Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Geography, Environment and Planning

Major Professor

William J. Mitsch, Ph.D.

Co-Major Professor

Mark Cable Rains, Ph.D.

Committee Member

Thomas L. Crisman, Ph.D.

Committee Member

Gary A. Lamberti, Ph.D.

Keywords

GIS, Harmful Algal Blooms, Suitability Analysis, Nutrients, Water Pollution, Wetlands

Abstract

The substantial increase among agricultural and urban land use over the past century is widely regarded as the leading cause in a variety of negative environmental impacts, particularly regarding eutrophication of both salt and fresh surface waters. Usage of both synthetic and natural fertilizers on agricultural fields and residential lawns has increased significantly in order to achieve maximum crop yield and green lawns. The nitrogen and phosphorus in these fertilizers inevitably run off the landscape and into downstream surface waters. The geographic scale of this issue makes it a challenge to overcome, as targeting specific sources/polluters is nearly impossible. Landscape-scale solutions to nutrient pollution such as treatment wetlands are necessary to handle the volume of water outflows from agricultural systems. Wetlaculture (wetlands+agriculture) is a relatively new concept that attempts to meet both the needs of the environment and the farmer via water treatment and soil enrichment by consistently rotating plots of land between wetland and agricultural usage. Small-scale pilot studies must be conducted to determine ideal hydrologic conditions, soil conditions, and placement before attempting a full-scale wetlaculture experiment.

Chapter 2 of this study reviews a small-scale wetland mesocosm experiment that tests the effect of several hydrologic variables on the nutrient retention capabilities of the mesocosms. The mesocosms experiment was conducted at Freedom Park in Naples, FL, and received inflow waters for 32 months, from June 2018 to February 2021. Sawgrass (Cladium jamaicense) was planted in each of the 28 mesocosms to mimic the dominant species in Everglades National Park. The 28 mesocosms were divided into 4 groups, each a different combination of two hydrologic variables: high and low hydraulic loading rate and high and low maintained water depth. Across all 4 scenarios, the mesocosms were overall nutrient sources for the first 1.5 years of the study due to elevated baseline N and P soil concentrations. The final year of the study showed nutrient percent retention rates trending positively for three parameters: SRP (53.11 ± 0.07%), NOx-N (98.58 ± 0.01%), and TN (53.91 ± 0.07%) and net export for two parameters: TP (-3.59 ± 0.21%) and TKN (-15.95 ± 0.09%). The high loading rate mesocosms exported more nutrients in the early years of the study than the low loading rate mesocosms, but sequestered more nutrients in the final year of the study than low loading rate mesocosms. The results of this study showed that stormwater runoff-fed treatment wetlands can be effective in removing some excess nutrients from inflow waters; that baseline soil conditions have a large impact on how quickly newly constructed wetlands achieve nutrient retention goals; and that the hydrologic regime implemented in future wetlaculture studies should be altered over time to account for initial nutrient export.

Because several of these mesocosm water quality parameters showed that the mesocosms were nutrients sources for part or all of this study, we explored the mechanisms responsible for this outcome. Chapter 3 of this study examines the impact of reclaimed irrigation wastewater on Naples sediments and surface waters in general and on the mesocosms specifically. While recycling reclaimed wastewater is an effective way of mitigating freshwater waste, the nutrient concentrations within this water are often ignored. The nutrient demands of common turfgrasses in southwest Florida, one of the most common land covers in the state, and of sugarcane are examined and compared to the nutrients supplied by reclaimed wastewater. Results show that reclaimed wastewater contains significantly more TP (P <0.05) than stormwater runoff that we were treating in the mesocosms from the city of Naples. This is notable because turfgrasses typically require no P fertilization at all, meaning all P from reclaimed wastewater (3.39 kg ha-1 yr-1) used to irrigate turfgrass likely runs off into nearby waterways.

Chapter 4 investigates the suitability of areas within and around the Everglades Agricultural Areas for wetlaculture implementation on a pilot or full scale. The purpose of identifying these areas is to locate prime locations for scaling-up the wetlaculture concept from the mesocosms. Agricultural fields in the Everglades Agricultural Area (EAA) were examined for potential conversion to wetland usage, while the Stormwater Treatment Areas (STAs) and Flow Equalization Basins (FEBs), areas that are currently wetlands, were considered for conversion to agricultural usage. A variety of parameters were evaluated to create a multiple-criteria suitability analysis within GIS software, including compound topographic index, land use, soil characteristics, and elevation. Results of the study showed that 347 km2 of STA and FEB land is either highly or moderately suitable for conversion to agricultural use, while 2.7 times that area (947 km2) of EAA land could be highly suitable for conversion from agriculture to wetland use. It is concluded that the EAA and surrounding lands could provide ideal sites for full-scale wetlaculture application.

Overall, my dissertation demonstrates that treatment wetlands, and the wetlaculture system in particular, can be effective in treating nutrient polluted wastewater, whether it comes from urban stormwater runoff or from agricultural runoff. Chapter 2 illustrates that wetland mesocosms in southwest Florida successfully sequester nitrogen and phosphorus after an initial delay, even with elevated soil nutrient baseline conditions. Chapter 3 explains the likely reasoning behind these baseline conditions by examining the impact of nutrient-laden reclaimed wastewater irrigation on City of Naples landscapes, ultimately determining that excess phosphorus inputs to turfgrasses are potentially dangerous to downstream ecosystem health. Finally, Chapter 4 establishes that there is more than enough viable land for the implementation of the wetlaculture system in the EAA by using GIS-based suitability modeling. The final conclusions of this dissertation indicate that increased usage of treatment wetlands are necessary to combat nutrient pollution and harmful algal blooms in Florida.

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