Principal Investigator: Bruce Beck (University of Georgia)
Sponsor: GWRI
Start Date: 2001-03-01; Completion Date: 2002-02-28;
Keywords: Surface Water, Iron, Nutrient Dynamics, Phosphorus
Introduction:
The complex interactions iron and phosphorus play a primary role in the availability of phosphorus in surface waters of the Georgia Piedmont. Exploration of these dynamics can provide information for nutrient management in surface water systems of this region. The soils of the Georgia Piedmont are rich in iron primarily as iron hydroxides (oxidized iron). Iron hydroxides form a ligand exchange with phosphate ions, making the phosphate biologically unavailable. Phosphorus, particularly inorganic phosphate, delivered through non-point source runoff to receiving waterbodies may be sorbed to iron hydroxides and not biologically available, while phosphorus, as organic phosphorus, delivered from a point source (such as an effluent pipe) may be immediately biologically available. Illuminating the biogeochemistry of phosphorus in surface waters rich in iron hydroxides will provide information useful in setting local water quality criteria and standards, and will help define the relationship between point and non-point pollution in surface waters receiving runoff from iron-rich soils.
The paradigm for phosphorus cycling was developed based on data from lakes in northern temperate regions. Lakes in north temperate regions tend to be glacial in origin. The phosphorus cycling paradigm in north temperate systems involves the sinking of inorganic particulates and organic material which result in a steady increase in dissolved phosphorus in the hypolimnetic waters of strongly stratified lakes. The dissolved phosphorus is then recirculated to the lake at fall mixis (Hutchinson 1957; Wetzel 1983; Goldman and Horne 1994). In contrast, Southeastern Piedmont lakes are primarily man-made impoundments. The climate in the southeastern US provides for a longer growing season and warmer annual average temperatures than those found in north temperate regions. This difference in climate affects the strength and length of summer stratification, and creates the conditions for monomictic rather than dimictic lakes in the southeastern Piedmont. The parent geology of the southeastern Piedmont is responsible for the differences in the cycling of phosphorus in southeastern Piedmont systems. The high iron content of the soils in the southeastern Piedmont provides transport of iron via runoff to aquatic systems in this region. The steady increase in hypolimnetic P during stratification, and the pulse of soluble P at fall turnover, is not found in southeastern Piedmont lakes. Oxidized iron in the water column binds phosphate via surface sorption and ligand exchange. We hypothesize that this sorption removes inorganic phosphorus from the biologically available fraction, thus creating a different lake phosphorus cycling regime for systems in the southeastern Piedmont.
We investigated the biogeochemical processes involved in the cycling of phosphorus as phosphate in the iron-rich waterbodies of the Georgia Piedmont. We explored the sorption chemistry of iron and phosphorus using the chemical equilibrium model MINTEQ. We conducted laboratory studies of the geochemical processes involved in phosphorus and iron interactions in surface water. We also conducted corresponding fieldwork on Lake Lanier sampling metals and phosphorus at depth four times in the annual cycle, to investigate the current roles of iron and phosphorus in the surface waters of Lake Lanier. The work conducted in this study will allow us to help identify appropriate in waterbody concentrations of phosphorus, given the local geochemistry, for local waterbody specific water quality criteria and standards, and may help evaluate appropriate parameters for monitoring significant changes in water quality of Lake Lanier.
The MINTEQ model program was released initially by USEPA in 1991 as a chemical equilibrium model for the calculation of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model can calculate the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions and gas phase partial pressures. MINTEQ comes complete with a comprehensive database, and also allows for user defined parameter input [http://www.epa.gov/ceampubl/minteq.htm]. We used the VMINTEQ model program, which is a modified form of the MINTEQ model to explore the iron-phosphorus chemistry of Georgia Piedmont lake systems. VMINTEQ has been modified by the addition of a Visual Basic interface and the Stockholm Humic Model sub-model to include dissolved organic matter interactions using the diffuse layer model rather than the Gaussian distribution for organic matter physical chemistry (Gustaffson 2001). The laboratory experiments we conducted utilized the results of the model runs to determine initial conditions for the sorption capacity experiments.
The laboratory experiments were conducted in multiple phases. The first phase involved 24 and 48 hour sorption capacity experiments. The second phase involved measuring changes in sorption of phosphorus to iron in the presence of elevated organic matter introduced as concentrated humate in the form of Agrolig powder. The final phase of the planned laboratory work involving algal response to additions of iron complexed phosphorus was not completed due to time and funding constraints.
The third component of our work included depth measurements of metals, nutrients, and basic water chemistry parameters taken four times in the annual cycle on Lake Lanier. We analyzed these data to evaluate the hypothesis that phosphorus cycling in Georgia Piedmont lakes differs significantly from the northeast temperate lake paradigm. Measurements at depth of iron, manganese, and phosphorus show the lack of phosphate in the anoxic bottom waters, and the lack of soluble iron at the sediment-water interface. These measurements help define the role of iron in the phosphorus cycle in Georgia Piedmont lakes.