Writing prompt Grade 9 Science
Should Vanbrough Chlorinate Its Wells? An Argument Prompt
A grade 9 science argument prompt where students take a position on whether a fictional groundwater town should begin chlorinating its three deep wells.
Objectives
- Students will state a defensible claim about whether a groundwater system should disinfect.
- Students will support the claim with evidence about pathogens, residual, and byproducts.
- Students will address a counterclaim and explain why their evidence outweighs it.
Vocabulary
- Disinfection
- Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.
- Chlorine residual
- Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.
- Confined aquifer
- A confined aquifer is an underground layer of sand, gravel, or rock filled with water and trapped between two layers of rock or clay that water cannot pass through easily. Because the water is squeezed under pressure, wells drilled into a confined aquifer can sometimes flow upward on their own.
Also: coliform, byproduct, counterclaim
Teacher overview
Not every public water system in the United States adds chlorine. Some small systems that draw from deep, protected groundwater have historically operated without continuous disinfection, and the question of whether they should keep doing so is a live one for their boards. This prompt gives students a fictional town in exactly that position, with a clean sampling record, two recent incidents, and a stated cost attached to either choice. Students take a position and defend it with technical evidence rather than preference.
Prompt
Background. Vanbrough is a fictional town of about 4,200 people. It gets all of its drinking water from three deep wells that draw from a confined aquifer, which means a layer of clay sits between the aquifer and the surface. The town has never added chlorine. Under its state's rules, a groundwater system can operate without continuous disinfection if it meets a set of conditions, including a long record of clean bacteria samples.
Vanbrough's record has been clean for thirty-eight years. Then two things happened last year. First, a storm knocked out power for eleven hours, and pressure in the distribution system dropped low enough that the state required the town to issue a precautionary boil notice. Second, one routine sample from a hydrant on Marlow Street came back positive for total coliform bacteria. The follow-up samples were clean.
Here is what the town's operators and its engineer have told the council.
Adding chlorine would put a disinfectant residual in the pipes. A residual is a small amount of chlorine that stays in the water all the way to the tap. Systems that carry a residual usually hold it between 0.2 and 2.0 mg/L, and the maximum residual disinfectant level for chlorine is 4.0 mg/L. A residual does two jobs. It kills organisms that enter the system through a break or a backflow, and it acts as a warning signal, because a sudden drop in residual tells operators that something has gone wrong in that part of the system.
Chlorine also has costs. When chlorine meets natural organic matter in water, it forms disinfection byproducts, and some of those are regulated because of long-term health concerns. Deep groundwater usually carries little organic matter, so byproduct formation in Vanbrough would likely be low, though the town would have to sample for it. Some residents already say they dislike the taste of chlorinated water. A fictional engineering estimate puts the equipment at about $180,000 to install, plus roughly $14,000 each year for chemicals, testing, and the extra operator hours.
The council's other option is to spend the same money on prevention instead: a backup generator at each well so pressure never drops, an expanded cross-connection control program, and a wellhead protection plan that limits certain land uses above the aquifer.
Your task. Write an argument addressed to the fictional Vanbrough town council in which you claim whether the town should begin chlorinating its wells or should not. Support your claim with at least two pieces of evidence from the background above. Address at least one counterclaim that the other side would raise, and explain why your evidence outweighs it. Close with a conclusion that tells the council what to do. Write four to six paragraphs in formal English.
Planning Organizer
Fill in every row before you begin drafting. Each row connects to one of the rubric criteria your teacher will use.
| Part of the argument | What to write here | My notes |
|---|---|---|
| Claim | One sentence stating what Vanbrough should do. | |
| Reason 1 | The first reason your claim is correct. | |
| Evidence for reason 1 | A specific detail from the background, such as the coliform sample, the pressure loss, the byproduct issue, or a cost figure. | |
| Reason 2 | A second, different reason. | |
| Evidence for reason 2 | A specific detail from the background that is not the one you already used. | |
| Counterclaim | The strongest point the other side would make. State it fairly and completely. | |
| Rebuttal | Why your evidence outweighs that point. Name the evidence again. | |
| Conclusion | What you want the council to do, and what you want them to measure or watch afterward. |
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
Vanbrough should begin chlorinating its three wells. The town's record is good, but a good record describes the past, and the events of last year show that the system can be exposed in ways the record does not cover.
My first reason is that the town has already had a day when its water was not protected. When the storm cut power for eleven hours, pressure dropped far enough that the state required a boil notice. Low pressure matters because water can be pulled into a pipe through a leak or a cross connection when the pressure inside falls. On that day, Vanbrough had nothing in the water to kill anything that got in. A residual between 0.2 and 2.0 mg/L would have given the system a defense while the power was out.
My second reason is that chlorine gives operators information they cannot get any other way. The positive coliform sample on Marlow Street was followed by clean samples, so the town never learned what happened. If Vanbrough carried a residual, operators could measure it at that hydrant in a few minutes. A drop in residual would point them to the part of the system with a problem, instead of leaving them to guess.
Some residents argue that chlorine creates disinfection byproducts, and that the town would be trading one health risk for another. That concern is reasonable, and byproducts are regulated for good reason. But the background explains that byproducts form when chlorine meets natural organic matter, and deep groundwater from a confined aquifer usually carries very little of it. Vanbrough would sample for byproducts and would almost certainly find low levels. A low, measurable risk that the town can monitor is a better trade than an unmeasured risk during the next outage.
Cost is the other objection, and the fictional estimate of about $14,000 each year is real money for a town of 4,200. Still, the generators, the cross-connection program, and the wellhead plan the council is considering all reduce the chance that contamination enters the system. None of them does anything once contamination has already reached the pipes. Chlorination is the only option on the table that works after something goes wrong.
The council should approve chlorination at all three wells, set a target residual near the low end of the usual range, and begin quarterly byproduct sampling in the first year. If byproduct levels stay low and taste complaints stay rare, the decision will have cost the town little. If the next storm takes the power out again, the town will be glad the residual was already there.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No clear position, or the position shifts | States a position but leaves it vague | States a clear, specific position in one sentence | States a clear position and previews the reasons that will support it |
| Evidence and reasoning | Uses opinion with no evidence from the background | Uses one detail from the background without explaining it | Uses at least two distinct details and explains how each supports the claim | Uses several details, explains each, and connects them to the underlying science of residual or byproducts |
| Counterclaim and rebuttal | No counterclaim appears | Names a counterclaim but does not answer it | States a counterclaim fairly and answers it with evidence | States the strongest counterclaim fairly and shows why the evidence still favors the claim |
| Organization and conventions | Ideas are hard to follow; frequent errors interfere | Some structure; errors distract | Clear paragraphs and few errors | Purposeful structure, controlled formal tone, and a conclusion that names a follow-up measurement |
Differentiation
Support
Give these students the claim sentence written two ways, one for each side, and let them choose. Provide the organizer with the counterclaim row already filled in, and reduce the requirement to three paragraphs. Highlight the four sentences in the background that carry the most usable evidence.
Extension
Ask these students to write a second, shorter argument for the opposite position, then a paragraph explaining which of the two they found harder to defend and why. They can also propose a decision rule the fictional council could adopt, such as a level of evidence that would trigger disinfection automatically.
ELL supports
Pre-teach "residual," "byproduct," "coliform," and "counterclaim" with short definitions and a diagram of a well, a pipe, and a tap. Provide sentence frames: "Vanbrough should ___ because ___." and "Some residents say ___, but the background shows ___." Allow a first draft in the student's stronger language before translation.
Extensions
Have students exchange drafts and mark, on their partner's paper, the exact sentence where the counterclaim begins and the exact sentence where the rebuttal begins. Partners who cannot find one of the two return the draft with that note. Follow with a five-minute revision window before collecting the final version.