Seasonal Variations of Water Quality of Downstream ...

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December 2013, P.P. 111-120

Seasonal Variations of Water Quality of Downstream Catchment of River Mahaweli A.G.P. ARAVINNA1,4, N. PRIYANTHA1,4, H.M.T.G.A. PITAWALA2,4 AND S.K. YATIGAMMANA3,4 1

Department of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka. Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka. 3 Department of Zoology, University of Peradeniya, Peradeniya, Sri Lanka. 4 Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka. Email: [email protected] 2

Abstract: Variations of pH, conductivity and major ions of the downstream region of Mahaveli River were assessed to understand the seasonal variations and the impact of agricultural activities on water quality. Although some variations of the levels of major ions were detected in tributaries, their impact was not apparent on the water quality of irrigation canals as well as cascade reservoirs. A positive gradient of concentration of cations was detected along the downstream of one irrigation canal (Minipe Yoda Ela) which is not lined. Conductivity values also increased along downstream direction of the river having the minimum of 106 µS cm-1 and the maximum of 200 µS cm-1. According to statistical analysis, variation of sodium adsorption ratio of the irrigation and drainage water was insignificant and levels were well belowthe minimum values of FAO guidelines for irrigation water. This indicates that the impact of agriculture and excised weather condition of the area studied does not significantly affect the ion balance of Na+, Ca2+ and Mg2+ in soil solution and water. Keywords: Water Quality, Mahaweli River, Major Cations, Sodium Adsorption Ratio, Irrigation Canals. 1.

Introduction:

Trends of water quality along the course of a river and its tributaries provide an important insight of the hydrogeochemical processes that prevail in a drainage basin. Components of this regime include inputs of dissolved ions from the main river and from weathering of the bedrock within the catchment and also the losses of surface water by evapotranspiration and recharge of groundwater. River Mahaweli (MR), the longest river with the largest river basin of Sri Lanka, covers a total area of 10420 km2 of which the Upper Catchment (UCMR) covers 5680 km2 (above the elevation of 150 m), and the Lower Catchment (LCMR) covers 4740 km2. Tea plantations and vegetable cultivation utilize 25% of the UCMR, while a larger portion of the LCMR covers conventional agricultural fields. Several major and minor tributaries feed the river, and as a result, 8.4 × 109 - 8.8 × 109 m3 of water is discharged to the Indian Ocean annually. Hulu Ganga (TL1), Galmal Oya (TL2), Uma Oya (TR5) and Badulu Oya (TR6) are major tributaries of the LCMR located within the study area, while others are peripheral (Fig.1). Victoria (MRR1), Randenigala (MRR2) and Rantambe (MRR3) are main storage reservoirs of MR constructed during 1980’s mainly for hydroelectric power generation and irrigation.

Downstream of these reservoirs of MR diverted with the Minipe diversion structure (MDS) to 74 km long left bank irrigation canal (LBC) and 86 km long right bank irrigation canal (RBC). LBC feeds relatively small reservoirs (capacity < 2.0 × 106 m3) while RBC feeds large reservoirs (capacity > 2.0 × 106 m3) [1], [2]. The supply area of both LBC (Mahweli System E) and RBC (Mahaweli System C) are 7020 ha and 30760 ha, respectively. Hydrological information of dominant reservoirs of RBC in LCMR are summarised in Table 1. In addition, two major cascade irrigation systems, Hepola Oya system (CS 1) (Fig. 1) and Diyabana Oya system (CS2) are located in the right bank of LCMR. Table 1: Morphometric Characteristics of Reservoirs of LBC, RBC and MR Reservoir

C / m3

CA / km2

A/ ha

Age/ year

Victoria reservoir

7.2 × 108

1891

1300

29

Randenigala reservoir

8.6 × 108

550

2350

28

Rantambe reservoir

1.1 × 107

1095

-

23

Loggaloya reservoir

5.0 × 107

250

-

31

Hepolaoya reservoir

1.7 × 107

70

-

31

#03040403 Copyright © 2013 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.

Seasonal Variations of Water Quality of Downstream Catchment of River Mahaweli

Ulhitiya reservoir

7.6 × 107

Ratkinda reservoir

2.2× 107

280

2020

31

C – Capacity, CA – Catchment area, A – Surface area

A.

Soil and Climate:

The supply area of LBC and RBC belongs to two major climatic regions, Intermediate Zone and Dry Zone of Sri Lanka [3], [4] and is located within the altitudinal range of 30 - 90 m above mean sea level (MSL). The main

112

soil type of the fed area of RBC and LBC is low humic glay soil, while small strip of the fed area of RBC is covered with reddish brown earth soil [3]. However, similar to other river environments, riparian zone of MR is covered with alluvial soils. Major clay mineral of the soils of the study area is kaolinite (40% - 60%), and others are illmonite, montmorillonite and vermiculite [4]. Even though long term mean annual rainfall in the area varies from 1500 mm to 2000 mm [3], recent records indicate elevated levels of annual rainfall exceeding 3000 mm [5].

N Reservoirs MRR1 - Victoria reservoir MRR2 - Randenigala reservoir MRR3 - Rantambe reservoir R4 - Loggaloya reservoir R5 - Hepolaoyareservoir R6 - Ulhitiya reservoir R7 - Ratkinda reservoir Tributaries: TL1 -Hulu Ganga TL2 -Galmal Oya TL3 -Heen Ganga TR1 -Thalathu Oya TR2 -Maha Oya TR3 -Belihul Oya TR4 -KurunduOya TR5 - Uma Oya TR6 -Badulu Oya TR7 -Loggal Oya TR8 -Heplola Oya TR9 -Diyabana Oya TR10 – Ulhitiya Oya Drainage Canal: DR1 –Moogamana Ela Diversion Structures: PO – Polgolla Dam MDS – Minipe Diversion Structure Irrigation Canal: LBC - Left Bank Canal RBC – Right Bank Canal Cascade Irrigation Systems CS1 CS2

Fig 1: Map of the study area of Mahaweli River basin in Sri Lanka showing MR and its reservoirs, tributaries, irrigation canals and their reservoirs, diversion structures and sampling locations. Insert: Location of Mahaweli River basin in the map of Sri Lanka. Usually, rainfall is maximum during the northeast– monsoon period (December – February) and minimum during the southwest-monsoon period

(May – September). The mean annual temperature varies from

International Journal of Biological Sciences and Engineering ISSN 0976-1519, Vol. 04, No. 04, December 2013, pp. 111-120

113

A.G.P. ARAVINNA, N. PRIYANTHA, H.M.T.G.A. PITAWALA AND S.K. YATIGAMMANA

26.5 °C to 28.5 °C [6] and the mean monthly pan evaporation rate varies from 2.6 – 5.4 mm/day, and thus the estimated annual pan evaporation reaches 1519 mm/year [3]. The current investigation was initiated to understand the variation of ionic composition and the impact of major cations and anions on agriculture of downstream of the Polgolla dam (PO) of MR during dry and wet seasons. In addition, other important limnological aspects were also evaluated to understand both point and nonpoint sources, and physical and chemical processes controlling major ionic composition. 2.

Methodology:

A.

Sampling Locations:

Mahaweli River (Polgolla to Manmpitiya), Thirteen tributaries of MR (with single sampling location), major irrigation canals (LBC and RBC), cascade systems (CS1 and CS2) of the right bank and a drainage canal (DR1) were selected for sampling. Thirteen locations along LBC, fourteen locations along RBC, six locations along CS1, seven locations along CS2, three location along DR1 and ten locations along MR were selected for water sampling in the study. All the sampling locations are given in Fig 1. B.

Water Quality Parameters:

Onsite measurements of pH and electrical conductivity (EC) were taken with the help of portable pH (Hanna HI 98101) and conductivity (Hanna HI98303) meters with standard calibrations. Water samples were collected in 500 ml acid washed polypropylene bottles and preserved according to APHA standard procedures [7]. All the samples were transported to the Analytical Research Laboratory of the Department of Chemistry, University of Peradeniya for chemical analysis. Concentration of Na+, K+, Ca2+ and Mg2+ were determined using Thermo M series flame atomic absorption spectrophotometer. SO42- and Cl- were determined using turbidimetric and volumetric method, respectively. All laboratory analysis were done according to the APHA (20th edition) and ASTM (D512-04 and D516-07) standard methods [7]-[9].

The water quality (WQ) measurements of MR (downstream of MDS), LBC and RBC during dry (Yala, August 2012) and wet (Maha, December 2012) seasons help understand the variation of ionic composition and related variables during cultivation and non cultivation periods (Table 3 and Table 4). The mean values of measured environmental data in study sites show the concentration variations throughout the monitoring period (Table 5 and Table 6). B. Water Quality Reservoirs:

Impact

of

Tributaries

on

According to the results of water quality measurements of tributaries of MR (downstream of Polgolla), relatively high levels of cations and anions were recorded in Thalathu Oya (TR1) as compared to that of the other tributaries of MR studied. The situation could be attributed to human activities of the catchment of TR1. However, the concentrations of both anions and cations in TR1, TR2 and TR5 (upstream tributaries of MDS) are slightly higher than those levels in downstream tributaries of MDS. The annual precipitation in the Victoria reservoir (MRR1) region is relatively low when compared to the total catchment of MR. Therefore, the concentration changes of ionic constituents of tributaries connected to MRR1 do not appear to have any influence on the ionic composition of MRR1 water. The study encountered that the ionic concentration of TR5 (Uma Oya) was slightly higher than that of MRR2 (Randenigala reservoir). However, as the larger portion of the inflow of MRR3 consists of the outflow of MRR2, the water from Uma Oya does not appear to affect the ionic composition of MRR3 water. Thus, the ionic composition of the major reservoirs of the Mahaweli cascade system is apparently similar to each other although the system receives water from deferent tributaries with variable water quality characteristics (Table 6 and Fig 2). In addition, as the reservoirs contain a large dead storage with the total capacity of 7.2×108 m3 in MRR1 and 8.6×108 m3 in MRR2, [10],[11] they function as buffer tanks to absorb seasonal variations of water quality in associated tributaries.

Initial sampling was carried out during Yala agricultural season (August 2012). Second and third samplings were carried during Maha agricultural season (in December 2012 and in March 2013). Maximum agricultural practices were observed during March 2013.

Changes of measured cations (K+, Mg2+, Na+ and Ca2+) and anions (SO42- and Cl-) of tributaries of the right bank region of MSD are relatively higher than that of MDS (Table 5 and 6 and Fig 2). But these changes do not appear to affect the average values of measured water quality parameters of RBC of MDS during both Yala and Maha seasons. This situation could be attributed to high water discharge (64 m3 s-1) from MDS to RBC

3.

Results and Discussion:

C. Impact of Irrigation Canal on Cascade Systems:

A.

Water Quality Data:

Measured WQ values of CS1 and CS2 decrease when water of the systems mix with RBC water with the

C.

Monitoring Period:



exception of pH (Fig. 3). A notable change is that electrical conductivity of the CS1 and CS2 reduced approximately to half of the initial measurements (257 to 135 µS cm-1 for CS1 and 280 – 122 µS cm-1 for CS2).

114

values of in CS1, CS2 and DR1 were always higher than those of the main river and its cascade reservoirs in the same period. As the MR flows through many of the climatic regions which experience rainfall at different periods of the year, it is apparent that the river experiences year around rainfall although minor changes of water level exist during a very short period. This situation helps MR to maintain unvarying water quality conditions, especially at the

D. Impact of Drainage Water on Latter Part of MR: Electrical conductivity (EC) along the latter part of MR (from MDS to Manampitiya) varies from 100 to 173 µS cm-1 during the rainy period. However, EC

Table 3: Statistics of pH, EC, Major Cations and Anions in Water of LBC, RBC and MR during the Yala (August 2012) and Maha (December 2012) Agricultural Seasons LBC Parameter*

Yala season ( L=13)

RBC Maha season (L =13)

Yala season (L =10)

MR Maha season (L =14)

Yala season (L=5)

Maha season (L = 5)

Mean

Range

Mean

Range

Mean

Range

Mean

Range

Mean

Range

Mean

Range

EC/µS cm-1

122.7

103.3190.0

125.9

72.2 212.4

122.0

120.0 140.0

122.2

66.1 138.6

165.6

106.7 200.0

163.0

155.8 168.3

pH

7.4

7.1-7.5

7.6

7.3 -7.7

7.1

7.1 -7.6

7.6

7.5 -7.8

7.9

7.1 -7.9

7.6

7.5 -7.7

SO42-

3.3

2.6 3.8

1.3

1.2 -2.3

3.4

3.1 4.0

1.4

1.2 -1.9

3.2

3.8 4.8

3.3

3.1-3.5

Cl-

5.7

4.0 8.2

-

BMDL – 6.3

5.9

BMDL - 6.6

-

BMDL – 6.3

6.9

4.0 9.9

-

BMDL - 5.2

Na

6.4

4.6 8.3

5.5

3.2 11.6

6.7

6.2 7.2

4.3

4.3 - 6.0

10.0

9.3 11.0

6.4

4.7 -7.8

K+

2.0

1.8 2.1

1.6

0.8 2.0

2.0

1.8 2.6

2.3

2.0 - 2.5

2.0

2.0 2.1

2.0

1.9 -2.0

Ca2+

13.9

8.3 23.9

14.7

9.0 26.5

11.3

10.6 13.2

14.1

11.9 20.3

20.4

13.2 24.8

16.2

12.7 18.4

Mg2+

6.7

5.2 9.7

5.6

4.3 8.3

6.6

6.2 7.2

6.2

5.5 -7.2

8.8

6.0 10.3

6.3

5.5 -7.0

*

-1

-Concentration of elements is in mg L , BMDL – below minimum detection limits, L – number of sampling location along the water way (from MSD)

Table 4: Statistics of pH, EC, Major Cations and Anions in Water of RBC, CS1, CS2, DR1 and MR during Maha (March 2013) Agricultural Season RBC (L=14)

CS1 ( L =6)

CS2( L =7)

DR1( L =6)

MR( L =6)

Parameter* Mean

Range

Mean

Range

Mean

Range

Mean

Range

Mean

Range

EC/µS cm-1

140.7

100.4 173.0

179.6

135.2 257.4

213.5

122.0 – 301.0

162.9

105.4 221.9

140.7

100.4 173.0

pH

7.9

7.9 - 8.0

8.0

7.9 - 8.0

7.8

7.6 -8.0

8.1

8.0 -8.1

8.0

7.9 -8.1

SO42-

2.8

2.0 - 3.5

3.4

3.1 - 3.7

2.6

2.3 - 3.2

3.0

2.0 - 4.8

3.4

2.5 - 4.1

Cl-

7.1

4.7 - 9.4

-

BMDL 6.3

6.6

4.7 - 9.4

6.3

4.7 -7.8

10.5

6.3 - 15.7

4.6

3.9 - 5.6

5.8

3.8 - 9.2

6.4

5.7- 6.8

8.2

4.4 -11.5

4.1

3.9 - 4.2

0.8

0.7 - 0.9

0.6

0.5 - 0.8

0.5

0.4 - 0.7

0.7

0.4 - 0.7

0.9

0.9 - 1.0

9.6

7.4 - 12.4

23.2

16.5 - 30.6

11.5

9.2 - 15.0

15.3

7.4 - 19.4

11.3

7.7 - 13.1

6.4

4.8 - 8.4

8.9

7.7 - 11.4

8.1

6.6 - 8.9

8.3

4.8 - 10.6

6.9

4.8 - 8.0

Na

+

+

K

Ca2+ 2+

Mg *

-1

-Concentration of elements is in mg L , BMDL – below minimum detection limits, L – number of sampling location along the water way (upstream to downstream)


115


Table 5: Mean Values of pH, EC, Major Cations and Anions in Water of the Tributaries of MR from August 2012 to March 2013

*

Parameter*

TL1

TL2

TR1

TR2

TR3

TR4

TR5

TR6

TR7

TR8

TR9

TR10

TL3

EC/µS cm-1

90.5 ± 11.8

85.2 ± 7.4

121.0 ± 10.1

125.1± 36.2

85.0 ± 7.1

108.3 ± 6.7

140.4 ± 71.6

266.8 ± 41.3

184.5 ± 36.1

245.5 ± 74.2

225.0 ± 52.7

246.6 ± 89.7

102.4 ± 25.0

pH

7.6 ± 0.1

7.6 ± 0.1

7.6 ± 0.1

7.6 ± 0.1

7.2 ± 0.7

7.5 ± 0.1

7.5 ± 0.3

7.6 ± 0.3

7.6 ± 0.3

7.5 ± 0.3

7.7 ± 0.1

7.5 ± 0.1

7.5 ± 0.0

SO42-

3.2 ± 0.4

3.3 ± 0.5

5.6 ± 2.1

3.6 ± 0.5

3.2 ± 0.2

2.8 ± 0.1

4.6 ± 1.2

3.5 ± 1.2

4.6 ± 1.1

3.7 ± 0.9

3.7 ± 0.7

3.4 ± 0.6

2.9 ± 0.7

Cl- #

BMDL

BMDL -5.2

12.8 ± 0.4

BMDL -7.8

BMDL -6.3

BMDL -5.7

5.8 ± 1.1

6.2 ± 2.4

BMDL - 6.6

6.8 ± 1.7

6.4 ± 1.3

6.0 ± 0.8

BMDL - 6.6

Na+

3.0 ± 0.7

2.5 ± 0.1

10.2 ± 2.6

4.5 ± 0.3

3.9 ± 0.4

3.0 ± 0.6

5.0 ± 1.2

5.1 ± 1.0

4.9 ± 1.8

8.4 ± 3.2

7.9 ± 1.5

8.7 ± 1.8

5.5 ± 3.1

K+

0.7 ± 0.2

0.5 ± 0.3

1.6 ± 0.8

0.8 ± 0.4

1.5 ± 0.6

1.2 ± 0.6

1.8 ± 0.8

1.8 ± 0.8

1.3 ± 0.6

1.4 ± 0.5

1.2 ± 0.3

2.4 ± 0.9

1.5 ± 0.8

Ca2+

4.1 ± 1.8

5.8 ± 2.9

34.1 ± 19.8

20.6 ± 4.7

11.3 ± 4.6

10.5 ± 2.4

13.7 ± 1.0

20.3 ± 13.1

13.7 ± 1.1

30.1 ± 8.9

23.8 ± 5.8

21.1 ± 1.0

16.7 ± 8.6

Mg2+

2.9 ± 1.8

3.1 ± 1.5

16.0 ± 8.4

9.8 ± 1.7

9.9 ± 0.8

4.5 ± 0.7

6.7 ± 0.8

6.9 ± 4.5

7.4 ± 1.9

11.5 ± 0.5

7.2 ± 0.7

8.6 ± 1.6

6.8 ± 3.3

-Concentration of elements is in mg L-1.# - range is reported when mean calculation is not possible.BMDL – below minimum detection limits

Table 6: Mean Values of pH, EC, Major Cations and Anions in Water of Cascade Reservoirs of MR from August 2012 to March 2013 MR0

MRR1

MRR2

MRR3

MDS

EC/µS cm-1

73.3 ± 7.6

112.3 ± 10.7

109.0 ± 8.5

106.7 ± 4.6

109.8 ± 9.0

pH

7.4 ± 0.1

7.4 ± 0.2

7.4 ± 0.1

7.4 ± 0.1

7.5 ± 0.1

SO42-

3.1 ± 0.2

3.1 ± 0.1

3.0 ± 0.1

3.1 ± 0.1

3.1 ± 0.4

Cl-

BMDL - 5.4

BMDL -5.2

BMDL -5.4

BMDL - 5.1

BMDL -5.5

Na+

3.4 ± 0.6

3.5 ± 1.0

3.5 ± 1.0

3.6 ± 0.6

4.1 ± 0.5

K+

0.9 ± 0.8

1.8 ± 0.4

1.7 ± 0.5

1.8 ± 0.5

1.8 ± 0.5

Ca2+

6.5 ± 0.2

9.7 ± 2.8

9.2 ± 1.6

9.5 ± 1.5

11.3 ± 2.0

Mg2+

2.9 ± 0.5

3.7 ± 0.3

4.6 ± 0.3

4.3 ± 0.5

5.5 ± 1.0

-1 #

-Concentration of elements is in mg L . - range is reported when mean calculation is not possible.BMDL – below minimum detection limits

(a)

14

Maximum

Mean

Minimum

12 10 8 Na+ / (mg L-1)

*

Parameter*

6 4 2 0

Water Body



(b)

116

30 25

Maximum

Mean

Minimum

Mg2+/ (mg L-1)

20 15 10 5 0

Water Body

(c)

60

Maximum

Mean

Minimum

Ca2+ / (mg L-1)

50 40 30 20 10 0

Water Body

Fig 2: Variations of levels of (a) Na+, (b) Mg2+and (c) Ca2+ in cascade reservoirs, tributaries and main canals of LCMR CS1 CS2 DR1

(a) 350 300 250

EC / (mS)

200 150 100 50 0

terminal regions (Fig 6). However, the impact of agricultural Drainage of LB and RB on MR (downstream of MDS) also appears to be minimum in the wet season when compared to the dry season. Yet, this study recorded that the maximum variation of EC of MR is 100 - 200 µS cm-1 in both dry and rainy periods, and further values are well below the minimum levels for irrigation water stipulated by FAO (Table 8).

 E. Possible Evaporation Impact:  The positive gradient of concentration of cations was observed long the downstream of LBC than RBC. This is the maximum in the latter region(50 km to 74 (b) 10 CS1 km)of LBC during the Maha (December 2012) season 9.5 CS2 (Fig. 4).These increasing gradients were estimated to 9 DR1 be 0.5 (R2 = 0.960), 0.1 (R2 = 0.769), 1.1 (R2 = 0.966) 8.5 and 0.3 (R2 = 0.958) in mg L-1 km-1, for Na+, K+, Ca2+ 8 and Mg2+, respectively (Fig. 4). However, slight 7.5 positive gradients of the concentration of cations were 7 observed in RBC as compared to those of LBC. It is 6.5 noted that the capacity of LBC is smaller than RBC, 6 and the latter region of LBC is relatively narrow and 0 5 10 15 20 25 shallow with dense mat of rooted macrophytes along Distance (upstream to downstream )/km the riparian zone. Such a condition could promote low velocity of water which facilitates high Fig 3: Variation of (a) pH and (b) conductivity along evapotranspiration, especially in an area with high pan the cascades systems and drainage canal studied evaporation rate (2.6 – 5.4 mm/day). 5 10 15 20 Distance (upstream to downstream )/km

25

pH

0


117


30

Concetration / (mg L-1)

25

Na

Mg

Ca

K

20 15 10 5 0 -100

-80

-60

-40

-20

0

20

40

60

80

Distance from MDS /km

LBC

100 RBC

Fig 4: Variation of Major cations along the left bank canal (LBC) and right bank canal (RBC) in Maha season F.

Impact of Na+on Soil and Agriculture:

Level of Na+ of the study area varies from 2.5 – 11.6 mg L-1. At the presence of high levels of sodium ion relative to Ca2+ and Mg2+ in the soil solution at high pH, clay minerals in soils tend to swell and disperse, and aggregates tend to slake [12]-[14]. [14]. Dispersed soil aggregate particles reduced the soil permeability which is essential for plant growth. In addition, soil surface becomes omes more crusted and compacted under dry conditionswhen Na+ present at high concentrations (Fig 5).

b) Dispersion of soil particles and breakdown into soil aggregates c) When soil is dried aggregate partials of soil becomes compact decreasing soil permeability

Fig 5: Illustration of decreasing soil permeability when increasing Na+ level in soil solution. Exchangeable sodium percentage (ESP) [15] and sodium adsorption ratio (SAR) [16] are closely related and common two indices that can be utilised to estimate sodium hazards of the soil solution or water [12]. ESP = (Exchangeable Na/CEC) × 100%

(1)

SAR= Na+/[(Ca2+ +Mg2+)/2]1/2

(2)

where CEC is cation exchange capacity, in meq dm-3 and Na+, Ca2+ and Mg2+ are concentrations of sodium, calcium, and magnesium, respectively respect in meq dm-3

a

b

c

Any increment of SAR of irrigation water causes increase in exchangeable sodium in soil solution [14]. [ Variation of SAR of the water bodies studied is 0.3 0.7 for both Yala and Maha seasons (Table 8a and 8b) and results indicate that the variation of SAR among the irrigation water and the drainage water is not insignificant. When CO32- or HCO3- is increase in soil solution or irrigation water, available Ca2+ and Mg2+ precipitate as carbonates causing an increase in SAR. For, such occasions, ions, adjusted SAR (adj. SAR) expressed in Equation 3 could be calculated [16], [17] to obtain correct value for SAR. SAR Adj. SAR = SAR [1 + (8.4 - pHc)] 2+

(3)

where pHc = (pk2 - pkc) + p(Ca + Mg ) + p(CO32- + HCO3-): pk2 and pkc are negative logarithms of the second dissociation constant of carbonic acid and the solubility constant of calcite, respectively. Ca2+, Mg2+, a) Na+ replaces Mg2+/Ca2+ adsorbed to the soil clays

International Journal of Biological Sciences and Engineering ISSN 0976--1519, Vol. 04, No. 04, December 2013, pp. 111-120

2+

A.G.P. ARAVINNA, N. PRIYANTHA, H.M.T.G.A. PITAWALA AND S.K. YATIGAMMANA 117

CO32- and HCO3- are concentrations of calcium and magnesium, carbonate and bicarbonate respectively in meq dm-3

Hulu Ganga

0.3 ± 0.1

GalmalOya

0.3 ± 0.1

ThalathuOya

0.3 ± 0.1

MahaOya

0.2 ± 0.0

BelihulOya

0.3 ± 0.0

KurunduOya

0.2 ± 0.1

Uma Oya

0.3 ± 0.0

200

BaduluOya

0.3 ± 0.2

0

LoggalOya

0.3 ± 0.1

HeplolaOya

0.7 ± 0.5

DiyabanaOya

0.5 ± 0.2

UlhitiyaOya

0.5 ± 0.1

Heen Ganga

0.4 ± 0.2

In the study area, the pH of river water varies from 7.1 to 8.1. This could be an evidence of low dissolved CO32in water. 1000 Aluthnuwara

800

Ulhitiya

600

Dec

Oct

Nov

Sep

Aug

July

June

May

Mar

April

Jan

400

Feb

Rainfall/mm

118

Month

Fig 6: Variation of monthly rain fall in two locations close to study area in Year 2011 (source: Meteorological department of Sri Lanka) It was noted that the SAR values of study sites are well below the minimum levels of FAO guideline for irrigation water (Table 8).

Table 8: Some FAO Guideline for Irrigation Water

Water Body

SAR Yala (August 2012)

Maha Maha (December 2013) (March 2013)

Degree of Restriction on Use

Potential Irrigation Problem

Indicator Parameter

Salinity (affects crop water availability)

Table 7(A): Variation of SAR of MR, Canals, Cascade Systems and Drainage Canal in Agricultural Seasons

Specific Ion Toxicity (affects sensitive crops)

None

Slight to Moderate

Severe

ECW / (dS m-1)

< 0.7

0.7 - 3.0

> 3.0

SAR*

9

Chloride * (mmol L-1 )

10

LBC

0.4 ± 0.1

0.3 ± 0.1

*

RBC

0.4 ± 0.1

0.3 ± 0.1

0.3 ± 0.1

CS1

*

*

0.3 ± 0.1

*- for surface water

CS2

*

*

0.4 ± 0.1

G.

DR1

*

*

0.4 ± 0.1

MR

0.4 ± 0.0

0.4 ± 0.0

0.2 ± 0.0

* Cation levels were not determined for SAR Table 7(B): Variation of SAR of Tributaries and Cascade Reservoir of MR (From August 2012 to March 3013) Water Body

SAR

Victoria

0.3 ± 0.1

Randenigala

0.3 ± 0.1

Rantambe

0.3 ± 0.1

Minipe

0.3 ± 0.1

Abundance of Major Ions:

The sequence of cations along LBC, RBC and MR is K+< Mg2+ ≈ Na+< Ca2+ in both seasons. However major soil types of catchment of MR and associated tributaries within the study area consist of red yellow podzolic, alluvial, reddish brown earth, immature brown loam and Radish brown latosolic [3], [18]. These soils contain Ca2+ and Mg2+ as dominant cations [18] while reddish brown latosolic soil has higher K+ level. Unlike the terrestrial environment, Na+ dominates in the surface water, especially in MR and associated systems, which is common to all head water streams of Sri Lanka [19]. A majority of the larger rivers draining tropical watersheds has high levels of Na+ and Cl- (Compared to Ca2+ and HCO3-) and the principal cause identified as the sea breeze [20]. Sri Lanka with the range of Na+ is 0.002 - 0.155 mg L-1 and the range of is Cl- 0.02 to 0.97 mg L-1 [19], [21]. Since Sri Lanka receives year around rain mainly from two monsoons, south-west and


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north-east, situation can be easily justified. In addition, considerable fraction of Na+ is generated through chemical weathering process of bedrock. Therefore, it is certain that all these natural processors and anthropogenic activities within the area are responsible for the cation sequence encountered in our study. Level of Cl- of the study area varies from, below MDL (4 mg L-1) to 12.8 mg L-1, and maximum level is indicated in TR1. However, the maximum detected Cl- level is well below the minimum guideline for irrigation water stipulated by FAO (4 mmol L-1 = 145 mg L-1) which is a good indication of the better health of irrigation water. Variation of SO42- of the water of the study area is 1.2 – 5.4 mg L-1and maximum detected SO42-level is well below the guideline value stipulated by FAO (200 mg L1 ). Therefore, this study helps to assure the better health of MR water for irrigation and other human consumption. 4.

Conclusions:

Levels of pH, electrical conductivity, chloride, sulphate and SAR in cascade reservoirs of River Mahaweli, and associated systems are well below the minimum gridlines for irrigation water stipulated by FAO for the period studied.Variation of SAR among the irrigation water and drainage is insignificant. Therefore, exchangeable sodium level of the soils of the left bank and the right bank of Mahaweli River has not reached to harmful levels due to the agricultural practices of last three decades. 5.

Acknowledgment:

Quality and Innovation Grants (QIG) of Higher Education for Twenty First Century (HETC) - Window 3funded by the World Bank is highly appreciated. 6.

References:

[1] E.S.P. Amarasingam, “Mahaweli - A taunting Challlenge ”, Tribune Ceylone News Review, vol. 27(46), pp.55-56, 1983. [2] The Master plan (2012) Home page of Mahaweli Authority of Sri Lanka [Online]. Available: http://www.mahaweli.gov.lk/The Master plan. [3] R.B Mapa, A. R. Dassanayake and H. B. Nayakekorale, Ed., Soils ofthe Intermediate Zone of Sri Lanka: Morpology,Charactarization and Classification,Soils Science society of Sri Lanka, 2005. [4] R.B Mapa, S. Somasiri, and A. R. Dassanayake, Ed., Soils of the Dry Zone of Sri Lanka: Morphology, Characterizations and Classification, Soils Science society of Sri Lanka , 2010. [5] Rainfall Data Sheet, Aluthnuwara and Ulhitiya stations, year 2011, Meteorological department, 383, Bauddhaloka Mawatha, Colombo 07, Sri Lanka.

[6] Climate in Sri Lanka, (2013) Climate page,[Online]. Available: http://www.meteo.gov.lk/Climate/ Climate in Sri Lanka [7] Standard Methods for Estimation of Water and Waste Water APHA, American Public Health Association, Washington DC, 17th Ed., 1989. [8] Test Method for Sulfate Ion in Water ((D516), Standard Methods of American Society for Testing and Materials (ASTM), 2007. [9] Test Methods for Chloride Ion In Water (D512:Method B), Standard Methods of American Society for Testing and Materials (ASTM), 2004. [10] Randenigala (2012) Homepage of Mahaweli Hyadro Power Complex, Ceylon Electricity Board [Online]. Available http://www.mahawelicomplex.lk/randdam. [11] Victoria (2012) Homepage of Mahaweli Hydro Power Complex, Ceylon Electricity Board [Online]. Avilablehttp://www.mahawelicomplex.lk/vicdam. htm [12] J. D. Rhoades, A. Kandiah and A. M. Mashali, The use of saline waters for crop production: Iirrigation and drainage paper 48, Food and Agriculture Organization of the United Nations, 1992. [13] C. W. Robbins, “Sodium Adsorption Ratio Exchangeable Sodium Percentage Relationships in a High Potassium Saline - Sodic Soil”, Irrigation Science, vol 5, 173-179, 1984. [14] H. D. Gunawardhana, “Water is our life – Chemistry for sustainable consumption of water”, Chemistry in Sri Lanka, Vol. 28 (1), pp. 49 -54, January 2011. [15] M. J. Flshman and L. C. Friedman, Ed.,Study and interpretation of the chemical characteristics of natural water: U.S. Geological Survey WaterSupply Paper, 3rd ed., 1989. [16] I.P. Abrol, J.S.P. Yadav and F.I. Massoud, SaltAffected Soils and their Management, Soil Bulleting 39, Food and Agriculture Organization of the United Nations, 1988. [17] R.S. Ayers and D.W. Westcot, Water quality for agriculture:Irrigation and Drainage Paper 29,Food and Agriculture Organization of the United Nations, Rev. 1,1985. [18] R.B Mapa, S. Somasiri, and S. Nagarajh Ed., Soils of the Wet Zone of Sri Lanka: Morpology, Charactarization and Classification,Soils Science society of Sri Lanka , 1999. [19] E.I.L. Silva and L. Manuweera, “Surface and rainwater Chemistry in Sri Lanka - A Risk of Acidification”, Asian journal of Water, Environment and Pollution, vol. 1, No. 1 & 2, pp. 79 – 86, 4004.



[20] Gibbs, R.J. (1970). “Mechanism controlling world water chemistry”, Science, Vol 170, pp. 1088-1090, 1970. [21] S.B. Jonnalagadda, J. Makadho, N. Matinde, R.P. Karimanzira and A. Makarau, “Chemical composition

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of rainwater and air quality in Zimbabwe, Africa”, The Science of the total Environment, Vol. 144 pp. 261 – 271, 1994.
