Arsenic in soil. In Denver, Colorado, environmentalists have discovered a link between hig...

Arsenic in soil. In Denver, Colorado, environmentalists have discovered a link between high arsenic levels in soil and a crabgrass killer used in the 1950s and 1960s ( Environmental Science&Technology , Sept. 1, 2000). The recent discovery was based, in part, on the accompanying scatterplots. The graphs plot the level of the metals cadmium and arsenic, respectively, against the distance from a former smelter plant for samples of soil taken from Denver residential properties.

a. Normally, the metal level in soil decreases as distance from the source (e.g., a smelter plant) increases. Propose a straight-line model relating metal level y to distance x from the plant. On the basis of the theory, would you expect the slope of the line to be positive or negative?

---

b. Examine the scatterplot for cadmium. Does the plot support the theory you set forth in part a ?

---

c. Examine the scatterplot for arsenic. Does the plot support the theory of part a ? ( Note: This finding led investigators to discover the link between high arsenic levels and the use of the crabgrass killer.)