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4.15 The influence of redox chemistry on groundwater
vulnerability
161
4.15 The influence of redox chemistry on groundwater
vulnerability
4.15.1 Background
Reduction and oxidation processes exert an important control on
the distribution of dissolved substances under natural conditions
in groundwater. They also play a major role in aquifer pollution
problems such as nitrate from fertilizers. Indeed, problems in
aquifers often concern the addition of an oxidant, like oxygen or
nitrate, to an aquifer system containing a reductant, like a
sulphide mineral or organic carbon (Appelo & Postma 1999). In
assessing the vulnerability of an aquifer to nitrate contamination,
it may be quite useful to consider the redox chemistry of the
groundwater. The same is true of data on the ability of the
subsurface sediments to chemically reduce nitrate. 4.15.2 Sediment
chemistry
The thickness and distribution of suitable clay cover layers
above an aquifer plays a most important part in protecting the
aquifer, as they form a physical barrier which to some degree
prevents polluted groundwater from reaching the aquifer. Clay cover
layers, and indeed most other Pleistocene and Miocene
unconsolidated sediments likely to be encountered above aquifers in
Northwest Europe, also provide a varying degree of chemical
protection of aquifers. The reason for this is that they, with the
exception of coarse-grained or monomineralical deposits such as
gravel or quartz sand, contain various amounts of minerals and
organic matter reductants capable of reacting chemically with
nitrate in the groundwater and, in effect, break it down. Minerals
such as pyrite (iron sulphide FeS2) as well as organic matter occur
in varying but appreciable amounts in the subsurface. For example,
a pyrite content of 0.11 % is quite common in many types of
Pleistocene and Miocene sediments in north-western Europe.
When groundwater with dissolved nitrate is moving through
subsurface sediments containing pyrite, bacterially assisted
chemical reactions break down the nitrate and pyrite into free
nitrogen + iron + sulphate + water. With organic matter as
reductant in stead of pyrite, a similar type of reaction produces
carbon dioxide and bicarbonate ions in addition to free nitrogen
and water. The nitrate reduction capacity of a given sediment
volume can be calculated from chemical analyses of their pyrite,
organic carbon and ferrous iron content. When this is combined with
data on nitrate-supply via the infiltrating groundwater, the number
of years it takes to use up the reduction capacity of one metre of
sediment in a given area may then be calculated. The finer grained
sediments the clays appear to offer the highest nitrate reduction
capacity. As water passes very slowly through clay layers, there is
ample time for the chemical reactions to use up the available
reagents. Some clays have nitrate reduction capacities running into
hundreds or even a thousand years per metre the redoxcline is
pushed (Ringkjbing Amt 2006), at a constant agricultural-level
nitrate infiltration rate. The redoxcline is the depth where
conditions change from oxidising to reducing, see Section 4.15.3.
However, the major part of aquifer recharge will happen where a
more coarse-grained lithology dominates the subsoil, favouring the
infiltration of groundwater to deeper levels. The nitrate reduction
capacities of silts and fine- to middle-grained sands appear to be
significantly lower than for clays, but even these coarser
sediments can have effective reduction capacities up to a hundred
or two-hundred years per metre the redoxcline is pushed (Ringkjbing
Amt 2002, 2006). 4.15.3 Water chemistry
Depending on its source and transport through the atmosphere,
rainwater will contain a number of ions in solution prior to
hitting the surface of the earth. Typical components of rainwater
will be chloride and sodium from natural seawater and also
anthropogenic nitrates and sulphates
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from industrialised areas, ultimately responsible for the acid
rain phenomenon. The chemical composition of groundwater starts out
with the rainwater chemistry. Its final composition results from
the chemical action the water has been exposed to on its way from
the surface to the aquifer. The water interacts with the sediments
through which it passes during the infiltration from surface to
aquifer. Various elements can be taken up by the water from sources
such as organic matter and minerals in the subsurface sediments.
The agricultural application of fertilizers and manure can add high
concentrations of nitrate to the infiltrating groundwater. This may
reach deeper-seated aquifers, if these aquifers are vulnerable
towards nitrate, i.e. are open to the introduction of an oxidised
environment where nitrate is stable. A redox-based chemical
classification system for groundwater is useful for assessing the
vulnerability and age of groundwater. The classification set up
below is a simplified version of the one used by the Danish EPA
(Danish EPA 2000). It is based on the waters content of
redox-sensitive elements like oxygen, nitrate, iron, sulphate and
methane. The system has four basic water classes ranging from the
most oxidised to the most reduced: A. The Oxygen zone. Recently
formed
groundwater, only a few years old at the most. It contains
considerable amounts oxygen as well as nitrate and sulphate.
Vulnerable in relation to nitrate.
B. The Nitrate zone. Characterised by its nitrate content but
has little or no oxygen. The development of this zone is controlled
by the presence of nitrate reducing substances like sulphide
minerals, iron and organic matter in the subsurface. The
depth/limit to which the nitrate-zone water has progressed is
called the nitrate front or Redoxcline. In the subsurface sediments
the redoxcline is usually accompanied by a change in colour from
grey and green (reduced) colours to yellow and red (oxidised)
colours.
A continuing supply of nitrate-containing groundwater will push
the redoxcline along the direction of the groundwater flow, thus
making nitrate stable in larger and larger groundwater volumes as
the nitrate reduction capacity of the subsurface sediments is used
up. Eventually the oxidised chemical conditions may reach the
aquifer used for drinking water supply. High sulphate content in
the nitrate zone water is a sign of possible nitrate reduction with
pyrite in the sediment, as this reaction among other things
produces sulphate. The reaction may also lead to an increase in the
content of nickel or arsenic in the water, because these unwanted
metals can be part of the pyrite mineral structure. As nitrate and
pyrite reacts with each other, this breakdown process will release
eventual metals to the groundwater. In some areas, including buried
valleys, serious problems with high concentrations of nickel or
arsenic in the groundwater may arise. Nitrate zone groundwater is
vulnerable with respect to nitrate and relatively young, usually
less than 50 years.
C. The Iron-sulphate zone. Moderately reduced conditions with
little or no nitrate, oxygen and methane. High content of dissolved
iron. The sulphate content may be as high as in the oxygen and
nitrate zones. The groundwater is relatively old, typically more
than 50 years. As a rule, the groundwater is not vulnerable and
pollution is rare. Increasing sulphate contents accompanied by a
decrease of the bicarbonate ( 3HCO ) ion in the water indicates
pressure on the zone in the form of a sliding redoxcline.
D. The Methane zone. A strongly reduced chemical environment
where methane occurs. There is no free oxygen or nitrate. The
sulphate content is low, less than 20 mg/l.
Brown water containing dissolved organic material or elevated
chloride concentrations may occur. The water is not very
vulnerable, and the risk of pollution is low. More than fifty and
up to hundreds of years old or more.
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4.15 The influence of redox chemistry on groundwater
vulnerability
163
Fig. 4.15.1: Depth of redoxcline, example of Tarm, Ringkjbing
County.
Common to the zones B, C and D, an inappropriately large water
extraction can pull an overlying zone or elements of this downwards
and initiate a slide in the water chemistry to a more oxidised and
thereby more vulnerable water type. Even in the short term this can
cause problems with nitrate and perhaps nickel or pesticide
metabolites like BAM in an aquifer. This process may be accelerated
if the aquifer in question is situated near-surface or in a
sand-dominated buried valley, without suitable clay cover layers or
any significant nitrate reduction capacity in the subsurface. Here,
the redoxcline will likely be pulled down in a matter of years,
causing a waterwork extracting groundwater from this particular
aquifer to experience nitrate and perhaps nickel problems.
Figure 4.15.1 shows a typical example. It is a contoured map of
the depth to the redoxcline in an area near the small town of Tarm,
Ringkjbing County, Denmark. The map is based on sediment
colour-changes recorded in well descriptions. A complex buried
valley system runs through the area. The waterworks of the nearby
town has its wells situated within this valley system, the infill
of which is strongly sand-dominated in this area. Outside the
limits of the buried valley system Miocene clays provide excellent
cover layers, and they, as well as the sands of similar age found
here, contain appreciable amounts of pyrite and organic material.
These Miocene sediments are found very close to the surface, and
here the redoxcline lies only a short distance below the
topsoil.
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PETER ERFURT
164
Inside the valley the redoxcline is clearly pulled down around
the waterwork well field, which contains several wells extracting
about 800,000 cubic metres of groundwater per year. Indeed, nitrate
and the pesticide metabolite BAM is found in wells at depths up to
100 m below surface. Another pulldown in the redox-cline surface is
seen to the southeast, situated across the border of the buried
valley system. This is due to pumping at a large landfill site
situated here. 4.15.4 Conclusion
In addition to the physical protection of aquifers accorded by
clay layers, the level of chemical protection may also be
important. Groundwater and sediment chemistry investigations can be
used to assess susceptibility of aquifers to nitrate pollution, and
form an important part of the vulnerability mapping process. 4.15.5
References
Appelo CAJ, Postma D (1999): Geochemistry, groundwater and
pollution: 239290. Balkema, Rotterdam.
Danish EPA (2000): Groundwater zoning. Guide nr. 3, 2000. Danish
EPA , Ministry of Environment, Copenhagen. In Danish.
Ringkjbing County (2002): Hydrogeological mapping at Brande.
Ringkjbing County, Department of Environment & Infrastructure.
In Danish.
Ringkjbing Amt (2006): Action planning in Ringkjbing County:
Hydrogeological mapping in Skjern and Egvad municipalities,
September 2006. Ringkjbing County Department of Environment &
Infrastructure. In Danish.
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