Septic tanks affect coastal water quality
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 effects on terrestrial ecosystems, Boehm explains. The value of this project is that it shows septic systems also can 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.
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.
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.
*The National Sea Grant College Program is part of the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce.
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