The near-field behavior of the Sand Island, Hawaii, ocean outfall plume was
modeled. The model, a modified version of the Environmental Protection Age
ncy Roberts-Snyder-Baumgartner model, used as input data simultaneous measu
rements through the water column of currents obtained from Acoustic Doppler
Current Profilers and density profiles obtained from thermistor strings. M
ore than 20,000 simulations were run for a modeling period of almost one ye
ar, and frequency distributions of plume characteristics were obtained. The
currents and density stratification change widely and rapidly, resulting i
n extreme variability in plume behavior. Rise height was predicted to vary
from deeply submerged to surfacing, and near-field dilution was predicted t
o vary from around 100 to several thousands within a few hours. The length
of the near field, or hydrodynamic mixing zone, also varies considerably, s
o that a fixed regulatory mixing zone may sometimes encompass all of the ne
ar field and some of the far field and sometimes only part of the near fiel
d. The combination of oceanographic data with suitable mathematical models
represents a significant improvement in our ability to predict the statisti
cal variability of ocean outfall plume behavior.