In each case, 10.00 mL of the sample solution and 15.00 mL of an organic additive were added to a series of five 50 mL volumetric flasks. The standard Eu(III) or Tb(III) solution (0, 5.00, 10.00, 15.00, and 20.00 mL) was then added to the flasks and they were diluted to 50.0 mL with water. The Eu(III) standards added to the groundwater sample contained 0.185 ng/mL (ppb) of Eu(III). Calculate the concentration of Eu(III) in the groundwater sample.