Gaikowski, M.P., W.J. Larson, S.M. Schleis, and W.H. Gingerich. 2002. Validation of a 
chemical dilution model for hatchery effluent using rhodamine WT and chloramine-T at 
the Upper Midwest Environmental Sciences Center. Submitted to the U.S. Food and 
Drug Administration Center for Veterinary Medicine PMF 005-637, October 30, 2002. 
261 pages.
Summary
Accurate estimates of hatchery environmental introduction concentrations (EIC) are 
critical to accurately assess the risk of hatchery drug discharge. This study measured the 
concentration of rhodamine WT and chloramine-T in the UMESC effluent to validate a 
chemical dilution model based on water flow to estimate the EICs of hatchery chemical 
effluents. Given accurate flow rate information, we hypothesized that the average effluent 
concentrations of either rhodamine WT or chloramine-T could be adequately predicted at 
a given location using the following dilution model:

average concentration = total chemical applied ÷ (time required for n volume-exchanges 
x location water flow)

For the hydraulic characteristics of our treated raceway, n equaled the time required for 
four volume-exchanges. To test our hypothesis, the concentration of rhodamine WT and 
chloramine-T were separately determined in water samples collected during and after 
continuous-flow treatments of a production type raceway. Water samples were collected 
from the raceway and two downstream locations (Sample points A and B) in the UMESC 
effluent stream to validate the dilution model. The predicted and observed mean 
concentrations were graphically and statistically compared for sample points A and B; 
sample point A represented about 47% of UMESC non-chemical wastewater flow 
whereas sample point B represented 100% of the effluent.
The mean observed 120-min average rhodamine WT concentration at sample points A or 
B (13.36 and 6.81 Fg/L, respectively) were not significantly different (|t|<12.706, ?=0.05, 
v=1; P>0.05) from the mean predicted 120-min average concentration at sample points A 
or B (14.80 and 7.35 Fg/L, respectively). The observed and predicted mean rhodamine 
WT concentrations at each sample interval were similar at both sample points A and B 
and the predicted mean rhodamine WT concentration was generally bounded by the 
observed 90% confidence intervals. 
Similar results were obtained when chloramine-T treatments were administered and 
effluent concentrations determined. The mean observed 120-min average chloramine-T 
concentrations at sample points A or B (2.68 and 1.23 mg/L, respectively) were not 
significantly different (|t|<3.182, ?=0.05, v=3; P>0.05) from the mean predicted 120-min 
average concentrations at sample points A or B (2.77 and 1.31 mg/L, respectively. The 
observed and predicted mean chloramine-T concentrations were similar at both sample 
points A and B at each sample interval and the mean chloramine-T concentration was 
generally bounded by the observed 90% confidence intervals. 
Although degradation of chloramine-T may have occurred during discharge past sample 
points A and B, the minimal difference between observed chloramine-T concentration 
versus predicted chloramine-T concentrations at sample points A and B suggests that 
degradation was not a significant factor in the reduction of chloramine-T during the short 
discharge period observed. Our data indicate that given accurate flow rates, the dilution 
model given previously will adequately predict the EIC of either rhodamine WT or 
chloramine-T in hatchery effluents.