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109
CHAPTER 7
IRRIGATION GROUNDWATER QUALITY
7.1 GENERAL
The exploitation of groundwater has increased greatly in the last
two to three decades in India, particularly for agriculture purpose, because
large parts of the area have little access to the surface water resources. Hence,
groundwater is used for irrigation and drinking purpose. The groundwater of
Tondiar basin is used for the domestic and agriculture purposes as the surface
water resources are limited. There has been increase in the demand for
groundwater agricultural practices and growth of the population. Groundwater
quality is strongly influenced by various hydrochemical processes as
discussed in the previous chapter. Increased agricultural activity in this region
is likely to have an impact on the groundwater quality. Groundwater is largely
contaminated by organic and inorganic pollutants in the rural area due the
modern agriculture, by way of application of agrochemicals. Hence, it is
necessary to determine the suitability of groundwater for the domestic and
irrigation purposes based on the presence of major ions, nutrients and trace
elements in the groundwater. There was no systematic variation in the quality
of groundwater throughout the study area, but the quality of the groundwater
is affected mostly where there is a human settlement, storage of the animal
waste and domestic waste.
The minimum, maximum and mean of the chemical analysis of
groundwater samples of this area during the months of May (summer) and
110
November (winter) 2006 is given Table 7.1. The concentration of Ca2+, Mg2+,
Na+, K+ HCO2-, Cl- increase from winter to summer month. NO3- and K+
concentration increase during the winter season due to the application of
fertilizer and shallow water table. Statistical data further confirms the role of
seasonal effect on the contributions of ions to the groundwater quality (Table
7.1).
Table 7.1 Min, max and aver concentration major ions in groundwater
Chemical Constituent (mg/l)
January 2006 (Winter) May 2006 (Summer)
Min Max Mean Min Max Mean
pH (no unit) 7.0 8.55 7.5 6.58 7.71 7.1
EC(μS/cm) 469 4531 1863 719 4844 2116
TH 154 761 338 155 826 340
Ca2+ 42 220 94 34 232 91
Mg2+ 4.8 59.4 25 9 68 27
Na+ 7.2 448 111 8.9 490 134
K+ 1 212 14 0.7 159 12.8
HCO2- 180 470 321 186 671 395
Cl- 22 638 192 26 899 218
SO42- 5 180 50 10 400 85
NO3- 4.5 62 20.83 4.0 43 17.50
7.2 DOMESTIC WATER QUALITY
Drinking water used for the domestic purpose should be free from
color, turbidity, odour, and micro-organisms. In most of the hydrogeological
conditions, groundwater can be put to direct use without treatment. The
domestic water quality indicates that a particular parameter may be useful at a
certain concentration but become toxic at higher concentration. According to
111
WHO (1984), about 80% of the disease prevalent are because of the
contaminated water. In the study area, the groundwater does not possess any
smell but there is some variation in taste. Drinking water specifications (Table
7.2) have been established by many organizations like ISI (1991), ICMR
(1975) and WHO (1984). The spatial distribution map of total dissolved solids
(TDS arrived from EC x 0.64) in Figure 7.1. Generally groundwater samples
in the study area is suitable for domestic purposes, except in Tondur,
Pennagar, Perumpoondi, Desur, C.M.Pudur, Melsithamur, Vallam,
Kongampattu, Indirasonkuppam, Elamangalam, Rettani, Vengathur and
Pelampattu where TDS is higher than the permissible limit of 1500 mg/l.
Hence, the water containing TDS more than 500 mg/l causes gastrointestinal
irritation (ISI, 1991) and about 80 % samples have TDS values above the
desirable limit of 500 mg/l. Most of the groundwater of this area is slightly
saline in nature with TDS greater than 1000 mg/l. Only in a few wells the
groundwater has TDS less than 1000 mg/l. Generally groundwater contains
TDS in more than 1000 mg/l make them unsuitable for ordinary water supply
purpose.
Table7.2. Standards for drinking Water
Chemical
Constituents
(mg/l)
ISI (1991) ICMR(1975) WHO(1984)
Highest
desirable
Limit
Maximum
permissible
Limit
Highest
desirable
Limit
Maximum
permissible
Limit
Highest
desirable
Limit
Maximum
permissible
Limit
pH (units) 7.0 – 8.5 6.5 – 9.2 6.5 – 8.5 6.5 – 9.2 7.0 – 8.5 6.5 – 9.2
TDS 500 1500 500 1500 500 1500
TH 300 600 300 600 100 500
Ca2+ 75 200 75 200 75 200
Mg2+ 30 100 50 100 50 150
Cl- 250 1000 250 1000 200 600
SO42- 150 400 200 400 200 400
112
Figure 7.1 Spatial distribution pattern of Total Dissolved Solids (mg/l)
of groundwater (May 2006)
The groundwater analysis of January 2006 plotted in the Durov’s
(1956) diagram is shown in Figure 7.2. Most of the groundwater samples fall
in the category C which is moderate quality from domestic purpose.
TDS (mg/l)
1500
2000
500
2500
3000
1000
300
May 2006
113
Figure 7.2 Durov’s classification of groundwater (January 2006)
7.3 WATER HARDNESS
Water hardness primarily depends on the amount of calcium and
magnesium present in groundwater. Water hardness in most groundwater
naturally occur due to weathering of limestone, sedimentary rock and calcium
bearing minerals. Hardness can also occur due to the application of lime to
soil in agricultural areas. Calcium and magnesium along with their sulphates,
chloride, bicarbonates and carbonates makes the water hard in nature. Safe
limit of hardness suggested by ISI (1983) for drinking water is 300 mg/l. Hard
water is unsatisfactory for household cleaning purpose, hence water-softening
process for removal of harness needed. Hard water is due to the unpleasant
taste. Hard water is generally believed to have no harmful effect on human
beings. Hard water leads to incidence of urolithiosis, anencephaly parental
mortality, some types of cancer and cardio-vascular disorder (WHO 1984).
114
Greater incidences of cardio-vascular diseases are reported to be more
confined to the area of soft water than hard water (Crawford, 1972). Hardness
is one of the important properties of groundwater because of its characteristics
influences on development of scales in water heaters, distribution pipes and
well pumps, boilers and cooking utensils and requires more soap for washing
clothes (Todd 1980; Hem 1991). The classification of the groundwater based
on hardness (Matthess 1982) and the representing wells are given in Table
7.3. Hardness of groundwater in the study area ranges from 154 to 761 mg/l
with an average value of 338 mg/l during winter season as shown in the Table
7.2. Majority of the groundwater samples fall in the hard water to very hard
category with allowable limit of total hardness for drinking water of 500 mg/l.
This may due to the geological formation of the rocks. In some agricultural
lands where fertilisers are applied to the land, excessive hardness may
indicate the presence of other chemicals such as nitrate.
Table 7.3 Classification of groundwater based on hardness (Matthness
1982) Hardness
Classification
Hardness as
CacO3 (mg/l)
Representing Wells
Very Soft 0-50 Nil
Soft 50-150 Nil
Average 150-250 2,3, 12,13,19,21,23,29,36,38,39
Hard 250-500 1,4,5,7,9,10,11,14,15,16,17,20,22,24,26,
Very Hard >500 6,8,18,27,28,30,31,32,33,34,35,37,40,41,42,
43,44,45.
7.4 IRRIGATION WATER QUALITY
The groundwater of the study area is extensively used for irrigation
in this area. The suitability of groundwater for irrigation depends upon the
mineral constituents present in the water. Irrigation water of good quality is
115
essential to maintain the soil crop productivity at a higher level. Water used
for irrigation always contains measurable quantities of dissolved substances,
which are generally called as the salts. The salts should contain small amounts
of dissolved solids originating from dissolution or weathering of the rocks.
EC and Na play a vital role in suitability of water for irrigation. Higher salt
content in irrigation water causes an increase in soil solution osmotic pressure
(Throne and Peterson 1954), which makes difficult for the plant root to
extract water for osmosis. The osmotic pressure is proportional to the salt
content or salinity hazard. The various salts present in the irrigation water not
only affect the plant growth directly, also affect the soil structure,
permeability and aeration which indirectly affect the plant growth (Mohan
and others 2000). The total concentration of soluble salts in irrigation water
can be classified into low (C1), medium (C2), high (C3) and very high (C4)
salinity zones and the values are shown in the Table 7.4. Higher EC in water
creates a saline soil. The important chemical parameters for judging the
degree of suitability of water for irrigation is sodium content or alkali hazard
which is expressed using EC, Sodium adsorption Ratio (SAR) and Sodium
Percentage, Residual Sodium Carbonate (RSC).
7.4.1 Electrical Conductivity
Electrical Conductivity (EC) is a measure of the degree of the
mineralization of the water, which is dependent on rock water interaction, and
thereby the residence time of the water in the rock (Eaton, 1950). EC of the
irrigation water becomes one of the important parameters to evaluate the
overall chemical quality of groundwater and it is being used to compare the
waters with one other in any region. Based on EC the water could be
classified as tasteless, sweet, brackish, saline and brine. As groundwater
moves and stays for a longer time along its flow path the increase in total
dissolved concentration and major ions normally occurs. It has been noticed
116
in many groundwater investigations that the groundwater in recharge area is
characterized by a relatively low EC than the groundwater in the discharge
area it is higher (Freeze and Cheery, 1979).
Figure 7.3 Spatial distribution pattern of Electrical Conductivity
(μS/cm) (May 2006)
Excellent
Good
Permissible
Unsuitable
LEGEND
Electrical Conductivity
Doubtful
117
Hence, irrigation water with high EC will affect the root zone and
water flow, due to high osmotic pressure. The United States Salinity
Laboratory has established a guideline for grouping of irrigation water based
on EC (U.S. Salinity Laboratory 1954). Groundwater of this area is grouped
based on these guidelines in Table 7.4 with corresponding well numbers
against each class. This shows that due to high EC the groundwater is not
suitable for irrigation in certain locations as specified in Table 7.4. However,
even in these areas groundwater can be used for irrigation with suitable
precautions as given in the Table 7.4. Similarly the regional variation of
Electrical conductivity of the groundwater as shown in Figure 7.3 falls in
permissible to doubtful in nature (May 2006).
118
Table 7. 4 USDA salinity laboratory (January 2006)
TDS (mg/l) EC in Μs/cm
at 25° C
SalinityClass Potential injury and necessary management for use in irrigation water
Representing Wells
119
7.4.2 Relation between SAR AND EC The SAR and EC values of water samples of the study area are plotted
in the widely used diagram for evaluating waters for irrigation purposes suggested
by the U.S.Salinity Laboratory (1954). This plot is shown in Figure 7.4. In this
USSL diagram (1954) waters of the study area are classified into C2, C3 and C4
types on the basis of salinity hazard and S1, S2, S3 types on the basis of sodium
hazard. The plot of the data on the US salinity diagram is shown for premonsoon
and postmonsoon seasons. This shows that there is slight improvement in the
water quality after the monsoon. This means in general they are classified as
satisfactory for irrigational use in almost all types of soils. Moderate and bad
quality types are due to enrichment of Na+ and EC concentrations. From (Table
7.5) it is found that most of samples fall in moderate water quality and few
samples 6, 8, 16, 17, 18, 25, 26, 27, 28, 30, 33, 35, 37 and 42 are undesirable for
irrigation. The causes of unsuitability are due to storage due to bedrock formation,
agriculture activities, storage of animal waste and local pollution of the villages.
The USSL diagram has shown both the premonsoon and postmonsoon period. The
good water can be used for irrigation in almost all types of soils. The moderate
waters can be used to irrigate salt-tolerant and semi-tolerant crops under favorable
drainage conditions. The bad waters are generally undesirable for irrigation and
should not be used on clay soils of low permeability. Bad waters, however, can be
used to irrigate plants of high salt tolerance, when grown on salty soils to protect
against under decline of fertile lands. The relative tolerance of crops to salt
concentration (After Sharma and Chawla 1977) is given in Table 7.7.
120
Figure 7.4 USSL Classification of groundwater during pre and postmonsoon
Table 7.5 Integrated Classification of groundwater
Groups USDA Classes Number of Wells Irrigation water classes
Premonsoon Postmonsoon Group I C1-S1,C2-S1 2 2 Suitable to use
Group II C1-S2,C2-S2, C3-S1,C3-S2
29 30 Conditionally suitable water
Group III C1-S3,C1-S4, C2-S3,C2-S4, C3-S3,C3-S4, C4-S1,C4-S2, C4-S3,C4-S4
14 13 Unsuitable
121
Table 7.6 Relative tolerance of crops to salt concentration
Salt-Sensitive Semi-tolerant High-tolerant
Gram, Moong, peas Rice, Wheat, Millets, Maize, Tomato,
Cabbage, Potato, onion, Mango,
Banana, Pears, Apple, orange, Lemon
Sugarcane, cotton, Mustard,
Sugarbeet, Tobacco, Barley
7.4.3 Sodium percentage
Sodium is important in classifying irrigation water, because sodium
reacts with soil thereby reducing the permeability. Percent sodium in water is a
parameter computed to evaluate the suitability of water quality for irrigation
(Wilcox 1948). The %Na is computed with respect to relative proportions of
cations present in water, where the concentrations are expressed in meq/l using the
formula
Soil containing large proportions of sodium with carbonate as the
predominant anions termed to alkali soil, whereas with chloride or sulphate as the
predominant cations termed as saline soil. Neither soil will support plant growth.
The percentage sodium computed for the postmonsoon and premonsoon period of
January 2006 and May 2006. Generally, %Na+ should not exceed 60% in
irrigation waters. In the Table 7.8 shows the most of groundwater samples fall
under the category of good to permissible quality during the premonsoon season.
A few samples fall under excellent and doubtful category. The Figure 7.5 indicates
the effect of monsoon rains on the irrigation water quality of the region. That is the
irrigation water quality improves in the post monsoon period. Groundwater
samples of the study area are plotted in the Wilcox’s diagram (Wilcox 1955) for
%Na+ = (Na+ +K+ ) X 100
(Ca2+ + Mg2+ +Na+ + K+)
122
the classification of groundwater for irrigation, wherein EC plotted against %Na
(Figure 7.5).
Table 7.7 Suitability of irrigation water based on sodium percent
Representing Wells in this category
Na% Suitability for
irrigation
Postmonsoon
(January 2006)
Premonsoon
(May 2006)
80 Unsuitable Nil Nil
The water samples of this area falls in all categories, however majority
of samples fall under good to permissible region. As explained earlier the
monsoon recharge results in the improvement of irrigation water quality. The
agricultural yields are observed to be generally low in lands irrigated with water
belonging to doubtful to unsuitable and doubtful. This is probably due to the
presence of sodium salts, which cause osmotic effects in soil plant system
123
Figure 7.5 EC Vs Sodium percent (Wilcox diagram)
7.4.4 Residual sodium carbonate
The quantity of bicarbonate and carbonate in excess of alkaline earth
(Ca +Mg) also influences the suitability of water for irrigation purposes. Residual
sodium carbonate (RSC) is frequently used to assess the water quality for
irrigation purpose, was not applied in present day. The RSC value is computed,
where ions are expressed in meq/l using the following formula.
RSC = (CO3 + HCO3) – (Ca + Mg)
The variation in RSC of the study area during pre and post monsoon
period is given in Table 7.8. However with respect to RSC all samples are within
the safe quality categories for irrigation this indicates that water is suitable for
irrigation purpose. From the table it is found that well no 36 is not suitable for
both seasons. This is clearly found from field studies the occurrence of alkaline
124
white patches of the soil. Further, continued usage high RSC will result in burning
of leaves of plants, affects crop yield. Similarly irrigation with high RSC water in
the fine textured soil will result in the development of alkali soil.
Table 7.8 Suitability of irrigation water based on residual sodium carbonate
Residual Sodium Carbonate of sample in this category
RSC (meq/l) Suitability for
irrigation
Premonsoon
Total wells
Postmonsoon
Total wells
2.5 Unsuitable 4 1
7. 4.5 Potential Salinity
This is defined as the chloride concentration and plus half of the
sulphate concentration. Doneen (1954) pointed out that the suitability of water for
irrigation is not dependent on the concentration of soluble salts. Doneen (1962) is
of the opinion that low solubility salts precipitated in the soil and accumulate with
each excessive irrigation, whereas the concentration of highly soluble salts
increases the salinity of soil. The potential salinity of water samples ranges from
2.5 to 27.85 with a mean of 8.50. The huge amount of potential salinity is due to
the presence of chlorides. From the Figure 7.6 shows the well nos. 33 with high
salinity and this well is surrounded by plantation crops like Casuarinas which is a
salt resistant plant should be cultivated in this region of high groundwater EC.
125
0
5
10
15
20
25
30
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 32 34 36 38 40 42 44
Well Nos
Pote
ntia
l Sal
inity
Figure 7.6 Potential Salinity of groundwater
7.4.6 Sodium adsorption ratio
The SAR is used to estimate the sodicity hazard of the water, the
sodium adsorption ratio (SAR) is used to predict the danger of sodium
accumulation in soil. Excess sodium in water produces the undesirable effects of
changing soil properties and reducing soil permeability and soil structure (Kelly,
1957). Hence, the assessment of sodium concentration is necessary while
considering the suitability for irrigation. The sodium or alkali hazard in irrigation
water is recommended by USSL which takes into account of the relative activity
in the exchange reaction with soil as expressed in terms of ratio known as SAR
(Sodium Adsorption Ratio). SAR is calculated by the following formula,
SAR = [Na+/√Ca2++Mg2+/2
The concentration is expressed in meq/l. While high salt content (EC) in
waters leads to development of saline soil, high sodium content (SAR) leads to
126
development of an alkaline soil. SAR can indicate the degree to which irrigation
water tends to enter cation-exchange reaction in soil. Sodium replacing adsorbed
calcium and magnesium is a hazard as it causes damage to the soil structure and
becomes compact and develops permeability problems. This will support little or
no plant growth. SAR is an important parameter for the determination of the
suitability of irrigation water because it is responsible for the sodium hazard
(Todd, 1980). The groundwater of the study area are classified with respect to
SAR values (Richard 1954) (Table 7.9). According to the above classification, the
SAR values in the study area range from 0.73 to 10.9 meq/l and the samples of the
study area have been classified as there is no danger of sodium consideration in
soil as per SAR. If the SAR values are greater than 9, the irrigation water will
cause permeability problems on shrinking and swelling in clayey soils (Saleh et
al., 1999). The higher the SAR values in the water, the greater the risk of sodium.
Table 7.9 Sodium adsorption ratio
Sodium adsorption ratio of sample in this category
SAR Suitability for irrigation Premonsoon
Total wells
Postmonsoon
Total wells
26 Unsuitable Nil Nil
7.4.7 Kelly’s ratio
Based on Kelly’s ratio waters are classified for irrigation. Sodium
measured against Calcium and Magnesium was considered by Kelly (1957) to
calculate this parameters. A Kelly’s ratio of more than one indicates an excess
level of sodium in waters. Therefore, water with the Kelly’s ratio less than one is
127
suitable for irrigation, while those with a ratio more than three are unsuitable for
irrigation. Kelly’ ratio of groundwater of the study area varies from 1.07 to 7.62
with an average 3.50 in the premonsoon while in the postmonsoon it varies from
0.65 to 6.07 with an average of 3.05. Therefore according to the Kelly’ ratio, all
the water samples are in the category of doubtful-unsuitable for irrigation as
shown in Figure 7.7.
Figure 7.7 Spatial distribution pattern of Kelly’s ratio (July 2006)
7.4.8 Permeability index
The soil permeability is affected by long term use of irrigation water as
it influenced by sodium, calcium, magnesium, and bicarbonate content of the soil.
Kelly's ratio
1
3
0
Legend
Permissible
Doubtful
Unsuitable
128
Doneen (1964), WHO (1989) gave a criterion for assessing the suitability of
groundwater for irrigation based on the permeability index (PI). Where
concentrations are in meq/l.
PI = (Na++√HCO3-)100
(Ca2+ + Mg2+ + Na+)
Accordingly, the permeability index is classified under class 1 (>75%), class
11(25-75%) and class 111(
129
The study area hazard of irrigation water sodium on soil infiltration
must be determined from SAR/EC interactions as shown in Figure 7.9 (Ayers and
Westcott 1985). From the Figure it is found most of water plot in the region of no
reduction in rate of infiltration. However, a few samples plot in the other regions.
Hence, it is concluded that the groundwater has moderate salt content and the
groundwater can be utilised for irrigation based on the capacity of infiltration.
7. 9 Plot of SAR Vs EC interactions
7.4.9 Magnesium hazard
Generally calcium and magnesium maintain a state of equilibrium in
groundwater. More magnesium present in waters affects the soil quality
converting it to alkaline and decreases crop yield. Szabolcs and Darab (1964)
proposed magnesium hazard (MH) value for irrigation water as given
MH = Mg/ (Ca + Mg) x 100
Where the are concentrations in meq/l.
130
MH > 50 is considered harmful and unsuitable for irrigation for irrigation purpose.
In generally groundwater samples have minimum value of 16.77 meq/l, maximum
value of 52.65 meq/l and average 32.65 meq/l. The spatial distribution of
magnesium hazard in groundwater as shown in Figure 7.10 indicates that almost
all the samples are with the range of less than 50 meq/l and suitable for irrigation.
Figure 7.10 Spatial distribution magnesium hazard (meq/l)
Magnesium Hazard
10-20
20-30
30-40
40-50
>50
Legend
131
7.5 NUTRIENTS AND TRACE ELEMENTS
As the study area is an intensive agriculture region, there is a possibility
of contamination of the groundwater by the inorganic fertiliser. Hence, the
groundwater samples were analysed for the concentration of nutrients and trace
elements to study the impact of agriculture and rocks formations.
7.5.1 Nitrate
The spatial variation of nitrate concentration in the groundwater of the
study area varies below detection limit to 45 mg/l (Figure 7.11). The nitrate
concentration in the groundwater samples varies between 4 mg/l to 43 mg/l with
an average of 17.5 mg/l. Thus it is within the recommended limit 45 mg/l
suggested for drinking water (ISI 1983; WHO 1984). Considering the intensive
agricultural activities and the application of the man-made and natural fertiliser in
this area, groundwater concentration of nitrate is reasonably less. Agricultural
activities, including both fertilizer nitrate and nitrate derived from increased
mineralization of soil nitrogen through cultivation, are the major sources for
nitrate in groundwater (Jackson and Sharma 1983, Flipse et al 1985). There is no
known geological source for nitrate for its presence in groundwater of this area.
The wells near agricultural land have high concentration due to irrigation return
flow.
Hence, fertilisers are considered to be the principle source of nitrates in
this area under intensive agriculture. The types of fertiliser that are in use in this
area are organic and inorganic chemicals. Organic fertiliser includes solid and
liquid manure, slurry and composite and inorganic fertiliser which are applied in
higher proportion than organic fertiliser. The commonly applied inorganic
fertiliser are urea, di-ammonium phosphate, ammonium sulphate, superphosphate,
132
Nitrate
20
010
30
40
Legend
potassium chloride, ammonium chloride and potash. Generally, people assume
that crop yield increases with higher fertiliser application without considering the
thickness and absorbing capacity of soil in the study area. From the study it is
found that groundwater of the basin is affected by nitrate due to the application of
fertiliser for agricultural purpose.
Figure 7.11 Spatial distribution of nitrate (mg/l) in groundwater (March
2006)
133
7.5.2 Boron
The spatial distribution (Figure 7.12) of boron in groundwater samples
varies from 0.43 to 0.76 mg/l with average of 0.66 mg/l. Boron is essential to
plants and it helps in the growth when it is present in a very small amount for
irrigation. Boron concentration of about 1 mg/l is good for the plants, but if it is
above 2 mg/l is injurious to the crops. The injury appears as a burning and
browning of the leaf top followed by yellowing of the margin. Boron
concentration in this area is within the permissible limit.
7.5.3 Silica
The regional distribution of silica (Figure 7.13) concentration in
groundwater varies between 10.8 mg/l to 39 mg/l and the concentration varies
seasonally. However, in most of the wells, high concentration of silica is observed
in the summer season due to the lowering of the water table. The concentration of
SiO2 decreases the due to the rise in water level. Silica concentration is
comparatively high in the central part of the basin, due the presence of composite
gneissic rocks. Exner and spalding (1979) reported that silica concentration in
groundwater is controlled by mineral solubility. Rock weathering is a major source
for high concentration of silica in the study area.
134
Figure 7.12 Spatial distribution of boron (mg/l) in groundwater (March
2006)
7.5.4 Fluoride
Fluoride is an essential element for maintaining normal development of
healthy teeth and bones. Deficiency of Fluoride in drinking water below 0.6 mg/l
contributes to tooth caries. An excess of over 1.2 mg/l causes fluorosis (ISI 1983).
High intake of fluoride results in physiological disorders, skeletal and dental
fluorosis, thyroxine changes and kidney damages (Latha et al., 1999). Bedrock
Boron
0
0.50
0.751.0
0.25
Legend
135
Silica
0
10
20
30
40
Legend
containing fluoride minerals is generally responsible for high concentration of this
ion in groundwater (Handa, 1975; Bardsen and others 1996). It is a common
constituent in most of the soil and rocks. The concentration of the fluoride in
groundwater of the basin varies between 0.08 to 0.3 mg/l in the month of
November 2006. All the samples exhibit suitability of this water for drinking
based on the concentration of F. The spatial distribution of fluoride concentration
in groundwater is shown in Figure 7.14. The concentration of the fluoride is within
the permissible limit of 1.5 mg/l drinking water (ISI, 1991; WHO, 1994).
.
Figure 7.13 Spatial distribution of silica (mg/l) in groundwater (March 2006)
136
Fluoride
0
0.1
0.2
0.3
0.4Legend
Figure 7.14 Spatial distribution of fluoride (mg/l) in groundwater
(November 2006)
7.5.5 Chromium
The concentration of chromium in the groundwater samples ranges from
0.12 mg/l to 0.67 mg/l with an average concentration of 0.49 mg/l. Out of 45
samples analysed almost all the samples exceed the desirable limit of 0.05 mg/l
and permissible limit of 0.1 mg/l as per BIS (2003) as shown in Figure 7. 15.
High concentration of Cr may therefore be due to their dissolution from the rock
137
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Well No
Cr (m
g/l)
material. This is possible because between the pH range of 5 to 13, chromium
dissolve from Cr2O3 (Hem, 1985). Thus Kerbyson and Schandorfn (1966)
reported that granite or metamorphic rocks in Ghana contain upto 0.03% of
Cr2O3and also the aquifer materials contain mica, hornblende and feldspar which
contain elevated level of chromium and Fe. Thus this basin contains hard rock
crystalline formations contain the higher concentration of chromium in almost all
the wells with the pH ranges from 6.5 to 8.5. The source of chromium in the area
is from the weathering of granite and subsequent leaching of ultrbasic rocks.
Chromium is highly toxic and in higher concentration it can be carcinogenic
(Swayer and MacCarty 1978).
Figurer 7.15 Chromium concentration in groundwater of all wells
138
0
0.005
0.01
0.015
0.02
0.025
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45Well No
Cu (m
g/l)
7.5.6 Copper
The copper in the groundwater ranges from 0 to 0.022 mg/l with an
average of 0.0099 mg/l. These values are within the desirable limit 0.5 mg/l and
permissible limit 1.5 mg/l of BIS (2003). Figurer 7.16 shows that the copper in the
groundwater samples related to agriculture practices. Furthermore, other potential
source of copper to agriculture land includes the application of pesticides. Copper
is essential for plant and animal metabolism, their limited occurrence in
groundwater is useful from the point of view water quality.
Figure 7.16 Copper concentrations in groundwater of all wells
7.5.7 Zinc
The zinc concentration falls below the desirable limit of 5 mg/l. Zinc is
essential element for plants metabolisms, their limited occurrence in groundwater
is useful from the point of view of water quality. Zinc occurs only in a few wells,
while in the rest of the wells it is absent. Zinc in the groundwater of this area
139
ranges from 0 to 0.372 mg/l. Agriculture activity is the major source for high
concentration of zinc in groundwater of the study area. Similar result was
observed by Pawer and Nikumbh (1999) in Behedi basin, Maharastra.
7.5.8 Nickel
The concentration of nickel is found in the groundwater is normally
from 0 to 0.034 mg/l. In most of the wells nickel is absent. The concentration of
nickel in drinking water is normally less than 0.02 mg/l (WHO 1993). High
concentration of nickel as both soluble and soluble compounds is now considered
to be a human carcinogen when related to pulmonary exposure (WHO 1993).
Generally, lower concentration of nickel is observed in the study area.
7.5.9 Manganese
The manganese in the groundwater ranges from 0.01 to 0.67 mg/l. The
permissible limit of manganese is 0.3 mg/l. as per BIS (2003). Agriculture
practices, fertiliser use, sewage and animal waste disposal contribute significant
amount of manganese to the groundwater of the study area and there is no
geological source. Similar results were observed by Pawar and Nikumbh (1999) in
the Bedi basin, Maharastra. Manganese is also essential element and is readily
absorbed by plants. This might be toxic at higher concentration and is usually
unpalatable in terms of taste, odour and discoloration of food (Figure 7.17).
Almost all the wells are within the limit and only the well no. 44 has exceeded the
limit and close to the well there is animal storage waste and also village sewage
pit, which would have resulted in this elevated concentration.
140
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mn
( mg/
l)
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Well No
Figure 7.17.Manganese concentration in groundwater of all wells
7.5.10 Lead
The concentration of lead is almost absent in all the wells except the
well no 1 where concentration is very low (0.005 mg/l). The concentration of lead
in natural water increases mainly through the anthropogenic activities (Goel
1997). Lead is extensively used some of the pesticides such as lead arsenate.
Table 7.10 Classification of Trace elements in the groundwater
Concentration
Of Trace
metals (mg/l)
Minimum Maximum Average BIS (2003)
Permissible
limit (mg/l)
Wells
exceeding
limit
Cr 0.122 0.669 0.4889 0.10 45 wells
Cu 0 0.022 0.0099 1.50 Nil
Zn 0 0.372 0.0275 15 Nil
Ni 0 0.034 0.0014 0.20 Nil
Mn 0.001 0.67 0.0679 0.30 1
Pb 0 0.005 0.05 Nil
141
7.6 SOURCES FOR NUTRIENTS AND TRACE ELEMENTS
From the groundwater studies it gives us preliminary information about the sources of nutrients and trace elements in the study area. Nutrient and trace metals in the groundwater of this area are mainly due to the agriculture activities, local pollution of the villages and rock formation. The nutrients like nitrate seeps from the agriculture land to the wells due to the fertilization of crop lands. The concentration of nitrate increases slightly with rise in water table during the monsoon and concentration decreases during the summer period. Silica is high in groundwater due to rock weathering of silicate minerals. The fluoride is from the country rocks like hornblende biotite gneiss and charnockite but the ionic concentration is within the limit, since no man made pollution is noticed in this area. Intensive and long term irrigation in the area is probably another factor that causes weathering and leaching of fluoride from soils/weathered rocks. The boron in groundwater occurring naturally, although its distribution varies widely among aquifers, also boron in groundwater also comes from over application agricultural fertiliser, improper manure management practice and storage of animal manure. Groundwater pollution has many sources in common, such as fertiliser, pesticides and animal waste in the rural area. Groundwater or the study area is permissible to unsuitable for domestic and irrigation purpose, except in a few locations where EC is high, the Na%. SAR and RSC. The source of trace elements in the groundwater is related to agricultural practices, included application of fertiliser, pesticides, rural sewage and the geological formation of the rocks due to the weathering of rocks, from which the released trace elements. However, the trace element chromium has been found at higher level than the permissible limits and is probably most harmful to the human beings. The remaining elements are within the limit. The seasonal variation in groundwater quality is due to agriculture and domestic activities through infiltration and percolation during monsoon. The groundwater in this basin is moderate quality suitable for irrigation and domestic purpose. The overall quality of the groundwater in the study area is controlled by agriculture and lithology.
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