Laserfiche WebLink
WATER QUALITY EV, _ IATIONS AT STATION A72 ON THE ANI. __3 RIVER USING <br /> CONTROL CHARTS <br /> 5. Regular processes - including measurement processes - can be described in terms of <br /> a probability distribution of the variables that describe the process (NBS91). That <br /> is, for each variable, the history of the process is characterized by the moments of <br /> the sample values, from which one can draw inferences about the distribution of the <br /> population from which the sample was drawn. <br /> 6. "Reference water quality" is intended to represent the history of water quality at a <br /> given point (say, A72) over some defined period of time (say, the period of record <br /> of WQCD and other Agency measurements for Station A72). From January 1989 <br /> through February 1995, there exist 54 measurements of Zn concentration at A72. <br /> Twenty-six of those sampling events, all between September 1991 and January <br /> 1995, also have simultaneous measurements of stream flow. The set of twenty-six <br /> measurements of coupled flow and Zn data collected from September 1991 through <br /> January 1995 is termed "the baseline set", representing the available history of Zn <br /> and Q at A72. The population from which this set of samples was drawn defines <br /> reference water quality. <br /> 7. Individual sample measurements of water quality in streams usually differ from <br /> previous and subsequent measurements for any given variable, due to (a) the variety <br /> of natural (and, in some instances, anthropogenic) factors that lead to differences in <br /> the release over time of elements from solid phases to the aqueous phase, (b) the <br /> amount of water that reports to the stream over time, (c) sampling variability, and <br /> (d) analytical variability. Two standard computations that can be used to describe <br /> the probability distribution for a range of measured values of the variable are the <br /> mean and variance of the data set. <br /> 8. Therefore, the set of water quality measurements at A72 should be described by a <br /> statistical distribution of values - not by any single value - for any given variable, <br /> say Zn. The distribution of values for the variable(s) of interest and standard tools <br /> of statistical inference allow evaluation of the probability that a new sample belongs <br /> to the same probability distribution as a set of measurements that are taken to <br /> represent "reference water quality". <br /> 9. The high-mountain portion of a river such as the Animas is subject to seasonal <br /> fluctuations in flow due to hydrologic response of the basin to climatic factors. <br /> Flow and the concentration of dissolved species in any river water are generally <br /> inversely related. Under low-flow conditions, water quality is dominated by the <br /> chemistry of base flow, whereas under high-flow conditions, prompt runoff mixes <br /> with the base flow to produce an integrated surface water sample that has lower <br /> concentrations than does base flow alone. Base flow represents the result of low <br /> water-to-rock ratio, long residence-time reactions between infiltrated precipitation <br /> and subsurface solids. In contrast, prompt runoff represents relatively high water- <br /> to-rock ratio, short residence time reactions. Thus, the water quality of these two <br /> components represents two different populations, and the in-stream flow represents <br /> a set of mixtures that would be described by yet another process distribution. <br /> 10. Based on measurements of stream flow at A72 from 1991-1995 (Figure 1), flow <br /> conditions at this point on the Animas consist of three (and perhaps a fourth) classes <br /> of flow conditions: <br /> Geochimica, Inc. 3 ANIMAS RIVER/9-Jun-95 <br />