1995-06-14_HYDROLOGY - M1977378 - Laserfiche WebLink

Home Browse Search

WATER QUALITY E...._,RATIONS AT STATION A72 ON THE AN,&MS RIVER USING CONTROL CHARTS • Low Flow (Q < 100 cfs) • High Flow (Q > 300 cfs) • Intermediate Flow (100 cfs < Q < 300 cfs) For the purpose of this evaluation, the single flow value greater than 2000 cfs is treated together with the other high flow values, as there is no way to develop a distribution related to a single sample value. 11. Because each of these classes of flow conditions represents a physical response of the upper basin to the complex factors that may govern interactions between meteoric water and the geology (and mining), it is appropriate to evaluate water quality separately for each of the three classes. Method for Evaluating Future Water Quality with Respect to Reference Conditions - Control Charts 1. Control charts are a standard tool for comparing continuing measurements from a process (including from a measurement process) to a stable probability distribution (NBS91). 2. Control charts are designed to detect specified departures in the process from the "assumed conditions" (NBS91). With respect to water-quality monitoring at A72, the "assumed conditions" are the probability distribution of Zn values in the baseline set, taken to represent "reference water quality". The "specified departures" would be values that are (statistically) discernibly higher than those expected for "reference water quality", based on the probability distribution that describes the baseline data set. 3. An X-chart for Zn at A72, which provides a running, graphical record of Zn concentration with respect to sampling event (time), allows reviewers to evaluate (a) gross errors in analysis; (b) shifts in average values; (c) trends in Zn concentrations (NBS91). The X-chart uses the mean and standard deviation of the baseline data set (for each flow class, in the case of the Animas River at A72) to establish the range of values within which there is a 95% probability that samples from the same population as the baseline set will fall (i.e., the level of the test is 0.05, and the test is one-sided because we are concerned primarily with water quality that exceeds "reference water quality"). 4. Table 1 presents and analyzes Zn data for the Low-Flow (Q< 100cfs) data set. Given that the population of Zn concentrations in the Animas River at A72 can be described by Student's-t distribution, the upper confidence limit for the 95 envelope and six degrees of freedom has a critical t value of 2.447. For a mean of 517 ug/1 and a standard deviation of 87 ug/l, the upper control limit for Q< 100 cfs would be 731 ug/l. 5. Table 2 presents and analyzes Zn data for the Intermediate-Flow (100 cfs<Q<300cfs) data set. Given that the population of Zn concentrations in the Animas River at A72 can be described by Student's-t distribution, the upper Geochimica, Inc. 4 ANIMAS RIVER/9-Jun-95