Quality criteria for rainbed substrates

Rainbeds are an increasingly common feature of the urban environment, managing stormwater while greening the cityscape and reducing impervious surfaces. The function of rainbeds can be both to retain and purify stormwater before the water is further infiltrated or led to a recipient. However, rainbeds can also leach nutrients, as shown by previous projects that have looked at the quality of outgoing water from rainbeds. Today, there are no quality criteria to ensure the functionality of rainbeds and rainbed substrates. In an interview study, and in a questionnaire study, different professional groups (designers, clients, stormwater experts and researchers, etc.) who work with rainbeds were asked how they see the need for quality criteria, which criteria are most important, how guideline values and reduction factors can be developed and desirable channels for communicating quality criteria. It has been clear that guidance is in demand by the industry and that it is desirable to have some form of industry standard that can be published in AMA or as a publication by Svenskt Vatten, or similar. The criteria that were considered most important varied with different professional groups, but they agreed that different requirements should be set for different purposes of the rain garden. In an international analysis, international representatives were also interviewed about how possible quality criteria are applied in each country. Four Swedish substrate producers also contributed a total of 10 different samples of their rainbed substrates. These were sent for analysis together with a substrate developed according to recommendations from research on rainbed substrates, which was used as a reference. One purpose of the analyses was to characterize the rainbed substrates. Another aim was to see how the different methods correlate - whether more complicated and timeconsuming analyses can be replaced by simpler ones. The analytical methods to be performed were decided after discussions with routine labs about suitable and available methods. The methods chosen were two different nutrient analyses: AL analysis and Spurway analysis, and two different leaching tests, namely shake tests and column tests. The shake tests were done at L/S 2 and L/S 10 and repeated twice with the leachate. The column tests were performed on three substrates at L/S 0.1; 0.2; 0.5; 1, 2, 5 and 10. The focus of the evaluation was on nutrient content and leaching. The nutrient content was evaluated based on guideline values for nutrient content according to AMA and Hässelby-Skälby garden laboratory. The leaching was evaluated based on guideline values for the city of Gothenburg and guideline values according to the guideline group, level 3VU. The elements that exceeded the guideline values were mainly cadmium, copper, phosphorus and nitrogen. The phosphorus leaching exceeded the guideline values in all substrates at L/S 2 and only the reference substrate was below the guideline values at L/S 10. In the repeated leaching, the phosphorus content gradually decreased but did not fall below the detection limit in any of the substrates except in the reference substrate at the last leaching. It should be emphasized that although the leached content was above the guideline limits, the leached amounts were in the order of mg/kg DM, which corresponds to g/ton DM substrate. One conclusion is that the substrates available on the market today have been developed mostly to contribute to the retention of stormwater and to get the vegetation to grow. The substrates have a completely different character than what is recommended by research. Another conclusion is that it has not yet been established which analysis method is best suited to form the basis of quality criteria.