WAC 173-351-420
Statistical methods for groundwater
monitoring. (1) The owner or operator must calculate and
evaluate all of the following statistics using background
groundwater quality data:
(a) The background mean;
(b) The background variance;
(c) The standard deviation of the background data;
(d) The coefficient of variation of the background data;
(e) The standard error of the background data; and
(f) Other statistics testing for homogeneity of variance
and the normality of the background data.
(2) The owner or operator must specify in the permit
application in accordance with WAC 173-351-730 (1)(b)(iii) one
of the following statistical methods to be used in evaluating
groundwater monitoring data for each hazardous constituent. The statistical test chosen shall be conducted separately for
each hazardous constituent in each well. The statistical
methods to be used are:
(a) A tolerance or prediction interval procedure in which
an interval for each constituent is established from the
distribution of the background data, and the level of each
constituent in each compliance well is compared to the upper
tolerance or prediction limit;
(b) A parametric analysis of variance (ANOVA) followed by
multiple comparisons procedures to identify statistically
significant evidence of contamination. The method must
include estimation and testing of the contrasts between each
compliance well's mean and the background mean levels for each
constituent;
(c) An analysis of variance (ANOVA) based on ranks
followed by multiple comparisons procedures to identify
statistically significant evidence of contamination. The
method must include estimation and testing of the contrasts
between each compliance well's median and the background
median levels for each constituent;
(d) A control chart approach that gives control limits
for each constituent; or
(e) Another statistical test method that meets the
performance standards of this section. The owner or operator
must place a justification for this alternative in the permit
application in accordance with WAC 173-351-730 (1)(b)(iii). The justification must demonstrate that the alternative method
meets the performance standards of this section.
(3) Any statistical method chosen under this section
shall comply with the following performance standards, as
appropriate:
(a) The statistical method used to evaluate groundwater
monitoring data shall be appropriate for the distribution of
chemical parameters or hazardous constituents. If the
distribution of the chemical parameters or hazardous
constituents is shown by the owner or operator to be
inappropriate for a normal theory test, then the data must be
evaluated to determine if nonnormal conditions are due to
laboratory or sampling error, poor well construction, seasonal
or spatial variability, or actual site conditions. Transformed or a distribution-free theory test may be used,
upon a determination of why nonnormal conditions exist. If
the distributions for the constituents differ, more than one
statistical method may be needed.
(b) If an individual well comparison procedure is used to
compare an individual compliance well constituent
concentration with background constituent concentrations or a
groundwater protection standard, the test shall be done at a
Type I error level no less than 0.01 for each testing period. If a multiple comparison procedure is used, the Type I
experiment wise error rate for each testing period shall be no
less than 0.05; however, the Type I error of no less than 0.01
for individual well comparisons must be maintained. This
performance standard does not apply to tolerance intervals,
prediction intervals, or control charts.
(c) If a control chart approach is used to evaluate
groundwater monitoring data, the specific type of control
chart and its associated parameter values shall be protective
of human health and the environment. The parameters shall be
determined after considering the number of samples in the
background data base, the data distribution, and the range of
the concentration values for each constituent of concern.
(d) If a tolerance interval or a predictional interval is
used to evaluate groundwater monitoring data, the levels of
confidence and, for tolerance intervals, the percentage of the
population that the interval must contain, shall be protective
of human health and the environment. These parameters shall
be determined after considering the number of samples in the
background data base, the data distribution, and the range of
the concentration values for each constituent of concern.
(e) The statistical method shall account for data below
the limit of detection with one or more statistical procedures
that are protective of human health and the environment. Any
practical quantitation limit (PQL) that is used in the
statistical method shall be the lowest concentration level
that can be reliably achieved within specified limits of
precision and accuracy during routine laboratory operating
conditions that are available to the facility.
(f) If necessary, the statistical method shall include
procedures to control or correct for seasonal and spatial
variability as well as temporal correlation in the data.
(4) The owner or operator must determine whether or not
there is a statistically significant increase over background
values for each parameter or constituent required in the
particular groundwater monitoring program that applies to the
MSWLF unit after each sampling event and as determined under
this section.
(a) In determining whether a statistically significant
increase has occurred, the owner or operator must compare the
groundwater quality of each parameter or constituent at each
monitoring well designated pursuant to WAC 173-351-430 or 173-351-440 to the background value of that constituent,
according to the statistical procedures and performance
standards specified under this section.
(b) Within thirty days after receipt of the analytical
data, the owner or operator must determine whether there has
been a statistically significant increase over background at
each monitoring well (at all hydraulically placed upgradient
and downgradient wells).
[Statutory Authority: Chapter 70.95 RCW and 40 CFR 258. 93-22-016, § 173-351-420, filed 10/26/93, effective 11/26/93.]