Nickel concentrations (ppb) at four monitoring wells (five monthly samples for each well).
A data frame with 20 observations on the following 3 variables.
a numeric vector of nickel concentrations (ppb)
a factor indicating the month of collection
a factor indicating the well number
USEPA. (1992c). Statistical Analysis of Ground-Water Monitoring Data at RCRA Facilities: Addendum to Interim Final Guidance. Office of Solid Waste, U.S. Environmental Protection Agency, Washington, D.C. p.7.