The long lists of microbiological and inorganic water quality data will be useful to a number of people who want to know the quality of the water in their neighborhood. For those who want more of an overview, descriptive statistics have been calculated for the monthly water quality data by water quality zone and on an overall basis. Descriptive statistics are simple calculations of the characteristics of the data sets. All of the terms shown on the descriptive statistics table are explained in the definitions section of this site.

Descriptive statistics give an overall picture of how the water quality varies in the distribution system of Tucson Water. For example, the September 1997 descriptive statistics for hardness show substantial variation of the mean (or average) and median in WQZ 1 as compared to WQZ 10. The water quality served in WQZ 1 is heavily influenced by a couple of wells that are high in hardness and other salts. WQZ 10 is comprised primarily of wells from the southern Avra Valley groundwater basin which is known to have low hardness and salt concentrations.

Besides the mean values, there are other descriptive statistics which illustrate how widely dispersed the data is about the mean (or average). For example, the standard deviation for hardness in WQZ 1 is much higher than the standard deviation in WQZ 10. This means that the scatter of data in WQZ 1 is much higher indicating that the wells serving that area have widely different hardness concentrations. It is well known that for WQZ 10, all of the wells are from the Avra Valley groundwater basin and the water quality is consistent from that source. Low values of the coefficient of variation also illustrate the low relative differences in data scatter about a mean (or average) value.

The minimum and maximum parameter values for each WQZ also indicate how widely varying the data can be. Again looking at WQZ 1, the minimum and maximum values for hardness in September 1997 are 79 and 240 mg/L, respectively. For WQZ 10, the minimum and maximum values are much closer together at 72 and 81 mg/L, respectively.

The count of data points used to calculate the descriptive statistics in each WQZ is important to show how many sampling points are used in each area. Only 7 sampling points are in WQZ 1 which is in the most northwestern part of the Tucson Water service area (which contains a relatively small population). By comparison, WQZ 10 contains 37 sample locations, is the westernmost zone and is composed of some of the more densely inhabited areas of the Tucson Water service area.

The descriptive statistics also show that some water quality parameters vary more than others. For example on the basis of the calculated coefficients of variation, pH varies little between the WQZs and has the lowest variability of all the parameters (about 3% system wide in September 1997). Hardness, on the other hand, had a system wide variability of 34% in September 1997.

If descriptive statistics are used in conjunction with the Water Quality Zone Map and the individual water quality data points, a more complete picture of the water quality in the Tucson Water service area can be obtained.

• View most recent Water Quality Descriptive Statistics