A CASE OF CONTAMINATION

Transcription

1 Water 1 page 1 A CASE OF CONTAMINATION Introduction In recent years, groundwater contamination has become an important problem, especially in urban environments. Subsurface contamination can have very severe consequences if it affects groundwater wells used for municipal water supplies. Groundwater contamination can also have adverse impacts on surface water bodies such as rivers, streams, or lakes. Legislative programs, such as the US EPA s Superfund, have been implemented to help address the problems associated with subsurface contamination. These programs allow state and federal agencies to force those who are responsible for contamination to pay for clean-up activities. The costs associated with these clean-ups often reach millions or tens of millions of dollars. One of the major challenges in urban environments is identifying those responsible for the contamination. Difficulties arise because potential sources are often in close proximity to one another. In today s exercise, you are asked to play the role of an environmental detective who is trying to identify the most likely culprit in a groundwater contamination crime. The tools at your disposal are those generally available to hydrogeologists: maps, monitoring wells, and some historical records. So grab your spyglass, give Watson a call, and let s prove, again, that crime does not pay! Background In early September of 2010, a stream monitoring station was installed along Cripple Creek and detected elevated levels of Chlorides in the stream water. The location of this monitoring station, which is identified as station #54, is in Figure 9.1. Monitoring station #53, located several hundred meters upstream from #54, did not show any changes in water quality during this time. Based upon these observations, the Environmental Protection Agency (EPA) installed seven groundwater monitoring wells at the locations shown on the map. Water samples collected from these wells were analyzed for contamination. Three types of contamination were found: chloride, trichloroethylene (TCE), and vinyl chloride. The concentrations that were observed are listed in Table 1.

2 Water 1 page 2 Table 1- Concentrations Observed in Monitoring Wells Well# Chloride Concentration (mg/l) TCE Concentration (mg/l) Vinyl Chloride Concentration (mg/l) # #2 150 below detection below detection # #4 20 below detection below detection # # # below detection below detection TCE is a commonly used solvent that is used to dissolve oil and grease. It is used in machine shops, automotive repair businesses, and dry cleaners. Both TCE and vinyl chloride are suspected human carcinogens. Chloride is the same ion that is found in table salt, and sea water. It is not particularly dangerous to human health, but it adversely affects aquatic life and, at high levels, it can render a water supply unusable. The groundwater monitoring wells also provide information in the near-surface geology at the site, shown in Figure 9.2. The hydrogeology of the site consists of two aquifers: and upper unconfined or water table aquifer, and a lower confined aquifer. It appears that the contamination has remained in the unconfined aquifer. The lower confined aquifer has been protected by an aquitard. Geologic samples collected when the monitoring wells were installed were used to estimate the permeability and porosity of the unconfined aquifer. The permeability, the speed at which water flows through the sample, is equal to 400 meters per year and the porosity, which is the measure of the ratio of air space in the sample, is equal to 0.1. The depth to the water table at the site was estimated using surface geophysical techniques. Contours of the water table are included on the map. The slope of the water table is in the direction of Cripple Creek. The magnitude of the slope is approximately This means the elevation of the water table changes by about 1 meter over 100 meters of distance. There are six businesses that are potential sources for the groundwater contamination at the Cripple Creek site. These are identified on the map with lettered squares. Table 2 summarizes the year in which each business began their operations. Table 2 - Start-up dates for Businesses that are Potential Sources Business Start-Up Date A 1975 B 1990 C 1998 D 2001 E 2002 F 1991 G 1997

3 Water 1 page 3 Some Clues: Groundwater Flow and Contaminant Transport The following pieces of information may be useful as clues in identifying the guilty culprit in the Cripple Creek contamination case. (1.) In general, the direction of groundwater flow is perpendicular to the contours of water level elevations. Groundwater will flow from areas with higher water levels to areas with lower water levels. (2.) The speed or velocity of groundwater flow is given by Darcy s Law: V=K(i/n) Where: K= permeability or hydraulic conductivity n = porosity i = slope of the water table (3.) Contaminants will move in the same direction as groundwater flow. As groundwater contamination moves away from the source, the concentration will generally decrease. this decrease in concentration is caused by the mixing of contaminated and uncontaminated waters in the subsurface. This process is called dispersion. (4.) Some contaminants move more slowly than others. Contaminants that do not interact with porous media will move at the same speed as the groundwater. These types of contaminants are termed conservative. Chloride is a conservative contaminant. Nonconservative contaminants are those that interact with the porous media. These contaminants spend part of the time attached to the porous media and part of the time moving in the groundwater. TCE and vinyl chloride are nonconservative contaminants. At the Cripple Creek site, the velocity of the TCE and vinyl chloride is about 1/2 the groundwater velocity. (5.) Contaminants can be destroyed in the subsurface by microbiological activity. Small microorganisms such as bacteria will actually eat some types of contamination. Often times, this is a very positive benefit, because the microorganisms convert the contamination into harmless carbon dioxide gas. Other times, however, the by-products of this microbiological activity can be more hazardous that the original contaminant. For example, it has been shown the microorganisms can convert TCE into vinyl chloride. Vinyl chloride is more carcinogenic than TCE. The population of microorganisms will only develop once TCE is introduced into the groundwater system.

4 Water 1 page 4 Your Assignment Your assignment is to identify the most likely business that is responsible for the contamination. Assume that there is only one source of contamination that is continuous and that the responsibility for this contamination lies with a single business. Please outline, very briefly, the evidence that you use in making your case. Finally, if you were allowed one more monitoring well, where would you recommend it be placed? Answer on a separate piece of paper as if you are a consulting firm hired to determine the polluter. Your reasoning behind who is not the guilty party should also be in your report and is just as important as your reasons behind the guilty party.

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