Field evaluation of mixing length and attenuation of nutrients and fecal coliform in a wastewater effluent plume.

We present an alternative method to dye tracer studies for mapping wastewater effluent plumes in rivers. A case study of a wastewater treatment effluent plume that was mapped in two different months (October and April) showed good resolution in determining where the plume reached the opposite river bank, the degree of mixing in a given river reach, and where the river was fully mixed with respect to the plume. Both electrical conductivity and chloride were useful in mapping domestic wastewater effluent plume mixing. Mixing lengths obtained by field mapping were consistent with previous studies. Electrical conductivity field readings facilitated real-time plume delineation and sampling locations, and identified a non-point source plume during the mapping exercise. The apparent difference in dispersion of the plume between to the two mapping dates (despite similar upstream river discharge) suggests that the calibration of a water quality model based on a single dye test may be inadequate. The river distributions of effluent nutrients and fecal coliform were also mapped. Nutrient attenuation rates were faster than that of the chloride tracer, indicating processes other than mixing occur. Substantial differences were observed in nutrient and fecal coliform distributions between the two mapping dates.