Point dilution method determination of groundwater velocities at the Bemidji research site

Groundwater velocity is the dominant physical parameter that controls the transport of contaminants in a aquifer. In the study of an crude oil contaminated, unconfined aquifer at the U.S. Geological Survey, Bemidji, Minnesota research site, groundwater velocities were determined by the point dilution method. The point dilution method is a tracer technique which relates observed tracer dilution in a monitor well to the undisturbed groundwater velocity. A conductivity probe is used to observe the change in concentration of a tracer (NaCI solution) due to groundwater flow through an isolated portion of a monitor well. The darcian groundwater velocity was measured in 19 wells. The measured darcian velocities ranged from 0.034 to 2.2 m/day with a median of 0.25 m/day and mean of 0.36 m/day. The distribution of measured velocities agree with the full range of previous velocity estimates by both direct and indirect methods. Hydraulic conductivities calculated from the measured groundwater velocities ranged from 1.9x10⁻⁴ to 2.8x10⁻³ m/sec. Median grain sizes were calculated from the hydraulic conductivities and correspond to sediment classes of fine to coarse sand. A two-dimensional steady-state contaminant transport model was used to match the dissolved organic carbon plume at the site. The measured darcian velocities were used to determine the constant or variable linear velocity used in the transport model. Dissolution of contaminant from the crude oil pool was simulated as a continuous point source. The linear velocity and the biodegradation decay coefficient were varied to fit the model plume to the observed plume. The model results showed that the observed plume can be generated with either variable linear velocity or variable biodegradation. The measured darcian velocities very greatly with position and depth at the research site, but examination of the stratigraphic, hydraulic and chemical data suggests that the aquifer can be described as homogeneous downgradient of the oil pool. In general, the distribution of measured velocities suggests that the discontinuous silt layer at the water table represents the major heterogeneity in the aquifer. In regions where the silt layer is found, the velocity is greatest at some depth below the water table.