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Reading passage Grade 8 Science

Why Hallam Reservoir Tasted Like Dirt in August

A grade 8 science reading passage about a summer algae bloom that made a fictional town's tap water taste earthy, and how the plant treated the symptom and the cause.

Objectives

  1. Students will explain how nutrients and warm water combine to produce an algae bloom in a reservoir.
  2. Students will distinguish between a taste problem and a safety problem in drinking water.
  3. Students will compare treating the symptom of a bloom with treating its cause.

Vocabulary

Activated carbon
Activated carbon is a special charcoal full of microscopic holes that grab and hold chemicals as water flows past. It is the same material used in many home water filter pitchers.

Also: cyanobacteria, bloom, nutrient, phosphorus, intake, buffer strip

Teacher overview

This passage follows a fictional town through a summer taste-and-odor event, a problem that many real utilities face every warm season. It is useful for teaching the difference between water that is unsafe and water that is unpleasant, and for showing how a nutrient problem on land turns into a taste problem at the tap. Pair it with a unit on nutrients, lake ecology, or drinking water treatment.

Why Hallam Reservoir Tasted Like Dirt in August — Grade 8 Science
Name: ____________________________ Date: ______________

Passage

The first calls came to the water office in Hallam Springs on a Monday in late July. A woman on Weir Street said her tap water smelled like a garden after rain. By Wednesday, the office had taken forty more calls. People used the same words again and again: earthy, musty, like dirt.

Rosa Delgado, the plant's lead operator, did what operators are trained to do. She pulled samples from the treated water and sent them to the lab. She checked the chlorine level in the pipes across town, and she read the pressure records for any sign of a break. Everything came back normal. The bacteria tests were clean, and the water met every standard the state sets, yet it still tasted bad.

The answer was waiting in the reservoir. Hallam Reservoir sits in a shallow valley, and by late July its top few feet of water had warmed to bath temperature. Spring rains had carried fertilizer from lawns, manure from a pasture upstream, and leakage from a few old septic systems into the feeder creek. Those sources all deliver the same thing: phosphorus, the nutrient that limits how much algae can grow in most freshwater lakes.

Warm water and extra phosphorus are exactly what cyanobacteria need. These microscopic organisms live in almost every lake in small numbers. Give them heat, sunlight, and nutrients, and their population can double in a day or two. Within two weeks the reservoir's surface carried a green tint, and satellite images showed streaks of it drifting toward the plant intake.

Some kinds of cyanobacteria release two compounds called geosmin and MIB as they grow and die. Neither compound is harmful in the amounts a reservoir produces. The trouble is that the human nose and tongue detect both of them at concentrations far below the level any health standard would set. Some kinds of cyanobacteria can also release toxins, so the lab tested the treated water for them as well and found none. That is why Delgado could tell residents that the water was safe while agreeing with them that it did not taste the way it should.

Her first response treated the symptom. The plant began feeding powdered activated carbon into the water as it entered the treatment train. Activated carbon is riddled with tiny pores, and molecules like geosmin stick to those surfaces and travel out with the settled sludge. Delgado also switched the reservoir intake from the shallow port to a deeper one, drawing water from below the bloom instead of through it. Within ten days the complaints dropped off.

Neither fix touched the reason the bloom happened. Carbon costs money every day it runs, and a deeper intake only helps while the bloom stays near the surface. So the town began working on the watershed instead. The county paid a farmer to fence livestock away from the creek and to plant a strip of grass and shrubs along the bank, which slows runoff and traps phosphorus before it reaches the water. The town changed its own fertilizer schedule at the parks and offered residents a soil test so they would stop applying phosphorus their lawns did not need. Three homeowners with failing septic systems got low-interest loans to replace them.

Delgado explains the difference to residents this way. The carbon feed handles the problem that is already in the water, and it will be ready again the next time a bloom drifts toward the intake. The watershed work reduces the phosphorus that starts the bloom, which is the only change that can make next August easier than this one.

Questions

  1. What did residents of Hallam Springs report about their tap water in late July?
  2. What two steps did Rosa Delgado take at the plant to remove the taste and odor?
  3. The passage says cyanobacteria live in almost every lake. Explain why their population grew so quickly in Hallam Reservoir that summer.
  4. The water met every state standard and still tasted bad. Explain how both statements can be true, using what the passage says about geosmin and MIB.
  5. What did the county and the town do in the watershed, and what problem were those actions aimed at?
  6. Trace the path phosphorus took from the land to the taste in a resident's glass. Cite the details from the passage that support each step.
  7. A nearby town draws its water from a fast, cold, shaded stream instead of a shallow reservoir. Would you expect that town to have the same summer taste problem? Use evidence from the passage to explain.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: The water smelled and tasted earthy or musty, like dirt.
    The passage opens with a caller describing a garden after rain, and it says people used the words earthy, musty, and like dirt over and over.
  2. Answer: She fed powdered activated carbon and switched to the deeper intake.
    The passage explains that geosmin sticks to the pores in activated carbon and leaves with the settled sludge, and that drawing water from a deeper port avoided the bloom near the surface.
  3. Answer: Warm surface water, sunlight, and phosphorus from runoff let the population double quickly.
    The passage says the top few feet had warmed to bath temperature and that spring rains carried phosphorus from fertilizer, manure, and septic leakage into the feeder creek, which is exactly what cyanobacteria need.
  4. Answer: The compounds are detectable by taste far below any level that would be harmful.
    The passage states that neither geosmin nor MIB is harmful in the amounts a reservoir produces, but that people can detect both at concentrations far below any health standard, so the water was safe and unpleasant at the same time.
  5. Answer: They reduced the phosphorus reaching the reservoir, which was the cause of the bloom.
    The county paid for livestock fencing and a planted buffer strip along the creek, the town changed its park fertilizer schedule and offered soil tests, and three failing septic systems were replaced.
  6. Answer: Fertilizer, manure, and septic leakage to the creek, then to the reservoir, feeding cyanobacteria that release geosmin.
    Accept any answer that follows the chain the passage lays out: spring rains washed phosphorus off lawns, a pasture, and old septic systems into the feeder creek; the phosphorus reached the warm reservoir; the cyanobacteria population grew quickly; the growing and dying cells released geosmin and MIB; and those compounds reached the tap because the plant draws from that reservoir.
  7. Answer: No, because the conditions the bloom needed are missing.
    The passage ties the bloom to warm, still, sunlit surface water plus phosphorus from the watershed. A fast, cold, shaded stream gives cyanobacteria neither the warmth nor the still water they need, so the same taste problem would be unlikely. Accept answers that note a shaded stream could still carry nutrients and cause other problems downstream.

Differentiation

Support

Read the first three paragraphs aloud while students follow along, pausing to mark where the problem is described and where the investigation begins. Provide a two-column organizer labeled "symptom" and "cause" for students to fill in as they read the last three paragraphs.

Extension

Ask students to write the short notice Rosa Delgado would mail to customers, explaining in plain language why the water tasted bad, why it was safe, and what the town is doing about the watershed. Require at least three specific details from the passage.

ELL supports

Pre-teach "bloom," "nutrient," "intake," and "buffer strip" with a labeled drawing of the reservoir, the creek, and the plant. Provide sentence frames such as "The cyanobacteria grew because ___ and ___." Allow students to answer question 6 by drawing and labeling the path before writing it in sentences.

Extensions

Have students find their own utility's yearly water quality report and look for any mention of taste and odor, algae, or source water protection, then write a paragraph comparing what they find with the choices Hallam Springs made.