Project Grade 12 Science
Road Salt and the Chloride Pulse in Havering Creek
A seven-session grade 12 field investigation measuring conductivity above and below salted roads, then evaluating a fictional town's winter salting practice.
Objectives
- Students will design a sampling plan with a named independent variable, a dependent variable, and controlled conditions.
- Students will build a conductivity-to-chloride calibration from prepared standards and use it to convert field readings.
- Students will evaluate a winter salting practice against a cited water-quality criterion and propose one testable refinement.
Vocabulary
Key words: specific conductance, calibration curve, chloride, reference site, baseline, refinement
Teacher overview
Winter road salt does not break down. Once it dissolves, the chloride travels with the water and stays in solution; only dilution lowers its concentration. This project asks students whether the salting practice of a fictional town raises chloride in the creek that runs through it, and if so, by how much and for how long after a melt. Students build their own conductivity-to-chloride calibration, sample five sites across three rounds, and deliver a data report and recommendation to the town's public works department. The investigation is science-fair ready, with a stated hypothesis, a defined comparison between the salted reach and an unsalted reference, and a written data-recording plan.
Project Brief
Driving question. Does the winter salting practice of the fictional town of Havering Mills raise chloride in Havering Creek, and if it does, how large is the rise and how long does it last after a melt?
Client. You are reporting to the Havering Mills Department of Public Works, which sets salting routes and spreader rates. Your teacher will play the department's engineer at the final presentation. If your own community salts its roads, your teacher may substitute a local creek and a local public works office for the fictional ones.
Your hypothesis. Write it before you sample anything. A workable form is this: specific conductance at sites downstream of the salted road network will exceed the upstream reference site, and the difference will be largest within 48 hours of a melt. You may write a different hypothesis, but it must name what you expect to change and in which direction.
Variables. The independent variable is the position of the sampling site relative to the salted road network, together with the timing of the sample relative to a melt. The dependent variable is specific conductance, which you will convert to chloride using your own calibration. Controlled conditions include the meter, the sampling depth, the time of day, and the person reading the meter.
Constraints. These are fixed and the rubric scores your work against them.
- Five sites: one upstream of every salted road, three at increasing distance downstream, and one on a tributary draining land with no salted roads.
- Three sampling rounds: a cold dry baseline, a round within 24 hours of a melt, and a follow-up round 5 to 7 days later.
- One meter shared by the whole class, so all readings must be logged with the time and the reader's name.
- Calibration standards prepared from sodium chloride and distilled water, at least five concentrations spanning the range you expect.
- Bank sampling only. No one enters the water. Every sampling pair works with an adult present, and no one steps onto creek ice or an icy bank edge.
Final deliverable. A technical report of six to eight pages plus a ten-minute presentation. The report must contain a site map, your hypothesis and variables, the calibration curve with its equation and coefficient of determination, a table of all raw readings, a time-series plot of chloride at each site, a comparison of upstream and downstream results with the test you used, a paragraph on what the data do not show, an evaluation of the current salting practice against a water-quality criterion you locate and cite for your state, and one refinement you recommend with the measurement that would show whether it worked.
Refinements worth considering. Pre-wetting the salt with brine, calibrating spreader rates to pavement temperature, reducing application on low-traffic streets, timing applications to the pavement temperature rather than the air temperature, and covering the salt storage pad. Choose one, and say what it changes about the amount of salt that reaches the creek.
Milestones
Session 1, question and design. Choose the five sites on a map and mark the salted road network on it. Write the hypothesis, name the independent and dependent variables, and list the conditions you will hold constant. Output: a marked site map and a one-page design statement your teacher initials before any sampling.
Session 2, calibration. Prepare at least five sodium chloride standards spanning the concentration range you expect, read each on the meter, and plot conductance against concentration. Output: a calibration curve with its equation, its coefficient of determination, and a sentence naming the range over which you will trust it.
Session 3, baseline round. Sample all five sites during a cold, dry stretch with no melt in the previous week. Record time, air temperature, water temperature, and conductance at each site. Output: a completed field data sheet for all five sites, signed by the pair who took the readings.
Session 4, melt round. Sample all five sites within 24 hours of a melt, under the same conditions as the baseline round. Output: a second completed field data sheet plus field notes describing the weather in the preceding three days.
Session 5, follow-up round and conversion. Sample all five sites 5 to 7 days after the melt round. Then convert every conductance reading to chloride using your calibration. Output: a third data sheet and a complete table of converted values.
Session 6, analysis. Plot chloride against date for each site on one set of axes. Compare the upstream reference site with the farthest downstream site using a test you can defend, and state the result plainly. Output: the plot, the comparison with its test named, and a written paragraph on the limits of three rounds at five sites.
Session 7, evaluation, recommendation, and presentation. Locate and cite your state's water-quality criterion for chloride, compare your highest measured value with it, and write the evaluation of the current practice. Choose one refinement and state the measurement that would show whether it worked. Output: the finished report and the ten-minute presentation to the department.
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 |
|---|---|---|---|---|
| Investigation design | No hypothesis, or sites chosen without regard to the salted road network | Hypothesis stated but variables unnamed, or the reference site is missing | Hypothesis names the expected direction of change, all five required sites are used, and controlled conditions are listed | Design also explains why the tributary reference site is necessary and what a result there would rule out |
| Data quality and calibration | No calibration, or field readings recorded without time and reader | Calibration built from fewer than five standards, or readings incomplete for one round | Five or more standards, a curve with its equation and coefficient of determination, and complete logs for all three rounds | Calibration range is checked against the field readings, out-of-range values are flagged, and repeat readings are used to estimate reading error |
| Analysis and inference | Values reported with no comparison or plot | Plot or comparison present but conclusions overstate what three rounds can support | Time series plotted, upstream and downstream compared with a named test, and conclusions kept within what the data support | Analysis also separates the melt signal from the seasonal baseline and quantifies how much of the downstream rise the salted network can account for |
| Evaluation and refinement | No criterion cited and no recommendation | Criterion cited but not compared with the measured values, or a recommendation with no reasoning | Measured maximum compared with the cited criterion, and one refinement recommended with a reason tied to the data | Recommendation also names the measurement that would test it, the time frame for that test, and what result would count as failure |
| Communication | Report missing required sections; presentation unclear | Most sections present; figures unlabeled or hard to read | All required sections present, figures labeled with units, and the presentation stays within time | Report reads as a technical document a public works engineer could act on, with a summary that leads with the finding and its uncertainty |
Differentiation
Support
Give these students a prepared field data sheet with every column already labeled, including units, and a calibration spreadsheet with the plot and trendline set up so only the standard values need entering. Provide a worked example of one conductance-to-chloride conversion before the analysis session.
Extension
Ask students to add a fourth sampling round in late spring, after the salting season has ended, and to use it to estimate how much of the winter chloride is still moving through the watershed months later. Have them explain what a persistent elevation would imply about where the chloride was stored.
ELL supports
Pre-teach specific conductance, calibration, reference site, and baseline with a labeled diagram of the creek showing the five sites and the salted roads. Provide a bilingual field card with the sampling steps in order, and allow the Session 1 design statement to be submitted as labeled diagrams with short captions.
Extensions
Have students request the winter salt tonnage applied on the routes in their study area and compare the pattern of application with the pattern of the chloride they measured. Discuss why the total applied is a poor predictor of the peak concentration in a creek, and what other measurement would predict it better.