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OOlH3 Speci~l emphasis is warranted in regard to adequacy of sampling for recordll of chemical quality. Inherently, a single analysis shows only the chemical constituents at a single point within the body of water sampled. Also, unless it represents a composite of samples taken ove~ a period of time, it shows only the constituents momentarily at that s8llIPling point. All too infrequently is a water body sampled at enough points or at enough times to represent all the potential variability within that body. All too few chemical-quality records define point and time of sampling in relation to features of the environment. Thus, an incautious hydrologist easily can be misled into undefendable chemical and geochemical conclusions. The sole defense is to "screen" all data for consistency, search out a full explanation of any seemingly abnormal data, and draw no conclusion by extrapolation. The difficulties just outlined are compounded in regard to ground- water quality. Here, available data generally are for samples withdrawn from wells that may range widely in depth and that, when pumped, may yield a blend of two or more waters of unlike chemical character, from distinct aquifers. The possible ramifications go far beyond the scope of this manual. Suffice it to point out that (1) a full explanation of all variations in chemical quality may require comprehensive informatioh on depth ahd casing records of wells, types and settings of pumps, and regimen of. withdrawals for some indeterminate period prior to sampling; but (2) rarely is such ancillary information reported as part of the chemical-quality record. I BIAS IN RECORDS OF LONG TERM Some hydrologic records have been published over a long term of years under a single station name when in actuality the statioh has been relocated one or more times during the term. In this situation, it is unwi~e to assume that the "same water" has been measured, and that the comp.osite record is not "biased" by" environmental differences among the several locations. For streamflow records in Water-3upply Papers of the Geological Survey, the latest station description should be scanned. to identify possible relevant changes in location. All such records through October 1950 have been compiled and summarized in Water- Supply Papers 1301 through 1319 and 1372; these reports include the history of gage changes, if any, at stations discontinued prior to the cut-off date of the compilation. For climatologic stations, changes in location are listed in the Weather Bureau's "Substation Histories" (Key to Meteorological Records Documentation No. 1.1). Each change of location should be treated as though a new and independent record had been started, unless the several partial records are shown to be consistent one with another, by double-mass plotting against records for one or more adjacent stations that are in the same environment and that have not been relocated. 3 "