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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~

- 198 -

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.

- 201 -

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 .