pages: AlamedaReuseandRedevelopmentAuthority/2011-11-02.pdf, 7
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AlamedaReuseandRedevelopmentAuthority | 2011-11-02 | 7 | Response to Comments on Status Update for the ARRA Alameda Reuse and Redevelopment Authority Meeting - November 2, 2011 Time Comment Marker Response First, underground bacteria and other microbes degrade many contaminants, often an important component of natural attenuation. However, initial site conditions may include contaminant levels that are so high that they are toxic to the very microorganisms that, with more dilute concentrations, would readily consume the contamination as food. Thus, following an initial phase of active cleanup of the higher concentration zones, natural attenuation can proceed more effectively. Second, with initial high contaminant concentrations, the effect of natural attenuation often is not discernible. This is because of the inherent variability of sampling and analysis. A simple example illustrates this point. With an initial contaminant concentration of 10,000 micrograms per liter (ug/L) and an inherent sampling variability of plus or minus ten percent, duplicate sampling and analysis of the same groundwater could yield analytical results anywhere between 9,000 ug/L and 11,000 ug/L. Say for discussion purposes, that natural attenuation reduces the contaminant concentration by 10 ug/L each year. This rate of natural attenuation would be difficult to detect in a reasonable period of time by sampling. On the other hand, after an initial phase of active remediation that reduces contaminant levels, to say 300 ug/L, then analysis of duplicate samples would yield results between 330 ug/L and 270 ug/L (+/- 10%). Now, natural attenuation at a rate of 10 ug/L would be both meaningful and discernible by sampling and analysis. 37:25 Planting trees will not be allowed. Gardening and fruit Restrictions on digging are used sparingly at Alameda Page 5 of 5 November 7, 2011 | AlamedaReuseandRedevelopmentAuthority/2011-11-02.pdf |