Copper concentrations (ppb) at two background and one compliance wells (six monthly samples for each well).
A data frame with 18 observations on the following 4 variables.
a numeric vector of copper concentrations (ppb)
a factor indicating the month of collection
a factor indicating the well number
a factor indicating the well type (background vs. compliance)
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.47.