Assessment of Natural Attenuation in the East Branch Canal Creek Area, Aberdeen Proving Ground, Maryland

WRD PROJECT #: MD135
PROJECT CHIEF: Fleck, William B.
BEGIN DATE: 01-May-2000
END DATE: 30-September-2001

Customers currently supporting the project:

U.S. Army, Aberdeen Proving Ground

Problem

Previous work at East Branch Canal Creek has probably provided enough data to demonstrate whether natural attenuation is occurring. There are three principle sources of data for the East Branch Canal Creek site. Nemeth (1989) collected a large amount of historical information. Of particular importance for the East Branch Canal Creek site are his estimates of the amount and location of discharges of 1,1,2,2-tetrachloroethane to the ground-water system. The second major source of data is a USGS 5-year study from 1985 to 1990 (Oliveros and Vroblesky, 1989; Lorah and Vroblesky, 1989; and Lorah and Clark, 1996). This study included the installation of about 150 wells, 5 synoptic water-level measurements, and 4 ground-water quality-sampling rounds. Some of this data was used to develop the hydrogeologic framework. Data from the second sampling round was used to define the location and horizontal extent of the contaminant plume for July through September 1988. Delineation of the contaminant plume was based on the average concentration of the contaminant across the full thickness of the Canal Creek aquifer. The third major source is the data collected from July 1994 to July 1995 by Jacobs Engineering (1996). They developed two-dimensional maps of the contaminant plume but in contrast to the USGS, their definition of the plume was based on maximum concentrations at any given vertical location within the Canal Creek aquifer.

Objectives

The objective of this project is to assess whether, on the basis of existing data, natural attenuation of organic contaminants is occurring within the Canal Creek aquifer under the East Branch Canal Creek area.

Approach

Three lines of evidence have been used in recent years to show that natural attenuation is occurring within a chlorinated solvent contaminant plume. These lines of evidence as defined by Wiedemeier and others (1999) are:

1. Historical data base showing plume stabilization and/or loss of contaminant mass over time
2. Chemical and geochemical analytical data, including:
   1. Depletion of electron acceptors and donors
   2. Increasing metabolic by-product concentrations
   3. Decreasing concentrations of parent compounds
   4. Increasing concentrations of daughter compounds
3. Microbiological data that support the occurrence of biodegradation and give estimates of biodegradation rates.

The first two lines of evidence are pertinent to this study and are discussed below. No information has been obtained from the East Branch Canal Creek area for the third line of evidence.

The first two lines of evidence are pertinent to this study and are discussed below. No information has been obtained from the East Branch Canal Creek area for the third line of evidence.

Line of Evidence # 1: Historical data base showing plume stabilization and/or loss of contaminant mass over time.
This line of evidence involves using historical data to demonstrate whether the contaminant plume is shrinking, stable, or expanding at a rate slower than that predicted by advective ground-water-flow calculations. To make this calculation at the East Branch Canal Creek site, data are needed to (a) calculate the rate and direction of ground-water flow, and (b) map the shape and extent of the contaminant plume over time. Data provided by Nemeth (1989) indicated that the flow system at the East Branch Canal Creek site from about 1950 to 1968 was controlled by ground-water withdrawals for water supply. This resulted in an elongated plume aligned along the axis of these supply wells. After the cessation of pumpage, the flow system reverted to its natural unstressed condition, and the plume has since migrated southeasterly in the direction of ground-water flow. The data provided by Oliveros and Vroblesky (1989), Lorah and Clark (1996), and Jacobs Engineering (1996) is sufficient to calculate the rate and direction of ground-water flow for the existing unstressed conditions in the Canal Creek aquifer.

The location and horizontal extent of the contaminant plume have been mapped for the 2^nd sampling round of July-September 1988 (Lorah and Clark, 1996) and for the sampling performed from July 1994 - July 1995 by Jacobs Engineering (1996). Lorah and Clark (1996) did not map the location and extent of the contaminants for either the 1^st, 3^rd or 4^th sampling rounds. As previously discussed, the delineation of the plume by Jacobs Engineering (1996) was based on the maximum concentrations. In contrast, plume delineation in Lorah and Clark (1996) was based on an average concentration for the full thickness of the aquifer. Neither Lorah and Clark (1996) nor Jacobs Engineering (1996) mapped the vertical distribution of the contaminants within the Canal Creek aquifer.

The mass of the contaminants for the 2^nd sampling round of Lorah and Clark (1996) has been calculated. To calculate the mass of the contaminants for the l^st, 3^rd, and 4^th sampling rounds of Lorah and Clark (1996), the plumes of the average vertical concentrations of the contaminants will have to be plotted. Because the contaminant plumes mapped by Jacobs Engineering (1996) are based on maximum concentrations, these plumes will need to be replotted based on average concentrations to calculate the mass. Mapping the contaminant plumes for all five sampling rounds will provide information on the changes that have occurred over the 7-year span from 1988 to 1995 in terms of (a) total mass of the contaminants and (b) individual masses of parent and daughter compounds.

The answer to the question of plume stability will be based on an analysis of plume movement as predicted by advective ground-water-flow calculations. Assumptions based on the locations and amounts of ground-water withdrawals (Nemeth, 1989) can be used to develop a model of the position of the contaminant plume in 1968 at the cessation of pumpage. Calculations of the positions of the plume for each of the 4 sampling rounds performed by the USGS from 1986 to 1989 and the 5^th sampling period performed by Jacobs Engineering from 1994 to 1996 will indicate the rate at which the plume is expanding. Velocity calculations to determine advective ground-water flow are based on head gradients, hydraulic conductivities, and porosities. These three parameters are well known for the East Branch Canal Creek site (Oliveros and Vroblesky, 1989; Lorah and Clark, 1996; and Jacobs Engineering, 1996). Thus, starting with the 1968 position of the contaminant plume, a series of calculations for both the actual plume positions and the positions calculated by advective ground-water flow will indicate if the contaminant plume is shrinking, stable, or expanding.

Line of Evidence # 2: Chemical and geochemical analytical data.
Microorganisms degrade organic contaminants in the subsurface, which results in measurable changes in the ground-water chemistry. The 2^nd line of evidence involves measurement of this biogeochemical signature. Wiedemeier and others (1999) list the geochemical parameters useful for evaluating natural attenuation as follows: parent and daughter chlorinated hydrocarbons; dissolved oxygen; nitrate; manganese; iron; sulfate; methane; carbon dioxide; alkalinity; oxidation-reduction potential (ORP); pH; temperature; conductivity; major cations; chloride; total organic carbon; and hydrogen. This list includes all the parameters necessary to determine if the following are occurring: depletion of electron acceptors and donors; decreasing concentrations of parent compounds (e.g. 1,1,2,2-tetrachloroethane, carbon tetrachloride, and possibly trichloroethene); increasing daughter compound concentrations (e.g.. dichlorethane, chloroform, methylene chloride, dichloroethene, and vinyl chloride); and increasing metabolic compound concentrations. Ground water from all the wells in the East Branch Canal Creek site has been collected and analyzed for all of the above listed parameters except carbon dioxide. These analyses will be plotted in cross section to better understand the interrelationships occurring as part of natural attenuation processes that may be taking place in the Canal Creek aquifer.