Theresa Wynn

Development of a constructed subsurface-flow wetland simulation model

Abstract This paper presents a mechanistic, compartmental simulation model of subsurface-flow constructed wetlands. The model consists of six submodels, including the nitrogen and carbon cycles, both autotrophic and heterotrophic bacteria growth and metabolism, and water and oxygen balances. Data from an existing constructed wetland in Maryland were used to calibrate the model. Model results reproduced seasonal trends well. Interactions between the carbon, nitrogen, and oxygen cycles were evident in model output. In general, effluent biochemical oxygen demand, organic nitrogen, ammonium and nitrate concentrations were predicted well. Because little is known about rootzone aeration by wetland plants, oxygen predictions were fair. The model is generally insensitive to changes in individual parameters. This is due to the complexity of the ecosystem and the model, as well as the numerous feedback mechanisms. The model is most sensitive to changes in parameters that affect microbial growth and substrate use directly. This dynamic, compartmental, simulation model is an effective tool for evaluating the performance of subsurface-flow constructed wetlands. The model provided insights into treatment problems at an existing constructed wetland. With further evaluation and refinement, the model will be a useful design tool for subsurface-flow constructed wetlands.

Can urban tree roots improve infiltration through compacted subsoils for stormwater management

Global land use patterns and increasing pressures on water resources demand creative urban stormwater management. Strategies encouraging infiltration can enhance groundwater recharge and water quality. Urban subsoils are often relatively impermeable, and the construction of many stormwater detention best management practices (D-BMPs) exacerbates this condition. Root paths can act as conduits for water, but this function has not been demonstrated for stormwater BMPs where standing water and dense subsoils create a unique environment. We examined whether tree roots can penetrate compacted subsoils and increase infiltration rates in the context of a novel infiltration BMP (I-BMP). Black oak (Quercus velutina Lam.) and red maple (Acer rubrum L.) trees, and an unplanted control, were installed in cylindrical planting sleeves surrounded by clay loam soil at two compaction levels (bulk density = 1.3 or 1.6 g cm(-3)) in irrigated containers. Roots of both species penetrated the more compacted soil, increasing infiltration rates by an average of 153%. Similarly, green ash (Fraxinus pennsylvanica Marsh.) trees were grown in CUSoil (Amereq Corp., New York) separated from compacted clay loam subsoil (1.6 g cm(-3)) by a geotextile. A drain hole at mid depth in the CUSoil layer mimicked the overflow drain in a stormwater I-BMP thus allowing water to pool above the subsoil. Roots penetrated the geotextile and subsoil and increased average infiltration rate 27-fold compared to unplanted controls. Although high water tables may limit tree rooting depth, some species may be effective tools for increasing water infiltration and enhancing groundwater recharge in this and other I-BMPs (e.g., raingardens and bioswales).

EFFECTS OF FOREST HARVESTING BEST MANAGEMENT PRACTICES ON SURFACE WATER QUALITY IN THE VIRGINIA COASTAL PLAIN

Three small watersheds located in Westmoreland County, Virginia, were monitored to evaluate the impact of forest clearcutting on surface water quality and to evaluate the effectiveness of forestry best management practices (BMPs) for minimizing hydrologic and water quality impacts associated with timber harvesting. One watershed (7.9 ha) was clearcut without implementation of BMPs, one watershed (8.5 ha) was clearcut with the implementation of BMPs and a third watershed (9.8 ha) was left undisturbed as a control. Forest clearcutting without BMP implementation reduced storm runoff volume and did not significantly change peak flow rates. Following site preparation, both storm flow volumes and peak flow rates decreased significantly. For the watershed with BMP implementation, storm flow volume decreased significantly following harvest, while peak flow increased. Site preparation did not change storm flow volumes over post-harvest conditions, but did significantly reduce storm peak flow rates. Disruptions in subsurface flow pathways during harvest or rapid growth of understory vegetation following harvest could have caused these hydrologic changes. Harvest and site preparation activities significantly increased the loss of sediment and nutrients during storm events. Storm event concentrations and loadings of sediment, nitrogen, and phosphorus increased significantly following forest clearcutting and site preparation of the No-BMP watershed. Both the BMP watershed and the Control watershed showed few changes in pollutant storm concentrations or loadings throughout the study. Results of this study indicate forest clearcutting and site preparation without BMPs can cause significant increases in sediment and nutrient concentrations and loadings in the Virginia Coastal Plain. However, these impacts can be greatly reduced by implementing a system of BMPs on the watershed during harvesting activities.

Transpiration and Root Development of Urban Trees in Structural Soil Stormwater Reservoirs

Stormwater management that relies on ecosystem processes, such as tree canopy interception and rhizosphere biology, can be difficult to achieve in built environments because urban land is costly and urban soil inhospitable to vegetation. Yet such systems offer a potentially valuable tool for achieving both sustainable urban forests and stormwater management. We evaluated tree water uptake and root distribution in a novel stormwater mitigation facility that integrates trees directly into detention reservoirs under pavement. The system relies on structural soils: highly porous engineered mixes designed to support tree root growth and pavement. To evaluate tree performance under the peculiar conditions of such a stormwater detention reservoir (i.e., periodically inundated), we grew green ash (Fraxinus pennsylvanica Marsh.) and swamp white oak (Quercus bicolor Willd.) in either CUSoil or a Carolina Stalite-based mix subjected to three simulated below-system infiltration rates for two growing seasons. Infiltration rate affected both transpiration and rooting depth. In a factorial experiment with ash, rooting depth always increased with infiltration rate for Stalite, but this relation was less consistent for CUSoil. Slow-drainage rates reduced transpiration and restricted rooting depth for both species and soils, and trunk growth was restricted for oak, which grew the most in moderate infiltration. Transpiration rates under slow infiltration were 55% (oak) and 70% (ash) of the most rapidly transpiring treatment (moderate for oak and rapid for ash). We conclude this system is feasible and provides another tool to address runoff that integrates the function of urban green spaces with other urban needs.

The Effects of Vegetation on Stream Bank Erosion

Riparian buffers are promoted for water quality improvement, habitat restoration, and stream bank stabilization. While considerable research has been conducted on the effects of riparian buffers on water quality and aquatic habitat, little is known about the influence of riparian vegetation on stream bank erosion. The overall goal of this research was to evaluate the effects of woody and herbaceous riparian buffers on stream bank erosion. This goal was addressed by measuring the erodibility and critical shear stress of rooted bank soils in situ using a submerged jet test device. Additionally, several soil, vegetation, and stream chemistry factors that could potentially impact the fluvial entrainment of soils were measured. A total of 25 field sites in the Blacksburg, Virginia area were tested. Each field site consisted of a 2nd-4th order stream with a relatively homogeneous vegetated riparian buffer over a 30 m reach. Riparian vegetation ranged from short turfgrass to mature riparian forest. Multiple linear regression analysis was conducted to determine those factors that most influence stream bank erodibility and the relative impact of riparian vegetation. Study results indicated that soil erosion is a complex phenomenon that depends primarily on soil bulk density. Freeze-thaw cycling, soil antecedent moisture content, the density of roots with diameters of 0.5 mm to 20 mm, and the interaction of soil pore water and stream water had a significant impact on soil erodibility and critical shear stress, depending on the soil type. Riparian vegetation had multiple significant effects on soil erodibility. In addition to reinforcing the stream banks, the streamside vegetation affected soil moisture and altered the local microclimate, which in turn affected freezethaw cycling. This study represents the first in situ testing of the erodibility of vegetated stream banks; as such, it provides a quantitative analysis on the effects of vegetation on stream bank erosion, relative to other soil physical and chemical parameters. It also represents the first measurements of the soil erosion parameters, soil erodibility and critical shear stress, for vegetated stream banks. These parameters are crucial for modeling the effects of riparian vegetation for stream restoration design and for water quality simulation modeling.

Variation in Root Density Along Stream Banks

While it is widely recognized that vegetation plays a significant role in stream bank stabilization, the effects have yet to be fully quantified. Many of the benefits of riparian vegetation are associated with the root systems. Roots bind the bank soil in place and act as fiber reinforcement, reducing soil erosion and increasing bank stability. The goal of this study was to determine the type and density of vegetation that would provide the greatest protection against soil erosion by determining the distribution and density of roots in stream banks. To quantify the distribution and density of roots along alluvial stream banks as a function of riparian buffer vegetation type and density, 25 field sites in the Blacksburg, Virginia area were sampled from June through August 2002. The riparian buffers varied from short turfgrass to mature riparian forests, representing the full range of possible vegetation types. Root-length density (RLD) with depth and above ground vegetation density were measured at each site. The sites were divided into two groups, forested and herbaceous, based on aboveground vegetation. Differences in root density and distribution were determined using nonparametric statistics. For both riparian buffer types, roots extended to depths in excess of one meter. At the herbaceous sites, very fine roots (diameter < 0.5 mm) were most common and over 75% of all roots were concentrated in the upper 30 cm of the stream bank. Under forested vegetation, fine roots (0.5 mm < diameter < 2.0 mm) were more common throughout the bank profile, with 55% of all roots in the top 30 cm. In the top 30 cm of the bank, herbaceous sites had significantly greater total RLD than forested sites (a = 0.01). While there were no significant differences in total RLD below 30 cm, forested sites had significantly greater concentrations of fine roots, as compared to herbaceous sites (a = 0.01). Research has shown that the ability of a soil to resist fluvial erosion is related to the quantity of fine roots in the soil (Kamyab, 1991). This suggests that forested vegetation may provide better protection against stream bank erosion than herbaceous vegetation. More research on the effects of root density on soil erodibility is required to confirm this conclusion.

Predicting Root Length Density in Stream Banks

Roots from riparian vegetation increase soil resistance to stream bank erosion; therefore, knowledge of root density and distribution in stream banks is useful in describing the resistance of stream banks to erosion. The objective of this study was to develop empirical models to predict stream bank root density in three size classes: very fine, fine, and big (diameters < 0.5 mm, 0.5-2.0 mm, and 2-20 mm, respectively) roots. Root length density, root volume ratio, soil physical and chemical properties, and aboveground vegetation density were measured at 25 sites on six streams in southwestern Virginia. Soil texture ranged from loamy sands to clay loam and riparian vegetation varied from mature forest to pasture. The sites were categorized as forested or herbaceous based on aboveground vegetation and differences in root density between vegetation types were determined using the Mann-Whitney test. Multiple linear regression was used to develop relationships between root density and site characteristics. Study results showed roots in all three diameter classes were evenly distributed across the bank face, with the majority of roots having diameters less than 2 mm. This may reflect the presence of herbaceous vegetation on the bank face and/or the preference of small diameter roots for the stream bank. Bulk density and aboveground vegetation density were a key factors influencing root density. While several statistically significant empirical relationships were developed to predict root density in stream banks, the predictive capabilities of the equations was low. Because of the highly variable nature of soil and vegetation properties, it is recommended at this time that soil erodibility be measured in the field for design and modeling purposes, rather than estimated based on empirical relationships.

Riparian Vegetation Effects on Freeze-Thaw Cycling and Desiccation of Stream Bank Soils

Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on stream bank erosion have not been quantified. Subaerial processes, such as desiccation cracking and freeze-thaw cycling, are climate-related phenomena that deliver soil directly to the stream channel and make the banks more vulnerable to flow erosion by reducing soil strength. This study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks. Soil temperature and soil water potential were measured at six paired field sites in southwest Virginia for one year. Differences in daily minimum and maximum soil temperatures, daily temperature range, and average daily soil water potential were compared for both summer and winter. Results of this study showed that stream banks with herbaceous vegetation had higher soil temperatures overall and a greater diurnal temperature range, as compared to forested stream banks. These differences decreased during the growing season as the herbaceous vegetation matured. Additionally, increases in daily average soil water potential of 13% to 57% were observed under herbaceous vegetation, as compared to woody vegetation, likely due to the evapotranspiration from the shallow herbaceous root system on the bank face. In contrast to summer conditions, the deciduous forest buffers provided little protection for stream banks during the winter: the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze-thaw cycles. With the absence of a dense canopy, the stream banks under mature forest cover were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze-thaw cycling. Results of this study suggest that in areas with soils susceptible to desiccation cracking, woody vegetation may provide the best protection against degradation by subaerial processes. In areas with silty soils prone to freeze-thaw cycling, a dense groundcover may provide more protection against soil loss due to freeze-thaw cycling than just deciduous woody vegetation.