CONTENTS-
Abstract
Introduction
Field Investigations
Experimental Details
Mass Balance Studies
Results and Discussions

The impoundment under study is called Mandakalli lake, which is situated, adjacent to the Ooty-Mysore highway at a distance of about, 7.5-km from Mysore on the way to Nanjangud. It is a small impoundment situated next to Dalvoi lake, which is the main feeder to the lake. In order to assess the quality of the impoundment in the beginning, a reconnaissance survey was conducted to gather data on the physical features and dimensions of the lake. Temperature, dissolved oxygen and conductivity were measured at regular intervals in the lake.   A depth contour map was   generated to supplement this. Then preliminary assessments were made for collecting the samples. Four sampling stations were selected for the study and samples were taken at regular intervals for 8 weeks. The samples were analysed for nitrates and phosphates leading to mass-balance analysis followed by prediction of the trophic status of the lake. Nitrates, phosphates and heavy metals were analysed in the roots of aquatic plants and sediments. Further aquatic plants around the lake were identified, which   are dominated by Cyprus and cropping was conducted to determine biomass density. Around 21% of the lake was   covered by biomass. Thermal heat budget was determined, which indicated the heat income of the entire lake expressed in heat units per unit area. From the results obtained average concentrations of nitrates and phosphates in the lake were found to be 11.92mg/L and 2.35 mg/L respectively. An annual seepage loss of 9474 mg of nitrates and 1114 mg of phosphates was determined. The heavy metal concentration was high in the sediments. The thermal heat budget of the lakes was 0.2966-cal sq. m. Lastly the trophic status of lake was determined from measured phosphate concentration thereby determining chlorophyll a concentration. All the results show that the lake has attained eutrophic status.

Untreated domestic sewage from various localities of Mysore city are let into Dalvoi lake directly, which in turn finds its way to Mandakalli lake. Thus controlling inputs into Dalvoi lake is one of the remedial measures to save this lake. Minimising the use of fertilisers and pesticides in agricultural fields can control considerable entry of pollutants through runoff. A clear indication of transition of lake from mesotrophic during 1990 to eutrophic state as of now clearly indicates that it would be surprising if the lake meets its end in the near future, if concrete steps are not taken immediately to save it.

Lakes are permanent features of landscapes, which are geologically transitory, usually born of catastrophes, to mature and die quietly and imperceptibly.   Most lakes are relatively shallow features of the earth's surface.   The conditions observed in the lakes reflect not only the processes operating within the body of water but also the metabolism and dynamics of the entire watershed and drainage basin.

Lakes are major recreational attractions for sport fishing, swimming and boating.   Large lakes and reservoirs are used as drinking water supplies.   They also have ecological and environmental values for moderating temperatures and affect the climate of the surrounding land.   They store water, thereby helping regulate stream flow, recharge groundwater aquifers and moderate droughts.   They provide habitat to aquatic and semi aquatic plants and animals, and they add to the diversity of the landscape.

Major categories of stresses include excessive eutrophication from nutrient and organic matter loadings; siltation from inadequate erosion control in agricultural, construction, logging and mining activities.   In addition, physical changes at the land-lake interface and hydrological manipulations (like damming outlets to stabilise water levels) also have major impacts on the structure and functioning of lake ecosystems.

1.2          General lake chemistry:

In absence of any living organisms a lake contains a wide array of molecules and ions from the weathering of salts in watershed, the atmosphere and the lake bottom, therefore the chemical composition of a lake is fundamentally a function of its climate (which affects hydrology) and its basin geology.   Each lake has an ion balance of the three major anions and four major cations,   the anions being HCO 3 , SO 4 and the cations being Ca 2+ , Mg 2 +, Na + , K + .

1.3        Wind and sunlight thermal stratification of lakes:

Lakes are assumed to be completely mixed both horizontally and vertically.   This assumption is justified on the basis   of wind stress on the water surface resulting in internal mixing, the condition also referred as homogenous condition.

The oxygen is introduced to a lake by the following sources:

*             Reaeration due to the agitation of lake water with the atmospheric air

*             Production of O 2 due to the photosynthesis from aquatic plants

The process utilising O 2 are:

*             Oxidation of organic materials

*             Respiration by aquatic plants and animals

*             Oxidation of sediments

Based on the above a balance in a segment of volume V can be written as V. dc/dt = Reaeration + (Photosynthesis - Respiration) - Sediment oxidation

1.4             Significance of N/p ratio:

*           The relative amount of Nitrogen and Phosphorus required by aquatic plants

*           The relative amount of Nitrogen and Phosphorus available for growth initially in the body of water.

Let N = available Nitrogen

a N = the nitrogen/chlorophyll ratio

p = available phosphorus

a p = the phosphorous/chlorophyll ratio

P is the resulting chlorophyll

Then we have P = min {N/a N : p/a p }    

The N/p ratio is thus a useful measure for understanding at first level the relationship between nitrogen, phosphorus and plant biomass.

1.5             Watershed Area:

The watershed, also called the drainage basin is all of the land and the water areas that drain towards a particular river or lake.   Thus a watershed is defined in terms of the selected lake.

The lake is a reflection of its watershed.   More specifically a lake reflects the watershed size, topography, geology, land use, soil fertility and erodibility and vegetation.   The impact of the water shed is eminent in the relation of nutrient loading to the watershed lake surface area ratio.

Typically, water quality increases with an increase in ratio of watershed area to the lake area.   This is obvious when one considers that as the watershed to lake area increases, there are additional sources of runoff to the lake.   Lakes with very small watersheds that are maintained primarily by ground water flow are known as seepage lakes.   In contrast, lakes fed primarily by in flowing streams or rivers are known as drainage lakes.

1.6             Eutrophication of lakes:

The addition of certain materials called nutrients to natural water often results in the growth of algae and higher aquatic plants, a process called eutrophication.   Eutrophication is a natural process, which can be greatly accelerated by those activities of man that introduce nutrients in the form of pollution.   Accelerated eutrophication results in loss of water clarity or colour, foul smell, build up of organic and nutrient rich sediments, loss of dissolved oxygen and changes in the lake's food web structure.

Lakes are often classified based on their trophic status as being either oligotrophic, mesotrophic or eutrophic.   Oligotrophic and mesotrophic lakes are relatively unproductive and receive only small amounts of plant nutrients, whereas eutrophic lakes are highly productive and receive nutrients in large quantities.

Eutrophication can have a significant effect on domestic, industrial and recreational uses of water.   Excessive growth of algae results in higher water treatment costs to make the water potable.   Even with proper treatment, certain residual tastes and odours are imparted to the water by algae.   Ultimately eutrophication may result in the filling up and disappearance of lakes.

Water Quality Impacts associated with Eutrophication:

*             Noxious algae

*             Excessive macrophyte growth (Loss of open water)

*           Loss of clarity

*             Possible loss of macrophytes (via light limitations by algae and periphyton)

*           Low Dissolved Oxygen (Loss of habitat for fish and fish food)

*             Extensive organic matter production (Smothering eggs and bugs)

*           Blue-green algae inedible by some zoo plankton (reduced food chain efficiency)

*           Toxic gases (Ammonia, Hydrogen Sulphide) in bottom waters (more loss of fish habitat)

*             Possible toxins from some species of blue green algae

*             Drinking water degradation from treatment disinfecting byproducts

*             Carcinogens such as chloroform (from increased organic matter, reacting with disinfectants like chlorine)

1.7             Objectives:

*           To find vertical variation of temperature, conductivity and dissolved oxygen

*           Analyse water samples for nutrients responsible for eutrophication

*           To identify and analyse aquatic biota around the lake and to analyse the bottom sediments

*           To carry out mass balance studies for nutrients and other water quality parameters

*           To assess the trophic status of the lake

2.1             Physical Features of the Lake:

The lake considered for the study is 'Mandakalli Lake".   It is a shallow lake situated on the way to Nanjandurg, at 7.5 kms from Mysore.   The geological location is at 12°15' N and 76° 31' E. The lake has a catchment area of 1.2 sq km and has a volume of 0.0012 cubic km.   The depth of the lake varies between 0.25 m to 2.3 m surrounding the lake.   The other aspects are shown in the Figure.

As already specified, the Mandakalli lake receives wastewater from Dalvoi lake in the upstream through two inlets, one of which comes directly to the lake by a small channel and the other comes through the agricultural fields.   Dalvoi lake receives waste water from more than 50% of the Mysore City Population, covering many parts of the city area.   They are St. Philomena's church, mandi Mohalla and Nazarbad, towards the southern parts of the city followed by the sewage farm at Chamundipuram, where only screening grit removal and sedimentation processes   are carried out.   The wastewater mainly contains sewage coming from residential houses, educational buildings, commercial and industrial areas in addition to the sewage during rainy seasons.   The rainwater picks up the waste derbis that is lying on the ground and streets during its course.   The runoff flows into the common   sewerage system.   Therefore, the potential sources of the nutrients are

*           The runoff from watershed drainage areas

*           The domestic and industrial wastewater discharges

*           The precipitation on the surface of the lake

*           Dry fallout like leaves, dust, seeds, pollen etc.

*           Ground water influx

*             Nitrogen fixation

*             Sediment cycling

*           Aquatic birds and animal wastes

The introduction of nutrients into the lake helps in the stimulation of algal   growth.   Algae play an important role in the self-purification of receiving water by photosynthesis.   Therefore, since the lake is productive, it is suggested that suitable   high yield fish species should be introduced and harvested periodically. This water not only increases   fish population but also eliminates organic chemical complexes through food chain, thus rendering the lake continuously productive.   Lake is chiefly used for the following activities: washing clothes,   domestic animals   and vehicles, bathing and fishing.

The people of Mandakalli village, situated about 1.5 km from the lake, come here   for the above said activities.   This lake is under supervision of fisheries department, Karnataka, which gives the contract to catch fishes annually.

2.3             Sampling

Stations were sampled weekly from April 18 to June 16, 2001.   Sampling was carried out at four monitoring stations in the lake as indicated in the map.   The sampling stations were chosen after extensive sampling done throughout the lake during preliminary studies and four distinct points showed much variation in concentration of nitrates and phosphates, and hence, these four stations were chosen for monitoring.   The samples were filtered in the laboratory and analysed for nitrates as N and phosphates as P. The samples were collected at the same time every week in the morning.

3.1             General:

Data collection related to this study includes:

*           Determination of temperature, DO and conductivity at different depths all along the lake

*           Routine weekly water quality sampling at four distinct monitoring stations identified for 8 weeks

*           Identifying aquatic plants and collecting root samples   and sediment samples in the lake

*           Cropping to determine aquatic plant density at dense, medium and sparsely grown areas in and around the lake

The analysis of data from the above was supplemented by mass balance studies to estimate factors such as heat budget, evaporation rate and inputs from agricultural runoff and from Dalvoi lake.

Results from these different sources are used to predict the trophic status of the lake system.

Water sampling is done beginning from summer season in mid April to the onset of monsoon by the end of May.  

3.2             Determination of temperature, DO and conductivity at different depths all around the lake:

The entire lake was divided into small grids; each grid measuring (20 X 10) m. At each point on the grid, direct reading DO meter with probe and equipped with temperature measuring feature, and a direct reading conductivity meter with probe was used .

3.3             Routine Weekly Water Quality Sampling:

There are four stations, which are monitored for 8 weeks, at weekly intervals.

*           At the bank of the lake.

*           At a small impoundment very near to the lake that has algal   growth.

*           Directly below the field which receives agricultural runoff (aquatic plants/weeds found).

*           Water hyacinth spreading at a point situated near the road side.

The concentration of phosphates as P and nitrates as N were determined spectrophotometrically at each of these stations, variations were studied and graphs plotted.

3.4             Identification of Aquatic Plants and Collecting Root Samples and Sediment Samples in the Lake:

The main aquatic plant in the lake is Cyperus.   Also, water hyacinth is   spreading gradually at the borders of the lake.   The roots of Cyperus were collected and analysed for the phosphates, nitrates and heavy metals.   Similarly, sediment sampling was done and analysed for phosphates, nitrates and heavy metals.   The analysis for heavy metals was carried out using atomic absorption spectrophotometer at Department of Chemistry, Central Coffee Research Institute, Balehonnur.

3.5             Cropping to determine aquatic plant density at dense, medium   and sparse growth areas in and around the lake:

Areas of dense, medium and sparse bio-mass growth were identified and the plants in (0.5x0.5) m area were weighed.   This was then multiplied by total area occupied by each category to find net bio-mass density.

4.1 Approach

Mass balance studies for nitrates and phosphates   were conducted thereby determining seepage losses from each. The following data are relevant for the study:

•  Total flow from agricultural fields (Q A ) into the lake.

•  Lake evaporation (E v )

•  Total flow from Dalvoi lake (Q D )

The detailed calculations for each are presented in the following paragraphs.

•  Total flow from agricultural fields into the lake (Q A )

Data:

Drainage area of the lake   = 12 x 105 m 2

Rainfall intensity = 0.55 m/year

Accounting 30% rainfall as runoff we get,

Q A = (Drainage area of lake x rainfall) x 0.3

Q A = 12 x 10 5 x 0.55x 0.3 = 198 x 103 m 3 /year

•  Lake evaporation (E v )

The following equation given by Thomann & Muller (1987) is used to calculate lake evaporation for a known intensity of rainfall:

E v = 1/A s [ D V/ D Z + Q in – Q + PA s ]

Where, A s = Lake surface area m 2

                   V = Lake volume, m3

            Q in = Net flow into the lake

                  P = Rainfall intensity

The change in volume with reference to depth was found to be uniform and hence the term D V/ D Z vanishes from the above equation. Also since net inflow is taken to be equal to net outflow for the mass balance studies, the above equation reduces to,

Ev = 1/ A s x PA s

Therefore E v = P

Hence net evaporation is taken equal to net precipitation which is equal to 0.55 m/ year.

•  Total flow from Dalvoi lake (Q D )

  Flow from Dalvoi lake is found to be 0.03 m 3 /sec or Q D = 946 x 103m 3 /year

4.2 Nitrate Mass Balance

Mass inputs

  The sources of inputs are

•  Agricultural runoff

•  Dalvoi lake water

Hence the total mass input is given by

W N = Q A N A + Q D + N D

Where NA and ND are concentration of nitrates in agricultural runoff and Dalvoi lake waters respectively.

N A = 32 mg/L                                                     N D = 48 mg/L

      = 32 x 10 -3 mg/m 3                                              = 48 x 10 -3 mg/m 3

Therefore,

W N = (198 x 103 x 32 x10-3) + (946 x 103 x 48 x10-3) W N = 51,744 mg/ year

Net accumulation:

Mass accumulates in the lake are

•  Lake water

•  Plant roots

•  Bed sediments

We have,

Concentration of nitrates in lake water = 11.92 mg/L

Concentration of nitrates in roots = 4.1 mg/L

Concentration of nitrates in bed sediments = 9 mg/L

Hence,

Total concentration = (11.92 x 10-3) + (4.1 x 10-3) + (9x 10-3) = 25.02 x 10 –3mg/m 3

Therefore, net accumulation = total flow x total concentration

                                             = (198 x 10 3 + 946 x 10 3 ) x 25.02x 10 -3

Net accumulation = 28, 634 mg/year

Mass outputs

Outputs from the lake are due to

•  Lake evaporation

•  Seepage

Accounting for lake evaporation, we have

Q = Q A + Q D + PA s -E v A s

Since P = E v , we have

Q = Q A + Q D

  = (198 x 103) + (946 x 10 3) Q = 11, 44, 000 m 3 /year

Therefore net output due to evaporation = Q x concentration of nitrates in lake water

                                                          = 11, 44, 000 x 11.92x 10 -3 Q = 11, 44, 000m 3 /year        

To determine mass output due to seepage, we apply mass balance equation as follows:

Input – Accumulation = Output

Therefore, 51, 744 – 28, 634 = 13, 636

LHS value = 23,110 mg/year    RHS value = 13, 636 mg/year

This value is accounted as seepage loss.

Hence seepage losses = 23, 110 – 13,636 = 9474 mg/year

Thus the net output = 13, 636 +9474 = 23, 110 mg/year

4.3. Phosphate Mass Balance

Mass Inputs

The sources of inputs are

•  Agricultural runoff

•  Dalvoi Lake water

Hence the total mass input is given by

W N = Q A P A + Q D P D

Where PA and PD are concentration of nitrates in agricultural runoff and Dalvoi lake waters respectively.

PA = 7 mg/L                                              PD = 32 mg/L

     = 7 x 10 – 3mg/ m 3                                    = 32 x 10 -3 mg/m 3

Therefore

W p = (198 x 103 x7 x10-3) + (946 x103 x 32 x 10-3) = 31, 658 mg/year

Mass Accumulation

Mass accumulates in the lake are

•  Lake water

•  Plant roots

•  Bed sediments

We have

Concentration pf Phosphates in lake water = 2.35 mg/L

Concentration of Phosphates in roots = 14 mg/L

Concentration of Phosphates in Bed Sediments = 8 mg/L

Hence, Total Concentration = (2.35x10 -3 ) + (14x10 -3 ) + (8x10 -3 ) = 24.35 x 10 -3 mg/m 3

Therefore Net accumulation = Total Flow x Total concentration

                                              = (198 x 10 3 + 946 x 10 3 ) x 24.35 x 10 -3 = 27.856 mg/ year

Mass Outputs

Outputs from the lake are due to:

•  Lake evaporation

•  Seepage

Accounting for the lake evaporation, we have

Q = Q A + Q D + PA s – E v A s

Since P= E v , we have

Q = Q A +Q D

   = (198 x 10 3 ) + (946x10 3 ) = 11,44,000 m 3 /year

Therefore net output due to evaporation = Q x Concentration of Phosphates in lake water

                                                                = 11,44,000 x2.35x10 -3 = 2,688 mg/year

To determine mass output due to seepage, we apply mass balance equation as follows:

Input – Accumulation = Output

LHS value = 3, 802 – 2,688 = 1114 mg/year

This excess value is accounted as seepage loss.

Hence seepage loss = 3, 802 – 2, 688 = 1114 mg/year

Thus the net output = 1,114 + 2, 688 = 3, 802 mg/year

•  Maximum depth = 2.35 m

•  Variation of temperature with depth was found to be negligible, hence thermal stratification does not exist

•  Conductivity value in the lake was not found to be fluctuating and thus the lake is in completely mixed condition

•  The high DO values during sunlight hours indicate lake's high productivity

•  Highly fluctuating variations of nitrates were found at each of the four stations. The station directly below the agricultural field showed maximum concentration.

•  A graph of mean chlorophyll a in mg/L vs median total phosphorous in mg/l was plotted and compared with standard graph.

•  Cropping results showed that 21% of the lake was inhabited by aquatic plants.

Table 1: Variation of Temperature and Dissolved Oxygen wit h Depth

Table 2:   Cropping Results

Area occupied by biomass: Dense Growth : 3800 m 2

                                            Medium Growth: 320 m 2

                                             Sparse Growth: 80 m 2

Total : 4200 m 2       Total Lake area: 20, 000 m 2 .

Therefore 21% of the entire lake is occupied by biomass

Table 3: Nitrates, Phosphates and Heavy metal Analysis in Roots and Bed Sediments

Table 4: Trophic Status of the lake

Department of Environmental Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore-570 006, Karnataka, India.
Phone: 0821-431133.
E-mail: vinubond@yahoo.com