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D. VASU*, S.K. SINGH, P. TIWARY, P.S. BUTTE AND V.P. DURAISAMIa
ICAR-National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur-440 033
a Directorate of Natural Resource Managment, TNAU, Coimbatore – 641 003.
Georeferenced groundwater samples collected from Thimmajipet mandal, Mahabubnagar district were analyzed and
evaluated for their suitability for irrigation. Out of 49 samples, 12 per cent samples recorded pH > 8.5 and electrical conductivity
varied from 500 to 5130 µS cm-1. All the samples have high Mg2+ content and 92 and 94 per cent samples have high Ca2+ and
Na+, respectively. Among the total groundwater samples 59 per cent have high sodium hazard, 80 per cent with high magnesium
hazard, 7 per cent with high Mg/Ca ratio and 100 per cent with very high permeability index. Water samples were classified as
C3S1 (29%), C3S2 (39%), C2S2 (2%) and C2S1 (4%). In general, the groundwater in Thimmajipet mandal is unsafe for longterm
irrigation use.
KEYWORDS: Groundwater quality, Irrigation suitability, Permeability index, Sodium hazard
Groundwater is one of the prime sources for irrigation especially in arid and semi-arid regions of India. Tube wells are major source of irrigation and their share has increased from 1 per cent in 1960–61 to 37 per cent in 1999–2000 (MoRD, 2006). Telangana, located in the southern Deccan plateau region receives very less mean annual rainfall (MAR) with uneven distribution and the agriculture sector registered a mixed growth rate varying from 25.2 to -10.0 per cent during the period 2005-06 to 2013-14 due to frequent droughts. The cropping intensity has increased from 109 per cent in 1993-94 to 127 percent in 2013-14 with a major shift in irrigation pattern making it more costly for the farmers, highly uncertain and unsustainable (Rao, 2014). The vagaries of climate change have put the seasonal agricultural system and agriculture dependent economy of Telangana in jeopardy.
The occurrence and behavior of groundwater is an
outcome of combined interplay of hydrological,
geological, structural and climatological factors. The
composition of groundwater is determined by their source
and the type of strata over which they flow (Rengaraj,
1996) and the kinds of salts found in them depends on
the laws of dissolution. The suitability of water for
irrigation is influenced by many factors such as quality
of water, soil type, salt tolerance characteristics of the
plants, climate and drainage characteristics of the soil
(Michael, 1990). The potential evapo-transpiration largely
exceeds precipitation in the semi-arid regions, thereby
increasing the probability of soil salinity or sodicity
development if the groundwater used for irrigation is poor
in quality. Telangana is now facing both depleting
groundwater table and deficit rains and allocation of
natural resources like groundwater to different sectors viz.,
agriculture, industry and drinking water use has become
a challenging task for policy makers. The steady decline
in groundwater level warrants immediate attention on both
quantity and quality of groundwater. The groundwater
quality is deteriorating particularly with high salt content
and limits its use for agriculture in Mahabubnagar district
(CWGB, 2013). In the context of above scenario, several
studies need to be conducted to assess the potential threat
of poor quality groundwater. The present study is one
such type, which attempts to evaluate the groundwater
for its quality and suitability for irrigation in Thimmajipet
mandal, Mahbubnagar district.
Description of the Study Area Thimmajipet mandal is part of Mahabubnagar district, located between 16°55′ N latitude and 78°20′ Elongitude with on elevation of 481 m above mean sea level in southern Telangana plateau, (Agro Ecological Sub Region 7.2).The total geographical area is 200.90 km2 and covered by granite, peninsular gneiss, basalt, and mixed granite and basaltic rocks. The MAR varies from 450-550 mm. Gross cropped and irrigated areas are 14,020 and 1,960 ha respectively. Major crops grown during kharif are cotton (Gossypium hirsutum), maize (Zea mays) followed by paddy (Oryza sativa), red gram (Cajanuscajan) and castor (Ricinus communis). In Rabi groundnut
(Arachis hypogaea) is the major crop followed by paddy
and maize. The length of growing period (LGP) is 90-
120 days. Tube wells are the major sources of water
irrigating more than 90 per cent of the irrigated area.
Sampling and Analysis
Forty nine geo-referenced groundwater samples were
collected from 19 villages of the Thimmajipet mandal
(Fig.I) during February 2015 from tube wells in stopper
fitted plastic bottles and stored at a temperature below
4°C prior to analysis in the laboratory. The samples were
analyzed for pH, electrical conductivity, cations (Ca2+,
Mg2+, Na+ and K+) and anions (Cl-
, CO3
2-, HCO3
–
, SO4
2-
and BO3
3-) using standard procedures (APHA, 2005).
Important indicators of water quality for irrigation were
derived using the guidelines given by Ayers and Westcot
(1985). Parameters such as pH, electrical conductivity
(EC), soluble sodium percentage (SSP), residual sodium
carbonate (RSC), potential salinity (PS), magnesium
hazard (MH) and permeability index (PI) were used to
assess the suitability of water for irrigation purposes.
SSP was calculated by the following equation (Todd,
1995).
2 2 100 Na
Ca M
SS
g
P
Na K
+
+ + ++ +
×
+ + =
RSC was calculated according to Gupta and Gupta
(1987):
RSC = (CO3
2- + HCO3
–
) – (Ca2+ +Mg2+)
PI was calculated according to Doneen (1964)
employing the following equation:
3
2 2 100 Na HCO
Na Ca M
P
g
I
+ −
+ ++ +
×
+
+ =
MH was calculated using the following equation
(Raghunath, 1987):
2
2 2 100 Mg
Ca
MH
Mg
+
+ + ×
+ =
PS was calculated based on the formula (Hammet,
1992) as:
PS =
1
2 SO4
2- + ClRESULTS
AND DISCUSSION
Hydrochemistry
pH ranged between 6.91 and 8.91. Among the 49
samples studied, 12% showed pH > 8.5 and may be
harmful for plant growth by (Table 1) inducing alkalinity
in soils. Electrical conductivity (EC) varied from 500 to
5130 µS cm-1 with a mean value 1897 µS cm-1 (Table 1)
and 20 per cent of water samples have EC more than
2500µS cm-1 which is unsafe for irrigation and may induce
strong salinity in irrigated soils (Ayers and Westcot,
1985).Total dissolved solids (TDS) varied from 320 to
3283 mg L-1 with a mean value of 1214 mg L-1. According
to FAO standards (Table 2), irrigation water with more
than TDS > 1500 mg L-1 are classified with severe
limitation for its use and 20 per cent of the water samples
are unsafe for irrigation.
Ionic concentration
The concentration of cations and anions in
groundwater depends on the geology, weathering of rocks
and soil and water management practices. Calcium (Ca2+)
contributes to the hardness of the water and the sources
of Ca2+ in ground water especially in sedimentary rocks
such as calcite, aragonite, gypsum and anhydride. In the
present study, Ca2+ varied from 11 to 114 me L-1 and Mg2+
from 14 to 129 me L-1. The usual range of Ca2+ and Mg2+
in irrigation water is 0-20 and 0-5 me L-1, respectively
and 92 per cent of water samples have Ca2+ more than 20
me L-1 and all the samples have high Mg2+. The high
concentration of Ca2+ and Mg2+ results in alkalinity of
water and induces osmotic stress to the plants. K+ ranged
between 0.13 and 1.97 me L-1 while Na+ varied between
35.65 and 117 me L-1 (Table 1). The safe range of Na+ is
0-40 me L-1 and 94 per cent of water samples have high
sodium content. Cl- varied from 5 to 18 me L-1and 39 per
cent samples are with moderate Cl-
toxicity, 53 per cent
with high toxicity and 8 per cent with very high toxicity
according to FAO standards (Ayers and Westcot, 1985).
CO3
2- varied from 0 to 10 me L-1, HCO3
– from 0.3 to 17.5
me L-1, SO4
2- from 0 to 63 me L-1 and BO3
3- from 0.2 to
1.25 me L-1.
Water quality indices
The sodium adsorption ratio (SAR) indicates relative
proportion of sodium to calcium and magnesium, which
influences soil physical properties especially hydraulic
conductivity. Results indicated that 6 per cent of water
samples have high (SAR > 18), 53 per cent with medium
(SAR 10-18) and 41 per cent with low (SAR < 10) sodium hazard. Also, soluble sodium in irrigation water is an important parameter influencing the soil properties and crop performance. High concentration of sodium in irrigation water may stunt the plant growth and reduces soil permeability (Joshi et al. 2009). The soluble sodium percentage (SSP) varied from 19.74 to 71.54 per cent and 17 per cent of water samples are unsafe for irrigation with SSP > 60 per cent.
The concentration of HCO3
–
and CO3
2- influences the
suitability of water for irrigation purpose. The water with
high residual sodium carbonate (RSC) will increase the
pH in soils. Since the calcium and magnesium content
were proportionately high, effect of HCO3
–
and CO3
2-
through RSC index is low as the RSC varied from -89 to
-5 (Table 3). Potential salinity (PS) is an indicator of ability
of water to cause salinity in soils and the suitability of
water for safe use in different textured soils. PS varied
from 7 to 40 me L-1 and the groundwater is unsuitable for
irrigating fine textured soils (PS > 3). However, 71%
samples are suitable for medium textured soils (PS 3 – 15
me L-1) and 84 per cent suitable for sandy soils.
Generally calcium and magnesium maintain a state
of equilibrium in most waters. High level of Mg2+ usually
promotes increase in Na+ concentration in irrigated soils.
Results showed that 80 per cent of water samples are
above the acceptable limit of 50 per cent MH (Ayers and
Westcot, 1985). The Mg/Ca ratio for analyzed water
samples varied from 0.36 to 3.18. Among the groundwater
samples 73 per cent is safe (Mg/Ca < 1.5), 20 per cent is moderately safe (Mg/Ca 1.5-3.0) and 7 per cent is unsafe (Mg/Ca >3.0) for irrigation. According to U.S. Salinity
Laboratory (USSL, 1954) classification, 29 per cent are
grouped in C3S1, 39 per cent in C3S2 and 2 per cent in
C2S2 (Fig. 2) indicating their unsuitability for long term
irrigation with moderate to high salinity and sodium
hazard.CONCLUSION
The groundwater samples of Thimmajipet mandal
were moderate to high in sodium hazard, highly saline
and medium in chloride toxicity. The ground water is
dominated by Na+, Mg2+ and Ca2+ and have the potential
to cause damage to the cultivated crops by inducing soil
salinity and alkalinity. Since most of the water samples
were classified as highly saline, use of groundwater
blended with fresh water for irrigation could be an option
to minimize the potential of soil salinity. Upon long term
use for irrigation the sodium dominated waters may force
the accumulation of sodium in soils and may impede the
hydraulic properties of soils. Hence, leaching methods
need to be improvised to remove salts from irrigated soils.