The purpose of this project was to: 1) validate sample collection methods and procedures that minimize variability in groundwater monitoring results; and 2) validate improved methods to optimize monitoring frequency and assess long-term concentration trends that better account for short-term variability in groundwater monitoring results. All three objectives of the project were met. The demonstration results indicated that the sample method (except active no purge) has only a modest impact on monitoring variability and concentration, suggesting sampling methods should be selected based on factors such as cost, ease of implementation, and sample volume requirements rather than concerns regarding data quality. At both sites, low flow standard (purging to parameter stability) and low flow alternative (small volume) showed the lowest variability. The results were consistent between the two sites except for the active no purge (HydraSleeve) method, which was more variable at the California site than the Texas site. Low flow small volume purge and passive no-purge (SNAP sampler) were the two best sampling methods based on the combined goals of minimizing monitoring cost and minimizing variability in monitoring results.

Distribution Statement A REPORT DOCUMENTATION PAGEForm Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. (From -To) REPORT DATE (DD-MM-YYYY) 07-11-2014 REPORT TYPE Draft Final Report DATES COVERED SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S) Strategic Environmental Research and Development Program 4800 MARK CENTER DRIVE, SUITE 17D08ALEXANDRIA VA 22350-3600 SERDP SPONSOR/MONITOR'S REPORT NUMBER(S) DISTRIBUTION / AVAILABILITY STATEMENTApproved for public release; distribution is unlimited SUPPLEMENTARY NOTES ABSTRACTContaminant redistributive effects in wells are nearly always present. Complete mixing appears to be very common; however, it is not universal. There is a continual balance between inflowing contaminant stratification (where present) and factors driving in-well mixing. Findings here imply common and very small drivers are responsible for slow but vigorous mixing relative to the residence time of water flowing through a typical well screen. Therefore, a tendency toward homogenization is anticipated to be common in field conditions. Most wells should experience strong redistribution effects, but some wells may maintain stratification or perhaps re-stratify differently from the surrounding formation. Ongoing technical transfer of these findings will promote better understanding in the environmental community that wells often represent a mixed flow-weighted average of the adjacent formation chemistry. This better understanding will yield cost savings in both short-term and long-term timeframes by accelerating the approval process for non-purge alternative sampling strategies, including passive sampling and in situ sensor technologies. SUBJECT TERMSGroundwater monitoring, passive sampling, purging,