Bramford's First Flush: Is the First Rain off the Roof Dirtiest?

Driving question. Is the first water off the Bramford Middle School roof dirtier than the water that comes later in the same storm?

The client. The grounds committee at Bramford Middle School is fictional, and so is every number in this brief. The committee installed a rain barrel last spring. The barrel waters the courtyard garden. Someone suggested adding a first-flush diverter, which is a small chamber that catches the first few liters and keeps them out of the barrel. The committee has $60 left for the year and wants evidence before it spends any of it.

What you will turn in. Each team turns in a two-page data report and gives a four-minute presentation to the committee, played by the teacher and two other students. The report must contain a hypothesis, the protocol, the full data table, one graph, the median and range of the clarity readings for each jar position, a written pattern statement, and a recommendation.

Constraints you must work within.

• Sample only from the north downspout, at ground level. Nobody goes on the roof.
• Use four jars per storm, all the same size. Jar 1 catches the first liter that leaves the downspout. Jar 2 is filled five minutes later, jar 3 fifteen minutes later, and jar 4 thirty minutes later.
• Collect from at least three storms. A storm counts only if it lasts long enough to fill jar 4.
• Let every jar stand undisturbed for 24 hours before you read it, so the settling time is the same for all samples.
• Record the number of dry days since the last rain for each storm.
• Measure clarity the same way each time, using the clear tube and the printed target card.
• Wear gloves when handling samples. This water is never tasted and never poured back into the barrel.
• An adult supervises every collection. Do not sample while there is thunder or lightning; wait indoors until it has passed.

Hypothesis and variable. Before you collect anything, write a hypothesis in this form: "If the first water off the roof carries more dirt, then jar 1 will be ___ than jar 4, because ___." The variable you are testing is how long the runoff has been flowing when the sample is taken. The things you hold the same are the downspout, the jar size, the settling time, and the way you read clarity.

1. Session 1, plan the investigation. Read the brief, write the team hypothesis, name the variable being tested, and list what is held the same. Draft the sampling protocol as numbered steps another team could follow without asking questions. Output: a signed one-page protocol.
2. Session 2, build and check the tools. Mark the clear tube in centimeters, prepare the target card, and label the twelve jars. Two teammates then read the same practice sample separately.
   Output: two independent readings of the same sample that agree within one centimeter, or a written note on what you changed until they did.
3. Session 3, collect the samples. Run the protocol during three storms, in short bursts outside class as the weather allows. Record the date, the dry days before the storm, the jar number, the minutes since runoff started, and the rain gauge reading. Output: a completed field data table with twelve rows and no blanks.
4. Session 4, analyze the data. Read the settled sediment depth in millimeters and the clarity in centimeters for every jar. Build a bar graph of clarity by jar position. Report how many samples you have, what you measured, and the median and range of clarity for each jar position.
   Output: the graph plus a written pattern statement of three or four sentences.
5. Session 5, report and recommend. Present to the grounds committee. Recommend whether to buy the diverter, state how many liters it should hold based on your data, and name one way the committee could check next spring whether the change worked. Output: the two-page report and the four-minute presentation.