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ANTHROPOGENIC ALLY REDUCEDGROUNDWATERS / Les eauxsouterraines réduites parl'activité de l'hommeFRIEDRICH SCHWILLE aa Federal Institute of Hydrology , PO Box 309, D-5400,Koblenz, German Federal RepublicPublished online: 25 Dec 2009.

To cite this article: FRIEDRICH SCHWILLE (1976) ANTHROPOGENIC ALLY REDUCEDGROUNDWATERS / Les eaux souterraines réduites par l'activité de l'homme, HydrologicalSciences Bulletin, 21:4, 629-645

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Hydrological Sciences-Bulletin-des Sciences Hydrologiques, XXI, 4 12/1976

ANTHROPOGENIC ALLY REDUCED GROUNDWATERS*

FRIEDRICH SCHWILLE Federal Institute of Hydrology, PO Box 309, D-5400 Koblenz, German Federal Republic

Received 11 November 1975, revised 15 June 1976

Abstract. Groundwaters, which have lost their original oxygen content as a consequence of reducing, i.e. oxygen-consuming microbial processes, are a common phenomenon in nature. But man, too, can change oxygen-containing waters in a variety of ways into oxygen-free groundwaters. The different ways of 'groundwater reduction' by man's activities are illustrated by way of selected examples.

Les eaux souterraines réduites par l'activité de l'homme Résumé. Les eaux souterraines ayant perdu leur teneur d'origine en oxygène à la suite des processus réducteurs, c'est-à-dire des processus microbiens consommant de l'oxygène se rencontrent très fréquem­ment dans la nature. L'homme peut également transformer de multiples façons des eaux contenant de l'oxygène en eaux souterraines sans oxygène. Les diverses possibilités de la formation 'd'eaux souterraines réduites' par l'activité humaine sont illustrées à l'aide des exemples choisis.

INTRODUCTION

In present-day groundwater research, the term 'reduced groundwater' denotes a water in which the oxygen-consuming processes continue after the original content of free dissolved oxygen has been depleted and recourse is taken to bound oxygen. 'Reduced waters' are the result primarily of microbial processes which are analogous to the corresponding processes in surface waters. Thus, reduced groundwaters are usually the result of the anaerobic degra­dation of organic substances under the ground.

The biochemical model of respiration presupposes a transfer of hydrogen in which the organic substances act as donors, and the oxygen as acceptor. In the aerobic process, the molecular (free) oxygen is the acceptor, and in the anaerobic process it is the bound oxygen, especially that of the nitrates and sulphates.

The transition from the oxidized to the reduced condition is stepwise, depending on the oxygen demand of the various bacterial species, when a partial oxygen pressure of 10-0.1 mm Hg, corresponding to about 0.7-0.01 mg/1. 0 2 at 8°C water temperature has been reached. However, field observations made so far seem to indicate that the 'capsizing' threshold is broader, because the characteristics of reduced water frequently appear already at consider­ably higher oxygen contents. The reason for this discrepancy is probably the fact that, for technical reasons, the groundwater samples are not quasi point samples but mixtures of waters of diversified degrees of reduction.

The range of influence of groundwater pollution (contamination) by organic substances can be subdivided into two zones, each characterized by specified microbial processes (Matthess, 1972; Minder, 1960), which may be described as follows (Fig. 1).

( 1) Reduction zone. Free oxygen is no longer available. The degradation of organic sub­stances is anaerobic. Due to the rich supply of organic substances, the total number of bac-

* This paper was presented at the International Symposium on the Geochemistry of Natural Waters organ­ized by IAHS and IAGC and held at Burlington, Ontario, Canada, 18-20 August 1975.

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teria is considerably increased. There is a strong tendency toward reduction; the redox po­tential is negative. Nitrate is reduced to nitrite, nitrogen and ammonium, sulphate to sulphur and hydrogen sulphide. Carbon dioxide develops. Iron, manganese and other heavy metals are transformed into the divalent, easily soluble state. Organic substances are transformed into low-molecular compounds.

(2) Oxidation zone. Free oxygen is available again. The further degradation of organic substances, as far as they are degradable, that may still exist is aerobic and continues down to the stage of complete mineralization into carbon dioxide and water. The total number of bacteria has returned, more or less, to normal. There is a strong tendency toward oxidation, the redox potential is positive. The inorganic reduction products oxidize: ammonium into nitrite and nitrate, hydrogen sulphide into sulphur and sulphate. Iron, manganese and other heavy metals return to the trivalent or a higher state and are precipitated. The incoming oxy­gen is continuously used for the oxidation of the reduction products and for the further de­gradation of organic substances so that, for a while, the oxygen content remains close to zero. Only after completion of the microbial oxidation processes will the oxygen content increase markedly.

Fig. 1 - Schematic view of the zone of influence of a groundwater contamination.

To allow a better description of the biochemical conditions, a transition zone can be interposed between the reduction zone and the oxidation zone. In this transition zone, free oxygen is available only temporarily; iron, for example, may be precipitated at this stage. The boundary between the reduction zone and the oxidation zone, and the outer limits of these zones shift in a complicated way. The outer limit of the oxidation zone must be drawn where the state of oxidation is approximately normal, i.e. in agreement with the background. Increases in the content of inorganic substances such as chlorides, alkaline earth salts, total dissolved solids, are not included in the corresponding considerations. A description of the microbial processes cannot be given here but a new summary description of the chemical and microbial processes is given by Matthess (1973) and Hall (1975), and the geochemical behaviour of the individual substances is described by Hem (1970).

A reduced groundwater can, therefore, be recognized from the following findings of a normal water analysis of medium extent: absence of free oxygen, absence of nitrate or at least considerably reduced nitrate content, occurrence of ammonium, nitrite and divalent iron and manganese; increased chemical oxygen demand, occurrence of aggressive carbonic acid, increased hydrogen carbonate content (or increased carbonate hardness), occurrence

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of phenol-like substances. Under conditions of extreme reduction, hydrogen sulphide occurs, and the sulphate content (or the non-carbonate hardness) is reduced. The smell and the taste of the water are usually extremely dull and mouldy, even foul. Not all of these characteristics must be simultaneously apparent; frequently, only some of them are positive.

Reduced waters are frequently encountered in nature. Every groundwater type specified from the chemical or penological point of view may occur in the oxidized as well as in the reduced condition. Gerb (1953), who introduced the term 'reduced water' into groundwater literature and defined it in detail, therefore contrasted the oxidation state, which he con­siders as the normal, original one, with the reduction state when laying down the various groundwater types. Thus, groundwater containing iron and manganese is not a special type of groundwater in the strict sense of the word but a specific state of the groundwater types.

GEOCHEMICAL, HYGIENIC AND TECHNICAL SIGNIFICANCE OF REDUCED WATERS

Geochemistry The migration ability of heavy metals depends inter alia on their valence. In the reduced form, most heavy metals are of higher solubility than in the oxidized state. Under certain conditions, the heavy metals introduced into the groundwater can migrate in the reduction zone. If iron or manganese compounds are dissolved under reduction conditions, trace metals co-precipitated or adsorbed by them may be released. Under anaerobic conditions, further­more, a partial migration of the heavy metals in the organically bound or complex state may be expected.

The most important mechanism in the elimination of heavy metals in reduced ground­water is the sulphidic precipitation. Only chromium, which has no sulphide phase, is an exception. Copper, which of all heavy metals forms the least soluble sulphides is, therefore, eliminated more intensively in the anaerobic zone than any other heavy metal.

An important role in the elimination of trace metals (e.g. copper, lead, zinc, tungsten, vanadium) in the oxidation zone is played by iron and manganese which may precipitate these metals in the form of hydroxides (Fôrstner and Miiller, 1975).

Thus, as far as the migration behaviour of the heavy metals is concerned, reducing con­ditions appear to be ambivalent. On the one hand, their mobility is increased in a reduced environment by inter alia intensified complexation. On the other hand, the low solubility of heavy metal sulphides acts as a retention mechanism which limits heavy metal concentration effectively.

The frequently simultaneous occurrence of divalent iron and H2 S seems to be a contra­diction at first glance, since FeS is highly insoluble. The cause must probably be seen in the large quantity of carbonic acid produced, which forms Fe(HC03)2. The weak H2S is not capable of precipitating FeS therefrom (Baars and Le Cosquino de Bussy, 1960).

It is still an open question whether nitrate and sulphate reductions begin simultaneously, or whether sulphate reduction starts only after the reduction of the nitrates, as seems to be the case. If the latter assumption is correct, this would mean that heavy metals mobilized in nitrate-rich waters can be precipitated as sulphide only after the nitrate^ have been exhausted.

Hygiene In the presence of free oxygen, microbial processes develop much faster than in a reduced environment. So far, the question of whether this is true also of pathogenous germs has not been sufficiently investigated but there is reason to believe that pathogenous germs remain virulent longer in a reduced environment. For hygienists, the presence of dissolved oxygen is, therefore, a sort of guarantee that possible bacterial groundwater pollution will be quickly eliminated. Free oxygen also excludes the development of nitrate-reducing bacteria which,

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in the case of the simultaneous presence of high nitrate contents in drinking water, may be­come the cause of methemoglobinemia of infants or older and tract-sensitive persons.

Hygienists usually consider ammonium and nitrite, besides other substances, as indicat­ors of fresh faecal pollution. However, in view of the wide distribution of reduced ground­water, ammonium and nitrite now have only little significance as pollution indicators.

Oxygen-saturated water has the fresh taste of spring water. Oxygen-free water tastes dull and mouldy and is instinctively rejected by the consumer. The emphasis which Central Europ­eans place on the pleasant taste of drinking water may seem extravagant to water suppliers in other regions but it is actually due to the fact that, in Central Europe, the water supply has always been derived from groundwater resources while in other countries, frequent re­course had to be made to surface water.

The cause of the fresh taste is, apparently, not the oxygen itself but the fact that the organic substances causing unpleasant smells and tastes disappear relatively fast in an oxi­dizing environment so that they cannot be determined organoleptically.

Dissolved oxygen is also needed to initiate the microbial degradation of certain substan­ces, such as paraffinic hydrocarbons, which may reach the groundwater as pollutants. The further degradation of intermediates, however, does not require free oxygen but can proceed anaerobically. With aromatic hydrocarbons, on the other hand, free oxygen is not needed for initiating degradation (Verstraete, 1975).

Technology In water supply technology, the value of free oxygen in groundwater and, thus, in drinking water is still in dispute even though oxygen-containing water is usually preferred. Oxygen plays a controversial role: on the one hand, it is an initial factor of material destruction, and on the other, its presence is desirable as it leads to the formation of a corrosion-inhibiting protective layer. In iron pipes, oxygen-containing water in conjunction with alkaline earth carbonates leads to the formation of a protective lime/rust crust which prevents further cor­rosion. The oxygen content in the pipelines should, therefore, be at least 6 mg/L; according to Schmassmann (1965), the formation of a protective crust may be expected with certainty only if there is at least 80 per cent oxygen saturation.

In Central Europe, where drinking water is obtained primarily from groundwater, oxy­gen-containing groundwater is preferred by the waterworks management. In the USA, on the other hand, oxygen seems to be not quite as important, as may be seen from Water Quality Criteria (Committee on Water Quality Criteria, 1972) which states 'No recommendation is made, because the presence of dissolved oxygen in a raw water supply has both beneficial and detrimental aspects'.

From the technical point of view, reduced groundwaters, on account of their content of iron, manganese and aggressive carbonic acid, are not desirable. Their use as drinking water or industrial water is possible only after proper treatment and aeration. In addition, they entail the danger of encrustation and the formation of slime as a result of the mass develop­ment of bacteria in wells, pipes and tanks. The costs of the erection and operation of treat­ment plants may be a considerable burden on the budget.

All in all, oxidized groundwater offers so many advantages that it pays in any event to maintain this condition.

EXAMPLES OF ANTHROPOGENICALLY REDUCED GROUNDWATERS

Man can produce reduced groundwater in a variety of ways. In principle, all of these possibi­lities can be traced to two main causes:

(a) excessive supply of organic substances, and (b) retardation or inhibition of gas exchange between the atmosphere and groundwater.

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Formerly, faecal infiltration was the main source of groundwater pollution in Central Europe. In entire settlement areas, the groundwater was in the reduced condition with all the mani­festations already described. The erection of central waste water disposal plants brought about a radical change and contributed to the considerable improvement that has been achieved in groundwater quality. It may even be said that in some densely populated indus­trial countries, central waste water disposal was the indispensable prerequisite for meeting the rapidly growing water demand to a considerable extent with groundwater. Modern socie­ty, however, has found a number of ways to ruin step by step the progress that was achieved at so high a price. In a variety of ways, usually without giving it even a thought, reduced groundwaters are now produced in many places to a far greater extent than before.

In many countries, individual home waste-water treatment systems are still frequently encountered. For example, by the sheer volume of waste water discharged, septic tanks and cesspools must be rated as the key potential source of groundwater contamination in the USA (Miller and Scalf, 1974).

The realization that waste water should not be allowed to infiltrate underground has not always led to practical consequences. It may not be economical to link remote farms with a central drainage system, but if the waste waters of larger villages or even towns are dis­charged underground, far-reaching consequences may ensue. Karst regions, so it seems, are preferred infiltration areas everywhere. Schmassmann (1965) reports that in the Swiss Jura, the waste waters of a town with a population of 40 000 are discharged into fissured lime­stone. About 4 km from the discharge point, the waste water reappears in two karst springs as clear but completely reduced water with a high hydrogen sulphide content. This is a good example of a subterranean anaerobic-biological waste-water purification system, which pro­duces clean but not potable water.

Leaky sewers are frequently the cause of organic groundwater pollution. According to Back and Langmuir(1975), such leaking sewers and the usual practices of waste disposal are responsible for the excessive iron content of groundwater at Camden and Philadelphia, USA. The original content of less than 0.5 mg/1. rose to 20 mg/1. and in some instances even be­yond 400 mg/1.

General opinion holds that in individual cases, a moderate extent of faecal infiltration outside the inner protection zone of an intake area may be tolerated if no other way of economically tenable disposal exists. Faeces, so the argument goes, are relatively easy degrad-able. However, organic pollution in its entirety may considerably reduce the oxygen content of a groundwater body and, thus, the latter's self-purification capacity, even if the individual load may seem insignificant. Waters so polluted may be highly susceptible to contamination by other organic substances.

Formerly, when assessing the influence of domestic and industrial refuse disposal sites on the groundwater, the increase in the content of dissolved inorganic substances caused by these sites was held to be the decisive factor. Even now, increased content of dissolved solids (measured via electric conductivity), chloride, nitrate and sulphate, and increased total hard­ness are considered important criteria of underground pollution.

However, variations in the inorganic content of a water are only a partial aspect. Micro­bial degradation of dissolved organic matter, which leads to the formation of reduced ground­water, may have much more far-reaching consequences. An interdisciplinary research team in the Federal Republic of Germany has thoroughly investigated the complex process at two disposal sites in the vicinity of Frankfurt and published a detailed report on the subject (Farkasdi et ai, 1969). According to these findings, there is no doubt that the cause of the reduction phenomena must be seen in the intensified microbial activity resulting from the increased supply of organic substances. Purely chemical reduction processes seem to play only a secondary role.

The existing comprehensive literature on the effects of refuse disposal sites cannot be discussed here. As for the formation of reduced groundwaters by refuse disposal sites see

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Apgar and Langmuir (1971), Coe (1970), LeGrand (1965), Lehman and WOson (1971), Miller and Scalf (1974), Zanoni (1972).

Investigation of mineral oil incidents leaves no doubt about the microbial degradation of dissolved oil products. This degradation is effected by the numerous species of organisms which are present even in pure groundwater and which are capable of acquiring, through adaptation, the ability to consume the hydrocarbons. While degradation is an extremely slow process in the oil body where the oil is in the liquid phase, it proceeds relatively fast in the case of dissolved oil components (Figs. 2 and 3). Systematic examination of oil incidents

Ground surface

Saturated zone Oil components dissolved in water Water table

Fig. 2 - Oil migration pattern (vertical cross section).

confirmed in all cases the formation of a marked reduction zone (Schwille and Vorreyer, 1969). It must be pointed out that the zone of the dissolved oil components, as shown in Fig. 3, does not always coincide with the reduction or oxidation zone. The former is defined under purely organoleptic and chemical aspects, the latter according to microbial and chem­ical criteria.

A textbook example of the reducing effect of dissolved aromatic hydrocarbons in groundwater and their influence on a drinking water well was provided by the experimental

Zone of

Oil zone

,

d is solv

) -.

--;-

ed c 0

.'.'/• ' s

m po nents

'—-—-̂ -

1 2 0 j d |

\ \ A

. . ^^* /

1

/ /

0 .25 50 [m]

Fig. 3 - Oil zone and zone of dissolved oil components (horizontal cross section).

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sealing of a shipping canal with tar concrete. Over a length of about 700 m the concrete, which was kept plastic with the aid of a fluxing agent, was distributed over the canal em­bankment and rolled down. As a result of the intimate contact between the dumped con­crete and the infiltrating canal water, and between the completed concrete work and the groundwater, aromatics were flushed into the groundwater. The well, which yielded about 5000 m3,/d, had to be inactivated for several years, due to the unpleasant smell and the pre­sence of iron and manganese. Figure 4 shows the flow net and the position of the concrete

Fig. 4 - Flow net of a well influenced by dissolved tar products.

sealing arrangement, Figure 5 gives a geological cross section through the well in the direc­tion of groundwater flow. Figure 6 shows the chemical consequences of the reduction pro­cess taking place in the groundwater. Where aromatics appear, free oxygen and nitrates dis­appear; manganese and iron dissolve in the divalent form.

In this context, mention must also be made of the industrial chemical products that are used for the conservation, insulation and protection from corrosion of subterranean installa­tions. Some of these products contain solvents or softeners which are soluble in water and which are flushed underground by seepage water or dissolved immediately by passing ground­water.

Organic degreasing agents such as, for example, trichloroethylene or perchloroethylene,

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River Pumping well Canal above datum

20

-10

-o

0 100 200 300 m

Fig. 5 — Geological-hydrological cross section along the line AB shown on Fig. 4.

0 100 500 [ m ]

Fig. 6 — Formation of 'reduced' groundwater through the influence of dissolved aromatic tar components.

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are needed in considerable quantities by the metal industry and the dry-cleaning trade. In many countries including the Federal Republic of Germany, there are, as yet, no legal pro­visions governing the disposal of such substances after use.

Out of ignorance of the resulting effects on the groundwater, these substances are usual­ly dumped somewhere in the open and then leached by infiltrating precipitation. The reduc­ing effect they may have has not been sufficiently investigated so far, but a number of obser­vations would seem to indicate that such an effect does indeed exist.

Phenols are often encountered in the groundwater. If they are diluted to the extent where they no longer exert a toxic influence on bacteria, they have a reducing effect which is similar to that of petroleum products.

In large-scale stock raising, animal excrement usually accumulates in quantities that can­not be utilized as manure on the breeder's own land. The storage of this excrement may lead to considerable difficulties. Excessively large quantities of organic substances infiltrate and lead to the formation of extensive reduction zones in the groundwater, as well as the infiltra­tion of silage waste waters.

At stock raising stations with surrounding pastures, liquid manure is often applied to an extent that exceeds the capacity of the meadows. The groundwater then reacts temporarily with a clear negative development toward the reduced state. This is true also of certain waste­water irrigation field practices. Schmassmann (1965) describes a case where the irrigation of meadows with waste-carrying water from a creek made it necessary to inactivate the well of a textile factory on account of manganese formation. This also goes for the deposition of fresh sludge from water treatment plants.

Well-aerated agricultural and forestry grounds are used to an increasing extent for hous­ing development. Sand and gravel pits are expanded. The top soil must be removed. Only a relatively small portion thereof is used to re-cultivate newly developed areas. The bulk of it is dumped into depressions and similar places, in some cases directly into the groundwater of abandoned sand and gravel pits. Due to insufficient oxygen input, the organic substances cannot be completely degraded, and extensive areas of reduced groundwater develop.

Sweet and Fetrov(1975) give an instructive example of incomplete degradation. In Oregon, USA, an abandoned gravel pit was filled with wood waste, including approximately 2700 t of hemlock bark. The groundwater dissolved organic substances, inter alia lignin-tannin, thus creating within a short time a reducing environment which was characterized by the presence of considerable manganese and iron contents. The same problem arises in connection with the disposal of dredging sludge from silted creeks, waste-water ditches and oxidation ponds, and of largely organic sediments dredged from the backwater area directly upstream of river dams, or of washing sludge accruing in sugar beet processing. As it is the least expensive solution, these masses are often used for filling abandoned river beds, de­pressions in the terrain, or sand and gravel pits. Whether or not and to what extent sludges polluted by municipal or industrial waste water can release again the heavy metals they have to a considerable extent absorbed has, as yet, not been clarified. Since reduced conditions exist in these sludges, it is probable that the pore water released during the consolidation of these watery masses also contains heavy metals.

An interesting example which describes the development of reduced waters with all its consequences is given by Minder (1960). During sugar beet processing, the untreated waste waters of a sugar factory in the western part of Switzerland are discharged in large quantities into a number of shallow basins where they seep into the ground. Below the basins, in the river gravel of the former bed of the Aare River, there is an abundant groundwater stream intended for drinking Water supply. More than 4 km downstream of the basins, the ground­water has not recovered from the reduction; the oxygen content remains below 1 mg/1. Thus, the water of the new waterworks erected there must be artificially aerated after cap­ture. It should also be mentioned that with the input of oxygen, in this case, the reduced iron is precipitated faster than the manganese.

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The damming up of a river, for example for shipping purposes, is frequently accom­panied by the simultaneous raising of the groundwater level in the vicinity of the river. As a result, layers which up to then were in the unsaturated, aerated zone come to be perma­nently situated in the saturation zone. The unsaturated zone contains significant quantities of organic substances even below the soil zone (soil in the pedological sense of the word). The oxygen content of the groundwater is too low to degrade the organic substances within a relatively short time. As a result, a reducing environment develops in the flooded zone, and this all the more quickly and pronouncedly, the lower the oxygen content of the ground­water is. It is, therefore, obvious that bank-infiltrated groundwater carrying a load of organic substances is converted to the reduced condition by floods much faster than the 'genuine' groundwater that streams into a well.

During the development of the Moselle River into a fully navigable waterway, pheno­mena of this kind were observed repeatedly (Schwille, 1973). Figure 7 gives a schematic view of the geological and hydrological conditions of a pumping well on an island in which

E 100-

Pumping well Moselle river

p Backwater level

o 50 wo m

Fig. 7 - Formation of reduced water as a result of river damming.

2-3 months after the raising of the river water level iron and manganese contents had risen to such a height that the well had to be inactivated. The oxygen content, which had been between 2 and 3 mg/L, dropped to values below 0.5 mg/1. (Fig. 8). In almost all cases in the Moselle, manganese occurrence predominated over iron which was quantitatively less im­portant than manganese. It was noted that an increased content of aggressive carbonic acid did not occur in any of the observed wells, while carbonate hardness increased considerably. Another surprising consequence of the reduction phenomena was the mass-occurrence of

m above datum

100 -

aH

Jan. 1963 Dec

i !•!«

ng/l]

95-Fig. 8 - Development of oxygen content in the well shown in Fig. 7.

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slime-producing bacteria in some of the wells. Obviously, these are aerobic species which are capable of utilizing the intermediate metabolic products originating from the anaerobic de­gradation of organic substances. The development of slime producers begins only where oxygen is available as a result of either the mixture of the reduced water with oxygen-rich water, or the inflow of air into wells and collection tanks. Cases similar to the ones observed in the Moselle have been known for quite some time in connection with damming-up at other places, e.g. the Danube, the Swiss rivers Aare and Limmat, and the Upper Rhine (Schwille, 1973).

The encrustation of well screens in naturally reduced groundwaters is also a result of bacterial processes. Encrustation occurs only in reduced water (Hâsselbarth and Ludemann, 1967). The redox potential must be positive. Krems(1972) explained these prerequisites in detail, using the wells of the Berlin waterworks as an example.

The effect noticed after raising the groundwater level may also occur in the case of land subsidence due to mining. This means that, as a rule, groundwater reduction as a consequence of land subsidence occurs only in the plains where e.g. organic alluvial deposits are flooded in this way.

Lowering of the groundwater level may entail the compaction of overlaying clay layers. The displaced pore water is in the reduced condition and may, therefore, contain heavy met­als which enter the aquifer.

The formation of reduced water as a consequence of bank infiltration is a well-known phenomenon. For the waterworks along the Rhine, in particular the Lower Rhine, growing pollution leads to a steadily intensifying change in the quality of the pumped water towards the reduced state. Probably more than one million people in the cities along the Rhine are supplied with reduced water which, as such, requires treatment. Back and Langmuir (1975) report similar effects of surface water pollution on groundwater from Rhode Island, USA. Any number of similar examples can be quoted.

Even in the case of artificial groundwater recharge with organically polluted surface water from infiltration basins, i.e. of a vertical infiltration process, reduced water may devel­op (Baars and Le Cosquino de Bussy, 1960; Haberer, 1968). The reducing effect can be pre­vented by intermittent operation which results in intermittent aeration.

The main problem of groundwater recharge through wells consists in eliminating the degradable organic substances from the raw water to the greatest possible extent so as to prevent the development of a reduced environment in the underground. This means that the water to be infiltrated must be of a quality which already approaches the quality of the groundwater. As a rule, mere mechanical filtration is, therefore, insufficient; it will be neces­sary to find ways to ensure an additional reduction in the content of colloidal or dissolved organic substances while simultaneously providing for adequate aeration. If, on the other hand, the water of the aquifer is already in the reduced state, it will be necessary to also infiltrate reduced i.e. oxygen-free water for reasons of compatibility.

Due to their constantly low temperature and the absence of iron and manganese, oxygen-containing waters are ideal for cooling purposes. On being heated they lose a considerable portion of their oxygen content. For example at 8°C and 760 mm Hg, the oxygen saturation concentration is 12 mg/L, and at 30°C it is 9 mg/1. Even if the heated cooling water, not otherwise polluted, is returned underground, it has an oxygen deficit which corresponds to a certain degree of pollution. It would, therefore, be quite correct in such a case to speak of groundwater contamination by cooling water.

Finally, a few examples of groundwater pollution by apparently harmless substances which are generally not considered as a danger to groundwater may be of interest. In one case, an accident involved a tank truck transporting raspberry syrup (Zwittnig, personal com­munication), another case concerned waste water from a biscuit factory (Hutchinson, 1975), and the third case residues from a whisky distillery (Montgomery and Wheatland, 1975). In -all three cases, a marked reduction effect on the groundwater was observed.

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During recent years numerous building and construction projects, frequently of very large proportions, hinder or completely prevent the air exchange between soil and atmos­phere, or the traffic of heavy trucks on levelled terrain may affect the structure of originally loose soils of high permeability to such an extent that the original permeability is greatly reduced. To fill such hardened horizons later on with relatively loose soils and to top the latter with humic soil will not remedy the situation. The percolating water is stopped at the compacted horizon which acts as an air-impermeable membrane.

It happens frequently that in order to save costs in land filling, the humus cover is not removed and the filling material is dumped directly on this. In this way, comparatively large quantities of live organic substances are buried and 'suffocate' slowly. As a result, reduced conditions develop, the consequence of which may be similar to the effect of disposal sites for domestic refuse.

There is no doubt that urban development (housing projects, streets, parking lots, etc.) in areas formerly reserved for agriculture and forestry almost completely prevents atmospher­ic air entering the ground. In this way, at least half of the developed urban area is 'sealed off, and 'shielded aquifers' are artificially created on a large scale. A great number of ex­amples in nature show the reducing effect of such sealed-off aquifers. It is, therefore, safe to assume that the frequently considerable oxygen deficit in the groundwater under our cities is a consequence not only of the pollution by organic substances but also of the artificial sealing.

In conclusion we should not fail to mention the activities in agriculture and forestry which, as a result of soil-improving measures of all sorts, permit a better aeration of the sub­soil. It would certainly be of interest to try to determine at least the order of magnitude of this positive contribution. In addition, it should be clarified to what extent the nitrates leached from the soil, which are an important oxygen reserve, prevent a reduction to hydro­gen sulphide formation.

RESULTS FOR SCIENCE AND PRACTICE

All experience so far with naturally or artificially reduced waters indicates that the reduction is almost exclusively due to microbial processes. Literature contains sufficient references or indirect proof, but generalization of individual cases or application of such findings to other similar cases has been rare.

It has been known for quite some time that oil field waters which have a certain organic content contain little or no sulphates, and where the sulphates disappear, hydrogen sulphide appears. The first observations of this phenomenon were made at the Caucasian and Califor-nian oil fields. Attempts at reducing the sulphates in the laboratory in a purely chemical way at the temperatures measured in the field failed. Today, there is general agreement that sul­phate reduction is the result exclusively of microbial processes (Schoeller, 1962). Shvets (1971) summarizes the present state of relating knowledge as follows. 'Numerous investiga­tions have proved that the micro-organisms are capable of consuming the hydrocarbons of various classes, of simple as well as complicated structure, i.e. practically all crude oil hydro­carbons. The wide distribution of bacteria in oil field waters in the absence of free oxygen clearly indicates the possibility of the anaerobic decomposition of hydrocarbons with the simultaneous formation of fatty acids'.

The formation of hydrogen sulphide in mineral waters as a result of bacteria sulphate reduction in an anaerobic environment is generally accepted as a fact (e.g. Baumann, 1973; Tsapenko, 1967; Shcherbakov, 1973).

Groundwaters with natural iron and manganese contents show a remarkable dependence on the aquifer's content of natural organic substances and the extent of the aquifer's cover­ing by impermeable or only slightly permeable layers. Reduced groundwaters are encountered

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most frequently in the plains where the water table is usually at shallow depths below the surface and the ground is rich in organic substances. Many examples could be pointed out in the North German Plain. Plochniewski (1973) who examined the groundwaters of the Quar-ternary deposits in North and Central Poland, writes 'The largest contents of iron and man­ganese occur in waters of fluvial deposits containing peats or muds, i.e. on low fluvial ter­races and within the delta of the Vistula. In fluvioglacial aquifers covered with till, the strong contents of iron and manganese are rather clearly marked.' Gerb (1953), and Fast and Sauer (1958), who tried to typify the Bavarian and South Baden groundwaters, came to similar conclusions. Apel (1971) describes the effect of impermeable covering layers on ground­water quality, using the karst region of the southern Frankenalb as an example. Karst water that is not covered by impermeable layers is usually rich in oxygen. In covered aquifers, on the other hand, the oxygen is consumed and iron and manganese occur; this water must, therefore, be treated.

A critical review of the literature which deals in one way or another with the anthropo-genous effects on groundwater shows that the considerations expressed there are in full agreement with the observations made in the case of naturally reduced waters. There is no longer any doubt about the microbial origin of the anthropogenically reduced groundwaters. Quite obviously, at least some of the many species of the bacteria which are present in every type of groundwater are capable of adapting themselves from case to case to the new nutri­tional situation and then to multiply on a mass scale, so to speak as 'specialists', displacing the other species. It seems that this process usually sets in very early. In laboratory tests, the microbial degradation of e.g. dissolved oil components can be indirectly proved after a few weeks or even days. In the case of field examinations, the same development can be proved after a few months by determining the reduction indicators through water analysis. Un­fortunately, only a few systematic examinations of the intensity and the development of degradation in time are known so far. It is, therefore, impossible to clarify the individual degradation process mathematically.

The development in time and space of the reduction and oxidation zones depends on a large number of factors:

(a) organic substances (type, quantity, degradability, toxicity, etc.); (b) groundwater hydrological conditions (all factors influencing the groundwater situ­

ation); (c) oxygen content of groundwater.

The original chemical/biological condition of the groundwater is, of course, of great import­ance. In water that has already been affected and lost most of its oxygen content, reduced conditions will develop much faster than in oxygen-saturated water. The heavier the original pollution of the groundwater, the larger will be the extension of the influenced zone and the longer the duration of the influence. Thews (1971) saw these interrelationships very clearly and drew attention to the cumulative effect of the diversified individual pollution sources.

From the hydrological point of view, hydrodynamic dispersion is the phenomenon that determines development and extension of the influenced zone. The theoretical basis for this assumption has been established. Practical application, on the other hand, usually fails for lack of handy formulae and the parameters that govern the dispersion. Yet even if these were sufficiently known, the diversified physico-chemical processes such as sorption, dissolu­tion and precipitation which occur simultaneously with the microbial processes, could hard­ly be incorporated in one formula. Nevertheless, it is very useful to depict the spreading process, under radically simplified assumptions, in the same way as if an ideal tracer of con­stant source capacity were spreading. In Fig. 9 the distribution of the concentration is de­termined in this way (Zilliox, personal communication). Only the concrete idea thus obtained of the two-dimensional or three-dimensional spread allows remedial measures, such as the

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1

Source of ^

contamination

1 y

k *1

v___

\

I * 2

/

/

* 3

\ /c *Cl

\ Row direction

y x

K c (x-x3 ,y)

Fig. 9 - Dispersion in a homogenous porous medium. C= concentration, Q= limiting boundary value.

installation of groundwater observation pipes and wells, and the sampling required for scien­tific investigation, monitoring and control, to be taken properly and effectively.

The increasing importance of the unsaturated zone, which has attracted the attention of groundwater hydrologists during the past decade, must be pointed out here. In this context, however, it would be preferable to speak of the aerated zone in agreement with Meinzer, who coined the expression, and some Russian groundwater hydrologists.

What are the conclusions that can be drawn from the findings in respect of groundwater protection? The basic principles and the technical measures cannot be discussed here. First of all, the decision-makers in water management must bi convinced that reduced waters considerably increase the cost of obtaining drinking water and industrial water. The total ef­fect of water quality deterioration cannot be easily quantified, and a cost/benefit analysis is hardly possible at this time. Furthermore, many measures which ultimately lead to the de­velopment of reduced water cannot be avoided because they are without reasonable alter­natives. For example, the advantages resulting from river damming to open a stream for ship­ping are certainly much more important than the detrimental effect on waterworks resulting therefrom. And the organic growth of a community cannot be stopped merely to ensure a certain water quality.

Unfortunately, water management is usually the weaker partner in planning, and must yield and adjust to the new situation. In any event, it is important that the responsible water­works manager foresees a possible detrimental effect on the chemical quality of the ground­water so that measures for raw water treatment, or the relocation of the pumping wells, can be planned in good time. On the other hand, the task of the disciplines concerned with groundwater is to compile all basic facts which lead to the development of reduced waters, to draw therefrom the consequences relating to the public water supply, and to submit them in a clearly understandable way to all who make the decisions in water management.

TASKS OF FUTURE RESEARCH

The above exposition shows that the problem of the reduced groundwaters is rather complex and covers the entire range of questions related to the anaerobic degradation of organic sub-

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stances underground with all the consequences for organic and inorganic water chemistry. Solutions can be achieved only in cooperation with microbiologists, geochemists, hydrogeo-logists, groundwater hydrologists, hydrochemists in the broadest sense of the word, and hygienists.

Fortunately, geochemists have comprehensive experience, in particular as far as the nat­ural laws are concerned which govern the migration of heavy metals and the transformation of hydrocarbons. This valuable experience should be made available to groundwater research­ers to a far greater extent than in the past.

There is a wide field open to microbiological research. Ecological and biochemical ground­water problems are only in the first phase of clarification. A detailed subdivision into bio­chemical zones, accompanied by a detailed description of metabolic processes, would be a valuable help to hydrogeological activities.

Hydrogeology and groundwater hydrology are now faced with the task of selecting suit­able cases from the multiplicity of actual contaminations and to provide the geological and hydrological skeleton for the research work of the associated scientific disciplines. More than in the past, hydrogeology should take up the task of studying naturally reduced waters so that the experience thus gained can be applied to anthropogenically reduced waters.

Groundwater chemistry should by all means give more attention to oxygen determina­tion at the laboratory and in the field, and to all factors relating to the redox potential. Hem (1970) justified this necessity as follows: 'Dissolved oxygen determination usually has not been made in geochemical studies of groundwater, but they have some value for such purposes and are increasingly utilized ... The concentration of oxygen in some respects is a better indication of oxidation potential than the measurement of electrical potential, which is an uncertain process in aerated solutions'. We go one step further and emphasize, that only oxygen determination will provide the key to the understanding of the self-purification processes underground. Groundwater chemistry should also devote more attention to the organic content of the groundwater. A quick quantitative estimate of dissolved organic sub­stances can be obtained through the determination of the dissolved organic carbon content, which can be easily effected (Leenheer et al, 1974, Shvets, 1971).

And, finally, hygienists should check their past findings with a view to the question of whether or not the behaviour of pathogenic germs shows marked differences as a function of life in an oxidized or a reduced environment.

The purpose of this report is to draw the attention of all disciplines concerned with groundwater to a water quality problem which, while already being considered by various activities, still awaits a comprehensive coverage. The term 'reduced groundwater' should be taken as a common denominator for a large number of research activities which concern the same problem under different aspects. Interdisciplinary cooperation is necessary.

To ease scientific communication, the term 'reduced groundwater' should be defined so comprehensively that all disciplines can agree on its meaning. An exact definition is also required to allow an equally brief and appropriate characterization of groundwaters that are free of contamination.

The above considerations are submitted from the point of view of a Central European living in a predominantly humid climate. His field of activity is a densely populated and heavily industrialized country where quality requirements in respect of drinking water and industrial water are high, and where it is still possible to make the fine difference between drinking water and potable water. It may well be that these concerns are only of secondary importance in other countries.

However, a comprehensive scientific treatment of the problem will be a decisive contri­bution to the better understanding of certain groundwater quality problems which have as yet not been solved satisfactorily.

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Apgat, M.A. and Langmuir, D. (1971) Ground-water pollution potential of a landfill above the water table. Ground Water 9, 76-96.

Baars, J.K. and Le Cosquino de Bussy, I.J. (1960) Infiltration von Flusswasser in Dunengebieten. Schweiz. Z. Hydrol. XXII, 380-395.

Back, W. and Langmuir, D. (1975) Influence of near-surface reactions and processes on chemical char­acter of ground water: A review of selected experiences in the United States. Mem. Int. Ass. Hydro-geol. 10, 31-37.

Baumann, M. (1973) Beitrag zur Klârung der Genèse der Schwefelwâsser an der Donau am Beispiel einer Schwefelwasserbohrung zu Bad Gôgging/Niederbayern. Heilbad und Kurort 25 , 8-13.

Coe, J.J. (1970) Effect of solid waste disposal on ground water quality./. Amer. Wat. Works Ass. 62, 776-783.

Committee on Water Quality Criteria (1972) Water Quality Criteria: The Environmental Protection Agency, Washington.

Farkasdi, G., Golwer, A., Knoll, K.H., Matthess, G. and Schneider, W. (1969) Mikrobiologische und hygienische Untersuchungen von Grundwasserverunreinigungen im Unterstrom von Abfallplàtzen. Stddtehygiene 20, 25-31.

Fast, H. and Sauer, K. (1958) Die chemische Zusammensetzung siidbadischer Grundwâsser, Herkunfts-fragen und Versuch einer Typologie. Vom Wasser XXV, 48-81.

Forstner, U. and Muller, G. (1975) Hydrochemische Beziehungen zwischen Flusswasser und Uferflltrat. GWF (Wasser-Abwasser) 116,74-79.

Gerb, L. (1953) 'Reduzierte' Wâsser. Beitrag zu einer Typologie bayerischer Grundwâsser. GWF (Wasser-Abwasser) 94, 87-92 and 157-161.

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Plochniewski, Z. (1973) Wystepowanie zelaza i manganu w wodach podziemnich utworow czwartorzedo-wych (na przykladzie wybranych obszarov polnocnej i centralnej Polski) (in Polish). Biul. Inst. Geol. 277,221-278.

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kogo basseina (in Russian). Problemui gidrogeologii i inzhenernogo gruntovedeniya, 58-78. Verstraete, W. (1975) Journée d'Etude sur les problèmes posés par le déversement accidentel des produits

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