Appendix 4. Quality assurance for volatile organic compound, methane, and 

inorganic analyses 

Quality assurance of water-quality data is an important step in data interpretation. 

This section describes the quality assurance of the groundwater and surface-water data 

collected from the West Branch Canal Creek area during 2005. The quality-assured data 

described in this section include the VOC, methane, and inorganic data. VOC data have 

undergone independent data-validation procedures and the results are described below. 

To determine the reproducibility of analyses, duplicate samples were collected for 

all methane samples, and VOC samples. Duplicate inorganic samples were collected 

from approximately 20 percent of the sampled locations. All methane and inorganic 

duplicates were analyzed, and about 20 percent of VOC duplicates were analyzed. 

Reproducibility of duplicate samples was determined by measuring the relative percent 

difference (RPD) between samples by the following calculation:

 | C1-C2 | x 100% = RPD,

 (C1+C2) / 2

where C1 is the concentration in the first sample, and C2 is the concentration in the 

duplicate sample.

Volatile Organic Compound Data

For the period of record, the following blanks were collected: 13 construction 

blanks for various diffusion samplers, 4 deployment blanks for diffusion samplers, 7 

equipment blanks for groundwater sampling, 36 trip blanks for various sampling rounds, 

and 3 surface-water ISCO VOC trip blanks. Results of these quality-assurance samples 

showed that seven low-concentration detections of carbon tetrachloride and one low-level 

concentration of tetrachloroethene in the data set may have been biased high due to 

detections in an associated blank, and those field values are qualified with a “B” in the 

data table. Chloroform (CF) was detected in 23 of 64 various blanks at concentrations

less than or equal to 1 ug/L but those were confirmed with further analyses to have come

from the office deionized water system that was used for all field blanks. These 
concentrations did not affect concentrations found in any field or field QA samples. 

After detecting the CF in the deionized water system, all water used for field and trip 

blanks were boiled before use and that process eliminated subsequent detection of CF in 

all the field blanks.

Individual VOC values may be qualified in the data table as being biased high (H) 

or low (L) based on response to internal calibration standards. Concentrations that may 

be biased high due to contamination found in associated laboratory blanks are qualified 

with a “V” in the table.

Duplicate VOC Data Pairs--For routine duplicate samples (not including 

multiple analyses from the same vials) where samples came from different (subsequent) 

sample vials, there were 172 data pair sets with 61 parameters giving 10,492 possible 

comparable data pairs. Of those 10,492 combinations, 973 pairs had detectable 

concentrations of a given compound for both analyses. Of the 973 pairs, the relative 

percent differences for 95.5 percent of the samples (910 pairs) were within 25 percent 

RPD. Sixty three pairs had RPDs greater than (>) 25 percent. Typically, small 

differences in lower concentrations can result in relatively high RPDs, however, for this 

data set, of the RPDS greater than 25 percent, a third of the pairs (22) had low 

concentrations (<10 μg/L), about a third (19) had medium concentrations (between 10 

and 100 μg/L), and the other third (22) had higher concentrations (>100 μg/L). 

For duplicate pairs where there was a detection in one sample but a „<‟ value in 

the associated duplicate (or a detection and a „>‟ value), most were the result of different 

dilution factors that were necessary to accurately measure both the low and high

concentrations. There were 379 data pairs with a detection in one but a „>‟ or ‟<‟ value 

in the associated sample. Of those, 320 were in agreement (i.e., a <10.0 μg/L in one 

sample vs. a concentration of 5.0 μg/L in the duplicate, or a >1,250 μg/L compared to 

3,740 μg/L). Thirty-seven pairs agreed within either 10 percent of the detection level or 

were within 1μg/L of the comparable analysis, and 22 pairs disagreed by more than 10 

percent.
Multiple Analyses--When a second analyses is required to be collected from a 

given vial, these samples are called „multiple analyses‟ and are designated as „MA‟ in the 

replicate column in Appendix III. These samples are required when either there was a 

failure of the initial sample based on internal calibration standards, or when the first 

sample was run at either too high or too low a dilution to get the best results. These 

samples are designated as such because of the possibility of loosing VOC concentrations 

from the vial due to either the extraction of the first sample, or because of the resulted 

head space after the initial sample was collected. The following compares the results 

when two samples were collected from the same vial.

There were 64 sets of samples where more than one sample was analyzed from 

the same sample vial. From those 64 sets, there were 303 pairs where there were

detections of a compound in both samples. The median RPD for each data pair from the 

same bottle was calculated, and the average of the 64 median RPDs was 10.3 percent. 

The RPDs exceeded 25 percent in 48 of the 303 data pairs and of those, two-thirds (32) 

were from samples with low-concentrations (less than 10 μg/L) where higher RPDs are 

generally more acceptable. Thirteen pairs with RPDs greater than 25 percent had 

concentrations between 10 and 100 μg/L, and 3 pairs with RPDs greater than 25 percent 

had concentrations greater than 100 μg/L. 

For pairs where there was a detection in one sample but a „less-than (<) value in 

the associated sample (or a detection and a „greater than‟ (>) value), most were the result 

of different dilution factors that were necessary to accurately measure both the low and 

high concentrations. There were 116 data pairs with a detection in one but a „>‟ or ‟<‟ 

value in the associated samples. Of those 116 pairs, 110 were in agreement (i.e., a <10.0 

μg/L in one sample vs. a concentration of 3.6 μg/L in the duplicate). Four pairs that did 

not agree but were within 10 percent of each other (detection level vs. detected 

concentration), or had a difference of less than 1.0 μg/L. Only two duplicate pairs did not 

agree. One of the two was for the highly volatile compound vinyl chloride. All the vinyl 
chloride was lost in that one sample between the times that the first and second were 

pulled. The other sample that did not agree was for TCE, and the concentration in the 

second pull from the vial showed an increase in concentration when run at the lower 

dilution.

Methane Analyses--There were 232 duplicate sample pairs with detectable 

methane concentrations in both samples. Of those 232 pairs, the RPD of 228 were

acceptable (less than 25 percent difference), and 187 of those had RPDs of less than 10 

percent. The median of all the RPDs was 4.3 percent. Four pairs exceeded the 

acceptable range and had RPDs ranging from 32 to 107 percent. The two highest RPDs 

were from PTN samples.

There were 165 sample pairs where methane concentrations of one or both were 

below the detection limits. Of those, 156 were in agreement where they were either both 

below the detection limit, or the one detection was less than the associated „less than‟ 

value (i.e., a <38.1 μg/L in one sample vs. a concentration of 36.6 μg/L in the duplicate). 

Six pairs agreed either within 10 percent, or within 5μg/L, and two pairs exceeded 10 

percent RPD. Of those two, the differences between the “<” value and the detected value 

were 7 and 12 μg/L.

Inorganic Analyses--Inorganic samples were analyzed by the U.S. Geological 

National Water Quality Laboratory in Denver, Colorado. For the March and August 

2005 groundwater sampling, there were 13 duplicate samples sent to the lab and the 

analyses are included in Appendix 2. In those 13 samples, there were 207 data pairs with 

detectable concentrations of a parameter in both samples. Of those 207 pairs, the RPD of 

20 samples were greater than 25 percent, and all those were the result of small variability 

from low concentrations.

Ion balances are used to determine if the cations and anions from the sample 

balance chemically. These balances are done by comparing milliequivalent 

concentrations of cations and anions. Given an accurate analyis of a sample and 
analyzing enough parameters, the balance should be approximately zero. Balances 

between -10 and +10 percent are acceptable. Balances greater than 25 percent, or less 

than -25 percent indicate that either not enough parameters were analyzed, or the 

analytical results were unacceptable. Negative balances indicate lower concentrations of 

cations (Ca, Mg, Na, K, Mn, Fe) compared to anions (Cl, SO4, F, HCO3). Positive 

balances indicate lower concentrations of anions compared to cations.

Of the 129 inorganic samples from both groundwater and surface water, all ion 

balances were within 25 percent, and 74 of those 129 samples were within 10 percent. Of 

the 81 groundwater samples, ion balances ranged from -16 percent to 5.7 percent, with all 

but 12 of the balances on the negative side. Of the 48 surface-water samples collected, 

ion balances, all balances were positive. The noticeable differences between 

groundwater and surface-water samples indicate that there are analytes that were not 

analyzed for that likely would have resulted in closer ion balances.

In Appendix II, some values are missing and described as either „O-deleted‟ or 

„U-deleted‟. The O-deleted qualifiers indicate that not enough water was available to 

complete the analyses. The U-deleted qualifier indicates that the lab was unable to 

determine the concentrations due to interference from other compounds.