Cost/benefit assessment of green infrastructure: Spatial scale effects on uncertainty and sensitivity.

Green infrastructure (GI) is becoming a common solution to mitigate stormwater-related problems. Given the uncertain costs of GI relative to other stormwater management strategies, stakeholders investing in GI need performance-analysis tools that consider the full suite of benefits and the impacts of uncertainty to help justify GI expenditures. This study provides a quantitative and comparative analysis of GI benefits, including nutrient uptake from stormwater and air pollutant deposition. Economic costs and benefits of GI are assessed using two metrics, benefit-cost ratios (BCRs) and nutrient removal costs, at three scales: household, subwatershed, and watershed scale. Results from a case study in the state of Maryland show that the costs of nutrient uptake at the subwatershed scale can be lower than those at either the watershed or household scales. Moreover, rain gardens are far more efficient in stormwater treatment at the household scale in comparison to watershed scale, for which large-scale dry or wet basins are more efficient. Using a BCR metric, smaller subwatersheds show more promise, while using a nutrient removal cost metric indicates that upstream subwatersheds are more suitable for stormwater treatment. The results also show that implementation of GI at all potential pervious locations does not necessarily increase nutrient removal costs and that self-installation of rain gardens greatly reduces nutrient removal costs. Finally, the results show that using numerous small-sized rain garden practices in front of residential buildings yields lower nutrient removal costs in comparison to permeable pavements placed in parking lots and commercial buildings.

Real-time emissions from construction equipment compared with model predictions.

The construction industry is a large source of greenhouse gases and other air pollutants. Measuring and monitoring real-time emissions will provide practitioners with information to assess environmental impacts and improve the sustainability of construction. We employed a portable emission measurement system (PEMS) for real-time measurement of carbon dioxide (CO), nitrogen oxides (NOx), hydrocarbon, and carbon monoxide (CO) emissions from construction equipment to derive emission rates (mass of pollutant emitted per unit time) and emission factors (mass of pollutant emitted per unit volume of fuel consumed) under real-world operating conditions. Measurements were compared with emissions predicted by methodologies used in three models: NONROAD2008, OFFROAD2011, and a modal statistical model. Measured emission rates agreed with model predictions for some pieces of equipment but were up to 100 times lower for others. Much of the difference was driven by lower fuel consumption rates than predicted. Emission factors during idling and hauling were significantly different from each other and from those of other moving activities, such as digging and dumping. It appears that operating conditions introduce considerable variability in emission factors. Results of this research will aid researchers and practitioners in improving current emission estimation techniques, frameworks, and databases.