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Instant gratification has, as most of us know, a strictly limited appeal:
the cake that we crave may well leave us bilious, the country cottage we
‘must have’ might be riddled with dry rot. So with experience, we can see
that the real consequences of our decisions often emerge long after they
are made. The environment reacts in a similar way to human actions – it
has built-in mechanisms which often delay the full effect of any damage
until much later.

One important mechanism is the ability of soils and sediments to retain
toxic substances. This is usually seen as a good thing because it means
chemical pollutants can be locked up in the soil, making them unavailable
to harm plants and animals. This view may be valid over the short term –
say, several decades – but not in the long term. Contaminated soils around
the world mean that many people are literally sitting on ‘chemical time
bombs’. To predict where and when they will go off we need to understand
that soils and sediments are dynamic systems whose capacities to buffer
the environment from the effects of toxic chemicals may not only change,
but change in ways that are unpredictable.

A good example of a chemical time bomb that has already exploded is
in the southeast of the Netherlands, where phosphates from animal manure
fertilisers have exceeded the natural buffering capacity of the agricultural
soils in which they are used. After four decades of continuous application,
these soils are so impregnated with phosphates that they are now polluting
ground and surface waters nearby. This problem didn’t come to light until
very recently but will persist for the next 50 years, even if phosphate
use is reduced.

A second example of a chemical time bomb that has exploded is in the
Adirondack Mountains of New York State. Here, the pH of the water in Big
Moose Lake (Figure 1) remained nearly constant for at least 200 years, until
1950. But from 1950 to 1980, severe contamination from acid rain resulted
in a drop in pH – from about 5.5 to 4.5 – which represented a tenfold increase
in the acidity of the lake water. This acidification was caused mainly by
the burning of coal containing high levels of sulphur several hundred miles
away in the Ohio River valley, causing formation of atmospheric sulphuric
acid which was deposited in lakes down wind. Emissions from these upwind
sources began around 1880, with the industrialisation of the American Midwest.
They rose sharply until around 1920, and remained relatively stable until
1980. Yet the acidification of the lake water lagged 70 years behind the
onset of the emissions, and 30 years behind their peak.

Why did this time lag happen? The buffering capacity of the soils in
the watershed of the lake was initially large enough to neutralise the acid
from the atmosphere. But after 70 years, this capacity was finally exhausted,
and the lake became vulnerable to acidification.

Understanding the balance between two key phases – accumulation and
release – is the key to predicting where the next chemical time bomb is
likely to go off, and trying to prevent it. This is important not least
because the phase where the toxic chemicals are released or dispersed can
happen quite quickly compared to the decades or even centuries it takes
them to accumulate in soils and sediments.

Toxic substances may interact with soil and sediments in two processes.
Both involve chemical exchanges between solid and liquid phases. Adsorption/desorption
is where toxins can be either deposited at selected sites on solid surfaces
or dissolve in the soil’s liquid phase. Precipitation/dissolution involves
the deposition of toxins as solids and their dissolution back into solution.
In lakes and rivers, adsorption/desorption takes place on the surfaces
of solid particles suspended in the water. In soils, these exchanges occur
between the surfaces of soil particles and the ‘soil solution’ – the moisture
that sticks to the soil particles or is held in pores within the soil structure.

Since most biochemical reactions go on in solution, chemicals associated
with solids are usually immobilised and biologically inert, while dissolved
chemicals are biologically available. If these dissolved chemicals are
toxic, environmental damage can result.

The solid surfaces that interact with substances dissolved in natural
waters and soil solutions include clay minerals, carbonates, quartz, feldspar,
organic solids and hydrated oxides of manganese and iron. They contain negatively
charged sites, which means that they attract positively charged ions, or
cations. These fall into three main groups: heavy metal ions such as cadmium,
lead, mercury, and zinc; protons (H+); and salts of sodium, calcium potassium
and magnesium, also called base cations.

The four base cations are nontoxic and are commonly found in unpolluted
soils. But when soils and water are polluted by acid rain, these cations
are replaced by protons, which bind more strongly to the negatively charged
sites on the solid surfaces. The base cations, released from the adsorption
sites, then dissolve in water. The process in soils polluted by heavy metals
is similar. It is precisely this exchange process that causes the protons
and heavy metals to be immobilised in the soil while releasing mobilised
base cations.

So how long can a soil continue to act as a buffer against toxic chemicals
by immobilising them in this way? One major determining factor is the number
of adsorption sites per unit of surface area – called the cation exchange
capacity (CEC). This can be measured, as can the percentage of the adsorption
sites occupied by base cations. Multiplying these numbers together gives
a figure that is a good indicator of a soil’s capacity to adsorb heavy metals.
For a given volume of soil this capacity depends on the overall surface
area of the particles in it. Soils made up of large particles tend to have
a lower CEC than those containing small particles. So sandy soils which
consist of fairly large quartz particles (around 100 to 200 micrometres)
have a low CEC compared with clayey soils where particles are very fine
(generally less than 2 micrometres).

What triggers the bomb?

The second important process for the accumulation of toxic chemicals,
precipitation/dissolution, depends on how soluble a compound is in water.
Table salt, for instance, is highly soluble, whereas mercury sulphide is
highly insoluble. Once again, it is the dissolved chemicals that are available
for biological activity. Their solubility can be measured. Chemists describe
it in terms of the ratio of dissolved to precipitated compound in a saturated
aqueous solution. For mercury sulphide, this equilibrium constant (K) is
extremely small, 10 ^-52, which means that most of the mercury
is present as insoluble sulphide. But things are not quite that simple.
Mercury sulphide, like other metal sulphides, is chemically stable only
under certain conditions. These depend on the redox potential of the soil
– how good an oxidising agent it is compared with others . Sulphides are
stable under reduced redox conditions, but when soils are exposed to air
the sulphides will be oxidised, increasing the solubility of heavy metals.

In general, if a relatively insoluble pollutant accumulates in soil
mainly by precipitation, adding more will not greatly affect the amount
of dissolved pollutant. Changes in soil conditions may, though. On the other
hand, when adsorption is the main mechanism by which a toxic chemical accumulates
in soils, adding more will gradually increase the ratio of adsorbed to desorbed
chemical until the ‘maximum sorption capacity’ of the soil is reached. From
this point, all added chemical is essentially available for leaching and
bioaccumulation (see Figure 2).

Large sorption capacities are common in nonacidic soils such as the
black chernozem soils of Ukraine and southwest Russia, which are rich in
clays, organic matter, ferric and manganese oxides, and base cations. These
soils can cope with high inputs of heavy metals, and will not become saturated
for a century or more, depending on the level of pollution. Whereas predominantly
sandy soils, such as the orthic podzols of Scan-dinavia, Finland, and northwestern
Russia – which are acidic, and have less organic matter and a smaller surface
area per unit weight, reach saturation point much more rapidly. Their adsorption
capacity is about five times lower than that of a typical chernozem soil.
However, chemical time bombs may be more serious in nonacidic, organic,
and clayey soils, because, although accumulation time is usually longer
here, the amount of stored pollutant is greater and they tend to explode
with greater impact.

So what triggers the bomb? Big Moose Lake is a clear example of what
happens when levels of pollution reach saturation point; but explosions
may also occur unexpectedly, in areas of moderate contamination. These happen
because the maximum soil sorption capacity is not constant: it changes,
depending on various chemical parameters. For heavy metals, the most important
of these are acidity, the amount of organic matter present, salinity, CEC
and redox potential. A change in any of these key parameters can reduce
the number of available adsorption sites in the soil, and hence decrease
the maximum adsorption capacity. These key soil properties are linked in
a complicated way to the geochemical cycles of sulphur, nitrogen, carbon,
iron, manganese and calcium. So major changes in these chemical cycles brought
about by human activities are also likely to significantly alter the balance
between adsorption and mobilisation of pollutants in the biosphere.

This suggests that current guidelines for classifying contaminated areas
give a false sense of security. If the contamination of an area is below
a certain level, it is considered safe, and all the attention and money
is directed towards highly contaminated, heavily publicised sites. At present,
none of the guidelines consider changing environmental conditions in moderately
contaminated sites which may tip the balance and release the accumulated
contaminants in the future . This is a major oversight.

Tipping the balance

Changes in redox potential seem to be particularly important in triggering
chemical time bombs. One example is in the Netherlands, where sediment spoils
dredged from the Rhine to keep navigation lanes open are causing a serious
problem. Until the early 1970s, these sediments were applied, several metres
thick, to agricultural land. Then people discovered that this practice was
contaminating the soil with toxic heavy metals. Investigations showed that
changes in the redox potential of sediments following dredging results in
the oxidation of metal sulphides, a process which creates the more soluble
metal sulphates. Today the spoils are considered hazardous waste and are
stored, at great expense, in special sludge depots in Rotterdam’s harbour.

A similar thing happens when wetlands containing iron sulphides, called
pyrites, are drained. Sulphide is oxidised to sulphate and then to sulphuric
acid. In 1986, Ingemar Renberg reported a striking example of this in Lake
Blamissusjon in northern Sweden. With a pH of about 3, it has been called
‘the most acidic lake in Sweden’. But here acidification was not caused
by acid rain, but by drainage of adjacent wetlands which were converted
to agricultural lands in the first half of the century.

As well as acting directly on toxic chemicals within soils, redox potential
also strongly regulates the behaviour of microorganisms. Peter Doelman,
senior adviser at International Water Supply Consultants, an independent
environment company in the Netherlands, believes that such regulation can
also help to trigger chemical time bombs. One example backing his view is
the methylation of inorganic metals by microorganisms in the absence of
oxygen. This kind of reaction was responsible for a notorious episode of
mercury poisoning in the mid-1950s among villagers at Minimata Bay in Japan.
Here a chemicals company discharged large amounts of a stable mercury compound
into the bay, assuming that it could have no harmful effects. However,
under the anaerobic conditions in sediments at the bottom of the bay, bacteria
converted the harmless inorganic mercury into methyl mercury. This toxic
form of mercury then accumulated in the food chain especially in fish, a
staple food among local villagers.

Methyl mercury is now fuelling another chemical time bomb. Mercury forms
an amalgam with gold, and is widely used in Brazil, Venezuela, the Philippines
and Indonesia to extract gold from low-grade deposits. In Brazil’s Amazon
Basin alone, 250 to 300 tonnes of mercury are used in gold mining every
year. In 1990 Olaf Malm and colleagues from the Federal University of Rio
de Janeiro reported concentrations of mercury in edible carnivorous fish
of as much as 2 to 3 micrograms per gram (mu g/g), wet weight, near the
mining sites, and 1.5 mu g/g at a distance of nearly 200 kilometres from
the sites. The WHO’s recommended limits for human consumption are between
0.2 to 0.3 mu g/g.

Mercuric rise and fall

Now Brazilian geochemist Drude La Cerda and Wim Salomons have worked
out the sequence of events that triggers the dispersal of mercury and its
subsequent conversion to methyl mercury. The gold-mercury amalgam is heated
to purify the gold. Vaporised mercury is then dispersed into the atmosphere
and oxidised to make ionic mercury (Hg2+) through reactions involving ozone,
solar radiation, and water vapour. Once formed, ionic mercury is removed
from the atmosphere by rainfall and deposited in the Amazon Basin, where
it is converted to methyl mercury. Methyl mercury leaches from soils into
the rivers and is carried into reservoirs on the Amazon built to generate
hydroelectric power.

These chemical time bombs have been isolated events. But in the future,
factors such as climate change could have huge effects on elemental cycles
which are linked to soil properties. This could in turn fundamentally alter
the way the biosphere adsorbs and releases toxic materials. Temperature
and rainfall are linked to soil variables such as organic matter content
(within the carbon cycle), nitrification (within the nitrogen cycle), soil
moisture and rate of leaching. These variables are in turn linked to CEC,
pH, and redox potential. If these were to change radically, toxic chemicals
stored out of harm’s way in soils and sediments could be mobilised and dispersed.

But if we could predict when chemical time bombs would explode, how
would this help? Conventional engineering solutions for the cleanup or isolation
of contaminated soils and sediments are far too expensive to be applied
over large areas. A few years ago, Salomons and Ulrich Forstner of the Technical
University of Hamburg-Harburg in Germany proposed ‘geochemical engineering’,
a concept that borrows much from natural mechanisms. For example, sulphide
oxidation could be prevented by keeping soils anaerobic through the management
of hydrology; toxic metals could be immobilised through the addition of
materials with adsorbing or neutralising capacities; wastes such as phosphogypsum
(sulphate) could be mixed with dredged material or sewage sludge to convert
the metals to insoluble sulphides; and oxidation of metal wastes from mines
to acids could be avoided by covering the mine spoil with clay-rich soils,
or ‘drowning’ the waste dumps with water.

Chemical time bombs are not caused solely by high inputs of toxic chemicals.
Rather, they are the result of complex interactions that transform soils
and sediments from sinks that accumulate toxic chemicals into sources that
disperse them. To tackle the problem we need to look at the whole picture
– acidification, eutrophication, soil salinisation and heavy metal pollution
– as part of the same problem. Research on chemical time bombs is just beginning.

William Stigliani is scientific leader of the industrial metabolism
project at the International Institute for Applied Systems Analysis in Laxenburg,
Austria. Wim Salomons is head of the soil chemistry department at the Institute
for Agro-biology and Soil Fertility in the Netherlands. Both are members
of the Scientific Advisory Board of the Chemical Time Bomb Project, a collaborative
effort between the International Institute of Applied Systems Analysis
and The Netherlands Ministry of Housing, Physical Planning and Environment.

* * *

1: Redox potential: the chemical switch

One of the fundamental requirements of life is the need to generate
biochemical energy by the oxidation of organic carbon to carbon dioxide.
The most efficient energy-producing mechanism is respiration, in which molecular
oxygen (O₂) is the oxidising agent. In soils, waters and sediments,
however, the supply of O₂ is often limited. Nonetheless, the
Earth’s aquatic and terrestrial ecosystems contain microorganisms which
can extract oxygen from other oxygen-containing compounds. These include
nitrate, manganese and iron oxides, sulphate and organic carbon.

The type of molecule used first depends on how good an oxidising agent
it is in relation to others – in chemical terms, its ‘redox’ potential.
Oxidation by molecular oxygen has the highest redox potential, so molecular
oxygen is the first compound to be consumed. Nitrate has the next highest
redox potential, so it is consumed next. The sequence continues with manganese
oxide, ferric hydroxide, sulphate, and finally to organic carbon. The redox
potential is a kind of ‘chemical switch’ which determines the order in which
oxygen-containing chemicals are used by microorganisms to extract oxygen.

* * *

2: The acid test

Can changing environmental conditions in moderately contaminated soils
tip the balance towards the explosion of a chemical time bomb? To test this
hypothesis William Stigliani and Peter Jaffe of Princeton University in
the US decided to investigate the build-up of cadmium in agricultural soils
of the Rhine basin. We considered not only the total cadmium concentration
in the soils, but also the effect of acidification, because we already knew
that metal concentrations in the soil solution depend to a large extent
on pH. Increasing the soil’s acidity will increase the amount of dissolved
cadmium, making this toxic metal more available for uptake by plants. The
two major sources of cadmium are the atmosphere (which contains cadmium
from industrial activity) and phosphate fertiliser, containing cadmium as
an impurity.

We estimated that since 1950 these sources have nearly doubled concentrations
of cadmium in the soil. One way to combat acidity is by applying lime to
agricultural soils, a long-standing practice among farmers for increasing
the productivity of their lands. Traditionally, liming has not been seen
as a way to offset acid rain, which now poses a major problem in the Rhine
basin. In areas suffering from acid rain, more lime must be used to compensate
for the added source of acidity. If additional liming is not done systematically,
the soils may acidify and the cadmium concentration in soil solution will
increase, raising the risk of crop contamination. Rhineland farmers, though
they may not realise it, have a big responsibility for ensuring that this
particular time bomb does not go off.

In our study we also estimated human intake of cadmium. We wanted to
look at the twin effects of increased cadmium in soil and increased soil
acidification (see Figure). Calculations based on cadmium increases since
1950, but assuming a constant pH of approximately 6 (which is typical of
agricultural lands in the basin), showed that human intake of cadmium, although
doubling between 1950 and 1990, would still be well below the WHO’s limits.
However, we also calculated that, given current cadmium levels, an increase
in the soil’s acidity of just half a pH unit – from 6 to 5.5 – would cause
the cadmium intake relative to 1950 to increase by about four-and-a-half
times, slightly exceeding the WHO limits. A decrease of one pH unit would
result in a ninefold increase – about double the WHO limit.

Further calculations showed that at 1950 cadmium levels, a decrease
of half a pH unit would result in an intake of cadmium well below the WHO
limit. Even with a drop of a whole pH unit, intake would exceed the lower
limit only slightly. So, one major impact of cadmium accumulation in the
soil since 1950 has been a loss in the soil’s capacity to immobilise cadmium
at safe limits during times of acidification. One way for soil acidification
to happen on a large scale is by converting farmland to forest. This could
easily happen in Europe in the next few decades.

Conversion to forest land in the European Union is estimated at 8.5
million hectares over the years from 1985 to 2020. If this change takes
place without the continuation of liming – which will certainly happen if
policy makers and land use planners are unaware of the potential for triggering
a chemical time bomb – the pH of the former agricultural soils is likely
to drop by between 1 and 1.5 pH units within a few decades over much of
the region. This could result in a significant increase of cadmium in soil
solution within 10 to 15 years, and increased leaching to ground-water.
Although the soil will purge itself of cadmium by the process of acidification,
problems could arise in areas where drinking water supplies come form shallow
groundwater reservoirs.