Dalecarlia Water Treatment Facility-Residental Sampling

Objectives

1. Describe the chemical properties of sediment discharged to the Potomac River by the Washington Aqueduct from the Dalecarlia water treatment facility.

2. Estimate the amount of sediment with associated contaminants that originates from the basins and deposits to the streambed in the Potomac River.

Statement of Problem

The chemical properties and volume and distribution of residental discharges to the Potomac River from the watertreatment plant at the Dalecarlia facility are not clearly understood. This study will provide information on the occurrence and distribution of residuals that will support other research by local, state, and federal agencies. This will facilitate future management decisions regarding the disposal of the sediment.

Some of the questions that remain to be answered are:

1. How far downstream and for how long does the sediment affect the habitat in the mainstem of the Potomac River?

2. How much of the bed sediment in the Potomac River adjacent to the outfalls can be directly attributed to the flushing of sediment from the settling basins?

Historically many water-treatment plants discharged their sediments directly into streams and lakes because there appeared to be little environmental significance. The main chemical components of this sediment are from the alum and ferric chloride that are used to coagulate colloidal materials in the water. Al and Fe are only sparingly soluble in the neutral pH range, so these are thought to be non-toxic in most stream environments. Because the pH in this section of the Potomac River is generally in the range of 7 to 8, aluminum is not expected to be a toxicant here for aquatic life.

A more detailed analysis of the chemistry of the residuals will provide further insight into their potential negative effects. Some of the previous testing done by other agencies has shown high concentrations of ammonia (2.2 to 16.4 mg/L), Kjeldahl nitrogen (3.0 to 52.0 mg/L), total phosphate (1.93 to 44.7 mg/L), and TOC (80 to 259 mg/L) (E.A. Engineering, Science, and Technologies, Inc., 2001). Five-day BOD measurements were as high as 92 mg/L and were normally elevated. All of these measurements suggest a strong potential for the development of anoxic conditions in sediments and the overlying water column in the deposition areas, but ammonia in particular has the potential for creating toxic effects in benthic environments. Chronic criteria based on pH would suggest an effects level of approximately 2 to 3 mg/L for the concentration of ammonia. Acute criteria for ammonia are higher, but if sediment persists on the river bed in the Potomac for any amount of time, then the chronic criteria may be relevant. Other chemicals of interest that have not be documented in the basin sediments include trace metals, Polycyclic Aromatic Hydrocarbons (PAHs), phthlatates, and organochlorine pesticides. These chemicals have been documented in Potomac River water (Foster and others, 2000) and will be carried into the basins and accumulate on the flocculated sediments, but concentrations also will be enhanced by loadings from atmospheric deposition and local runoff directly to the Dalecarlia and Georgetown basins in the urban Washington D.C. area. Established water and sediment-quality guidelines for the protection of aquatic organisms may be used to determine the potential toxicity of the sediment material (Long and Morgan, 1990; MacDonald, 1994; CCME, 1999).

Strategy and Approach

To accomplish the objectives of this study, the USGS proposes to conduct the following tasks.

1. During the first year of the project, USGS staff will measure the concentrations of nutrients including phosphorus and ammonia, trace metals, Polycyclic Aromatic Hydrocarbons (PAHs), phthalates, and organochlorine pesticides in sediment that is discharged from the settling basins. To capture some of the seasonality in these parameters, there will be two sampling events that will be coordinated with the timing of discharge events. Three samples will be collected from bed sediment in each of the basins prior to a release. Samples will also be collected from the Potomac River bottom upstream and downstream of one of the outfalls as well as prior to and one week after a release. A total of 5 samples will be collected in the river for each sampling event. Samples will also be collected for quality assurance including blanks and replicates, making the total number of samples equal to 30. These data will be evaluated and compared to publishedpotential to impact aquatic organisms and/or communities in the river.

2. During the second year of the study, USGS staff will measure the spatial concentrations of aluminum in Potomac River sediments and use a two-end-member mixing model to map the percentage of sediment on the streambed that originated from the settling basins. Some samples will also be analyzed for selected chemicals if they were detected in significant concentrations in the settling-basin sediments during the previous year's sampling events. The purpose of sampling during the second year of study will be to determine if and to what extent contaminated sediments affect the benthic environment of the Potomac River in the vicinity of the outfalls.

3. Before each flushing event, the USGS staff will calculate the mass of sediment that will be delivered from the settling basin during the event. This will be done using the area of stored sediment in the settling basin based on information from the Washington Aqueduct personnel, and the average density of the sediments based on calculations made on sediment samples collected for the chemistries. This information will be compared to other sediment loadings that have been estimated for the basin flushing events as well as the background load of suspended sediment in the Potomac River.