Soil Pollution in Wadis Investigated with New Method

Lead pollution is a major issue in climate adaptation in the Netherlands, where public spaces are being redesigned to better manage heavy rainfall. Wadis, or water drainage and infiltration zones, are replacing traditional infrastructure to store and infiltrate rainwater. However, the runoff infiltrated into these wadis contains heavy metals, including lead.

Climate adaptation is high on the agenda in the Netherlands and requires a different layout of the public space. Large rain showers do not fit in sewer pipes, so hardening is replaced by green areas. Green areas are given functions such as water storage and infiltrate rainwater into the soil, such as at wadis (water drainage and infiltration). The run-off rainwater that infiltrates contains impurities such as PAH and heavy metals that are captured in the top layer of the wadi. However, the concentrations are so low that pollution can only be measured after years.

Because the oldest Dutch wadi in 2019 is 20 years old, an exploratory study has been conducted into soil quality of 30 wadis with a new research method. This study shows that the XRF (X-ray Fluoresence) is a cost-effective method of performing a soil quality scan in wadis and assessing the environmental functioning of wadis. Charging of the soil with heavy metals has been observed at various locations and measures are desirable in some cases. A workshop on these research results generated many reactions.

This article provides a brief overview of 20 years of Dutch literature on the quality and infiltration of run-off rainwater and the first results of exploratory research into the soil quality of wadis in the Netherlands with a new in situ sampling method.

Wadis

The first wadis were built twenty years ago to store, infiltrate and purify rainwater and to reduce negative effects such as sewer overflows. In 2019, more than 250 residential areas with wadis were mapped across the Netherlands by the open source platform climatescan.nl (Boogerd et al. 2017).

However, that data is not complete, there are probably more than 500 residential areas where rainwater is visibly drained to wadis. A single municipality even has more than 200 wadi compartments. Much research has been conducted over the years into the hydraulic functioning of wadis. Hydraulic functioning of most overgrown wadis is judged to be good by municipalities and water boards. The rainwater almost always infiltrates within 24 hours, even in places in the low Netherlands with high groundwater levels and poor permeable soil (Boogaard et al. 2000 and 2018).

Less is known about the long-term environmental functioning of the Dutch wadis. It is known from the rainwater database (STOWA, Boogaard and others 2007) that run-off rainwater contains impurities such as PAH and heavy metals. In 2019, more than 6000 measurements of the heavy metal copper, lead and zinc are known. The average concentrations in the run-off rainwater from residential areas (roof and road) are 5, 28 and 47 micrograms per liter, respectively.

These metals are recorded in the top layer of wadis (Boogaard et al. 2004). That is why 20 years ago (water board dependent) guidelines were published that indicate, among other things, which paved surfaces you could connect to infiltration facilities (Boogaard et al. 2003) and which soil mixture and which filter layer thickness this top layer should have (RIONED 2006). In addition, these guidelines contain practical recommendations for nature-friendly vegetation of wadis (STOWA 2003, 2007).

The degree and speed of pollution of the top layer of wadis depends on many factors (design, construction, use and management of wadi and surroundings) and location specific, making the environmental behavior of wadis difficult to predict. For this reason, guidelines for infiltration facilities recommend examining the soil quality of the top layer of wadis every 5 years. However, this is hardly ever done in practice, partly because of the costs and unfamiliarity with the purifying effect of the wadi system.

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Results

At approximately 1 in 5 locations, the intervention values for soil remediation for copper, lead or zinc have been exceeded (in particular zinc). High concentrations in the bottom of wadis are generally found in rainwater intakes, where a lot of sediment with associated contaminants accumulates and most water infiltrates. In most cases, the heavy metals come from a clear source or application.

In almost all cases charging takes place: higher concentrations have been found in the mudflats than in the bottom just above the gullet or just next to the wadi (reference soil quality). The reference soil quality is a place in or near the wadi where no run-off rainwater infiltrates, but where the soil is exposed to the same atmospheric deposition and other conditions.

Additional investigations will be conducted into the origin and depth of contamination at locations where intervention values have been exceeded. Preferably not only heavy metals and PAH will be included, but also other substances.

Read the article on H2O.