A delicate balance: Understanding underground water reserves

The sun has passed the halfway mark on its annual northward journey. Days are getting shorter and nights longer, as we say goodbye to the dry summer and await the arrival of the wet winter. Nobody can predict with any certainty how much rain we are going to have over the next six months. Does a slightly wetter March mean anything? I don’t know. I am told a wet April may mean a wet winter, but we will just have to wait and see.

Proclamations of the drought being over are premature. Last hydrological year (October – September) we received 654 mm of rainfall, against my average of 774 mm/a. Dam levels in October 2018 were much better than the two preceding years, but our dams were by no means full. Currently, rainfall is slightly above the average to March, but this doesn’t mean much. Less than 30% of rainfall occurs in the summer months. Good rains in 2019 will provide some comfort, but below average rains will reignite the narrative of water restrictions and Day Zero.

My disquiet on the water situation was awakened by a recent article in The Conversation by Professor Jasper Knight and repeated on other online platforms. In his comment on the water strategy of the City of Cape Town currently under review, he remarked that aquifers are depleted. He supported this with a link to the website of the Department of Water and Sanitation (DWS).

The good professor misinterpreted the presented information and he is at odds with some 50 members of the Ground Water Division who met in November 2018 to consider the impact of the drought on groundwater. The Ground Water Division has a paid-up membership of about 300 members who are either professionals in the field or have an interest in the promotion of the science of groundwater.

That gathering concluded that the drought had had no impact on groundwater resources, but that a drought signature was evident in some groundwater levels monitored across the Western Cape Province and elsewhere. The drought signature observed during the droughts of 2003 – 2004 and 2010 – 2012 are now being observed in data monitored from 2015 onwards.

Following good rains, aquifers quickly recharged back to normal or above average conditions. These seasonal and inter-annual fluctuations are nothing more than a shift in the balance between inflows, outflows and a change in aquifer storage which occur continuously. They are certainly not a sign of aquifer depletion.

In this regard, groundwater suffers from two drawbacks. First, it cannot be seen and has to be measured by indirect methods. And second, being the Cinderella of water supply in South Africa contributing only 15% of the country’s total supply, efforts to monitor and report on the state of aquifers are not comparable to surface water supplies.

However, since the early 1990s, DWS has been establishing formal groundwater monitoring points across the country. This monitoring network currently consists of 1,459 boreholes. While it is widely accepted that the national department is not in a great space, credit must be given where credit is due. The collection and collation of this data is a mammoth task and in spite of the challenges that their staff endure, they continue to dutifully collect this vital data.

Colleagues within the DWS are now attempting to analyse the monitored data and report on the status of our aquifers. It is this information to which Professor Knight provided a link. In trying to develop a means of communicating a vast amount of information in an understandable manner, DWS developed an index called “Water Level (%)” which it fails to properly describe. In the case of a dam, we know when a dam is empty and when it is full. With aquifers, this is less clear and defining the base of aquifers at a national scale is not easy.

Consequently, it developed this index where the shallowest measured groundwater level on record was assigned a value of 100% and the deepest recorded level was assigned 0%. If my school maths doesn’t fail me, this is called normalisation. This allows officials to compare aquifers in which groundwater levels only fluctuate annually by 2m to those that might fluctuate by 20m, with the difference in fluctuation largely being a function of hydrogeological properties. So instead of providing a measure of the aquifer being full or empty, it merely provides a measure of the range in groundwater level observed in the monitoring record.

It then assigned colour codes to the different percentages and plotted these on a map of South Africa. So, for example, boreholes with groundwater levels within the deepest 0%-10% normalised range were coloured red. This might all be a bit technical, but the current map has a very red colour as a result of groundwater levels now being at or near the deepest observed in a 20- to 30-year record, but this is simply a drought signature and not a sign of aquifer depletion.

This national-scale effort is corroborated with local-scale monitoring and I have heard of only one aquifer faltering as a result of the drought. It is true that some well points in parts of Cape Town deliver less water – or even dry up – at the back-end of summer, but this is because the well points are generally shallow (less than 8m) and susceptible to seasonal variations in groundwater level. The same does not hold true for deeper boreholes.

The aquifer under threat is at Beaufort West.

Groundwater levels in and around Beaufort West are on a downward trajectory, to an extent that both DWS and the Beaufort West Local Municipality urgently need to implement groundwater management measures to nurse the aquifers and their water supply through the drought.

Beaufort West is a groundwater-dependent town, with dam water and recycled wastewater only contributing a small volume to the town’s water supply. The hydrogeology of the area has been well studied, being of particular interest to DWS since the mid-1970s. Sound scientific intervention is now required.

The irony of the Beaufort West situation is that this is the first instance during the current drought where hydrogeologists are concerned about the state of a groundwater-based water supply. Our concern about the state of dams over the past 18 months is well documented.

Interpretation of groundwater level monitoring data allows hydrogeologists to predict that groundwater levels will quickly lose their drought signature after good rains. In the case of Beaufort West in the semi-arid Karoo, we may have to wait a couple of years for the reset button to get pushed – but it will get pushed.

While we cannot predict how long the current drought will continue, we can predict aquifer behaviour with some certainty once the drought is broken. Very little in science is certain, but we know that more than 60 towns and village across the Western Cape have been reliant on groundwater for decades without aquifer depletion. It is a pity that Professor Knight failed to appreciate this, rather only addressing groundwater and aquifers in the context of their perceived depletion.

An analysis of monitored groundwater level data in Cape Town and across the province debunks the notion that our groundwater resources are depleted or face any immediate threat. Clearly, aquifers are not infinite in capacity, and just like any resource, need to be appropriately managed. But as of now, our aquifers are in good shape – despite the drought. DM

Dr Roger Parsons is a hydrogeologist with more than 30 years’ experience. He is the Director of Parsons & Associates Specialist Groundwater Consultants^cc and former provincial branch chair of the Ground Water Division. He writes this article in his private capacity.