Sedimentation and internal phosphorus loads in ... - Wiley Online Library

Lakes and Reservoirs: Research and Management 2014 19: 161–173

Sedimentation and internal phosphorus loads in Krishnagiri Reservoir, India Elangovan Arunbabu,* Seetharaman Ravichandran and Paulraj Sreeja Centre for Water Resources, Anna University, Chennai, India

Abstract This study examines sedimentation rate and its consequences on the bathymetry, capacity and internal phosphorus loading of Krishnagiri Reservoir in Tamil Nadu, South India, utilizing an acoustic Doppler profiler and remote sensing data in an ArcGIS environment. There was a significant change in the reservoir bathymetry for the year 2012, compared with the 2007. The sedimentation rate was 0.818 MCM from 1960 to 1990 and 0.83 MCM over past 5 years. The present reservoir volume is 35.57 MCM, having been reduced to nearly half of its original capacity over a 55 year span, pointing to a seriously threatened lifespan. The sediment total phosphorus (TP) load spatially varied from 6.84 to 23 394 kg, depending on the sediment deposition zones. Sequential extraction indicated the dominance of phosphate fractions to be Al-P> FeP> Ca-P> SRP, with an average TP value of 27.27 mg g1 dry weight. Aluminium- (35%) and iron(25%)-bound forms are the major sediment phosphorus fractions, suggesting temperature, pH and redox or related chemical reactions may be important means of sediment P release in Krishnagiri Reservoir. The sediment phosphorus load in Krishnagiri Reservoir is estimated to be 44.50 tons, with an average TP release of 40.97 mg m2 (range of 10.22–70 mg m2). The measured pore water TP concentration and calculated sediment phosphate release exhibited a linear relation. Even with a reduced external P load, the eutrophication of Krishnagiri Reservoir cannot be reduced immediately because of its high internal load and nutrient remobilization.

Key words internal phosphorus load, Krishnagiri Reservoir, phosphate release rate, reservoir capacity, sedimentation.

INTRODUCTION Reservoirs are created to impound water for multiple human uses, including water supply, irrigation, flood control, hydropower and navigation. The World Commission on Dams reported a rapid increase in large dam construction during the 20th century, with about 45 000 dams available in 140 countries (WCD 2000). India ranks among the top five dam-building countries, with about 4500 large reservoirs (i.e. >15 m height), as well as many small to medium reservoirs, which have stabilized irrigation in India, substantially increasing its food security and self-sufficiency. Dams constructed at appropriate locations in the natural course of rivers can create large water storage capacity. Under natural conditions, the river reaches are approximately balanced with sediment inflows and out*Corresponding author. Email: [email protected], [email protected] Accepted for publication 21 July 2014.

Doi: 10.1111/lre.12069

flows. This natural balance can be altered dramatically when a hydraulic structure is raised across a river, eventually decreasing water flow velocities, increasing the water spread area, and increasing deposition of suspended particles carried by the river flow in the reservoir storage zone. This process is unavoidable, resulting in the loss of water storage created, affecting water supply, irrigation, navigation, power generation, fishing and recreational uses. An estimated 1% of dam storage capacity worldwide is lost annually by sedimentation (WCD 2000). Indian reservoirs also face serious sedimentation threats (Garg & Jothiprakash 2008), with large Indian dams silting up at an average annual rate of about 0.5%. It also is predicted that India’s reservoirs, having about 24% of the national storage capacity, may be affected by sedimentation by the year 2020. Another sedimentation consequence is accumulation of nutrients, pollutants and chemicals on surface sediments which can affect reservoir water quality, with

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non-point source pollution from the catchment being a major threat to reservoir water quality.

Reservoir sedimentation Sediment transport from the watershed, especially from agricultural land, into rivers and reservoirs is a major problem throughout the world. Means of calculating sedimentation range from subtracting measured surfaces of the original reservoir bottom profile to direct measurements of sediment volumes. Furthermore, sedimentation and its impacts on the quantity and quality of water vary, with different methodologies applied around the world, including the United States (DeWitt et al. 2007; Furnans & Austin 2008; Ortt et al. 2000; Odhiambo & Boss 2004; Snyder et al. 2004), Sri Lanka (Weerakoon 2005), India (Goel et al. 2002; Jain et al. 2002; Garg & Jothiprakash 2008; Sabeti 2011), Zimbabwe (Sawunyama 2005)t, China (Kummu & Varis 2007), Czech Republic jo et al. 2006; (Krasa et al. 2005) and Brazil (de Arau Alcântara et al. 2010). Krasa et al. (2005) reviewed methods of measuring reservoir sedimentation, providing a simple algorithm for selecting appropriate methods for a given water system. The use of multifrequency echo sounders, GPS and GIS have proved to be an innovative, accurate method by many researchers (Goel et al. 2002; Jain et al. 2002; Odhiambo & Boss 2004), compared with conventional methods requiring more onsite survey and data collection. Computing capacity loss and sediment volumes also require comparison with previously conducted surveys, with such data either not always available or not accurate. Loss of reservoir water storage capacity because of sedimentation can significantly impact water availability. jo et al. 2006) A recent Brazilian study by de Arau assessed the impacts of reservoir siltation on water availability through stochastic modelling and open source data. Alcântara et al. (2010) proposed a rapid, inexpensive method to determine the bathymetry of hydroelectric reservoirs in Brazil, integrating historical and Shuttle Radar Topography Mission (SRTM) data to estimate the bathymetry of Itumbiara Reservoir with high accuracy (R2=0.98). The spatial resolution of Shuttle Radar data, however, may become a limitation for small- and medium-size reservoirs and reduce the accuracy of volume estimates. Systematic sedimentation surveys of reservoirs in India only commenced in 1958, with a Central Board of Irrigation and Power survey of 28 reservoirs (Kothyari 1996). The most common technique (hydrographic survey) during this early period was direct in situ measurements of reservoir bed profile. Many reservoirs were


later surveyed with an echo sounder along predetermined range lines, being both time-consuming and labour-intensive. Several researchers recently attempted to map reservoir bathymetry using remote sensing data (Goel et al. 2002; Jain et al. 2002; Alcântara et al. 2010), with temporal satellite imageries used to determine the water spread area at different water levels. The reservoir capacity between two consecutive water levels was computed using a prismoidal equation. While the volume estimates of these studies may be reasonable for large reservoirs, they cannot totally replace presently ground-based observation methods. Acoustic Doppler current profiler (ADCP) is likely to produce more accurate data, bathymetry profiles and better volume estimates than any currently available method.

Sediment quality In addition to reduced water capacity, sediment inflows can cause water quality deterioration. Many studies emphasize the role of sediments in affecting the water quality for beneficial uses and the dangers posed by eutrophication. Although many previous studies address the natural and total phosphorus load contained in reservoir sediments, and the reactions/mechanisms at the sediment–water interface that control phosphorus release and internal loading in reservoirs (Carter & Dzialowskiv 2012), different approaches have been utilized under different field conditions in addressing these issues. One approach is the sequential extraction of different phosphorus chemical forms in sediments to assess potential lake eutrophication problems. The distribution and mobility of total and other forms of phosphorus appears to be important in different lakes (Fytianos & Kotzakioti 2005). The forms and abundance of specific chemical forms appear to depend on such factors as catchment geology, grain size and sediment mineral content (Kapanen 2008), as well as their enrichment by organic matter (Beidou et al. 2011).

Sediment–water interface Pore water release and phosphorus dynamics are important factors in assessing phosphate release from sediments, using microcosm studies (Steinman & Ogdahl 2010) and field investigations. Pore water total phosphorus is an important indicator of its release from sediments, which may be site specific (Thornton et al. 2013) and must be monitored to calculate reasonable load estimates. Phosphate turnover and circulation may be influenced by hydrological dynamics, perhaps becoming the driving force, as reported for some Polish reservoirs

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(Trojanowska & Jezierski 2011). Seasonal and spatial phosphorus concentration variations in pore waters are significant, with their impacts on internal loading having been reported for another Polish lake, Swarz Edzkie (Kowalczewska-Madura & Goldyn 2011) and for Baiyangdian Lake in North China (Dong et al. 2011).

Internal phosphorus loading Nurnberg (1994) made the first attempt to classify lakes based on their anoxic sediment surfaces especially prone to high internal phosphorus loading. She identified the morphometric, hydrological and geochemical characteristics of several lakes, using them to calculate the internal phosphorus load as the function of the area of the lake anoxic/oxic zones, to estimate phosphorus release rates. Phosphorus concentrations generally increase during summer (Pettersson 1998; Graneli, 1999), when external additions are minimal, and possible effects such as fish bio-manipulation and sediment bacteria processes (Jeppesen et al. 2003) may influence annual internal phosphorus loads. Nevertheless, laboratory experiments do not explicitly consider oxic/anoxic conditions, instead indirectly assessing total phosphorus releases and loads. Being more relevant, and providing better phosphorus estimates, lake and reservoir field conditions (Nurnberg 1994) were the focus of the present study.

Scope of present study Information on sedimentation or sediment phosphorus loads in subtropical reservoirs is limited. Studies of catchment processes, reservoir bathymetry and sedimentation surveys can provide necessary information for long-term reservoir planning and management (Thornton et al. 2013). Better tools, such as bathymetric and sedimentation surveys, are now available, providing reliable data on

water storage loss and water quality deterioration. Accordingly, this study investigates sedimentation and internal phosphorus loads in Krishnagiri Reservoir in the Ponnaiyar basin (Fig. 1), a medium-scale reservoir in Tamil Nadu, South India. The objectives of this study are to: (i) conduct a bathymetric study by integrating acoustic Doppler Q liner data, remote sensing data, and Global Positioning Systems and Geographical Information Systems to derive the present water capacity and spread area at different levels of reservoir water storage; (ii) investigate sediment physical and chemical characteristics, especially for phosphorus partitioning; and (iii) study phosphorus dynamics in the sediment–water interface and calculate Krishnagiri Reservoir internal phosphorus loading.

STUDY AREA Krishnagiri Reservoir is one of the oldest dams on the Ponnaiyar River, near Krishnagiri in Tamil Nadu, South India, being constructed in 1954. It had an initial water storage capacity of 68 million m3, but has been reduced by almost half because of environmental perturbations (Ravichandran 2002; IHH 2007; Jasmine & Ravichandran 2008). It is located between 77°410 56.4″and 78°160 51.6″ east longitude and 12°230 45.6″ and 13°20 27.6″ north latitude, with a designed capacity of 68.2 MCM (Fig. 1; Mohanakrishnan 1988). The reservoir catchment area contains eight subwatersheds with an area of 2500 km2, being influenced by south-west and north-east monsoon rainfall seasons. A tropical hot climate prevails, with a maximum and minimum temperature range of 34–37 °C and 22–24 °C, respectively, and an annual mean rainfall of about 800 mm. It has two main canals (Left Main Canal (LMC); Right Main Canal (RMC)) running parallel to the Ponnaiyar River, irrigating an area of 3642 ha. The 1000 m dam has eight spillways, three rivers and two

KRISHNAGIRI RESERVIOR CATCHMENT

KRISHNAGIRI RESERVIOR

Fig. 1. Index map of Krishnagiri Reservoir Project.


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canal sluices. The water spread area is 12.32 km2 at full reservoir level (FRL). Previous studies indicate Krishnagiri Reservoir may be hypereutrophic because of heavy siltation and poor sediment quality. Ravichandran (2002), for example, reported soil erosion and sediment-bound nutrient transport could be main sediment phosphorus load contributors, with subsequent studies (Jasmine & Ravichandran 2008) indicating soil erosion from Veppanapalli (one of eight reservoir subwatersheds) significantly contributes to a higher siltation rate. These environmental degradation impacts are likely to affect the reservoir’s beneficial uses in the future, unless the sedimentation and sediment phosphate load are properly addressed.

METHODOLOGY Bathymetry survey A River Discharge Monitoring System (Q liner), which is an acoustic Doppler profiler, was used to measure the reservoir depth at various locations to prepare a bathymetry map. The Q liner has a Doppler current profiler mounted on a miniature catamaran, with a aquapro sensor mounted on the bottom of the boat used to measure velocities and depth, thereby acting as both a transducer and receiver. It propagates four beams at 2 MHz, with sufficient energy to penetrate the water column, but also reflected from the sediment–water interface. The first three beams are used for velocity measurement, while the fourth beam is configured as a high accuracy echo sounder for depth measurements. A very narrow beam, combined with a very short pulse, gives accurate high resolution depth measurements. A special algorithm is incorporated to discriminate false echoes. Returning echoes are taken through a digital matched filter which increases the quality of depth measurements between 0 (no match) and 255 (perfect match), and any echo with a quality figure between 50 and 100 is accepted as a good measurement. The bathymetry surveys were conducted between February and March 2012, with the reservoir water level being 480.3 m above MSL (14.3 m). The Q liner was deployed from a 5 m boat, with the accuracy of the total depth measured increased by averaging measurements over a period of 30–45 seconds. Survey lines consisted of south-west, north-east, north-east–south-west and northwest–south-east trending tracks, with an average offset of 30–40 m and a total track line distance of the reservoir surveys being 72 km. Navigation data were acquired with a Magellan path finder global positioning system, and measured depth was corrected for the heading, pitch and


roll by the Q liner review software. The survey yielded 700 soundings spread over the reservoir water spread area. The data file was postprocessed to generate a contour plot of the entire reservoir in MapInfo GIS environment. The network of traverses in this study was much denser than in a traditional range survey method, to minimize the interpolation errors during postprocessing, and because contouring algorithms perform better with increasing data density. The water level during the survey was 2.81 m below the FRL. Remote sensing imageries were used to determine the reservoir water spread area between the water level during the survey and the FRL (483.11 m). The water spread area obtained via satellite imagery was overlaid with the contour map drawn from the bathymetry survey. The volume between any two adjacent elevations was determined by prismoidal formula, with the resulting volumes summed to create a stage–storage curve. The reservoir depth contours for 2007 (IHH 2007) and 2012 (present study) were processed using cut/fill tool in ArcGIS 9.3 to determine the areas and volumes of change between two surfaces, thereby identifying the areas and volume of the surface modified by the removal or addition of sediments within the reservoir.

Sediments Sediment physical and chemical characteristics were assessed via field investigations. In situ measurements were made with an YSI Professional plus field probe for pH, temperature, dissolved oxygen concentration (DO), electrical conductivity (EC), and total dissolved solids (TDS). Sediments were sampled with a simple grab sampler (Fig. 2), and samples transferred to the laboratory under cold, air-tight condition for further analysis. Samples were centrifuged to separate pore water and sediments, with the pore water refrigerated until further analysis. Sediment samples were air-dried in the shade and homogenized by grinding with mortar and pestle. Dry sediments were passed through a 0.075 mm sieve to separate silt/clay particles. The chemical speciation of sediment phosphorus was analysed by sequential extraction procedures (e.g. Nurnberg 1994; Kapanen 2008). The total phosphorus (TP) and various inorganic phosphorus forms in the sediment (i.e. soluble reactive phosphorus (SRP); calcium-bound phosphorus (Ca-P); aluminium-bound phosphorus (Al-P); iron-bound phosphorus (Fe-P)) were estimated. The sediment organic matter content was analysed by loss on Ignition method (550 °C; 3 h), with the weight difference indicating the sediment organic matter content (expressed as percentage of sediment by weight).


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RESULTS AND DISCUSSION Bathymetry survey

Fig. 2. Krishnagiri Reservoir sediment sampling locations.

Phosphate release rate The sediment phosphate release rate was calculated by estimating the variation of phosphorus fractions and total phosphorus in the sediment and pore water. The formula of Nurnberg (1994) was used to calculate the release rate for in situ conditions from the reservoir sediments, with the total phosphorus release rate calculated as follows: log RR ¼ 0:8 þ 0:76 log TPsed ;

ð1Þ

where RR = release rate; and TPsed = total phosphorus in sediment;

Internal phosphorus load The internal phosphorus loading in Krishnagiri Reservoir also was calculated with the Nurnberg (1994) formula for in situ study, as follows: IPL ¼ Phosphate release rate anoxic factor;

ð2Þ

where anoxic factor = (anoxic area 9 anoxic period)/reservoir area. The DO depth profile was measured monthly with a YSI Professional plus field probe in selected reservoir sampling locations, with the anoxic area calculated by interpolation in ArcGIS 9.3 environment.

The bathymetry of Krishnagiri Reservoir from the present study and during 2007 is shown in Figure 3, with a significant change in the reservoir bathymetry from sedimentation processes being evident. For the present survey, the reservoir water level was 480.3 m above mean sea level, with a spread area of 8 km2. The mean and maximum reservoir depths were 5.4 and 10.5 m, respectively, the latter near the dam shutter. Sediment accumulation displaces reservoir volume, causing both its storage and area curves to shift. The stage capacity curves exhibited a decreased volume at all water levels, suggesting sediment accumulation at all levels within the reservoir (Table 1), with the specific accumulation pattern varying from one location to another. The dead storage volume is often termed a sediment storage pool, although this is inaccurate because only a portion of the sediment inflow will be deposited in the dead storage zone. In long reservoirs, where turbidity currents are absent, however, or where most sediment is deposited in delta areas, virtually all the initial sedimentation will be focused in the higher-elevation active pool area, with little dead storage accumulation (Morris & Jiahua 1998). The topographical profile down the long axis of the reservoir indicated a generally smooth, low relief reservoir floor sloping gently from the headwater to the dam. The topographical cross section at 30, 1000, 2000 and 3500 m indicated deeper cross sections along the original river course. The reservoir was deepest near the dam and along the original Ponnaiyar river course. Measured depths were compared with those of the IHH Poondi (IHH 2007) study, which reported a maximum reservoir depth of 12 m. The different depth values may be attributable to 5 years of reservoir sedimentation, compared with the IHH study (2007). Comparison with the previous survey suggests a significant sediment deposition during the last 50 years (Table 2). The increased water retention time, as inflows from the catchment area lose their velocity entering the reservoir (Sabeti 2011), results in the coarser sediments settling in the inflow region, and fine sediments settling near the dam, a typical pattern for flat longitudinal reservoirs. The water storage capacity curve analysis indicates the present capacity of Krishnagiri Reservoir is 35.57 MCM (million m3). Table 2 indicates the percentage loss of capacity from 1957 to 2012, with the difference between the capacity recorded immediately after impoundment (in 1957) and that derived through the successive sedimentation surveys indicating the lost capacity


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Fig. 3. Krishnagiri Reservoir bathymetry from 2012 to 2007.

Table 1. Krishnagiri Reservoir water spread area and capacity at different levels, 1957–2012 Water spread area (km2) Level of contours

Height (m)

Capacity (MCM)

1957

1976

1981

1983

2007

2012

1957

1976

1981

1983

2007

2012

466

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

466.34

0.34

0.00

0.00

0.00

0.00

0.00

0.00

0.06

0.00

0.00

0.00

0.00

0.00

468

2.00

0.36

0.00

0.00

0.00

0.00

0.00

0.41

0.00

0.00

0.00

0.00

0.00

470

4.00

0.75

0.10

0.00

0.00

0.00

0.00

1.50

0.00

0.00

0.00

0.00

0.00

472

6.00

1.22

0.56

0.30

0.70

0.16

0.30

3.42

0.26

0.30

0.09

0.00

0.05

474

8.00

2.06

1.72

0.84

1.94

0.75

1.51

6.64

2.06

0.87

1.14

0.39

1.46

476

10.00

2.88

2.44

1.31

2.64

1.63

2.89

11.62

6.07

3.56

4.41

3.96

4.05

478

12.00

5.58

4.35

4.63

5.06

3.16

4.50

21.24

12.92

8.95

10.32

6.31

7.03

480

14.00

8.12

6.80

6.90

7.57

5.79

7.51

34.66

23.46

19.98

20.23

14.21

11.59

482

16.00

10.65

9.70

9.92

8.83

10.09

10.01

53.31

39.30

36.28

36.08

28.59

23.94

483.11

17.11

11.94

10.55

11.01

10.73

10.96

10.96

68.20

50.48

47.79

47.18

39.70

35.57

Table 2. Krishnagiri Reservoir capacity and percentage losses Survey year

Survey details

Reservoir capacity (MCM)

Loss in capacity (%)

1957

Preliminary Survey

68.20

0.00

1976

First capacity Survey

50.47

26.00

1981

Second capacity Survey

47.78

29.94

1983

Third capacity Survey

47.18

30.82

2007

Fourth capacity Survey

39.70

41.79

2012

Present study

35.57

52.16

attributable to sediment accumulation. The estimated present reservoir capacity has been reduced from 68.2 to 35.57 MCM over a period of 55 years (Table 2), equating to a capacity loss from the dam commissioning to as much as 52.16%.


The water elevation versus original and revised reservoir capacities are shown in Table 1, noting 32.64 MCM of gross storage has been lost to sedimentation. The capacity loss between 1957 and 1976 was 17.73 MCM, with a sedimentation rate of 0.933 MCM year1 until


1976. The Tamil Nadu Government implemented soil conservation plans during the late-1970s in many subcatchments under joint forest management programme, and in the Krishnagiri Reservoir catchment (Mohanakrishnan 1988). Check dams and percolation ponds also were subsequently constructed in all subwatersheds by the Agricultural Engineering Department (TAWDEVA 2002). The sedimentation rate was determined to be 0.826 MCM year1 during the last 5 years (Table 2), possibly attributable to soil and water conservation practices adopted in the catchment area to control soil erosion (Ravichandran 2002), although the recent increase in agricultural activities in the catchment area (Sabeti 2011) also may have enhanced soil erosivity. The water spread area at FRL at the time of impoundment was 11.94 km2, subsequently being reduced to 10.96 km2 (Table 1). The reduced water spread area at FRL is due to bank encroachment, mostly by the fishing community, along with deposition of coarser sediments. The storage capacity have significantly reduced from 466 MSL to 472 MSL (Table 1), indicating the siltation is increasing at an alarming rate, which will eventually decrease the reservoir’s useful life span. Accordingly, more attention is needed in catchment conservation programmes to prevent further soil loss and reservoir sedimentation, including for eutrophication management plans.

Sediments The sediment survey indicates heavy sedimentation (>0.75 m) near the deepest part of the reservoir at the dam and at the nearby boat jetty area on the north-western side. The middle part of the reservoir showed light to medium sediment deposition (0.25–0.5 m) along the north–south axis in the direction of the river inflow. The upper reaches of the water spread area have a rocky bottom, being devoid of sediments. About 70% of the water spread area (5.95 9 103sm2) exhibited low sediment deposition, while an area of 3197 9 103 m2 (30%) located around the dam shutters and sluices had higher sediment deposition.

Pore water phosphorus Total phosphorus (TP) in the pore water varied from 0.30 to 3.98 mg L1, while the orthophosphate ranged from 0.23 to 1.75 mg L1. The maximum TP values were detected in the deep zone near the dam shutters, while the minimum values occurred in the reservoir inflow areas. The major part of the inflow phosphate may be in sediment-associated particulate form. The range of TP measured in the pore waters is very high, compared with

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other lakes/reservoirs (Yang et al. 2005; Kapanen 2008; Dong et al. 2011; Trojanowska & Jezierski 2011), suggesting Krishnagiri Reservoir is hypereutrophic. Table 3 summarizes other chemical and physical parameters measured in pore water, noting the pH varied from 6.7 to 8.8, while the EC varied from 773 to 910 lS cm1. The pH variations were wide, covering an acidic to alkaline range, which may be important in regard to the phosphate release/deposition processes in the sediment-water interphase (Steinman & Ogdahl 2010), and might complicate the sediment P-release mechanisms. The loosely sorbed phosphorus in the pore water is generally transferred to the overlying water by windinduced resuspension and water body disturbances (Steinman & Ogdahl 2010). With an average depth of 5.4 m, Krishnagiri Reservoir can be classified as a medium-scale shallow reservoir (Wetzel 2001), meaning wind also may be a significant external factor for re-suspension of sediments and particulate materials. Other external factors for enhanced phosphorus release to the water column by decreasing the solubility threshold of the silt fraction may be increased temperature and pH (Holdren & Armstrong 1980). High TDS, EC and pH values also were observed in pore water samples collected near the deep (dam) zone. The pore water phosphorus concentration was high in January 2012, decreasing towards (summer season) March 2012. Bacterial breakdown of organic matter may be significant in deeper sediments than other zones, and the mineralization process (Wetzel 2001) may have increased the TDS release in the pore water. Figure 4 illustrates the spatial variation of the reservoir sediment’s organic matter content, with the content being 15.15%, although varying from 7% to 40%. A higher organic matter and clay content was observed in the dee-

Table 3. Krishnagiri Reservoir sediment pore water physical and chemical characteristics Average Parameter pH Total dissolved solids

value 7.75

SD 1.05

Minimum

Maximum

value

value

6.70

8.80

549

14.37

520

578

841

17.88

773

910

(TDS; mg L1) Electrical conductivity (EC; lS cm1) Temperature (°C) Total phosphorus

25.75

1.17

23.60

27.90

2.14

0.27

0.30

3.97

(TP; mg L1)


23.383 9.460 2.663 5.643 6.460 Max

6.522

17.226

11.070 1.910

5.685 2.380

2.100 3.500

4.550 3.590

0.650

3.310

0.150

Avg

Min

3073 8.85 347.046 9 103 Deep Near-dam

shutters

4.204

27.273 19.223 3.135 11.68 5.350

14.561

15.740

Max

4.580

0.175 0.016 0.259 0.010 Min

0.030

9.699

5.633 3.221

1.666 5.950

3.936 1.430

2.305 2.680

0.380 Max

Avg 23 394 20.41 Medium Middle zone

999.06 9 103

9.166

3.765 2.443

4.038 1.619

0.016 0.432

2.184 0.762

0.093

0.220

0.058

Avg

Min

6.84 11.05 Shallow Inflow region

(kg) (mg g1)

0.6198 9 103

Org-P

(mg g1) (mg g1) (mg g1)

Ca-P Fe-P Sediment TP load TP concentration

Zones

(kg)

Values

SRP

(mg g1)

Al-P


Location

Phosphate fractions Sediment TP varied from 1.89 to 24.0 mg g1. It was very high in January 2012, decreasing in March 2012, and being confined to the deep sediment zone with a high quantity of organic matter. The variation of total phosphorus and phosphorus fractions in the reservoir shallow, medium, deep sediment zones are shown in Table 4. Orthophosphate (OP) is the phosphorus form readily released from sediments. The sediment OP varied from 0.09 to 6.45 mg g1. The Ca-P content is about 25% of

Average sediment

Sediment phosphorus mass The sediment TP content was calculated by classifying the sediment deposition areas into deeper (near-dam shutters), moderate (middle zone) and shallow (inflow area) zones, based on sediment depth. The calculated variation of sediment depth total phosphorus load for each zone is illustrated in Table 4, with the total sediment phosphorus content varying from 6.85 to 23 394 kg per zone.

Sediment mass

per zones, suggesting sedimentation of finer particles in the inflowing sediment primarily takes place in this zone.

Table 4. Total phosphorus load and sediment phosphorus fractions in three Krishnagiri Reservoir sediment deposition zones

Fig. 4. Spatial variation of sediment organic matter.

(mg g1)

TP

E. Arunbabu et al.

(mg g1)

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the sediment TP, ranging from 0.16 to 3.22 mg g1 in the sediment. The Al-P spatial variation ranged from 0.25 to 11.68 mg g1. The increased sediment pH can accelerate Al-P release in deeper sediments (Yang et al. 2005). The Fe-P varied from 0.03 to 6.52 mg g1 in the sediment. Although iron-bound phosphate can become soluble under anoxic conditions, especially in reducing situations, it can become insoluble in oxic conditions and coprecipitate phosphate, thereby removing it from the pore water (Pettersson 1998). The average phosphorus fractions in the three sediment layers are summarized in Figure 5. The phosphate content was very high near the dam area (deeper zone) and middle part of the reservoir, with the order of abundance among the different fractions being Al-P> Fe-P> Ca-P> SRP. 20 18 16

mg g–1

14 SRP (mg/g) Fe-P (mg/g) Al-P (mg/g) Ca-P (mg/g) Org-P (mg/g) TP (mg/g)

12 10 8 6 4 2 0

Shallow

Medium

Deep

Fig. 5. Distribution of sediment phosphate fractions in Krishnagiri Reservoir.

The predominant phosphorus fractions in tropical and subtropical water bodies are compared in Table 5. Although CA-bound phosphorus is the dominant form of phosphorus fractions in most studied reservoirs, Al-bound phosphorus is the predominant form in Krishnagiri Reservoir, suggesting a different P-release mechanism. A large quantity of long-term available phosphorus (Al-P) is stored in the sediments. Even though the organic matter is high, physical and chemical processes may dominate over other processes in phosphate release from the reservoir sediments, with potential influence on Krishnagiri Reservoir internal phosphorus loading processes.

Phosphorus release The calculated sediment phosphorus release rate varied from 10.19 to 77.83 mg m2, with an average release of 40.97 mg m2, a higher rate than for many other lakes/ reservoirs (Wetzel 2001). The sediment sample temperå ̊ ture varied from 25 to 29.5 °C, and the sediment pH value ranged from 6.9 to 8.9. The former may have enhanced the sediment phosphorus release (Pettersson 1998). The phosphorus release rates for the three reservoir zones of the reservoir are provided in Table 6 and Figure 6, with the average release rate being 40.91 mg m2, similar to that reported by Holdren and Armstrong (1980) for highly eutrophic reservoirs. Krishnagiri Reservoir also is hypertrophic with a high sediment phosphate load, the major part (40%) being bound with alumina, which may remain for long periods. Sediment TP release plotted against actual TP concentration measured in pore water extracted from sediments,

Table 5. Comparison of sediment phosphate fractions in selected reservoirs Source Kapanen (2008)

Study area Tallinn, Estonia

TP and fractions

Trophic condition Eutrophic and mesotrophic

Ca-P>Al&Ca-P>SRP. TP=890.22 lg g1

Yang et al. (2005)

Dianchi Lake, south-western China.

Trojanowska and Jezierski

w Reservoir, Poland Sulejo Lake Volvi and Lake Koronia

2.27 g kg1 Ca-P>Fe-P> SRP.

Shallow eutrophic reservoir

Ca-P >Al-P > Fe-P >SRP.

Meso- to eutrophic lake

TP= 0.9–1.30 mg g1

(2005) Dong et al. (2011)

Eutrophic lake

TP=3105.71 lg g1

(2011) Fytianos and Kotzakioti

lakes

Ca-P > Al-P > Org-P > Fe-P > SRP. TP =

Baiyangdian Lake, North China

Ca-P>Al/Fe-P>SRP.

Hypereutrophic

TP = 443–611 mg kg1 Bolalek et al. (2002)

Pomeranian Bay, Southern BalticPoland

Present study

Krishnagiri Reservoir, Tamil Nadu, India.

Ca-P> Fe-P> Al-P> SRP.

Eutrophic water body

TP= 0.039–0.470 mg g1 Al-P> Fe-P> Ca-P> SRP.

Hypereutrophic water body

TP = 27.273 mg g1


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E. Arunbabu et al.

Table 6. Calculated P-release rates in three Krishnagiri Reservoir zones P release (mg m2) Zones

Average

STD

Minimum

Maximum

Shallow

29.57

12.94

10.84

48.30

Medium

44.01

23.94

10.19

77.83

Deep

54.20

15.02

39.17

69.24

using the Nurnberg (1994) methodology, is shown in Figure 6, indicating a linear relation (R2 = 0.59) and suggesting the internal TP load calculations based on the release rate assumed in the present study may provide realistic estimates. Table 7 compares Krishnagiri Reservoir phosphorus release rates to various tropical and subtropical water bodies. The release rates in the present study are higher than for other studies, suggesting chemical processes may be important in controlling reservoir TP concentrations.

Internal phosphorus load Internal P loading is a major problem in many water systems and a significant eutrophication management factor (Xiaojing et al. 2006). The P-release rate factor and anoxic factor were used to calculate the internal P load in the present study. The oxic and anoxic zones during the different seasons were monitored via the reservoir DO

concentration and distribution profile (Fig. 7), with a DO value of ≤0.1 mg L1 considered to be an anoxic condition. The reservoir internal phosphorus load is high in the winter (24.78 tons) and low in summer (2.21 tons). The Krishnagiri Reservoir internal phosphorus loading rate is relatively high, being directly proportional to its anoxic area. Table 8 summarizes the estimated Krishnagiri Reservoir oxic and anoxic areas, average P-release rates and internal phosphorus loads over three seasons. The pore water SRP concentrations generally depend on sediment redox conditions (Yang et al. 2005), with the actual sediment phosphorus release governed not only by the store of exchangeable phosphorus, but also by such factors as pH, redox potential (Eh) and bioturbation (Yang et al. 2005). The predominance of Fe- and AlP fractions and the high P-release rate is a probable reason (Xiaojing et al. 2006) for the observed high pore water concentrations and enhanced release rates. Shallow water bodies such as Krishnagiri Reservoir are vulnerable with respect to internal loading because nutrients accumulated in sediments (40%) can have a greater impact on water quality for a longer period of time, in contrast to other water bodies/deep lakes reported to date.

CONCLUSIONS The present study objective was to examine sedimentation processes and the eutrophication status of Krishnag-

Fig. 6. Actual vs. calculated sediment TP release in Krishnagiri Reservoir.



171

Table 7. Comparison of sediment P release from Krishnagiri Reservoir and other lakes/reservoirs Source Holdren and Armstrong (1980)

Study area

Value range

Mendota and Wingra lakes, Wisconsin, Madison. (Experimental study)

Average Release rate= 51 mg m2 day1 (hypereutrophic lake) Release rate = 0.004–0.016 mg L1 day1 (in pore water)

Xiaojing et al. (2006)

Lake Taihu and Lake Chaohu, China.

Nurnberg (1994)

Great lakes, Canada

Release rate = 1.4–14 mg m2 day1 (eutrophic lakes)

Present study

Krishnagiri Reservoir, Tamil Nadu, India

Release rate = 10.19–77.83 mg m2 day1 (Hypereutrophic

(Experimental study)

(meso-eutrophic reservoir)

reservoir)

Fig. 7. Spatial Variation of dissolved oxygen concentration in Krishnagiri Reservoir.

iri Reservoir. The reservoir has lost as much as 52% of its capacity since 1957, at a rate of about 0.81 MCM for about 3 decades (1960–1990), and 0.83 MCM over the last 5 years. It has deeper zonation zones along the original river course. The FRL water spread has been reduced to 10.96 km2 because of decreased inflows over the last 5 years. Heavy sedimentation has occurred in 40% of the water spread area, comprising mostly silt and clay in the middle part of the reservoir. The average sediment organic matter content was 15.15%, which increased up to

46%, especially in the deeper reservoir zone, which might characterize the nature of the external load retained from river inflows. The Krishnagiri Reservoir sediment TP load varied from 6.84 to 23 394 kg, depending on the deposition zones. Al-bound total phosphorus, a long-term resident form in the sediment, is the major fraction (35%) of the four sediment phosphorus fractions. Fe-P is the second predominant sediment phosphorus form (25%), which is highly mobile and can release phosphate under both oxic and anoxic conditions. Ca-P and SRP are minor sediment phosphorus components (10% and 5%, respectively). Although Ca-P in the surface sediments is relatively stable under alkaline conditions, it is very low in Krishnagiri Reservoir. Al-P and Fe-P are susceptible to physical and chemical changes, and their predominance suggests temperature, pH and redox or other chemical reactions may be important means of sediment P release. The phosphorus release rate from sediments to pore water varied from 10.22 to 70 mg m2, which may be related to Fe-P and Al-P dominance. Krishnagiri Reservoir internal phosphorus loading was estimated as 43.36 tons during the present study. Even if the external P load to Krishnagiri Reservoir is reduced, eutrophication may not be reduced immediately because of its high internal phosphate load and its likely mobilization in the reservoir. These results suggest soil conservation programmes at the catchment level must be improved and intensified to prevent soil erosion from the catchment, which would eventually reduce the reservoir eutrophication problems. To this end, a long-term planning of simultaneous catchment and reservoir management is required to increase the lifetime of the reservoir and improve its water quality.

ACKNOWLEDGEMENTS We wish to acknowledge an anonymous reviewer that helped us reorganize the manuscript structure to


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Table 8. Krishnagiri Reservoir internal phosphorus load Oxic zone (m2)

Anoxic zone (m2)

Avg. P-release rate (mg m2)

Load (tons)

Premonsoon

7.733 9 106

2.5775 9 106

70.988

16.467

Winter

6

4.124 9 10

6.186 9 106

44.5131

24.784

Summer

3.093 9 106

7.217 9 106

3.406

2.212

Season

enhance its readability. We also thank Dr. G K Nurnberg for constructive comments on an earlier version that improved the clarity of the discussion. An anonymous reviewer that improved English language usage also is acknowledged. One of the authors (SR) thanks UGC, New Delhi, for providing financial assistance for field work. We are also thankful to Ms Metilda for assistance in the field and data analysis.

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