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| Location of Kaikondrahalli lake |
This is a photo-narrative blog of a trip to Kaikondrahalli lake in Bangalore. This is the first post of an ongoing narrative about a project that we are involved in. The intent behind this specific post is to expose the readers to some sensors that can be used to measure lake data, what they measure, the use of this data, and some practical difficulties that we encounter in the field. The next post will be about the results coming out of these sensors and what they are telling us. These sensors were installed 3 weeks ago and this is the first trip to check on them.
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| A view of the lake from the bridge close to the lake entrance |
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| Sayan has brought an intriguing “mobile laboratory” and we will soon see this put in action in the middle of the lake! |
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| The coracle that we will be boarding soon.
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It has become very hot by now and there will be no shade once we get on board
A quick primer about dissolved oxygen (DO): It is a critical component of the water which keeps aquatic life alive.
It can drop to low levels due to domestic sewage flowing into the lake, which has organic matter that increases the oxygen demand. A combination of low DO and ammonia toxicity due to the input of nutrients from domestic sewage can lead to “fish kill” in a matter of minutes like it has happened in Ulsoor and few other Bangalore lakes in the past.
We approach the DO (dissolved oxygen) sensor which is anchored in the lake bed and kept afloat at the other end with the ATREE buoy. This is ATREE’s first experience with continuous monitoring of DO concentration in a lake
The units of measurement are in milligram per litre (mg/l). If DO level falls below 5 mg/l, it can affect aquatic life. This DO sensor has been set to take a reading every half an hour.
Venkatesh eagerly volunteered to collect the water sample in a bottle supplied by Sayan. He had to bend down until the bottle was almost at the level of the sensor and then fill the bottle with the lake water.
The dissolved oxygen in the lake water sample won’t stay stable until it is taken back to the lab. Hence the oxygen needs to be “fixed” before its concentration changes. This method of testing for DO is called the Winkler test. The chemically inclined can look up the details at https://en.m.wikipedia.org/wiki/Winkler_test_for_dissolved_oxygen
The first step is to add Manganese Sulphate (MnS04) solution to the lake water sample. Sayan is taking out the MnS04 from its container using a pipette. Brought back dim & distant high school memories of chemistry lab experiments 🙂
After that, an Alkali iodide is added. The lake water sample can be seen in the bottle kept in front of Sayan.
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| The bottle is shaken vigorously! |
The dissolved oxygen oxidises the Manganese ions and a brown precipitate starts forming. Sayan critically examining his handiwork 🙂
Now sulphuric acid is added. The acid facilitates the conversion of the iodide ions into iodine. The iodine concentration is what gets measured in the lab (through a process called titration) and is used for working out the dissolved oxygen concentration, through chemical equations.
Pretty darn fascinating! And all this happening on a coracle floating in an urban lake under the hot sun 🙂
Sid diligently notes down various readings including time, temperature, pressure etc.
Measuring Venkatesh’s arm length – this is the depth of the water sample. He stuck in his whole arm to get the lake water into the bottle.
A second sample is taken. This time Venkatesh goes down deeper (based on the prior arm measurement). And then the same process of “fixing” the dissolved oxygen is repeated. The 2 samples are kept in contact with ice. The average of the lab results from the 2 samples will be taken for calibration. The adjustment is done automatically by the sensor software.
Taking out the casing from the water, with the sensor inside. A fair amount of biofouling is observed.
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| Removing the sensor from the casing. |
The sensor end from where data is transferred. The communication from the sensor to the computer is through light. Also, the principle behind the functioning of the sensor while measuring DO is based on light as it is an optical sensor.
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| Attached to the “Coupler” |
The other end of the coupler plugs into the “Shuttle” which reads the data. This shuttle is particularly useful if you don’t want to bring a computer to the field to download the readings. You can transfer to this device, take it to your office or home, and then transfer the data to the computer to analyse the readings.
A closer look at the HOBO Shuttle made by Onset which is a US based company. This entire instrument chain – DO sensor, Shuttle, coupler and the software to read and display the data belongs to the HOBO product series.
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| The yellow lit LED indicating that data transfer is taking place |
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| Venkatesh did not bargain for this endless sitting around. He looks lost in thought |
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| Cleaning the sensor end with distilled water |
Another kind of field calibration is being carried out. This is to set the 100% saturation reading. A sponge is soaked with fresh water, excess water is squeezed out and kept inside the black casing called the calibration boot. This is then affixed to the sensor.
After attaching, we have to wait for about 15-20 mins to take the reading so that temperature equilibrium is reached. This would correspond to 100% saturation value. Several readings are taken in succession until the readings stabilise.
While we are doing the 20 mins waiting, Sayan has transferred the DO data for the last 3 weeks from the HOBO Shuttle device to his laptop and is examining it. This is the first time he is viewing data from this sensor (for this lake).
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| Something doesn’t quite look right! Is the connection ok? |
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| Something is really looking fishy about this data!! |
Sayan discovers from the data that after 1 week, the DO sensor’s readings have drifted to a very low erroneous range. The conclusion is that the biofouling has caused this. The only remedy is that once every week the sensor needs to be taken out, cleaned and put back, to make sure it continues to function correctly.
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| 2 hours later Venkatesh looks thoroughly fed-up of this coracle sit-out!! |
Phew – we are finally done with the lake measurements. Our resident kayaker takes charge of rowing us to the shore. It’s about 2 pm by now.
#ORCLCitizenship #tripszee #OracleGiving #atree_org