USING TEMPERATURE DATA AS AN ALTERNATIVE METHOD FOR DETERMINING THE PLACEMENT OF PASSIVE DIFFUSION BAGS IN FRACTURED BEDROCK AQUIFERS

This study looks at using groundwater temperatures as an indicator for effective placement of passive diffusion bags (PDB's) for monitoring groundwater at fractured bedrock sites. Groundwater monitoring at hazardous waste sites effected by chlorinated volatile organic solvents (chlorinated's) often requires remediation and monitoring efforts that last for decades, and those managing the sites seek effective ways to reduce operation and maintenance costs without sacrificing data accuracy. Adoption of PDB's for collecting groundwater samples at chlorinated sites occurred in the early 2000s and are now commonly accepted by regulatory agencies. PDB's consist of permeable membranes; they are initially filled with deionized water which equilibrates with the surrounding groundwater resulting in representative groundwater samples.

This study site is in central Maryland. It overlies a highly fractured sedimentary bedrock unit (New Oxford formation), that consists of inter-beds of sandstone and shale. Permeability in the different layers varies by 2 to 4 orders of magnitude. Within the more permeability layers, permeability varies significantly between wells which encounter interconnected fractures and those that do not. Previous studies at the site demonstrated the relation between fractures and fluctuations in the water level, temperature and water quality data during pumping cycles. Building on those findings, this study looks at multi-level water temperature fluctuations in the wells and uses that data as a tool for evaluating the appropriate interval for placement of the PDB's.

Temperature recording data loggers were placed in discrete intervals in select monitoring wells where fracture flow was and was not evident. Temperature recordings were collected on alternating 1-minute and 4-hour intervals and fluctuations of temperatures across the depth intervals were noted. Based on variations of groundwater temperatures, intervals where temperature fluctuations occur within the well were identified. This data was compared to the long-term monitoring data. It was determined this data can be used to define intervals for effective PDB's placement in fracture flow aquifers. The temperature monitoring described in this study is another tool for assessing the accuracy of groundwater monitoring networks.