Lab / experiment Grade 6 Science
Dissolved Oxygen and pH Above and Below a Storm Drain Outfall
A grade 6 lab testing pH and dissolved oxygen at two sites on a fictional creek to see whether a storm drain outfall is lowering water quality downstream.
Objectives
- Students will measure pH, dissolved oxygen, and temperature at two sites and calculate an average for each measurement at each site.
- Students will calculate the percent decrease in average dissolved oxygen between an upstream and a downstream site.
- Students will use their data and a real dissolved-oxygen threshold to evaluate whether a storm drain outfall is affecting a stream's ability to support fish.
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.
- Storm drain
- A storm drain is the grate at the edge of a street that swallows rainwater and sends it through pipes to the nearest creek or river. That water is usually not treated, so only rain belongs down the drain.
- 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
Storm drains along most streets do not connect to a treatment plant; they carry rainwater, and anything the rain picks up along the way, straight to the nearest stream or river. This lab has students compare two sites on Cobble Creek (a fictional creek used for this lab): Site A, upstream of a storm drain outfall behind the Aldergate Plaza shopping center, and Site B, just downstream of that outfall. By measuring dissolved oxygen, pH, and temperature at both sites and comparing the results, students build the same kind of before-and-after monitoring method a water-quality technician uses to judge whether a specific outfall is affecting a stream.
Watch for this misconception: students often assume that clear-looking water is automatically healthy water. Dissolved oxygen and pH are invisible to the eye, and a downstream site can look just as clear as an upstream site while carrying measurably less oxygen for fish and other aquatic life. Questions 5 and 8 in the Data & Analysis section confront this directly by asking students to reason from numbers rather than appearance.
Procedure
This lab can be run two ways: as a real field trip to a local stream, pond, or ditch with two accessible sites, or as an indoor version using two samples the teacher prepares in advance. Either way, every group tests the same two labeled samples, Site A and Site B, twice each.
Step 1. Discuss as a class: where does water that enters a storm drain actually go? Establish that in most towns it flows untreated to the nearest stream, carrying along whatever oil, soap, fertilizer, or sediment the rain washed off nearby streets and parking lots.
Step 2. Collect or receive two labeled water samples per group from the fictional Cobble Creek: Site A (upstream of the outfall) and Site B (downstream of the outfall). For an indoor version, the teacher can prepare "Site B" by leaving an uncovered jar of tap water in a warm, sunny spot for several hours, and "Site A" as a jar of cold tap water drawn just before class, to model how runoff warmed by pavement lowers dissolved oxygen. Adding fertilizer or soap to the jar would not lower dissolved oxygen in a day, because that effect comes from algae and bacteria growing and decaying in a stream over days to weeks.
Step 3. Safety note. No sample is ever tasted or sipped, even the "clean" Site A sample. Students should wash their hands after handling any sample and treat every jar as non-potable throughout the lab. On an outdoor visit, an adult supervises at all times, students collect samples from the bank and do not wade into fast or deep water, and no one enters or reaches into the outfall pipe.
Step 4. Measure and record the water temperature of each sample using the thermometer.
Step 5. Test the pH of each sample using a test strip, comparing the strip's color to the reference chart, and record the reading.
Step 6. Test the dissolved oxygen of each sample using the classroom DO test kit, following the kit's instructions exactly, and record the reading in mg/L.
Step 7. Repeat Steps 4 through 6 with a fresh test strip and a fresh DO test for a second trial of each sample, to check whether the readings are consistent.
Step 8. Dispose of all samples down a sink drain, never a storm drain, and clean up test materials as directed.
Data & Analysis
| Site | Trial | Temperature (°C) | pH | Dissolved oxygen (mg/L) |
|---|---|---|---|---|
| A (upstream) | 1 | 16 | 7.2 | 9.0 |
| A (upstream) | 2 | 16 | 7.4 | 8.8 |
| B (downstream) | 1 | 18 | 6.8 | 4.6 |
| B (downstream) | 2 | 19 | 6.6 | 4.4 |
Use the sample data above (or your own collected data, if you ran this lab outdoors) to answer the following.
Calculate the average pH for Site A across its two trials, and the average pH for Site B across its two trials.
Calculate the average dissolved oxygen (DO) reading, in mg/L, for Site A and for Site B across their two trials.
Find the difference between Site A's average DO and Site B's average DO. Which site has higher dissolved oxygen?
Calculate the percent decrease in average DO from Site A to Site B. Round to the nearest whole percent.
Most fish need dissolved oxygen above 5 mg/L to survive comfortably. Based on your average DO values, which site would better support fish, and does either site's average fall below that level?
The secondary standard for drinking-water pH is 6.5 to 8.5. Do both sites' average pH values fall inside that range? Would you expect a stream to be held to the same standard as tap water leaving a treatment plant? Explain your reasoning.
Site B's average water temperature was slightly higher than Site A's. Colder water can hold more dissolved oxygen than warmer water. Explain how temperature might be adding to the DO difference you found, in addition to any effect from the storm drain outfall itself.
Based on all of your data, does Cobble Creek show signs that the storm drain outfall is affecting water quality downstream? Support your answer with at least two specific numbers from your data table.
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: 7.3 pH (Site A average); 6.7 pH (Site B average).
Site A: (7.2 + 7.4) ÷ 2 = 7.3. Site B: (6.8 + 6.6) ÷ 2 = 6.7.Answer: 8.9 mg/L (Site A average); 4.5 mg/L (Site B average).
Site A: (9.0 + 8.8) ÷ 2 = 8.9 mg/L. Site B: (4.6 + 4.4) ÷ 2 = 4.5 mg/L.Answer: 4.4 mg/L; Site A has higher dissolved oxygen.
8.9 mg/L − 4.5 mg/L = 4.4 mg/L.Answer: 49 percent decrease.
(8.9 − 4.5) ÷ 8.9 = 4.4 ÷ 8.9 ≈ 0.494, or about 49.4%, which rounds to 49 percent.Answer: Site A would better support fish; Site B's average (4.5 mg/L) falls below the 5 mg/L level, while Site A's average (8.9 mg/L) does not.
Fish at Site B may experience oxygen stress based on this sample data.Answer: Yes, both averages (7.3 and 6.7) fall within the 6.5-8.5 range, though Site B sits closer to the low edge.
A stream is not legally required to meet the drinking-water secondary standard, since that standard applies to treated water leaving a plant, not a natural waterway; however, comparing a stream's pH to that familiar range is still a useful way to judge whether a reading looks unusual. Accept any answer that makes this distinction.Answer: Because warmer water holds less dissolved oxygen, Site B's slightly higher temperature would be expected to lower its DO somewhat even without any pollution, meaning part (but likely not all) of the 4.4 mg/L difference could come from temperature alone rather than the outfall.
Accept any answer that correctly separates the temperature effect from a possible pollution effect and does not claim temperature explains the entire difference.Answer: Yes, the data suggest the outfall is affecting the creek downstream.
Strong answers cite the roughly 49 percent drop in average DO between the sites and the fact that Site B's average (4.5 mg/L) falls below the 5 mg/L threshold most fish need, while Site A's does not; a supporting detail is Site B's slightly lower average pH and higher temperature, both consistent with runoff entering through the outfall.
Differentiation
Support (below grade level)
Provide the four averages (Site A and Site B pH and DO) already calculated on a reference card, so students can focus on questions 3 through 8 instead of the arithmetic in questions 1 and 2. Allow question 4's percent decrease to be answered as "Site B is about half of Site A" rather than requiring the precise 49 percent figure.
Extension (above grade level)
Ask students to decide (using only the data and thresholds given in this lab, not outside sources) what a water utility or environmental agency might do next if it found a result like Site B's during routine monitoring, and to write a short paragraph proposing one specific next step, such as additional testing dates or notifying the town's public works department.
ELL supports
Pre-teach dissolved oxygen, pH, and storm drain with a simple diagram showing rain flowing off a parking lot into a drain and then into a stream. Provide the data table with the two averages for each site already labeled in both English and, where possible, students' home languages, and allow answers to questions 5, 6, and 8 to be given as short phrases supported by pointing to specific numbers in the table.
Extensions
If a real stream, pond, or drainage ditch is accessible near the school, repeat this lab there before and after a rain event (waiting until high, fast water has dropped, with an adult collecting from the bank) and compare the results with the sample data used in class. Many local watershed groups or public works departments will lend classroom water-testing kits or send a volunteer to help a class collect a real upstream/downstream data set, turning this lab into an authentic local water-quality record the class can track over the school year.