Date of Award
Doctor of Philosophy
Ecology, Evolution, and Organismal Biology
William G. Crumpton
Matthew J. Helmers
Wetlands developed to intercept agricultural drainage and runoff differ from more traditional constructed treatment wetlands, such as tertiary treatment systems, in several key ways. For one, agricultural wetlands are typically built at the termini of tile-drained farmed watersheds, and as such receive unregulated nutrient mass and hydrological loads. Further, these systems are subjected to continually time-varying atmospheric and ecological conditions, and tend to operate as passive, unmanaged, systems. In some respects, these wetlands can be regarded as small and shallow run-of-the-river type impoundments. The long-term ability of constructed agricultural wetlands to reduce nutrient loads from farmed watersheds is dependent upon several overarching factors including landscape position; 2), wetland area, and the residence time distribution of the system, which characterizes internal mixing patterns. However, mixing processes in agricultural treatment wetlands are poorly understood even though mixing behavior has been shown to have considerable impact on the constituent removal performance of shallow flow-through basins in general, and strongly influences some wetland biogeochemical and ecosystem processes. Mixing influences wetland treatment performance by directing the spatial distribution of influent materials throughout the system, thereby determining the area and volume of the basin which is active in treatment; and by dictating the quantity and rate of delivery of materials to reactive media. For a system operating under steady flow conditions, the rate and extent of mixing is primarily governed by wetland bathymetry and the presence of submersed elements or structures, such as from vegetation. However, mixing in these systems is also highly affected by environmental factors such as the ambient momentum of the inflow channel, wind speed and direction, and the complex interactions between these effects and the seasonal growth and senescence of emergent and submersed aquatic vegetation. How these factors influence mixing in these systems individually and in concert is an under-explored topic, particularly with respect to the shallow run-of-the-river type systems exemplified by wetlands developed in agricultural watersheds. A more fully developed understanding of mixing processes in these types of wetlands can aide wetland designers and modelers by providing information on the potential range of mixing conditions within a given system over time (and thus account for these effects in wetland nutrient performance models) and provide guidance, in conjunction with existing wetland design best practices, on how to mitigate for deleterious environmental mixing effects, and enhance desirable hydraulic performance in the wetland design process, prior to wetland development. This dissertation explores the topic of mixing in constructed agricultural wetlands, and the environmental factors which control mixing behavior. Further, this dissertation addresses the issue of tracer decay, a common occurrence in natural flow systems, on estimation of the moments of developed residence time distributions, and by extension, on common measures of mixing. The structure of this dissertation is centered around a set of hydraulic tracer studies conducted on a sub-sample of wetlands currently enrolled in the Iowa Conservation Reserve Enhancement Program. Studies were conducted to examine the hydraulic and mixing behavior of these systems over a range of environmental and vegetated conditions. This information was collected in support of an ongoing effort at Iowa State University to monitor and model the nutrient removal performance of constructed agricultural wetlands, with the explicit intent of understanding the role of mixing on wetland nutrient removal performance.
David Immanuel Green
Green, David Immanuel, "Environmental factors affecting the residence time distribution dynamics of constructed agricultural wetlands" (2016). Graduate Theses and Dissertations. 15920.