Dames & Moore, 1999 - USDA Forest Service

data sets where the total value was not detected but the dissolved result was above the detection limit.
The total zinc results in these instances are assumed to be more representative of actual dissolved zinc
concentrations. Table 5.3- 14 lists these specific data points.
The background module of the MTCAStat program was used to determine the distribution of data. Based
on the data distribution, the 90th percentile, mean, standard deviation, median, minimum, and maximum
were calculated.
The results of the statistical analysis for the total data set, the spring seasonal set and the fall seasonal data
set are summarized in Tables 5.3-1, 5.3-1 5 and 5.3- 16, respectively. The statistical summary for Sets 1 A
and B are summarized in Table 5.3-17.
The 90th percentile value is a default value for the MTCAStat background evaluation. The data
distribution allowed the calculation of a 90th percentile, with few exceptions. Statistical analysis was not
performed for dissolved beryllium, chromium, selenium, silver, and thallium data sets because these
metals were not detected. A 90th percentile was not calculated for arsenic in the fall data set due to a
limited number of data points. Generally, data distribution was lognormal with an occasional normal
distribution. There were several data sets that were distribution free (non-parametric). The non-
parametric distribution generally results from data sets that include a wide range of detected
concentrations or data sets with concentrations (detects andlor non-detects) that are similar and tend to
bunch rather than spread. The 90th percentiles for the total data set are summarized in Table 5.3-1. The
90th percentiles for the seasonal data sets and sets 1A and 1B are summarized below,
Metal @issolved)
Aluminum
Arsenic
Cadmium
Copper
Iron
Lead
Magnesium
Manganese
Zinc
Total
37.4
0.9
0.07
1.06
40
0.54
626
2.42
7.8 1
Spring
48
0.84
0.06
1.02
30
0.3 1
540
2.64
4.84
90th Percentiles
(~gn)
The 90th percentile concentrations of metals in the total data set include spring, summer, fall and winter
data points. The spring and fall data sets generally bracket the range of 90th percentile concentrations for
the total data set. Limited variation exists between the 90th percentile metal concentrations in the total
data set as compared to the spring and fall seasonal data sets for dissolved aluminum, arsenic, cadmium,
copper, iron, lead, magnesium, manganese, and zinc. Additionally, limited variation exists between area
background Sets 1A (Railroad Creek stations) and 1B (other background stations). Based on the
statistical analysis, it is appropriate to use the 9 0 percentile ~ values in the total data set for area
background assessment. The data also justify using the upstream Railroad Creek station(s), RC-1, RC-6,
and/or RC- I 1, to monitor area background surface water quality in Railroad Creek.
\KIM-SEA IWOL I\COMMOMWmWDATAWN(EWRTS W d
17693-005-0 19Uuly 28. 1999; 1 1 :09 AM;DRAFT FINAL RI REPORT
Fall
26
0.82 (50th YO)
0.08
0.9
56
0.56
744
2.9
13
5- 1 8
Set lA
Railroad Creek
39
0.92
0.07
1.13
44
0.53
- _ _ _ _ _ _ _ - 660
2.47
8.1
Set 1B
Other Background
36
0.97
0.07
0.81
26
1.2
703
2.57
9.4
DAMES & MOORE