Writing prompt Grade 12 English Language Arts
Why Bidwell Creek's Chloride Keeps Rising: An Explanatory Prompt
A grade 12 explanatory writing prompt: students explain to a fictional watershed council why creek chloride rises while the town spreads less road salt each winter.
Objectives
- Students will explain a delayed cause without implying that the delay disproves the cause.
- Students will define a technical term for a lay audience at the point where the reader needs it.
- Students will organize an explanation so each paragraph depends on the one before it.
Vocabulary
- Baseflow
- Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.
Also: chloride, dissolved, shallow groundwater, conventional treatment, controlling idea
Teacher overview
Explaining a lagged cause is one of the harder tasks in informative writing, because the audience's own reasoning works against the explanation. Readers who learn that a town cut its road salt by 40 percent and that creek chloride went up anyway will conclude that salt is not the cause. This prompt asks students to write the article that keeps those readers reading, using a fictional town's monitoring record. The writing problem is one of order: the explanation only works if the reader understands what dissolved chloride does before being shown the numbers that seem to contradict the cause.
Prompt
Background: the Bidwell Creek briefing sheet
Stanwyck is a fictional town, and Bidwell Creek and the Bidwell Creek Watershed Council are invented for this task. All figures below come from the fictional council's own monitoring program.
Bidwell Creek runs through Stanwyck and past the town's drinking water intake. For twelve years the council has sampled the creek every summer at the Ashford Road gauge, when the creek is running on groundwater rather than on rain or snowmelt. Over the same period the town has cut how much salt it spreads on winter roads, using calibrated spreaders, brine pre-treatment, and a plow route redesign.
| Year | Road salt applied that winter | Median summer chloride at the gauge |
|---|---|---|
| 2016 | 2,400 tons | 41 mg/L |
| 2019 | 2,050 tons | 58 mg/L |
| 2022 | 1,700 tons | 76 mg/L |
| 2025 | 1,450 tons | 88 mg/L |
What the council knows about chloride. Road salt is mostly sodium chloride. Once it dissolves, chloride stays dissolved. It does not break down, settle out, evaporate with the water, or get taken up by plants in any meaningful amount. It travels wherever the water travels, and it stays until it is diluted or flushed out to sea.
Where the summer water comes from. In summer, Bidwell Creek is fed mainly by baseflow, which is shallow groundwater seeping into the channel. Water that soaked into the ground after a winter storm may not reach the creek for months or years. The council's hydrologist estimates that some of the water reaching the creek in a dry August fell as snow three to seven winters earlier.
What the town's treatment plant does. Stanwyck's drinking water plant is a conventional plant: it uses coagulation, sedimentation, filtration, and disinfection. Those steps remove particles and pathogens. None of them removes a dissolved salt. Membrane treatment can remove chloride, but it is a much larger energy investment; seawater reverse osmosis uses roughly 11 to 15 kWh per 1,000 gallons (about 3 to 4 kWh per cubic meter), against about 1 to 2 kWh per 1,000 gallons for conventional treatment and delivery.
What the council has been asked. At the council's last meeting, three members said the data prove that road salt is not the source, since the town keeps spreading less of it. The council's chair has asked for a newsletter article that explains what is actually happening.
Your task
Write an explanatory article of 700 to 900 words for the Bidwell Creek Watershed Council newsletter. Explain why summer chloride in the creek has risen while winter salt application has fallen, what the town's treatment plant can and cannot do about it, and what the trend means for the next decade. Your readers are attentive volunteers who are not chemists. Define any technical term at the point where a reader first needs it. Do not argue for a policy; explain the situation so that a reader could form a view.
Planning Organizer
Complete every box before drafting. Together, the boxes cover every row of the rubric.
| Box | What goes here | My notes |
|---|---|---|
| Controlling idea | One sentence stating what the article will explain. It must account for both trends, not just one. | |
| Reader's starting belief | The conclusion your readers have already drawn, stated in their words. | |
| Element 1 | What chloride does once it dissolves, and the two or three sentences a reader needs before any data will make sense. | |
| Element 2 | Where the creek's summer water comes from, and the time lag that follows from it. | |
| Element 3 | What the data show when Elements 1 and 2 are applied to them. | |
| Element 4 | What conventional treatment does and does not remove, and why membranes are a different scale of decision. | |
| Definitions | The terms you will define, and the sentence in which each definition will appear. | |
| Closing | What the trend implies for the next decade, stated without recommending a policy. |
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
Bidwell Creek carried more chloride last summer than it has in any year the council has measured, and Stanwyck spread less road salt last winter than in any year on record. Both statements are true, and the second does not disprove the first. What connects them is time, and the way a dissolved salt moves through the ground.
Start with the salt itself. Road salt is mostly sodium chloride, and when it dissolves in meltwater the chloride separates and stays dissolved. It does not break down into something else. It does not settle to the bottom, evaporate with the water, or wash out of the system in any single storm. Whatever chloride entered the ground under Stanwyck in the past twenty years is still chloride, and it is still moving with whatever water it entered.
Where it is moving is the second half of the explanation. The council samples the creek in summer on purpose, because a summer creek is not carrying runoff. It is carrying baseflow, the shallow groundwater that seeps steadily into the channel between storms. Groundwater is slow. The council's hydrologist estimates that some of the water reaching the creek in a dry August fell as snow three to seven winters earlier. When the council measures chloride in August, it is not measuring last winter's salt. It is measuring the salt from a set of winters that ended years ago, when the town was spreading more salt than it does now.
With those two ideas in place, the numbers stop looking contradictory. Application fell from 2,400 tons in 2016 to 1,450 tons in 2025, a reduction of about 40 percent. Median summer chloride at the Ashford Road gauge rose across the same period from 41 mg/L to 88 mg/L, more than doubling. The rising line is the arrival of older salt. The falling line is a decision that has not reached the creek yet. A reduction that began in the past decade will show up in summer baseflow over the decade ahead, which is why the council should expect the two lines to keep moving in opposite directions for some time before they converge.
That leaves the question of what happens at the treatment plant, and the answer is less comfortable. Stanwyck's plant is a conventional one. Coagulation gathers fine particles into clumps, sedimentation drops them out, filtration catches what remains, and disinfection handles pathogens. Every one of those steps works on something that can be separated from water as a solid or destroyed. Chloride is neither. It passes through the plant unchanged, at whatever concentration the creek delivers it.
Removing chloride requires membranes, which push water through a barrier fine enough to hold back dissolved salts. That technology exists and is used at scale, but it is a different order of commitment. Seawater reverse osmosis runs at roughly 11 to 15 kWh per 1,000 gallons (about 3 to 4 kWh per cubic meter), against about 1 to 2 kWh per 1,000 gallons for conventional treatment and delivery, and it produces a concentrated waste stream that has to go somewhere. Adding membranes is therefore a major decision for any plant.
For the next decade, then, the council should read its own data with the lag in mind. A rising summer chloride number does not mean the town's winter program failed, and a year of lower numbers would not by itself mean it succeeded. The measurement that would actually test the program is the trend in the youngest water, the chloride the creek carries during and just after a winter storm, which reflects the salt spread that season. Summer baseflow answers a different question, one about the past. Both are worth measuring, and confusing the two is what turned a straightforward record into an argument.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Controlling idea and focus | Restates the topic without stating what will be explained. | States a controlling idea that accounts for one trend but not both. | States a controlling idea that accounts for both trends and governs the article. | Frames the controlling idea around the reader's mistaken inference and keeps returning to it. |
| Accuracy and selection of information | Uses briefing figures inaccurately or omits the ones the explanation needs. | Reports the figures accurately but includes material the explanation does not use. | Selects the figures the explanation requires and reports each accurately with its unit. | Selects figures for what they prove, and says what each one cannot show. |
| Organization and building of ideas | Paragraphs could be reordered without loss, because none depends on another. | Some sequence is visible, but the data arrive before the reader has the ground for them. | Each paragraph depends on the one before it, and the data arrive after the explanation. | Sequence is controlled throughout, and the closing reframes the record rather than summarizing it. |
| Audience and precision of language | Technical terms are used without definition or replaced by vague wording. | Terms are defined, but in a glossary block rather than where they are needed. | Each term is defined at the point of first need, in plain language, without losing accuracy. | Definitions do double duty, carrying the explanation forward while they define. |
Differentiation
Support
Give these students the briefing sheet with the two trends already graphed on a shared time axis and the four elements of the organizer pre-labeled in the order the article should follow. Reduce the length requirement to 500 words, and provide the opening sentence of the exemplar's second paragraph as a model for how to begin an element with the reader's need rather than with a fact.
Extension
Ask students to write the 150-word sidebar the fictional newsletter would run beside the article, aimed at a reader who wants to know what the council should measure next. The sidebar must not repeat any sentence from the article and must name one measurement, one reason, and one limitation of that measurement.
ELL supports
Pre-teach "dissolved," "baseflow," "lag," and "conventional" with a simple diagram showing a winter storm, the ground, and a summer creek, and post it for the whole task. Provide sentence frames for the causal chain, such as "Because chloride stays dissolved, it ..." and "The water reaching the creek in summer fell as snow ...". Let students draft Elements 1 and 2 orally with a partner before writing.
Extensions
Have students exchange articles and mark, in the margin of a partner's draft, the exact sentence at which a skeptical council member would be convinced. Students then revise so that sentence arrives at least one paragraph earlier, and write a two-sentence note explaining what they had to move and what they had to cut to make room for it.