Lab / experiment Grade 9 Science
Bell Creek Nutrient Pollution: A Grade 9 Dissolved Oxygen Lab
A grade 9 lab where students test three simulated creek samples for nitrate, pH, dissolved oxygen, and turbidity to investigate how farm runoff drives eutrophication.
Objectives
- Students will measure nitrate, pH, dissolved oxygen, and turbidity in three simulated water samples representing different points along a waterway affected by agricultural runoff.
- Students will use their data to explain how nutrient pollution can lower dissolved oxygen through the process of eutrophication.
- Students will evaluate a proposed mitigation, a vegetated buffer strip, and predict its effect on future water-quality readings.
Vocabulary
- Eutrophication
- Eutrophication is what happens when a lake or bay gets overfed with nutrients: algae grows out of control, and when it dies and rots, the water loses the oxygen that fish need.
- Dissolved oxygen
- Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.
- Nitrate
- Nitrate is a form of nitrogen that plants use as food. It gets into water from fertilizer, animal waste, and septic systems, and too much of it is unhealthy for both people and streams.
- Algal bloom
- An algal bloom is a fast, thick growth of algae that can turn a lake green and scummy. Blooms happen when the water gets lots of nutrients and warm sunshine, and some blooms make toxins.
Teacher overview
Bell Creek (a fictional waterway used for this lab) runs past Hollow Run Dairy, a fictional farm whose fields border the creek before it flows through town. After a heavy rain, farm fields can send fertilizer and manure into a nearby stream, adding nitrogen and phosphorus that the stream did not have before. This lab uses three teacher-prepared samples to model that process safely in the classroom, so that students can measure real changes in water chemistry without needing an actual field sample from a working farm. Students often assume that added nutrients are harmful only because they are unpleasant chemicals in the water; this lab is built to correct that assumption by showing that nutrients themselves do not remove oxygen directly. Instead, nutrients feed algae growth, and it is the decomposition of dead algae that consumes the oxygen fish and other aquatic organisms need.
Teacher preparation (lab day). Use a houseplant fertilizer whose label lists nitrate nitrogen, since the test strips read nitrate. Sample A: tap water left to stand overnight to lose its chlorine, then shaken hard in a half-full closed jar to aerate it. Sample B: the same water with a small pinch of dissolved fertilizer. Sample C: water you have boiled and cooled in a container filled to the brim and sealed (boiling drives out dissolved oxygen), with more fertilizer and a pinch of fine soil for cloudiness. Keep all three sealed until students test them.
Procedure
Step 1. Put on safety goggles. Your teacher has prepared three labeled water samples that model three points along Bell Creek. Sample A represents water upstream of Hollow Run Dairy, Sample B represents water midstream, and Sample C represents water just downstream of the farm after a heavy rain.
Step 2. Safety note. Do not taste or drink any sample. Although these samples are prepared with safe classroom materials, treat every lab sample as you would an unknown water source.
Step 3. Using a nitrate test strip, test each sample according to the strip manufacturer's timing instructions and record the reading, in parts per million, in your data table.
Step 4. Using pH strips or the pH testing kit, test the pH of each sample and record it.
Step 5. Using the dissolved oxygen test kit, follow the kit's steps to test each sample's dissolved oxygen level in milligrams per liter (mg/L). Safety note: if your kit uses a chemical reagent, work over a tray and tell your teacher immediately if any reagent touches skin or eyes.
Step 6. Pour a portion of each sample into a clear cup and compare it against the turbidity card, assigning a cloudiness rating from 1 (clear) to 5 (very cloudy). Record each rating.
Step 7. Once all three samples are tested, compare your group's data table with another group's results before moving to the analysis questions.
Data & Analysis
| Sample | Location represented | Nitrate (ppm) | pH | Dissolved oxygen (mg/L) | Turbidity (1-5) |
|---|---|---|---|---|---|
| A | Upstream of the farm | ||||
| B | Midstream | ||||
| C | Downstream, after rain |
- Compare the nitrate readings across the three samples. Which sample had the highest nitrate level, and where along the creek was that sample collected?
- Calculate the difference in dissolved oxygen between Sample A (upstream) and Sample C (downstream). What does that difference suggest about the effect of nutrient runoff on dissolved oxygen?
- Most aquatic organisms, including the fish that live in Bell Creek, need a dissolved oxygen level above about 5 mg/L to survive comfortably. Suppose a downstream sample reads 3.2 mg/L. By how many mg/L does that reading fall short of the 5 mg/L benchmark?
- Explain, using the process of eutrophication, why a nutrient-rich sample like Sample C often ends up with lower dissolved oxygen than a nutrient-poor sample like Sample A, even though the nutrients themselves do not remove oxygen directly.
- Based on your turbidity ratings, does higher nitrate content appear to be associated with higher turbidity in your samples? Propose one reason algae growth could increase turbidity.
- Suppose Hollow Run Dairy planted a vegetated buffer strip along the creek to filter runoff before it reaches the water. Predict how you would expect Sample C's nitrate and dissolved oxygen readings to change the following season, and justify your prediction using what you observed in this lab.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: Answers will vary based on collected data. Sample C (downstream, after rain) typically shows the highest nitrate reading, since it sits closest to the farm's runoff path and after a rain event that washes fertilizer and manure toward the creek.
- Answer: Answers will vary based on collected data. Dissolved oxygen is typically lower in Sample C than in Sample A, often by several mg/L, which suggests that nutrient-rich runoff is associated with reduced oxygen downstream.
- Answer: 1.8 mg/L. 5 mg/L minus 3.2 mg/L equals 1.8 mg/L, meaning the sample falls 1.8 mg/L short of the level most aquatic organisms need.
- Answer: Nutrients such as nitrogen and phosphorus cause algae to grow quickly, a bloom. When the algae die, bacteria decompose the dead algae, and that decomposition process consumes dissolved oxygen from the water, which is why a nutrient-rich sample ends up with less oxygen even though the nutrients did not remove any oxygen directly.
- Answer: Accept any reasoned response noting that samples with higher nitrate often show higher turbidity, since an algal bloom fueled by nutrients adds tiny living particles to the water that increase cloudiness.
- Answer: Accept a prediction that nitrate would decrease and dissolved oxygen would improve, reasoning that a buffer strip's plants and soil filter and absorb nutrients before they reach the creek, reducing the algae growth that would otherwise deplete oxygen.
Differentiation
Support (below grade level)
Provide a completed example row (a fourth, already-recorded sample) showing what a full data-table row looks like before students begin testing their own three samples. Reduce the analysis set to questions 1, 2, 3, 4, and 6, and allow question 4 to be answered by putting a provided list of steps (nutrients, algae growth, algae death, decomposition, oxygen use) into the correct order rather than writing a full explanation.
Extension (above grade level)
Have students describe, using only the dissolved-oxygen benchmark given in this lab and not outside sources, how a fish kill event might be reported in local news, then write a two-paragraph explanation connecting their own Sample C data to that kind of real-world event, including what a water manager might measure first to confirm the cause.
ELL supports
Pre-teach nitrate, dissolved oxygen, turbidity, and eutrophication with a labeled diagram showing the sequence: fertilizer runoff, algae growth, algae death, decomposition, oxygen loss. Allow data-table entries and short-phrase answers for questions 1, 2, and 5, reserving full sentences for question 3, which has a single fixed numeric answer.