Simulation of watershed-scale practices for mitigating stream thermal pollution due to urbanization.

Thermally enriched runoff from urban impervious surfaces can be harmful to aquatic life; however, only limited information is available on how to mitigate these impacts at the watershed-scale. This study evaluates the effects of retrofitting an urban watershed with thermal mitigation practices (TMPs) relative to thermal toxicity thresholds for aquatic species. The Minnesota Urban Heat Export Tool (MINUHET) and Storm Water Management Model (SWMM) models were used to evaluate TMPs that help reduce temperature and total heat loads (THL) from the Stroubles Creek watershed in Blacksburg, Virginia. We used the aquatic health criteria for brook trout (Salvelinus fontinalis), the most sensitive species present downstream of the watershed, as a performance measure. TMPs included bioretention systems, methods for reducing the albedo of surfaces (cool surfaces), and increasing forest canopy. Performance metrics included Event Mean Temperature (EMT), and the Percentage of Time Temperature Exceeded the 21 °C Acute Toxicity Threshold [Percentage of Time above the Threshold (PTAT)] for brook trout; these metrics were used to quantify reductions in heat loads and temperatures. TMPs were evaluated during continuous simulation and selected storm events. Increased forest canopy alone produced the greatest reduction of stream temperature, as quantified by EMT and PTAT metrics during continuous and event-based simulations. In contrast, cool surfaces reduced THL more than any other individual TMP for the continuous simulation. A comprehensive mitigation plan (CMP) integrating all three TMPs reduced THL by 62.3%, and PTAT by approximately 12%, for the entire summer of 2015. The CMP was also applied to select storm events, during which streamflow EMT was reduced up to 9%, and PTAT was reduced nearly to zero. This study, which is the first to simulate watershed-scale TMPs for a large, complex urban area, demonstrates the application of appropriate strategies for restoring aquatic habitats in the thermally impacted Stroubles Creek.

Thermal evaluation of urbanization using a hybrid approach.

Urban development increases runoff temperatures from buildings and pavement, which can be harmful to aquatic life. However, our ability to predict runoff temperature as a function of land use is limited. This paper explores available tools for simulating runoff temperature with respect to brook trout (Salvelinus sp.), a sensitive species. The Minnesota Urban Heat Export Tool (MINUHET) and the Storm Water Management Model (SWMM) were applied to a 14.1 km2 portion of the Stroubles Creek watershed near Blacksburg, Virginia for two summers. Streamflow, water temperature, and weather data were acquired from the Virginia Tech StREAM Lab (Stream Research, Education, and Management) monitoring stations. SWMM and MINUHET were calibrated and validated for streamflow, and stream temperature, respectively. The models were sensitive to imperviousness (SWMM-predicted streamflow) and dew point temperature (MINUHET-predicted water temperature). While the models output time-step was 15 min, the model performance in simulating streamflow was evaluated using Nash-Sutcliffe Efficiency (NSE) on hourly time-steps. NSE values were 0.67 and 0.65 for SWMM and 0.62 and 0.57 for MINUHET during the calibration and validation periods, respectively, indicating that SWMM performed better than MINUHET in streamflow simulation. Stream temperatures were simulated using MINUHET with NSE value of 0.58 for the validation period, demonstrating a satisfactory simulation of water temperature. Since SWMM is not capable of stream temperature simulation beyond simple mixing. Hydrologic and thermal outputs from SWMM and MINUHET were combined in a hybrid approach that emphasized the strength of each respective model, i.e. SWMM for runoff and streamflow and MINUHET for water temperature. Heat loads were simulated using the MINUHET and the Hybrid models; the Hybrid model (0.56) had a greater NSE than MINUHET (0.45) alone. MINUHET predictions indicated water temperatures would exceed the trout toxicity threshold of 21 °C during 39% and 38% of calibration and validation periods, respectively. Since the observed temperature exceeded the toxicity threshold 59% and 53% of the time for the calibration and validation periods, respectively, MINUHET was not a conservative predictor of the duration of temperatures exceeding the toxicity threshold value.

Engineered and Environmental Controls of Microbial Denitrification in Established Bioretention Cells.

Bioretention cells (BRCs) are effective tools for treating urban stormwater, but nitrogen removal by these systems is highly variable. Improvements in nitrogen removal are hampered by a lack of data directly quantifying the abundance or activity of denitrifying microorganisms in BRCs and how they are controlled by original BRC design characteristics. We analyzed denitrifiers in twenty-three BRCs of different designs across three mid-Atlantic states (MD, VA, and NC) by quantifying two bacterial denitrification genes ( nirK and nosZ) and potential enzymatic denitrification rates within the soil medium. Overall, we found that BRC design factors, rather than local environmental variables, had the greatest effects on variation in denitrifier abundance and activity. Specifically, denitrifying populations and denitrification potential increased with organic carbon and inorganic nitrogen concentrations in the soil media and decreased in BRCs planted with grass compared to other types of vegetation. Furthermore, the top layers of BRCs consistently contained greater concentrations and activity of denitrifying bacteria than bottom layers, despite longer periods of saturation and the presence of permanently saturated zones designed to promote denitrification at lower depths. These findings suggest that there is still considerable potential for design improvements when constructing BRCs that could increase denitrification and mitigate nitrogen export to receiving waters.

Passive Biological Treatment of Mine Water to Reduce Conductivity: Potential Designs, Challenges, and Research Needs.

The remediation of mine water to preserve receiving water quality has advanced substantially over the past half century, but prospective regulations to limit the conductivity of mining-impacted waters pose a significant new challenge. Conventional approaches to reduce high levels of conductivity in these mine waters are often costly, requiring high levels of maintenance and significant inputs of energy and refined chemicals. In contrast, passive biological treatment (PBT) systems are a relatively low-cost, low-maintenance treatment technology for mine waters that have been used for over three decades. However, their practical ability to reduce conductivity is unclear, given previous research reports focused on the removal of metals, acidity, and solids. A systematic literature review to identify previous reports of PBT systems at the laboratory or field scale that include evaluations of changes in conductivity suggests that decreases in conductivity of 30 to 40% are achievable. Substantial variability in performance is common, however, and conductivity increased markedly in some systems. This variation may be associated with the dissolution of limestone, which is a key treatment material in some systems. Although the development of PBT to serve as pre-, post-, or stand-alone treatment systems targeting conductivity may reduce overall treatment cost in some settings, optimization of these designs requires an increase in the number of published conductivity datasets from similar systems, detailed reports on the key ions contributing to elevated conductivity region to region, and further investigation of the underlying biochemical processes responsible for conductivity reductions.

Plant biodiversity effects in reducing fluvial erosion are limited to low species richness.

It has been proposed that plant biodiversity may increase the erosion resistance of soils, yet direct evidence for any such relationship is lacking. We conducted a mesocosm experiment with eight species of riparian herbaceous plants, and found evidence that plant biodiversity significantly reduced fluvial erosion rates, with the eight-species polyculture decreasing erosion by 23% relative to monocultures. Species richness effects were largest at low levels of species richness, with little increase between four and eight species. Our results suggest that plant biodiversity reduced erosion rates indirectly through positive effects on root length and number of root tips, and that interactions between legumes and non-legumes were particularly important in producing biodiversity effects. Presumably, legumes increased root production of non-legumes by increasing soil nitrogen availability due to their ability to fix atmospheric nitrogen. Our data suggest that a restoration project using species from different functional groups might provide the best insurance to maintain long-term erosion resistance.

Relationship between woody plant colonization and Typha L. encroachment in stormwater detention basins.

We studied stormwater detention basins where woody vegetation removal was suspended for 2 years in Virginia, USA to determine if woody vegetation can control Typha populations and how early woody plant succession interacts with Typha, other herbaceous vegetation, and site factors. Distribution and composition of woody vegetation, Typha and non-Typha herbaceous vegetation biomass, and site factors were assessed at 100 plots in four basins ranging in age from 7 to 17 years. A greenhouse study examined the interaction of shade and soil moisture on Typha biomass and persistence. Principal component analysis identified an environmental gradient associated with greater water table depths and decreased elevation that favored Typha but negatively influenced woody vegetation. Elevation was correlated with litter layer distribution, suggesting that initial topography influences subsequent environmental characteristics and thus plant communities. Soil organic matter at 0-10 cm ranged from 5.4 to 12.7%. Woody plants present were native species with the exception of Ailanthus altissima and Pyrus calleryana. In the greenhouse, shade and reduced soil moisture decreased Typha biomass and rhizome length. The shade effect was strongest in flooded plants and the soil moisture effect was strongest for plants in full sun. Typha in dry soil and heavy shade had 95% less total biomass and 83% smaller rhizomes than Typha in flooded soil and full sun, but even moderate soil moisture reductions decreased above- and below-ground biomass by 63 and 56%, respectively. Suspending maintenance allows restoration of woody vegetation dominated by native species and may suppress Typha invasion.