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Evaluation of Natural tlttenuation: 7500 West Eleventh Street, .Tracy,CA. Page 43 i, 8.2 Efficient Sampling Frequency Unnecessarily frequent sampling from arrays of groundwater-quality monitoring wells > and sampling from a larger number of wells than is necessary to provide sufficient statistically-reliable data to efficiently monitor groundwater quality at a given site can generate very high project costs without significantly improving understanding of conditions within a plume of contaminated groundwater. 8.2.1 ASTM Standard E 1943-98 Guidance At sites where a plume of contaminated groundwater has been demonstrated to be stable € ' and natural attenuation has been confirmed to be active, as has been demonstrated for the Navarra site in. Sections 7.1 and 7.2 above, via presentation of example cases, ASTM Standard E 1943-98 recommends reducing the scope of a groundwater monitoring 1 program to annual sampling from wells along the longitudinal axis of the plume (American Society for Testing and Materials 2004). F>;; Based on that ASTM Standard, the scope of future groundwater-quality monitoring at the 7500 West Eleventh Street site could be reduced to annual recovery of samples from Monitoring Wells MW-3, MW-13, MW-7, MW-14, MW-16, MW-18, MW-11 and MW- 9. Although an argument to adopt that monitoring program could be supported, more sophisticated protocols for optimization of groundwater-quality monitoring programs have been developed and, as is discussed below, SJC has performed such analyses and developed recommendations for future monitoring at the Navarra Site based on a methodology that is advocated by the US-EPA. 8.2.2 US-EPA Guidance The US-EPA, in an effort to reduce wasteful expenditures, advocates analysis of monitoring well arrays and sampling programs by statistical procedures to achieve optimized monitoring programs (United States Environmental Protection Agency 2005). That policy has also been endorsed by the California Department of Toxic Substances it='I Control (DTSC). Several protocols for selecting the number of wells and the frequency of sampling ;? required for an efficient groundwater-quality monitoring program under site-specific conditions have been developed. These include the Monitoring and Remediation Optimization System (MAROS), which was developed for the United States Air Force Center for Environmental Excellence (United States Air Force 2004). The MAROS system is endorsed by the US-EPA and software that permits site-specific computations to be made is freely distributed for the US-EPA by Groundwater Services, Inc. of Houston, Texas. SJC has completed applicable elements of the MAROS optimization for the 7500 West Eleventh Street site and the results, which are presented in Appendix A, are discussed below. SJC ' I