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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 41 8.0 OPTIMAL MONITORING DATES AND FREQUENCY The groundwater-quality monitoring data that has been gathered at the 7500 West ! Eleventh Street site since May 2000 can be analyzed to provide guidance for designing a cost-effective, optimal monitoring schedule that provides the data necessary to monitor the groundwater-quality trend with time and to check for unexpected changes in plume 13 geometry. f : 8.1 Optimal Monitoring Dates It has long been observed that at many sites where groundwater is affected by components of fuel hydrocarbons, the concentrations of analytes of concern in the ` groundwater rise and fall periodically at a remarkably consistent frequency. Because these fluctuations were often associated with seasonal changes, it became traditional for F= 3 groundwater-quality monitoring programs to call for a quarterly monitoring schedule. However, although such scheduling can be useful during the initial phase of a groundwater-monitoring program to investigate the actual frequency of cyclical changes '. in contaminant concentration, there is no scientific basis for extending quarterly groundwater-quality monitoring tinto a long-term monitoring program (United States Environmental Protection Agency 2005). A cost-effective, long-term monitoring program should be based on a groundwater-sampling frequency that permits the concentrations of analytes of concern to be measured at the time of the year when concentrations are likely to be at their highest, or, if additional data density is required, at both that time and at the time of year when concentrations of analytes of concern in groundwater are likely to be their lowest. Monitoring schedules of this type permit the long-term trend of groundwater quality to be monitored and the trends reliably observed while monitoring costs are reduced to a practical minimum without loss of statistical confidence in regression and other groundwater-quality trend analyses. The periodic fluctuations in the concentrations of components of fuel hydrocarbons in groundwater at affected sites are caused by groundwater rising and falling in elevation relative to the zone of subsurface soils that are the most severely affected by the fuel >� hydrocarbon contamination. Because fuel hydrocarbons are less dense than water, when they are released at a source, light non-aqueous phase liquid (LNAPL) floats on the surface of the groundwater and there are high concentrations of components of the fuel adsorbed onto soil particles in a zone that extends some distance above the highest elevation of periodic groundwater table depth down to the depth of the greatest depth to } which the groundwater table recedes. Within that zone, the highest concentrations are present close to the high groundwater table elevation and the lowest are present at the low groundwater table elevation. As fluctuations in groundwater flow cause the groundwater table to rise and fall over this "smear zone," contaminants are slowly desorbed from the soil and pass into solution in the groundwater. Therefore, when groundwater is high, the total volume of heavily-affected soil in contact with groundwater is large, so that } concentrations of dissolved components of hydrocarbons reach their high periodic levels. Conversely, when groundwater levels are low, the volume of soil that serves as a source of contamination to groundwater is at a minimum and concentrations fall. k sic