The U.S. Geological Survey conducts research and collects hydrologic data relating to the Nation's water resources. Two water quality laboratories in Atlanta, Georgia, and Denver, Colorado, support the national research programs, and provide chemical analyses of natural waters for the data program. Additional chemical water quality data are provided by cooperator and contract laboratories.

Continuous quality assurance efforts with these laboratories require several thousand reference samples each year. Reference samples approximating actual field samples provide the most realistic quality assurance for the laboratories. Seven types of natural matrix reference water samples are prepared for use in the Survey's quality assurance program. These include samples containing major constituents, trace metals, nutrients, herbicides, insecticides, trace metals in a water and suspended-sediment mixture, and precipitation (snowmelt). To prepare these reference samples, natural water is collected in plastic drums and the sediment is allowed to settle. The water is then filtered, selected constituents are added, and if necessary the water is acidified and sterilized by ultraviolet irradiation before bottling in plastic or glass. More than 1000 1-L samples of a given type may be prepared at a time. These reference samples are distributed twice yearly to more than 100 laboratories for chemical analysis. The most probable values for each constituent are determined by evaluating the data submitted by the laboratories using statistical techniques recommended by ASTM.

A stockpile of several thousand reference samples is maintained by the preparation of new samples as needed. Periodically, some of these samples are submitted to laboratories as “unknowns”. When the analytical data for these samples are reported and evaluated, problem areas detected are promptly reported to the respective laboratory.

Use of both identified and unidentified reference samples provides some of the quality assurance data necessary to ensure the continuing accuracy of chemical analyses obtained to support the Survey's basic water-data collection and research program activities.