Groundwater infiltration and recharge: what we know

As a result of climate change, we are increasingly faced with long droughts. Solutions are needed to maintain adequate supplies of fresh water during dry periods. Retaining water and actively replenishing groundwater – or ‘infiltration’ – is one way of maintaining groundwater levels. Nevertheless, this subsurface solution was introduced for different reasons in the past.

Backbone of drinking water supplies

“The first infiltration operations in the Netherlands were near the coast,” says Klaasjan Raat, a geohydrologist with KWR. “The dunes were a wonderful source of fresh water but, when people began to extract more dune water than was brought in in the form of rain, that led to salinisation. In 1940, the then Leidsche Duinwatermaatschappij was the first to introduce the infiltration of water from the polders into the dunes. Many more dune filtration systems followed in the provinces of Noord-Holland, Zuid-Holland and Zeeland. They are still the backbone of the drinking water supplies in the west of our country. The dunes are a wonderful stage in the treatment process and they are huge water buffers.”

Waves

To describe the development of infiltration and recharge, Raat speaks in terms of waves. “I call the beginning in the 1940s the ‘first wave’. It was immediately accompanied by research conducted by the drinking water utilities and Kiwa, the research institute established by those utilities in 1948. One of the first Kiwa reports, which was published in 1956,looked at surface water infiltration. A small wave followed in 1970, with the establishment of the Veluwe Infiltration Committee. So this was right in the centre of the country in view of the need to compensate for the effects of groundwater extraction. The second wave followed in the 1980s and 1990s. Drinking water demand rose rapidly at that time, and water utilities expected the trend to continue. That led to the rise of deep infiltration, a method for infiltrating water with a well at a greater depth, by definition under an impermeable clay layer. With deep infiltration, you use the soil as a treatment phase and you set up an extra water buffer in the system. There were fantastic pilot projects in this domain throughout the country. But measures such as eco-friendly shower heads, more economical washing machines, dual flushing systems in toilets and using water sources for livestock other than drinking water led to a levelling off of water demand. The drinking water infrastructure in place turned out to be adequate and, with the exception of one system, deep infiltration systems were not introduced on a large scale at the time. The situation is very different for greenhouse horticulture. More water was also needed there in the 1980s. Growers use rainwater for irrigation because its low sodium content is best for plant growth and it can be reused several times. In Aalsmeer and Oostland, over a hundred systems with infiltration wells for rainwater have been in operation since that time. During that second wave, Kiwa Water Research, as KWR was then called, and the drinking water utilities conducted a lot of joint research into deep infiltration. That knowledge was published in the well-known Kiwa Mededelingen 105 en 106.”

Fresh and salt

The third wave of infiltration and recharge was seen at the beginning of this century, says Raat. “The Netherlands was preparing for climate change and that resulted in, among other things, the Knowledge for Climateresearch programme. The increasing frequency of long dry periods means that we must look to local freshwater facilities. Storing surpluses in the subsurface, for example, is an interesting alternative here. But how do you do that in brackish soil along the coastline? You need to have a sound understanding of how freshwater and salt water behave. At that time, the academic world came up with groundwater models that allowed us to describe fresh and salt locations and transport. Working in the national research programme, we then teamed up with Deltares and the consultancy Acacia Water to set up the first pilot projects, among other places in the Westland area and on the creek ridges of Zeeland. The system in the greenhouse horticulture area in Dinteloord, Brabant, was one of the results. Treated residual water from the nearby Cosun sugar factory was transformed into water for irrigation and stored in an underground field of wells. During prolonged water shortages, growers use this water to supplement the regular water supply. The results that we published on this in 2017 won the H2O award for best article that year.”

Showcased knowledge

When he is asked about more KWR knowledge that can be showcased relating to the underground water storage in the third wave, Raat gives an example from the urban environment. “I like the Urban Water Buffer.That is a TKI Water Technology project that looked at how rainwater in the urban area can be retained effectively and longer in the subsurface. The system at ‘Het Kasteel’, the Sparta soccer stadium in Rotterdam, is a genuinely iconic project in the Netherlands. The Urban Water Buffer project has also produced several spin-offs, such as this year at Ahoy, another stadium in Rotterdam.And of course, we must not forget the COASTAR knowledge programme. The ongoing brackish water pilotat Dunea has emerged from this, as has the Westland Water Bank.”

Breaking the deadlock

At the same time, Raat identifies 2018 as the year that ushered in the present phase in groundwater infiltration and recharge. “We are now in the fourth wave: a renewed interest in infiltration and recharge, particularly in the high-lying areas with sandy soils. We knew climate change would bring longer dry periods. But when this actually happened in 2018, it was something of a shock. The water authorities also started thinking about this, for example in Brabant. They came up with the question of whether they could disconnect rainwater drains in residential areas or business parks to divert water into the ground. Or surface water, or even treated effluent. How does that work in technical terms? And above all: what about the water quality? At the same time, the new Environment Act, which went into effect on 1 January 2024, resulted in a lot of uncertainty. The act provides a lot of discretion for recharging groundwater locally. This would seem to be useful but it actually creates a deadlock. People behind initiatives no longer dared to move ahead and permit authorities also got cold feet. The Soil Protection Infiltration Decree of the 1990s was not a good basis either because it had been designed at the time for large dune water infiltrations for drinking water supplies and not for all types of water or small systems. At KWR, we saw this problem coming and I am very pleased that STOWA has taken it on. With the Responsible Infiltration and Recharge report that was published recently, I hope that the deadlock will be broken and that the competent authorities will have the clarity they want. If that happens, they can go into action.”

Water system as the basis

The report just mentioned provides an in-depth overview of infiltration and recharge measures in the Netherlands with the focus on a range of water systems and objectives. For example, it includes a detailed table of measures and explains the legal framework. A framework that can be applied flexibly provides concrete guidelines for the safe and effective implementation of infiltration and recharge measures with a specific focus on groundwater quality. “Instead of taking the substances found in water as the basis for responsible infiltration and recharge, we decided to look at the water system as a whole,” says hydrologist Gijsbert Cirkel, who works at KWR and was involved in the drafting of the STOWA report. “That simplifies the issues because you don’t have to look at all the substances separately and you can work independently of new substances that emerge. If, as an organisation behind the initiative or a permit authority, you look at the vulnerability of the water system and the reversibility of your intervention from the outset, you are doing a good job. For example, with a creek ridge in Zeeland enclosed by clay layers and draining surface water, you are looking at a very different situation than if you have a relatively non-confined groundwater system in the Veluwe. The water you introduce can go in every direction there. Reversing a situation like that is by no means straightforward.”

Street water

Following up on the cases in the ‘Responsible Infiltration and Recharge’ study, RIONED – the umbrella organisation for urban water management – is picking up the thread by taking a deeper look at the urban environment, says Cirkel. The researcher is happy to see this happening because he believes that rainwater run-off is still quite a challenge in that situation. “You have little control over the substances that rainwater brings in to the urban environment. Examples include worn tyre rubber, lubricating oil or other substances found on the street or that leach out of building materials. According to Dutch legislation and regulations, rainwater is clean. In this perspective, directing rainwater through the drainage system to a wastewater treatment plant is considered undesirable because of the risk of overflows and a reduction in the effect of the treatment plant during peak rainfall. The preferred order is therefore not to direct rainwater into the drainage system but to infiltrate it into the soil or divert it into the surface water. Obviously, this is not done where there is a lot of transport or where there is a high risk of spillages and leaks. But it is not entirely clear where the lines should be drawn. A literature review we conducted for the TKI Street Water Filtration for Infiltration project showed that rainwater can contain significant amounts of organic micropollutants. In the same project, we are working with municipal authorities, Vitens and a range of commercial companies to develop simple treatment steps before rainwater is infiltrated or enters the surface water. For example, we are currently conducting experiments in KWR’s test hall using rock wool to which we have ‘glued’ activated carbon and iron hydroxides. The challenge is to find material through which a lot of water can flow in a relatively short time, while at the same time bonding undesirable compounds. The solution must also be practical in terms of installation in public areas, and easy to maintain. I look forward to reading the test results. With the materials that do well, we will also conduct pilot projects in cities such as Apeldoorn, Nijmegen and Amsterdam.”

Story Map for deep infiltration

The fact that the waves described by KWR researcher Klaasjan Raat can also look back to previous developments is illustrated by his colleague Gilian Schout. “The deep infiltration systems that never took off on a large scale in the 1980s are the focus of renewed interest,” he says. “We are seeing an increase in drinking water demand after a long period of stability. Drinking water utilities are therefore looking for new sources and deep infiltration is one such possibility.” The drinking water utilities needed to brush up the lessons learned with deep infiltration late last century to see where the current challenges and opportunities are located. This resulted in a ‘Story Map’that was drawn up as part of Waterwijs, the collective research programme of the water sector. Schout: “We interviewed employees of drinking water utilities who were present during the initial developments in deep infiltration. We have published the results in an accessible way so that they can be consulted by a wide range of people. An interactive map of the Netherlands shows all the locations where there has ever been a trial with deep infiltration. There are about 25 of them. If you move your mouse over a location, a pop-up appears with in-depth information.”

On the shoulders of your predecessors

The deep-infiltration technologies from the 1980s can still be used, continues Schout. They involve underground water storage, aka Aquifer Storage and Recovery (ASR). Or Aquifer Storage Transfer and Recovery (ASTR). Schout: “With ASR, you infiltrate water into a well at a certain depth and then take it out later. So this is a temporary water buffer. In ASTR, you infiltrate water in a specific location and then take it out in another. Here, you use underground transport for treatment purposes. In the past, deep infiltration was a way to reduce the effects of existing extraction and therefore to relax permit restrictions for drinking water utilities. In the story told by the Story Map, we look at the challenges and opportunities afforded by this technology to meet water demand in the future. One of its uses is as a buffer for emergencies, or to cope with peak demand. Deep infiltration can also be used locally as a logistical solution, or as an alternative to enlarging a water main or expanding a treatment plant. I have had all these reports from 45 years ago in my hands. And that gets me to thinking: people were working on the same issues back then. They had the same questions. It feels like standing on the shoulders of your predecessors.”

Future research

As far as Schout is concerned, we need to build on, and further optimise, existing knowledge about deep infiltration. “With the drinking water utilities, one of our aims is to look at the optimal design of a well field. How do you get as much of the available water into the soil as possible, and can you extract it with the right quality? How can you tackle clogging in the well? And how do you determine how useful infiltration is to tackle the effects of water shortages? My main hope is that the Story Map will lead drinking water utilities to see deep infiltration as a serious option. And that they will find out how much knowledge is already available. So they don’t have to reinvent the wheel, and this Story Map gives a nice initial overview of everything that has been done before in this area.”

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