Intermittent rainstorms cause pulses of nitrogen, phosphorus, and copper in leachate from compost in bioretention systems

Abstract

Bioretention systems rely on vegetation and mixtures of soil, sand, and compost to filter stormwater runoff. However, bioretention systems can also leach metals and nutrients, and compost may be a major contributor to this leaching. To safely implement bioretention systems, it is crucial to determine the composition of compost leachate. We characterized and quantified the leachate composition of compost following intermittent, simulated storm events. Columns of municipal compost were irrigated to simulate 6-month, 24-hour rain storms in the Seattle-Tacoma region. Outflow was analyzed for pH, electrical conductivity (EC), particulate concentration, surface tension, dissolved organic carbon (DOC), nitrogen, phosphorus, and copper. Results indicate a decrease of chemical concentrations over the course of individual storms and following repeated storms, but each new storm released another peak of constituents. The decrease in phosphorus, copper, and DOC concentrations with repeated storms was slower than for nitrate and EC. Nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) showed that the DOC consisted mainly of aliphatic and aromatic components typical of fulvic and humic acids. Less than 3% of the original copper content from the compost leached out even after nine storm events. Nonetheless, copper concentrations in the leachate exceeded regulatory discharge standards. Our results show that compost can serve as a sustained source of leaching of nutrients and metals.

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

“…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).…”

“…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]. When designing filter soil it is therefore relevant to look at the opportunities to improve the composition of the filter soil in order to retain organic matter.…”

Interpolation and 3D visualization of soil moisture

“…In the current paper, we described homogeneous flow; whereas, in field cases preferential flow may also contribute to subsurface moisture conditions and solute pulses during individual rainstorms as indicated by former studies (e.g. Hendrickx -Flury 2001;Mullane et al 2015).…”

Experimental Study of Nitrogen Removal Efficiency of Layered Bioretention Under Intermittent or Continuous Operation

“…The filler surface had faster adsorption points in initial rainfall, while the filler surface fast adsorption area was occupied gradually over time, such that water concentration gradually increased. Columns of municipal compost were irrigated to simulate 6-month, 24-hour rain storms in the Seattle-Tacoma region, and the results showed that compost could serve as a sustained source of leaching of nutrients and metals [23]. Lucke and Nichols [24] also found the negative TN removal for the performance of street-side bioretention basins after 10 years in operation.…”