USGS Internship Program: Biogeochemistry of Nutrients and Metal in Sediments
USGS Internship Program: Phosphorus Geochemistry in Reservoirs Historically Treated with Copper Sulfate for Control of Algae
Dr. Nancy Simon
2005 graduate, Environmental Science & Visual Arts
The American University
Eric is currently a graduate student in the Oceanography Department at the University of Rhode Island. His experience as a WRRI intern contributed to his being selected for the program at URI.
For the past 30 plus years copper sulfate has been spread in the waters of these reservoirs to eliminate algal growth that interferes with water filtration and which is thought to affect the smell and taste of the drinking water produced by the treatment plants. As a result, large concentrations of copper, a metal that can be highly toxic, are found in the sediment. Phosphorus comes into the reservoirs with the Chickahominy River water that is pumped in to optimize water levels. Pumpage is as much as 40 million gallons per day from the Chickahominy River. There are algal mats in the area of the reservoir where pumpage occurs (Figure 1). Also, large concentrations of phosphorus are found in the bottom sediments of the reservoirs. Under conditions of low dissolved oxygen, bottom sediments can release phosphorus to the overlying water column. At issue is whether the major portion of phosphorus that fuels the growth of algae is coming from Chickahominy River water that is pumped into the reservoirs or from bottom sediments.
To determine the contribution of bottom sediments to the phosphorus concentrations in the water column, bottom material was collected using either a grab sampler (resembles the working unit on a steam shovel) or a box corer (Figure 2). The sediment was freeze-dried, ground and sieved to produce very small, uniform particles. This called for dedicated work by Eric. Samples were analyzed for total phosphorus and phosphorus bound by poorly crystalline iron, and from these numbers organic phosphorus was determined. Auxiliary information included iron, copper, aluminum, manganese, and calcium concentrations in sediment samples.
Examination of the data indicated that there are very large concentrations of copper in the bottom sediments and that the bulk of the phosphorus is bound with the poorly crystalline iron oxides. The phosphorus is bound with the poorly crystalline iron oxides will release phosphorus only when sediment bacteria no longer have oxygen or nitrate available as electron acceptors and turn to ferric iron as an electron acceptor. The use of insoluble ferric iron as an electron acceptor produces ferrous iron which is much more soluble. The reduction of iron releases phosphate from the solid to the aqueous phase and can fuel the release of phosphorus from bottom sediment to the overlying water column.
Figure 1. Algal mats are found in the area of pumped inflow from the Chickahominy River to Lee Hall Reservoir.
Figure 2. Collection of bottom sediment from reservoir using a box corer.