INTRODUCTION:

Background - Past Industrial and Waste Management Activities

In recent events the public was made aware of the hazards of contaminated land through incidents throughout the country. Waste treatment plant at Aravallis, Faridabad, has led to 8 lakhs tonnes of untreated soil waste dumped in eco-sensitive water recharge zones. Populations feeding off the ground water are at threat of consuming polluted water. Bhandwari waste treatment plant has not been operational since two years (figure-1). However the unprocessed waste has led to leachate seepage into the hills. TerraFirma waste processing unit in Karnataka receives waste generated from urban areas such as Bengaluru. Following heavy rains polluted water from the unit has flowed into tanks and ponds in nearby villages. Cattle have been reported dead as a result of consuming contaminated waters. Also, Government’s plans to landfill quarrying pits may end up destroying water recharge zones.

Figure-1: Bhandwari waste treatment plant - Slurry leaching out of the waste.[1]

Containment Technologies:

Physical barriers are used where a waste mass is too large for practical treatment, where residual from other treatment processes are land filled, and to contain soluble and mobile constituents that pose an immediate and imminent threat to a source of drinking water. The two types of physical containment are 1) In-ground barriers and 2) At-ground liners and covers. Such barriers consist of vertical trench excavated along the perimeter of a site, filled with bentonite slurry to support the trench and subsequently backfilled with a mixture of low permeability material (Figure 2). Such walls are keyed into an aquitard, a low permeability soil or rock formation.

Figure 2: Soil bentonite slurry trench cross-section

In-ground barriers comprise of vertical and low hydraulic conductivity in-ground trenches, walls or membranes installed around contaminated material to prevent lateral migration, usually penetrating into a naturally occurring low permeability basal stratum. Slurry trench cut-off walls are where the soil is excavated and the trench supported by stabilizing slurry. Construction can be carried out in single or double phase process. The single phase process is where the bentonite is left to self-harden in the trench. The excavation may consist of continuous trench (for smaller projects in flat-lying areas) or a series of panels (for larger projects where the ground is uneven). The double phase process is shown in (figure-3) where the cut-off extends to depth and where for practical reasons it would be detrimental to the slurry to excavate through it repeatedly for a significant time period. In this case the final cement-bentonite slurry often replaces bentonite excavation slurry via a tremie tube. The hydraulic conductivity achieved by cement-bentonite cut-offs is of the order of 10^-9 m/s [2].

Figure 3: Double phase slurry walls [3 ] Figure 4: Slurry-wall cut-off with geo-membrane [3]

Slurry trench cut-offs with geomembranes insertions are a recent development. They employ flexible membranes of very low permeability, usually HDPE, installed with cement-bentonite slurry (figure-4). Such systems may be considered essential when gas migration control is required and when high levels of contamination or aggressive chemicals are present. The hydraulic conductivity of these systems is of the order of 10^-11 m/s and 10^-12m/s.

Liners and Covers are used where contaminated material is subsequently placed and are constructed over contaminated material. They are designed to prevent contact between the contaminated grounds, prevent upward migration of gases or liquids, and the downward ingress of rain or surface water in order to limit the formation and migration of leachate [4]. Used in conjunction with a low permeability cover system, liner systems provide total encapsulation of the contaminated ground.

Laboratory Test method

Darcy's Law forms the basis for understanding the groundwater flow in the subsurface. Figure 5 is the schematic of Darcy's experiment to determine the laws of flow of water through clean sand. The flow of water through a column of sand is proportional to the change in hydraulic head and the cross-sectional area of the flow, and inversely proportional to the length of the sand column. The flow rate is related to the hydraulic head and the area of flow required by a constant of proportionality termed k [5]. This term is referred to as hydraulic conductivity or permeability.

Figure-5: Schematic of Darcy experiment.

Q = k i A --------------------(1)

where,

Q = flow rate (cm²/sec)

k = hydraulic conductivity (cm/sec)

i = hydraulic gradient (cm/cm)

A = cross-sectional area of flow measured perpendicular to the flow direction (cm²)

The hydraulic gradient, i, is described as the rate of change in which the head is lost as water flows through the porous media and is defined as:

i = (h₁-h₂)/l -----------------------(2)

where,

h₁ = head at location 1 (cm)

h₂ = head at location 2 (cm)

l = length of sand column (cm)

Also, contaminant transport through the low-permeability soils is governed by 1)advection 2) Diffusion 3) Loss or Gain of Solute Mass.

Advection is the process, whereby the contaminant (solute) is transported by flowing water in response to hydraulic gradient. For instance, if a mass of contaminant of concentration C_o is placed at one end of pipe, then in time t₁ it will travel a distance d=vt₁ as a plug due to advection alone.

In porous media such as soils, the flow passes through the voids. Therefore the average velocity, v, will be seepage velocity, V_s and can be expressed as follows:

V_s = V/n = V_d/n = Q/An -----------------(3)

where,

V_d = discharge velocity

n = porosity=volume of voids/total volume

Q = discharge through cross-sectional area, A

But, according to Darcy's Law from equation eq.(1)

v_d = k dh/dl = k i -----------------(4)

where dh/dl = i , is the hydraulic gradient and k is the hydraulic conductivity

Then from eq.3 and eq.4

v_s = k i/n ------------------(5)

The contaminant is also known to spread laterally. This spreading phenomenon is known as diffusion or dispersion. Dispersion can be due to motion of liquid i.e. requires a hydraulic gradient. This is called mechanical dispersion and occurs in pore channels due to seepage velocity. Dispersion can also be due to diffusion that occurs due to concentration gradients.

Solute transport is influenced by chemical reactions. The amount of concentrations (s) that is absorbed by the solids is commonly a function of the solute concentration (C) The partitioning of solutes between solids and liquid phases can be determined by experiments. The relationship can be represented by following equations:

S = K_d C ------------------(6)

Where,

S = mass of the solute adsorbed or precipitated on the solids per unit dry mass of the soil medium

C = solute concentration

K_d = distribution coefficient

K_d, represents the partitioning of the contaminants between the solution (liquid) and the solids. The chemical reactions and the adsorption result in the transfer of contaminants mass from pore water to solid mass. The transfer causes retardation in the movement of contaminant.

Engineering practices requires designing the cut-off walls with low hydraulic conductivity and are compatible with the leachate. The contaminants from waste may migrate through the barrier into the surrounding groundwater system with time. The rate of migration will be controlled by advection, dispersion and the loss and gain of solute mass as a result of the reactions. Laboratory tests were performed in a tri-axial cell (Figure 6). The hydraulic conductivity is determined based on Darcy’s Law. In constant-head method the hydraulic head is maintained constant through a sample of length, L and area A. The soil sample was placed in a column. The contaminant of interest was placed above the soil and allowed to migrate through the soil at a known hydraulic gradient.

Figure 6: Soil testing equipment (Permeameter)

MATERIALS:

The mixture ratios, in mass, used to realize a containment barrier were as follows: Bentonite: 35; Water: 934; GGBS: 120; Cement: 30. The use of tap water, as opposed to distilled water, recreates the usual site condition more accurately. The bentonite is a Volcay product (CE grade). It is a natural sodium-bentonite from India processed in UK. The slag references are Appleby Frodingham GGBS. Granulated slag is produced by fast cooling of blast-furnace slag in a slag-granulation pit filled with water. The cement is ordinary Portland cement, Castle OPC class 42.5 N. The bentonite is first mixed with water using a high speed colloidal shear mixer for five minutes. The slurry is stored overnight in an airtight container. Then the slag and cement are added to a Hobart mixer. The mixer is necessary because the mixture is too thick for the blender. The cement-bentonite mixture is poured into plastic moulds of 70-mm diameter and stored in a tank of tap water. Their characteristics are water content of 355% and 0.78 porosity (n).

RESULTS:

Results of the permeability were observed to be stable at 1.5*10^-9 m/s. Firstly, the hydraulic gradient was doubled (i=200) to observe permeability and flow of effluent. This was achieved by doubling the pressure on the influent line and keeping the same pressure in the effluent line. The pH and conductivity (µS) of the samples were collected at each location in the permeameter (influent line, effluent line, triaxial cell, in-burette and out burette). The results have been plotted for the influent and effluent lines versus time intervals as a basis for comparing the two parameters (Figure 7 and 8). The pH results have shown a typical value for cementations material of the order of 12. The increase in pH and conductivity from the influent to the effluent has underscored the adsorptive capability of cement-bentonite.

Figure 7: pH vs. time intervals Figure 8: Conductivity (µS) vs. time intervals

CONCLUSION AND RECOMMENDATIONS:

With the constant-head method, confining pressures were applied to observe any substantial changes in the permeability of the sample. The cement-bentonite sample has shown a reduction in the permeability with an increase in confining pressures. The permeability tests should be carried out with the falling-head test method. This ensures a rapid rate of results. The constant head permeameter testing simulated in-situ flow conditions in the sample and allowed a quantification of a range of metals extraction by the cement-bentonite sample. The capacity of the slurry wall to behave as an active barrier could not be verified. It is recommended that sections of sample be analyzed using scanning electron microscope (S.E.M.) to determine concentrations of chemical composition at various depths. S.E.M makes it possible to examine all manners of surfaces over a wide range of magnifications. X-Ray diffraction, XRD, could be used to examine the chemical compounds and crystalline nature of the specimen.

REFERENCES:

1. The Times of India (Delhi) Sep 2015 “Landfill in Fbd hills will make NCR aquifers toxic” http://epaperbeta.timesofindia.com/Article.aspx?eid=31808&articlexml=Landfill-in-Fbd-hills-will-make-NCR-aquifers-09092015002010.

2. Hester, R.E. and R.M. Harrison (1997) Contaminated Land and Its reclamation, Thomas Telford Publishing, London.

3. CIRIA (1996) Barrier Liners and Cover systems', Special publication 124, CIRIA, London.

4. Dianee Cox-Tramel, Revised: March 29, 1999 'Superfund Program' FOIA (Freedom of Information Act) URL: www.epa.gov/region02/superfnd/site_sum/0201290c.htm

5. Michael D. Lagrega, Phillip L. Buckingham, Jeffery C Evans (1994) Hazardous Waste Management, McGraw Hill Inc, New York, New York

Received on 15.10.2015 Accepted on 06.11.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 192-196

DOI: 10.5958/2231-3915.2015.00020.6