BACTERIA MOBILIZATION AND DIVERSITY AS A FUNCTION OF RATE OF EXTRACTION FROM A MONITORING WELL Abstract by Nicholas Michael Petruzzi Various physical, chemical and biological processes can be responsible for groundwater colloid mobilization. Earlier studies by other investigators demonstrated that rate of production at a well can have significant impact on total colloid concentration observed in groundwater samples. However, little work has been done to quantify the impact of pumping rate at a sampling point on bacterial and inorganic colloid concentrations, as well as the diversity of the bacteria population observed in a groundwater sample. It was hypothesized that total bacteria concentrations will increase with increasing pumping rate (consistent with the previously observed increase in total colloids with pumping rate). It was further hypothesized that bacterial diversity, as reflected by the DNA of the microbial population present in the water sample, will also show variation with the pumping rate. These hypotheses were addressed experimentally by collection of natural groundwater samples at various rates of production (passive to 2.0 l/min.) from two monitoring wells at a field site in the St. Joseph aquifer, northern Indiana. A passive sampler was designed, tested, and implemented for collection of groundwater under natural hydraulic gradients. Active sampling was accomplished using a dedicated pumping setup and monitoring water quality in order to determine appropriate purge time prior to collection. Enumeration of total colloids was performed with an environmental scanning electron microscope (ESEM), total bacteria concentrations were determined by fluorescent microscopy using the nuclear stain acridine orange, and bacteria diversity was identified by denatured gradient gel electrophoresis (DGGE). The results of these experiments provided quantitative data supporting the hypotheses. Respective total colloid and bacteria concentrations were comparable between the two wells, with total colloid concentrations approximately 2 orders of magnitude greater than bacteria concentrations. Total colloid and bacteria concentrations increased significantly at the higher pumping rates. It appeared that bacteria diversity varied (appearance and disappearance of community bands in the DGGE results) as a function of pumping rate. Diversity appeared to remain approximately constant at low-flow rates (excluding passive) but showed variation at higher pumping rates (coincident with the pumping rates for which the colloid concentrations were observed to increase). Laboratory and field experiments provided confidence in the ability of the passive sampler to collect water samples. Application of the passive sampler at the field site demonstrated that the water existing within a wellbore under zero pumping is not representative of water quality in the pore fluids in the aquifer. Total colloid and bacteria concentrations were higher in samples obtained passively than for samples obtained at all pumping rates (excluding total colloids collected at 2.0 l/min.). Bacteria communities also appeared to be less diverse in samples collected using the passive sampler than in samples collected at each of the pumping rates.