You are measuring lead by atomic absorption spectroscopy, a technique that is known to respond linearly to lead concentration. One 10.0 mL portion of river water is added to a 25.00 mL volumetric flask and diluted to the mark. A second 10.0 mL portion is added to another 25.00 mL volumetric flask. 5.00 mLs of a standard solution containing 4.0 ppm Pb are added to the second flask which is then diluted to the mark. The measured absorbance for the first flask is 0.178. The measured absorbance for the second flask is 0.287. Use this information to calculate the concentration of lead in the river water.