INNOVATIVE IN-SITU FIELD MEASUREMENT OF HIGH HYDRAULIC K VALUES USING PNEUMATIC SLUG TESTING IN A COARSE GRAVEL AQUIFER (TRENTON GRAVEL) NEAR THE DELAWARE RIVER

It has become increasingly important to be able to accurately estimate the hydraulic conductivity (K) of unconsolidated sediments for groundwater transport and mass flux models. Many groundwater transport models have discrepancies noted in areas where heterogeneity is greater, suggesting a connectivity cause to non-uniform groundwater flow. Gravel sediment acting as aquifer porous media are dependent on grain size and the variability of the connectivity structures or preferential pathways through the open framework, enabling a great deal more water to move through the system relative to a homogeneous system.

The Trenton gravel consists of gravel and sand and is found in the Delaware River Valley. A temporary well with pneumatic slug testing apparatus was designed for use with a sonic drill rig. The temporary well consisted of a 2-inch stainless steel riser casing and 4-inch 20-slotted stainless steel screen. To minimize short circuiting of formation water, an inflatable packer was placed at the top of well screen. The temporary well screen, packer, and riser pipes were lowered into the annulus of an 8-inch sonic driven cased borehole. The casing was raised approximately 10 feet allowing for the natural formation to collapse around the well. The pneumatic test apparatus was attached to the top of the riser pipe. Nitrogen lines were then attached to both the packer and apparatus. A transducer was lowered to the middle of the screen and was used to capture water level changes. The packer was inflated, and water levels were allowed to equilibrate while water inside the casing was checked for signs of leakage (bubbles) which would indicate packer seal failure. Two rising head pneumatic slug test were completed. A final falling head slug test was completed by pouring a bucket of water into the well. The cost of the temporary well materials was minimal since it was cleaned and re-used during the entire program. The tests were typically completed within 1.5 to 2 hours of field time. Results were evaluated using industry standard program. The in-situ site specific hydraulic K ranged from 16 to 737 ft/day with a geometric mean of 104 ft/day. K values are consistent with pump test documented K value range of 3.9 to 2,900 ft/day and a geometric value of 142 ft/day, calculated when Trenton gravel was in contact with the Delaware River (USGS, 2002).