## Description

Forced-gradient tracer tests are utilized in aquifers to estimate the transport parameters controlling solute movement. Previous work at the Bonita well field indicated low tracer concentrations at monitoring wells in tests using an injection rate of 1.87 l/min and an extraction rate of 544 l/min (flow rate ratio of 291). Such tracer breakthrough concentrations are not easily utilized to estimate transport parameters other than advection. In an effort to increase the breakthrough concentrations within monitoring wells, an injection system was designed and implemented that is able to inject at rates up to 113.5 l/min. This system was used to determine the effects of varying the flow rate ratios of groundwater extraction (Qe) to groundwater injection (Qi) on tracer breakthrough concentrations and transport parameter estimates. Initial laboratory column experiments indicated that fluorescein is an economically viable and conservative tracer based on maximum breakthrough concentrations and mean breakthrough times compared to rhodamine, bromide, chloride, and iodide. Fluorescein was then used in four forced-gradient tracer tests with flow rate ratios (Qe/Qi) ranging from 4.4 to 25.1. Each tracer test was conducted for 24 hours. Tracer breakthrough was continuously monitored through a 1.5 meter section in a monitoring well, located 4.72 meters from the injection well, using an inflatable straddle packer. Effluent concentrations were also monitored from the pumping well in order to calculate tracer recovery. Breakthrough data was then analyzed using a temporal moment analysis and a one-dimensional, finite difference model. Comparing the two methods indicates that the one-dimensional model, which takes into account variable flow velocities within a two-well system, is able to more accurately estimate dispersive transport parameters. The results of the tracer tests indicated that the higher flow rate ratios yielded the highest maximum tracer breakthrough concentrations. Furthermore, the results from using the one-dimensional model indicates that there is an increase in dispersivity estimates with increasing flow rate ratios from 2 meters (flow rate ratio of 4.4) to 4.5 meters (flow rate ratio of 25.1). The previous data from Thorbjarnarson et al. (in press) was also analyzed as a comparison, and yielded a very high dispersivity estimate of 30 meters (flow rate ratio of 291). The error, or changes in dispersivity estimates between the tracer tests conducted in this thesis are the result of applying a one-dimensional model to a two or three-dimensional flow phenomenon. This error in estimated dispersivities is much greater for high flow rate ratios. Therefore, the resultant dispersivity estimates are apparent values, and are not truly representative of the aquifer. Additional modeling, that takes into account multidimensional flow and transport needs to be conducted in order to more thoroughly define the dispersive transport parameters from each of the tracer tests conducted at the field site.