Septic systems topic of discussion at CCP meeting
Septic systems were discussed in length at the Captiva Community Panel meeting on Sept. 14, as the panel continues to seek research indicating if the systems are having an impact on nearshore waters.
Chief engineer and project manager for Captiva’s sewer project Doug Eckmann, of the firm Kimley-Horn, said the major source of nutrients to nearshore waters in an island community comes from stormwater runoff, which is a problem. He said then there are septic tanks that rely on drain fields to defuse the liquid.
“Septic systems historically have done a great job in eliminating bacteria. Septic systems rely on treatment in dry soil and they are generally ineffective in biological nutrient reduction in wastewater,” Eckmann said. “In sandy soils like Captiva, water just flows through it and then the high water table doesn’t allow much detention time for biological processes.”
Eckmann said stormwater causes for 64 percent contribution to nitrogen loading, while septic systems contribute 34 percent.
“Very little nitrogen is removed in the saturated sandy soil conditions,” he said.
CCP President Jay Brown said there has to be at least 24 inches of separation between the drain field and underlying water table. It was found in sampling of all the island’s septic tanks that at least 24 percent did not have separation.
“In recognizing that we have very sandy porous soil, septic systems would really be a problem,” he said, adding that it becomes a nitrogen loader because of the lack of conformity.
Brown said in studying all the permits for septic systems, they should not be located on a lot less than one and a half acres. He said the lots in the Village area are less than a half-acre to a quarter-acre.
“We don’t have enough of vertical separation between drain field, groundwater and lots are too small,” he said.
The panel also heard from Sanibel-Captiva Conservation Foundation research associate Mark Thompson, who shared research about septic tanks. He explained that discharge from a septic tank goes into a drain field, which in the soil receives nitrogen treatment. The bottom of a drain field and at the top of the water table is where nitrogen occurs.
“When talking about nitrogen load or nutrient load from a septic tank, the load can be calculated at the bottom of the drain field after it travels through the soil right before it goes into the water table,” he said.
Thompson said the average water table elevation on Captiva was 39 inches from the soil surface based on the SCCF study. A minimum of 42 inches is required.
“On average, the average septic system on Captiva is interacting with the water table,” he said, adding that impacts on the home will start as mechanical things can happen when the septic systems are down to the water table.
Thompson said the worry is the older septic systems, especially with sea level rise.
He said about half of the nitrogen loading from Captiva is coming from septic systems, while the other is coming from stormwater, as the island is very lush. Thompson said Captiva does have an impact within a kilometer offshore, which is where there is fish habitat, invertebrate habitat and ecosystems.
“Just to start with 50 percent coming from septic systems and you go to sewer right away you are reducing that load coming from Captiva by 50 percent,” he said. “DEP will be looking at our calculations and show what they have for numbers. Since this has all been done by Kimley-Horn and us, they will use that as guidance to get started. If they find problems, they correct it. They do a good job of estimating.”
Thompson added that the Pine Island Sound is impaired water due to nitrogen and chlorophyll.
“In the next 10 to 15 years, DEP will have a plan for total maximum daily load calculated for this body of water. We need to reduce nitrogen loads going into this body of water. How do we do it? They will look at stormwater runoff, groundwater and septic systems,” he said. “Septic systems are always an easy target as far as blame goes.”
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