
More recently, the author encountered cat sand in the Gelderse Vallei, a depres-
sion in the area of sandy soils in the central part of The Netherlands. The pre-
sence of sulphur in the subsoil has also been reported from other upland places
in The Netherlands, such as southeast Drenthe, Vriezenveen, and "De Peel" (per-
sonal communications). In all these areas newly laid tile drains became clogged
by a slimy sulphurous deposit.
3 . The recognition of sandy soils rich in sulphides
In contrast to the cat-clay soils in the western part of The Netherlands, which
are easily recognisable by the typical colours of the basic ferric sulphate, cat
sand lacks conspicuous visual characteristics and until lately it could only be
identified by laboratory investigation. Therefore the author developed a method
( 1 3 ) by means of which cat sand could be rapidly identified in the field++). This
method has been applied in various studies of soils in which cat sand occurs. It
appears from these studies that a strong accumulation of sulphides in sandy soils
can be recognized by a cyanic-blue to blackish colour of the soil. These colours
occur frequently in the immediate vicinity of material rich in organic matter
such as remnants of roots, but also appear as a fine mottling distributed through-
out a mineral layer.
Within these study areas cat sand has been encountered only rarely in hydromor-
phic podzol s o i l s , and more often in associated half bog soils and bog soils. In
the half bog s o i l s the sulphide concentrations occur mainly in and close under
the topsoil and in the bog soils in and below the transition from peat to sand.
++) A description of the method is given as an appendix to this paper
199
The distribution pattern of the sulphide in specific soil bodies and horizons
appeared to be quite irregular.
4 . The formation of sulphide accumulations in sandy soil material in non-coastal areas -
The formation of sulphides in soil material is generally accepted to occur in
wet anaerobic conditions in the presence of sulphate, iron, and organic matter
suitable to serve as a substrate for sulphate reducing microorganisms. Modern
literature on this subject ( 1 , 1 4 ) generally associates sulphide formation with
slow sedimentation of clayey soil material in shallow brackish water simultaneous
with vegetative growth e.g. reeds, rushes, mangrove. This relation holds indeed
for the most common, and the best known, acid sulphate soil formations.
The older literature (2, 5 , 18, 19) , based less on field surveys and often on
laboratory experiments only, allows more often for a supply of sulphates from
other than marine origin, and formation of sulphides independent of sedimentary
processes. Van Bemmelen ( 2 ) in his study of "acid soils" of 1886 discusses secon-
dary salinization by brackish seepage water as a possible factor in sulphide for-
mation in some polder soils.
In his descriptions of the sandy "acid soils" of Reutum, in the earlier mentioned
cat sand area near Ootmarsum, van Bemmelen pointed out the presence of iron sul-
phate in local seepage water ( 1 1 , 1 3 ) . Recent surveys have confirmed the presence
of sulphates in groundwater in other cat-sand areas (16, 1 7 ) . It is now known
that in the hills flanking the cat-sand valleys, outcrops of tertiary marine clays
occur, which are most probably the source of the sulphates in the seepage water.
Very favourable conditions for new formation of sulphides might be expected to
exist in the valleys where this seepage water is accumulating, creating at the
same time swampy sites with peaty topsoils. This is especially true for the zone
of contact between the water bearing sand and the overlying peat. In fact, this
is where the pyrite accumulations in the cat sand are concentrated.
The origin of the sulphides in the other cat sand areas mentioned has as yet not
been investigated, but their geographical position suggests that here, also, a secondary formation of pyrites, conditioned by a lateral flow of groundwater con-
taining some sulphates and feeding swampy depressions with accumulations of orga- nic matter, might be a plausible hypothesis.
200
5. Some concluding remarks
Although the above genetical hypothesis remains to be verified by analyses and
experiments, it can already be concluded that acid sulphate soils and sulphidic
soil material have a wider geographical range than is generally assumed. It is
moreover highly probable that laterally moving, relatively dilute sulphatic ground-
water can give rise to high concentrations of sulphides at specific sites along
its course. Therefore it may be expected that the scala of acid sulphate soil
phenomena is wider than is known at present. Quick field tests for sulphides,e.g.
a s described in the appendix, enabling more inductive surveying, might well be
instrumental in widening the scope of this specific chapter in pedology.
REFERENCES
(1) BEERS, W.F.J.van. 1962. Acid sulphate soils. 1nt.Inst.Land Reclamation and
Improvement. Wageningen. Bu11.3.
(2) BEMMELEN, J.M.van. 1886. Bijdragen tot de kennis van den alluvialen bodem
in Nederland. Natuurkundige Verhandelingen der Koninklijke Akademie van
Wetenschappen, deel XXV. Amsterdam.
(3) CHENERY, E.M. 1954. Acid sulphate soils in Central Africa. Transactions
(18 ) WOLZOGEN KUHR, C.A.H.von, VLUGT, L.S.van der. 1934. De grafiteering van
gietijzer als electrobiochemisch proces in anaerobe gronden. Water 18:147-165.
(19 ) WOLZOGEN K u H R , C.A.H.von. 1938. D e eenheid van het anaerobe en aerobe
ijzercorrosieproces in den grond. Water 22:33-38 and 45-47.
204
APPENDIX
METHOD FOR DEMONSTRATING THE PRESENCE OF SULPHATES AND SULPHIDES
Reagents
H 2 0 2 30 %
BaClz - 2 Hz0 IO % (w/w)
HC 1 a 1 : l dilution of conc. HC1
N a n S O h - 10 n20 (0.908 normal) ( 1 4 6 g per litre)
Apparatus
1 rack of test tubes or a number of 100-cc. flasks, 2 pipettes ( I O cc.each), a
number of funnels, filter paper, 1 pair of rubber gloves, pH-meter.
Procedure
Measure about 1 cm3 of soil into a test tube and add 10 cc. H 2 0 2 . Shake several
times to speed up the reaction (use rubber gloves) and allow about half an hour
for it to take effect. Then filter off and acidulate with several drops of hydro-
chloric acid. Add 5 cc. BaClz to the filtrate. In the presence of sulphate and sulphides oxidized to sulphates, a milky-white turbidity of BaS04 will appear.
Standard series
The degree of turbidity can be compared with the standard series, which is made
up as follows: Measure quantities of 1/10 cc. 4 cc. 1 cc. and 5 cc. of NanSOi,
into test tubes; add distilled water to make 5 cc. Acidulate with a few drops of
HC1 and then add to each test tube 5 cc. of BaC12 solution. Shake before measu-
ring. The quantity of SO4 in the four test tubes is then 4 . 4 - 22 - 44 - 220 mg respectively.
Explanation
This prescription is intended to describe a simple field method of demonstrating
the presence of sulphides. When sulphates are already present, this can be indi-
cated directly without the need of using the strong oxidant. In the event that
salt water (containing sulphates) is present, this can he checked with the pH-
meter. A high pH (8 and above) will point in this direction. It must also be
taken into account that there is a possibility that the H2O2 contains traces of
sulphates.
In the course of the treatment gas may develop, perhaps to such an extent that
the contents will "boil" out of the test tube. This can be prevented either by
205
cooling or by the use of a flask.
The difference in the degree of turbidity before and after the oxidation gives
an idea of the quantity of sulphates present in both cases. The degree of turbi-
dity can be compared with the standard series, from which a semi-quantitative
conclusion can be drawn.
The greater the quantity of sulphides present, the more the pH level will drop
after the oxidation. This can be measured with the pH-meter. However, certain
humus matter can also have an acidifying effect, through the oxidation with
H,Oz (Poelman) (13 ) ' ) . If, on the other hand, carbonates (CaC03, MgCO,) are pre-
sent, they will either partially or completely compensate for the drop in pH. The
presence of carbonates can be shown by the well-known method using HCl.
+) See the reference-list of the preceding paper
206
Smmary
Several examples are given of acid sulphate soils occurring in s i t e s beyond recent marine inf luences. Special. emphasis is given to sandy p y r i t i c soi2s i n the eastern part of The Netherlands. I n an appendix a rapid f i e l d t e s t for determination of
sulphides is described.
Résumé
L ' a r t i c l e donne plusieurs exemples de sols s u l f a t é s acides dans s i tua t ions l o i n de l ' in f luence marine récente. L'accent e s t sur des sols sablonneux r iche en p y r i t e dans l a par t ie e s t e de l a Hollande. Une appendice ddcrit üne épreuve de champ rapide pour l a dgtermination du su l fu re .
Resumen
Se da varios ejemplos de suelos de su l fa tos úcidos en sitios l e j o s de l a in f luen- c ia marina reciente . Se hace hincapid en suelos arenosos sulfurosos en e l parte e s t de los Países Bajos. En un apéndice se describe una prueba para l a determina-
ción rapida en e l campo de los s u l f i t o s .
Zusammenfassung
Verschiedene Beispiele schwefelsaurer Böden i n Lagen ausserhalb rezenter Meeres-
e i n f l l i i s s e werden gegeben. Insbesondere werden pyr i tha l t ige Sandböden im Osten
der Niederlanden hervorgehoben. Im Anhang wird e i n Schnellverfahren zur Bestim- mung von S u l f i d e n i m Felde beschrieben.
20 7
P H Y S I O G R A P H Y
C L A S S I F I C A T I O N
r4 A P P I N G
PROBLEME BE I DER KARTIERUNG VON "MAIBOLT" UND "PULVERERDE" - BESONDEREN FORMEN DER SULFATSAUREN BBDEN - I N DEN MARSCHGEBIETEN NIEDERSACHSEN (BRD)
J . H. BenzLer Nicdersüchsisches Landesamt fiir Bodenforschung Hannover-Buchholz
In den vergangenen 15 Jahren wurde durch die Ahteilung Bodenkunde des Nieder-
sächsischen Landesamt für Bodenforschung in Hannover das Gehiet der niedersach-
sischen Marschen im Masstah 1:25 000, an vielen Stellen auch im Masstab 1:5 000
kartiert. Dabei ist ein umfangreiches Material an Profilheschreibungen und ho-
denkundlichen Laboruntersuchungen angefallen, das es erlaubt, auch zur Frage
der sulfatsauren BÖden einige Aussagen zu machen.
Schwefelreiche Schichten kommen in den niedersachsischen MarschbÖden in zwei For- men vor: als Maibolt und als Pulvererde. Maibolt ist die regionale Bezeichnung
fÜr ein schwefelgelb geflecktes Material im Unterboden und Untergrund der Marsch-
bÖden im ubergang vom Klei zu unterlagernden Torfen oder im Ühergangsbereich
zwischen MarschbÖden und randlichen Mooren. Der Name ist gehildet aus Mai =
Maifeld (gemahtes Feld) und Bold = Kobold (Erdgeist). Nach altem Volksglauhen
macht ein bÖser Erdgeist (Kobold) das Maifeld unfruchtbar. Dieser Vorstellung
liegt die Beobachtung zu Grunde, dass maibolthaltiges Material, mit Grahenaushuh
oder bei anderen Massnahmen auf die Oberflache aufgehracht, den Pflanzenwuchs
fÜr langere Zeit, oft fÜr mehrere Jahre, schadigt oder sogar verhindert. Pulver-
erde (auch Bettelerde oder Gifterde genannt) ist die regionale Bezeichnung fÜr
ein hlauschwarzes Material im Untergrund der MarschbÖden, das nach Aufbringen
auf die Oherfläche unter Lufteinfluss zu einer pulverigen Vasse zerfällt und
ahnlich wie Maibolt wirkt.
Die Forschungen der letzten Jahrzehnte haben die Natur dieser Schadstoffe geklärt.
Maibolt ist ein basisches Eisen-Kalium-Sulfat, Pulvererde enthalt Schwefeleisen.
I n beiden Fällen wird bei BelÜftung Schwefelsäure freigesetzt, wodurch das pH der
BÖden zu extrem sauren Werten absinken kann. Die chemischen Fragen sollen hier
nicht näher besprochen werden.
Die Feststellung des Vorkommens von Maiholt und Pulvererde ist a l s o fÜr praktische
Belange sehr wichtig. Bei der Kartierung wurde deshalb besonders darauf geachtet.
Ein anderer Grund kommt heute noch dazu. Bei der Handarbeit in frÜheren Zeiten
wurde das Auftreten von Maibolt und Pulvererde von den Arbeitern sofort bemerkt
und entsprechende Massnahmen ergriffen. Beim modernen Maschineneinsatz ist die
21 1
Gefahr viel grosser, dass unbemerkt, etwa bei der Grabenraumung, grössere Men-
gen dieser Schadstoffe auf die FlSche gebracht und starkere Schäden verursacht
werden. Um so wichtiger ist es daher heute, dass auf den Bodenkarten genaue
Hinweise auf das Vorkommen von Maibolt und Pulvererde gemacht werden.
Auf einige Besonderheiten, die bei der Kartierung von BÖden mit Maibolt und Pul-
vererde zu beachten sind, so l1 noch naher eingegangen werden.
Bei bodenchemischen Laboruntersuchungen maibolthaltiger Roden wurde festgestellt,
dass die hÖchsten Sulfatgehalte keineswegs immer in den Schichten gefunden wer-
den, die im Bohrer die starkste Maiboltfleckung zeigen. Oftmals war in den dar-
unter liegenden, nicht schwefelgelb gefleckten Schichten ein weit hoherer Sul-
fatgehalt festzustellen. Verursacht wird diese Erscheinung dadurch, dass die
schwefelgelbe Fleckung erst bei Oxydation auftritt, also im G -Horizont, wahrend
sie im G -Horizont fehlt, solange dieser vollig reduziert bleibt. Sowie das farb-
lose Material des G -Horizontes kurzfristig der Luft ausgesetzt ist, etwa beim
Ausheben von Gruben, stellt sich die Farbung ein. Aus der durch die Kartierung
erlangten Kenntnis, dass mit dem Auftreten von Maibolt immer dann zu rechnen ist,
wenn Kleischichten und Torfe ineinander Übergehen oder eng benachbart sind, er-
gibt sich die Notwendigkeit, in diesen Gebieten vorsorglich auch dann Proben aus
den in Frage kommenden Schichten zu entnehmen und etweder einige Tage an der Luft
zu beobachten oder im Labor auf Sulfat zu untersuchen, wenn im Bohrer keine An-
zeichen von Maiboltfleckung zu sehen sind. Es kÖnnte leicht sein, dass die Mai-
bolt fÜhrenden Schichten, besonders bei standig hohem Grundwasserstand in den
niedrigen Sietlandgebieten, noch vollstandig im G -Horizont liegen und daher noch
nicht gefleckt sind. Trifft man diese Vorsichtsmassnahme nicht und schliesst al-
lein aus der fehlenden Fleckung auf das Fehlen von Maibolt, so kÖnnte bei spate-
ren Meliorationsmassnahmen, die sich auf die Angaben der Rodenkarte verlassen,
leicht der Fall eintreten, dass unbemerkt maibolthaltiges Material, zum Reispiel
bei der Vertiefung des Grabennetzes zur besseren Entwasserung, auf die Flachen
aufgebracht wird und schwere Schaden verursacht werden.
A l s Pulvererde kann man nicht einfach jedes blauschwarze Untergrundmaterial an-
sprechen. Ausschlaggebend ist seine Kalkfreiheit oder zumindest Kalkarmut. Bei
ausreichendem Gehalt am Kalk (CaC03) wird die freigesetzte Schwefelsäure zu Gips
neutralisiert und damit unschädlich gemacht. Es genügt also nicht, nur das Vor-
kommen blauschwarzer Schichten festzustellen, sondern es muss in jedem Fall das
Fehlen von Kalk nachgewiesen werden, um die Aussage "Pulvererde, das heisst
Gefahr von Pflanzenschäden" rrachen zu kÖnnen. Andrerseits kann das lleraufbringen
212
von kalkreichem Untergrundmaterial zur Bodenverbesserung (Blausandmelioration,
WÜhlen oder KÜhlen genannt) dadurch beeinträchtigt werden, dass aus den kalkhal-
tigen blauschwarzen Schlicken ein betrachtlicher Teil des Kalkes zur Neutrali-
sation freigesetzter Schwefelsäure verbraucht wird und damit fÜr die Aufkalkung
der Krume nicht mehr zur Verfügung steht. Dadurch kann die Wirtschaftlichkeit
dieser Massnahme in Frage gestellt sein.
Auf den modernen Bodenkarten des Niedersächsischen Landesamtes fÜr Bodenforschung
sind daher fÜr die Marschengebiete die Vorkommen von Maibolt und Pulvererde nach
Ort und Tiefenlage der Schichten angegeben und zusatzlich in der Tabellenlegende
der Karten zur naheren Beschreibung der Kartiereinheiten unter der Spalte "sonsti-
ge wichtige Eigenschaften und besondere Beimengungen" aufgeführt. Der Meliorati-
onstechniker kann sich also leicht an Hand dieser Bodenkarte Über das Vorkommen
dieser Schadstoffe unterrichten.
213
Swmnary
In t he lower Saxony coastal area, acid sulphate soils and p y r i t i c sol:Í! material
occur pre f e ren t l y in t r a n s i t i o n s betueen peat and marsh depos i t s . For adequate
planning of soil reclamation the precise depth of p y r i t i c s o i l layers should be
known. Modern de ta i l ed s o i l maps of the area provide t h i s information.
Résumé
Dans l a &ne co'tidre de l a Saxe Basse, les sols sulfate's acides e t l e s roches
mDre pyr i t i que se trouvent de préférence dans l e s t rans i t i ons entre tourbes e t
dép6t.s de marée. Pour l a récupération de ces s o l s i1 f a u t connactre l a profondeur
des couches pyr i t i ques e t e e t information ei a été incorporée dans l a légende
des cartes de sol d é t a i l l é e s de l a région.
Resumen
E n l a Sajonia Ba's en e l l i t o r a l , l o s suelos de s u l f a t o s ácidos y las capas
su l fu r i eas se encuentran en las transiciones entre tourbas y sedimentos marinos.
Para l a recuperación de esos suelos se neces i ta conocimiento de l a profundidad
de las capas su l fu r i eas y e s t u informaeidn se da en las mapas de suelo detal ladas.
Z usammen fas s ung
In den Marschgebieten Niedersachsens bef inden s i ch d i e sul fatsauren Böden und d i e
schwefeleisenhaltigen Untergriinde vornehmlich an den S t e l l e n , wo Tonablagerungen
und T o r f ineinander übergehen. Für Meliorationsmassnahmen braucht man d ie T i e f e n -
lage, besonders d i e der schwefeleisenreichen Schichten, zu kennen. Betreffende
Einzelhei ten sind in den modernen und ausführlichen Rodenkarten angegeben.
2 1 4
ACID SULPHATE SOILS I N HONG KONG
Charles J . Grant Hong Kong University Hong' Kong
Introduction
Soils that develop extreme acidity on drying have been reported in the Pearl
River estuary (Huang 1958; Kung & Chou 1964) and in Hong Kong (Grant 1960), but
it was n o t until the rapid expansion of fish farming in the past 5 years that
problems associated with these soils became acute in relation to economic deve-
lopment in this area.
The Pearl river, with an annual liquid discharge of 668 O00 millions of cubic metres (Lu 1956) flows through areas of severe erosion, and in the Canton delta
deposits 28 million tons of silt annually (Kovda 1959). Extensive mudflats have
formed along the shores of the estuary, and many formerly prosperous ports (e.g.
Macau, Fatshan, Nam Tau, Yuan Long) have become silted up. There are some in-
dications that the rate of coastal progradation in this delta has increased in
the past 300 years, but there has been massive sedimentation since the Han colo-
nisation of Kwangtung at the end of the Sung dynasty (12th century) initiated a
period of widespread deforestation.
In Hong Kong recent mudflats and acid sulphate soils occur principally in the
Deep Bay area to the northwest of the Colony. Formerly,these areas were barren
wastelands with dwarf mangrove (Rhizophoracea, Aegiceras, and various Cyperaceae).
In areas not regularly covered with tidal waters but with the watertable near the
surface, occasional crops of seagrass (Eleocharis spp.) were grown for matting,
and on the slightly drier areas brackish water rice was grown. Between 1958 and
1968 the fishpond area has increased from 500 to 2000 acres and many more ponds
are under construction. Most of this development has taken place in the mudflats
of Deep Bay.
Location and geomorphological characteristics
The Pearl river, flowing into the South China Sea immediately to the west of Hong
Kong, is formed by the confluence of three rivers: the Peikiang (North River),
the Sikiang (West River), and the Tungkiang (East River). Canton lies at the apex
of the estuary while the territories of Macau and Hong Kong are situated on either
side of the entrance to the channel some 70 miles to the south (Fig.1).
215
Deep Bay is a landlocked inlet in the northeast of Hong Kong and sedimentation
in the bay is markedly influenced by marine action and salinity. Mudflats are
extensively developed on the southern (Hong Kong) shore of the embayment, but
within the past 300 years there was another outlet from Deep Bay to Tuen Moon
5 miles to the south. Silting cannot have continued at the present rate for many
centuries and it is probable that a major shift in the courses of the Peikiang
(Hui 1956) and of the Sikiang resulted in the bypass of Deep Ray inlet by main
channel flow of the Pearl River estuary.
Resultant on change of current, the island of Nam Shan at the mouth of the
inlet became linked to the coast of Po On County by a tombolo complex, cutting
off Deep Bay from the direct riverine scouring influence of the Pearl River.
Sedimentation is most active around the outlets of the Shum Chun river and Yuen
Long creek but the catchments and rate of flow of these watercourses are inade-
quate to account for the volume of sediment accumulated within the past three
hundred years. The materials appear to be primarily derived from the Pearl river,
but are brought into the embayment in dispersed suspension by saline waters from
the estuary mouth at high tide. The sediments are flocculated and precipitated
on contact with the freshwater of Shum Chun and prevented from re-entry to the
estuary at low tide by the seaward flow of Pearl river water past the bay mouth.
Intertidal flats occur in an arcuate area between the Shum Chun and Yuen Long
streams. There is a marked break of slope at the 10 m contour, which may be taken
as roughly corresponding to the pre-Han coastline, and a less well defined break
at 3-5 metres. Below 3 metres lies the wide expanse of recent sediments with le-
vee and backswamp undulations but no definable terrace edges that might indicate
isostatic readjustment or independent diastrophic movement (Fig.2).
The sediments of the 5-10 m terrace are in general coarse textured with numerous
quartz grains and are clearly directly derived from erosion of adjacent granitic
and metamorphic rocks, whereas the more recent deposits are silt loam or silty
clay loam materials with distinctively marine characteristics. Construction of
bunds for fishponds and brackish water paddy fields has obscured the morphologi-
cal features of the recent sediments, but the main natural subdivision of the area
is a series of sluggish drainage channels sub-parallel to the coast midway between
the 3 m contour and mean sea level.
All of the soils below the 3 m contour may be described as acid sulphate soils
as defined by Moormann (1963) i.e. they are marine floodplain soils that, upon drainage and aeration, show definite ar.d severe acidification due to the oxidation
of sulphides. Within this broad category Moormann proposes the term "cat-clay" to
216
P
be applied to acid soil material in its oxidized form, showing straw-yellow mott-
lings, and streaks of basic ferric sulphate, and "mud.clay" to refer to the non-
drained unoxidized phase with high potential acidity. Both the mud-clay and cat-
clay phase are represented in Hong Kong as shown in the transect diagram (Fig.3).
Intermediate forms between the two phases are frequent in the fishpond area;
the upper profile is oxidized and acid, whereas the subsoil is formed of reduced,
near-neutral mud-clay.
Environmental conditions
Acid sulphate conditions are found only on sediments below the 5m contour, but
the distribution of sulphur compounds and of acidity, or potential acidity, is
variable both with depth, within a single sediment core, and areally, from one
sampling site to another. These variations may be associated:
a) with activity and differences in metabolic rates of heterotrophic sulphate
reducing bacteria that under anaerobic conditions reduce seawater sulphate
to H 2 S (Berner 1964).
b) with the distribution of mangrove (Huang 1958), and
c) with the degree or nature of drying out of the soil.
So long as seagrass and brackish water paddy were grown, drying out of the soils
was gradual and acidification was not a problem, though efflorescences of sulphur
and sulphates formed on the low bunds between fields. When fishponds began to be
constructed on a large scale in the 1960's, oxidation of the soils was accelera-
ted. There is no clear pattern Of susceptibility to acidification and some fish-
ponds have been unaffected while adjacent ponds have had to be abandoned.
Coastal emergence in Deep Bay has been partly natural and partly accelerated
by the construction of seawalls of mud to form kei wat or tembaks, which are
large enclosures formed on mudflats and surrounded by bunds up to 2 m high. Each
kei wai is about 25 acres (IO ha) in area and acts as a tidal fish trap. Most of the present kei wai (Fig.2) were constructed in 1940/41 but are largely inope-
rative now or have been converted to brackish water fishponds.
Coastal reclamation in this area has followed the sequence: tidal flat, mangrove
colonisation, kei wai construction, fishpond conversion. The speed of colonisa-
tion by mangrove may be gauged by the fact that two mangrove patches totalling
16.3 acres (6.5 ha) in 1963 had coalesced and extended to 20.4 acres (8.2 ha) in
1968.
Slow sedimentation from impounded water in the kei wai has raised the land surfa-
. ". . . ,. , .. . . . . . .x.. " _. , .. . .w .
ce within the enclosures 30-50 cm higher than the average land level in the fish
pond area.
Physical and chemical characteristics of the soils
Sediments in the top metre of the areas occupied by mudflats, mangrove, kei wai,
and fishponds (Fig.2) have been affected by subaerial alteration and organic
activity so that they may be properly considered as soils.
Textures are uniformly fine in the Silty Clay - silty clay loam range and have distinct horizon differentiation (Fig.4) in the areas not affected by urban or
agricultural drainage. Depth to water-table and length of daily or seasonal
exposure to drying between tides is closely related to the thickness of the sur-
face horizon. At the more inland locations near the edge of the 3 m terrace the
surface horizon is sufficiently thick (20-30 cm) to permit shallow cultivation of
vegetables provided that adequate fresh water is available for surface irrigati-
on and that upwelling of toxic water from the subsoil is avoided.
The subsoil is uniformly bluish black (5B 2/1) when wet, but if permitted to dry
out the soil becomes grayer (IOGY 6 / 1 , greenish gray) with numerous mottles and
streaks of basic ferrisulphate. In the wet condition the soil reaction is pH 7.5
throughout the profile but, on drying, all horizons, but particularly the G be-
come more acid and values of pH 2.3-3.0 have been noted in Hong Kong.
Sulphur is present in appreciable amounts throughout the profile but is seldom
less than 0.8% (in the surface horizon) and is normally 1.5-2.52. The environment
is similar in many respects to the intertidal flats of The Wash in England
(Love 1967) and the Netherlands Waddenzee (van Straaten 1954). The high proportion
of sulphur in Deep Ray soils may be due partly to the influence of mangrove ve-
getation. Huang (1958) reports a total S content of 4.69-6.62% in the leaves of
Rhizophoraceae and Aegeciras in nearby Kwangtung Province. The more commonly at-
tributed cause of sulphur concentration in these soils is the reduction of sul-
phate from seawater by micro-organisms (Starkey 1950).
Copper is present (10-100 ppm Cu) in all profiles, mainly in the form Chalcopy-
rite, but investigations into diagenetic formation of copper compounds in these
A l l of the soils in Deep Bay below the 3 m terrace edge are liable to become acid on drying and oxidation, but the degree of acidification is not directly related
to total sulphur percentage in the soil.
A total of 540 samples representing 90 profiles have been examined to determine the extent and rate of acidification. The detailed results of this study are the
subject of a separate paper, but briefly it may be noted that almost all the
samples showed a marked fall of pH within the first week of air-drying. The de-
crease continued in more than 85% of the samples, but began to stabilize at the
end of 5-6 months. Air drying at 25 O C for periods of several months is not re-
presentative of field drainage and oxidation conditions with probable alternation
of wetting and drying, but does give some indication of the relationship between
profile characteristics and soil reaction.
In the graph representing profile location S6 (Fig.5) the oxidized topsoil sample
(175) and the GFeSn bottom sample (180) show little change in pH over the 7-month period. Samples 176-179 became moderately acid and showed the black monosulphide
coloration. At profile location R 3 (Fig.6) whitish subsoil was not reached in
profile depth, and at profile location S 4 (Fig.7) the oxidized layer at the sur-
face was too thin to be separately sampled. Berner (1963, 1967) has shown that
the first-forming iron sulphide in many marine sediments is black, amorphous Fes.
219
Transformation of sulphate from seawater through phases of monosulphide (Hydro-
troilite, or mackinawite and melnikovite or griegite) to pyrite is indicated by
colour changes in the soil and is related to the extent and rate of acidification
on drying.
The downward sequence of brown, black, gray, and grayish white indicate stages in
progression of pyrite diagenesis.
minant and the sulphur content is generally low. Soils of the black horizon
quickly become acid on drying and react strongly with dilute H C 1 to provide H z S .
Downward the soil becomes progressively grayer and, in spite of high total sulphur
content, acidification on drying is less extreme and dilute H C I produces little
react ion.
Where drainage has occurred in the fishpond area, a variable thickness of yellow
mottled cat-clay directly overlies the G and G horizons. The water-table
is high throughout the fishpond area and cat-clay only occurs on drained and dis-
turbed building sites or on fishpond bunds, so that natural profiles could not be
positively established. However, soil samples from cat-clay material give consis-
tently lower pH values (pH 2.5-3.5) than have been recorded in air-dried mud-clay.
The reason for this difference is believed to be incomplete or slow drying in the
field.
To test the effect of wet storage of mud-clay on acidification, some samples have been kept in wet field condition. The samples were prepared in identical
volumes and with identical surface areas to equalize any effect of surface oxida-
tion and diffusion. Oxidation was prevented in one batch of samples by storing
under nitrogen.
The results of this test (Fig.8) indicate that a l l samples kept in field condi-
tion become more acid than air dried soil and that exclusion of oxygen accentuates
the acidification.
The reasons for this phenomenon are not clear but the implications are that in
the reclamation of these soils drainage should be complete and thorough. Labora-
tory preparation of the air-dry soil by grinding to pass a 2 nun sieve apparently
allows oxidation and gaseous loss of sulphur compounds that would be re-absorbed
if trapped in clods of clay or damp soil.
In the brown topsoil, ferric hydroxide is do-
FeS FeSz
220
2- CK 6
m W
CK W
Lli
2
2
1 CK 6 W a W I c
O c
z 2 c
5 W CK
o Z O Y
c3 Z O I
O Z Q
t a m
a W W O
7
m - LL
2 2 2
L A N D U S E
VEGETATION
GEOMORPHOLOGY
SOILS
I 1 2 CROP R I C E K V E G E T I B L E S I B R A C K I S H W A T E R F R E S H WATER
F I S H P O N G F I S H POND I O Y S T E R F I R M I N G 1 T E M B A K l K c i W a i l
C O I I U V ' A I 1 5m T E R R I C E I H I L I F O O T I R E O YELLOW
M U O C L I V p o G s o L , c PAOOV S O I L S
5 O0 1000 M c f r c r '"1 O-
PRINCIPAL OAlUU
F i g . 3 TRANSECT DIAGRAM ACROSS DEEP BAY ALLUVIAL FLATS,
N N w
I
a 3 I
3 u)
a J
A
2 c
Y) - c L" a w + U 4 a 4 I U
L Z L OL
N:
ai - 0 - I:
a 3 O
O u
2
* o
1 N
o
* o o
I "7
N
5 o m
*-
4 - W Y
azI u) I-
-I LL O 3 I IA O
u) t
o I O 3 z
a
3
z I- z W 5 a O -i w > W O
W
IA O cs a
='
U
m LL. .-
224
1 6 9 ~ 5 7 0
1 7 0
-
i 5 7 6
5 LO 6 LO
F ig . 5 PROFILE
Ghur ‘I.
O 1 0 -
1 2 6
220
2 2 6
2 L5
0 1 6
_ _
- . ani<
2.35
AV* --
6 1 0
7 20
7 65
6 65
6 L5
-~
4 , Y e -
6 L O
6 LO
6 72
6 88
7 3 6
6 88
-
.- impli mbei
175 -- ~
1 7 6
177
178
179
1 6 0
- --
5-6
m p l e mber 1 3 3 ~~ -
13L
135
136
137
1 3 8
-~
-~ I e p t h
0-6
6-12
12-18
18-21,
21,-30
30-36
F ig . 6 PROFILE R--3
S a m p l i n g D a t e 2 n d Dec.1967
-‘I-- 1-- r-
I ,“C
6 -12
12-18
18-21
30-36
3
- F l e l d Condi t ion
~ Air O r y 1 Week
c Air Dry I M o n t h
~ A i r Dry 3 Months
. . . . . . . . A8r Dry 6 M o n t h s
- F i e l d Condition
~ Atr Dry 1 Week - A i r Dry 1 M o n t h
Atr Ory 3 Months
.. . . . . . . Atr Dry6 Months
225
_ _ ilphui
011
I l l ~~
1 65
2.20
2 o9
1.86
2.03
-
~~
ga"#< , t t i . r5
7 LO ~~ ~
8 2 0
7 3 5
7 20
5 5 5
5 90
~
.,O,'/< ~~
6 O8
6 2 1
6 LO
6 08
6 O8
6 72
~
~~
imple i m b e r
1 6 3 -
1 6 1
165
166
167
168
~
Sampling Dote 2"d Dec ,1967
3 ~ 5 6 7 8
F i g . 7 PROFILE S-L PH -
S a m o l i n a D a t e 12thFeb.1968 -
ulphui o/.
O 13 -
o 63
O 62
a 5 L
o L I
0 7 3
-
lrganir ~ dllii'i
3 o0
3 65
3 2 5
3 O0
2 3 5
3 O0
__
imp: Im_bg
2 9 1
2 9 2
293
2 9 L
295
296
- F l e l d Condi t ion
~ A # r Dry I Week - Air Dry 1 Month
- ~~ A i r Dry 3 Months
. . . . . .. . ~ l r Dry 6 Months
- Field Condition - Air D r y 1 Month
- Air Dry 3 Months
- .. ... Alr Dry 6 Months _ _ F i e l d Wetness
6 Months in a i r
6 Months under N 2 F i e l d W e t n e s s .~~._
F i g . 8 PROFILE U-6 p H -
226
REFERENCES
BERNER, R.A. 1963. Experimental studies in the formation of sedimentary iron
