Water Quality and Robots: Educating Minnesota Youth�

Depts. Of Computer Science and Engineering; Soil, Water and Climate, and Extension's Center for Youth Development at the University of Minnesota;

and High Tech Kids

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This material is supported by the Environmental and Natural Resources Trust Fund (ENRTF) of the State of Minnesota.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the ENRTF.

Acknowledgement

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Minnesota Lake Water Quality�

Module 2.3: Lake Water Quality Sensors, Monitoring Lakes with Satellite Remote Sensing, and Lake Water Quality Trends

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Lake Water Quality Data Module

• Minnesota Lake Water Quality Data Module LWQ2.3 Objectives:

1) Learn how water quality sensors on underwater robot work

2) Introduce remote sensing web browsers for assessing lake water clarity and chlorophyll a

3) Learn how to interpret trends in lake water quality over time

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Lake Water Sensors for Underwater Robot

• A series of four low-cost sensors will be mounted on the underwater robot
• These sensors will measure
• Water temperature
• pH of water
• Dissolved oxygen in water
• Turbidity of water
• The following slides provide an overview for each sensor, and some background on how they work

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Lake Water Temperature

• The low-costs analog temperature sensor we will mount on the underwater robot:
• Measures water temperature in units of degrees Celsius
• It does this by measuring voltage changes across a resistor when a constant current is applied
• Voltage is linearly correlated with water temperature

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Lake Water Temperature

• Ohm’s Law describes the relationship between voltage (V), current (I) and resistance (R)
• V = I x R
• Accuracy is +/- 0.5 °C

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Lake Water pH

• pH is a measure of the how acidic or basic water is on a scale ranging from 0 to 14, with 0 being extremely acidic and 14 being extremely basic
• A pH of 7 is neither acidic, nor basic… it is neutral
• Water with a pH less than 7 is acidic, while water with a pH greater than 7 is acidic

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• pH is the logarithm of hydrogen ion concentration
• pH = -log[H+], where [H+] is the concentration of hydrogen ions

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On a log scale, pH readings that differ by

1 unit have hydrogen ion concentrations

that differ by a factor of ten. For example,

a pH of 7 has [H+] = 0.0000001, while a pH

of 6 has [H+] = 0.000001

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Lake Water pH

• The low-cost pH meter we will mount on the underwater robot estimates hydrogen concentration in water by measuring the voltage between two electrodes
• Electrodes are separated from water by a thin, impermeable glass membrane
• Only hydrogen ions can move between electrodes
• When the sensor is placed in water, hydrogen ions move between electrodes until the concentration of ions becomes equal to the concentration of hydrogen ions in the water
• This process of equilibration takes about 4 seconds
• Accuracy is +/- 0.2 units of pH

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Lake Water Dissolved Oxygen

• The low-cost dissolved oxygen (DO) sensor we will mount on the underwater robot estimates the relative amount (% saturation) of dissolved oxygen in the test solution, by measuring the partial pressure of oxygen in the test solution
• Relative partial pressure is expressed in units of percent (a fraction relative to a calibrated air pressure value in air)
• Voltage measurements with the sensor decrease as the % saturation of oxygen decreases

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Lake Water Dissolved Oxygen

• Air is made up of different gases, and oxygen is one of them (what are some other gases in air?)
• About 21% of the air is oxygen
• At sea level (near the ocean), the total pressure of the air is 760 mm of mercury (which is just a way to measure pressure)
• To find how much of that air pressure is from oxygen, we multiply: 760 x 0.21 = 160 mm Hg, so, the pressure from oxygen alone is 160 mm Hg
• This amount is considered 100% oxygen saturation in the air at sea level
• In water, though, oxygen doesn’t dissolve as easily as in air, so the pressure from oxygen is lower than 160 mm Hg
• That means oxygen saturation in water is usually less than 100%, as shown in the examples on the right

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Lake Water Turbidity

• The low-cost analog turbidity sensor we will mount on the underwater robot measures the quantity of total suspended solids (TSS), expressed in units of nephelometric turbidity units (NTU)
• It does this by measuring the amount of light scattered in the water sample

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Lake Water Turbidity

• Higher turbidity (NTU) indicates lower water clarity (i.e. cloudy water)
• There is an exponential decrease in turbidity as voltage on the sensor increases
• At about 3.5 volts, with no errors in voltage (red line), the turbidity reading is 2,000 NTU
• However, this method has large errors, the actual turbidity could be as high as 2,500 NTU for a voltage error of +0.3 (blue line) or as low as 1,200 NTU for a voltage error of -0.3 (green line)
• As voltage increases, turbidity decreases, meaning water clarity increases (better water quality)

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Lake Water Quality Remote Sensing

• Satellite remote sensing is a powerful tool for assessing lake water quality, particularly water clarity as measured by Secchi disk and chlorophyll a concentrations
• Sentinel 2 satellites orbit the earth from an altitude of 488 miles, at a spatial resolution of 65ft x 65ft in area (4,305 sq. ft) for each pixel in the image
• Each path is 180 miles wide
• These satellites can view any location on earth every 5 days, assuming there are no clouds

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Lake Water Clarity Remote Sensing

• Sentinel 2 measures reflectance of solar radiation at 12 different wavelengths ranging from the visible to the near infrared to the shortwave infrared regions of the electromagnetic spectrum
• Secchi disk data (from citizen scientists) at 79 Minnesota lakes strongly correlates (88% accuracy) to the Sentinel 2 remotely gathered data
• These data were strongly correlated (88% accuracy) to a Sentinel 2 remote sensing model of Secchi depth (SD) based on lake reflectance in the blue (band 2), red (band 4) and red-edge(band 5) wavelengths

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Lake Water Quality Remote Sensing

• University of Minnesota LakeBrowser (https://lakes.rs.umn.edu/)
• Contains water clarity and chlorophyll a remote sensing data for over 10,500 individual Minnesota lakes from:
• 1975-2008 at five-year intervals
• 2017-2021 for annual data

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Water Clarity Trends (Watershed Avg)

• Trends in lake water clarity and chlorophyll a estimates using remote sensing are averaged for all lakes within any Minnesota major watershed (Hydrologic Unit Code HUC8) from 1975 to 2021 at the University of Minnesota LakeBrowser (https://lakes.rs.umn.edu/)

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Lake Water Quality Trends

• A trend in lake water quality over many years can take one of three forms:
• Stabilizing trend means no change in water quality
• Degrading trend means a worsening in water quality
• Improving trend means water quality is getting better

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Lake Water Quality Trends (Ex: Clarity)

• Water quality data have scatter, making interpretation of trends difficult; e.g., transparency trend tab at https://webapp.pca.state.mn.us/surface-water/impairment/48-0002-00

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Mille Lacs Lake

Shaokatan Lake

Upper Twin Lake

Lake Elmo

Activity #11: Which lakes show stable, decreasing or increasing trends in water clarity?

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