Writing prompt Grade 6 Science
Ellerby Reservoir's Musty August Water: Which Intake Depth?
A grade 6 science argument prompt using a fictional reservoir's summer depth profile to decide which intake depth a plant should use and why algae grow where they do.
Objectives
- Students will interpret a reservoir depth profile with five measured variables.
- Students will argue for one intake depth using at least three pieces of data.
- Students will explain how light and phosphorus supply controls where algae grow.
Vocabulary
- 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.
Also: intake, layer, phosphorus, algae, profile
Teacher overview
Students read a fictional summer depth profile from a reservoir and write a science argument choosing the depth a treatment plant should draw from, then explain what the data show about where algae grow. The misconception worth naming first is that deeper water is always cleaner water. Students often reason that dirt sinks and that cold water is fresh, so the bottom must be best. In a lake that has separated into layers for the summer, the bottom is the one place with no oxygen at all, and water without oxygen pulls iron and manganese out of the mud and takes on an earthy smell. The best water is usually in the middle, and the profile in this prompt is built so students can find that themselves rather than being told.
Prompt
Background. Birchlow is a fictional town that takes its drinking water from Ellerby Reservoir. Every measurement below was written for this assignment. The reservoir is 40 feet deep at the intake tower. The tower has three gates, at 3 feet, 20 feet, and 40 feet below the surface. The operator can open any one of them.
Every August, customers call to say the water smells musty. The smell is not dangerous, but people do not want to drink it. Some kinds of algae release a compound that smells earthy and musty even in tiny amounts. The plant's filters do not remove it well.
In April the operator measured the reservoir. The temperature was about 9 degrees Celsius from top to bottom. There was plenty of oxygen at every depth. In August the operator measured again and found this:
| Depth (feet) | Temperature (degrees C) | Light | Phosphorus (mg/L) | Algae (cells per mL) | Dissolved oxygen (mg/L) | Musty smell, scale of 1 to 5 |
|---|---|---|---|---|---|---|
| 3 | 25 | bright | 0.02 | 42,000 | 9.1 | 4 |
| 10 | 23 | dim | 0.02 | 18,000 | 8.4 | 3 |
| 20 | 14 | dark | 0.03 | 900 | 4.0 | 1 |
| 30 | 9 | dark | 0.09 | 100 | 0.5 | 2 |
| 40 | 8 | dark | 0.21 | 0 | 0.0 | 3 |
The operator also pulled water from 40 feet. It came out gray. After a few minutes in air it left brown stains in the sink. Iron and manganese dissolve out of the mud when the water above it runs out of oxygen.
Your task. Write an argument that answers this question: which gate should the Birchlow plant open in August, and why?
Your argument must:
- state a clear claim naming one depth,
- support the claim with at least three rows or columns of data,
- include one paragraph explaining why the algae grow where they do, using both the light column and the phosphorus column,
- explain why you rejected each of the other two gates,
- end with one sentence naming a measurement the operator should take next August.
Write four to six paragraphs. Everything you need is above, so do not use outside sources.
Planning Organizer
Complete every row before drafting. Each row becomes at least one sentence.
| Part of the argument | What I will write |
|---|---|
| My claim (which gate, in one sentence) | |
| Evidence 1 from the profile | |
| What evidence 1 shows | |
| Evidence 2 from the profile | |
| What evidence 2 shows | |
| Evidence 3 from the profile | |
| What evidence 3 shows | |
| Algae paragraph: where the light is | |
| Algae paragraph: where the phosphorus is | |
| Algae paragraph: why the counts are highest near the top | |
| Why I rejected the first of the other two gates | |
| Why I rejected the second of the other two gates | |
| Ending: what to measure next August |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response
The Birchlow plant should open the gate at 20 feet in August.
The 20-foot water is the cleanest of the three choices. Its musty smell rating is 1, the lowest in the whole profile, while the top water rates a 4 and the bottom water rates a 3. It holds 900 algae cells per milliliter, far below the 42,000 cells near the surface. It still has 4.0 mg/L of dissolved oxygen, so it is not the dead water found at the bottom.
The algae are packed near the top because that is where the two things they need overlap. Light is bright at 3 feet, dim at 10 feet, and dark below that, so algae cannot grow in the deep water no matter what else is there. Phosphorus is a nutrient algae need, and there is more of it at the bottom, 0.21 mg/L at 40 feet compared with 0.02 mg/L at the surface. The deep phosphorus does the algae no good because no light reaches it. Near the surface the phosphorus number is low partly because the algae have already taken it up. Where both light and some phosphorus are available, the counts are highest, and that is the top ten feet.
I rejected the 3-foot gate because it draws water straight from the algae layer, which is what causes the musty complaints in the first place. Its smell rating is the highest in the table.
I rejected the 40-foot gate because that water has 0.0 mg/L of dissolved oxygen. Water with no oxygen dissolves iron and manganese out of the mud, which is why the sample came out gray and stained the sink. Its smell rating is 3, and cold alone does not make water clean.
Next August the operator should measure dissolved oxygen at 20 feet every week, because if that layer loses its oxygen too, the middle gate will stop being the safe choice.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No depth named, or the claim is unclear | Names a depth but does not answer why | Names one depth clearly in a single sentence | Names one depth and previews the main reason without listing the entire essay |
| Evidence | Uses no data from the profile | Uses one number, or copies numbers without explaining them | Uses at least three data points and explains what each shows | Uses at least three data points, explains each, and compares the three gates directly |
| Reasoning about algae | No explanation of where algae grow | Says algae grow near the top without using the data | Uses the light column and the phosphorus column to explain where algae grow | Explains the overlap of light and phosphorus and accounts for the low surface phosphorus reading |
| Organization and conventions | Missing paragraphs or ideas out of order | Paragraphs present but the order is hard to follow | Four to six ordered paragraphs, few errors | Clear paragraphs, smooth transitions, precise use of terms such as profile and dissolved oxygen |
Differentiation
Support
Give these students the profile with the smell column and the algae column highlighted, and cross out the 10-foot and 30-foot rows so only the three gate depths remain. Provide the claim starter "The plant should open the gate at ___ feet because ___." Allow three paragraphs, and let the algae paragraph be a labeled drawing of the reservoir showing where light reaches and where the nutrient sits, with two sentences underneath.
Extension
Ask students to write a short paragraph predicting what the profile would look like in November, after the surface cools and the layers mix, and to say which gate they would choose then. A second option is to graph algae counts against depth and to explain in writing what the shape of that graph shows.
ELL supports
Pre-teach "intake," "profile," "layer," "dissolved oxygen," and "phosphorus" with a cross section drawing of the reservoir, the three gates, and arrows showing sunlight entering the top. Provide the column headings in both languages. Allow partner talk through the organizer before drafting, and accept a labeled drawing plus two sentences for the algae paragraph.
Extensions
Have students find out whether the reservoir or lake that supplies their own community has an intake tower with more than one gate, using the utility's annual water quality report or website, and write one paragraph on what the operator there would gain from a choice of depths. Students may also research why some utilities add powdered activated carbon in late summer and explain what problem that treatment step is meant to solve.