Project Grade 12 Science
Dissolved Oxygen Below Cedar Mill Outfall: A Capstone Investigation
A grade 12 capstone project where students sample dissolved oxygen above and below a fictional outfall, then use statistics to recommend action to a watershed council.
Objectives
- Students will design and carry out a paired-site water-quality investigation testing whether dissolved oxygen differs upstream and downstream of a wastewater outfall.
- Students will calculate a sample mean, standard deviation, and margin of error for dissolved oxygen readings at each site.
- Students will write an evidence-based monitoring report that compares field data to the federal secondary-treatment standard and recommends a course of action.
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.
- Biochemical oxygen demand (BOD)
- Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.
- Effluent
- Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.
- 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
The fictional town of Cedar Mill sits along Cedar Mill Creek, a short distance downstream from the town's wastewater treatment plant. This capstone investigation asks students to test whether the plant's effluent measurably lowers dissolved oxygen in the creek, mirroring the paired upstream-downstream sampling design that state water-quality agencies use below real treatment plant outfalls. Working in teams across five sessions, students collect or analyze replicate dissolved-oxygen readings, apply sample statistics to judge whether an observed difference is likely real rather than ordinary variation, and submit a written monitoring report and recommendation to the fictional Cedar Mill Watershed Council.
Project Brief
Driving question: Does effluent from the Cedar Mill wastewater treatment plant measurably lower dissolved oxygen (DO) in Cedar Mill Creek? If so, what should the watershed council recommend?
Scenario: The fictional town of Cedar Mill operates a wastewater treatment plant. The plant discharges treated effluent into Cedar Mill Creek. The fictional Cedar Mill Watershed Council is a volunteer group that advises the town on water issues. The council has asked an environmental science class to investigate whether the plant's discharge affects the creek's dissolved oxygen downstream.
Hypothesis: Dissolved oxygen concentrations at the downstream site will be lower, on average, than dissolved oxygen concentrations at the upstream reference site.
Variable tested: Sampling site (upstream reference versus downstream of the outfall), as it relates to dissolved oxygen concentration in mg/L. Record water temperature and time of day with every reading. Temperature sets how much oxygen the water can hold, and time of day matters because algae and aquatic plants add oxygen in daylight and use it at night.
Constraints:
- Each team collects at least three replicate DO readings per site on each of two separate monitoring days. That is a minimum of six readings per site. A team may instead analyze a provided fictional practice data set.
- Compare all DO readings against the standard that most fish need dissolved oxygen above 5 mg/L to survive.
- The final report must state a margin of error for each site's mean. It must also reference the federal secondary-treatment standard. That standard requires effluent biochemical oxygen demand (BOD) of 30 mg/L or less, a roughly 85 percent reduction from raw sewage's typical 200 to 300 mg/L BOD.
Final deliverable: A written monitoring report of two to three pages, submitted to the fictional Cedar Mill Watershed Council. The report includes the data table; the calculated mean, standard deviation, and margin of error for each site; a graph comparing the two sites; and a specific, evidence-based recommendation. Each team also gives a five-minute presentation of its findings.
Milestones
Session 1 (60 min) — Frame the investigation: Teams read background on secondary-treatment standards and dissolved-oxygen thresholds. Each team selects an upstream reference point and a downstream point below the outfall. Teams then write their hypothesis and identify the variable tested. Finally, they draft a data-recording plan listing exactly what each reading must include: site, date, time, DO in mg/L, water temperature in degrees Celsius, and observer initials. Output: a completed research and sampling-plan brief.
Session 2 (60 min) — Monitoring day one: Teams collect three replicate readings at each site, using a DO meter or test kit. Teams working without field access use the teacher-provided fictional practice data set for Cedar Mill Creek. An adult supervises all creek work; students sample from the bank or from shallow, slow water away from the outfall pipe, wear gloves, never drink or taste creek water, and wash their hands afterward. Log temperature and time alongside each reading. Output: a completed Day 1 data log with six readings.
Session 3 (60 min) — Monitoring day two: Teams repeat the sampling procedure on a second day, under different weather or flow conditions. Again they collect three replicate readings per site. They then combine both days into a single data table. Output: a combined two-day data table with a sample size of six per site.
Session 4 (60 min) — Statistical analysis: Teams calculate the sample mean, the sample standard deviation, and a 95 percent margin of error for each site. Use margin of error approximately equal to 1.96 times the standard deviation divided by the square root of the sample size. Teams then compare the two sites' means and margins of error. The question is whether the downstream reading is meaningfully lower, or whether ordinary sampling variation explains the gap. Output: a completed analysis worksheet with a bar graph comparing site means and error bars.
Final session (60 min) — Report and presentation: Teams write the monitoring report. It states the hypothesis, the method, and the statistical findings. It then compares those findings to the 5 mg/L fish-health benchmark and to the federal secondary-treatment standard. The report closes with one specific recommendation for the watershed council. A team might recommend that the plant upgrade its aeration or effluent-treatment process to reduce the BOD load reaching the creek. A team might instead request the plant's BOD removal-efficiency data, or recommend additional downstream monitoring. Each team then presents its findings and recommendation in a five-minute presentation, role-playing a meeting of the watershed council. Output: a completed written report and team presentation.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Data quality and method | Sampling plan lacks a clear hypothesis or variable, or fewer than six readings per site are recorded | Hypothesis and variable are stated but the sampling plan or data log is incomplete | Hypothesis, variable, and a complete two-day, six-reading-per-site data log are all present and consistent | All of the Proficient criteria are met, and the team also records and discusses a possible confounding variable, such as recent rainfall or time of day |
| Statistical reasoning | Mean, standard deviation, or margin of error is missing or calculated incorrectly | Calculations are attempted but contain errors that change the conclusion | Sample mean, standard deviation, and margin of error are calculated correctly for both sites and compared | All of the Proficient criteria are met, and the team explains in its own words why a margin of error matters when comparing two small samples |
| Evidence-based recommendation | Report does not connect data to a recommendation | Report states a recommendation without clearly tying it to the data or standards | Report ties its recommendation directly to the calculated data and to the cited federal standard or fish-health benchmark | All of the Proficient criteria are met, and the recommendation anticipates a likely objection the watershed council might raise |
Differentiation
Support (below grade level)
Provide the mean and standard deviation formulas already set up in a spreadsheet template so students enter only their raw readings, and walk through one complete worked example of the margin-of-error calculation using a sample data set different from the class data before Session 4 begins.
Extension (above grade level)
Ask students to explain, using their own data, what would happen to the margin of error if the team had collected twice as many replicate readings per site, and to recompute the margin of error for a doubled sample size (using the same mean and standard deviation) to show the relationship directly.
ELL supports
Pre-teach "dissolved oxygen," "effluent," "outfall," and "margin of error" with a labeled diagram showing the outfall, the upstream site, and the downstream site along the creek. Allow the written report's recommendation section to be organized with a provided sentence frame: "Because our downstream mean was ___ mg/L with a margin of error of ___, we recommend that the watershed council ___."
Extensions
Invite a representative from a real regional or state water-quality monitoring program, such as a volunteer stream-monitoring coordinator, to review the class's sampling design and offer feedback before or after the investigation. Teams that finish early can also research how a real state agency's monitoring network reports dissolved-oxygen data to the public, and compare that reporting format to their own written report.