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Worksheet Grade 10 Science

Kelbury Road Salt: Chloride Loading in Tarnwick Creek

Grade 10 students convert a winter's road salt into chloride mass, calculate stream loading in pounds per day, and judge whether a salt policy change would work.

Objectives

  1. Students will calculate the mass of chloride applied to a road network from the tonnage of rock salt spread.
  2. Students will use the pounds formula to compare chloride loading above and below a town.
  3. Students will evaluate a proposed change in winter road maintenance using their own loading results.

Vocabulary

Baseflow
Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.

Also: chloride, loading, conservative pollutant, dilution

Teacher overview

Chloride from winter road salt behaves differently from most pollutants a class studies. It does not settle, break down, or get removed by coagulation and filtration, so once it reaches a stream the only thing that lowers its concentration is dilution. Two points are worth stating plainly before the class begins. A drinking water plant does not strip chloride out along with turbidity and bacteria, and spring rain does not flush the salt away for good, because a large share soaks into shallow groundwater and returns to the stream through baseflow all summer. This worksheet uses the fictional town of Kelbury and its stream, Tarnwick Creek, so that every figure is a scenario value rather than a published statistic.

Kelbury Road Salt: Chloride Loading in Tarnwick Creek — Grade 10 Science
Name: ____________________________ Date: ______________

Student Worksheet

Suppose Kelbury is a town of about 9,000 people that maintains 48 lane-miles of road. Over one winter the public works department spreads 220 tons of rock salt, which is sodium chloride. The county samples Tarnwick Creek at two stations: one upstream of town and one just below the last storm drain. Use the molar masses sodium 22.99 g/mol and chlorine 35.45 g/mol, and use 1 ton = 2,000 pounds, 1 MGD = 1.547 cfs, and the loading formula pounds per day = concentration (mg/L) x flow (MGD) x 8.34.

  1. Rock salt is sodium chloride. Using the molar masses sodium 22.99 g/mol and chlorine 35.45 g/mol, calculate the percent of the compound's mass that is chloride. Round to the nearest tenth.
  2. Kelbury spread 220 tons of rock salt this winter. Using 2,000 pounds per ton and your percent from item 1, calculate the pounds of chloride applied to the roads.
  3. On the February sampling day, Tarnwick Creek was flowing at 15.47 cfs. Convert that flow to million gallons per day, using 1 MGD = 1.547 cfs.
  4. At the downstream station the chloride result was 168 mg/L. Using the flow you found in item 3 and the formula pounds per day = mg/L x MGD x 8.34, calculate the chloride load passing the downstream station each day.
  5. At the upstream station the same morning, chloride measured 24 mg/L. Using the same flow of 10.0 MGD, calculate the chloride load passing the upstream station each day.
  6. Subtract your item 5 answer from your item 4 answer to find the chloride load the town adds to Tarnwick Creek each day.
  7. Suppose the state guideline for long-term chloride in a stream is 230 mg/L. During a thaw the downstream station measured 310 mg/L. Calculate how many milligrams per liter that reading sits above the guideline.
  8. Using the same two numbers, 310 mg/L and the 230 mg/L guideline, calculate the percent by which the thaw reading exceeded the guideline. Round to the nearest tenth.
  9. The town adds about 12,010 pounds of chloride to the creek per day at the February rate. Using your total from item 2, calculate how many days at that rate it would take to move the whole winter's chloride past the downstream station.
  10. On the thaw day the creek still ran at 10.0 MGD and carried 310 mg/L of chloride. If the load stayed the same, calculate the flow in MGD that would dilute the creek to the 230 mg/L guideline.
  11. Kelbury's plant uses coagulation, sedimentation, filtration, and chlorination. Explain why none of those steps lowers the chloride concentration in the finished water.
  12. The public works director proposes two changes: calibrating every spreader so it applies a set pounds per lane-mile, and pre-wetting the salt with brine so less bounces off the pavement. Using your loading numbers, explain what evidence would show the change worked, and name one measurement the town should keep taking.

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: 60.7 percent.
    The molar mass of sodium chloride is 22.99 + 35.45 = 58.44 g/mol. The chloride share is 35.45 / 58.44 = 0.6066, or 60.7 percent when rounded to the nearest tenth.
  2. Answer: 267,000 pounds.
    First convert tons to pounds: 220 tons x 2,000 pounds per ton = 440,000 pounds of rock salt. Then take the chloride share: 440,000 x 0.607 = 267,080 pounds, which rounds to about 267,000 pounds of chloride.
  3. Answer: 10.0 MGD.
    Divide the measured flow by the conversion factor: 15.47 cfs divided by 1.547 cfs per MGD = 10.0 MGD. Students who multiply instead of dividing will get 23.9 and should check which unit needs to cancel.
  4. Answer: 14,011 lb/day.
    Apply the pounds formula: 168 mg/L x 10.0 MGD x 8.34 = 14,011.2 pounds per day. Report about 14,011 pounds per day.
  5. Answer: 2,002 lb/day.
    Apply the same formula to the upstream station: 24 mg/L x 10.0 MGD x 8.34 = 2,001.6 pounds per day, or about 2,002 pounds per day.
  6. Answer: 12,010 lb/day.
    Subtract the upstream load from the downstream load: 14,011.2 minus 2,001.6 = 12,009.6 pounds per day, which rounds to about 12,010 pounds per day added between the two stations.
  7. Answer: 80 mg/L.
    Subtract the guideline from the measurement: 310 minus 230 = 80 mg/L above the guideline.
  8. Answer: 34.8 percent.
    Divide the excess by the guideline and multiply by 100: 80 / 230 = 0.3478, so the thaw reading was 34.8 percent above the guideline. Accept 34.8 percent or 35 percent.
  9. Answer: 22.2 days.
    Divide the winter total by the daily added load: 267,080 pounds divided by 12,009.6 pounds per day = 22.24 days, or about 22.2 days. The result is worth discussing, since it shows the creek can carry an entire winter's salt in about three weeks of high loading rather than spreading it evenly across the year.
  10. Answer: 13.5 MGD.
    The load on the thaw day is 310 x 10.0 x 8.34 = 25,854 pounds per day. To reach 230 mg/L with that same load, solve 25,854 = 230 x flow x 8.34, so flow = 25,854 / 1,918.2 = 13.48 MGD, or about 13.5 MGD. Students may also scale directly: 310 / 230 x 10.0 = 13.5 MGD.
  11. Answer: Chloride is dissolved, so those steps cannot capture it.
    Coagulation and sedimentation remove suspended particles by clumping them until they settle, and filtration strains out what remains. Chloride is a dissolved ion spread evenly through the water, not a particle, so it passes through all three steps. Chlorination adds a disinfectant and does nothing to remove chloride. Only reverse osmosis, ion exchange, or dilution with a low-chloride source would lower it.
  12. Answer: Lower pounds per day at the downstream station for the same flow and weather.
    A fair comparison holds flow and weather constant, because a wetter winter dilutes chloride and can make concentrations look lower without any real reduction, while a snowier winter needs more salt and can hide one. Students should propose tracking chloride concentration and creek flow at both stations on matched storm days, calculating the load added between them, and comparing it with the tons of salt applied that season. Keeping the upstream station is essential, since it establishes the background the town is measured against.

Differentiation

Support

Give students a worked example that uses the same formulas with different values before they start: a stream in another town of 5,000 people flowing at 5.0 MGD with a chloride result of 90 mg/L, worked all the way to 3,753 pounds per day. Provide a conversion card listing tons to pounds, cfs to MGD, and the pounds formula, and let students use it openly on items 2 through 6.

Extension

Have students research their own state's winter maintenance guidance for pounds of salt per lane-mile, then estimate how much chloride the town of Kelbury would apply if it followed that rate on 48 lane-miles for 20 storm events, and compare that estimate with the 220 tons in the scenario.

ELL supports

Pre-teach "load" in its technical sense, since students often read it as weight rather than mass per unit time, and post the phrase "pounds per day" next to the formula. Pair "upstream" and "downstream" with a simple sketch of the town of Kelbury and the two sampling stations, and allow students to answer items 11 and 12 in a labeled diagram with short captions before writing full sentences.

Extensions

Ask students to design a one-page memo to the fictional Kelbury town council that recommends a winter maintenance change and states the monitoring the town would need to prove it worked. Require the memo to include the February load calculation, the thaw-day comparison against the 230 mg/L guideline, and one sentence explaining why a drinking water plant cannot solve the problem downstream.