Agriculture: Ground Water Unfortunately, arsenic occurs naturally in some ground water (Reference: Union Carbide Technical Report K/UR-1). A mean arsenic level of m 5 8.0 parts per billion (ppb) is considered safe for agricultural use. A well in Texas is used to water cotton crops. This well is tested on a regular basis for arsenic. A random sample of 37 tests gave a sample mean of x 5 7.2 ppb arsenic, with s 5 1.9 ppb. Does this information indicate that the mean level of arsenic in this well is less than 8 ppb? Use a 5 0.01.