Title: Characterization of water and sediment of a tropical shallow lake in the Xochimilco urban wetland system, Mexico
Author: Alejandra Gulfo Gonzalez
Supervising Institution: IHE Delft - Institute for Water Education
Xochimilco wetland in Mexico City has gone through a long historical land use change since pre-Hispanic times, related with agricultural use, water abstraction, and urban development. This has led to environmental degradation in terms of water quality and biodiversity, threatening endemic species such as the Mexican salamander axolotl (Ambystoma mexicanum). In efforts to restore its historic and ecological importance, Xochimilco wetland currently has different protection and conservation status: it is a UNESCO historic world heritage centre, and a RAMSAR site. In 1992 a Natural Protected Area was created in order to conserve the traditional agriculture system “chinampas” (raised beds for crop production surrounded by canals) as well as the ecological equilibrium of the site. Inside this protected area, a lake has developed as a result of the land subsidence and a series of inundations. This lake has become an important ecological site providing different ecosystem services. However, little is known about the ecological functioning of this site, and how it is been affected by the urban population growth of Mexico City.
In order to characterize the protected area, four contrasting water bodies (the lake itself, a small adjacent lake, the surrounding wetland and a nearby chinampa) were selected. To relate the characteristics of water and sediment with the land use, hydrology and history of the area, physico-chemical parameters and nutrients of water and sediment samples were measured in each water body. Additionally, the bathymetry of the lake and an analysis of the land use and land cover of the sampling area were done. The hydrology of the lake was analysed based on stakeholder interviews.
Results show a high environmental degradation related with the land use, hydrology and history of the sites. All sites are eutrophic systems, however they differ in terms of the physico-chemical characteristics of water and sediments. The small lake and the wetland have a high content of organic substances and low content of inorganic nutrients, while the chinampas have a high content of inorganic nutrients and low content of organic substances. The lake is similar to the small lake and the wetland in terms of the inorganic nutrient content, but resembled the chinampas more with respect to the organic substances content. The hydrology of the lake is not well understood. During the dry season, inflows and outflows are minimal or non-existent. The inflow of water during the rainy season is likely to change the dynamic of these systems.
Chinampas are the sites most affected by the land use. The high content of inorganic nutrients in the water column is related to the addition of N and P fertilizers, the constant removal of sediments from the bottom of the canals to be used as a natural fertilizer, the transport system used in the canals, and the quality of the treated wastewater discharged into the canals. The lake, small lake and the wetland are less affected by agricultural practices, but could be highly affected by the input of polluted water during the rainy season. Additionally, these three sites are former agricultural land flooded, with sediments rich in organic matter and nutrients.