Septic Tanks Affect Coastal Water Quality

Date: March 11, 2009

Septic
<http://www-csgc.ucsd.edu/NEWSROOM/NEWSRELEASES/2009/CoastalWaterQuali...
l>  Tanks Affect Coastal Water Quality

PALO ALTO - California Sea Grant researchers have strong evidence that
septic tanks in Northern California are leaking nitrogen and phosphate into
coastal waters that can trigger algal blooms.

Reporting in the journal Limnology and Oceanography, they report finding
elevated levels of these "nutrients" in the surf zone during periods of high
groundwater flows to the beach.

Following one of these freshwater pulses, they observed a four-day elevation
in chlorophyll-a levels - a proxy for phytoplankton concentrations. Though
it is extremely difficult to attribute any single algal bloom to the
presence of higher than normal nutrient levels, the general link between
nutrification and algal blooms is widely recognized for both marine and
freshwater ecosystems.

"Our project is one of the first in California to show definitively that
septic tanks can affect coastal water quality through submarine groundwater
discharge," says Alexandria Boehm, a professor in the Department of Civil
and Environmental Engineering at Stanford University.

Most research on septic systems has focused on their effects on terrestrial
ecosystems, Boehm explains. The value of this project is that it shows they
can also impact marine ecosystems via polluted groundwater discharging
directly to the ocean.

In theory, the nutrient spikes detected in Northern California could have
come from polluted creeks or runoff. This, however, is not what the
scientists believe is happening because their fieldwork was conducted in
summertime when, they say, groundwater is the only source of freshwater to
the coast.

Fertilizers spread on lawns and crops could also potentially be sources of
the nutrients they detected. Again, however, scientists rule out this
possibility because of the concomitantly high levels of human fecal
indicator bacteria detected in groundwater samples collected between the
septic systems and shoreline.

Interestingly, bacteria counts in beach water samples did not rise and fall
with changes in groundwater fluxes, suggesting the beach aquifer removes
pathogens, says Stanford doctoral student, Nicholas de Sieyes, the lead
author of the journal article. "Some of our current research is focusing on
this point."

From a scientific perspective, the researchers were not surprised to find a
link between septic systems and beach water quality. "It is what we
expected," Boehm says.

The unanticipated discovery was the way in which tidal cycles modulate
freshwater fluxes to the coast. Indeed, their prediction was that fresh
groundwater flows would peak during spring tides, when the tidal forces of
the Sun and Moon reinforce each other. Instead, the greatest pulses of
exiting groundwater occurred during neap tides, when weak tidal forcing
results in minimal differences between high and low tides.

In particular, they measured a fresh groundwater discharge rate of 1.2 to
4.7 liters per minute per meter during neap tides, compared with .1 to .5
liters per minute per meter during spring tides. During neap tides, nitrogen
levels rose 35 percent, phosphate levels 27 percent and silicate levels 14
percent, as compared with spring tide measurements.

Their technical explanation for the pattern is outlined in detail in their
peer-reviewed work. The gist of it is that ocean water fills the beach
aquifer during high spring tides, creating a sort of hydraulic mound in
front of fresh groundwater. During low tides, all of this saltwater must
drain back to sea before fresh groundwater can begin to exit. During neap
tides, the absence of a formidable hydraulic mound results in a greater
release of fresh groundwater to the beach during low tide, hence their
results.

All of the fieldwork, which will continue into the summer of 2009, was
conducted at Stinson Beach in Marin County because of the community's
interest in protecting its beach water quality.

"We don't think our findings are unique to Stinson Beach," de Sieyes says,
noting that septic systems are common along coastal counties north of San
Francisco, as well as in more densely populated areas such as Morro Bay,
Malibu, Rincon and Los Osos.

In recognition of the potential environmental implications for beach, ocean
and river ecosystems, the California legislature has directed the State
Water Resources Control Board to establish regulations on septic systems.
California and Michigan are the nation's only two states without statewide
regulations on septic systems.

Critics of the proposed changes cite a lack of data showing septic systems
contribute to actual water-quality problems. "I think it's really important
for people to know that we've quantified the impact of septic systems on the
coastal ocean at one location, that there were documentable effects on
groundwater and coastal water quality and that, in general, on-site
wastewater treatment is indeed an important environmental concern and may
require additional regulatory attention," de Sieyes wrote in an email
exchange.