Executive Summary
Background, Goal, and Objectives
A research project was conducted by an interdisciplinary team from the University of
California, Santa Barbara (UCSB), with assistance from researchers at the University of
Southern California (USC) and Lawrence Berkeley National Laboratory (LBNL),
between July, 2006 and December 31, 2009, involving a one-year field sampling and
analysis program between November, 2007 and November, 2008. The project was
funded by Heal the Ocean (HTO) with support from the California State Water Resources
Control Board (SWRCB). The overall objective of the project was to examine of the fate
and transport of the treated effluent plume from a wastewater treatment plant (WWTP)
discharging through a diffuser at the terminus of a short, shallow outfall. The project is
important because its results are applicable to the many short outfalls discharging into
California’s coastal waters. The approach of the project employed coordinated
microbiological and chemical analysis of effluent composition with observations of
effluent plume motion to quantify plume dilution, to identify plume tracers, and to
describe trajectories of the plume. The WWTP discharging the effluent is operated by
the Montecito Sanitary District (MSD) in Montecito, CA, which also was the site for field
sampling. The specific goals of the project were to:
1. Describe the likely pathways of the WWTP effluent plume based on GPS-tracked
drifter data.
2. Determine if waters located farther offshore from the WWTP outfall location move
similarly.
3. Quantify culturable and DNA-based fecal indicator bacteria, amounts of DNA-based
human waste markers, bacterial abundances (by extracted DNA, and also cell counts),
and nutrient concentrations at the sampling locations.
4. Describe the differences and similarities in the microbial communities at the diffuser
versus offshore, nearshore, and shoreline sampling locations.
5. Identify microbial taxa that appear to be tracers of the plume, and describe their
abundances along the plume path.
6. Determine if and where treated effluent is most likely to enter the surf zone.
7. Estimate the plume concentration if and when it enters the surf zone.
Project Approach
The project approaches included mathematical modeling, field observations, laboratory
sample analysis, and data analysis of physical oceanographic, chemical, and
microbiological water characteristics. Field data were collected in four ways: 1) from
moored instrumentation installed for this project near the outfall diffuser, 2) from global
positioning system (GPS)-tracked drifters released from a small research vessel, 3) from
vertical profiles of water properties, and 4) from boat-side sampling of ocean waters for
microbiological and chemical constituents. Temperature and current profiles were
measured at the mooring continuously for a year to provide data for modeling the plume
vertical rise and to monitor ocean water masses and waves. Other field measurements
were collected one day each week for a year. The GPS-tracked drifters recorded how
ocean currents moved effluent and surface waters away from each of three release points:
1000 meters (m) offshore of the diffuser, 500 m offshore of the diffuser, and above the
diffuser. An instrument called a CTD was used to measure vertical profiles of
temperature and salinity. Water samples were collected 1000 m offshore (Offshore1000)
of the diffuser, above the diffuser (Diffuser), and at up to three locations (Lagrangian a,
b, c) along the paths of drifters. On the same days as the offshore sampling, effluent
water samples (Effluent) were collected from the WWTP discharge upstream of the
outfall, and water samples were acquired from the shoreline (Shoreline) in ankle-deep
water at beach locations aligned with the drifters’ ending positions.
Current and temperature data collected at the mooring were inputs to a model predicting
plume rise over the diffuser. CTD data from the vertical profiles allowed estimation of
salinity differences and dilution as the fresher effluent plume mixed into saltier ocean
waters. Drifter data tracked the plume motion over the sea surface. Water samples were
analyzed for culturable fecal indicator bacteria (FIB), DNA-based markers of
Enterococcus and of human waste-associated Bacteroides, enterovirus, ammonia,
nitrate+nitrite, phosphate, total bacterial counts, and microbial community profile and
composition using terminal restriction fragment length polymorphism (TRFLP) and
PhyloChip analyses, respectively.
Quarterly stakeholder meetings were conducted throughout the project to convey
progress and results to date. Attendees routinely included representatives of UCSB,
HTO, MSD, and the SWRCB.
Summary of Results
Plume modeling indicated that the buoyant effluent plume always reached the sea surface
within about a minute after discharge from the diffuser. Drifters deployed over the
diffuser moved mainly alongshore and eastward or westward away from the diffuser.
Drifters also moved in the onshore direction on 49 of the 50 sampling days. Drifter
movements were consistent with known patterns of the local coastal circulation. Drifters
released at the diffuser reached the surf zone over a region extending from about 0.6 km
east, to about 1.6 km west, of the diffuser. Roughly half of all diffuser drifters reached the
surf zone waters west of the diffuser, and the other half east of the diffuser.
Drifters deployed offshore of the diffuser moved both eastward and westward with
generally higher speeds. Drifters offshore of the diffuser were less likely to reach the surf
zone during the three to five hour sampling times of this study. About half of all drifters
released at the diffuser location reached the surf zone while only 15% released 500 m
offshore of the diffuser and 3% released 1000 m offshore of the diffuser did so.
The effluent plume was almost always detectable in surface salinity. Dilution estimates
over the diffuser and following drifter motions were always greater than 100, and
typically greater than 400, consistent with design criteria of WWTP effluent outfall
diffusers.
Phosphate and nitrate+nitrite concentrations in effluent samples were greater than in the
ocean, but ammonia concentrations were similar to the ocean. Dilution estimates based
upon phosphate and nitrate+nitrite concentrations at the diffuser were similar in
magnitude to estimates from salinity.
Concentrations of culturable fecal indicator bacteria (total coliforms, E. coli and
Enterococcus) were generally low across all samples and, while higher at the shoreline
than in effluent samples, did not exceed State of California beach water quality criteria.
Total bacteria were also typically very low in the effluent, but were higher in the
nearshore region relative to offshore. DNA-based measures of Enterococcus and of
human waste-associated Bacteroides were generally low and undetectable for many
samples. Enteroviruses were not quantifiable in any of the samples.
Terminal restriction fragment length polymorphism (TRFLP) analyses, performed for 26
sampling events, allows for “profiling” bacterial communities using total DNA extracted
from water samples. A high density phylogenetic microarray (PhyloChip), developed by
LBNL, was also used to profile bacterial communities and to also potentially identify
which taxa, from over 8000 microbial taxa detected by PhyloChip, were present in the
samples. Due to its high cost, PhyloChip analysis was performed for a subset of 8
sampling events that were also analyzed by TRFLP.
By TRFLP and PhyloChip analyses, microbial communities in effluent samples,
regardless of sample DNA content, varied but overall were distinct from all other
samples. By PhyloChip analysis, several taxonomic groups, or “families” of bacteria
were identified as contributing to the distinctiveness of the effluent microbial community
relative to offshore. One family was shared with the nearshore region, including the
shoreline. There were also many distinct taxa in the shoreline samples, meaning that the
shoreline harbored bacteria that were not detected in either the effluent or the ocean.
Conclusions and Future Recommendations
A highly interdisciplinary research program was successfully performed to characterize
the fate and transport of a WWTP effluent plume offshore of Montecito, CA. Effluent
was extensively diluted following discharge, consistent with modeled diffuser
performance and design. The plume typically moved towards shore. However, no beach
water quality criteria were exceeded. The effluent fertilized the nearshore environment
with phosphate and nitrate+nitrite which may have been responsible for increased overall
bacterial biomass in the nearshore waters. While the shoreline microbial community
shared some microbial taxa with the effluent and ocean that were not found further
offshore, the exact origins of these taxa at the shoreline remain unknown. Further, the
shoreline appeared to have a number of taxa that were unique to that environment,
suggesting other, possibly land-based, sources. Additional data and sample analysis
could provide more insight.