As I have shown in previous blogs, a large fraction of Africa’s low-income population is strictly reliant on groundwater for its freshwater supply. These aquifers can be relatively thin in the low-lands and form adjacent to the coastline. The Ghyben-Herzberg function explains the existence of a freshwater lense “floating” on top of seawater, separated by the saltwater-freshwater interface (for 1 feet of freshwater above the interface, 40 feet of saltwater lie below). This interface is vital in determining the quantity of freshwater storage in the coastal aquifer, and the mixing of the layers effects the quality. Many studies have evaluated the pressures existing on these vulnerable freshwater reserves, leading to sea water intrusion (the encroachment of the interface) and excessive chloride contamination of the freshwater.
While the unsustainable abstraction for human use has been evaluated to be a greater threat in many coastal aquifers than climate change (e.g. Ferguson and Gleeson, 2012), I want to highlight the ways climate change may exacerbate this.
The diagram below shows how a rise in sea-level affects a coastal aquifers’ characteristic by reducing the freshwater lens (Figure 1). An area identified as being under significant pressures from this mechanism is Southern Africa. A modelling study was conducted by Ranjan et al (2006) that estimated the loss of freshwater by the encroachment of the interface – also taking into account predicted changes in aquifer recharge. The rate of change of groundwater loss (% per year until 2100) in South African coastal aquifers was named to be 0.027 (A2 scenario) – 0.022 (B2 scenario).
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| Figure 1: click for Source |
Besides this reduction in reserves, climate change also is predicted to increase cyclonic activity and storm events. Storm surges cause coastal flooding, salinisation of surface waters and ecosystems but also will result in instantaneous events of saltwater intrusion to coastal aquifers from above into the freshwater lens (Anderson, 2002). Groundwater chemistry is thus adversely affected by increases in chloride concentrations, adding to the stress on coastal water reserves.
In a comparative impact study of the climate change effects described above (sea-level rise and storm surges), Dasgupta et al (2009) from the World Bank Development Reasearch group, find what I had expected while examining the impacts on Africa. The vulnerability to these threats is found to be concentrated to populations and large cities at the lower end of the international income distribution (for Africa e.g. Djibouti, Tanzania, Mozambique). Once again, this raises the point that the most socio-economically vulnerable are those to expect the largest negative impacts of anthropogenic climate change, while having contributed least to it. A very recent publication by Barbier (2015) effectively highlights the threat climate change poses to African low-elevation coastal zone populations by increasingly pushing these into the ‘poverty-environmental trap’ and calls for a rethinking of policy and development strategies.
Really interesting post. Are there any cities that are already planning to deal saltwater intrusion? And if so how?
ReplyDeleteThanks for the question - I've looked into this accordingly.
DeleteSo, what is quite obvious from all posts regarding groundwater resources is that management is generally severely limited by the lack of data, monitoring and mapping of the resources themselves. The process of salt-water intrusion (even though simplified in this post) is highly dependent on the locally specific ground geology, faulting and density parameters involved. Thus, what has generally been the first step into "better management" of the effected aquifers is to invest in research. Cotonou, a coastal city in Benin in West Africa is severely dependent on the underlying coastal aquifer but has been experiencing dramatic salt water intrusion. In response to this problem, a large seismic imagining study has been done that helps to understand the system, model it, and make water management effective. (See: https://www.onepetro.org/conference-paper/SEG-2013-1481 for the study if you are interested).
Regarding the methods of management: firstly, reduced freshwater usage and effcient water supply networks are the obvious choices. Methods to combat the extension of the salt-to-freshwater interface inland are to increase recharge to the aquifer. These engineering projects require large funds and effective planning and are thus less likely to be an option in poor regions of Africa.
Hi Loulou, a very interesting blog post. I was thinking, as they are socio-economically vulnerable, with relatively little power, in what way would you suggest that this issue could be resolved? Unfortunately, I personally feel that it is extremely difficult to achieve this and although highly inefficient, the only way to partially resolve this issue to use surface water more sustainably as these aquifers will diminish. What do you think?
ReplyDeleteThanks for your interest Maria! As I have mentioned in my answer to Oliver's comment above, the actual effective prevention of salt water intrusion needs funds for 1)detailed research on the relevant aquifer 2) effective management of the salt water intrusion. If this occurs in economically impoverished locations, these strategies are not likely to work out due to this economic barrier. If the salt water intrusion is caused mainly by rising sea levels rather than unsustainable abstraction, it is even less likely to be able to resolve the problem. Therefore yes, I must agree with you, that these coastal regions are going to have to find alternative water resources and reduce their dependency on the diminishing groundwater.
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