Whether it confirms that design and operational practices are preventing groundwater contamination or signals the need for corrective action, statistical analysis can help a landfill owner prevent costly remedial action.
Subtitle D regulations require solid waste landfill owners to statistically analyze the surrounding groundwater samples to determine if it has been contaminated with volatile organic compounds (VOCs) or metals.
A statistical analysis of groundwater data gives a more accurate way to determine groundwater contamination. Not only can it show if the landfill - or another facility - is responsible, it also can provide warning of future impacts, which can be costly in terms of fines, litigation and remediation expenses.
In the past, contamination was determined by a governing agency who would use the laboratory chemical data to determine if VOC and metal concentrations were high or low. In some states, they were compared almost directly with a groundwater standard.
However, laboratory chemical data alone are insufficient to determine if the landfill has contaminated the groundwater. The groundwater beneath a landfill may contain one or more of the indicator chemicals due to natural occurrences or from other sources.
Statistical analysis compares laboratory data between up- and down-gradient wells and within a single monitor well over time. Only a statistically significant increase in the chemicals or metals detected in the laboratory chemical analyses indicates contamination.
To perform an analysis, all of the groundwater monitoring data from the inception of monitoring, including quality assurance and quality control data, are gathered and arranged chronologically.
Data from specific wells should be selected. These wells, chosen by a hydrogeologist familiar with the site's design and hydrogeology, should represent two primary areas: the up-gradient, or background areas, and the down-gradient, or compliance areas.
The wells' selection also may require input from the lead regulatory agency. The hydrogeologist will determine whether to compare data from up-gradient versus compliance wells or data over time from a single well -a decision that depends on the type and volume of monitoring data available at a specific site.
The data can be analyzed by one of two software programs created for landfills: GRITS STAT, developed by the U.S.; and Chempoint and Chemstat, developed by Starpoint Software Inc., West Lafayette, Ind.
These programs create a table that shows contamination by specific chemicals at specific wells. The report can discuss the likely migration pathways and may predict the contamination's fate and potential impacts.
Most states require annual statistical analysis, but semiannual or quarterly monitoring is recommended. If no contamination is indicated, detection monitoring usually can continue. If contamination is noted, assessment monitoring may be required.
Assessment monitoring is three or four times more expensive than detection monitoring because of increased monitoring frequency and an expanded list of chemical parameters. Performing regular statistical analysis, however, may help avoid contamination and its costs, including future assessment monitoring.
If, for example, contamination appears imminent, the owner can install additional monitoring wells and increase the monitoring frequency in the "hot spot." Other operational changes can be considered, such as reducing the working face's size, controlling run-on water more closely or providing less permeable daily cover. Or, the owner might purchase additional buffer zones, which might be less expensive than assessment monitoring.
If the statistical analysis indicates that the landfill is contaminating the groundwater at the site, this may indicate that the liner system is not functioning properly. As a result, it may be necessary to change the design of future cells.
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