Lesson plan Grade 9 Science
Bickerstaff Lake Swim Advisory: Reading Bacteria Counts
A 50-minute grade 9 lesson where students summarize bacteria sample data from two fictional beaches and decide whether the county should post a swim advisory.
Objectives
- Students will calculate the mean, median, and range of a set of bacteria sample results.
- Students will apply a stated advisory rule to sample data and justify the decision.
- Students will explain how stormwater runoff raises bacteria counts at a lake beach.
Vocabulary
Key words: colony-forming unit, advisory, outfall, runoff, median, range
Teacher overview
County health departments close swim beaches when bacteria counts rise, and the counts usually rise after rain, because runoff carries animal waste and soil off the land and into the water. This lesson uses the fictional town of Lowmoss and its beaches on Bickerstaff Lake so students can summarize a real-looking set of laboratory results and apply a posted advisory rule to them. A common misconception is that a single high result proves the water is unsafe on every day of the season; the advisory rule in this lesson uses both a single-sample limit and a five-sample average, which lets students see why one spike and a sustained problem are treated differently. Students also test the explanation that the storm drain, rather than the lake itself, is the source of the difference between the two beaches.
Procedure
Hook (5 min): Ask students who has seen a beach or pool closed for water quality, and what they think the closure was based on. Record two or three guesses on the board. Tell them that a county laboratory counts bacteria colonies grown from a measured volume of water, and reports the result in colony-forming units per 100 milliliters.
Direct instruction (12 min): Explain that a colony-forming unit, or CFU, represents one bacterium that grew into a visible colony on a lab plate. Higher counts mean more fecal contamination reached the water. Model the arithmetic on the board with a short practice set that is not on the handout: samples of 30, 50, 90, 110, and 220 CFU per 100 mL. Show the mean as (30 + 50 + 90 + 110 + 220) divided by 5, which equals 100, and the median as the middle value of the sorted list, which is 90. Point out that the one large value pulls the mean above the median, and ask why a health officer might want to see both numbers.
Guided practice (13 min): Distribute the handout for the fictional Lowmoss beaches. Work item 1 together so students see how to total five values before dividing. Then have pairs finish items 2 through 5. Circulate and check that students sort the values before choosing a median, and that they subtract the smallest from the largest for the range.
Independent practice (12 min): Students complete items 6 through 8 on their own. Item 8 asks for an explanation rather than a calculation, so remind them to point to specific numbers from the table in their answer.
Closure and exit ticket (8 min): Formative check. On an index card, each student answers this question: "A beach's five most recent results are 40, 60, 80, 100, and 320 CFU per 100 mL. What is the mean, and is it above the fictional five-sample limit of 126?" Collect the cards and scan them. The mean is 120, which is below 126. If more than about a quarter of the class divides by four instead of five, reteach that step at the start of the next class.
Student Handout
The fictional town of Lowmoss runs two swim beaches on Bickerstaff Lake. North Beach sits about 60 feet from a storm drain outfall that carries runoff from a parking lot and a lawn area. South Beach sits on the far side of a wooded point, with no storm drain nearby. The county lab reports results in colony-forming units per 100 milliliters, written CFU per 100 mL.
Under the fictional county beach rule, the health officer posts an advisory when a single sample is above 235 CFU per 100 mL, or when the mean of the five most recent samples is above 126 CFU per 100 mL.
Here are the five most recent results from each beach. The lab collected all ten samples on the same five days.
| Sample day | North Beach | South Beach |
|---|---|---|
| Day 1 (dry) | 18 | 12 |
| Day 2 (dry) | 42 | 20 |
| Day 3 (1.4 inches of rain) | 380 | 30 |
| Day 4 (dry) | 96 | 44 |
| Day 5 (dry) | 64 | 24 |
- Find the mean of the five North Beach results, in CFU per 100 mL.
- Find the median of the five North Beach results, in CFU per 100 mL.
- Find the mean of the five South Beach results, in CFU per 100 mL.
- Find the range of the five North Beach results, in CFU per 100 mL.
- Find the range of the five South Beach results, in CFU per 100 mL.
- How many of the ten samples in the table are above the single-sample limit of 235 CFU per 100 mL?
- Using the five-sample means you found in items 1 and 3, does either beach exceed the mean limit of 126 CFU per 100 mL? Name the beach or state that neither does.
- The county is deciding whether to reroute the storm drain outfall away from North Beach. Use at least two numbers from the table to explain whether the data support that change.
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: 120 CFU per 100 mL. The five North Beach results total 18 + 42 + 380 + 96 + 64 = 600, and 600 divided by 5 is 120.
- Answer: 64 CFU per 100 mL. Sorted, the North Beach results are 18, 42, 64, 96, 380, and the middle value of five sorted values is 64.
- Answer: 26 CFU per 100 mL. The five South Beach results total 12 + 20 + 30 + 44 + 24 = 130, and 130 divided by 5 is 26.
- Answer: 362 CFU per 100 mL. The largest North Beach result is 380 and the smallest is 18, so 380 minus 18 is 362.
- Answer: 32 CFU per 100 mL. The largest South Beach result is 44 and the smallest is 12, so 44 minus 12 is 32.
- Answer: 1 sample. Only the North Beach result of 380 CFU per 100 mL on Day 3 is above 235; every other value in the table is below it.
- Answer: Neither beach exceeds the mean limit. North Beach averages 120 and South Beach averages 26, and both are below 126 CFU per 100 mL.
- Answer: The data support rerouting the outfall. On the four dry days the two beaches are fairly close, with North Beach at 18, 42, 96, and 64 against South Beach at 12, 20, 44, and 24. On the rainy Day 3, North Beach jumped to 380 while South Beach only reached 30. The beach next to the outfall spikes on the day it rains and the beach away from it does not, which points to the storm drain rather than the lake as the source.
Differentiation
Support
Give students a version of the table with the North Beach values already sorted from smallest to largest, and provide a worked frame for the mean that shows the sum written out with blanks for each addend. Allow calculator use throughout, and let students answer item 8 in two sentences rather than a paragraph.
Extension
Ask students to calculate what the North Beach mean would be if the Day 3 result were removed from the set, then explain in writing why a health officer would not simply throw out a high result. Students can also propose one additional measurement, such as sampling one day after each rain, that would test the storm-drain explanation more directly.
ELL supports
Pre-teach "sample," "mean," "median," "range," and "outfall" using a labeled sketch of a parking lot, a pipe, and a beach. Provide sentence frames for item 8: "On dry days, North Beach measured ___ and South Beach measured ___." and "After rain, North Beach measured ___, which suggests that ___."
Extensions
Have students graph the five North Beach results and the five South Beach results on the same set of axes, with sample day on the horizontal axis, then mark the rainy day. Ask them to describe in writing what the two lines show that the mean by itself hides. Students who finish early can work out, using only the fictional data in the table, how many additional dry-day samples it would take to pull the North Beach mean below 100.