Perception studies to determine receiving-stream color objectionability due to effluent discharge

In the State of Tennessee, the Tennessee Department of Environment and Conservation (TDEC) is the NPDES permitting authority who sets color limits for a recreational water use color narrative under the auspices of EPA review that requires effluent discharge to be free of objectionable color, but the narrative lacks numeric guidance on how to determine an acceptable discharge color. The City of Memphis, Tennessee had concerns about the subjectivity of the narrative and initiated a color study to provide TDEC with recommendations to establish numeric limits for NPDES permit compliance. This color study links human perception of river color contrasts (subjective) with measured apparent color, dissolved and suspended particles, true color, dissolved particles and environmental data (objective) through four psychological experiments that show perception of objectionable river color is primarily a result of cloud cover (sky reflections) and seasonal leaf foliage, not the background color of the river or contributing effluent color. When Experiment 1 participants visited three riverside locations once a month for a year only one person noticed an objectionable color, while the remaining perceived color contrasts related to cloud/sky reflections. Similar results were obtained in Amazon’s Mechanical Turk (AMT) Experiments 2-4, where the results show that the significance of environmental factors, cloud cover and seasonal foliage, overshadow any effect the wastewater effluent color may have on perceived objectionable river color differences. Since an individual’s perception of objectionable color is the result of environmental factors and not the contribution of discharge from M.C. Stiles’ plant into the Mississippi River, this suggests that a numerical limit for the M.C. Stiles’ discharge is unnecessary as this will not affect objectionable color perception at this location.