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Muddy Creek: Monitoring Runoff from a Building Site

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Muddy Creek: Monitoring Runoff from a Building Site

Use turbidity data to judge ways to keep soil out of a stream.

Monitoring the creek

Read the passage and study the storm table. Then answer the questions.

Brown Water Below the New Road

1Every spring, the Alder River Watershed Club at Fairmont Middle School helps with a cleanup along the river and the streams that drain into it. This year, club members Beatriz Castillo and Jonah Whitfield noticed a new problem. A construction crew had cleared 4 hectares of land beside Cobble Creek, a stream that flows into the Alder River, to build a road and a parking area. After heavy rains, the creek below the site turned brown.

2During a rainstorm, water running over bare soil can pick up loose bits of soil, called sediment, and carry them into streams. Cloudy water blocks some of the sunlight that underwater plants need. Sediment can also clog the gills of fish and stream insects and bury the gravel where some fish lay their eggs. Scientists measure how cloudy water is with a turbidity meter, in units called NTU. The higher the number, the cloudier the water. The town set a goal of keeping Cobble Creek below 50 NTU after storms.

3With help from a town water specialist, the club set up a way to monitor the creek. Within two hours after each storm that dropped at least 2 cm of rain, members used the same meter to test water from two bridges: Site U, upstream of the construction site, and Site D, downstream of it.

4The town also tested ways to keep sediment out of the creek. After the first two storms, the crew put up a silt fence, a low wall of tough fabric staked along the bottom edge of the site. Water can seep through the fabric, but much of the soil it carries is trapped behind it. Before the fifth storm, the crew also covered the bare soil with straw and sowed grass seed in it. The silt fence cost $1,400, and the straw and grass seed cost $1,100.

Cobble Creek Turbidity After Storms
Storm	Rain (cm)	Site U (NTU)	Site D (NTU)	Solution in place
1 (April 3)	3	12	240	None
2 (April 19)	2	10	210	None
3 (May 8)	3	14	90	Silt fence
4 (May 22)	2	11	70	Silt fence
5 (June 10)	3	13	30	Fence + straw and seed
6 (June 25)	2	9	22	Fence + straw and seed

1.

In Storm 1, how much higher was the turbidity at Site D than at Site U? NTU

2.

Why did the club test the water at Site U as well as at Site D?

A. To make the average turbidity of the creek look lowerB. To see how cloudy the creek was before it passed the siteC. Because sediment from the site floats upstream after rainD. Because a turbidity meter needs two samples to work

3.

Why did the club test the creek within two hours after storms of at least 2 cm instead of on any day of the week?

A. Runoff happens during rain, so similar storms give readings that can be compared.B. Rain washes the sediment out of the creek, so it is the best time to clean it.C. The creek water is clearest right after rain, so the meter reads it more easily.D. The construction crew works only on rainy days, so that is when soil moves.

4.

Which statement best describes the Site D data?

A. Turbidity was highest after the smaller 2 cm storms in each pair.B. Turbidity went up after the silt fence went in, then went down in June.C. Turbidity fell after the fence went in and fell more once the soil was covered.D. Turbidity stayed about the same no matter which solution was in place.

5.

The Site D water after Storm 1 was times as cloudy as the Site D water after Storm 5, which had the same amount of rain.

Reducing the impact

Use the passage and the storm table to judge each solution.

6.

Sort each action by what it does about the muddy runoff.

7.

What best explains why covering the soil with straw and grass seed helped?

A. The grass seed soaked up the mud that was already in the creek.B. Covered soil is harder for rain and runoff to loosen and carry away.C. The straw cooled the creek water, so the mud in it sank to the bottom.D. The straw made the creek flow faster, so mud washed farther downstream.

8.

Next year the town will clear land for a school parking lot near the same creek. Its plan must keep the creek below 50 NTU after storms of 2–3 cm and cost no more than $3,000. Which plan do the data support best?

A. A silt fence plus straw and grass seed on the bare soilB. A silt fence plus a $6,500 pond that lets the sediment settleC. A silt fence alone, since it was the cheapest solution testedD. A silt fence put up after the first heavy rain shows a problem

9.

When the construction ends, which plan would best show whether Cobble Creek has recovered?

A. Ask the construction crew whether the creek looks clean to them from the new road.B. Test the water once at Site U on a dry, sunny day next summer and compare it to Storm 1.C. Test the water at Site D after one storm and stop if the reading is below 100 NTU.D. Keep testing at both bridges after similar storms and count stream insects below the site.

10.

A crew member says the creek was clearer in June only because the June storms were smaller, not because of the straw and grass seed. Do you agree? Write a claim, use at least two pieces of data from the storm table, and explain your reasoning.