Fellow: Kasi Murthy, Department of Civil Engineering, NDSU
Advisor: Wei Lin, Assistant Professor of Civil Engineering, NDSU
Matching Support: NDSU
Degree Progress: Ph.D. expected in May 2005.

Wetlands are valuable as sources, sinks, and transformers of a multitude of chemical, biological, and genetic materials (Mitsch & Gosselink, 2000). Prairie pothole marshes in North Dakota, South Dakota and eastern Minnesota are considered as one of the dominant areas of freshwater marshes in the United States. The Prairie upland, almost all of the prairie wetlands, and some of the timberlands have been converted into croplands. Wetland restoration efforts and return of the agricultural lands to natural habitats are increasing in this particular region. Phosphorus is believed to be the biggest problem in returning the reclaimed wetlands to a healthy condition. Enormous application of fertilizers has caused high phosphorus levels in soil, and water in this wetland region. These phosphorus levels can be a serious problem in causing imbalance in plant species growth and thereby a problem to water levels and also to some wildlife. This research is mainly concentrated on phosphorus transport in wetlands ecosystem.

Introduction

Wetlands in prairie pothole region (PPR) span portions of Minnesota and the Dakotas, small region in Northern Montana and Iowa, and three Canadian provinces (southwestern Manitoba, southern Saskatchewan and southeastern Alberta). These wetlands are small glacial depressions formed thousands of years ago due to scouring action of Pleistocene glaciation interact with mid-continental climate variations. The PPR region covers approximately 715,000 km² (Euliss, et.al. 1999) and are known to support diverse species of biota, regulate flooding, control water quality, and recharge ground water (Mitsch and Gosselink, 2000; Van der valk, 1989). Due to immense agricultural activities during past couple of centuries about 50% of these wetlands are lost (Van der valk, 1989; National Research Council, 1992). After recognizing the functions and values of wetlands, state, federal and various other national agencies, that are interested in protecting these natural resources, have increased their efforts in reclaiming and restoring the lost wetlands.

Earlier agricultural activities in PPR have altered the hydrology of wetlands, contaminated the wetlands with fertilizer application, and disturbed the habitat due to wetland drainage and conversion of native prairie grasslands into agricultural fields (Euliss, et.al. 1999). Nutrients applied (in excess than required for plant growth) through agricultural chemicals are among the other factors that have considerable after-restoration impacts. Excess nutrients in water can cause excess biological (such as algae, macrophytes, etc.,) growth, which is also known as eutrophication (USEPA, 2000). Among nutrients, phosphorus is considered as major limiting nutrient in most of the lake eutrophication (Nitrogen is considered as limiting nutrient where sewage treatment plant effluent is involved (USEPA 2000)). The reason for this is, the other nutrients such as nitrogen and carbon are difficult to control because of their exchange interactions with atmosphere (Sharpley 1995).

Thus, phosphorus is selected as target element in this research. Since these wetlands have been restored where agriculture was practiced for more than 100 years, it is of major interest to many scientists and engineers to study the impact of the high levels of phosphorus already present in various compartments of wetlands (soil, water and biota). First of all, it is important to understand the transformations of phosphorus within a wetland that might lead to transport of phosphorus to the downstream lakes and streams. However, the phosphorus transformations and transport are influenced by hydrology, landscape and climatic conditions. This leads to a complex situation and needs a mathematical modeling.

Literature review and past work in the research

Literature review

To determine the factors that influence phosphorus transport in a wetland, numerous models have been devloped (Prescott and Tsnasis 1997; Kadlec R. H., 1997; Wang N and Mitsch W. J., 2000; Tsanis et. al., 1998; Brown 1988; Hammer and Kadlec 1986; Kao et.al. 1998). Literature review of these existing models was conducted. The review of these models indicated that the phosphorus transport and transformations within a wetland are influenced by kinetics of various physical, chemical and biological processes. The processes are adsorption-desorption, dissolution-precipitation, and biological uptake and release. Sedimentation and resuspension are ignored for the selected study area due to absence of considerable sediments in the wetland water column. The following discussion describes the kinetics of the processes in a wetland.

Phosphorus usually available in two forms, particulate and dissolved. Among these forms, each one can be present in either organic or inorganic form. Phosphorus cycling between these forms is controlled by the rate of the each process. 

1.   Adsorption and Desorption: Kinetics of adsorption and desorption of phosphorus can be described by transforming Langmuir equillibrium equation into a kinetic form (Van der zee 1986).

(1)

Where Q is the quantity of P sorbed in g [P] kg^-1[soil], C is the concentration of P in solution in mg/l, Q_max is maximum sorption capacity in g[P]/kg, and k_a and k_d are adsorption and desorption rate constants. At equilibrium (∂Q/∂t = 0), the Langmuir equation can be obtained as below:

  (2)

Rate constants in equation (1) are strongly effected temperature and can be modified using van’t Hoff-Arrhenius relationship

 (3)

Where 0 is temperature coefficient.

Desorption is expected to occur when the influent flow has less phosphorus dissolved than the phosphorus sorbed onto soils (McGechan, 2002).

2. Dissolution and Precipitation: Equilibrium equations for different phosphate minerals have been useful in analysis of water quality and wastewater treatment (Ferguson and McCarty 1971; House 2003; Musvoto et.al. 2000; Garcia 2002). A kinetic based approach is chosen to model dissolution and precipitation of various reactions. Since, in natural waters under normal pH conditions (ranging between 6 and 9) calcium phosphates are most insoluble (Stumm and Morgan 1981; Lindsay 2001). Although, apatites (Ca₁₀(PO4)₆(X)₂) are the most insoluble among calcium phosphates, crystallization of apatites requires high energy and their formation is quite unusual (Musvoto et.al. 2000; House 2003; Maurer et.al.1999). However, anhydrous calcium phosphate (ACP) is considered as intermediate formation, which has higher solubility product among calcium phosphates (excluding apatites). Following is a summary for equilibrium equations for controlling solid phases for calcium and magnesium. Carbonate concentration is controlled by CO₂ equilibrium with water.

For calcite: CaCO3 → Ca2+ + CO3-2      Ksp = 8.48                         (4)

Dolomite: MgCa(CO3)2  → Mg2+ + Ca2+ + 2CO3-2   Ksp = 17.09    (5)

ACP: Ca3(PO4)2.xH2O → 3Ca2+ + 2PO4-3 + xH2O   Ksp = 25.5      (6)

Precipitation rate for the above reactions can be written as below (Arvidson and Mackenzie 2000).

r =k (Ω -1)n                                                                             (7)

where Ω is the ratio of the ion activity product (IAP) to the thermodynamic equilibrium constant, n is the order of the overall reaction and k is rate constant.

3.   Biological uptake and release: Macrophytes’ (such as cattails) growth, death and decomposition create a cycle for uptake and release of phosphorus from wetland soils and water (Kadlec, 1997). The rate of each process (growth, death and decomposition) in the biological cycle have an effect on amount of phosphorus in wetland in a long period of time (Wang and Mitsch, 2000).

Biological uptake (V) kinetics can be related with inorganic phosphorus (Pi) and the maximum uptake rate (V_max) (Spijkerman & Coesel 1996).

V = V_max * (P_i / (K_m + P_i))                                                         (8)

Where K_m is half saturation constant for uptake.

Phosphorus mineralization is described as first order kinetic model that relate the change in mineralized phosphorus (P_m) in the soil with time (dP_m/dt) to the amount of mineralizble substrate (P_o) as below (Pierzynski et. al. 2000):

dP_m/dt = k (P_o)                                                                        (9)

Constants K_m and k both are temperature dependent and can be related with temperature as described in equation (3). 

Prior work

Wetlands in Hamden Slough National Wildlife Refuge (NWR) were selected for the proposed study. Wetlands in two different locations, three in one location and six in another location are selected for the study. In summer of 2001, a preliminary surveying, construction of wells for groundwater sampling, and a preliminary soil and water sampling was done. Soil samples were analyzed for total phosphorus, and some cations and anions (cations such as Ca, Mg, K, Na and Fe; anions such as sulfide and chloride). Surface and ground water samples were analyzed for Ca, Mg, Carbonates, Sulfates, total phosphorus and orthophosphate. pH, temperature and dissolved oxygen for both surface and ground water were measured in the field. Depth of the water in each well was also measured to locate ground water table.  

Scope and Objectives of the proposed research

Scope of the Ph.D. research

The scope of the research is to study the various forms and concentrations of phosphorus and its transport in water, soil, and biomass (plant, fish, macrophytes, etc.,) in wetlands and thereby develop a water quality model to analyze future scenarios.

Objectives of the proposed fellowship research

The specific objectives of the project include:

• Studying seasonal water quality changes in the selected wetlands;
• Determining phosphorus forms and concentrations in wetland water, plants, and sediments;
• Performing a mass balance analysis on phosphorus and developing a mathematical model to simulate the phosphorus dynamics in the wetland systems; and
• Sampling and data collection at the wetlands and model evaluation.

Progress to date

The analysis of existing data and field sampling will help determining the current status. Water and sediment sampling and testing will help study the phosphorus dynamics in wetlands. A water quality model will be used to analyze phosphorus inflow and outflow due to point and non-point sources.

Soil samples taken for determining the profile of phosphorus concentration and other compounds along transects of the wetland. The results from soil sample testing will be analyzed for available phosphorus and total phosphorus. Preliminary water sampling completed and analyzed for basic water quality parameter including phosphorus. The area was surveyed using GPS and level for preparing topographic map, which will be used to assess the non-point source flow to these wetlands.

The research is divided into 3 parts. In the first part, phosphorus transport due to seasonal changes will be studied and modeled. The other parts will be modeling hydrology surface and ground water movement in the wetlands, then integrate hydrological model with model for seasonal changes. The final part will be solving the mathematical equations using numerical techniques. So far, literature review for processes involved in phosphorus transport and transformations is completed and a conceptual framework of the model is done. The conceptualized model is converted into mathematical equations (from the literature review).

Presentation to the NDWRRI Advisory Committee