The link between groundwater and marine nature on Saba
Expert(s) |
Tess Davids
|
Perry de Louw
Saba wants to take targeted measures to protect its unique marine environment. To do so, there is also a need to understand how pollution occurs via groundwater. A new study by Deltares, commissioned by the Public Entity of Saba, provides the first complete picture of how nutrients, such as nitrogen and phosphorus, end up in the sea from land, and what role groundwater plays in this.
The coastal waters of Saba are among the most valuable nature reserves in the Caribbean part of the Netherlands. They are part of the protected Saba National Marine Park and border the ecologically unique Saba Bank. These areas are rich in biodiversity, but also vulnerable. Nutrient concentrations in coastal waters are approaching ecological limits, posing a direct threat to vulnerable marine life.
Importance for coral reefs
Coral reefs are particularly sensitive to excess nutrients in the water. High nitrate and phosphate levels can lead to excessive algae growth, which suffocates the coral. Although Saba's total nutrient input is lower than on larger islands, the measured values in seawater are close to ecological thresholds. This research helps to better understand where best to intervene to protect the vulnerable marine life around Saba.
Research process
At the time of the research, there was hardly any data available and little insight into the sources of pollution. In fact, no groundwater research had ever been conducted on Saba. "Initially, the question was: can you create a groundwater model for Saba?" says Tess Davids, geohydrologist and groundwater modeler at Deltares.
“But it soon became clear from discussions with Sarah van der Horn and Niké Dekkers of Public Entity Saba that it wasn't just about a model. They mainly wanted to understand how the (ground)water system on the island works. If you know how water flows and where pollution comes from, you can take targeted measures to protect the vulnerable marine ecosystem" Tess explains. “So that became the goal of this study: to better understand the water system, identify the main sources of pollution and quantify the role of groundwater.”
Recommendations for the future
The research shows that groundwater on Saba is an important route through which nutrients such as nitrogen and phosphorus enter seawater. Wastewater from households plays a particularly important role in this. Although the total nutrient load is relatively low compared to larger islands, the concentrations in coastal waters are already close to ecological limits. This makes it even more important to take targeted action.
Based on the research, it is recommended to investigate the possibilities of incorporating better wastewater management into new construction projects. And: more monitoring. By better monitoring rain, groundwater and runoff, the island can take increasingly targeted measures to protect the vulnerable reef. In addition, it is important that the successful goat control program is continued so that the restored vegetation, reduced erosion and significantly reduced nutrient input are maintained.
Challenges of groundwater research on a tropical volcanic island
To understand Saba's groundwater system, Tess and her colleague Perry de Louw, specialist groundwater systems, first collected all available relevant data and read the literature in the Netherlands. In this way, they tried to get as good a picture as possible before going on a two-week field trip in April. The aim was to collect additional data and deepen their understanding of the system there.
Still, that was quite a challenge at times, says Tess: "As expected, finding (ground)water during the dry season was quite a challenge. As on most volcanic islands, the subsoil is very permeable and there is hardly any surface water to be found. For groundwater, you have to rely on existing wells or springs. When you are researching the water system and there is no water available, you focus mainly on geology: what does the subsoil consist of and where does it flow when it rains? We also spoke to many locals, who shared valuable knowledge. For example, about old groundwater wells that are sometimes no longer marked on maps. Thanks to that information, we were ultimately able to take samples at several locations."
Perry: “My motivation is to understand groundwater systems in order to come up with effective solutions. This research confirmed once again that understanding water systems starts in the field; there is an incredible amount of information to be gleaned there. I also found the collaboration with marine biologists and the connection between our fields of expertise very valuable.”
From mountain to reef
Saba's steep slopes and volcanic subsoil ensure that rainwater quickly finds its way to the sea. According to the research, this happens in two ways: via a slow, constant groundwater flow and via rapid underground drainage during heavy rainfall, a few times a year. The groundwater only emerges from the ground again in the sea. A historical description even mentions that such an undersea groundwater source was used to tap fresh water.
The researchers found elevated nitrate levels in various groundwater wells and springs. Cesspits, simple wells into which untreated domestic wastewater sinks into the ground, appear to be the largest source of nutrients. Historically, the large feral goat population was the largest source of nutrients.
Fewer goats, more recovery
Just a few years ago, an estimated 5,000 goats roamed freely on Saba. Their grazing behaviour led to bare slopes, causing rainwater and sediments to wash away more quickly towards the sea. In addition, goats are a major source of nutrients (nitrogen and phosphorus).
“A goat emits about twice as much nitrogen and phosphorus as a human, so 5,000 goats are equivalent to 10,000 people in terms of nutrients. Given that Saba has only 2,000 inhabitants, the goats were therefore a relatively large source,” explains Tess.
Thanks to an intensive goat control program, the population has now been reduced to around 150 animals. The difference is visible: more vegetation, less erosion and, in the future, probably better groundwater quality in the soil.
Going forward
With this research, Saba is taking an important step towards sustainable water management. It offers administrators, residents and nature managers concrete tools to work together on a healthy marine ecosystem, now and in the future.
Tess: “This was one of the most enjoyable projects I have done at Deltares. Due to the scarcity of data and knowledge, the results have a real impact for Saba; it feels like we have really added something. In addition, it was a privilege to work on such a beautiful island, rightly called the “unspoiled queen”. I am grateful that I was able to contribute to its preservation.”
