SINGLE-WELL, PUSH-PULL TEST FOR IN-SITU DETERMINATION OF MICROBIAL ACTIVITIES

A single-well, ''push-pull'' test method is proposed for the in situ d etermination of microbial metabolic activities in groundwater aquifers . The method consists of the pulse-type injection (''push'') of a test solution into the saturated zone of an aquifer through the screen of an existing monitoring well followed by the extraction (''pull'') of t he test solution/ground-water mixture from the same well. The test sol ution contains a tracer and one or more reactive solutes selected to i nvestigate specific microbial activities. During the injection phase, the test solution flows radially away from the monitoring well into th e aquifer. Within the aquifer, biologically reactive components of the test solution are converted to various products by the indigenous mic robial community. During the extraction phase, flow is reversed and so lute concentrations are measured to obtain breakthrough curves, which are used to compute the quantities of reactant(s) consumed and/or prod uct(s) formed during the test and reaction rates. Tests were performed to determine rates of aerobic respiration, denitrification, sulfate r eduction, and methanogenesis in a petroleum contaminated aquifer in we stern Oregon. High rates of oxygen, nitrate, nitrite, and hydrogen uti lization and nitrite, and carbon dioxide production support the hypoth esis that petroleum contamination has resulted in an increase in micro bial activity in the anaerobic portion of the site, The results sugges t that the push-pull test method should be useful for obtaining quanti tative information on a wide range of in situ microbial processes.