EnvironmentalMedia

EnvironmentalMedia - HPR-EB-Ul THU 14:58 BREN UCSB FHX N0....

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Unformatted text preview: HPR-EB-Ul THU 14:58 BREN UCSB FHX N0. 8058937812 . Ul CHAPTER 4 The Nature of Environmental Media 1 4-1 inrnooucrtou The objective of this chapter is to present a qualitative description of environ- l menial media, highlighting some of their more important properties. This is done l because the fate of a chemical depends on two groups of properties: those of the chemical and those of the environment in which it resides. We find it useful to assemble “evaluative” environments, which are Hand in later calculations. We can consider, for example, an area of' 1 a: l lcnay consisting of some air, water, soil, and sediment. Volumes and properties can be assigned to these media. which are typical hut purely illustrative and will, of Course, require modification if chemical fate in a ‘ specific region is to be treated, The sequence is to treat the atmosphfirfi’ am hyde ! sphere tie. water), and then the lithosphere (bottom sediments and terrestrial soils), ' each with its resident biotic community. ‘ It transpires that it is convenient to define two evaluative environments First is a simple four-compartment system that is easily understood and illustrates the application of the genera} principles of environmental partitioning. Second is a more complex, cigl'tT-eorrlpanment system that is more representative of real environ- : meats. It is correspondingly more demanding of data and leads to more lengthy calculations. t The environments 0: “unit worlds" are depicted in Figure 4.1. Details are dis- ; cussed by Neely and Maelcay. 1932. l E l ‘ 4.2 THE ATMOSPHERE ‘[ 4.2.1 Air The layer of the atmosphere that is in most intimate contact with the surface of t the Earth is the troposphere, which extends to a height of about 10 ion. The tem- pcrature, density, and pressure of the atmosphere fall steadily with increasing height, 55 ram—2001000 14:51 BREE} 0030 FI—III [\{g 8058937812 56 MULTIMEDIA ENVIRONMENTAL MODELS AND FUGACIW AIR 10001112 IOOOmx'SDOOm “5!: 109m3 i' SOIL 1000mx300meJSm =45000m3 WATER IDDOmx'I'OOmxlflm u7x105m3 SEDNENT 1060 m x 700 m x 0.03 m =21000 m3 I 'I I I I i. i i i I" I E ‘ Simple Four—Compartment Environment I I; a AIR 6): 1091113 5; g; I AEROSDLS 0.12 m3 I 3 ; 2.2.500 I nouns I son, 45000013 0000:: AIR ‘ mom “5”” m’ “TEE a» mom 3000 m3 1% LIPID WATER 7 x 106 m” AQUATIC mom (015m 7 m3 5% LIPID SUSPENDED 5513mm 35 m3 BOTTOM SEDIMIENT 21000 m3 ; Eight-Compartment Environment Figure 4.1 Evaluative onvironmontn. ; which is a nuisance in subsequent calculations. If we assume uniform density at a % 1; prcssurc of one atmm'aplmm, Lhcn the entire: troposphoro can be viewed as being E comprcsmd Into a laugh: of about 6 km. Exchange of matm from the troposphere THE tho: U31! froI per. OV-E pro th n-. dLI a I: CD a t of dc 1:0 131'. VC W'UCI hPR-EB-Ul THU 14:58 BREN UCSB THE NATUFIE OF ENVIRONMENTAL MEDIA 57 through. the tropopause to the stratosphere is a relatively slow process and is rarely important in environmental calculations, except in the ease of chemicals such as the freons, which catalyze the destruction of stratospheric ozone, thus facilitating the penetration of UV light to the Earthls surface. A reasonable atmospheric volume over our 1 km square world is thus 1600 >< 1000 x 6000 or 5 x 109’ m3. If our environmental model is concerned with a localized situation (e.g., a state, province, or metropolitan region), it is unlikely that most pollutants would manage to penetrate higher than about 500 to 2000 in during the time the air resides over the region. It. therefore may be appropriate to reduce the height of the atmosphere to 500 to 2000 m insuch cases. In extreme cases (e.g., over small ponds or fields), the accessible mirted height of the atmosphere may be as low as 10 In. The modeler must make a judgement as to the volume of air that is accessible to the chemical during the time that the air resides in the region of interest. 4.2.2 Aerosola The atmosphere contains a considerable amount of particulate matter or aerosols that are important in determining the fate of certain chemicals. These particles may range in size and composition from water in the form of fog or cloud droplets to dust particles horn. soil and smoke from combustion. They vary greatly in size, but a diameter of a few tttn is typical, Larger particles tend to deposit fairly rapidly. The concentration of these aerosols is normally reported in ugh-hi A rural area may have a concentration of about 5 pig/m3, and a fairly polluted urban area a concentration of 100 pig/1rd. For illustrative purposes, we can assume that the particles have a density of 1.5 gfcm3 and are present at a concentration of 30 rig/m3. This corresponds to volume fraction of particles of 2 x 10'”. The density of these particles is usually unknowns, thus the volume fractions are only estimates. It is, however, conVenient for us to calculdtc this amount in the form of o. voltuno fraction. In an evaluative air volume of 6 x 109 m3, there is thus 0.12 m1 or 120 L of solid material. These aerosols are derived from numerous sources. Some are mineral dust particles generated from soils by wind or human activity. Some are mainly organic in nature, being derived from combustion sources such as vehicle exhaust or wood fires, i.e., smoke. Some are generated from oxides of sulfur and nitrogen. Some “secondary” aerosols are formed by condensation as a result of oxidation of hydro- carbons in the aunosphere to less volatile species. These hydrocarbons can be generated by human activity such as fuel use, or they can be of natural origin. Forests often generate large quantities of isoprene that oxidize to give a blue haze, hence the terms “smokey” or “blue” mountains. These aerosols also contain quantities of water, the amount of which depends on the prevailing humidity. 4.2.3 Deposition Processes Aerosol particles have a very high surface area and thus absorb (or adsorb or sorb) many pollutants, especially those of very low vapor preSSurc, such as the PCBS or potyaromatic hydrocarbons. In the case of henzo(a)pyrene, almost all the chemical present in the atmosphere is associated with particles, and very little exists in the gas phase. This is important, because chemicals associated with aerosol particles FHX N0. 8858837812 HPR-EB-Ul THU 14:59 BREN UCSB 515 MULTIMEDIA ENVIRONMENTAL MODELS AND FUGACITY are subject to two important deposition processes. First is dry deposition, in which the aerosol particle falls under the influence of gravity to the Earth’s surface. This falling velocity, or deposition velocity, is quite slow and depends on the turbulent condition of the semaphore. the size and properties of the aerosol particle, and the nature of the ground surface, but a typical velocity is about 0.3 cur/s or 10.8 mm, The result is deposition of 10.8 m/h it 2 x 10*” (volume fraction) it 10“ m2 or 0.000210 nil/l1 or 1.89 mifyear. Second, the particles may be scavenged or swept out of the air by Wet deposition with raindrops. As it fails, each raintlrop sweeps through a volume of air about 300,000 times its volume prior to landing on the surface. Thus, it has the potential to remove a considerable quantity of aerosol from the atmosphere. Rain is therefore often highly contaminated with substances such as PCBs and There is a warmer: fallacy that rain water is pure. In reality, it is often much rrrore contaminated than surface water. Typical rainfall rates lie in the range 0.3 to .1 to per year but, of course, vary greatly with climate. We adept a figure of 0.8 mr'year for illustrative purposes. This results in the scavenging of 200,000 x 0.8 rniyeeu- x '2 :t 10‘” :t 10‘" m2 or 3.2 m3/ycar, about twice the dry deposition. Snow is an even more efficient scavenger of aerosol particles. It appears that one volume of snow (as solid ice) may scavenge about one million volumes of atmosphere. five times more than rain, presumably because of its flaky nature with a high surface area and a sloWer. more tortuous downward journey. In the four-compartment evaluative environment, ignore aerosols, include them in the eight~coinpartment version. but we 4.3 THE HYDRDSPHEHE OH WATER 4.3.1 Water SeVenty percent of the Earth’s surface is covered by water. ln sonic evaluative models, the area of water is taken as 70% of the 1 million tn2 or 700,000 mi. Similarly to the atmosphere. only near—surface water is accessible to pollutants in the short term. in the oceans, this depth is about 100 in but, since must situations of environ- marital interest involve fresh or estuarine water, it is more appropriate to use a shallower water depth of perhaps 10 m. This yields a water volume of about 7 X W1 m3. If the aim is to mimic the proportions of water and soil in a political jurisdictioa, such as‘a state or prevince, the area of water will normally be consid— erably reduced to perhaps 10% of the total, or about 10" m3. We normally regard the water as being pure. i.o.. containing no dissolved electrolytes, but we do treat its content of suspended particles. 4.3.2 Particulate Matter Particulate matter in the wamr plays a key role in influencing the behavior of chemicals. Again, We do not normally Know if the chemical is absorbed or adsorbed to the particles. We play it safe and use the vague term screed. A very clear natural Water may have a concentration of particles as low as l gin-13 or the equiVaient l mg/L. ‘ hPR-EB-Ul THU 15:00 BREN UCSB FHX N0. 8058937812 THE NATURE OF ENVIRONMENTAL MEDIA 59 ill in most cases, hOWcVCl‘, the concentration ishigher, in the range of 5 to 12.0 gins-l. i ‘ VCW turbid: muddy WEIR-“FPS may have coucentrations over 100 3/1113. Assuming a - concentration of 7.5 g/m3 and a density of “LS g/cm3 gives a volume fraction of ‘ particles of about 5 x 10"“. Thus, in the 7 x 10“ m3 of Water, there is 35 m3 of particles. i This particulate matter consists of a wide variety of materials. It contains mineral , matter, which may be clay or silica in. nature. lt also contains dead or dctrital organic a l - matter, Which is often referred to as hrmzin, humic acids, and falvic acids or, more vaguely, as organic atelier: .It is relatively easy to measure the total concentration of organic carbon (DC) in water or particles by converting the carbon to carbon dioxide and measuring the amount spectroscopically. Altemativcly, the solids can be dried to remove water, then heated. to ignition temperatures to burn off organic ' matter. The loss is referred to as [err on ignition (LO!) or as organic matter (0M). Thus, there are frequent reports of the amount of dissolveaorganic carbon (DOC) or total organic carbon (TOG) in Water. These humic and fulvic acids have been the subject of intense. stutly for many years. They are organic materials of variable composition that probably originate from the ligneous material present in vegetatioa They contain a variety of chemical structures including substituted alksne, eyeloal- ltane, and aromatic groups, and they have acidic properties imparted by phenolic or carbosylic acids. They arel therefore, fairly soluble in alkaline solution in Which they are present in ionic form, or they may be precipitates antler aciclie conditions. The operational difference between humic and t'ulvic acids is the pH at which precipitation occurs. It is important to discriminate between organic matter (OM) anti organic carbon (0C). Typically, OM contains 50 to 60% thus an OM analysis of 10% may also be 5% DC. A mass basis, i.e., gflClG g, is commonly used. For convenience in our evaluative calculations, we will treat GM as 56% DC, and we will assume the density of both DM and DC as being equal to that of water. Concentrations oi." these suspended materials may be defined operationally by 1 using filters of various pore size, for example, 0.45 tun There is a tendency to 3 l -‘ describe material that is smaller than this, i.e., that passes through the filter, as being E operationally “dissolved.” it is not clear how we can best discriminate between “clissolvocl” and “particulate” toms of such material,-since there is presumably a continuous sure spectrum ranging from molecules oi“ a few nanoinetres to relatively large particles of MO or lOGO 11m. it transpires that the organic material in the 3 suspended phasas is of great importance, because it has a. high sorptive capacity for i V organic chemicals. It is therefore common to assign an Organic carbon content to S these phases. in a fairly productive lalcc, the ions content may he as high as 50% but, for illustrative purposes, a figure of 33% for GM or 15.7% DC is convenient. in each. cubic metre of“ water, there is thus 2.5 g or cni 1‘ of 3M and 5.0 g or 2.5 cm-3 of mineral matter, totaling 7.5 g or 5.0 cut-l, giving an average particle density of LS g/cm3. 4.33 Fish and Aquatic Blots ‘ Fish are of particular interest, because they are oi“ commercial and recreational . ;" importance to users of water, and: they tend to bloconcentrate or hioaccumulate l ' fiPR-EB-Ul THU 15:01 BREN UCSB FHX P. 08 ED MULTIMEDIA ENVlFlONMENTAL. MODELS AND FUGACITV f TH metals and organic chemicals from water. They are thus convenient monitors of the 3 by contamination status of lakes. This raises the question, “What is the volume fraction 53' of fish in a lake?“ Most anglers and even aquatic biologists greatly overestimate this ;' “3 number. It is probably, in most cases, in the region of 10—3 to 10-9. but this is somewhat “I: misleading. because most of the biotic material in a lake is not fisht—it is material E 9" of lower ttdphic levels, on which fish feed. For illustrative purposes. we can assume ‘ Of that all the biotic material in the water is fish, and me total concentraan is about Pf 1 part per million, yielding a volume of “fish” of about 7 of. It proves useful later 111‘ to define a lipid or fat content of fish, a figure of 5% by volume being typical. 01‘: In summary, the water thus consists of ’7 x 105 m3 of water containing 35 m3 of ‘ mi particulate matter and “I in3 of "fish" or biota. . f": In shallow or near-shore water, there may be a considerable quantity of aquatic ‘ 51'“ plants or macrophytes. These plants provide a substrate for a thriving microbial I f” connnunity, and they possess inherent soi'ptive capacity. Their importance is usually it ' cadet-estimated. Because of the present limited ability to quantify their sorptive 1'3? properties, we ignore them here. co 4.3.4 Deposition Processes gr a The particulate matter in water is important, because, like aerosols in the atmo— W‘ sphere, it serves as a vehicle for the transport of chemical from the bulk of the water 111' to the bottom sediments. Hydrophobic substances tend to partition appreciably on to the particles and are thus subject to fairly rapid deposition. This deposition velocity {3" is typically 05 to 2.0 to per day or 0.02 to 0.08 mfh. This velocity is sufficient to “1’ cause removal of most of the suspended matter from most lakes during the course of a year. Thus, under ice—covered lakes in the winter, the water may clarify. Some 4' of the deposited particulate matter is resuspended from the bottom sediment through the action of currents, storms, and the disturbances caused by bottom—dwelling fish 1 and invertebrates. During the summer, there is considerable photosynthetic fixation fa- ? of carbon by algae, resulting in the fonnation of considerable quantities of organic 5‘3 carbon in the water column. Much of this is destined to fall to the bottom of the 3 - lake. but much is degraded by microorganisms within the water column. V3 Assuming, as discussed earlier, 5 x 10—6 m3 of particles per mi of water and a m - f deposition velocity of 200 in per year. we active at a deposition rate of 0.001 1113/1111 l?r i of sediment area per year or, for. an area of 7 x 105 m1, a flow of 700 tin/year. We 1‘ It ‘ examine this rate in more detail in the next section. 138 ; "VB ‘ I w: l {l i i 4.4 BOTTOM SEDIMENTS l l I I OI 4-4-1 Sediment Solids 5 ‘ cc Inspecu'on of the state of the bottom of lakes reveals that there is a fairly fluffy m 5 1 0t nepheloid active layer at the water—sediment interface. This layer typically eon- th ' sists of 95% water and 5% particles and is often highly organic in nature. It may 1“ 01' consist of deposited particles and fecal material from the water column. It is stiIIEd hPR-EB-Ui THU 15:02 BREN UCSB FHX N0. 8058937812 P. 7 THE NATURE or: ENVIRONMENTAL MEDtA oi . " i by currents and by the action of the various biota present in this henthte regiOn. The sediment becomes more consolidated at greater depths, and the water content tends to drop toward 50%. The top few centimetres of sediment are occupied by burrowing organisms that feed on the organic matmr (and on each other) and generally turn over (bioturbate) this entire “active layer” of sediment. Depending on the condition of the water column above, this layer may be oxygenated (aerobic or code) or depleted of oxygen (anaerobic or anoxic). This has profound implications for the fate of " inorganic substances such as metals and arsenic, but it is relatively unimportant for organic chemicals except in that the oxygen status influences the nature of the microbial community, which in turn influences the availability of metabolic pathways for chemical degradation. The deeper sediments are less accessible, and ultimately the material becomes almost completely buried and inaccassible to the aquatic environment above. Most of the activity occurs in the top 5 cm of the sediment. but it is misleading to assume that sediments deeper than this are net accessible There remains a possibility of bioturbation or diffusion reintroducing chemical to the water column. Bottom sediments are difficult to investigate, can be unpleasant. and have little or no commercial value. They are therefore often ignored. This is unfortunate, because they serve as the depositories for much of the toxic material discharged into water. They are thus very important, are valuable as a “sink” for contaminants, and merit more sympathy and attention. Fast-Howin g rivers are normally sufficiently turbulent that the bottom is scoured, exposing rock or consolidated mineral matter. Thus, their sediments tend to be less , important. Sluggish rivers have appreciable sediments. , 4.4.2 Deposition, Resuspeneion, and Burial ‘ .3“ It is possible to estimate the rate of deposition, i.e., the amount of material that falls annually to the bottom of the lake and is retained there. This can be done by sediment traps, which are essentially trays that collect falling particles. or by taking a sediment core and assigning dates to it at various depths using concentrations of various radioactive metals such as lead. Nuclear events provide convenient dating ‘ markers for sediment depths. The measurement of deposition is complicated by the I {a presence of the reverse process of resuspension caused by currents and biotic activity. I, It is difficult to measure how much material is rising and falling, since much may .t be merely cycling up and down in the water column. Burial or net deposition rates ‘ vary enormously, but a figure of abouc 1 mm per year is typical. Much of this is water, which is trapped in the burial process. Chemicals present in sediments are primarily removed by degradation, burial, or resuspcnsion back to the water column. For iiiustrative purposes we adopt a sediment depth of 3 cm and suggest that it consists of 67% water and 33 ‘72: solids. and these solids consist of about 10% organic matter or 5% organic carbon. Living creatures are included in this figure. Some of this deposited material is resuspendcd to the water column, some of the organic matter is degraded (Le, used as a source of energy by benthic or bottom-living organisms), and some is destined to be permanency buried. The low 5% organic 8937812 hPR-EB-Ul THU 15303W__ BREN UCSB I F x N0. 805 '. ea MULTIMEDIA FNVIRGNMENTAL MODELS AND FUGAClTY carbon figure for deeper sedimentseompared to high 17% for the depositing material implies that about 75% of the organic carbon is degraded. It is now possible to assemble an approximate mass balance forthe sediment 4 mineral matter (MM) and organic matter (OM) and thus the organic carbon (DC). This is given in Table 4.1. Tehle 4.1 illustrative Sediment-Water Meec Balance on a 1 nt2 Area Basie H ' Organic 11: Mineral matter rganie matter Total carbon m _- ._ __H _.__.__,_ elm3 ' a m“ s cm“ s s I in Deposrtlcn 500 1200 500 500 1000 1700 250 c: I I Hesuspeneion 200 480 200 200 400 €380 100 pa in " OM Conversion — _ sec 233 ass see 117 at its. a Burial (solids) sec 720 s? 67 as? 78? as W th Total burial is 1000 omit/year or 1420 g/yeer, corresponding to st “velocity” of 1 mm/‘yeat‘. . pr The Sedll’ttertt thus has a density of 1.42 g/cm3 or 1420 leg/mi. w; Assumed densities are: mineral matter 2.4- g/cm9. organic matter 1 g/cm3. I Organic matter is 50% (mass) organic carbon. _. int _ ___- .—..._M—uu_nm—n.._ on. I On a 1 mi basis, the deposition rate is 0.001 1113 per year or 1000 ctrr3 per year. i With a particle density of 117 g/cml, this corresponds to 1700 glycar of which 500 g “3 i is 0M, and 1200 g is MM. We assume that 40% of this is resuspendedt i.e., 200 g ab 9 of OM and 480 g of MM Of the remaining 300 g OM we assume that 233 g is } digested or degraded to C02, and 67 g is buried along with the remaining 720 g of “r ' MM. Total burial is thus 1420 g, which consists of 720 g of MM, 67 g of OM, and “"3 633 g of water. The total volumetric burial rate of solids is 367 cm3/year. Now, _ .- § associated with these solids is 633 cm3 of pore water; thus, the total volumetric m burial rate of solids plus Water is approidmately 1000 cmia’yesr, corresponding to a the .l rise in the sediment—water interface of 1 min/year. The mass percentage of DC in d“ ‘ the depositing and resuspcnding material is: 15%, while in the bruied material. it is by} .3 J." 4.2% The‘bullc sediment density, including pore water, is 1420 kg/m3. 2dr: - 5 On a 7 X 105 in2 basis, the deposition rate is 700 mil/year, resuspcnsion is 'u' . 51 2301n31’year1 burial is 257 m3/year, and degradation accounts for the remaining -;1. 5 163 Ina/year. The organic and mineral matter balances are thus fairly complicated. P“: 5 i but it is important to define them, because they control the fate of many hydrophobic PM chemicals. can. ; It IS noteworthy that the burial rate of 1 min/year coupled With the sedIment mg} i depth of 3 cm indicates that on aVorage, it will take 30 years for sediment solid: 33; 9 l to become buried During this tune, they may continue to release sorhed chemical esp! ‘ back to the water column This is the crux of the “in-place contaminated sediments" cud t problem, which is unfortunately very common, especially in the Great Lakes Basin. ‘ -' g: In the simple four—compartment environment we treat only the solids but 11] the 45 ‘ g eight-compartment version We include the sediment pore water. in the interests of . I: simplicity, we assign a density of 1500 kg/m3 to the sediment in the four—compart— ' ‘f 3' meat model. i sea hPR-EB-Ul THU 15:04 BREN UCSB FHX N0. 8058937812 THE NATURE oF ENVIHQNMENTAL MEDIA ‘ as l i 4.5 SOILS 4.5.1 The Nature of soil Soil is a complex organic matrix consisting of air, water, mineral matter (notably clay and silica), and organic matter, which is similar in general nature to the organic matter discussed earlier for the water column. The surface soil is subject to diurnal and seasonal temperature changes and to marked variations in water content, and thus in air content. At times it may be completely flooded, and at other times almost completely dry. The organic matter in the soil plays a crucial role in controlling the retention of the water and thus in l ensuring the viability of plants. The organic matter content is typically 1 to 5%, but peat soils and forest soils can have much higher organic matter contents. Depletion , of organic matter through excessive agriculture tends to render the soil infertile, l l, which is an issue of great cancer-n in agricultural regions. Soils vary enormoust in their composition and texture and consist of various layers, or horizons, of different properties. There is transport vertically and horizontally by diffusion in air and in water, flow, or advection in water and, of course, movement of water and nutrients into plant roots and thence into stems and foliage. Burrowing animals such as worms can also play an important role in misting and transporting chemicals in soils. A typical soil may consist of 50% solid matter, 20% air, and 30% water, by volume. The dry soil thus has a porosity of 50%. The solid matter may consist of ‘ about 2% organic carbon or 4% organic matter. During and after rainfall, water flows vertically downward through the soil and may carry chemicals with it. During periods of dry weather, water tends to return to the surface by capillary action, again moving the chemicals. Later, we set up equations describing the diffusion or permeation of chemicals in soils. When doing so, We treat the soil as having a constant porosity. In reality, there are channels or “inacroporous” areas formed by burrowing animals and decayed roots, and these enable water and air to flow rapidly through the soil, is bypassing the more tightly packed soil matrix. This phenomenon is very difficult to address when ccrnpiling models of transport in soils and is the source of considerable frustration to soil scientists. in Most soils are, of course, covered with vegetation, which stabilizes the soil and prevents it from being eroded by wind or water action. Under dry conditions, with poor vegetation cover. considerable quantities of soil can be eroded by Wind action, carrying with it sorbcd chemicals. Sand dunes are an extreme example. In populated regions, of more concern is the loss of soil in ivater runoff. This water often contains very high concentrations of soil, perhaps as much as a volume fraction of 1 part per thousand of solid material. This serves as a vehicle for the movement of chemicals, especially agricultural chemicals such as pesticides, From the soils into water bodies such as lakes. 4.5.2 Transportin Soils It In most areas, there is a net movement of water vertically from the surface soil to greater depths into a pervious layer of rock or aquifer through which groundwater . 1r . w sPR—as—Ut tau gains“ BREN UCSB Fax N0. 8058937812 r. 10 64 MULTIMEDIA ENVIRONMENTAL MODELs AND PUGACITY 3 THE flows. The quality of this groundwater has become of considerable concern recently, i spe especially to those who rely on walls for their water supply. This water tends to move _‘ soil very slowly (i.c., at a velocity of metres per year) through the porous sub—surface strata. E SDI" If contaminated, it can talte decades or even enemies to recover. Of patucular concern - atrr. are regions in which chcmical leachate from dumps or landfills has seeped into the mic groundwater and has migrated some distance into rivers, Wells, or lakes. It is quite difficult and expensiVe to investigate, sample, and measure contaminant flow in ground— enc water. It may not even be clear in which direction ithe water is flowing or how fast it see is flowing. Chemicals associated with groundwater generally move more slowly than des the velocity of the groundwater. They are retarded by sotption to the soil to an extent ‘ Thi expressed as a “retardatitm factor," which is essentially the ratio of (a) the amount of Th: chemical that is sorbed to the solid matrix to (h) the mount that is in solution. Sorption . par of organic: chemicals is usually accomplished preferentially to organic matter; however, (an days also have considerable SOt'pfiVc capacity, especially when dry. Polar, and espe- cor eially ionic, substances may interact strongly with nunersi matter. The eheuaetericuti on of migration ofchetnicals in groundwater is difficult, and especially so when a chemical ‘ out is present in an non—aqueous phase, for example, as a built oil or emulsified oil phase. ' Considerable effort has been devoted to understanding the fate of nonaqueous phase liquids (NAPLs) such as oils, and dense NAPLs (DNAPLs) such as chlorinated sol— Vents that can sinlc in the aquifer and are very difficult to recover. For illustrative purpOSCs, we treat the soil as covering an area 1000 in X 300 m x 15 cm deep, which is about the depth to which agricultural soils are plowed. This . in ' yields a volume of 45,000 m3. This consists of about 50% solids, of which 4% is get organic matter content or 2% by mass organic carbon. The porosity of the soil, or roll void space, is 50% and consists of 20% air and 30% Water. Assuming a density of the the soil solids of 2400 ltg/rn3 and Water of 1000 ltglm3 gives masses of 1200 kg All solids and 300 kg water per tn3 (and a negligible 0.2 kg air), totaling 1500 kg, ' l i , corresponding to a built density of 1500 leg/mi Rainvvater falls on this soil at a rate ecu 2‘ g of 0.8 In per year, i.e., 0.3 niJJm2 year. Of this, perhaps 0.3 In evaporates, 0.3 in runs are i off, and 0.2 in percolatcs to depths and contributes to groundwater flow. This results rot g I in water flows of 90,000 mifyear by evaporation, 90,000 mil/year by runoff, and ma l 60,000 mfilyear by percolation to depths totaling 240,000 m3/year. With the runoff Chi l : is assoeisted 90 tug/year of solids, i.e., an assumed high concentration of 0.1% by pat f Q volume. Again, it must be emphasized that these numbers are entirer illustrative. nit- ‘ i E This soil runoff rate of 90 m3/ycar does not correspond to the deposition rate of ms ' 700 aid/year, partly because of the contribution of organic matter generated in the of water column, but mainly because of the low ratio of soil area to water area. . .‘ an! 4 4.5.3 Terrestrial Vegetation are l {13' Until recently, most environmental models have ignored terrestrial vegetation The reason for this is not that vegetation is ununportant, but rather that modelers I 1 ‘ currently have enormous difficulty calculating the partitioning of chemicals into ,: ‘. plants. This topic is receiving more attention as a result of the realization that i i. consumption of contaminated vegetation, either by humans, domestic animals, or ' wildlife, is a major route or vector for the transfer of toxic chemicals from one ‘ Sta HPR-EB-Ul THU 15:08 BREN UCSB FHX N0. 8058937812 ‘ P. 11 t , ,. . cyan-s...“- . THE Naruse or ENVtaoNMENTAt. MEDtA as , species to another, and ultimately to humans. Plants play a critical role in stabilizing soils and in inducing water movement from soil to the aunosphete, and they may ‘ SflIVE as collectors and recipients of toxic chemicals deposited or absorbed from the ‘ I . atmosphere. They can also degrade certain chemicals and increase the level of microbial activity in the root zone, thus increasing the degradation rate in the soil. t Amounts of vegetation, in terms of quantity of biomass per square metre, vary enormously from near zero in deserts to massive quantities that greatly exceed 5, I 'I t”? accessible soil volumes in tropical rain forests. They also vary seasonally. If it is 5 desired to include vegetation, a typical “depth” of plant biomass might be 1 cm. This, of course, consists mainly of stator, cellulose, starch, and ligneous material. There is little doubt that future, more sophisticated models will include chemical partitioning behavior into plants. But at the present state of the art, it is convenient (and rather unsatisfactory) to regard the plants as having a. Volume of 3000 m3, ,: containing the equivalent of 1% lipid-like material and 50% water. . ,. We ignore vegetation in the simple four—Compartment model, treating the soil as only a simple solid phase. 4.6 SUMMARY These evaluative volumes, areas, compositions, and flow rates are summarized in Table 4.2. From them is derived a simple four-compartment. version. Also sug— - gested is an alternative environment that is more terrestrial and less aquatic, and it reflects more faithfully a typical political jurisdiction. It is emphasized again that the quantities are purely illustrative, and site-specific values may be quite different. All that is needed at. this stage is a reasonable basis for calculation. Scientists who have devoted their lives to studying the intricacies of the structure, . a composition. and proCesses of the atmosphere, hydrosphere. or lithosphere will . ! a l? undoubtedly be offended at the simplistic approach taken in this chapter. The envi- p. ‘ ronrnent is very complex, and it is essential to probe the fine detail present ‘31 its many compartments. But, if WE. are to attempt broad calculations of multimedia chemical fate, we must suppress much of the media-specific detail. When the broad patterns of chemical behavior are esrablished, it may be appropriate to revisit the media that are important for that chemical and focus cn detailed behavior in a specific : medium. At that time, a more detailed and site—specific description of the medium, , of interest will be justified and required. Our philosophy is that the model should be only as complex as is required to answer the immediate question, not every question that could be asked. As questions are answered, new questions will surface and new, more complex models can be 3 , developed to answer these questions. 4.7 conctuomo EXAMPLE Select a region with which you are familiar; for example, a county, watershed, state, 0r province. Calculate the volumes of air to a height of 1000 to; soil to a depth HPR-Efiffll THU .1510? BREN UCSB FHX N0. 8058937612 P.12 060,000 0010 water: 240,000 m1 10 00“ Aerosol 000001000 H0100 (total) 2 00 MULTIMEDIA ENVIRONMENTAL MDDELs AND FUGACITY 5 TH Table 4.2 Evaluative Environments I A. Fourhcompartmeni, 1 10112 envlronment Areas (01’) ( W — Alem‘l 3 X 105 A Water-sediment 7' X 105 e 3 4: 5 Depths (m) Volumea (m3) Densllles (Ruin-13) Con-100010000 “ 3 " 0000 0 x 101 1.2 E 1; W0th 10 7 x 109 1000 ‘ 1 Soil 0.15 4.0 x 10* 1500 2% 00 F Sediment 0.03 2.1 K 10‘“ 1500 5% OC F H~E10htvcnmpar1menh1 km’envimnmem, areas as In A above of Volumes Densities I da _' ‘ {1113) L (kglm3) 0001100010000 3i] Air 0 x 10* 1.2 Air m W010i 7 X 105 1000 Water be 0011(50% 001100. 415 x 10‘ 1500 Soil {50% solids, 20% air, 30% waver) T0 20% air. 30% water) . Sediment (30% 2.1 x 10 1500 Sediment (00% solids) fa” L ‘ 00005) I. 5 300000000 35 1000 10.7% 00 ; ‘ Sedimnt . ‘ ' Aer000ls 0.12 1500 2 K 10-1T v0lume fraction Or 30 09110113 ‘I {' Aquatic Biola 7 1000 5% lipid Vagetation 0000 1000 1% lipid l f Plain 09.10 0.0 mfyear or 000,000 miiyear i _ Dry deposition 210 >1: 106 I113 01 or 1.09 r11J {year ' 1 W01 000001000 005 2-: 104‘ m3 Ih or 0.2 m: /year I” Qedimem Deposition Rates 33 _ Deposition 700 m3 lyear solids 171/5 00 00505001101011 200 m1 fyear solids 17% 00 % N0: 0000011100 (burial) 207 01a lyear 00000 5% QC 1 F010 01 Water in Soil - Evaporation 00,000 mJ [year 1“ ‘l H0000 10 water 00,000 I113 {year I I ‘ Percolation 10 groundwater 00,000 m3 [year 1‘ if S0000 runoff 90 m5 lyeal' ; ‘ a—u _ FHX N0. 8058937812 HPR-EB-Ul THU 15:08 BREN UCSB THE NATURE OF ENVIRONMENTAL MEDIA 57 Table 4.2 (continued) 0. Regional, 100,000 km? environment as used in the EEOC: model of Maekay et at. (19950) Volume this) Area (m2) Composition Air " 10” 100 x 10° Aerosols. 2000 - (2 X “Hir-ll vol frn) Water 2 X10“ ‘lD X109 Soil I o x 109 90 x 109 2% or: Sediment 103 10 x 109 4% QC. Suspended sediment 10'l — 20% DC Fish 2 x 105 — 5% lipid For delete of other properties See Monkey et al. 199613. of 10 em; water and bottom Sediment to a depth of 3 cm, and vegetation. Obtain data on average temperatme, rain rate, water flows, and wind velocity, and calculate air and water residence times. Attempt to obtain infonnation on typical cementin- tions of aerosols, suspended solids in water, and the organic carbon contents of 501134, bottom, and suspended sediments. Prepare a summary table of these data similar to Table 4.2. ‘ ‘ These basic environmental data can be used in subSequent assessments of the fate of chemicals in this mginn. ...
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