The objective of this project was to identify key sources of variability that influence volatile organic chemical (VOC) concentration measurements in water samples collected from groundwater monitoring wells using current sampling and analysis techniques. By understanding the sources of variability in groundwater monitoring results, improved sampling and analysis methods can be developed to reduce and control for sampling variability.
The project was implemented through the completion of three project tasks:
: The initial project task utilized large existing groundwater monitoring datasets to identify key sources of variability in groundwater monitoring results. The data mining study involved identification of large databases of groundwater monitoring results, selection of datasets from these databases that were suitable for statistical analysis, and exploratory analysis and statistical analysis of the datasets to identify factors associated with monitoring variability. For this task, three large groundwater monitoring databases were identified: (i) the Hill Air Force Base (AFB) database, (ii) the Marine Corps Logistics Base (MCLB) Albany database, and (iii) a database of monitoring results from 48 underground storage tank sites compiled by GSI. Because the Hill AFB database included the most comprehensive documentation, many of the analyses could only be conducted using the Hill AFB database.
: For this task, sources of short-term variability in groundwater monitoring results were characterized through a field program that involved collecting and analyzing a large number of groundwater samples from a set of monitoring wells over a short period of time. Eighteen wells at Hill AFB were identified for sampling, including six low variability wells, six medium variability wells, and six high variability wells as determined by the analysis of the Hill AFB groundwater database conducted in Task 1. An intensive sampling program was conducted at each of the wells consisting of the following elements: (i) collection of a series of samples from the middle of the well screen by low-flow sampling over a defined range of purge volumes, (ii) measurement of ambient vertical flow within the screened interval of the well, (iii) collection of samples from the top, middle, and bottom of the well screen by passive sampling, (iv) collection of samples from the top, middle, and bottom of the well screen by low-flow sampling, and (v) collection of a second series of samples from the middle of the well screen by low-flow sampling over a defined range of purge volumes.
: For this task, the effect of sample collection method on monitoring variability was evaluated, as well as whether improved sampling methods and procedures could reduce the short-term variability in groundwater monitoring results. The field program involved collecting samples from a set of eight monitoring wells using five different sample collection methods. Each method was used for three sampling events, resulting in a total of 15 sampling events. The five sampling methods evaluated were Low-Flow Sampling with Purge to Parameter Stability (reference method), Low-Flow Sampling with Constant 24L Volume Purge, No Purge Low-Flow Sampling without In-Well Mixing, SNAP (No Purge Passive Sampling), and No Purge Low-Flow Sampling with In-Well Mixing.
The results support the following observations:
Based on the results of this project, the following changes are recommended to make long-term monitoring programs more efficient and cost effective: