Table 5-2-3 Division Standard of Redox Zone in Vejen Landfill, Denmark (unit: mg/L)
Fig. 5-2-2 Redox Zoning in Pollution Halo Downstream of Vejen Landfill in Denmark
5.2.2. 1 volatile organic compounds
Halogenated aliphatic compounds are the most common volatile organic pollutants in industrial fields. They are the most commonly used solvents in machine lubrication, so they are used in many industrial sectors. For example, in Coventry, England, an industrial equipment began to use trichloroethylene and related organic solvents in the 1930s (Bishop et al., 1993), and the pollution was caused by careless operation around the surface storage tank. In the past, when a tanker was used to fill the oil storage tank, once the oil storage tank was full, the driver would put the remaining oil in the hose on the ground. As time goes on, such a bad habit will leave a lot of solvents on the ground. The aquifer in the lower part of the site consists of a series of sandstone strata and shale interlayers. When the production well was polluted, people drilled several test wells on the spot. The test hole is exposed, so the packer sampler can be used to sample each sandstone layer in the hole. However, the use of bare holes proved to be a mistake, because the concentration of pollutants in production wells increased to a certain extent, Bishop et al. (1993) speculated that pollutants moved to lower aquifers through bare holes. Because the aquifer is a dual medium system, pores and cracks exist at the same time, and pollutants will enter a single pore through cracks. If the pollutants are finally removed from the high permeability flow path, the diffusion process will be reversed, and the pollutants in a single pore will be transported to the main flow path, which will affect the aquifer by organic solvents for a long time to come.
In the literature, industrial waste sites related to a special industry are more common, which contain more specific compounds. Goerlitzer( 1992) describes six sites studied by the Organic Matter Project of the US Geological Survey, in which one site contains chlorinated hydrocarbons used to produce pesticides, two sites contain explosive wastes, and the other three sites are timber storage sites. A very interesting phenomenon is the migration difference of pentachlorophenol (PCP) between two wood storage sites. Wood tar and pentachlorophenol are commonly used to treat wood poles. Wood tar is a mixture of more than 200 compounds extracted from coal tar, of which 85% are polycyclic aromatic hydrocarbons, 12% are phenolic compounds, and 3% are nitrogen, sulfur and oxygen heterocyclic compounds (Goerlitz, 1992). Because of its high octanol/water partition coefficient, pentachlorophenol is not expected to migrate far in groundwater. However, this proved to be incorrect in Victoria Cella, California. Since it was put into use in the early 1950s, PCP has moved 500 meters from leaking storage tanks to groundwater (Figure 5-2-3). Polycyclic aromatic hydrocarbons, mainly naphthalene and methylnaphthalene, also moved the same distance as PCP. Phenolic compounds and heterocyclic compounds were not detected in groundwater, and there were water-insoluble liquids on the water surface near the pollution source.
Fig. 5-2-3 Isogram of Pentachlorophenol Concentration in Underground Water of Visalia Wood Treatment Site in California (1autumn of 976)
Pensacola Wood Treatment Plant in Florida uses the same compounds as Visalia site, but the ultimate fate and migration of pollutants are completely different. Pentachlorophenol in this site is relatively stable, but the concentration of phenolic compounds and nitrogen heterocyclic compounds is very high. An important difference between these two locations is the difference in pH value. The pH value of groundwater in Visalia site is 7.9 ~ 8.6, while that in Pensacola site is 5.0 ~ 6.3. Goerlitz( 1992) thinks that the solubility of pentachlorophenol is much less at low pH value, so it is only detected at the pollution source of Peng Sacola site. However, under the condition of high pH value in Visalia site, PCP is more soluble, and there is no obvious adsorption and biodegradation during migration. The difference of biodegradation between the two places may also affect the different behaviors of pollutants.
Heavy metals in 5.2.2.2
Another common component in industrial waste is heavy metal, and its content in leachate may be many times higher than that in natural groundwater. The limited trace metal elements in drinking water standards mainly include chromium, lead, copper, silver, cadmium, zinc, nickel and mercury. In order to accurately predict the migration of these elements, it is first necessary to analyze their equilibrium distribution according to the redox potential. Most metal elements have different oxidation states, so it is necessary to understand the geochemical characteristics of pollution halos in order to predict the equilibrium distribution of metals.
Chromium is a metal, which will cause serious groundwater pollution. Under oxidation conditions, chromium exists in chromate in the form of hexavalent chromium (Figure 5-2-4). At this time, the migration ability of chromium is very strong. The first reported halo of chromium pollution in groundwater is caused by chromium electroplating waste in Long Island, new york (Perlmutter and Lieber, 1970). The surface material of Long Island is permeable glacial sediment, and there are shallow and deep aquifers under it. Groundwater is the main drinking water source in this area. With the continuous expansion of urbanization in this area, wastewater is often discharged into the ground through surface depressions, septic tanks and rainwater retention ponds. During World War II, a chromium alloy electroplating waste storage tank was built for an aircraft factory in this area. In the downstream direction, the pollution halo of chromium and cadmium in groundwater extends about 1300 m and is discharged into the river (Figure 5-2-5).
Fig. 5-2-pE-pH diagram of 4cr-o-H2O system at 25℃ and 1 atm (solubility is defined as chromium activity equals 10-6).
Henderson( 1994) studied the pollution halo of chromate in the aquifer near Odessa, Texas, USA. During the 6-year monitoring period, the maximum concentration of hexavalent chromium in pollution halo decreased by 10 times. According to the volume of pollution halo, the total amount of dissolved chromium can be estimated. By measuring the adsorption partition coefficient (KD) of Cr6+, Henderson also estimated the adsorption capacity of Cr6+. It was found that the total amount of hexavalent chromium in the aquifer (the sum of Cr6+ content in solution and solid phase) decreased in a first-order reaction (Figure 5-2-6). In order to explain the decrease of hexavalent chromium content in aquifer, Henderson plotted the measured Eh value of water sample into an Eh-pH diagram of chromium (Figure 5-2-7). It can be seen that most of the points are located in the stable region of Cr3+. At this time, the hexavalent chromium in the aquifer will be reduced to trivalent chromium, and will be precipitated or adsorbed on the surface of oxide or Fe(OH)3. The reaction formula is:
hydrogeochemistry
Fig. 5-2-5 Halo of chromium pollution caused by electroplating waste in Long Island, new york (according to Perlmutter and Lieber, 1970).
Figure 5-2-6 Cr6+ content in Trinity sandstone aquifer in Texas decreases due to decrease.
The bivalent iron or dissolved organic carbon in the aquifer is reduced by crops, and trivalent chromium is adsorbed on the surface of hydroxide. Referring to the concept of half-life of the first-order reaction model, Henderson (1994) can predict when the concentration of hexavalent chromium will fall below the drinking water standard. This is an example of purifying aquifer pollution through natural attenuation. In order to actively control aquifer pollution artificially, it is necessary to deeply understand the geochemical environment of aquifers.
Status of organics and metals in 5.2.2.3.
The most complicated geochemical problem occurs when organic matter and metal react with each other in industrial waste disposal sites, and Davis et al. (1994) give a good example to illustrate this situation. The factory is located in Woburn, Massachusetts, and has been engaged in leather processing and coloring since 1927. Previously, since 1853, the site has also produced pesticides containing arsenic and lead, and hundreds of tanneries are located in the basin where the site is located.
See Figure 5-2-8 for the geochemical reaction and process of the site. Leachate rich in organic matter released from fur piles forms a reductive pollution halo very similar to landfill, in which arsenic and chromium come from pesticides and tanning processes respectively. In the reductive pollution halo, the mobility of arsenic and chromium is not expected to be very strong, but the actual situation is just the opposite. Davis et al. (1994) think that organic complexes containing arsenic and chromium greatly enhance their mobility. Figure 5-2-8 shows hypothetical chemical reactions with letters. Arsenic (a-b-c-d sequence) reacts with methane in polluted halos to generate easily-migrated compounds-methyl arsenic acid (MMAA, CH3AsO(OH)2) and dimethyl arsenic acid (DMAA, (CH3)2AsO(OH)). Among these compounds, arsenic is+V. With the pollution halo moving to the area with higher oxidation degree downstream of the fur pile, MMAA and DMAA are demethylated, and arsenic is also reduced to trivalent arsenic near the discharge tank.
Fig. 5-2-7 Eh-pH diagram of chromium
Figure 5-2-8 Schematic diagram of groundwater flow system and chemical reaction in the lower part of Woburn leather manufacturing site in Massachusetts.
The chemical reactions affecting chromium are shown in e-f-g sequence in Figure 5-2-8. Hexavalent chromium is used in the coloring process, and it is reduced to trivalent chromium in the pollution halo at the lower part of the reduced pile. As mentioned above, this process will precipitate chromium. However, in the presence of organic acids, trivalent chromium will combine with it to form soluble complexes, thus enhancing the mobility of trivalent chromium and transporting it to the drainage pond.
Other chemical reactions have occurred in the pollution halo, such as the precipitation reaction (H) of gypsum in supersaturated state, which comes from the oxidation of sulfide in pyrite, and Ca2+ comes from the dissolution of calcium carbonate. In addition, a part of it may be reduced to sulfide and combined with iron to form amorphous ferrous sulfide precipitate (I). The above process shows the complexity of geochemical reaction in the pollution halo of mixed waste sources, and also shows that in order to fully describe the characteristics of pollution halo, it is necessary to analyze a large number of compounds, so as to fully understand the geochemical control process of the migration of various components.