A Dry Year in Prairie Hollow: Why Soil and Water Are Uneven

Use soil and well data to explain where a farm community's resources are.

Answer Key

Soil on five farms

Read the passage and study Table 1. Then answer the questions.

The Summer Without Rain

1Prairie Hollow is a farm community on the plains east of the Cassel River. This year almost no rain fell from May through August. On some farms the corn stayed green, while on others it dried up by midsummer. Some wells dropped fast, and others hardly changed. At a meeting of the farm co-op, soil and water scientist Ms. Philippa Obuya used survey records to explain why the county's soil and water are spread so unevenly. Seventh grader Zofia Petrosyan took notes for her science fair project.

2The topsoil in the county formed in loess, a fine yellow-brown silt carried by wind. During the last ice age, melting glaciers to the north sent floods of meltwater down the Cassel River valley each summer. The floods left wide bars of silt that dried out in the cold, windy months. Strong winds, mostly from the west, picked up the dry silt and dropped it over the land. Loess holds water well in the tiny spaces between its grains. Under the loess lies a gravelly, rocky layer that holds much less water.

3Farms here get water from two layers. Shallow wells reach sand and gravel along the river, which river water and rain refill each spring. Deep wells go down about 300 m to the Pembina sandstone, a layer with many small connected spaces. The sandstone tilts up and reaches the surface in hills 150 km to the west, where rain and snowmelt soak into it. Above it lies a thick layer of shale, which water barely passes through, so the water in the sandstone is trapped under pressure.

4Scientists can estimate how long ago groundwater fell as rain or snow. Water in the Pembina sandstone is about 2,000 years old in wells 20 km from the hills, about 8,000 years old in wells 80 km away, and about 15,000 years old under Prairie Hollow, 150 km away. Table 2 shows four of the co-op's wells this summer.

Table 1. Five farms in August of the dry year
Farm	East of river (km)	Loess thickness (m)	Corn still green (%)
A	2	18	85
B	10	8	70
C	25	3	55
D	50	1	30
E	80	0.3	15

Which statement best describes Table 1?

A. Farther east of the river, the loess gets thicker and more of the corn stays green.
B. The loess is about the same everywhere, but the corn dries more on farms near the river.
C. The loess gets thinner farther east, but the corn stays about equally green on each farm.
D. Farther east of the river, the loess gets thinner and less of the corn stays green.

The loess on Farm A is how many times as thick as the loess on Farm C? 6 times as thick

Use paragraph 2. Which explanation of the loess pattern in Table 1 fits best?

A. West winds lifted silt off the river's ice-age bars and dropped the most of it nearest the river.
B. Glaciers spread silt evenly over the county, and later farming wore it thin near the river.
C. Each spring today, the river floods across the plains and spreads silt up to 80 km east.
D. The bedrock under each farm crumbled into silt, and it crumbled fastest near the river.

The co-op is mapping a new farm 40 km east of the river. Based on Table 1, about how thick is its loess most likely to be?

A. About 0.2 m
B. About 5 m
C. About 2 m
D. About 10 m

Water under the farms

Use Table 2 and paragraphs 3 and 4.

Table 2. Four co-op wells, May to August of the dry year
Well	Layer it draws from	Depth (m)	Water age (years)	Level drop (m)
W1	River sand and gravel	12	about 1	4
W2	River sand and gravel	15	about 1	3.5
W3	Pembina sandstone	310	about 15,000	0.2
W4	Pembina sandstone	330	about 15,000	0.3

Which statement best describes Table 2?

A. The four wells dropped by about the same amount during the dry summer months.
B. The shallow wells dropped several meters, while the deep wells dropped less than half a meter.
C. The deep wells dropped the most, about ten times as much as the shallow wells.
D. The shallow wells hardly changed, while the deep wells each dropped about 4 m.

Use paragraph 3. Which explanation of the pattern in Table 2 fits best?

A. The shale above the sandstone let this summer's rain soak straight down into the deep wells.
B. The deep sandstone sits nearer Earth's hot interior, which makes new water underground.
C. The gravel relies on this year's rain and river water; the sandstone holds old stored water.
D. The river gravel is older than the sandstone, so it has had more time to lose its water.

Use paragraph 4. The water under Prairie Hollow soaked into the sandstone in the hills 150 km away about 15,000 years ago. (1 km = 1,000 m)

On average, the water moved through the sandstone about 10 m per year.

The co-op pumps water from the Pembina sandstone. Which statement is best supported by paragraph 4 and your answer to question 7?

A. Water pumped out this year will be replaced by next spring's snowmelt in the hills.
B. The sandstone's water is younger than the river water, so it refills more quickly.
C. The shale above slowly makes new water, so the supply grows as the shale gets older.
D. New water takes thousands of years to arrive, so heavy pumping can use up the supply.

Sort each resource by the process that put it where it is.

The 18 m of loess on Farm AThe water in Well W1The 15,000-year-old water in Well W3The thin loess on Farm EThe water in Well W4The water that refills Well W2 each spring

Ice-age winds carrying silt	River water and rain each year	Slow flow from the western hills
The 18 m of loess on Farm A The thin loess on Farm E	The water in Well W1 The water that refills Well W2 each spring	The 15,000-year-old water in Well W3 The water in Well W4

10.

The co-op wants one new well for its cattle that will keep giving water in a dry year like this one. Which choice is best supported by Table 2?

A. A shallow well into the river gravel, since it dropped the least
B. A deep well into the sandstone, since its level barely dropped
C. A second well near W2, since W2 dropped less than W1 did
D. A well into the shale, since the shale sits above the sandstone water

Explaining the dry year

Use the whole passage and both tables.

11.

A neighbor says, "Every farm in the county got the same lack of rain, so the soil and water under every farm should be the same. The farms that did badly just farmed poorly." Do you agree? Make a claim, use evidence from Table 1, Table 2 and the passage, and explain how geoscience processes spread soil and groundwater unevenly.

I disagree. The farms got the same weather this year, but past geoscience processes left them with very different soil and water. Table 1 shows loess 18 m thick on Farm A, 2 km from the river, but only 0.3 m on Farm E, 80 km away, and 85% of Farm A's corn stayed green compared with 15% on Farm E. During the last ice age, west winds blew silt off the river's meltwater bars and dropped the most near the river, so farms near the river got thick loess that holds water well. Table 2 shows that shallow wells in river gravel dropped 3.5 to 4.0 m because that layer depends on each year's rain and river water, while deep wells in the Pembina sandstone dropped only 0.2 to 0.3 m. The sandstone's water soaked in at hills 150 km away about 15,000 years ago and is trapped under shale, so only farms with deep wells into it have a steady supply, and it refills so slowly that it can be used up.