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Quiz & test questions Grade 7 Science

Willow Creek Water Quality Monitoring Assessment

A grade 7 science assessment on dissolved oxygen, pH, and sampling design, using data from two fictional Willow Creek monitoring sites to test water-quality reasoning.

Objectives

  1. Students will calculate and interpret the mean dissolved oxygen level from repeated water samples.
  2. Students will explain why taking multiple samples at a site produces a more reliable estimate than a single sample.
  3. Students will use given data to estimate the probability of a future water-quality 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.
pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.
Watershed
A watershed is all the land where rain that falls will drain to the same stream, river, or lake. Everyone lives in a watershed, no matter how far from the water they live.

Teacher overview

This 30-minute assessment (22 points total) checks whether students can interpret water-quality data the way a monitoring team actually collects it: in repeated samples, at more than one location, compared against a known benchmark. It mixes multiple-choice, short-answer, and computation items built around dissolved oxygen and pH readings from a small watershed. Most fish need dissolved oxygen above 5 mg/L to thrive, and public water systems use a pH range of 6.5 to 8.5 as a secondary standard, two real benchmarks this assessment asks students to apply rather than memorize.

Willow Creek Water Quality Monitoring Assessment — Grade 7 Science
Name: ____________________________ Date: ______________

Assessment

A fictional group, the Willow Creek Watershed Alliance, monitors water quality at four sites along Willow Creek, and this assessment uses data from two of them. At Site A, upstream of any farmland, and at Site B, downstream of pastureland, the group collects three dissolved oxygen samples during one visit and a single pH reading. Use this data for all items.

Site A dissolved oxygen samples (mg/L): 9.0, 8.6, 8.8

Site B dissolved oxygen samples (mg/L): 3.2, 3.6, 3.4

Site A pH reading: 7.2

Site B pH reading: 6.1

  1. (2 points) Which gas dissolved in stream water is most directly needed for fish to survive?

    • A) Nitrogen
    • B) Carbon dioxide
    • C) Dissolved oxygen
    • D) Chlorine

  2. (2 points) Most fish need dissolved oxygen above 5 mg/L. A reading well below that level is best described as which of the following for fish in that stretch of stream?

    • A) Ideal
    • B) Stressful or harmful
    • C) Unrelated to fish health
    • D) Only a problem in winter

  3. (3 points) Calculate the mean (average) dissolved oxygen level at Site A from the three samples given.

  4. (3 points) Calculate the mean dissolved oxygen level at Site B from the three samples given, and state whether that mean falls above or below the 5 mg/L level most fish need.

  5. (3 points) The Alliance takes three dissolved oxygen samples at each site instead of just one. Explain why collecting several samples gives a more reliable picture of a site's water quality than a single sample would.

  6. (3 points) Public water systems use a pH range of 6.5 to 8.5 as a secondary standard. Compare Site A's pH reading (7.2) with Site B's pH reading (6.1) against that range, and identify which site's reading falls outside it.

  7. (2 points) Based on the pattern in the data, which explanation is best supported by the difference between Site A and Site B?

    • A) Site B is downstream of pastureland, where animal waste and soil runoff can lower oxygen and pH.
    • B) Site A is downstream of pastureland, so its water should be more affected.
    • C) Dissolved oxygen and pH are not related to land use near a stream.
    • D) The difference is caused by the stream being wider at Site A.

  8. (4 points) Over the past year, the Alliance recorded that 2 of its 10 total visits to Site B measured dissolved oxygen below 5 mg/L. Based on this record, is the probability that a randomly chosen future visit to Site B will record a reading below 5 mg/L closer to 0, 0.2, 0.5, or 1? Explain your choice.

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: C) Dissolved oxygen. Fish take in dissolved oxygen through their gills. Carbon dioxide is a waste gas fish give off, nitrogen is not used for breathing, and chlorine is a disinfectant added to drinking water.
  2. Answer: B) Stressful or harmful. The assessment states that most fish need dissolved oxygen above 5 mg/L, so a reading well below that level puts fish under stress.
  3. Answer: 8.8 mg/L. (9.0 + 8.6 + 8.8) ÷ 3 = 26.4 ÷ 3 = 8.8 mg/L.
  4. Answer: 3.4 mg/L. (3.2 + 3.6 + 3.4) ÷ 3 = 10.2 ÷ 3 = 3.4 mg/L, a level below the 5 mg/L most fish need to thrive.
  5. Answer: A single sample can be unusually high or low just by chance, so it may not represent what the water is really like at that site. Taking three samples and averaging them reduces the effect of any one unusual reading and gives a more dependable estimate of typical conditions.
  6. Answer: Site B. Its pH reading of 6.1 falls below the 6.5-8.5 range, while Site A's reading of 7.2 falls within it.
  7. Answer: A) Site B is downstream of pastureland, where animal waste and soil runoff can lower oxygen and pH. Runoff carrying waste and sediment can add material that consumes oxygen as it decomposes and can shift a stream's pH.
  8. Answer: 0.2. Two out of ten visits recorded a low reading, so 2/10 = 0.2 is the fraction of past visits that matched that outcome, making 0.2 the best estimate of the probability of it happening again, closer to 0 than to 0.5 or 1.

Differentiation

Support (below grade level)

Provide the addition already set up for items 3 and 4 (9.0 + 8.6 + 8.8 = ___, then ___ ÷ 3 = ___) so students complete only the final division step, and allow a calculator for all computation items. For item 8, provide a simple fraction strip showing 2 out of 10 shaded to support the probability estimate.

Extension (above grade level)

Ask students to calculate the range (difference between the highest and lowest reading) at each site and explain what a small range versus a large range might suggest about how stable a site's water quality is from sample to sample.

ELL supports

Pre-teach the words dissolved, sample, and watershed with a simple diagram showing a stream, a bottle collecting a sample, and small bubbles labeled "oxygen." Allow item 5 and item 7 to be answered with a labeled diagram or a short spoken response in addition to writing.

Extensions

Have students design a simple monitoring schedule for a real local stream or pond, deciding how many sites, how many samples per site, and how often they would test if the Alliance's model, three samples per site, were applied to a place they know.