Award Date

5-1-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Zhongbo Yu

Second Committee Member

Kumud Acharya

Third Committee Member

Matthew Lachniet

Fourth Committee Member

Ganqing Jiang

Fifth Committee Member

Thomas Piechota

Number of Pages

194

Abstract

The Las Vegas Wash (LVW) has undergone significant wetlands degradation and soil erosion over the past thirty years due to increasing flow resulting from urbanization and large rainfall events in the Las Vegas Valley Watershed. The increased flow and associated pollution load in the LVW and its adverse impact in Lake Mead have alerted stakeholders to pay a greater attention to explore alternative measures for rehabilitation of wetland ecosystems. This dissertation, using the case of changes in LVW, analyzes and describes ecological and engineering services provided by wetlands in arid and semi-arid regions and provides a knowledge base that can be used to improve water quality and enhance stream restoration respectively. The dissertation includes three separate studies that are organized into three independent chapters.

In the first study, constructed and naturally created wetlands in the LVW and its tributaries were studied to characterize and understand their potential role for improving ecosystem services (i.e., water purification). Excess nutrients and harmful metalloids removal was assessed at four wetlands, including Flamingo Wash Wetland, Pittman Wash Pilot Wetland, Demonstration wetlands at the city of Henderson water reclamation facility, and Las Vegas Wash Wetland. The study showed that the nutrient removal capacity of wetland vegetation in the four wetland sites correlated well with ambient nutrient concentrations in the sediments and water columns, irrespective of the type of plant present. For example, cattail and bulrush plant species have different nutrient uptake capacities, with these capacities mostly determined by the ambient nutrient and hydrologic conditions. Both species were equally efficient for nutrient uptake with high phosphorus concentration in below-ground and high nitrogen in above-ground plant parts. The below-ground parts of both species were capable of storing arsenic and selenium more efficiently than above-ground parts. However, bulrush species seem particularly efficient for removing metalloids as compared to cattail. These findings have important implications for improving our ability to engineer ecological solutions to the problem associated with common pollutants in the Las Vegas Valley.

The second project analyzed the structural and functional attributes of increasing common reeds (Phragmites australis (Cav.) Trin. ex Steud.) and native cattails (Typha domingensis Pers.) for the best ecosystem services from large scale wetlands such as the one in LVW. The entire LVW vegetation was analyzed through mapping and ground truthing to estimate areal coverage of P. australis Vs. T. domingensis. The results from this study compared with the previously published data showed that P. australis population is increasing in most of the places. P. australis in comparision to T. domingensis, appears to thrive better in areas with altered hydrology and high nutrient inputs. In addition to its structural dominance, our data showed that P. australis plays a significant role in nutrient storage in wetlands. The net above-ground standing stock of nutrients in LVW wetlands was estimated to be approximately 26418.7 kg TN and 1264.1 kg total phosphorus (TP) for P. australis and 5183.8 kg total nitrogen (TN) and 272.8 kg TP for T. domingensis. Despite management concerns over P. australis dominance and growth, they fared quite well in nutrient storage in LVW wetlands compared to T. domingensis. The study concluded that in LVW, both T. domingensis and P. australis could be utilized for water quality improvement. It should be noted, however, plant uptake alone is not enough to improve water quality below regulatory thresholds from large scale wetlands, and managing dominant vegetation may be required for better nutrient removal efficiency.

The third project studied the riparian wetlands function for their engineering services on streambank stabilization. The mechanical properties of native species (Artiplex lentiformis, Lycium andersonii, Larrea tridentata, and Allenrolfea occidentalis) were studied to understand their suitability in revegetation purpose on banks that are easily erodible. Field experiments were conducted to estimate root length, root length density, root area ratio, and root tensile strength.

Finally, the root cohesion values were assessed using a simple perpendicular model and Fiber Bundle Model. The maximum root cohesion in the present study was estimated for A. lentiformis (97.6kPa) followed by L. andersonii (89.3kPa), L. tridentata (35.6 kPa), and A. occidentalis (34.8 kPa). These values were estimated to rank the native species for their potential use in bank stabilization. The results showed that these native and most prevalent species were more suitable for shallow bank slope stabilization, since their root distributions were significantly higher in topsoil depth (0-0.5 m) in comparison to subsoil depths (>0.5 m). This information could be utilized for revegetation and restoration purposes in the arid and semi-arid regions where these plants are abundant.

Keywords

Arid and Semi-arid regions; Arid regions; Constructed wetlands; Ecological engineering; Ecological Services; Phragmites; Natural wetlands; Nevada – Las Vegas Wash; Typha; Water – Purification; Wetland plants; Wetlands vegetation

Disciplines

Biogeochemistry | Desert Ecology | Environmental Health and Protection | Environmental Indicators and Impact Assessment | Environmental Sciences | Natural Resources and Conservation | Plant Sciences | Terrestrial and Aquatic Ecology

Language

English