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Lesson plan Grade 8 Science

Cressett Point's Salty Wells: Modeling Seawater Intrusion

A grade 8 science lesson using a fictional coastal town's chloride records to model seawater intrusion and design a monitoring and pumping plan for the aquifer.

Objectives

  1. Students will determine a constant rate of change from a chloride record and write the linear equation it describes.
  2. Students will solve a linear equation to find the year a well reaches a stated chloride alert level.
  3. Students will describe a monitoring plan and a pumping change that would slow seawater intrusion.

Vocabulary

Aquifer
An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.
Drawdown
Drawdown is how far the water level in a well drops when a pump pulls water out. The harder and longer you pump, the bigger the drawdown gets.

Also: chloride, seawater intrusion, freshwater lens, sentinel well, alert level

Teacher overview

The fictional coastal town of Cressett Point draws its drinking water from sandy ground called the Drumlin Sands aquifer, and its oldest well has been getting saltier for years. Fresh water floats on top of salt water underground because it is less dense, so the fresh supply sits in a thin lens that thins further whenever pumping pulls it down. This lesson gives students a chloride record with a constant rate of increase, has them write and solve the linear equation behind it, and ends with the monitoring and pumping decisions a small utility actually faces. Students frequently picture the aquifer as an underground lake with a hard edge between fresh and salt water; the boundary is a wide mixing zone that moves inland when pumping increases and back toward the shore when it eases.

Procedure

Hook (5 min): Fill a clear jar with salt water dyed blue, then pour fresh water slowly down the side onto a spoon so it layers on top. Ask students which layer a well screened near the bottom would draw from, and what would happen to the layers if a straw took water quickly from the middle of the fresh layer.

Direct instruction (12 min): Sketch a cross section of the fictional Cressett Point shoreline: the ocean on the right, the sand aquifer beneath the town, and a fresh lens sitting on salt water. Explain that chloride is the ion operators track because it is easy to measure and it rises long before anyone can taste salt. Explain that geology controls where salt water can reach, since coastal sand deposits let both waters move freely, while inland clay layers can seal an aquifer off from the sea entirely. Model finding a rate of change over a multi-year interval using a different well than the handout, such as a record rising from 30 to 42 over three years.

Guided practice (13 min): Pairs complete handout items 1 through 3 and graph the record. Circulate and check that pairs divide by the number of years, not the number of rows, when finding the annual rate. Ask one pair to explain why the graph's vertical intercept is the 2016 reading rather than zero.

Independent practice (17 min): Students complete handout items 4 through 8 individually. Item 6 asks them to redo the calculation for a slower rise, which is the number the district would use to argue for a pumping limit.

Closure and exit ticket (8 min): Collect the handout and ask students to hold up one finger if the district should keep pumping as it does now, and two fingers if it should cut back and monitor. Call on one student from each group to give the number from their own work that supports the choice. A response that cites no number from the handout is the signal to reteach item 5.

Cressett Point's Salty Wells: Modeling Seawater Intrusion — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

The fictional Cressett Point Water District measured chloride in Well 3 every two years. Well 3 sits close to the shore, where salt water can move inland into a fresh aquifer, which is called seawater intrusion. The district set its own alert level at 200 milligrams per liter, the point at which it would take the well out of service.

Year Chloride in Well 3 (mg/L)
2016 40
2018 56
2020 72
2022 88
2024 104
  1. Using any two rows of the table, find the rate of change of chloride in milligrams per liter per year.
  2. The 2016 reading was 40 milligrams per liter. Write an equation for the chloride level c, where y is the number of years after 2016.
  3. Explain how the table shows that the increase is constant rather than speeding up.
  4. Use your equation to find the chloride level the district would expect in 2030.
  5. Solve your equation to find the year Well 3 reaches the district's alert level of 200 milligrams per liter.
  6. Suppose the district cuts pumping at Well 3 starting in 2024, when chloride reads 104 milligrams per liter, and the rise slows to 3 milligrams per liter per year. In what year would Well 3 then reach 200 milligrams per liter?
  7. Well 7 sits farther inland. It read 22 milligrams per liter in 2024 and rises 2 milligrams per liter per year. What chloride level would you expect in Well 7 in 2044?
  8. Describe one monitoring step and one pumping change the district could adopt to slow the intrusion, and state what measurement would tell the district the change is working.

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: 8 mg/L per year.
    Each row is two years apart and rises 16 milligrams per liter, so 16 divided by 2 equals 8 milligrams per liter per year. Every pair of rows gives the same rate.
  2. Answer: c = 8y + 40.
    The rate of change 8 multiplies y, and the 2016 reading of 40 milligrams per liter is the initial value.
  3. Answer: Each two-year step adds the same 16 mg/L.
    Accept any answer noting the equal differences between consecutive readings. If the rise were speeding up, the differences would grow from one interval to the next.
  4. Answer: 152 mg/L.
    The year 2030 is 14 years after 2016, so c equals 8 times 14 plus 40, which is 112 plus 40, or 152 milligrams per liter.
  5. Answer: 2036.
    Solving 8y plus 40 equals 200 gives 8y equals 160, so y equals 20 years after 2016, which is the year 2036.
  6. Answer: 2056.
    From 104 milligrams per liter the well must rise another 96 to reach 200, and 96 divided by 3 equals 32 years after 2024, which is 2056. Cutting pumping in this fictional scenario buys the district twenty extra years.
  7. Answer: 62 mg/L.
    The year 2044 is 20 years after 2024, so 2 times 20 equals 40, and 22 plus 40 equals 62 milligrams per liter.
  8. Answer: Sample sentinel wells on a set schedule and lower the pumping rate at the well nearest the shore.
    Accept any monitoring step that names what is measured, where, and how often, such as quarterly chloride sampling at two wells between Well 3 and the beach. Accept any pumping change that reduces the stress causing the intrusion, such as capping daily withdrawal at Well 3, shifting production to Well 7, or resting the well during the dry season. The measurement that would show success is a chloride trend that flattens or falls at the sentinel wells over several years.

Differentiation

Support

Provide the graph already plotted with the axis labels filled in, so these students read the rate of change off the graph rather than computing it. Supply the equation frame c equals ___ times y plus ___ and allow a calculator throughout. For item 6, give the subtraction step 200 minus 104 already done.

Extension

Suppose the district finds that pumping at Well 3 also lowered the water level by 1.5 feet per decade. Ask students to explain, in a short paragraph, why a falling water table and a rising chloride level are two views of the same problem, and to sketch how the mixing zone moves in each case.

ELL supports

Pre-teach "chloride," "aquifer," "lens," and "alert level" with the labeled cross-section sketch from the direct instruction step, and keep it posted during independent work. If a student cannot yet write a full explanation for item 8, accept a labeled drawing of the monitoring wells plus these sentence frames: "The district should measure ___ at ___ every ___" and "Pumping less at Well 3 would help because ___."

Extensions

Students research how a coastal community in their own state or region tracks salt in its wells, then compare that program with the plan they wrote for item 8. Where no coastal aquifer exists nearby, students can instead compare their plan with a program that monitors road salt in a shallow inland aquifer, which produces the same rising chloride pattern from a completely different source.