Results of the U.S. Geological Survey's analytical evaluation program for standard reference samples : T-151 (trace constituents), M-144 (major constituents), N-55 (nutrient constituents), N-56 (nutrient constituents), P-29 (low ionic strength constituents), GWT-2 (ground-water trace constituents), GWM-2 (ground-water major constituents), AMW-4 (acid mine water constituents), and Hg-25 (mercury) distributed in September 1997 / - Farrar, Jerry W.