SKILL: MAKING A DOUBLE BAR GRAPH
Tracking Contaminants
MATH TUTORIAL
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Click on Math Tutorial
for more help with
bar graphs.
The layered sediments at the bottom of the ocean have formed over
time. The particles in the deeper layers settled to the floor long ago,
while those in the top layers settled out of the water more recently.
By studying the amounts of pollutants in different layers of sediment,
scientists can see how the water quality has changed over time. In
1991, scientists collected sediment cores north of Dash Point in Puget
Sound. The table below shows levels of two pollutants, lead and
arsenic, in the sediment layers for 1880, 1960, and 1990. The levels
are measured in milligrams per kilogram dry weight (mg/kg d.w.).
Levels of Lead and Arsenic in Sediments
Year
Lead (mg/kg d.w.)
Arsenic (mg/kg d.w.)
1880
10
6
1960
62
22
1990
45
17
You can use a double bar graph to analyze the data. A double
bar graph shows two sets of data on the same graph. The first two
bars of the graph are drawn for you below.
Example
(2) Draw bars for the lead
data. Use the scale to
determine the height of
each bar, as shown.
(3) Draw the arsenic bars
next to the lead bars.
Machines mounted on
boats drill down into
the ocean floor to collect
sediment cores.
(4) Shade the bars in differ-
ent colors. Include a key.
Levels of Lead and
Arsenic in Sediments
Amount of lead and arsenic
(mg/kg d.w.)
(1) Copy the axes and labels.
Lead
Arsenic
80
60
40
20
10
0
1880
1960
Year
1990
Make a double bar graph of the data by following the steps
above. Use your graph to answer the following questions.
1. What happened to the levels of lead and arsenic between
1880 and 1960?
2. What happened to the levels of lead and arsenic between
1960 and 1990?
This tube contains a sediment core.
160 Unit 1: Earth’s Waters
CHALLENGE Because lead can be harmful to humans, the
use of leaded gasoline in new cars was banned in 1975 and
the sale of lead-based paint was banned in 1978. How might
these bans have affected the amount of lead in Puget Sound?
Use evidence from your graph to support your answer.