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S ome scientists and regulatory agencies point to fertilizers as the main source of the <br />nutrient runoff generating heavy algae in the lagoon. But Harbor Branch professor <br />B rian LaPointe believes sewage carries more of the nutrients spurring algae growth. <br />"It's really unclear how much fertilizer is reaching the lagoon," LaPointe said. "But one <br />septic tank on 4 acres — that s enough to create a nutrient problem." <br />Algal blooms block sunlight that sea gras s needs to thrive. As the algae decompose, <br />they deplete oxygen, which can suffocate sea grass and fish, turning clear, biodiverse <br />waters into a murky dead zone. <br />Local treatment plants discharge some effluent, though most wastewater in the lagoon <br />comes from septic tanks, said LaPointe, who has studied sewage impacts on <br />waterways for 30 years. <br />Indian River County: <br />37,000, roughly hal( <br />issued betore 1983 Of <br />the 900 systems on the <br />barrier island — where <br />they're more likely to be <br />near waterways — 747 <br />are more than 30 years <br />old <br />St. Lucie County: <br />45,000. about 18,000 <br />date back betore 1983 <br />There are about 120,000 septic systems on the Treasure <br />Coast, the newspaper investigation found. As many as half <br />were installed before stricter regulations were enacted in <br />1983, making them more likely to drain sewage into <br />groundwater that ends up in the lagoon, according to data <br />from the counties and Harbor Branch. <br />No one knows how many systems affect the lagoon, and <br />recordkeeping is sketchier on older septic tanks that could <br />cause the most harm. <br />One thing is certain: sewage taints the estuary. <br />Martin County: 40,000, LaPointe's research team took a total of three lagoon -wide <br />officials didn't know how samples in 2011 and 2012 and found nitrogen isotopes in the <br />many predate 1983 <br />algae, an element directly linked to sewage. Elevated levels <br />of ammonium and nitrate also were detected, LaPointe said, <br />noting anything above 3 parts per million indicates sewage. <br />H e called the findings a smoking gun. <br />All three counties on the Treasure Coast showed at least 5 parts per million Indian <br />River County had as much as 9 parts per million, putting it on par with troubled water <br />bodies such as Boston Harbor, according to the research. <br />67-6a <br />