Washington State's Department of Ecology measured MTBE and BTEX at 70 LUST sites around the state, using groundwater samples taken from monitoring wells. To test the hypothesis that groundwater contamination by organic compounds at these sites was caused by spillage or leakage of gasoline, one might compare the concentration of MTBE with the concentration of benzene in the groundwater sample. The values of MTBE and benzene range over about 5 orders of magnitude, making them difficult to compare on one graph. Logarithms of both measurements were calculated and are displayed on the scatterplot. As is apparent visually, there is little to nor correlation.
Students can practice their regression techniques to confirm and quantity the lack of correlation.
However, groundwater contamination and sampling are not so simple. MTBE might migrate faster than benzene, causing changes in the ratio of these two contaminants with time and distance.