f THREE-DIMENSIONAL RESISTIVITY IMAGING FOR DETERMINATION OF AIR-FILLED PORE SPACE IN THE DESIGN OF GROUNDWATER AIR SPARGING SYSTEMS

Typical methodologies for determining well spacing in groundwater air sparge systems, such as water table response, dissolved oxygen measurements, and conservative gaseous tracers have been known to give semi-quantitative estimates of the radius of influence. Placement of observation wells that intersect established air channels can lead to data that are very different than wells that do not intersect, while two such wells could be located only a short distance apart. Non-invasive geophysical methods such as electrical resistivity (ER) can provide more detailed data over a larger area, alleviating the data density issues associated with typical test methods. Data are presented from two sites with existing air sparge systems that were not performing to expectations, ER surveys were conducted to evaluate the air-filled pore space induced by the operating air sparge wells. The air sparge systems were turned off in advance for a baseline survey with no air-filled pore space. Multiple repeat surveys were then collected at progressively increasing air flow rates to evaluate differences in radius of influence or volume of air-filled pore space. An increasing area of resistivity values during air sparging at an increased flow rate is attributed to pore-water displacement by injected air, resulting in decreased relative electrical conductivity. This conclusion is supported by direct measurments at various observation wells of dissolved oxygen concentration and changes in depth to water.