6. ANALYTICAL METHODS AND QUALITY ASSURANCE/QUALITY CONTROL
The equipment selection process should take into account what water quality parameters are of primary interest. Parameters such as oil and grease, VOCs, TPH, and bacteria cannot be collected using automatic samplers, and a grab sampling program must be used in these cases.
The use of automatic samplers for monitoring suspended sediment or total suspended solids may not be representative of water column concentrations particularly in areas where the flow is not well mixed. Using an automatic sampler requires the selection of a location in the water column for the intake port. If the port is placed too low in the conveyance, bed load may be taken up in a sample. However, for low flow situations, placement of the sample port as low in the pipe as possible is helpful for ensuring that the intake port is below the water line. Suspended solids and sediment measurements therefore may be problematic using automatic sampling equipment. If detailed water quality information is desired for suspended solids and sediment, a grab sampling program may be required to compliment an automated sampling program by providing verification of the representativeness of automatic samples.
6.1. Quality Assurance/Quality Control (QA/QC) Process Overview
The QA/QC process is used to assure that data collected are of acceptable quality to enable reliable plans and/or decision-making. QA/QC involves planning, field procedures, laboratory procedures, and reporting. Typically, a QA/QC plan is developed prior to sample collection. During plan development, specific aspects of the sample collection and laboratory procedures are worked out with the program manager, the field crew, and the analytical laboratory. Specifics include labeling and communication protocols, the number and type of bottles to be filled, laboratory performance objectives, collection of QA/QC samples, preparation of blanks, reporting requirements, and validation procedures. Plan development also serves to initiate communication with the personnel involved in collection and analysis. After sample collection and analysis, data should be checked against the laboratory performance objectives and project data quality objectives to ensure the quality is acceptable prior to reporting. Corrections to reports, qualification of the data, or corrective actions with the laboratory (reanalysis) or field (re-sampling) should be used to resolve any problems prior to reporting the data.
6.2. Data Quality Objectives and Process
Data collection for environmental studies generally involves four phases of activities including planning, implementation, data assessment, and drawing conclusions (usually to support planning and/or decision-making). For many data collection efforts, lack of sufficient planning results in data of limited use in the assessment phase. As a result, such data do not provide the necessary information to draw sound conclusions. The use of Data Quality Objectives process (DQO) (USEPA; 1993, 1994a) in the planning stage is an important aid to develop data adequate to support planning and decision-making.
The first step in the DQO process is to identify the problem or concern to be addressed. The next steps are to identify the decisions to be made and the necessary inputs to planning and decision-making. The physical or geographic study boundaries are then defined and a decision rule is developed. Next, the acceptable limits of the decision errors are agreed upon. All of this information is then used to optimize the study design for obtaining the necessary data.
The results of the DQO process are specific recommendations on the frequency, location, and quality of data, which are needed to make a specific decision. The DQO process also ensures that data not necessary to make the decision are omitted. For example, if the question is whether additional BMPs are necessary, determining whether water quality objectives are attained in the receiving waters becomes important. An additional question may be: Is the exceedance of water quality objectives due to the site in question or other factors? The monitoring program will need to use sampling and analysis methods that produce data that are appropriate for comparison with water quality objectives. Additionally, the sample locations should be selected to enable evaluation of other possible sources. Specifically, field and laboratory methods will need to have sufficient control over contamination and sensitivity to allow comparison with the lowest expected water quality standard that may apply to the site.
Specific data quality objectives for monitoring programs were described in Chapter 2. Data quality efforts should be reviewed based on the ability to meet program goals.
6.3. Precision, Accuracy, Representativeness, Completeness, and Comparability
Precision, accuracy, representativeness, completeness, and comparability are several different measures of data quality. These measures can be affected by factors in the field or in the laboratory. Each term is explained below.
Precision is the measure of the repeatability of a given measurement. Imprecise data are generally a problem because individual samples are not a reliable measure of the mean site conditions making it necessary to gather more data to characterize a given site. Often, poor precision is due to field variability, problems with the sampling and sub-sampling procedures, contamination, or poor sensitivity of the laboratory methods. Variability in the field can often be minimized through the use of compositing procedures.
Precision is assessed through analysis of laboratory duplicate samples or matrix spike duplicate samples. Laboratory duplicates are prepared by splitting one sample into two and performing a separate analysis on each split. Matrix spikes and matrix spike duplicates are prepared by adding a known concentration of analyte to a sample or to a laboratory duplicate and determining the concentration of the sample plus the spike. The two values (sample and duplicate, or spike and spike duplicate) are compared to provide an estimate of the precision of the laboratory method.
Accuracy is the degree to which the measurement reflects the true value of the sample. Accuracy may be monitored using matrix spikes, standard reference materials, or performance evaluation samples. Factors that influence the accuracy include laboratory calibration procedures, sample preparation procedures, and laboratory equipment or de-ionized water contamination. Accuracy is usually expressed as a percent recovery, where the measured value is divided by the true value.
Representativeness is the degree to which the samples represent site conditions. Typically, representativeness is assessed through the analysis of field duplicate samples. Compositing is sometimes used to minimize field variability. Composite samples generate an average of site conditions.
Completeness measures the success of the field and laboratory efforts by comparing the final validated data with the planned data collection activities. Completeness is used to assess how field situations and laboratory problems affected the overall success of the data collection efforts. If specific data are critical for a given decision, a goal of 100% completeness should be established.
Comparability expresses the confidence with which one sample set can be compared to another sample set measuring the sample property. Comparability is generally evaluated by evaluating "check samples," which are well-characterized samples that have been evaluated by a number of analysis methods and laboratories.
6.4. Detection Limits/ Quantitation Limits
Method detection limits (MDL) and practical quantitation limits (PQL) are measures of the sensitivity of the laboratory analysis methods. The method detection limit is defined as the "minimum concentration of analyte that can be measured and reported with 99% confidence that the analyte concentration is greater than zero" (Federal Register, 40 CFR 136.2). The PQL is the minimum concentration of analyte that can be accurately and precisely quantified. Guidance for deriving PQLs from MDLs indicates the PQL is generally 5 to 10 times the MDL, depending on the analyte and the degree of confidence that is required. If sample values are reported between the range of the MDL and PQL, the reported value has more uncertainty than values reported above the PQL.
If the goal of the data collection activity is to determine specific concentrations for comparison with a numerical objective, then every effort should be made to ensure the PQL is below the expected water quality objective. It should be noted that the freshwater objectives for some metals (e.g., cadmium, copper, chromium (+3), lead, nickel, silver, zinc) are a function of the hardness of the receiving water. It is recommended the PQL of the analysis method be set at or below the expected water quality objective for the receiving water. Note that the hardness value selection will affect water quality objectives for freshwaters. One can choose to utilize the hardness measured in the stormwater or that of the receiving water. There is no specific guidance on how to take into account hardness. In most cases, the receiving water hardness should be used.
Both MDLs and PQLs are often specific to a given type of sample. Often the MDL for a sample is elevated due to the presence of interfering compounds. For example, MDLs for metals in salt water are generally 5 to 10 times higher than MDLs for fresh water due to salt interference with the atomic absorption instrumentation utilized for metals analyses.
Control over sample contamination is critical when attempting to measure concentrations of compounds at the parts-per-billion level. If contamination occurs, USEPA recommends that the detection limit for the affected compounds be raised to five times the level of contamination. Often this will invalidate the sample collection effort, making the data not very useful for comparison with the required objective or standard.
Contamination can be introduced either during the bottle/equipment preparation steps or during the sample collection, transport, or analysis steps. Control over all of these steps can be achieved through the use of standardized equipment cleaning procedures, clean sampling procedures, and clean laboratory reagents. The level of contamination introduced during each of these steps is determined by analysis of different types of blank samples. Each of these different types of blanks is described below:
- Method Blanks are prepared by the laboratory by analysis of clean Type II reagent water. They are used to determine the level of contamination introduced by the reagents and laboratory processing.
- Source Solution Blanks are determined by analysis of the deionized or Type II reagent water used to prepare the other blanks. The source solution blank is used to account for contamination introduced by the deionized water when evaluating the other blanks.
- Bottle Blanks are prepared by filling a clean bottle with source solution water and measuring the solution concentration. Bottle blanks include contamination introduced by the source solution water and sample containers. By subtracting the source solution blank result, the amount of contamination introduced by the sample containers can be determined.
- Travel Blanks are prepared by filling a sample container in the laboratory with Type II reagent water and shipping the filled water along with the empty sample containers to the site. The travel blank is shipped back with the samples and analyzed like a sample. The bottle blank result can be subtracted from the travel blank to account for contamination introduced during transport from the laboratory to the field and back to the laboratory.
- Equipment Blanks are usually prepared in the laboratory after cleaning the sampling equipment. These blanks can be used to account for sample contamination introduced by the sampling equipment, if the bottle blank results are first subtracted.
- Field Blanks account for all of the above sources of contamination. Field blanks are prepared in the field after cleaning the equipment by sampling Type II reagent water with the equipment. They include sources of contamination introduced by reagent water, sampling equipment, containers, handling, preservation, and analysis. In general, field blanks should be performed prior to or during the sample collection. Because the field blank is an overall measure of all sources of contamination, it is used to determine whether there are any blank problems. If problems are encountered with the field blank, then the other components of the sampling process should be evaluated by preparation of other blanks to identify and eliminate the specific problem.
EPA's recent guidance on the use of clean and ultra-clean sampling procedures for the collection of low-level metals samples (USEPA 1993) should be considered to ensure bottles and equipment are cleaned properly and samples are collected with as little contamination as possible. While ultra-clean techniques throughout are not necessary for stormwater runoff samples, some of the laboratory procedures should be employed. Metals levels in highway runoff are typically much greater than introduced errors associated with in-field clean sampling techniques. These techniques are typically employed in receiving waters where their applicability is more relevant.
6.6. Reconnaissance and Preparations
Reconnaissance and preparation are important components of any field sampling program. Proper reconnaissance will help field operations go smoothly and ensure field personnel are familiar with the sampling locations.
During the planning stage, a site visit should be performed by the field personnel, prior to conducting sampling. The purpose of the site visit is to locate access points where a sample can be taken and confirm that the sampling strategy is appropriate. Because of the transient nature of meteorological events, it is possible sites may need to be sampled in the dark. For this reason, the actual persons involved in the field sampling should visit the site during reconnaissance as a complement to a training program for the monitoring effort.
The training program should include:
- A discussion of what the programs goals are and why their efforts are important;
- Familiarization with the site;
- Training on the use and operation of the equipment;
- Familiarization with field mobilization, sampling, and demobilization procedures;
- Health and safety requirements; and
- QA/QC procedures.
Coordination with the laboratory is a critical step in the planning and sampling process. The laboratory should be made aware of specific project requirements such as number of samples, required laboratory performance objectives, approximate date and time of sampling (if known), required QA/QC samples, reporting requirements, and if and when containers or ice chests will be required. Laboratory personnel should be involved early in the process so they can provide feedback on methods and performance standards during the planning phase. Notifying the laboratory that stormwater sampling is planned is also important to allow the laboratory to plan for off hours sample delivery and to set up any analysis with short holding times.
6.7. Sample Containers/ Preservation/Holding Times
USEPA recommends that samples be collected and stored in specific types of sample container materials (e.g., plastic, glass, Teflon). For analysis of certain parameters, the addition of specific chemical preservatives is recommended to prolong the stability of the constituents during storage. Federal Register 40 CFR 136.3 lists recommended sample containers, preservatives, and maximum recommended holding times for constituents.
If composite sampling procedures are to be used to collect one large sample that will be sub-sampled into smaller containers, the composite sample bottle should be compatible with all of the constituents to be sub-sampled. In general, the use of glass containers will allow sub-sampling for most parameters (with the exception of fluoride).
Sample volumes necessary for the requested analysis should be confirmed with the laboratory prior to sample collection. Extra sample volume must be collected for field and laboratory QA/QC samples. As a general guide, if one station is to be used for field and laboratory QA/QC measurements, four times the normal volume of water should be collected.
6.8. Recommended Field QA/QC Procedures
Listed below are the recommended quality control samples and field procedures to be used during a sampling program.
Field blanks should be prepared at least once by each field sampling team to prevent or reduce contamination introduced by the sampling process. It is recommended that field blanks routinely be prepared and analyzed with each sampling event. In addition, it is desirable to prepare field blanks prior to the actual sampling event as a check on procedures. This will ensure field contaminated samples are not analyzed. Additional field blanks should be prepared if sampling personnel, equipment, or procedures change.
Field Duplicate Samples
Field duplicate samples should be collected at a frequency of 5% or a minimum of one per event, whichever is greater. Field duplicate samples are used to provide a measure of the representativeness of the sampling and analysis procedures. These types of duplicates are recommended, but they often are not done due to the expense.
Field Sample Volumes
Sufficient sample volumes need to be collected to enable the required laboratory QA/QC analysis to be conducted. In general, one station should be targeted for extra sample volume collection and identified on the chain-of-custody as the laboratory QA/QC station. If possible, this station should be the one where the data quality is most critical.
Chain of Custody
All sample custody and transfer procedures should be based on USEPA-recommended procedures. These procedures emphasize careful documentation of sample collection, labeling, and transfer procedures. Pre-formatted chain-of-custody forms should be used to document the transfer of samples to the laboratory and the analysis to be conducted on each bottle.
6.9. Recommended Laboratory QA/QC Procedures
For each batch of samples, method blanks should be run by the laboratory to determine the level of contamination associated with laboratory reagents and glassware. Results of the method blank analysis should be reported with the sample results.
For each batch of samples, one site should be used as a laboratory duplicate. For the laboratory duplicate analysis, one sample will be split into two portions and analyzed twice. The purpose of the laboratory duplicate analysis is to assess the reproducibility of the analysis methods. Results of the laboratory duplicate analysis should be reported with the sample results.
Matrix Spike and Spike Duplicates
Matrix spike and spike duplicates should be used to determine the accuracy and precision of the analysis methods in the sample matrix. Matrix spike and spike duplicate samples are prepared by adding a known amount of target compound to the sample. The spiked sample is analyzed to determine the percent recovery of the target compound in the sample matrix. Results of the spike and spike duplicate percent recovery are compared to determine the precision of the analysis. Results of the matrix spike and spike duplicate samples should be reported with the sample results.
External Reference Standards
External reference standards are artificial standards prepared by an external agency. The concentration of analytes in the standards are certified within a given range of concentrations. These are used as an external check on laboratory accuracy. One external reference standard appropriate to the sample matrix should be analyzed and reported at least quarterly by the laboratory. If possible, one reference standard should be analyzed with each batch of samples.
6.10. Data Validation
Data reports should be reviewed for completeness. Reports should be checked to ensure all requested analyses were performed and all required QA data are reported for each sample batch.
Compliance with QA Objectives
Sample holding times should be compared to recommended maximum holding times listed in the Federal Register. Laboratory quality control sample data should be compared to target detection limits, and precision and accuracy goals and qualified according to USEPA functional guidelines for data validation (USEPA, 1988).
Data should be reviewed as soon as it is received from the laboratory. If problems with reporting or laboratory performance are encountered corrective actions (re-submittal of data sheets or sample re-analysis) should be performed prior to final data reporting or data analysis.
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