Soil nitrogen accumulation, denitrification potential, and carbon source tracing in bioretention basins.

Bioretention basins are one of the most commonly used green stormwater features, with the potential to accumulate significant levels of nitrogen (N) in their soil and to permanently remove it through denitrification. Many studies have investigated the N removal potential of bioretention basins through the assessment of inflow and outflow concentrations. However, their long-term N removal through soil accumulation and denitrification potential is less known. This study investigated the temporal variation of total N (TN) accumulation and denitrification potential in soils of 25 bioretention basins within a 13-year soil chronosequence, in a subtropical climate in Australia. The denitrification potential of a subset of seven bioretention basins was investigated in accompaniment with nutrient and soil characteristics. Additionally, stable isotopes (δ13C and δ15N) were used to assess temporal changes in the soil composition as well as to identify the sources of carbon (C) into these basins. Over 13 years of operation, TN accumulated faster in the top 5 cm of soil than deeper soils. Soil TN density was highest in the top 5 cm with an average of 1.4 kg N m-3, which was about two times higher than deeper soils. Site age and soil texture were the best predictors of soil TN density and denitrification (1 to 9.7 mg N m-2 h-1). The isotope values were variable among basins. Low δ15N values in young basins (-1.02‰) suggested fixation as the main source of N, while older basins had higher δ15N, indicating higher denitrification. Bioretention plants were the primary source of soil C; although the occurrence of soil amendment also contributed to the C pool. To improve the performance of these bioretention basins, we recommend increasing vegetation at initial years after construction, and enhancing more frequent anaerobic conditions in the high soil profile. These two conditions can improve denitrification potential, and thus the performance of these basins for improving water quality.

A process-based method of estimating exponential decay parameters for particulate removal in a gravel trench.

Observed reductions in pollutant concentrations through stormwater treatment devices commonly display the characteristic form of exponential decay, in which the rate of decrease of pollutant concentration with distance is proportional to the concentration. The observation of an apparently irreducible or background pollutant concentration, C*, in many devices has led to development of the two-parameter "k-C*" model. It is known that this model is too simplistic because the parameters k and C* are not constant but can vary greatly with pollutant concentration and hydraulic conditions. This paper presents an alternative exponential decay model for filtration of particulate pollutants, which is based on simple mathematical descriptions of key removal processes. The model delivers a process-based method for estimating the exponential decay constant. Moreover, the need to specify a background concentration is eliminated. To test the theory, the model is applied to the removal of clay and silica particles from horizontal flow through an experimental gravel trench. Particle concentrations were measured at nine locations along a 7.2 m long flume. The model agrees very well with the observed change in suspended solids concentration for the two pollutant materials and the range of flow rates tested. A single model parameter, notionally representing the "stickiness" of pollutant particles, is required for different pollutant materials.

Determination of the structures of trisaccharides by 13C-n.m.r. spectroscopy.

A literature survey of the 13C-n.m.r. chemical-shift data for aqueous solutions of monosaccharides, disaccharides, oligosaccharides, and their methyl derivatives is reported. Analysis of these data reveals a set of empirical rules which may be used in the elucidation of the structure of trisaccharides of known monosaccharide composition, and an example is reported. However, it is not possible to extend the rules to tetrasaccharides and higher saccharides without additional chemical-shift data for related model compounds.