How SGDs help and hurt reefs<\/h2>\n\n\n\n
In some contexts, groundwater can support reef systems, delivering the right combination of nutrients at modest levels, or cooling<\/a> marine heat.<\/p>\n\n\n\n However, rising coastal population densities, changing agricultural practices and ageing wastewater systems impact this balance.<\/p>\n\n\n\n At two small coral islands in the northern South China Sea, researchers found<\/a> that even a small amount of SGD carries carbon compounds that can degrade the shells or suppress the growth of certain marine animals.<\/p>\n\n\n\n In many coastal areas of the Philippines, harmful algal blooms \u2013 commonly known as \u201cred tides\u201d \u2013 have become increasingly frequent<\/a>, with over 44<\/a> distinct coastal locations across the archipelago experiencing outbreaks since the first major bloom was recorded in 1983.<\/p>\n\n\n\n \u201cI recall the feeling of the hot viscous layer of water sitting on top of the colder, clear water below, the first time I swam in the Philippines,\u201d says Amelia Wenger, conservation scientist and water pollution programme lead for coral reefs at the Wildlife Conservation Society, describing an algal bloom.<\/p>\n\n\n\n Scientists link red tides to a combination of climate change and eutrophication \u2013 where an overconcentration of nutrients stimulates excessive algal growth. SGDs are increasingly recognised sources of these nutrients, which include nitrogen, phosphorus and dissolved silica.<\/p>\n\n\n\n \u201cThe science shows very clearly that wastewater is one of the two big killers of coral reefs \u2026 in Hawaii and in other places in the world,\u201d Greg Asner, director of the Center for Global Discovery and Conservation Science at Arizona State University, said in a radio interview<\/a> in January. \u201cThe only other one that\u2019s competing with wastewater is climate change. And when the two combine, it is a terrible mix.\u201d <\/p>\n\n\n\n In many island territories, porous volcanic rock or limestone\u2019s high permeability limits<\/a> the soil\u2019s natural filtration capacity, allowing rapid transport of water and pollutants. Private houses and holiday resorts alike often rely on private cesspools or septic tanks in these territories, where sewage systems are typically insufficient or non-existent. These simple holes, or containers in the ground, collect the wastewater but are often badly sealed. They use some form of chemical treatment but are often outdated, poorly maintained or vulnerable to flooding, allowing contaminants to seep into groundwater with minimal intervention. From there, pollutants travel unseen to coastal waters, with consequences ranging from coral disease<\/a> to unsafe swimming<\/a> conditions.<\/p>\n\n\n\n A study<\/a> on the reefs around Santiago Island in the north-western Philippines found that groundwater emerging beneath reef flats had nutrient concentrations much higher than those of normal seawater. In other sites, such as the resort island of Boracay, researchers found<\/a> residues of caffeine and painkillers travelling from land to sea via groundwater. These results clearly point to untreated wastewater, especially in busy tourist areas.<\/p>\n\n\n\n \u201cContaminants can also be agrochemicals and petrochemicals, and science barely knows the impacts of the huge range of those chemicals,\u201d Asner said in an email interview with Dialogue Earth.<\/p>\n\n\n\n In Metro Manila, the Philippines\u2019 largest metropolitan area, less than 15%<\/a> of the city\u2019s population is connected to a sewage system. From the majority of households, untreated waste<\/a> flows directly into rivers or leaks into the ground. From there, they are an easy reach<\/a> to Manila Bay, a natural harbour on the western border of the region.<\/p>\n\n\n\n The bay is now heavily polluted<\/a> from household wastewater, with faecal contamination far exceeding safe limits \u2013 a hallmark of the faulty human waste system.<\/p>\n\n\n\n Amid mounting water pollution<\/a> and these high levels<\/a> of faecal bacteria, in 2025 the Metro Manila Development Authority, alongside the Department of Environment and Natural Resources and the Metropolitan Waterworks and Sewerage System, began drafting a plan<\/a> to expand sewerage infrastructure. Their target is to service around 80% of the national capital region by 2047. Early progress has been visible: Manila Water, the main concessionaire involved, added over 43,000 new sewer accounts in September 2025 alone.<\/p>\n\n\n\n However, SGD remains largely absent from policy frameworks.<\/p>\n\n\n\n \u201cWe know that SGDs play a key role in marine pollution,\u201d Von Hernandez, vice president of the conservation NGO Oceana Philippines, tells Dialogue Earth. \u201cWe need governments to add it to their priorities for research and investigation, to make the issue more visible to the public so that appropriate actions could be taken.\u201d<\/p>\n\n\n\n Researchers are pushing to change that. \u201cThere is not enough attention from policymakers in the region on the phenomenon, and that tells us that there is still much work that needs to be done,\u201d says Fernando Siringan, research professor at the University of the Philippines\u2019 Marine Science Institute.<\/p>\n\n\n\n His team discovered<\/a> storms and heavy rainfall events can temporarily boost groundwater flow into the ocean for several days after the event. As climate change intensifies storm events in the region, the pulse of pollution could become more dynamic and less predictable, adding another layer of concern.<\/p>\n\n\n\n \u201cCoral reefs are already suffering from climate change consequences. Polluted hidden effluents are increasing the impact,\u201d Siringan tells Dialogue Earth.<\/p>\n\n\n\n For coastal countries reliant on healthy reefs for fisheries, tourism and shoreline protection, the stakes are high. Reef degradation lowers<\/a> fish stocks, reduces tourism and causes significant income declines for coastal households. For example, the six-month closure<\/a> of the Philippines\u2019 tourist gem Boracay in 2018, due to environmental degradation, was projected to have affected over 36,000 workers and caused losses of nearly USD 1 billion. Beyond tourism, over 2 million people<\/a> work directly or indirectly in the country\u2019s fisheries sector.<\/p>\n\n\n\n \u201cAn invisible problem is becoming visible,\u201d says the chemical oceanographer Gil Jacinto. A 2024<\/a> global meta-analysis of coastal groundwater nutrient concentrations identified SGD as a significant source of nitrogen and phosphorus in the ocean of many coastal areas. Yet it is rarely included in official nutrient budgets. These budgets balance the input, output and storage of such nutrients, making them critical to identifying sources of eutrophication.<\/p>\n\n\n\n \u201cThere is good data and evidence that SGD pollution affects coastal environment health in the Philippines. Still, most policy responses here only focus on river discharge, while the contribution of SGD remains largely unaccounted for,\u201d Jacinto tells Dialogue Earth.<\/p>\n\n\n\n As evidence mounts, local communities and scientists in island territories are pushing to integrate SGD monitoring into environmental management at tourist sites \u2013 a first step toward recognising groundwater as part of the land-sea pollution continuum.<\/p>\n\n\n\n One example can be found in Hawaii, where researchers from Arizona State University including Asner worked with the \u02bb\u0100ko\u02bbako\u02bba<\/a> Reef Restoration Project. The team mapped<\/a> over 1,000 locations where groundwater contaminated with faecal bacteria enters reef systems on the western coast of the state\u2019s largest island. This data is now informing policymakers when considering new land reformation efforts, wastewater infrastructure upgrades, and community-led efforts to replace cesspools and repair septic systems.<\/p>\n\n\n\n \u201cWhen I started working on pollution issues, there weren\u2019t that many people who were interested,\u201d Wenger tells Dialogue Earth. \u201cIn the last five years, there\u2019s been a real shift. The more we know about it, the more we can raise awareness and adapt solutions for other countries.\u201d<\/p>\n\n\n\n Wenger is now developing guidance for the sanitation and production sectors (agriculture, livestock and aquaculture), with plans to expand this to encompass plastic pollution, coastal erosion and urban runoff.<\/p>\n\n\n\n This follows from a previous guide<\/a> and toolkit<\/a> she and her colleagues developed to help conservationists and sanitation workers reduce wastewater pollution. Released in 2024, the toolkit includes factsheets to help such professionals diagnose, assess and monitor specific types of pollution within their systems, and to design ad hoc solutions.<\/p>\n\n\n\n \u201cI split my time talking to ocean people and then talking to sanitation people about what they both need to think about pollution. Coastal communities are generally receptive, especially when shown that reducing wastewater pollution directly benefits local reefs, fisheries and public health,\u201d she says. \u201cBut often the high costs are a limitation. Working alongside the WASH [water, sanitation and hygiene] sector can help on this front.\u201d<\/p>\n\n\n\n She adds: \u201cI wish we\u2019d started this 20 years ago, but we didn\u2019t. Here we are now, and we’re thinking about the solutions.\u201d<\/p>\n\n\n\n Submarine groundwater flows might be invisible, but their impact is not. As scientific understanding deepens, researchers say governments must widen their scopes beyond rivers and pipes and confront the hidden currents shaping coastal systems.<\/p>\n","protected":false},"excerpt":{"rendered":" Groundwater brimming with excess nutrients and other contaminants is quietly flowing into the ocean, posing a threat to vulnerable reefs<\/p>\n","protected":false},"author":40000299,"featured_media":60129391,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[50039901,760],"tags":[20000060,50042407,20000341],"hashtags":[],"country":[50040715],"class_list":["post-60129379","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-ocean","category-pollution","tag-groundwater","tag-waste","tag-water-pollution","country-philippines"],"acf":[],"yoast_head":"\n![\"waste](\"https:\/\/dialogue.earth\/content\/uploads\/2026\/04\/Manila-waste-water-treatment_Danilo-Pinzon_World-Bank_Flickr_6359024323-3.jpg\")
<\/div>Wastewaters threaten the coral triangle<\/h2>\n\n\n\n
High stakes<\/h2>\n\n\n\n
From mapping pollution to integrated solutions<\/h2>\n\n\n\n
I wish we\u2019d started this 20 years ago, but we didn\u2019t. Here we are now, and we\u2019re thinking about the solutions<\/blockquote>Amelia Wenger, water pollution programme lead, Wildlife Conservation Society<\/cite><\/div><\/div>\n\n\n\n