A map under heaven

Tuesday 15 Jun 10
by Morten Aagaard Krogh

Sian Ka’an in Mexico is a precious wetland area. But it is fragile. To help protect the natural environment, a PhD student from DTU has mapped the paths by which water flows into the area.


Corals, mangrove swamps, tracks of rainforest, an extremely diverse wildlife: It is not surprising that in Maya language Sian Ka’an means ‘where heaven was born’. And for an increasing number of tourists Sian Ka’an is heaven – as reflected in the plans to build a new tourist mecca and add another nought to the 12,000 inhabitants of Tulum.

Sian Ka’an is on the Yucatan peninsula. Its life source, water, flows from the uplands towards the coast through a hidden network of underground caves and channels. This is where Bibi Gondwe from DTU Environment comes into the picture. In her PhD, she has been mapping the water volume underground, how fast the water moves and not least the paths which it follows. „The purpose of the project was to identify the inland areas which contribute water to the Sian Ka’an wetlands. Armed with this knowledge, it will be easier to protect Sian Ka’an, for example by passing laws stipulating which areas can be used for what.“

The work was carried out in collaboration with Amigos de Sian Ka’an, a local NGO which advises the government institution CONAGUA on how to best protect the area against pollution.

Unpredictable subsurface

Quite apart from the size and inaccessibility of the area, a major challenge in mapping the groundwater in Sian Ka’an is its special geology. It is a so-called karstland, which can be likened to a much-used bath sponge. „The groundwater flows through masses of small pores in the limestone bedrock. Some of these pores have turned into caves where the water flows even faster.“ The pores and the caves have been formed by thousands of years of rainwater which has bonded with atmospheric carbon dioxide. When the slightly acidic drops of water hit the ground, they gather and pass through cracks and holes into the subsurface. Slowly, very slowly, the rain dissolves the lime in the ground, and the cracks develop: Paths turn into roads and finally one day into motorways. Below ground, the water creates an infrastructure through which it flows easily and effortlessly.

In some places, the system of caves through which the groundwater flows is connected with the surface through so-called cenotes. These are like open wounds, small lakes where the surface water and the groundwater become one. As a result of this special geology, water and pollution are transported around the area very quickly, and as the unique ecosystem is dependent on the groundwater, the area is extremely sensitive.

Extensive research

To map the flow of water through this complex subsurface, Bibi and her partners have had to study Sian Ka’an from space, from the air, on the ground and below ground. This has involved using an array of methods: „I have been looking at a specific problem from the real world, and unlike many other PhD fellows, I have not focused on a specific method, but on combining a variety of supplementary methods,“ Bibi Gondwe explains.

The methods can fundamentally be divided into electromagnetic measurements (EM) and the analysis of satellite data combined with the more earthbound methods which took Bibi out into the dusty landscape, among the Mayas in their rush huts. To gather more precise data about the geological make-up of the area and to measure the thickness of the groundwater lenticle, she and her colleagues had to criss-cross Sian Ka’an and especially the uplands in a pick-up truck looking for wells into which they could sink their measuring equipment.

The future of Sian Ka’an depends very much on the extent to which the groundwater can be protected against the mounting pressure it will undoubtedly face as the tourist industry moves from the northern part of the Yucatan peninsula south towards Sian Ka’an. „It is important to take action now as the area is still relatively undeveloped. It will therefore be possible to prevent the fatal pollution which is seen further north in Merida, where the topmost 15 metres of a 60-metrethick freshwater lenticle are so contaminated that the water is unfit for drinking,“ says Bibi Gondwe. Her approach is pragmatic. Her recommendation is that when deciding which restrictions should apply to the individual areas, the starting point should not be the time it takes for water to travel through the subsurface. The areas which would require maximum protection, using this method, would simply be too large, and the rules would be difficult to enforce. Instead, she recommends drawing up a vulnerability map based on our knowledge of the water’s passage through the subsurface.

There are already two cases which her research could influence: plans for an airport near Tulum and a waste dump in the same  area. Both are alarmingly close to zones where pollution can easily seep through to the groundwater and from there spread to Sian Ka’an.