Liver stiffness values are lower in pediatric subjects than in adults and increase with age:A multifrequency MR elastography study

Purpose: To determine if healthy hepatic mechanical properties differ between pediatric and adult subjects at magnetic resonance (MR) elastography. Materials and Methods: Liver shear moduli in 24 healthy pediatric participants (13 children aged 5-14 years [seven boys, six girls] and 11 adolescents aged 15-18 years [six boys, five girls]) and 10 healthy adults (aged 22-36 years [five men, five women]) were obtained with 3-T MR elastography at 28, 56, and 84 Hz. Relationships between shear moduli and age were assessed with Spearman correlations. Differences between age groups were determined with one-way analysis of variance and Tukey multiple comparisons tests. Results: Liver stiffness values (means ± standard deviations) were significantly lower in children and adolescents than in adults at 56 Hz (children, 2.2 kPa ± 0.3; adolescents, 2.2 kPa ± 0.2; adults, 2.6 kPa ± 0.3; analysis of variance, P = .009) and 84 Hz (children, 5.6 kPa ± 0.8; adolescents, 6.5 kPa ± 1.2; adults, 7.8 kPa ± 1.2; analysis of variance, P = .0003) but not at 28 Hz (children, 1.2 kPa ± 0.2; adolescents, 1.3 kPa ± 0.3; adults, 1.2 kPa ± 0.2; analysis of variance, P = .40). At 56 and 84 Hz, liver stiffness increased with age (Spearman correlation, r = 0.38 [P = .03] and r = 0.54 [P = .001], respectively). Stiffness varied less with frequency in children and adolescents than in adults (analysis of variance, P = .0009). No significant differences were found in shear moduli at 28, 56, or 84 Hz or frequency dependence between children and adolescents (P = .38, P = .99, P = .14, and P = .30, respectively, according to Tukey tests). Conclusion: Liver stiffness values are lower and vary less with frequency in children and adolescents than in adults. Stiffness increases with age during normal development and approaches adult values during adolescence. Comparing pediatric liver stiffness to adult baseline values to detect pediatric liver mechanical abnormalities may not allow detection of mild disease and may lead to underestimation of severity

Accumulation of heavy metals in stormwater bioretention media:A field study of temporal and spatial variation

Short term studies have found that stormwater biofiltration systems, also known as bioretention systems or raingardens, are very effective in reducing heavy metal concentrations. However, their long-term treatment performance, as well as the spatial and temporal accumulation of metals within these systems remain uncertain. This paper reports on a large scale field study that assessed the changes over time of cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) levels in biofilters varying in age, design, and catchment characteristics. The survey incorporated 29 biofilters in 2006/7 and 49 biofilters, in 2014/15, located in three major Australian cities (Melbourne, Brisbane, and Sydney). Noting that these results are influenced by having just one industrial site with 25 filters measured at that site Catchment characteristics were significantly correlated with metal accumulation rates. Biofilters in catchments with current or past industrial activities had elevated heavy metal concentrations in the filter media. Zinc concentrations in the surface 0–100 mm exceed both soil quality and ecological guidelines. In contrast, heavy metal concentrations in residential catchments are unlikely to (ever) reach levels that exceed soil quality guidelines for human health, although zinc concentrations approach ecological guideline criteria.</p

Processes and drivers of nitrogen removal in stormwater biofiltration

Biofiltration systems harness the treatment capabilities of plants, microorganisms, and soil to mitigate impacts of polluted stormwater. However, their effectiveness for nitrogen removal can vary, from concentration reductions exceeding 70% to net leaching. Performance is particularly sensitive to plant species selection, presence of a saturated zone with carbon source, and the frequency of inflows. The authors review controls on nitrogen cycling in natural and modified environments to identify important processes and influences within biofilters. Key factors include plant-microbial interactions, root architecture, plant strategy, and moisture heterogeneity. They note a critical lack of studies comparing nitrogen removal through denitrification and plant assimilation.</p

Biofilter design for effective nitrogen removal from stormwater - Influence of plant species, inflow hydrology and use of a saturated zone

The use of biofilters to remove nitrogen and other pollutants from urban stormwater runoff has demonstrated varied success across laboratory and field studies. Design variables including plant species and use of a saturated zone have large impacts upon performance. A laboratory column study of 22 plant species and designs with varied outlet configuration was conducted across a 1.5-year period to further investigate the mechanisms and influences driving biofilter nitrogen processing. This paper presents outflow concentrations of total nitrogen from two sampling events across both 'wet' and 'dry' frequency dosing, and from sampling across two points in the outflow hydrograph. All plant species were effective under conditions of frequent dosing, but extended drying increased variation between species and highlighted the importance of a saturated zone in maintaining biofilter function. The saturated zone also effectively treated the volume of stormwater stored between inflow events, but this extended detention provided no additional benefit alongside the rapid processing of the highest performing species. Hence, the saturated zone reduced performance differences between plant species, and potentially acts as an 'insurance policy' against poor sub-optimal plant selection. The study shows the importance of biodiversity and inclusion of a saturated zone in protecting against climate variability.</p

Which species? A decision-support tool to guide plant selection in stormwater biofilters

Plant species are diverse in form, function and environmental response. This provides enormous potential for designing nature-based stormwater treatment technologies, such as biofiltration systems. However, species can vary dramatically in their pollutant-removal performance, particularly for nitrogen removal. Currently, there is a lack of information on how to efficiently select from the vast palette of species. This study aimed to identify plant traits beneficial to performance and create a decision-support tool to screen species for further testing. A laboratory experiment using 220 biofilter columns paired plant morphological characteristics with nitrogen removal and water loss for 20 Australian native species and two lawn grasses. Testing was undertaken during wet and dry conditions, for two biofilter designs (saturated zone and free-draining). An extensive root system and high total biomass were critical to the effective removal of total nitrogen (TN) and nitrate (NO3 −), driven by high nitrogen assimilation. The same characteristics were key to performance under dry conditions, and were associated with high water use for Australian native plants; linking assimilation and transpiration. The decision-support tool uses these scientific relationships and readily-available information to identify the morphology, natural distribution and stress tolerances likely to be good predictors of plant nitrogen and water uptake.</p

Temporary storage or permanent removal? The division of nitrogen between biotic assimilation and denitrification in stormwater biofiltration systems

The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a ‘black-box’ approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer (15)NO(3) (−) (nitrate) over the course of one inflow event. The immediate partitioning of (15)NO(3) (−) between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO(3) (−) under typical stormwater concentrations (∼1–2 mg N/L), contributing an average 89–99% of (15)NO(3) (−) processing in biofilter columns containing the most effective plant species, while only 0–3% was denitrified and 0–8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of (15)NO(3) (−), and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems

Inside Story of Gas Processes within Stormwater Biofilters: Does Greenhouse Gas Production Tarnish the Benefits of Nitrogen Removal?

Stormwater biofilters are dynamic environments, supporting diverse processes that act to capture and transform incoming pollutants. However, beneficial water treatment processes can be accompanied by undesirable greenhouse gas production. This study investigated the potential for nitrous oxide (N₂O) and methane (CH₄) generation in dissolved form at the base of laboratory-scale stormwater biofilter columns. The influence of plant presence, species, inflow frequency, and inclusion of a saturated zone and carbon source were studied. Free-draining biofilters remained aerobic with negligible greenhouse gas production during storm events. Designs with a saturated zone were oxygenated at their base by incoming stormwater before anaerobic conditions rapidly re-established, although extended dry periods allowed the reintroduction of oxygen by evapotranspiration. Production of CH₄ and N₂O in the saturated zone varied significantly in response to plant presence, species, and wetting and drying. Concentrations of N₂O typically peaked rapidly following stormwater inundation, associated with limited plant root systems and poorer nitrogen removal from biofilter effluent. Production of CH₄ also commenced quickly but continued throughout the anaerobic interevent period and lacked clear relationships with plant characteristics or nitrogen removal performance. Dissolved greenhouse gas concentrations were highly variable, but peak concentrations of N₂O accounted for <1.5% of the incoming total nitrogen load. While further work is required to measure surface emissions, the potential for substantial release of N₂O or CH₄ in biofilter effluent appears relatively low