Division of Engineering and the Built Environment - Essay Example

Faculty of Arts, Design & Technology Division of Engineering and the Built Environment H291 - BEng (Hons) Civil and Infrastructure Engineering Geotechnics and Environmental Engineering Contamination and Remediation Submitted by : Your name here... Student number: Your student number here... Submitted to : Instructor's name here... Submission date: 10 December 2008 Introduction Contaminated sediments have been singled out as a major environmental concern in many areas. Persistant toxic substances that have accumulated in sediments from various sources are a threat to the survival and health of bottom living organisms and their consumers, and can also impair the quality of the surrounding water. Lack of adequate guidelines for evaluation of the significance of contaminated sediments creates problems for environmental managers. Groundwater is water that is found underground in cracks and spaces in soil, sand and rocks. The area where water fills these spaces is called the saturated zone. The top of this zone is called the water table. The water table may be only a foot below the ground's surface or it may be hundreds of feet down. Groundwater can be found almost everywhere. The water table may be deep or shallow; and may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise, or an extended period of dry weather may cause the water table to fall. Groundwater is also one of our most important sources of irrigation water. Unfortunately, groundwater is susceptible to pollutants. Groundwater is generally a safe source of drinking water; however, there are concerns that contamination may increase as toxins dumped on the ground in the past make their way into groundwater supplies. Pollutants that contaminate groundwater may be some of the same pollutants that contaminate surface water. Compounds from the surface can move through the soil and end up in the groundwater. For example, pesticides and fertilizers can find their way into groundwater supplies over time. Road salt, toxic substances from mining sites, and used motor oil also may seep into groundwater. In addition, it is possible for untreated waste from septic tanks and toxic chemicals from underground storage tanks to contaminate groundwater. Drinking contaminated groundwater can have serious health effects. Contaminated site poses considerable risk to human health and the environment also contaminated soil functions are endangered because of impacting contaminant levels (Spalton 2008). Diseases such as hepatitis and dysentery may be caused by contamination from septic tank waste. Poisoning may be caused by toxins that have leached into well water supplies. And it is important not to forget that wildlife, too, can be harmed by contaminated groundwater. Deterioration of soil quality, loss of soil functions and water resources degradation should be avoided as prevention and remediation is to restore land use and soil functions (or natural background levels) (Spalton 2008). Laboratory, soil and water, rock, chemical (for effect on construction materials), contamination (Analytical - pH, organics, inorganics, metals, asbestos) tests are applied in order to define the current pollution situation of the specified site and for applying the proper remediation methodology (Spalton 2008). Contaminated Land Exposure Assessment (CLEA) Model Contaminated Land Exposure Assessment (CLEA) Model describes the conceptual exposure models for each standard land-use. CLEA model is shown in Figure 1. Figure 1. Illustration of potential exposure pathways in CLEA model (Spalton 2008) Remediation of Soil and Contaminated Groundwater Remediation, in this sense, is a broad term that refers to the reduction of risk caused by exposure to contaminated groundwater. Although there were various reasons for the failure of the systems to meet their goals, the data strongly suggest that, for most circumstances, best available technologies are not capable of restoring groundwater to its original quality. However, returning groundwater to its useful purpose or eliminating health and environmental risks does not always require restoration to original quality. Accordingly, a growing number of remediation systems are being designed with the goal of either containment or remediation to health-based standards. A typical approach for achieving this remediation goal is, first, to estimate the risk associated with exposure to the contaminated groundwater and then design a remediation system that will improve groundwater quality to a level that is protective of human health and the environment. After a remediation goal has been selected, a remedy must be implemented to achieve the goal. In general, groundwater can be remediated in one of the following three manners. 1. Containment of a plume of contaminated groundwater involves preventing the plume from migrating to a location where receptors can be exposed to it. 2. For some remediation approaches, groundwater must be extracted from the ground, either to allow ex situ treatment or to provide hydraulic control of contaminated groundwater. 3. Contaminated groundwater can be treated, either in situ or ex situ, using a variety of physical, chemical, or biological methods. One of the main problems associated with ex situ remediation is its high operating cost for activities like soil excavation and groundwater pumping. In recent years, in situ techniques have become popular. Bioventing is a used to stimulate in situ biodegradation of aerobically degradable contaminants in soil by providing oxygen to existing soil microorganisms (Chen et al. 2003). The first step in the design of the groundwater remediation system is to define the problem in adequate detail to allow the design of an efficient and cost-effective groundwater remediation system. The problem should be defined in terms of the extent of contamination, the risk associated with the contaminated groundwater, the regulations that apply to the problem, the degree of remediation required, and the subsurface conditions. If the vertical and horizontal extents of contamination have not been adequately evaluated during the site exploration or groundwater monitoring program, then additional field exploration data should be collected to complete this evaluation. The various manners in which groundwater can become contaminated are summarized in Figure 2. Figure 2. Groundwater contamination by dissolved vapor and nonaqueous phase constituents (Houlihan & Lucia 1999) Beyond doing nothing, seal and dig and dump there are some other important remediation practices as mentioned in the following. Solidification/Stabilization (S/S) Solidification/Stabilization (S/S) Involves mixing portland cement into contaminated media such as soil, sediment, sludge or industrial waste. S/S treatment protects human health and the environment by immobilizing hazardous constituents within treated material. For Genzyme Building construction site, the challenge was Coal tar residue behaving as A dense nonaqueous phase liquid (DNAPL) and Light Non-Aqueous Phase Liquid (LNAPL) (See Figure 3). As a solution In-situ S/S treatment for risk based closure treatment to 6 meters depth by auger for 3.3 acres. Cement-based remediation technology and concrete can help in achieving Leadership in Energy and Environmental Design (LEED) certification or increasing the certification level of a building/development (See Figure 4). Figure 3. Genzyme Building, Cambridge, MA, 2001 (OBOA 2007) Figure 4. US National Brownfield Award, LEED Platinum Certified (OBOA 2007) Screening (Grading/Sieving) This involves using large plant on site and passing material through the grader (Spalton 2008). Excavation and Soil Removal Excavation and soil removal is one of the most common activities in groundwater remediation and cleanup. Excavation involves removing contaminated soil from the unsaturated zone to prevent further groundwater contamination by the residuals present in that zone (Chae & Hamidi 1999). Grouting This is also known as soil mixing, the basic requirement is to add grout mortar to a soil in order that it becomes solid and inert (Spalton 2008). Chemical reagents According to Spalton (2008), chemical reagents are used as a pre or post-treatment process. To complete the mineralization of the target contaminants eg: PolyChlorimated Biphenyls (PCBs) NaCl + CO2 + H2O. Chemical treatment techniques can be listed as substitution, oxidation and precipitation, respectively. Soil washing Spalton (2008) indicates that the soil washing is used to remove contaminants from soils by using water. The principle of soil washing lies in separating the most polluted portion from the cleaner portion of the soil by scrubbing it and scrubbing process reduces the amount that needs to be cleaned, and we get two fractions of soil; clean and polluted (Spalton 2008). Separation Seperation is another alternative remediation process. The flowchart of separation is shown in Figure 5. Figure 5. Separation flowchart (Spalton 2008) Thermal treatment According to Spalton (2008) the temperatures vary but the science is quite simple. Raise the temperature high enough to burn of chemicals or create extractable vapor. Issues come from the collection of the gases and high energy inputs. Drill cuttings generated during borehole excavation have been routinely dumped into the Sea, where they now represent a significant environmental hazard owing to contamination by oil residues and heavy metals. An alternative method of removing oil from the water surface is in situ burning. This process offers the advantage that the oil is rapidly removed from the surface of the water, thus preventing shoreline damage (Cohen et al. 2001; Eames et al. 2002). Using Crops For all mass flux balances copper inputs were found to exceed the export with crops. Remediated agricultural land should be protected from further metal accumulation by controlling the relevant copper pathways. These are also copper pesticides for special crops and pig manure. Decisions to remediate the soil should take the uncertainty of mass flux balances both on field scale as well as on regional scale into account (von Steiger et al. 1998). Using Sorption Previous studies indicate the high sorptive capacities of humic materials (such as peat) for petroleum hydrocarbons, and peats with low fiber contents, high lignin pyrolysis material and ash content have been shown to be the most effective peats to remove free and dissolved phase petroleum hydrocarbon from groundwater. The ability to both adsorb dissolved and free phase petroleum hydrocarbons from groundwater, and to provide a catalytic surface on which microbial activity can occur, indicated the suitability of peat for use in groundwater treatment (Guerin 2001). More studies should be undertaken to assess the metal concentrations detected in the water and sediment, and possible accumulation in biota or vegetation. This can provide valuable information to address the question of metal toxicity (Jeng & Hong 2005). The sorption mechanisms are variable in the reactions between the apatite and dissolved Pb, Cd, and Zn. The Pb removals primarily resulted from the dissolution of the apatite followed by the precipitation of hydroxyl fluoropyromorphite. Minor otavite precipitation was observed in the interaction of the apatite with aqueous Cd, but other sorption mechanisms, such as surface complexation, ion exchange, and the formation of amorphous solids, are primarily responsible for the removal of Zn and Cd (Chen et al. 1997). Metals and organic micro pollutants frequently show a strong particle affinity. As a consequence sediments in the vicinity of a discharge point will characteristically contain high levels of contaminants. Often, very little attention is paid to these sediments as they are frequently excluded from sediment monitoring programs as anomalies (Skei 1992). For certain trace elements like Cu, Pb and Zn adsorption on minerals surfaces like in quartz seems to be a valuable mechanisms for the attenuation of toxic elements from the mine-tailings porewater even the surface immobilization of trace elements by pyrite is much higher than for quartz (Muller et al. 2003). Mineral dissolution is the key process controlling Cr(VI) concentration. It is only valid at very high pH and that, as the pH decreases, other processes, such as co-precipitation, play an increasingly important role (Geelhoed et al. 2001). Bioremediation Bioremediation can be divided into several parts; Aerobic, where oxygen in some form is added to the treatment environment and Anaerobic, where nitrate, iron, or other electron acceptor is added to the treatment environment. Combinations are also exists, where a combination of the above is used, often in pulses. Bioremediation is also applied as in situ Bioremediation (ISB) or enhanced bioremediation, natural bioremediation, biopiles, bioreactors, bioventing/biosparging, engineered treatment cells. Using Bacteria and Microorganisms Microorganisms play an important role as particulate contaminants in groundwater systems. Important categories of microbial groundwater contaminants include viruses, bacteria, and protozoan cysts. Based on the arbitrary definition given above, viruses are representative of the smallest particles which are likely to be present in groundwater systems. Their small size facilitates mobility in groundwater systems; however, even though viruses are smaller than the pore size openings common to groundwater systems, their movement in aquifers is not unimpeded by soil particles. Like all constituents in groundwater systems, movement is largely governed by association with soil grains. Viruses display surface characteristics (large surface: volume ratio, net surface charge) which allow them to associate with soil grains. In the general case, groundwater supplies are assumed to be subject to possible contamination by pathogenic viruses, and disinfection processes for public water supplies are designed to address this source of contamination. Microorganisms from the other categories (e.g., bacteria and protozoan cysts) will also move in groundwater systems; however, the larger size of microorganisms from these categories dictates that their movement will be generally slower than that of viruses (Blachley & Thompson 1999). Shakoori & Muneer (2002) applied a case study in which six copper-resistant bacterial strains were isolated from wastewater of tanneries of Kasur and Rohi Nala. The isolates were observed to be very efficient in cleaning the copper polluted environment provided optimum growth conditions were ensured. Remediation of Radionuclide Contaminated Sites Remediation of radionuclide contaminated sites can be a costly and difficult endeavor. The chemical behavior of an atom in the environment is dictated by its atomic structure (not nuclear structure). Therefore, different isotopes of the same element behave identically in terms of chemical reactions. The essential difference between isotopes is that unstable elements will undergo radioactive decay. A fundamental difference between radionuclide and inorganic wastes is that radionuclides do not require direct contact to cause damage to an organism; radiation provides its own transport mechanism to reach a target. Because of this, providing for public and worker safety at a site is often more difficult than for conventional pollutants. It is sometimes possible to use the unstable characteristics of radionuclides to facilitate remediation. For radionuclides with short half-lives, it may only be necessary to contain the waste for an appropriate period of time before the unstable nuclei have decayed below the background radiation. In this manner, it is possible to use the decay properties of the nucleus to facilitate safe disposal of some radionuclide wastes. Air Sparging Air sparging involves introducing air into the aquifer and allowing the air to rise through contaminated groundwater to the ground surface (See Figure 6). Because sparging introduces oxygen into the subsurface, it stimulates aerobic biodegradation; therefore, the technique is sometimes also referred to as "bioventing" to convey its value to the bioremediation processes. In general, the physics of air movement in the saturated zone are currently not well understood, which limits the predictability of air sparge system performance; in fact, the decisions regarding sparge well location and spacing are usually based on experience rather than calculation. Air sparging can be a cost-effective approach to groundwater remediation. As presently understood, air discharged to groundwater by a sparge well typically flows upward to the vadose zone through air channels that are established at first operation of the system. These air channels directly impact only a very small quantity of groundwater and soil. Contaminants partition between the sparge air, groundwater, and soils; the portion that partitions to vapor in the sparge air rises with the sparge air to the ground surface. For air sparging to be effective, adsorbed contaminant mass must be transported by diffusion from the surface of soils through the groundwater to the air-water interface of the air channels, which is a slow process. Figure 6. Air sparging in stratified sands for site remediation (Houlihan & Lucia 1999) Groundwater is one of the most important resources that nature has given us. We should do our best to avoid all the harm to be given to the groundwater because the human nature won't be healthy at all unless the groundwater is clean. References Blatchley, E. R. and Thompson, J. E. 1999, 'Groundwater Contaminants' in The Handbook of Groundwater Engineering, editor in chief J. W. Delleur, CRC Press, Boca Raton, FL, USA. Chae, Y. S. and Hamidi, A. 1999, 'Groundwater and Surface Water Pollution' in EnvironmentalEngineers' Handbook, 2nd edn, editor in chief I. Liu, CRC Press LLC, Boca Raton, FL, USA. Chen, X., Wright, J. V., Conca, J. L. and Peurrung, L. M. 1997, 'Evaluation of heavy metal remediation using mineral apatite', Water, Air, and Soil Pollution, vol. 98, pp. 57-78. Chen, Z., Huang, G. H., Chan, C. W., Geng, L. Q., and Xia, J. 2003, 'Development of an expert system for the remediation of petroleum-contaminated sites', Environmental modeling and assessment, vol. 8, pp. 323-334. Cohen, A. M., Nugegoda, D. and Gagnon, M. M. 2001, 'The effect of different oil spill remediation techniques on petroleum hydrocarbon elimination in Australian bass (Macquaria novemaculeata)', Archives of environmental contamination and toxicology, vol. 40, no. 2, pp. 264-270. Eames, I., de Leeuw, B. and Conniff, P. 2002, 'Formation and remediation of drill-cutting piles in the North Sea', Environmental geology, vol. 41, pp. 504-519. Geelhoed, J. S., Meeussen, J. C. L., Lumsdon, D. G., Hillier, S., Roe, M. J., Thomas, R. P., Bewley, R. J. F., Farmer, J. G. and Paterson, E. 2001, 'Modelling of chromium behaviour and transport at sites contaminated with chromite ore processing residue: Implications for remediation methods', Environmental Geochemistry and Health, vol. 23, no. 3, pp. 261-265. Guerin, T. F. 2001, 'Commercial activities in remediation of soil and sediments and trends in the asia pacific region', J. Soils & Sediments, vol. 1, no. 1, pp. 50-57. Houlihan, M. F. and Lucia, P. C. 1999, 'Remediation of Contaminated Groundwater' in The Handbook of Groundwater Engineering, editor in chief J. W. Delleur, CRC Press, Boca Raton, FL, USA.. Jeng, H. and Hong, Y. J. 2005, 'Assessment of a natural wetland for use in wastewater remediation', Environmental Monitoring and Assessment, vol. 111, pp. 113-131. Muller, B., Axelsson, M. D. and Ohlander, B. 2003, 'Analyses of trace elements on quartz surfaces in sulfidic mine tailings from Kristineberg (Sweden) a few years after remediation', Environmental Geology, vol. 45, pp. 98-105. OBOA (Ontario Building Official Association) 2007, Role of Solidification/Stabilization in Sustainable Development of Contaminated Brownfield Sites, Available at: http://www.oboa.on.ca/files//Cement%20Assoc-CDickson-Brownfield&Sustainable%20Devel..pdf Shakoori, A. R. and Muneer, B. 2002, 'Copper-Resistant Bacteria from Industrial Effluents and Their Role in Remediation of Heavy Metals in Wastewater', Folia Microbiol., vol. 47, no. 1, pp. 43-50. Skei, J. M. 1992, 'review of assessment and remediation strategies for hot spot sediments', Hydrobiologia, vol. 235/236, pp. 29-638. Spalton, D., 2008. Geotechnics and Environmental Engineering module lecture support notes. Beng (Hons) Civil and Infrastructure Engineering. University of Derby. Vangronsveld, J. and van der Lelie, D. 2003, 'The use of alternative techniques for remediation of polluted sites and groundwater in flanders: Plant-based strategies', Soils & Sediments, vol. 3, no. 4, pp. 250-251. von Steiger, B., Keller, A. and Schulin, R. 1998, 'Regional mass flux balancing for controlling gentle soil remediation operations', Nutrient Cycling in Agroecosystems, vol. 50, pp. 303-306. Read More