Geoenvironmental Engineering - Essay Example

GEOENVIRONMENTAL ENGINEERING WORK Introduction Contamination of aquifers has become a great concern in many countries where groundwater resources is the major source for drinking water supply. Currently, the ground water contaminants that are of greatest concern are synthetic compounds. These man-made contaminants are usually divided into organic substances, i.e., compounds based on carbon, and inorganic substances, which are not based on carbon. Organic contaminants can reach the groundwater zone either dissolved in water or as organic liquid phases that may be immiscible in water. Sources of contamination include: gasworks, coal gasification plants, refineries, abandoned industrial areas, storage tank areas, military areas, bus-stations and airports. Seepage of gasoline and other petroleum-derived fuels from these facilities is one of the major sources of groundwater contamination (Shevah and Waldman, 1995). Dissolved contaminants can result from spills or leaks of aqueous solutions or from the leaching of solid phases or immiscible organic liquids present in the vadose zone or land disposal areas. Organic liquids can be introduced to the subsurface by spills, leaks, or intentional disposal. The subsurface transport of immiscible organic liquids is governed by a set of factors different from those for dissolved contaminants (Mackay, et al, 1985). List of organic contaminants in ground water Synthetic Organic Contaminants, including pesticides & herbicides 2,4-D 2,4,5-TP (Silvex) Acrylamide Alachlor Atrazine  Benzoapyrene Carbofuran Chlordane Dalapon Di 2-ethylhexyl adipate Di 2-ethylhexyl phthalate Dibromochloropropane Dinoseb Dioxin (2,3,7,8-TCDD) Diquat Endothall Endrin Epichlorohydrin Ethylene dibromide Glyphosate Heptachlor Heptachlor epoxide Hexachlorobenzene  Hexachlorocyclopentadiene Lindale Methoxychlor  Oxamyl [Vydate] PCBs [Polychlorinated biphenyls]  Pentachlorophenol  Picloram  Simazine Toxaphene  Volatile Organic Contaminants Benzene Carbon Tetrachloride Chlorobenzene  o-Dichlorobenzene p-Dichlorobenzene  1,1-Dichloroethylene cis-1,2-Dichloroethylene trans-1,2-Dicholoroethylene Dichloromethane 1,2-Dichloroethane 1,2-Dichloropropane Ethylbenzene Styrene Tetrachloroethylene 1,2,4-Trichlorobenzene 1,1,1,-Trichloroethane  1,1,2-Trichloroethane Trichloroethylene  Toluene Vinyl Chloride Xylenes  Source: EPA, 2006 Many other contaminants not synthesized by man are also of concern. These include various naturally occurring elements, e.g., arsenic and radionuclides, and microbiological contaminants. Microbiological contaminants include bacteria, viruses, and parasites such as Cryptosporidium. For example, it is estimated that at least 65,000 synthetic organic chemicals are in common use in the U.S. today, and this number continues to grow each year. Organic chemicals have become a more frequently detected contaminant in ground water supplies. The 1984 OTA report listed 175 different organic compounds that have been found in ground water, and EPA ground water surveys conducted over the last decade confirm the widespread occurrence of organic contaminants (EPA, 1998). Overview of the biodegradation process Bioremediation provides cost-effective, contaminant- and substrate-specific treatments equally successful in reducing the concentrations of single compounds or mixtures of biodegradable materials. In-situ groundwater bioremediation is a technology that encourages growth and reproduction of indigenous microorganisms to enhance biodegradation of organic constituents in the saturated zone. In-situ groundwater bioremediation can be effective for the full range of petroleum hydrocarbons. In-situ bioremediation of groundwater can be combined with other saturated zone remedial technologies (e.g., air sparging) and vadose zone remedial operations (e.g., soil vapour extraction, bioventing). The key parameters that determine the effectiveness of In-situ groundwater bioremediation are: hydraulic conductivity of the aquifer, which controls the distribution of electron acceptors and nutrients in the subsurface; biodegradability of the petroleum constituents, which determines both the rate and degree to which constituents will be degraded by microorganisms; and location of petroleum contamination in the subsurface. Contaminants must be dissolved in groundwater or adsorbed onto more permeable sediments within the aquifer (EPA, 2006). Biodegradation process of groundwater has been successfully carried out in various parts of the world by different organizations. For instance, studies show that the former manufacturing site in New Jersey, the manufacturing site in Ohio, the industrial facility located in the Blue Ridge physiographic province of Virginia, etc. has utilized biodegradation process to decontaminate groundwater (Borum, 2002). References Borum, E. (2002) Bioremediation of Chlorinated Solvents in Fractured Bedrock: Characterization and Case Studies. Retrieved December 1, 2006, from http://cluin.org/download/studentpapers/nnems_borum.pdf EPA, (1998) Groundwater contamination in United States, Retrieved December 1, 2006, from http://www.purdue.edu/dp/envirosoft/groundwater/src/overview.htm EPA, (2006) In-Situ Groundwater Bioremediation, Retrieved December 1, 2006, from http://www.epa.gov/OUST/cat/insitbio.htm Mackay, D.M., Roberts, P.V. and Cherry, J.A. (1985) Transport of organic contaminants in groundwater. Environ. Sci. Technol., Vol. 19, No. 5. pp 384. Shevah, Y. and Waldman, M. (1995) In-Situ and On-Site Treatment of Groundwater. Pure & Appl. Chern., Vol. 67, Nos 8/9, pp. 1549-1561. GEOENVIRONMENTAL ENGINEERING COURSEWORK: 2 Introduction Groundwater, under most conditions, is safer and more reliable for use than surface water. However once groundwater is contaminated, it is an extremely costly operation to decontaminate. Any chemicals that are easily soluble and penetrate the soil are prime candidates for groundwater pollutants. Groundwater contamination by petroleum products and organic solvents is a serious problem in both developed and developing countries. Underground petroleum storage tanks account for a large portion of the problem. Groundwater contamination occurs when man-made products such as gasoline, oil, road salts and chemicals get into the groundwater and cause it to become unsafe and unfit for human use. Some of the major sources of these contaminants are storage tanks, septic systems, pipeline leaks, surface spills, hazardous waste sites, landfills, and lagoons and the widespread use of road salts and chemicals. Various organic contaminants can be found in a contaminated subsurface zone, including the groundwater and soil, such as hydrocarbon compounds including volatile organic compounds (VOCs), semi-volatile organic compounds, and non-volatile materials, and the like. Contaminants can exist in subsurface soil and in groundwater, below the water table, in various phases as discrete substances and mixed with and/or dissolved in groundwater and soil gases. Such contaminants can occur in the vapour phase in the vadose (unsaturated) zone, in the free (separate) phase floating on top of the groundwater (or dense non-aqueous phase liquid (DNAPL) at the base of an aquifer), dissolved phase in the groundwater, and in the absorbed phase in the unsaturated (vadose) zone and saturated groundwater zone below the water table (Athens and Goodrich, 2000). List of Synthetic Organic Contaminants, including pesticides & herbicides 2,4-Dichlorophenoxyacetic acid (2,4,D) 2 (2,4,5-Trichlorophenoxy) propionic acid (2,4,5,T TP) (Silvex) 2-propenamide (Acrylamide) 2-chloro-2’,6’-diethyl-N-methoxymethylacetanilide (Alachlor) 2-chloro-4-(ethylamine)-6-(isopropylamine)-s-triazine (Atrazine) Benzo[a]pyrene 2,3-Dihydro-2,2-dimethyl-7-benzofuranol methylcarbamate (Carbofuran) 1,2,4,5,6,7,8,8-octachloro-2,3,3a,4,7,7a-hexahydro-4,7-methanoindene [9] (Chlordane) Dalapon Di 2-ethylhexyl adipate Di 2-ethylhexyl phthalate Dibromochloropropane (DBCP) 2-(sec-butyl)-4,6-dinitrophenol [1] (Dinoseb) 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Dioxin) 1,1-ethylene-2,2-bipyridyldiylium dibromide salt [1] (Diquat) 3,6-endoxohexahydrophthalic acid (Endothall) Endrin Epichlorohydrin 1,2-dibromoethane [53] (Ethylene dibromide) N-(phosphonomethyl) glycine [1] (Glyphosate) 1,4,5,6,7,8,8-heptachloro-3a,4,7,7a-tetrahydro-4,7-methanoindene [9] (Heptachlor ) Heptachlor epoxide hexachlorobenzene [9] (Hexachlorobenzene) Hexachlorocyclopentadiene gamma-1,2,3,4,5,6-hexachlorocyclohexane [9] (Lindane) 1,1,1-trichloro-2,2-bis(4-methoxyphenyl)ethane [9] (Methoxychlor) N,N-dimethyl-2-methylcarbamoyloxyimino-2-(methylthio)-acetamide [10] (Oxamyl) PCBs [Polychlorinated biphenyls]  pentachlorophenol [9] (Pentachlorophenol ) 4-amino-3,5,6-trichloropyridine-2-carboxylic acid [1] (Picloram) 6-chloro-N2,N4-diethyl-1,3,5-triazine-2,4-diamine [6] (Simazine) Toxaphene  List of Volatile Organic Contaminants Benzene Carbon Tetrachloride Chlorobenzene  o-Dichlorobenzene p-Dichlorobenzene  1,1-Dichloroethylene cis-1,2-Dichloroethylene trans-1,2-Dicholoroethylene Dichloromethane 1,2-Dichloroethane 1,2-Dichloropropane Ethylbenzene Styrene Tetrachloroethylene 1,2,4-Trichlorobenzene 1,1,1,-Trichloroethane  1,1,2-Trichloroethane Trichloroethylene  Toluene Vinyl Chloride Xylenes  Overview of Soil Vapour Extraction Technique Soil Vapour Extraction (SVE) is one of the most effective and cost-efficient methods of removing VOCs from unsaturated soils. An SVE system consists of one or more extraction wells screened in the unsaturated zone, blowers or vacuum pumps, and often also includes air injection or pressure venting wells, a low permeability cap at the ground surface, an air/water separator, and an offgas treatment system (US Army Corps of Engineers, 2002). Vacuum extraction techniques have also been proposed for removing VOCs from soil. Known soil vapour vacuum extraction provides an apparatus and process by which volatile vapours may be extracted from the soil through subsurface vacuum application above the resting water table, thereby extracting vapours present in the vadose zone. However, there are many problems associated with the soil vapour vacuum extraction process. When a single tube extraction well or well casing is disposed to a depth below the water table surface and a vacuum is applied at the unsubmerged end of the well casing, the resting water table surface can become dynamic resulting in the water table surface moving upward in the area adjacent the well casing, essentially forming a peak and resulting in "upwelling." This upwelling can cause floating contaminants just above the water table surface (in the capillary fringe where up to 80 or 90 percent of the contaminants can reside) to float or gravitate away from the well casing, resulting in a less treatable capillary fringe and the undesired consequence of migration of the floating contamination. Further, the upward movement of the water table surface adjacent the well casing can effectively "seal" the extraction well and prevent the well casing from ingesting the air necessary to maintain the vacuum for extraction (Athens and Goodrich, 2000). Soil vapour extraction (SVE) has been shown to be effective at removing hydrocarbons from the unsaturated zone. However, at many spill sites significant fractions of the mass are at or below the water table, in which case SVE is far less effective. To improve its efficiency in cases where gasoline is trapped below the water table, SVE can be used in conjunction with other techniques to get at that trapped mass. In the last few years the direct injection of air into the formation below the water table (i.e., in situ sparging) has become a popular technique. Another approach is to lower the water table to improve air flow in the vicinity of the trapped product. This can be accomplished either in the localized area of a ground water draw down cone or as the result of larger scale dewatering. Experiments to date have examined SVE operating as a stand-alone technique, as well as in conjunction with air sparging below the water table, dewatering of the "smear zone" (i.e., where product is trapped as residual below the water table), and air injection into the dewatered smear zone (Johnson, et.al, 1992). References Athens, N. and Goodrich, D.M. (2000) Soil and groundwater decontamination system with vacuum extraction. Retrieved December 1, 2006, from http://www.freepatentsonline.com/6158924.html Johnson, R.L et.al, (1992) Experimental Examination of Integrated Soil Vapor Extraction Techniques: Published in Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Groundwater: Prevention, Detection, and Restoration, p. 441-452. US Army Corps of Engineers, (2002) Soil Vapor Extraction and Bioventing, Retrieved December 1, 2006, from http://www.lwrri.lsu.edu/downloads/JohnPardue_Class/basdoc.pdf Read More