Category WATER IN ROAD STRUCTURES

Complexes are chemical compounds consisting of a central atom (metal) and ligands (consisting of a group, molecule or ion) tied to the central atom with at least one co-ordination bond. A chelate is a special form of complex where the ligand is attached to the central atom by at least two bonds. The most common ligand in water solutions is the water molecule itself but anions such as hydroxide, carbonate, hydrocarbonate, sulphate and organic acids also form ligands. The formation of a complex from a metal and a ligand is a balanced reaction characterized by a constant (Kc) that is often pH dependent. Some complexations can be considered as “surface complexation” reactions (e. g. of a metal with an iron oxide) as opposed to “aqueous complexation” reactions.

Organic and inorganic complexes are present in all natural waters. Organic acids such as humic acids (originating in humus formation upon decay of plant litter) make up one of the most important types of ligands in natural waters. Humic acids and other types of humic substances greatly affect the solubility and thus the availability of heavy metals to biota. In soil water, humic substances occur in dissolved form and in more or less insoluble aggregates. Compared to heavy metals occurring as insoluble aggregates, heavy metals occurring in the dissolved form are much more mobile and available and therefore more toxic to biota (Berggren Kleja et al., 2006).

Among the inorganic complexes, hydroxides of Fe and Mn are common in nat­ural soils. From a pollution point of view, it is of great importance whether the hy­droxides are present in dissolved or precipitated form because hydroxides regulate the mobility of heavy metals. The stability of the hydroxide complexes is greatly governed by the pH. Depending on the soil type, but also on the degree and charac­teristics of the pollution load, roadside soils vary greatly in pH. In many cases, pH is higher close to the road than further away (James, 1999).

Components present in the road/soil environment and likely to form complexes with heavy metals include hydroxides, carbonates, hydrocarbonates, sulphates and organic acids. They originate from deposition, road materials and infiltrating water.

Many chemical reactions imply the transfer of electrons from one chemical species to another. These reactions are called redox reactions and they are usually rather slow. In soil and water, redox reactions involve hydrogen ions and are thus greatly pH dependent. The most important redox reactions involve oxygen, carbon, nitro­gen, sulphur, manganese and iron. In polluted soils, arsenic and mercury can also participate.

The redox potential, and changes thereof, play a crucial role in the behaviour of metals in soils. For instance, iron oxides are formed at high redox potentials. Iron oxides and hydroxides are capable of adsorbing heavy metals onto their surfaces, which will greatly reduce the mobility of the heavy metals. When the redox potential is lowered, the iron oxides dissolve and the adsorbed heavy metals are released and will be available for leaching further down the soil profile. Many redox reactions in nature are speeded up by certain bacteria, however. The bacteria utilise the energy released from redox reactions (Berggren Kleja et al., 2006).

Reactions Between Acids and Bases (Proton Exchange)

The amount of protons (H+) in solution greatly influences most chemical reactions. Proton transfer reactions are usually very fast. According to the Brensted definition, protons are provided by an acid and captured by a base. To each acid Ac there is a corresponding base Ba:

Aci Bai + H+

The acid and the corresponding base constitute an acid/base couple (Ac1 /Ba1).

In most natural waters, the pH lies within the range from 5 to 8. All the sub­stances dissolved into water (gases, mineral and organic compounds) contribute to the acid-base equilibrium of water. All components of the carbonate system make a major contribution to the acid neutralizing capacity (called alkalinity) of the water and to its base neutralizing capacity (acidity). The buffering capacity of water (the ability of the water to maintain its pH despite any addition of H+ or OH-) is also largely determined by the carbonate system. However, dissolved silicates, ammonia, organic bases, sulphides and phosphates also contribute to the alkalinity. In like manner, non-carbonic acids, polyvalent metal ions and organic acids contribute to the acidity.

Rainwater often contains strong acids originating in atmospheric pollutants (dis­solution of gases leading to HCl, HNO3, H2SO4). Acid rain may increase the heavy – metal solubility in soils. The pH effect of strong acids on soil and water will depend on the buffering capacity of the soil or water, however. Oxidation reactions lead to a decrease in pH whereas reduction tends to increase the pH.

Exchange reactions take place between two reactants, usually meaning that both are in the liquid phase (although some surface complexation reactions may involve an exchange reaction, too). They include electron exchanges (reactions between ox­idizers and reducers), proton exchanges (reactions between acids and bases) and
“particle”[13] exchanges (formation of complexes from ions or molecules) (Stumm & Morgan 1996).

On its way from the road surface downwards, the infiltrating seepage (carry­ing chemicals accumulated during rainfall and runoff) will encounter and interact with varying redox-potential and acidity conditions in the various layers of the road construction and soil layers beneath. The resulting more or less steady conditions will govern the equilibria of chemical reactions. More or less oxidizing or reduc­ing road/soil materials will, through dissolution, create more or less oxidizing or reducing conditions. This will influence the toxicity of some chemicals (chromium for example). In like manner, road/soil materials will influence the acidity/alkalinity of the medium and its buffering capacity. Under special conditions, e. g. where very alkaline man-made road materials are present, percolating water can reach very high pH levels followed by more neutral conditions in subsequent layers. In this way, the buffer capacity of the road/soil materials can mitigate the influence of an acid or base spillage, should it occur.

Rainwater is able to dissolve gas present in the atmosphere (leading to acid rain, for example; see Section 6.2.6). Rainwater is also able to dissolve chemicals present at the road surface (e. g. metals, salts and some organics). Road materials are of course selected for not being soluble but trace elements present in natural and alternative materials can be released by dissolution when leached by seepage. The rise in a water table can also bring about dissolution. Dissolved elements can precipitate downstream where hydrous, pH and/or redox conditions differ from those upstream. Dissolution/precipitation sequences are also part of the circulation of chemicals. Dissolution of CO2, whether from the air or biological activity, is of great impor­tance to the pH of the soil solution, also in the road context.

Dissolution is the process by which a solution is formed when a soluble substance (a solute) is dissolved in a liquid (a solvent). A true solution is a uniform molecular or ionic mixture of one or more solutes in a solvent, as distinguished from a colloidal solution or dispersion in which the dispersed material is in the form of extremely small particles, 1 ^m or less. The solute can be a solid or a gas.

As a polar molecule, water can dissolve ionic substances such as salts but also substances consisting of polar molecules with which the water molecule forms hy­drogen bonds. The solubility (i. e. the maximal quantity of a chemical compound that can be dissolved per litre of solvent) is dependent on temperature, pH and activity coefficient.

Whether a substance is present in the dissolved or in the precipitated form is of crucial importance to its mobility and transport in the soil. This is especially true of heavy metals and other micro-pollutants (Ramade, 1998). Depending on variations in the chemical and physical properties of its environment, a given pollutant present in the soil can repeatedly change from being dissolved to being precipitated, and thus from being mobile to being less mobile. Some salts (ionic solids) are very soluble, for instance NaCl and CaCl2 which are used for de-icing and dust-binding, respectively.

The degree of solution of any salt MpXq is governed by the dissolution equilibrium:

MpXq(s) <-> pMq+ + qXP – (6.17)

where the solubility product Ks = (Mq+)p ■ (XP-)q. Thus, the greater the solubility product, the greater the solubility of the salt. This equilibrium can be coupled with other equilibria, e. g. acid-base, redox or complexation equilibria. The solubility of a salt is, e. g., dependent also on the pH and the redox status of the soil.

Carbonates are important to the mobility of heavy metals. Carbonates are dis­solved upon the interaction with water and with the carbon dioxide present in air, water and soil. Calcium carbonate (CaCO3) is a major constituent of calcareous rock. Where enough free carbonate ions (CO32-) are present, they will react with heavy metal ions to form immobile precipitates, e. g. lead carbonate. The mobility of many heavy metals is low in calcareous soils. On the contrary, heavy metals are often more mobile in acidic soils where carbonates are largely absent (Selim & Sparks, 2001).

Hydroxides of Fe and Mn also play a major role in natural waters and soils. The solubility of hydroxides depends on the acidity (pH) of the water or the soil solution. The solubility of hydroxides decreases when pH increases, passes through a minimum and then increases at higher pH.

Organic molecules that contain polar groups or create hydrogen bonds are to a great extent soluble in soil solution and water. This is the case for organic molecules with groups such as hydroxyl, amine, carboxylic acid, carbonyl, ester or ether.

In the road situation, adsorption and desorption will happen routinely whereas precipitation will depend on the ion concentration. At low concentrations, many metals are under-saturated with respect to their associated mineral phases so that their mobility/retardation is governed by adsorption/desorption. At higher concen­trations, both adsorption and precipitation may be occurring to take ions out of solu­tion, but it is the dissolution/precipitation processes that will determine the aqueous concentration of the metal.

In the road context, adsorption/desorption phenomena greatly influence the fate of pollutants entering the road construction, present therein or transported through road-construction layers and further down. Sorption phenomena are also of importance regarding pollutants possibly leached (dissolved) from some road ma­terials (e. g. alternative materials) under the effect of infiltration, and adsorbed on a surface downstream. Sorption/desorption sequences (under the effect of surface characteristics and seepage pH, for example) can lead to a progressive downward transfer of substances.

Adsorption can be defined as the attachment or adhesion of a molecule or an ion in the gaseous or liquid phase to the surface of another substance (an adsorbent) in the solid phase or to the surface of a soil particle. Desorption describes the process by which molecules or ions move in the opposite direction. Adsorption/desorption is a universal surface phenomenon. It can occur at any surface, e. g. surfaces formed by any type of opening, capillary, crack, depression or other type of physical irregu­larity. The nature of the adsorbing surface plays an essential role in the process. The smaller the size of the soil particles, or the greater the porosity, the more efficiently the adsorption will occur because of the increase in surface area provided. Road pollutants are therefore leached much more quickly through a coarse-textured soil than through a clayey soil (Brencic, 2006). This feature is of particular relevance when traffic accidents involve cargoes of harmful or toxic compounds.

The adsorption/desorption of substances between the liquid form and the sur­face of solid-state materials, such as soil particles, is one of the processes of great­est importance for the behaviour of inorganic and organic substances in the soil. The degree of adsorption increases with the concentration of the substance in the solution outside the adsorbent until a maximum is gradually approached. As the reaction kinetics depend on temperature (adsorption decreases with higher tem­perature because the molecules are more energetic and less easily held by their potential sorbent), the quantitative assessment of adsorption is done by means of so – called isotherms. Various models can be used to interpret isotherms, e. g. Langmuir, Freundlich or Brunauer-Emmet-Teller (BET) (Fig. 6.4) a variant of which is given inEq. 6.16.

S = Qt в C/(1 + в C)

Fig. 6.4 Variation of the sorbed quantity (S) as a function of the concentration of sorbate (C) for different temperatures (T1 >T2>T3) – Langmuir isotherm (adapted from Bontoux, 1993 and Selim & Sparks, 2001)

where S = mass of sorbate sorbed per mass of sorbent (typically in units of mg/kg); QT = maximum sorption capacity of the sorbent at temperature, T(°), в = a variable that is only a function of the temperature, T, and C = aqueous concentra­tion of sorbate (typically in units of mg/l).

Desorption can occur when a “new” ion (or other chemical) arrives at a sorption site and is sorbed, preferentially, over a previously sorbed ion of a different type. Less readily, sorbed species can be desorbed if the concentration of that species decreases in the groundwater around the sorbent.

Time is required for sorption/desorption reactions to become complete. Therefore the approach adopted both in analysis and in testing is to allow sufficient time for equilibrium to develop. Often this will take hours, perhaps days, to complete. Care is required when the input or output condition is changing due, for example, to flow bringing more contaminant. Then, true equilibrium may not be possible. The use of an isotherm approach necessitates the assumption of equilibrium conditions.

More important, though, is that the adsorption is also pH dependent; cations such as most metal ions are more strongly adsorbed at increasing pH. The degree of adsorption rises sharply in a short interval of increasing pH. This is due to the fact that the charge of the particle surfaces is greatly pH dependent. The pH of the soil thus largely regulates the mobility of heavy metals occurring in the soil. With the exception of some amphoteric compounds (e. g. some metal hydroxides) and some oxyanions (e. g. MoO42-, AsO43-), the general rule is that many heavy metals are more mobile at lower pH (Berggren Kleja et al., 2006).

The road construction is a multi-component system which is not isolated but open to physical, chemical and biological interaction with its surroundings. Reactions taking place in the road construction thus influence and are influenced by adjacent systems. For instance, the washing of the road surface by run-off brings organic and inorganic compounds (from sources mentioned in Section 6.2) to road shoulder materials and to neighbouring soils with which they may interact when water infiltrates. Seepage from the road surface into the road structure will also lead to chemical reactions with materials in the various road layers and the underlying soil.

Chemical reactions occurring in the road construction and adjacent soil systems commonly involve the solid and the liquid phases, but the gas phase can also play a role. The most significant chemical processes are sorption/desorption, dissolu – tion/precipitation and ion exchange reactions.

Any transformation occurring during the chemical reaction induces a decrease in the total energy of the system. Under constant conditions, systems tend to evolve more or less quickly (depending on the chemical kinetics) towards a lower energy level. Chemical processes will occur as long as an equilibrium state is not reached or as long as the system is modified. Modifications can be induced by inputs and outputs of material or energy.

In natural waters, metals occur in various forms, so-called species. The speciation (the distribution of the various forms of a metal in a solution) depends on a wide range of factors. One of the most important factors is the presence of compounds capable of forming complexes. Other important factors are the acidity and the re­dox potential. The speciation greatly influences the solubility and mobility of heavy metals in soils.

In water, metals mainly occur either in ionic form or are associated with particu­late matter. For practical reasons, filtering with a mesh size of 0.45 ^m is often used to define a limit between ions and particulates.

Organic chemicals may also be present in natural waters, sometimes from nat­ural sources, for example animal carcasses and excreta, decaying vegetation, etc., but often from the consequences of human actions – deliberate or accidental. The concentration of organic solids in the porous media greatly affects the partitioning of organic compounds between the aqueous and the solid phase as dissolved organic substances are, usually, preferentially sorbed to (or released from) organic solids.

The brief introduction given in this sub-section can only provide a few brief pointers to the complex description that would be required to fully explain the inter­action between chemicals carried in groundwater and each other and their interac­tion with the solid structure through which they travel. It is sufficient, here, to make readers aware of the complexity of these and to be aware that both inorganic and organic chemicals, too, can undergo a wide range of reactions and transformations that can result in unexpected outcomes, both good and bad from an environmental point of view.

Non-aqueous liquids, such as petroleum-based fluids, are not, in general, soluble in water so their movement must be considered separately. Although some of the liquid may be soluble or miscible in groundwater to such an extent that it is, thereby, subject to advection, diffusion and dispersion processes as described above, much may remain separate due to its different density and chemistry. These are termed non-aqueous phase liquids (NAPLs). Such fluids with densities less than that of wa­ter (light NAPLs) will float on top of groundwater in unconfined situations and their movement will, therefore, be controlled by the gradient of the top of the ground­water – which will act as the stimulus for movement – and the non-hydraulic per­meability coefficient for that fluid and soil combination. Fluids with densities greater than that of water (dense NAPLs) will tend to flow vertically or sub-vertically through the groundwater until arrested by a soil stratum which is essentially im­permeable to that fluid. Its movement will then be largely controlled by the gradient of the top of that stratum and the non-hydraulic permeability coefficient for that fluid and the soil in which it is contained.

Two of the more common sources of NAPLs in the road environment are spills from (e. g.) tankers and leaking storage tanks. It can be difficult to remove the NAPL from the ground by flow methods as the poor miscibility of the NAPL in water and the particular wettability characteristics between soil particles and the NAPL often means that small droplets are left behind in the soil pores from which the bulk of the NAPL has departed. These small droplets may present a continuing source of low-level contamination over long periods given their low miscibility with/solubility in the surrounding groundwater. A schematic of a light NAPL flow following a spill is illustrated in Fig. 6.3.

Residual LNAPL in soil from spill

Mobile LNAPL above water table

Mobile LNAPL
in water-saturated
stratum

—Щ. Ц, . ШШМш

Diffused fringe

of LNAPL in Ground water flow

groundwater

Fig. 6.3 Schematic illustration of the movement of a light NAPL (LNAPL) in the ground following a spill

In most saturated soils, advection and diffusion/dispersion do not transport contam­inants as fast as might be expected from a consideration only of these processes. Of­ten, there is a movement of contaminant from the liquid phase to the solid phase due
to various physio-chemical processes (see Section 6.3.2). Together, these processes act to retard the contaminant flux. Where the soil solids are, in effect, clean with respect to the contaminant prior to the contaminant’s arrival, this retardation may be expressed using a very simple equation:

fR = 1 + Pdkd

where fR is the retardation factor (no units), kd (almost invariably expressed in units of l/kg = mL/g) is the partition factor which is discussed in the next paragraph and pd is the dry density of the soil (for which units of Mg/m3 will allow Eq. 6.14 to be used directly if kd is expressed in units of l/kg).

The rate of contaminant flux is slowed by a factor of 1/R from that which would be expected assuming only advection, diffusion and dispersion have an effect. This approach allows the effects of physio-chemical processes to be simply modelled by adapting the advection-dispersion Eq. 6.9 as follows:

Dl d2C Dt d2C vx dC _ dC

fR dx2 + fR dx2 fR dx dt

The partition factor, kd, is a very simple means of describing the concentration of a contaminant in the solid phase to the concentration of a contaminant in the fluid phase at equilibrium conditions. At low concentrations such as those normally experienced in the highway environment (except, perhaps, after certain spillages from vehicle accidents), a linear “isotherm” (relationship between the two concen­trations) may not be too inaccurate and is a commonly used characterisation having the benefit of simplicity. Therefore, in such situations, a constant value of kd is used.

Values of kd are highly dependent on soil type, fluid and contaminant species. Values vary by several orders of magnitude for apparently small changes in some of these factors. Even with specific laboratory testing, the natural variability of ground conditions, mineral composition, particle size, etc. from place to place in a soil profile means that prediction of the retardation effect is very imprecise. Accordingly it is common to use published values and to compute the most and the least likely retardations that are credible. Values of kd are available from many sources, notably from the US EPA (EPA, 1999).

It is possible for enhancement, the inverse of retardation, to occur, for example when a spillage changes the fluid chemistry causing leaching of contaminants pre­viously bound into the soil or aggregate. When enhancement takes place, the con­taminant flux is higher than would otherwise be anticipated. This can be modelled by a value of fR of less than 1.0, although Eq. 6.15 will not be directly applicable as it assumes that the soil is initially clean as far as the contaminant of interest is concerned.

Where precipitation falls mainly as storm events, the majority of mass transport in surface runoff is connected with the start of the storm water runoff. This so-called first flush will mobilise pollutants having accumulated on the pavement surface since the previous storm event (Barbosa & Hvitved-Jacobsen, 1999). Concentra­tions and masses decrease with time, and the relationship between the mass and the contamination pulse depends on many factors (Sansalone & Cristina, 2004). The amount of pollutant in the storm runoff depends on several conditions be­fore the rain. The consideration of the first-flush phenomenon, inclusive of con­taminant fluxes, in stormwater treatment is of much concern among practitioners (Hager, 2001).

The transport of pollutants accumulated during dry weather can be described using the theory of sediment transport with water combined with semi-empirical equations. The wash-off rate of pollutants is directly proportional to the amount of material remaining on the surface. During a storm event, the mass of pol­lution present on the pavement is decreasing exponentially with time (Hall & Hamilton, 1991). The relation can be described as:

M(t) = M0e-JRpwt (6.13)

where M(t) = pollutant mass on the pavement surface (M/L2) at time t (T); M0 = pollutant mass on the pavement surface at the beginning of the storm hydrograph (M/L2); J = rate coefficient (L2/M); R = runoff (L/T) and pw = density of water (M/L3).

At the beginning of the storm runoff event, various particles from dry deposition are remobilised. As a consequence of the interaction between water and dry sed­iment during the storm, the concentrations in the diluted phase are also changing with time.

Part of the runoff water is mobilised by the traffic to form splash and aerosols which will be wind-transported away from the road. The vast majority of the pol­lutants so mobilised will be deposited close to the road but the smaller particles will be wind transported further away from the road, at least some hundred me­tres (Blomqvist & Johansson, 1999; Folkeson, 2005). To what extent pollutants are transported in the form of splash/spray or in the form of pavement-surface runoff is governed by factors such as traffic characteristics, weather conditions, topography and the type and condition of the pavement surface. For instance, aerial transport is more limited where porous asphalt is used as compared to conventional asphalt (Legret & Colandini, 1999; Legret et al., 1999; Pagotto et al., 2000).

Mass transport in the unsaturated part of the road construction (the sub-base and upper part of the subgrade) strongly depends on the soil moisture distribution inside the pores. Where the mass transport is principally by advection then the water move­ment direction will control the contaminant flux direction. As the principal fluxes in the vadose zone are those due to evaporation and percolation, it follows that the
direction of the mass transport will then be essentially vertical, upwards or towards the lower part of the subgrade.

Soil moisture travelling through the unsaturated part of the road construction moves at different velocities in different pores due to the fact that saturated pores through which the moisture moves have different-sized pore throats and differ­ent thickness of the water film on the mineral grains of the soil. The theory of mass transport inside of unsaturated soil is much more complicated than in satu­rated media. The processes are described in standard textbooks (e. g. Fetter, 1993; Hillel, 2004).

In general, the structure of the equations for mass transport in unsaturated soil is similar to the equations for saturated soil. They differ in that the diffusion and dispersion coefficients and flow velocities for unsaturated soil depend on the water content.

The rate of change of the total pollutant mass present inside the unsaturated part of the road construction must be equal to the difference between the pollutant flux going into the road pavement and that leaving it and going into the saturated subgrade. Due to the complex processes inside the unsaturated road layers, several sources and sinks of pollutants can exist. These processes can be associated with biological decay (for organic contaminants) as well as chemical transformations and precipitation.

If occurring in large quantities, organic compounds originating in petroleum products form a special case of great concern in connection with roads. Spillages of petroleum products from traffic accidents or from petrol-filling stations, often situated adjacent to roads, may result in large quantities of organic compounds en­tering the road surface of roadside soils. The different interaction of these organic fluids with the soil’s chemistry will frequently increase the effective permeability and enhance the flux of the contaminants through the soil – behaviour referred to as incompatibility. Such situations are largely undesirable from an environmental and from other points of view.

Diffusion

The steady-state diffusion of solute in soil moisture is given by

„ dC

F = -D* (в) (6.10)

dx

where F = mass flux of solute (units of M/L2T); D*(e) = soil diffusion coefficient which is a function of the water content, the tortuosity of the soil, and other factors related to the water film on grains (units of L2/T); C = the concentration of the contaminant (units of M/L3), x = the distance in the direction of travel (units of L) and dC/dx = the concentration gradient in the soil moisture.

The second-order diffusion equation for transient diffusion of solutes in soil wa­ter is defined as

Advection

In road aggregates that are between saturated and residual saturation, some advec­tion can occur with higher saturation allowing advective transport to increase. In terms of contaminant destination, the influence of advection is important. Diffusion will occur evenly in all directions in which the differences in contaminant con­centrations exist, while advection will transport contaminants wherever the water is draining, either into a fin drain, or down the vadose zone towards the phreatic surface.

In an unsaturated soil, some of the void space is filled with gas. Due to evapora­tion, some contaminants will pass from the liquid phase into the gas phase both by volatilisation and by transport in water vapour. Contaminants within the gas will then be transported by diffusion and advection within the gas phase. Exchange processes transferring contaminants between the gas and liquid phases in the road construction are very complex. The transport in the gas phase inside a road construc­tion can be substantial, especially when incidental spills appear on the road surface. However, the extent to which these processes occur inside the road construction is not well known and may be small for most metals. Certainly, as a soil or aggregate becomes less saturated the opportunity for advective transport reduces markedly, particularly because of the substantial reduction in permeability as described in Chapter 2, Section 2.8.

Dispersion

Rather as in Eq. 6.7, the soil-moisture dispersion coefficient, D(e), is defined as the sum of the mechanical and diffusion mixing and is now expressed as:

D (в) = £ M + D* (в)

where £ = an empirical dispersivity measurement (units L) that depends on the soil moisture and v = the average linear soil moisture velocity. This definition of D(e) may be contrasted with that for Dt given above for saturated conditions (see Eq. 6.7) which includes a soil tortuosity term, a, in place of the £ term which is also controlled by the water content.

In a road aggregate where pores are only partially saturated, especially during dry periods, the capillary suction of the aggregate can increase very significantly, mov­ing the aggregate towards its residual saturation condition and hindering advective contaminant transport. In these conditions, contaminant transport will, therefore, be very slow and primarily occur by diffusion.