Development and testing of a model for predicting contaminant transport in leaching beds
Septic tank leaching-bed systems used for on-site wastewater treatment have been identified as sources of bacterial and nitrate contamination of groundwater. With an estimated one million septic tank leaching-bed systems in Ontario, evaluating the performance of existing systems and improving leaching-bed design practices are essential. The primary objectives of this research were to develop a leaching-bed design tool to predict the performance of alternative leaching bed designs and to obtain field-scale data to test how well the design tool predicts the potential of these systems to be sources of enteric-bacterial contamination. The field experiments also provided a means to evaluate how two typical systems were performing. To meet the first objective, a performance-based leaching-bed design tool has been developed in Microsoft (MS) Visual Basic® with MS-Excel® automation. The leaching-bed area is defined using two- and three-dimensional finite elements. Flow and contaminant transport equations are solved for each element to obtain contaminant concentrations in the vicinity of a leaching bed. The design tool was tested with data from field-scale tracer experiments conducted at two typical mounded systems in Southern Ontario and was able to successfully simulate the systems. In these experiments, the transport of both a conservative tracer (bromide), and a non-conservative tracer (' Escherichia coli' resistant to nalidixic acid) through the systems was monitored. Simultaneous introduction of the tracers ensured they would be influenced by the same climatic conditions and that direct comparisons between the results could be made. The predictive software was also used to evaluate the sensitivity of model results to parameter values to determine which parameters must be known most accurately. The results of the tracer experiments concur with the conclusions of other studies, in that conservative tracers cannot be used to predict transport of non-conservative contaminants since their observed transport is very different; the peak biotracer concentration consistently appeared earlier than the peak concentration of the conservative tracer. The experiments also indicated that when background 'E. coli' contamination was evident, the leaching-bed was the primary source. Application of the design tool to the field experiment results illustrates that use of this tool for performance-based design is feasible and incorporation into the design process would be beneficial.