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Classroom activity Grade 8 Science

Hollowmere Tabletop Watershed: Point and Nonpoint Pollution Sources

Grade 8 groups compare pipe and runoff pollution on a paper watershed, classify six sources as point or nonpoint, and weigh permits with a phosphorus budget.

Objectives

  1. Students will use a tabletop model to compare how pollution from a pipe outfall and pollution spread across the land reach a stream in dry weather and during rain.
  2. Students will classify watershed pollution sources as point or nonpoint and justify each classification by how the pollution enters the water.
  3. Students will use a fictional phosphorus budget to explain why permit limits on pipes cannot by themselves solve a nonpoint pollution problem.
  4. Students will design a way to reduce and to monitor pollution from one point source and one nonpoint source.

Vocabulary

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.

Also: runoff, point source, nonpoint source, outfall, discharge permit, phosphorus

Teacher overview

This is a model-and-analysis activity for groups of three or four. Each group builds a crumpled-paper model of the land around the fictional town of Hollowmere and marks two kinds of pollution on it. Red dots placed right at the stream bank stand for pipe outfalls, the end of a pipe where a facility releases water, and they are point sources. Blue stippling spread across whole hillsides stands for fertilizer on fields and lawns, which is nonpoint pollution. Groups first run a "dry day," in which only the pipes flow, and then a storm, and they record when each color reaches the stream and at how many separate places it enters. The difference they observe, one known spot that flows in any weather compared with many scattered entry points that appear only when it rains, is the same difference that makes point sources straightforward to permit and test while nonpoint runoff is much harder to control. Students then classify six Hollowmere sources, work with a fictional phosphorus budget for Hollowmere Lake, and design a reduction and monitoring plan.

Teacher background. Under the federal Clean Water Act, a facility that releases pollutants into a stream or lake through a pipe, ditch, or other single, discrete channel must hold a discharge permit. The permit limits what the outfall may release, and the facility must sample its discharge and report the results. Farm-field runoff is excluded from the law's definition of a point source, and most runoff that reaches water across a wide area is addressed through state programs, grants, and voluntary practices rather than through permit limits. City storm sewers that empty through outfall pipes fall in between: each outfall is treated as a point source, and many cities hold stormwater permits, yet the pollution in the pipe comes from thousands of streets, roofs, and yards, so these permits usually require the city to carry out a pollution-reduction program rather than meet a fixed limit at each pipe. Source E in the Student Handout is built around this case.

Activity Directions

  1. Before class, build one test landscape following steps 3 and 4, then run steps 5 and 6 on it. Confirm that 5 drops of water on a red dot sends a thin line of color into the nearest crease, and that 10 sprays from about 20 cm make the blue stippling run downhill. If the color barely moves, plan on a second set of 10 sprays in step 6. Most school-brand washable markers run well; permanent and "ultra-clean" markers do not.
  2. Introduction (4 minutes). Group students in threes or fours and explain what each part of the model stands for. The crumpled paper is the land around Hollowmere, the low creases are streams, and the lowest edge of the tray is Hollowmere Lake. Red marks a pollutant released through a pipe. Blue marks a pollutant spread across the land surface. The dropper is the pipe's own flow, which runs whether or not it rains, and the spray is rain.
  3. Build and map (6 minutes). Each group gently crumples its sheet, opens it partway so it keeps clear ridges and valleys, and lays it in the tray. In pencil, they trace the main channel, label it "Hollow Creek," and write "Hollowmere Lake" at the low end where the creek leaves the paper.
  4. Mark the sources (5 minutes). Groups mark four of the six sources from the Hollowmere source cards in the Student Handout. For Sources A and B, they draw a short pencil line for the pipe and then a red dot about the size of a pencil eraser at the pipe's end, touching the edge of Hollow Creek. For Sources C and D, they choose a slope that drains toward Hollow Creek, at least 3 cm from the creek, outline a patch about 5 cm by 5 cm, and fill it with about 25 small blue dots spread evenly through the patch. Groups label each source with its letter in pencil.
  5. Dry day (3 minutes). Groups write a one-sentence prediction in Part A of the handout. Then one student places 5 drops of water from the dropper directly on each red dot. Do not spray. Groups watch for one minute and record in the Part A table which colors moved and how many separate places each color entered Hollow Creek.
  6. Storm (6 minutes). Each group mists its whole landscape evenly from about 20 cm above with 10 sprays. Groups watch for one minute, then complete the storm columns of the Part A table. An entry place is any point where a separate trickle of color reaches the main channel. If the blue has not reached the creek, groups run a second set of 10 sprays and note it.
  7. Model questions (5 minutes). Groups answer questions 1 through 3 in Part A while the models are still wet and the color paths are visible.
  8. Analysis (20 minutes). Pairs complete Parts B and C (questions 4 through 9) and then, individually, Part D (question 10).
  9. The activity ends when every group has run its dry day and storm and every student has completed question 10. For the closing debrief (6 minutes), ask two students to share their answers to question 9, then ask the class: "Most of the phosphorus reaching Hollowmere Lake comes from sources that no single permit can limit. What does that mean for how the town has to work on the problem?" Guide students to the idea that point sources can be controlled by setting limits on a few known pipes, while nonpoint pollution requires changes by many landowners and in-stream monitoring during storms to show whether those changes are working.
Hollowmere Tabletop Watershed: Point and Nonpoint Pollution Sources — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

Hollowmere source cards

Hollowmere is a fictional town beside Hollow Creek, which flows into Hollowmere Lake. Every summer the lake develops thick mats of algae, because too much phosphorus, a nutrient found in fertilizer, manure, soil, and wastewater, reaches the water. These are the six main sources of phosphorus in the watershed.

Source A: Hollowmere Wastewater Treatment Plant. The plant cleans sewage from every home and business in town. It releases the treated water through one concrete pipe on the bank of Hollow Creek, every hour of every day. The treated water is far cleaner than raw sewage, yet it still carries some phosphorus.

Source B: Hollowmere Creamery. The creamery makes cheese and butter. Water used to rinse tanks and floors is collected, partly treated on site, and released through one pipe into Hollow Creek on weekdays, rain or shine.

Source C: Westslope Farms. Corn and soybean fields cover the west slopes. Farmers spread fertilizer and manure each spring. When heavy rain falls on bare or freshly fertilized ground, water runs across the fields and carries soil and phosphorus downhill to the creek.

Source D: Eastside neighborhood. Several hundred homes sit on the east slope, with lawns, gardens, and driveways. Some residents fertilize their lawns, and grass clippings and pet waste are left in many yards. The neighborhood has no storm drains, so rain runs across the yards and down the slope to the creek.

Source E: Downtown storm drains. Rain washes oil, litter, leaves, and lawn fertilizer from downtown streets and parking lots into about 300 curb drains. Underground pipes collect water from all of those drains and release it through one outfall pipe at Hollow Creek, mostly during and just after rain.

Source F: Ridge Road logging site. Last winter a timber company cut the trees on a steep slope below the ridge and built temporary dirt roads to haul the logs out. Until new plants grow, rain washes soil from the bare slope and the dirt roads into the small streams that feed Hollow Creek. Phosphorus is attached to the soil particles.

Part A: Model observations

Prediction (write before the dry day): On the dry day, I expect ___ to move, because ___.

Source Color Moved on dry day? Places it entered the creek (dry day) Moved in storm? Places it entered the creek (storm)
A: Treatment plant Red
B: Creamery Red
C: Westslope Farms Blue
D: Eastside neighborhood Blue
  1. On the dry day, which sources sent color into Hollow Creek, and which did not? Explain what this shows about when a pipe outfall releases pollution compared with when runoff from the land does.
  2. During the storm, compare the number of places where red entered the creek with the number of places where blue entered it. Explain why the two kinds of sources produced different numbers.
  3. Imagine you are a state inspector with one sample bottle. Where on your model would you fill it to capture everything Source A released, and why? Could you do the same for Source C? Explain.

Part B: Classify the sources

A point source releases pollution into the water from one single, identifiable place, such as the end of a pipe or a ditch. A nonpoint source releases pollution that is picked up by runoff as it flows across a wide area of land and reaches the water at many places.

  1. Complete the table for all six source cards. In the evidence column, describe how the pollution enters the water, not just what kind of business or land it comes from.
Source Point or nonpoint? Evidence from the card
A: Treatment plant
B: Creamery
C: Westslope Farms
D: Eastside neighborhood
E: Downtown storm drains
F: Ridge Road logging site
  1. Source E is the hardest card to classify. Give one reason it can be called a point source and one reason its pollution behaves like nonpoint pollution.

Part C: The Hollowmere Lake phosphorus budget

The table gives a fictional estimate of the phosphorus that reaches Hollowmere Lake from each source in an average year.

Source How it enters the water Phosphorus (kg per year)
A: Treatment plant Pipe 180
B: Creamery Pipe 60
C: Westslope Farms Runoff over land 420
D: Eastside neighborhood Runoff over land 150
E: Downtown storm drains Pipe 90
F: Ridge Road logging site Runoff over land 100
Total 1,000
  1. Which single source adds the most phosphorus to the lake, how many kilograms does it add each year, and is it a point or a nonpoint source?
  2. What percent of the lake's phosphorus enters through pipes (Sources A, B, and E), and what percent enters as runoff over land (Sources C, D, and F)?
  3. Suppose the state rewrites the discharge permits for the treatment plant and the creamery so that each must cut its phosphorus in half. By how many kilograms per year would the total fall, and what percent of the original 1,000 kg is that?
  4. Suppose instead that the farmers at Westslope Farms plant grass buffer strips along their streams and grow winter cover crops, and that these practices cut the phosphorus from their fields by one third. How many kilograms per year would that remove? Compare it with your answer to question 8, and explain which of the two plans would be easier for the state to require and to check, and why.

Part D: Design a plan

  1. Choose one point source (A or B) and one nonpoint source (C, D, or F). For each, describe one change that would reduce the phosphorus it sends to the lake and one way to monitor whether the change worked. Then explain why your monitoring plan for the nonpoint source has to be different from your plan for the point source.

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: Only the red sources, A and B, should have sent color into the creek on the dry day, while the blue patches stayed in place. This shows that a pipe outfall releases pollution whenever the facility is running, in any weather, while runoff pollution from the land moves only when rain falls hard enough to flow across the surface. If a group's blue moved on the dry day, check whether drops splashed onto the patch.
  2. Answer: Counts depend on the model. Each red source should have entered the creek at one place, the end of its pipe, while each blue patch should have entered at several places spread along the creek. The pipe collects its pollution and releases it at one point, while the rain picks up blue color across the whole patch and carries it downhill along many small paths. If a group's blue patch drained into only one crease, compare its result with other groups' models, and note that a real field borders a creek along its whole length, so its runoff enters at many places.
  3. Answer: The bottle should be filled at the red dot where Source A's pipe meets the creek, because everything the plant releases passes through that one point. The same cannot be done for Source C, because its color entered the creek at several places along the bank and only during the storm, so no single sample captures all of it. An inspector would have to sample the creek upstream and downstream of the farm area, and during storms, to estimate what the farms add.
  4. Answer: A is a point source, because treated water leaves through one pipe. B is a point source, because rinse water leaves through one pipe. C is a nonpoint source, because rain carries soil and fertilizer across the fields and into the creek at many places. D is a nonpoint source, because water runs across many yards and reaches the creek along the whole slope. E is a point source by the way it enters the creek, through one outfall pipe, although its pollution starts across all of downtown (see question 5). F is a nonpoint source, because soil washes off a whole slope and several dirt roads into many small streams.
  5. Answer: Source E is a point source in the sense that all of its water enters the creek at one place, the outfall, where it can be sampled, and storm sewer outfalls like this are treated as point sources under the law. Its pollution behaves like nonpoint pollution because it starts as runoff from hundreds of streets, parking lots, and yards, it flows mainly when it rains, and reducing it means changing what happens across all of downtown, not adjusting one machine or treatment process at the end of the pipe.
  6. Answer: 420 kg per year, from Source C, Westslope Farms, which is a nonpoint source.
  7. Answer: 33 percent enters through pipes and 67 percent enters as runoff over land. Pipes: 180 + 60 + 90 = 330 kg, and 330 ÷ 1,000 = 0.33. Runoff: 420 + 150 + 100 = 670 kg, and 670 ÷ 1,000 = 0.67.
  8. Answer: 120 kg per year, which is 12 percent of the original total. The treatment plant falls from 180 to 90 kg (a cut of 90 kg) and the creamery falls from 60 to 30 kg (a cut of 30 kg), so the total falls by 90 + 30 = 120 kg, to 880 kg per year, and 120 ÷ 1,000 = 0.12.
  9. Answer: 140 kg per year, which is 20 kg more than the 120 kg removed by the permit plan in question 8. One third of 420 kg is 420 ÷ 3 = 140 kg. The permit plan would be easier to require and check, because it involves two known facilities whose outfalls can be sampled directly, and their permits can set enforceable limits. The farm plan depends on many farmers agreeing to change their practices, its results vary from storm to storm, and no single place exists where the farms' phosphorus can be measured. Accept answers that also note that the farm plan removes more phosphorus, which is why towns like Hollowmere usually need both kinds of action.
  10. Answer: Answers vary. A strong answer for the point source names a change at the facility, such as adding a phosphorus removal step at the treatment plant or capturing and reusing rinse water at the creamery, and monitors it by sampling the outfall regularly and comparing results with the permit limit and with earlier samples. A strong answer for the nonpoint source names a land practice, such as buffer strips and cover crops on the farms, a lawn fertilizer and pet waste program in the neighborhood, or replanting and blocking the dirt roads at the logging site, and monitors it by sampling Hollow Creek just upstream and just downstream of that area during several storms, before and after the change, and by tracking how much land is using the practice. The explanation should state that nonpoint pollution has no single outfall to sample and arrives mostly during storms, so it has to be measured in the stream across many rain events.

Differentiation

Support

Give groups a landscape with Hollow Creek already traced and the four source patches already outlined in pencil, so the group can focus on marking and observing. For Part B, provide the sentence frame "Source ___ is a ___ source, because its pollution enters the water ___." For Part C, allow a calculator and point out that the total is 1,000 kg, so each kilogram figure can be turned into a percent by moving the decimal point.

Extension

Hollowmere sets a goal of cutting the phosphorus reaching the lake from 1,000 to no more than 700 kg per year, a cut of at least 300 kg, in this fictional scenario. Ask students to build a combination of cuts from the budget table that meets the goal and to label each cut as one a permit could require or one that depends on landowners. One combination that works is halving Sources A and B (120 kg), cutting Source C by one third (140 kg), and cutting Source D by one third (50 kg), for a total of 310 kg. Students who finish can add a buffer strip of folded paper towel between a blue patch and the creek on their model, rerun the storm, and compare the number of blue entry places with and without the strip.

ELL supports

Teach "point source" with one finger touching one spot on the model, and "nonpoint source" with a flat hand sweeping across a hillside. Post the words outfall, runoff, and discharge permit with a small sketch beside each (a pipe end, arrows running down a slope, a document with a stamp). Allow students to answer questions 1 through 3 by pointing to the color paths on their model and completing the frame "The red entered at ___ place(s); the blue entered at ___ place(s)."