Reading passage Grade 9 English Language Arts
Testing the Chemistry of Cobalt Reservoir Before Treatment
A grade 9 informational passage on the daily raw-water testing protocol technicians follow at a fictional reservoir, with questions on procedure and inference.
Objectives
- Students will identify the sequence and purpose of a daily raw-water testing protocol.
- Students will explain why dissolved oxygen, pH, and turbidity readings matter before treatment begins.
- Students will draw an inference about why testing consistency matters as much as any single reading.
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.
- Turbidity
- Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
- Raw water
- Raw water is water taken from a river, lake, or well before it has been treated. It may look clean, but it is not ready to drink yet.
Also: calibration
Teacher overview
A common misconception this passage addresses is that testing water quality means checking a single number against a single limit, when in practice a water system runs several tests in a specific order for specific reasons. This original passage follows the daily raw-water testing protocol technicians use at the fictional Cobalt Reservoir before water ever reaches the treatment plant, tracing why the sequence of tests matters as much as the individual readings. The questions move from locating stated facts to inferring why testing consistency protects the whole system downstream.
Passage
Testing the Chemistry of Cobalt Reservoir Before Treatment
Raw water gets tested before it ever reaches a treatment plant. At Cobalt Reservoir, a fictional water source, technicians test the raw water every morning, well before it reaches the intake pipe leading to the plant. These tests do not check the water against finished-water limits, since raw water is expected to need treatment. Instead, the tests warn plant operators how much treatment that day's water will require.
Technicians run three tests each morning, always in the same order, and that order is not random. Dissolved oxygen comes first. A dissolved oxygen reading begins to change the moment a sample meets air, so technicians use a calibrated probe and take the reading within seconds of collecting the sample, before bubbling or warming can throw the number off. Most fish need dissolved oxygen above 5 mg/L to survive, and a reading below that level can mean more than one thing. It may reflect a warm, still week on the reservoir. Or it may signal that algae or decaying material is using up oxygen faster than it can be replaced, a case a technician treats as an early warning and passes along right away.
pH testing comes second. Technicians calibrate their pH meter each morning using two buffer solutions of known pH, since a meter that drifts out of calibration can report a reading that looks normal but is not. The secondary standard for finished drinking water sets a pH range of 6.5 to 8.5. Raw reservoir water can sit closer to the edges of that range, or briefly outside it, especially after heavy rain washes acidic runoff into the reservoir. A raw pH reading outside the expected range does not mean the water is unsafe. It tells the plant how much to adjust the coagulation and disinfection chemistry that day, so the finished water lands back within range.
Turbidity testing comes last. Turbidity is read from its own sample bottle, filled without stirring up the reservoir bottom and handled gently so that air bubbles do not distort the result. Finished water leaving the plant must stay at or below 0.3 NTU in 95 percent of monthly samples, but raw water entering the plant is often far cloudier, especially in the days after a storm washes soil and debris into the reservoir. A raw turbidity reading well above normal tells operators to add more coagulant and expect to backwash the filters more often that day, since the 0.3 NTU standard has to be met by the finished water, not by the reservoir itself.
Every reading gets logged right away, and any number outside the expected range for that time of year gets flagged for a supervisor to review before the day's treatment plan is finalized. Roughly once a month, two technicians independently sample the same spot on the same morning and compare results. That cross-check exists to catch a miscalibrated instrument or a sampling mistake before it becomes a pattern nobody notices.
The order, the timing, and the cross-checks all exist for the same reason. A single wrong number, treated as correct, could lead a plant to under-treat or over-treat that day's water. A slow dissolved oxygen reading, an uncalibrated pH meter, or a stirred-up turbidity sample would not just be a bad data point. That bad number would feed directly into decisions about how much chemical treatment to apply that day, decisions the rest of the plant's process depends on being right the first time.
Questions
According to the passage, why do technicians test raw reservoir water separately from the water leaving the treatment plant?
According to the passage, in what order do technicians run the three tests each morning? Why does dissolved oxygen come first?
What is the secondary standard range for drinking water pH mentioned in the passage? How does the passage describe raw reservoir pH sometimes differing from that range?
According to the passage, why can a dissolved oxygen reading below 5 mg/L be concerning for more than one reason?
Why do raw turbidity readings after a storm matter to plant operators, even though the 0.3 NTU standard applies only to finished water?
Based on the passage, what is one specific step in the daily testing protocol designed to catch a technician's possible error?
Based on the passage as a whole, why might a single uncalibrated instrument or rushed sample lead to bigger problems later in the treatment process?
Writing task. In one paragraph, explain why the order and consistency of the daily testing protocol matters as much as the individual test results themselves. Use at least two details from the passage.
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: Raw water is tested to give plant operators advance warning of how much treatment that day's water will require, rather than to check it against a finished-water limit, since raw water is expected to still need treatment.
Answer: Dissolved oxygen, then pH, then turbidity. Dissolved oxygen comes first because a reading begins to change the moment a sample is exposed to air, so it must be measured within seconds of collection before the number drifts.
Answer: The secondary standard is 6.5 to 8.5. The passage explains that raw reservoir water can naturally sit closer to the edges of that range, or briefly outside it, especially after heavy rain washes acidic runoff into the reservoir.
Answer: A low dissolved oxygen reading might simply reflect a warm, still week on the reservoir, or it might signal that algae or decaying organic material is consuming oxygen faster than it can be replaced, which is a different and more serious problem.
Answer: A high raw turbidity reading tells operators to add more coagulant and expect to backwash the filters more often that day, since the 0.3 NTU standard must be met by the finished water on the far side of treatment, not by the reservoir water itself.
Answer: Roughly once a month, two technicians independently sample the same spot on the same morning and compare results, a cross-check built into the protocol to catch a miscalibrated instrument or a sampling mistake.
Answer: Accept any answer noting that a wrong reading, treated as correct, would feed directly into decisions about how much chemical treatment to apply that day, meaning the plant could under-treat or over-treat the water based on one bad data point.
Writing task exemplar. The daily testing protocol at Cobalt Reservoir depends on order and consistency as much as on any single number, because each test is timed and sequenced to protect its own accuracy. Dissolved oxygen is measured first because its reading changes within seconds of a sample meeting air, while pH and turbidity depend on a properly calibrated meter and an undisturbed sample. A monthly cross-check, where two technicians independently sample the same spot, exists specifically to catch a mistake before it becomes a pattern, since a single wrong reading treated as correct could lead the treatment plant to under-treat or over-treat that day's water.
Differentiation
Support (below grade level)
Provide a simple numbered flowchart (1. Dissolved oxygen → 2. pH → 3. Turbidity) with a one-phrase reason next to each step, for students to reference while answering question 2. Allow question 7 to be answered as a short spoken response instead of writing.
Extension (above grade level)
Ask students to research, using only the passage, what might happen to the day's treatment plan if the monthly cross-check revealed that one technician's pH readings had been consistently off by half a point, and to explain their reasoning using details from the fifth paragraph.
ELL supports
Pre-teach "dissolved oxygen," "calibrate," "turbidity," and "raw water" with simple definitions and a labeled image of a reservoir and intake pipe before reading. Allow students to answer the literal questions (1, 2, 3, 6) with short phrases rather than full sentences.