Variation in the patterns of toxicant distribution corresponds with variation in the frequency, magnitude, and duration of toxicant exposure events for stream-dwelling organisms. The aim of this study is to quantify the three-dimensional spatial and temporal variability of chemical distributions within a stream environment. More specifically, this study will examine how temporal and spatial patterns of chemical distribution are influenced by flow regime and mode of toxicant entry into the stream. This experiment will examine the impacts of mode of toxicant introduction (ground water introduction or runoff introduction) and flow velocity (2, 6, and 10 cm/s) on toxicant distribution patterns using a 2 x 3 fully factorial design. An artificial stream will be constructed (17.4 x 0.98 x .39 m: Lx W x H) using cinder blocks lined with 0.1 mm thick plastic sheeting (Harrigan and Moore 2017). Minimally filtered water from the Maple River will be pumped into the stream at a velocity specified by the treatment, velocity will be held constant for the duration of each trial. Water will be filtered through nylons to remove any large particulate matter that could influence flow within the stream (Ludington and Moore 2016). After filtration, water will flow into a 4.3 m zone of mixing. In the zone of mixing, water will be passed through two collimators spaced 1.78 m apart to minimize the stochastic elements of turbulence due to water entry into the flume (Harrigan and Moore 2017). Water will then flow to a delivery system, where a dopamine tracer element will be introduced. Depending on treatment, dopamine will be fed into a sampling area by one of two delivery systems. To imitate runoff entry, dopamine will be introduced onto the sampling area at the level of the water’s surface. To imitate groundwater entry, water will be introduced via a vinyl tube at the bottom of the flume. Dopamine will be introduced into the flume via head tank delivery at a constant rate of approximately 60 ml/min across all treatments. Concentrations of dopamine throughout the sampling area will be recorded at 9 sampling sites (3 x 3 grid) using an Epsilon electrochemical detection system. For each sampling site, measurements will be taken at 3 heights within the water column (5 cm, 20 cm, and 35 cm above the bottom of the flume). This will allow for the creation of an image of dopamine concentrations in the benthic region, mid-water column, and at the surface waters. After concentration measurements have been completed, the measurements will be used to describe concentration characteristics which are likely to influence the degree of deleterious effects resulting from an exposure event. These characteristics include the frequency, duration, and magnitude of concentration peaks at a given sampling site. Data describing these characteristics at each sampling site will then be imported into ArcGIS. Using an Inverse Distance Weight technique to interpolate, an approximation of the three-dimensional distribution of dopamine under each treatment will be created. Using this approximation, areas of the stream in which organisms are likely to experience the most severe impacts of anthropogenic chemical introduction will be identified. The distribution of these “hot spots” will be compared across treatments to examine the influence of flow regime and toxicant mode of introduction.