Europium is a lanthanide element found in parts per billion levels in natural waters. It can be measured from the intensity of orange light emitted when a solution is illuminated with ultraviolet radiation. Certain organic compounds that bind Eu(III) are required to enhance the emission. The figure below shows standard addition experiments in which 10.00 mL of a sample and 20.00 mL containing a large excess of organic additive were placed in 50 mL volumetric flasks. Then Eu(III) standards (0, 5.00, 10.00 or 15.00 mL) were added and the flasks were diluted to 50.0 mL with water. Standards added to tap water contained 0.160 ng/mL (ppb) of Eu(III), but those added to pond water were 100 times more concentrated. (a) From the x intercepts in the graph, calculate the concentration of Eu(III) (ng/mL) in tap water.