Investigation of the Effects of Biofouling on the Hydraulic Properties of Wells in Fractured Bedrock Aquifers
Cooke, Brenda Elizabeth
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The objective of this study is to improve the understanding of the effect of biofilm growth and biofouling on well performance in a fractured rock aquifer. The study was conducted by investigating biofouling in several recently drilled and much older wells. The study was conducted at three field sites using six groundwater wells in Perth, Portland and Cambridge Ontario. The well located in Perth was drilled in the spring of 2006 and after preliminary hydraulic testing; biofilm was grown in the well by the addition of nutrients (steel wool, Difco beef peptone and gelatin). The wells located in Portland and Cambridge are more than 10 years old and were already biofouled. In each case, the well was hydraulically tested using open- well pumping methods and via slug testing using a straddle packer system (1.7m spacing) in which measurements were obtained contiguously from the bottom to the top of the well. This provided distinct measurements of the hydraulic properties of the individual fracture zones along the length of each hole. The five older wells in Portland and Cambridge were cleaned following the initial testing using recirculation of hydrochloric acid followed by recirculation of 35% hydrogen peroxide. Once the cleaning was completed, the hydraulic testing was repeated. Each of the wells were videotaped using a downhole camera at various stages during the study. Samples were obtained on three occasions for bacterial typing using the BART™ system. Transmissivity data from the different states of fouling (drilled, fouled or cleaned) were compared and it was determined that fractures with low transmissivity values are more susceptible to the effects of biofouling than those with high transmissivity. The reduction in transmissivity was as much as 50% percent. Fractures with low transmissivity are of small apertures which are easily plugged by biofilm. The cleaning process did not markedly improve total wellbore performance for those wells where a comparison was made following cleaning. The presence of any form of iron, (i.e. steel well casing or steel pump parts) in the well will contribute significantly to the biofouling, based on the results of the laboratory study.