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4.3 OVEREXPLOITATION OF GROUNDWATER
4.3.1 Groundwater recharge methods
4.3.1.1 Current situation
Whether artificial or natural, recharge is the flow of water into
aquifers. Artificial recharge of groundwater is the augmentation of
the natural infiltration of precipitation or surface water by
appropriate methods. These include spreading of water on the ground,
pumping to induce recharge from surface water bodies and injection
through boreholes, wells, or other suitable access features.
In Europe artificial recharge dates from the early nineteenth century
where the first infiltration basin for recharging was constructed at
Goteborg (Sweden) in 1897.
In that country, such basins are common to a great number of municipal
water supplies and are located mostly on glacial eskers which, of
course, function very efficiently as conduits conveying recharge water
to pumping installations.
Adjacent river or lake waters transit through mechanical or rapid sand
filters prior to recharging. Most plants utilize rectangular basins
with a layer of uniform sand up to one metre thick on the bottom.
The success of these installations led to the widespread application
of the method in Sweden, Germany, and the Netherlands.
In Germany artificial recharge by means of basins and ditches and,
more recently, wells, is frequently utilized. Installations are
prevalent along the Lippe, Main, Rhine, and Ruhr rivers whose waters
are polluted and natural groundwater supplies are insufficient to meet
the industrial and municipal demand.
In the Netherlands, the water supply systems of Amsterdam, Leiden and
the Hague include basins for recharging water into coastal sand dunes.
The largest regional artificial recharge scheme is being constructed
in California, (USA), where as early as 1895 flood waters were spread
over the alluvial fan at the mouth of the San Antonio Canyou to
sustain the flow of wells in the Upper Santa Ana Valley. By the late
1950's more than 50 different agencies were involved with artificial
recharge, most of the projects being in the San Francisco Bay, Tulare,
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and southern coastal regions. About 65 percent of all the projects
utilize recharge basins or pits and these account for about 60 percent
of the water recharged.
Nearly 40 percent of the water used in Los Angeles is derived from
aquifers that have been artificially recharged with storm runoff and
other surplus water. The water is recharged through 36 spreading 2
basins with a combined total area of 13 km .
Examples of successful recharge projects and experiments in the USA
have been reported in Illinois, Ohio, North Dakota, Michigan, Arizona,
New York, and particularly California, amongst others.
By far the widest use of artificial recharge in the world is the
supplementing of dwindling municipal and industrial groundwater
supplies or the improvement of their quality. Advanced techniques are
utilized in Algeria, Egypt, France, Germany, Iran, Israel, Spain,
Sweden, Switzerland, USA and the Latvian, Lithuania, Turkman, Uzbek,
and Ukrainian Soviet Socialist Republics.
Artificial recharge is also extensively used to control salt-water
intrusion in coastal areas of Australia, Israel, Japan, Morrocco,
the Netherlands, Senegal, Togo and the USA.
In Japan, artificial recharge is also used to reduce land subsidence
in areas of excessive pumping and in Bulgaria, France and Romania it
is used to supplement irrigation water supplies.
The case of China
Groundwater recharge began in Shanghai (China) in 1969 to control land
subsidence which at that time had attained 2.4 m. The main productive
aquifers are the second and third layers which are situated at 60 and
90 m below the ground surface respectively. After five years of
recharge, the water level of the aquifer more than recovered its
original position but the 1st compressible layer had not recovered.
The amount of subsidence was almost halted. Recharge can only halt or
reduce subsidence, it cannot restore land to its original elevation.
Beijing, the capital of the People's Republic of China, draws most of
its water from the alluvial fan of the Yungting River. The annual
decline of the groundwater level has varied between 1.5 and 2 m since
1970. The total decline now attains about 20 m.
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Various groundwater recharge experiments have been undertaken since
1978:
a Flooding 3
Along the channel of the Yungting River 0.7 to 1 Mm of water are
released daily from the reservoir upstream, along a reach of river
17 km long. The groundwater level has risen by more than 2 m on each
side of the channel, over a distance of about 2 km.
b Infiltration basin
There are more than 40 abandonned gravel pits on the alluvial fan of 2
the Yungting River and their total area is about 2 km . Artificial
recharge by infiltration has been experimented in one of these pits.
The water comes from a reservoir and has low turbidity. After
chlorinization the water was infiltrated in the pit at a rate of 3 -1
0.60 to 1.07 m .s for 80 days and the total volume of recharge was 3
3.85 Mm . The rise of the water level in the observation wells is
indicated in Table 4.3.1.
Table 4.3.1 The rise of water in observation wells following
artificial recharge with infiltration basin
Distance of observation well (m)
Rise of the water level (m)
0
4.5
100
3.5
200
2.0
300
1.5
500
0.75
700
0.25
1 000
0.05
A much larger infiltration basin was also used. It has an irregular 2
shape 280 m by 150 m with a surface of nearly 28 200 m . The first 3 -1
experiment revealed an infiltration rate of 1.07 m .s and lasted for
16 days from 9 to 25 December, 1978. The total volume infiltrated was 3
1.48 Mm and the influence was observed 3 900 m downstream. After
excavation of deposits resulting from the first experiment a second
experiment took place in 1982 lasting 25 days, from 7 August to 3
1 September. The total recharge volume was 3.86 Mm . The area
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influenced attained 20 km , and the maximum rise of groundwater level
in the vicinity of the basin was 3 m.
c Well recharge
A recharge well 8 m in diameter and 25 m deep was excavated at the top
of the alluvial fan. From 17 June to 6 July 1980 the recharge rate 3 - 1 3 - 1
varied from 0.5m.s t o O . l m . s . The results are shown in
Table 4.3.2.
Table 4.3.2 Results of recharge by means of a well
Dates
17-27/6
28/6-2/7
4-6/7
Recharge time
(h)
143
51
17.5
Rechargi rate 3 -1
m . s :
0.5
0.3
0.1
e volume
3 3 K 10 m
257
55
6
Observation well
No.
1 2
1 2
1 2
Dist rech
(m)
36 116
ance arge
from well
Rise of groundwater
level (m)
3.47 2.90
1.89 1.27
0.80 0.64
Groundwater recharge has greatly supplemented the natural infiltration
in the Beijing area and recharge is used in many places in China since
the 1970's in order to compensate the overdraft of groundwater.
In Hauntai, a county of Shantung Province, a total of 600 km of
channels and ditches have been excavated which together with shafts,
basins and small lakes receive river water in order to recharge
groundwater. It is reported that the groundwater level of the whole
county had been raised by 2 to 3 m. Many abandonned wells have come
back into use.
In Shansi Province groundwater recharge is practised in Chixian county
both by water spreading and well injection. The total annual recharge, 3
lasting for 2 to 3 months each year, amounts to about 0.93 Mm . For
more than 10 years the irrigation demand has been satisfied and the
groundwater level of this region has remained unchanged.
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Gaocheng, Chaoxian and Luancheng counties of Hopei Province depend
upon groundwater for irrigation and the groundwater level dropped
about 5 m between 1972 and 1976 creating a serious problem in these
regions. Since then, channels and ditches totalling about 578 km have
been excavated to recharge groundwater and the groundwater level has
risen by 2 to 4 m.
In China, in conjunction with injection recharge an energy
conservation method has been developed since the 1970's called "winter
recharge-summer use and summer recharge-winter use". In winter a
certain amount of low temperature water is injected into the aquifer
for use in the summer time to lower temperatures. Most of the injected
water remains at a low temperature. This is a good way to save water.
The réverse process consists in injecting warm water to maintain a
certain temperature in industrial plants in the winter season.
The Third Cotton Factory of Beijing offers a typical example of
"winter recharge-summer use". From 1970 to 1982 the total amount of 3
water injected in winter was 8.5 Mm , representing about 78.6 MWh of 3
energy. The total volume extracted in summer is 8.5 Mm which is equal
to the volume injected in winter, but the energy gained is about
12.7 MWh. The average energy gain is about 15 percent and the maximum
about 28 percent. The latest recharge began on 25 November 1981 and
lasted until 20 March 1982 a period of 118 days. The total recharge 3
volume amounted to 209 507 m with an average water temperature of 3
4.5°C. In summer the total volume extracted was 155 832 m with an
average temperature of 13.3°C. Many factories in Shanghai and Tienjin
practice this method and realize considerable savings of energy.
In Zhangsi province experiments of "winter recharge-summer use" took
place between 1974 and 1979 at Zhangsi Cotton Factory, situated near
the Gan river on the lower terrace. This terrace consists of
Quaternary sand and gravel 15 to 20 m thick at the lower part and 5 to
10 m of loam at the upper part. In winter the temperature of the river
water is about 10°C, which is about 10°C lower than that of the
groundwater. Taking 1976 as an example, from January to March the
recharge water had a mean temperature of 10.7°C and a total volume of 3
0.18 Mm , equivalent to 1.76 MWh of energy. From June to October
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0.35 MWh of energy were extracted from the system whose efficiency is
about 20 percent. The method offers the benefit of combining energy
conservation and a source of water.
A.3.1.2 Negative effects
- Repercussions of recharge on groundwater quality
Although groundwater recharge is an effective means of augmentating
natural recharge, negative side effects can emerge under certain
conditions such as pollution, thermal pollution or blockage of the
aquifer. At Beijing, together with observations of groundwater
level, the quality of the water was monitored. Bacteria are not
present outside a radius of 80 m from the recharge point and it
seems that there is no real problem.
In the southern suburb of Beijing, sewage including factory
effluents, has been used for irrigation for about twenty years. The
results reveal that hardness, chloride, sulphate and total dis
solved ions have all increased with time. It appears that chloride
contents of over 200 mg.l and total dissolved solids contents of
over 1 000 mg.l have a harmonic distribution. This is due to
percolation of the irrigation water through overlying sediment
whose thickness varies from 5 to 10 m.
The conclusion is that contamination of groundwater by recharge is
a long term process, even when use is made of polluted water as is
the case in the suburb of Beijing where negative side effects
appeared in scattered regions only after ten years. No conclusions
can be drawn from experimental recharge of limited range over short
periods of time. In order to avoid negative side effects, the
recharge water must be pre-treated, especially in the case of the
well injection method in which water enters the aquifer directly.
Effect on groundwater temperature
In 1959 the temperature of groundwater at the top of the Yangting
River alluvial fan was 14°C or slightly higher than the mean air
temperature of 15°C for that year. In 1979, however, the
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temperature of groundwater rose to 17.6°C and whilst the mean air
temperature was lower than in 1959, dropping to 14.1°C. This
phenomenon may be due to the establishment of an iron and steel
plant as well as other factories. Since the beginning of recharge
operations in the alluvial fan it is obvious that temperature of
the observation well downstream has been influenced, the
temperature having attained 22.3°C and then gradually dropped,
stabilizing for ten days at 18.6°C. The natural variation of
groundwater temperature is sinusoidal with a high peak occurring in
summer and a low temperature in winter. The temperature of
thermally polluted groundwater depends upon the temperature
variation of the recharge water.
Due to the increasing temperature of the water both the hardness
and sulphate content increase. When the temperature of groundwater
increases by 5°C, the total solids increase by 35 percent in the
Beijing region. This not only increases the cost of treatment but
is also harmful to public health.
Blockage of the aquifer
a Silting of the aquifer
The most common problem associated with recharge is the blockage of
the aquifer resulting in the reduction of infiltration rates due to
silt-size particles which fill the interstices of the aquifer.
Experience in the Beijing region shows that the silting process
accompanying basin infiltration occurs in three steps. The first
step is the entry of water loaded with suspended particles to the
basin. Fine particles and water under pressure flows from the
recharge basin to the aquifer. The depth of silting varies
according to the pressure as well as the size and the concentration
of particles.
High pressures, fine grains and low concentrations can result in
silting-up to depths of more than 20 cm from the surface, whilst
low pressures and high concentrations give rise to silting in a
superficial zone of only a few centimetres in depth. Another
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controlling factor is the grain size of the aquifer itself.
The silting depth is usually greater in coarse grain aquifers than
in fine grain ones. After the silting-up of the superficial zone of
the aquifer, the second step begins. Most of the suspended
particles stop at the surface of the aquifer forming a filter
layer. This is when the third step begins. Almost all the suspended
particles stop above the surface of the aquifer forming a thin
layer of sediment. By this time the infiltration rate of the
recharge water is greatly reduced and solely depends upon the
vertical permeability of the layer of sediment and on its
thickness.
b Air blockage of the injection well
In a well injection experiment the recharge took place at 3
atmospheric pressure. The daily recharge rate was 1 054 m . After
25J5 hours, the water level in the well rose quickly to within 0.14
m of the surface when the experiment had to be stopped. The rise is
attributable to air dissolved in the water and a corresponding
decrease of the density of the recharge water in the well and the
formation of an air blockage.
Other side effects
Groundwater recharge may result in some unexpected effects. For
instance recharge into an alluvial fan with too much water may
result in the formation of swamps at the fan periphery.
Injection into deep wells, especially in the bedrock in the
presence of active faults, may induce earthquakes. In the 1960's a
reservoir was established on the Xinfung River in the Kungtung
Province (China). A few months after filling of the reservoir,
earthquakes began. The intensity of these was 1 to 3 degrees with a
frequency of 200 to 500 per day.
This was a serious situation and after monitoring and geological
investigation it was concluded that a great fault tending NNE was
present across the left abutment of the dam, extending into the
reservoir. The depth of water in the reservoir exceeded 20 m,
leading to recharge into the fault. Water in the fault acted as a
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lubricant promoting the release of shear stress in the fault in the
form of earthquakes.
4.3.1.3 Procedures for the elimination or reduction of negative side
effects
Among the side effects of artificial recharge, blockage of the aquifer
is the most common, whether the method used is water spreading or well
injection. When water spreading is adopted, the most convenient and
economic remedial method is the pretreatment of the water. If this
cannot be considered, in the long run a reduction of the infiltration
rate, is unavoidable. In order to maintain the infiltration rate,
recharge should cease periodically to break up the surface layer of
deposited sediment when dry. Weather conditions as well as the grain
size of the deposit should be taken into consideration. In Beijing
recharge proceeds for three weeks and ceases for one week in the
summer whilst in winter recharge continues for three weeks and stops
for three weeks. Another way of maintaining the infiltration rate is
to remove the superficial layer with machines when dry. Table 4.3.3
shows infiltration rates before and after excavation of the
infiltration basin in Beijing.
Table 4.3.3 Effect of excavation of sediments on recharge rates
Before excavation of sediments
Water level in infiltration basin (m) 60.3 59.52
Infiltration
area (m2) 10 700 8 600
Infiltration
rate (m3.day-1) 8.64 7.43
Average infiltration
3 -1 rate (m .day ) 7.25
After excavation of sediments
59.32 58.0 58.0
7 000 12 300 12 300
5.68 12.19 8.13
10.16
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In the case of water spreading, changes in groundwater quality only
appear in the long run but with well injection the effects are
revealed in a relatively short time. Pretreatment of the recharge
water avoids the pollution of the groundwater. Pretreatment avoids not
only chemical changes but also biological effects.
4.3.2 Blockage of aquifers
4.3.2.1 Types and causes of aquifer blockage.
Two general methods of groundwater recharge are widely used: surface
infiltration and well injection. Both methods are subject to a
reduction of efficiency due to progressive blocking of the aquifer.
This paper lays emphasis on aquifer blockage associated with well
recharging and the remedial measures used to combat this phenomenon.
Based on many years of experience of recharging and extracting
processes, six factors are seen to have a bearing on the problem:
Blocking of the sand filters and well screens with turbid material,
blocking of the pores of the sand aquifers.
Air entrainment during recharge resulting in blocking of the pores
of the sand aquifers.
- Chemical and electrochemical corrosion of metal screens and well
pipes, due to changes of temperature and pressure of the water in
the recharging wells.
- Increase of the dissolved oxygen in groundwater resulting in
chemical changes, salt forming sediments which are insoluble in
groundwater.
- Iron and sulphate reducing bacteria introduced with the recharging
water reproduce and biochemical blockage occurs.
After long periods of recharging and extraction, changes occur in
the arrangement and composition of the sand grains in the sand
filling body and in the aquifer resulting in a decrease of the
porosity.
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The side effects can be classified as physical, chemical and
biochemical blockages and these can be either temporary or permanent
(Table 4.3.4).
Table 4.3.4 Classification of blockage in recharge wells
Type Nature Comments
physical mechanical, sand bed compaction, porosity and gaseous and permeability reduced; air entry forming suspended air-water mixture closing the pores; matter entry of turbid matter blockages
precipitated in the pores
chemical iron compound dissolved oxygen increase, Fe changes precipitate and calcium, chemically with precipitation of the
magnesium salts oxide and hydroxide; temperature and precipitate pressure change: C0_ is given off and
the calcium and magnesium carbonate precipitate, cementing sand or filter screen
electro- electro- electrochemical corrosion of well-tube chemical chemical and screen with perforation or corrosion corrosion precipitation of iron.
bio- micro-organisms a large amount of iron sulphate reducing chemical bacteria are reproduced and their
secretion block screens and pores
a Permanent blockage
When the extraction volume exceeds recharge, there is a decrease of
the level of the groundwater table near the well or of the well field.
The grains in the artificial sand packing and round the screens
gradually become organized to suit the direction of flow. On
the contrary, when recharge takes place the water passes through the
sand packing towards the sand aquifer and disturbs the original
arrangement and composition formed under extraction conditions and a
new arrangement of the sand grains is realized to suit the new flow
direction during recharge. Under the repeated changes of direction the
sand packing and the sand aquifer near the screens become more closely
packed and the porosity is decreased. Thus penetration of recharge
water is reduced together with the yield of the wells. These signs
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indicating permanent blockage are most apparent in two-way wells. The
processes of permanent blockage however, are generally very slow,
often taking ten years or more.
Table 4.3.5 shows data from wells used for winter recharging and
summer extraction.
Table A.3.5 Change of recharge rates in Shanghai two-way wells
Well 1974 1975 1976 1977 1975
Shanghai Q at end of winter 3 , -1 -1
m .hr .m
Cotton Q at end of summer
Mill 2 m3.hr _1.m _ 1
Well 5 Q at end of winter/
Q at end of summer
1.04 1.25 1.01 0.76 0.60
3.20 3.40 2.48 1.99 2.13
0.33 0.37 0.41 0.38 0.28
Shanghai Q at end of winter 3 , - 1 -1
m .hr .m
Cotton Q at end of summer
Mill 19 m3.hr -1.m _ 1
Well 12 Q at end of winter/
Q at end of summer
7.35 9 .45 5 .72 4 . 4 1 4 .54
15 .71 20 .22 17.10 13.93 14.47
0 .47 0 .47 0 . 3 3 0 .32 0 . 3 1
b Temporary blockage
Experience in China and elsewhere shows that there are four general
causes of temporary blockage: physical, chemical precipitation,
electrochemical corrosion, biochemical blockage.
4.3.2.2 Physical blockage
a Air blockage
The recharge water may carry air into the aquifer if the recharging
tube is ineffectively sealed. The water forms a milky mixture of gas
and water and the minute air bubbles fill in the pores of the sand
layer, only part of the gas being dissolved in the water. The recharge
capacity drops markedly due to a decrease in the permeability of the
sand layer.
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Pumping should be stopped until the gas has been given off and the
water becomes clear. When the gaseous blockage is associated with
turbid material and biochemical blockage, large quantities of air
bubbles flow out of the discharge pipes, accompanied by a very strong
smell.
b Blockage by suspended matter
The recharge water often contains suspended matter which act as
cementing materials. A concentration of 5 mg.l suspended matter in
the recharge water represents 5 kg of suspended matter entering the 3
well if the recharge rate is 1 000 m per day. The suspended matter
collects in the filter screen and the sand layer in the vicinity of
the well-tubes, resulting in blockage.
4.3.2.3 Blockage due to chemical precipitation
Soluble corrosive salts cause chemical and electrochemical corrosion
of the metal well-tubes and screen, resulting in chemical
precipitation and blockage. The lower is the Ph of the water, the
higher is the hydrogen electrode potential, leading in some cases to
depolarized hydrogen corrosion. Dissolved oxygen in water is also
corrosive. The CI ion accelerates corrosion, when concentrations
exceed 300 mg.l . SO, is a strong depolarizer which favours
corrosion as the concentration increases. The dissolved oxygen in the
groundwater increases if air enters the aquifer during recharge.
Changes of the physical state (temperature and pressure) and of the
chemical components of the groundwater surrounding well screens give
rise to a series of complex chemical reactions causing a variety of
salts to be precipitated. The chemical precipitation, with the passage
of time accumulates in the gravel/sand pack and in the pores of the
aquifers and the wells are gradually blocked up.
When the chemical precipitation blockage occurs in injecting wells,
the chemical constituents of the extracted water should be analyzed
since they are, at a given temperature and pressure, the product of
the synthesis of recharge water, groundwater, metal well-tube and
aquifer strata. The extracted water may give an indication of the
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composition of the chemical precipitation blockage. The chemical
constituents of the extracted water are very complex however. As the
recharge water is pumped up, only pure water is observed, then water
with minute air bubbles and finally pure water once more. The chemical
constituents of turbid water are quite different from those of pure
water:
- The content of turbid matter in the recharge water is low (not more
than 2 mg.l ), and the content of suspended matter in groundwater
does not exceed 40 mg.l . The turbid matter content in the pumped
water attains 200 to 400 mg.l however. The composition of the
turbid matter is mainly iron and mud.
- The iron content of the recharge water is close to zero and the
iron content of the groundwater is generally less than 1.5 to
3.0 mg.l . The iron content of the extracted turbid water is more -1 +++ ++
than 10 mg.l . The principal substance is Fe with some Fe The concentration of dissolved oxygen in the recharge water is low
(about 2 mg.l ). The content of the dissolved oxygen in the
groundwater is more than 6-8 mg.l . As the air enters the aquifer
during recharge, the higher the dissolved oxygen content and the
lower the temperature at a given pressure the higher will be
content of the dissolved oxygen. The highest values are found
during winter recharging. As the extracted water is a mixture of
recharge water and groundwater, the content of the dissolved oxygen
reflects the values of concentration observed above with a maximum
value of 14 mg.1
The Ph value of the extracted water is less than 7.
The chemical precipitation causing blockage is mainly composed of
iron compounds. The two principal factors causing iron compound
precipitation are:
1 Iron compound precipitation increases with the concentration of
dissolved oxygen. When the Ph of groundwater lies between 6.5 and I i i
8.5, the water does not contain soluble Fe , but only soluble
Fe . However, when the recharge water contains comparatively large
amounts of dissolved oxygen, the oxygen undergoes a kind of
deterioration:
h 0 +H 0+2Fe -> 0H~
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and the oxygen reacts with the Fe to form iron hydroxide, which
is insoluble in water. The iron hydroxide is precipitated onto the
mesh opening of the screens or into the pores of the sand layer,
with a resulting blockage by chemical precipitate:
2Fe + 0 •* 2FeO 4FeO+02 -> 2Fe2<33
Fe_0o+3H.0 -> 2Fe(OH),4-
++ 2 Electrochemical corrosion of the well tubes increases the Fe
component in groundwater which is a natural electrolyte. Although
the degree of ionization of water is very slight, it is ionized
into H or OH ions. Moreover, when C0„ is present in the water,
there is an increase in the number H ions.
C02+H20 j H2C03 t H + + HC°3
The screens of recharge wells are generally steel or cast iron
tubes in which holes have been punched and reinforcing ribs welded
to the outside, copper wire being wound around the ribs. In ground
water, this type of screen bound with wire forms a primary electric
cell placed in a solution of OH H and HCO_ ions.
The corrosion rate of screen is generally controlled by the
negative pole (iron tube). If there is no dissolved oxygen in the
groundwater, the colloidal Fe(OH) continues to collect in the
vicinity of the screen and begins to block the continuous movement
of metal ions into the groundwater, thus slowing corrosion. This
stage is called "polarization". However, in the well, the dissolved
oxygen content of the water increases. At this point a reaction
between the oxygen and the iron forms a precipitate (Fe OH) which
is not soluble in water and causes blockage
4 Fe(OH) + 0 + 2H 0 •+ 4Fe(0H)
Because recharging results in high concentrations of dissolved
oxygen, depolarization occurs, increasing the corrosion rate. At
the same time, the flowing water has a washing function and the
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protective skin of the screen can be broken, resulting in a
decrease in the concentration of the Fe ions at the negative
(iron)pole accelerating the transfer of the Fe ions to the
groundwater and forming an Fe precipitate. Fe(OH) precipitate is
the principle agent of precipitation blockage. There may also be a
calcium carbonate precipitate in the chemical blockage since
changes of pressure and temperature often occur in the groundwater
due to repeated extraction and recharge. C0„ is given off and CaCO
precipitate collects in the sand/gravel pack and screens or in the
pores of the aquifer, producing precipitate blockage. Precipitated
CaCO often cements the calcium horizon to the water-bearing sand
strata. This is a serious form of blockage.
4.3.2.A Biochemical blockage
Biochemical blockage is caused by the activity of micro-organisms.
Communities of micro-organisms frequently compose fungus algae, an
important factor in the biochemical blockage and the production of
iron bacteria and anaerobid sulphate reducing bacteria.
The sulphate reducing bacteria have a strong corrosive action on well
pipes and enter with the dissolved oxygen, NH. NO. ions etc. present
in recharge water. In the absence of oxygen and at mild temperatures,
the anaerobic sulphate reducing bacteria can survive and produce the
corrosion of a metallic surface in polluted water, resulting in
biochemical blockage:
S0~~ + 4H+ + 8e -»• S~~ + 4H 0 + energy
Fe"*"1" +S~~ ->• FeS +
The iron bacteria are maintained by the energy of Fe and Fe
oxidation. They often collect in communities invisible to the naked
eye, accelerating the oxidation of Fe to Fe (HCO„)„. Thus Fe in
solution and Fe(OH). precipitate are formed. Blockage is caused by the
accumulation of Fe(OH) due to activity of these bacteria, known as
iron bacteria.
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1 Environmental conditions necessary to the iron bacteria
On the basis of observations made in China and elsewhere iron bacteria
are considered to propagate rapidly in the following conditions:
a The recharge well has undergone chemical or electrochemical
corrosion so that the content of Fe increases in the groundwater.
b The temperature of the groundwater lies between 10 and 13° C.
c The concentration of dissolved oxygen is less than 10 mg.l
d The Ph values range from 6.5 to 7.5.
2 Damage caused by iron bacteria
The iron bacteria is a kind of autotroph (nutrition) micro-organism
which obtains carbon from CO and relies on the energy of the
oxidation of Fe to Fe to live. The iron bacteria lives in ground-
water poor in oxygen and rich in Fe . It multiplies by cell division
at an amazing speed. In suitable circumstance, the cells split once
every 20 minutes. One iron bacteria can thereby produce a population 9
of 472 x 10 each day. Iron bacteria can absorb the iron in water and
convert Fe into cotton fibre or bean curd forms of brownish-red, I I i
rust-yellow Fe precipitates. Moreover, the propagation of the iron
bacteria accelerates the electrochemical corrosion of the well pipes
and increases the Fe content in groundwater. In addition, if the
recharge water is poor in dissolved oxygen and contains other salts
and organisms, suitable conditions are created for the multiplication
of iron bacteria. The continuous oxidation of Fe in water produces a
large amount of cotton fibre form Fe (OH) precipitate. Recharge well
blockage caused by iron bacteria collecting in the porous layers is a
source of serious trouble and should be treated as soon as it is dis
covered, for the speed of propagation is rapid and the well screens
can suffer considerable damage due to corrosion.
4.3.2.5 Remedial measures
The usual remedies employed to deal with well blockage and restoration
are the pump reversal and chemical methods.
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a The pump reversal method
The method consists of reversal of flow direction at regular intervals
in order to remove the blockage, thus ensuring normal recharge.
The recharge periods of each well are determined according to the
permeability of the aquifer, the characteristics of the recharge well,
the quality of the recharge water, the recharge volume and the
recharging technique employed.
- Successive recharge periods without pump reversal.
Recharge with water containing suspended matter such as mud, sand
and micro-organisms in these conditions leads to blockage of gravel
pack and aquifer, accompanied by a rapid rise of recharge level and
overflow from the well-head.
Indefinite periods of pumping alternating with intermittent
recharge.
After pumping, the rate of recharge recovers to some extent. If the
length of the pumping period is increased the recharge rate is
further improved as the difference between dynamic and static
levels drops, indicating reduced blockage.
Intermittent recharge with fixed pumping intervals.
Based on experience of recharging in water-bearing strata of medium
porosity together with observations of pumping from a number of
recharge wells, the suggested intervals for recharge are: once a day
for vacuum recharge, once or twice a day for pressure recharge and
2 to 3 times a day for pumped recharge. The time intervals will depend
on the quality of water, varying from pure to turbid water, but 15 to
30 minutes is common practice.
b Reversal of recharge wells
In general, when recharge wells show signs of blockage it is usual to
resort to successive pumping periods. Recharge is not resumed until
the yield per unit height of drawdown has recovered. During pumping
the flow, static and dynamic water level variations with time must be
determined together with the ion analysis of the water. The technical
methods available include two types of vacuum recharging and pressure
recharging which differ from the pumping method. The pumping method of
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vacuum recharging calls for turn-on, cut-off and control valves to
extract water. Three different methods can be employed during pumping,
involving complete sealing of the well pipeline, the riser pipe and of
the well casing, whilst recharging proceeds simultaneously. The choice
of method depends upon the strength of the stratum and its structure,
and the type of blockage (Table 4.3.6).
Table 4.3.6 Remedial measures for production well blockage
Pumping Conditions of application method
vacuum strong suction of sand-bearing strata, good for removal of sand precipitate, suitable for general recharge blockage
gas medium suction of strata, suitable for general blockage of the recharge well
reflux small suction of strata, long-term pumping suitable for sand-bearing wells subject to intermittent use
To treat seriously blocked wells, pumping, intermittent-pumping-
recoil, vacuum pumping and intermittent-reflux-recoil, combined
pumping and pressure recharge are employed:
1 Pumping and "intermittent stop-pump-recoil"
With the so-called "intermittent stop-pump-recoil" method extraction
is stopped for 3 to 5 minutes intermittently so as to transmit the
shock of the falling column of water in the riser to the blocked sand
strata.
The method should be used in the following conditions:
the time interval between successive recoils should not be too
short, 3 to 5 minutes being suitable;
the method is suitable for use with filter screens of low strength,
pumps of low quality and when there is a large output of sand;
the recoil should not be attempted before the extracted water has
become pure, in order to avoid entry of impurities and chemical
precipitate into the water-bearing strata, with the risk of further
blockage.
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2 Vacuum pumping and intermittent reflux-recoil
This method makes use of the original pressure recharging equipment
(the vacuum pumping method with closed air valve). When the water
level in the well drops, a vacuum is created in the well-tube above
the surface of the water with the result that a suction force is
applied to the aquifer. To apply the vacuum pumping and intermittent
reflux recoil method, the reflux valve is opened at intervals of 5 to
10 minutes during vacuum pumping so that the returning water recoils
violently into the screen pipe and the sand layer, the precipitates
that have filled the pores of the sand stratum around the filter
screen being removed together with the water.
3 Combined pumping and pressure recharge
The pressure is gradually increased by assisting the recharge pump
with a centrifugal pump, the pressure obtained varying between 1 and -2
3 kg.cm . The well pump is first started then stopped after the water
has become pure. Recharge then commences using the pressure pump,
stopping after 10 to 15 minutes, starting again after 15 minutes.
Observations are made of the rate of recharge and of the static and
dynamic levels.
c The chemical method
If the calcium or iron agglomerates have become cemented, they form a
hard scale on the screens. In general these precipitates react with
hydrochloric acid to form soluble salts. The reactions are the
following
CaCO +2HC1 -> CaCL + H CO ; H CO •*• CO + +H 0
Before a hydrochloric acid treatment can be employed, a knowledge is
required the bore size and depth and of the materials of the well tube
and screen pipe as well as the static and dynamic levels of the
groundwater. Experience shows that the corrosive action of hydro
chloric acid on steel pipes and trapezoid copper wire can be avoided
by using 10 percent hydrochloric acid to which 2 percent acid-washing
anti-corrosive agent has been added.
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The method of acid-injection makes use of a metal drum in the bottom
of which there is a small opening to which the acid-injecting tube is
connected. The acid-injection tube is 20 to 50 mm in diameter and is
inserted directly into the screen pipe at the well mouth in order to
prevent any back flow of the acid solution through the well mouth.
Small holes 3 to 5 mm in diameter are drilled in the acid-injection
tube so that the acid solution will flow uniformly into the screen
pipe.
A rubber cover reinforced with an iron plate is installed on the top
opening of the screen pipe, the diameter of the rubber cover being
3 to 5 mm greater than that of the well-tube. This cover isolates the
water in the upper and lower parts of the screen pipe. The quantity of
the acid to be injected is a function of the length and diameter of
the screen pipe. The following example from Shanghai First Cotton Mill
illustrates the method:
The inside diameter of the screen pipe is 77.6 mm, its cross sectional 2
area is 0.025 m and its length is 15 m. The volume of water present 3
below the rubber cover is 0.37 m . Therefore 37 kg of pure acid and
7.4 kg of acid-washing anti-corrosive agent will be required.
Injection of the acid should take place in three stages. Each time,
one-third of the volume of the hydrochloric acid and of the acid-
washing anti-corrosive agent should be diluted with cold water. All of
the acid is introduced into the well-tube and the acid remaining in
the drum is washed out with pure water and also introduced into the
well-tube. The valve is closed and the acid is allowed to stand for 3
to 5 days. Then the acid is washed out mechanically and the impact
force of the water is used to promote removal of scale. In addition,
repeated scrubbing is carried out using a special wire brush, having a
diameter equal to the internal diameter of the well-tube and which is
lowered into the well, after which the well is washed again. During
the first few minutes, the water has grey-white appearance as it
contains a large number of air bubbles. After a few minutes, it turns
bright yellow and after 4 hours it becomes pale yellow or milky-white.
Pumping is continued for 32 hours, and gradually becomes normal, the 3 3 hourly discharge rising from 12 m to 60 m .