The rate is not provided in the stimulus, but the argument implies that in a place where the traffic pattern is similar to the one in Albritten 20 years ago the salt content is only 10 mg/ liter, hence the salt content must have increased from 10 mg to 100 mg over 20 years. The fact that the true rate of salt increase is a lot lower - only 90 to 100 mg over 20 years suggests that the concerns regarding groundwater becoming too salty are unfounded.