The distribution of dye tracer immediately after injection into a heterogeneous aquifer under pumping and non-pumping conditions was studied. The tests were conducted on an alluvial aquifer adjacent to the Sweetwater River in Bonita, CA. Borehole induction and geologic logging show the site to be vertically heterogeneous. The upper 28 ft of the aquifer consists of sands overlying a 3 ft thick clay layer. Below this clay layer, there are more sands with gravel layers at 52-54 ft and 60-64 ft. Two injection pulses occurred under pumping conditions. The production well was pumped at a constant rate of 148 gpm. In the first tracer injection under pumping conditions, iodide was injected into MW2A and MW2B, while rhodamine WT was injected into MW2A. In the second tracer injection under pumping conditions, fluorescein and rhodamine WT was injected into MW2B, while rhodamine WT was injected into MW2A. Under non-pumping conditions, rhodamine WT was injected into both MW2A and MW2B. It was seen that a distinct vertical stratification of the tracers occurred under pumping conditions, which is related to the natural gradient and higher flow rate though the most permeable layer in each borehole. Under non-pumping conditions, this distinct stratification did not occur, and no trends of tracer distribution within a borehole was observed. The fundamental implication of this study emphasizes the importance of measuring tracer concentration at several depths within a borehole during a tracer test.
