“Water, water, everywhere, nor any drop to drink” Samuel Taylor Coleridge (1772-1834)
The right of all humans to safe drinking water is a key focus of the Sustainable Development Goals to be agreed at COP21 in Paris this year. Groundwater currently provides the main source for human consumption around the world and, given its comparative resilience to variations in climate, our reliance on this precious resource is only likely to increase as populations grow. This dependence will be greatest in coastal areas, where the average population density is three times the global average.
Obvious man-made threats to groundwater quality include the increasing prevalence of chemicals in agriculture and the use of various anthropogenic contaminants in industry and transport. Indeed, most of us are within touching distance of these substances every day as we commute to work or operate electronic devices. By their very nature, they can be monitored and controlled, and incidences of contamination can be identified relatively easily, often before major impacts occur.
Keeping track of seawater
A less obvious threat is the contamination of groundwater by natural sources, catalysed by human activities. In coastal areas, seawater lies beneath all of our aquifers. It is pervasive in nature, but as a natural commodity, its extent is extremely difficult to determine. As weather patterns, sea levels and lunar cycles vary, its boundaries change and as we exploit it further to cope with patterns of drought and increasing population, its extent becomes ever more dynamic.
Saline drinking water can cause a variety of health problems. Once seawater enters an aquifer, it can take years for rainfall to wash out remnant salts and bring the resource back to a suitable drinking standard. Most existing methods of monitoring rely on identifying the start of seawater incursion into boreholes, but by that time there can be little or no time to respond. Traditional geophysical surveys can define the saline front at a point in time, but it is not feasible to constantly scan our coastline with costly and labour intensive surveys. In short, the way we provide drinking water may be unsustainable in many areas, but how can we tell before it is too late?
The self-potential (SP) method relies on detecting small, naturally-occurring voltages in the ground, caused by changes in pressure, temperature and groundwater salinity. Modelling suggests that monitoring electrodes placed in a single borehole can respond to changes in groundwater quality several hundred metres away. To date, SP monitoring has never been used to warn of impending seawater intrusion, but research at Imperial College London suggests that it may be our most powerful tool to protect coastal groundwater. My own research will seek to develop an early warning methodology to protect coastal groundwater supply boreholes from salinisation, using SP data collected at a field site near Brighton and a series of computer simulations.
Towards a better understanding of resources
We can only manage resources sustainably when we understand them. If we don’t know where they are, the job of custodian becomes little more than a game of chance and one which is stacked against us in the face of increasing demand and more variable weather patterns. By knowing where the resource starts and ends, and how it changes over time, pumping regimes can be optimised to make the most sustainable use of our coastal aquifers.
It is interesting to note that much of the funding for research in SP comes from the petroleum industry, where there is a need to prevent saline water entering oil production wells. This may seem ironic to many environmentalists, who see petrol companies as the enemy of sustainable development. Perhaps the pragmatism of industry and the idealism of conservationists can make for a potent blend, if only we can get past the dogmatism that is common to us all.