Ariz. Admin. Code § R18-11-603 - General Data Interpretation Requirements

A. The Department shall use the following data conventions to interpret data for impaired water identifications and TMDL decisions:

1. Data reported below laboratory detection limits.

a. When the analytical result is reported as i. Use these statistically derived values in trend analysis, descriptive statistics or modeling if there is sufficient data to support the statistical estimation of values reported as less than the laboratory detection limit; or

ii. Use one-half of the value of the laboratory detection limit in trend analysis, descriptive statistics, or modeling, if there is insufficient data to support the statistical estimation of values reported as less than the laboratory detection limit.

b. When the sample value is less than or equal to the laboratory detection limit but the laboratory detection limit is greater than the surface water quality standard, shall not use the result for impaired water identifications or TMDL decisions;

2. Identify the field equipment specifications used for each listing cycle or TMDL developed. A field sample measurement within the manufacturer's specification for accuracy meets surface water quality standards;

3. Resolve a data conflict by considering the factors identified under the weight-of-evidence determination in R18-11-605(B);

4. When multiple samples from a surface water or segment are not spatially or temporally independent, or when lake samples are from multiple depths, use the following resultant value to represent the specific dataset:

a. The appropriate measure of central tendency for the dataset for:

i. A pollutant listed in the surface water quality standards 18 A.A.C. 11, Article 1, Appendix A, Table 1, except for nitrate or nitrate/nitrite;

ii. A chronic water quality standard for a pollutant listed in 18 A.A.C. 11, Article 1, Appendix A, Table 2;

iii. A surface water quality standard for a pollutant that is expressed as an annual or geometric mean;

iv. The surface water quality standard for temperature or the single sample maximum water quality standard for suspended sediment concentration, nitrogen, and phosphorus in R18-11-109;

v. The surface water quality standard for radiochemicals in R18-11-109(G); or

vi. Except for chromium, all single sample maximum water quality standards in R18-11-112.

b. The maximum value of the dataset for:

i. The acute water quality standard for a pollutant listed in 18 A.A.C. 11, Article 1, Appendix A, Table 2 and acute water quality standard in R18-11-112;

ii. The surface water quality standard for nitrate or nitrate/nitrite in 18 A.A.C. 11, Article 1, Appendix A, Table 1;

iii. The single sample maximum water quality standard for bacteria in subsections R18-11-109(A); or

iv. The 90th percentile water quality standard for nitrogen and phosphorus in R18-11-109(F) and R18-11-112.

c. The worst case measurement of the dataset for:

i. Surface water quality standard for dissolved oxygen under R18-11-109(E) . For purposes of this subsection, worst case measurement means the minimum value for dissolved oxygen;

ii. Surface water quality standard for pH under R18-11-109(B) . For purposes of this subsection, "worst case measurement" means both the minimum and maximum value for pH.

B. The Department shall not use the following data for placing a surface water or segment on the Planning List, the 303(d) List, or in making a TMDL decision.

1. Any measurement outside the range of possible physical or chemical measurements for the pollutant or measurement equipment,

2. Uncorrected data transcription errors or laboratory errors, and

3. An outlier identified through statistical procedures, where further evaluation determines that the outlier represents a valid measure of water quality but should be excluded from the dataset.

C. The Department may employ fundamental statistical tests if appropriate for the collected data and type of surface water when evaluating a surface water or segment for impairment or in making a TMDL decision. The statistical tests include descriptive statistics, frequency distribution, analysis of variance, correlation analysis, regression analysis, significance testing, and time series analysis.

D. The Department may employ modeling when evaluating a surface water or segment for impairment or in making a TMDL decision, if the method is appropriate for the type of waterbody and the quantity and quality of available data meet the requirements of R18-11-602 . Modeling methods include:

1. Better Assessment Science Integrating Source and Nonpoint Sources (BASINS),

2. Fundamental statistics, including regression analysis,

3. Hydrologic Simulation Program-Fortran (HSPF),

4. Spreadsheet modeling, and

5. Hydrologic Engineering Center (HEC) programs developed by the Army Corps of Engineers.