Assessment of Existing Roadside Swales with Engineered Filter Soil: II. Treatment Efficiency and in situ Mobilization in Soil Columns

Abstract

Use of roadside infiltration systems using engineered filter soil for optimized treatment has been common practice in Germany for decades, but little documentation is available regarding their long-term treatment performance. Here we present the results of laboratory leaching experiments with intact soil columns (15 cm i.d., 25-30 cm length) collected from two German roadside infiltration swales constructed in 1997. The columns were irrigated with synthetic solutions of unpolluted or polluted (dissolved heavy metals and fine suspended solids) road runoff, as well as a soluble nonreactive tracer (bromide) and a dye (brilliant blue). The experiments were performed at two irrigation rates corresponding to catchment rainfall intensities of approximately 5.1 and 34 mm/h. The bromide curves indicated that preferential flow was more pronounced at high irrigation rates, which was supported by the flow patterns revealed in the dye tracing experiment. Nonetheless, the soils seemed to be capable of retaining most of the dissolved heavy metals from the polluted road runoff at both low and high irrigation rates, except for Cr, which appears to pass through the soil as chromate. Fluorescent microspheres (diameter = 5 μm) used as surrogates for fine suspended solids were efficiently retained by the soils (>99%). However, despite promising treatment abilities, internal mobilization of heavy metals and P from the soil was observed, resulting in potentially critical effluent concentrations of Cu, Zn, and Pb. This is mainly ascribed to high concentrations of in situ mobilized dissolved organic carbon (DOC). Suggestions are provided for possible improvements and further research to minimize DOC mobilization in engineered filter soils.

Controlling Stormwater Quality with Filter Soil—Event and Dry Weather Testing

“…It seems like the filter soil is removing particulate P while at the same time leaching dissolved P. As for SS and OC, this points to mobilization of the internal P-pool in the filter soil, as was also shown in the studies by Ingvertsen et al [16]. In parallel with this, swale-trenches leached more particle-bound P than the low SS-leaching curb extensions, where P was mostly in dissolved form, probably reflecting the higher plant-available P in the landfill soil.…”

“…When comparing the soil from swale-trenches and curb extensions it is observed that the local soil in the swale-trenches was leaching more OC than the landfill soil in curb-extensions, similar to what was observed for SS. Leaching of organic matter was also observed by [16] from German filter soil, 15 years old, but even young systems like the ones monitored here leach organic matter. Organic matter is not a pollutant as such, but can be a source of P and complex metals such as Cu and cause leaching of these e.g., [39,40].…”

“…However, when adding dissolved Cu in a concentration of approximately 50 µg/L, the effluent concentrations of dissolved Cu was significantly reduced to below the detection limit and to 2.7 µg/L, but with a total Cu concentration of 8 and 8.1 µg/L, indicating that the Cu found in the effluent stems from the soil. A linear correlation of R 2 of 0.5 (Pearson's correlation test p-value of 0.04) for swale-trench West, and 0.9 (Pearson's correlation test p-value < 0.001) for East between total Cu and DOC points to complexation of Cu by the mobile DOC as the mechanism, as also observed by [16,40]. This trend was, however, not observed at Moellebakken and Lindevang where the correlation between TOC and dissolved Cu was 0.2 and 0.4 respectively (Pearson's correlation test with both p-values of 0.27 and 0.3 respectively).…”

“…Studies performed by [16] showed that even small particles are well retained in filter soil, and the main part of the particles present in the percolate at both locations are probably particles that have been mobilized in the filter soil, rather than the particles entering with the stormwater runoff. This mechanism of internal mobilization of particles is confirmed in the present study when the addition of particle-free synthetic influent resulted in effluent concentrations of SS in the same range as observed during rain events.…”

“…The OC found in percolates is likely to stem from mobilization of the internal organic pool in the soil [16], while incoming OC is retained. Unfortunately OC was not measured in effluent from the synthetic pulse addition, which could have given an idea of the mechanisms.…”