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FATE OF NITRATE AND PHOSPHATE THROUGH AGRICULTURAL SUB-SOIL ENVIRONMENT Sadashiva Murthy.B.M1, Ramesh. H.S.2& Mahadevaswamy. M.3 1,*) Assistant Professor, Department of Environmental Engineering, 2) Professor & Head, Department of Environmental Engineering 3) Professor, Department of Environmental Engineering

Sri Jayachamarajendra College of Engineering, MYSORE – 570 006, KARNATAKA, INDIA FAX: +091 821-2548290 EMAIL: [email protected]

Abstract: The connection of the groundwater pathway in the hydrologic cycle to the land surface provides the opportunity for anthropogenic impacts on groundwater and devalues the resource. Several sources of groundwater pollution have been identified, including leachate from sanitary landfills, industrial waste seepage from storage basins, industrial waste introduced through groundwater recharge, domestic waste from septic tanks, agrochemicals (fertilizer and pesticides) irrigation salts leached. So, with concern increasing over nitrates and phosphate contamination of ground water, today research is oriented towards various processes affecting the transport of chemicals in soils, which is ultimately determined by the relative rates of percolation, leaching, sorption and degradation within soil profile and is evaluated by different methods. As accurate quantification of solute under field conditions is inherently difficult, leaching characteristics of pollutants (nitrates and phosphates) have been analyzed by laboratory soil column and channel studies. The analysis of the breakthrough curves showed that, the dispersion co-efficient for sandy soil was 0.202 m2/d for column and 0.276 m2/d for channel studies. The study of leaching characteristics of nitrates and phosphates conducted upto 40 days through agricultural soil column revealed that the concentration of nitrates and phosphates increased steadily in the first few days (around 5 days) and then it increased rapidly after 8 to 10 days. However, it attained equilibrium after 20 days. Similarly, the channel studies revealed steady increase in concentration of nitrates and phosphates in the first few days (1 to 2 days) and rapid increase upto 15 days and thereafter it attained equilibrium. In the contemporary study, one-dimensional analytical model has been developed which include the advection, dispersion, and a characteristic term called “total elimination rate”. The comparative study of soil column and channel studies, and model output (in which the term total elimination rate “K” is considered as zero) exhibited a variation of 30 to 50 % in leaching characteristics of pollutants (nitrates and phosphates). Keywords: Vadoze Zone, Contaminant, Transport, Plume Model, Total Elimination Rate.

INTRODUCTION In the last few decades groundwater contamination has increased dramatically. Among the various land uses, agriculture is reported to be the main source of groundwater contamination. Therefore, it is vital to understand the various complex factors such as dispersion, advection, sorption, degradation and quantity of seepage water, which control the movement of contaminants in the porous medium (soil). The most evident broadly based scientific advances groundwater hydrology includes the transport of fluid, energy, mass in porous media, ground-water microbiology, theoretical and practical knowledge related to its contamination, etc. Groundwater is one of the earth’s widely distributed and most important resources. The existing demand for groundwater as a source of conventional water supply will continue to grow (Metry, 1976). The merits of groundwater over surface water includes, little or no need of treatment to ensure potable quality as natural groundwater has a minimal suspended solids, small concentration of bacteria and viruses, and often meager

concentrations of dissolved mineral salts, easy availability and natural annual replenishment by rainfall (Agrawal et al., 1999).To understand the mitigation and control of various pollutant sources, development and application of a wide range of mathematical modeling techniques are essential. Modeling techniques are thus used for assessing the probable impact on groundwater quality. It is also used to assess remedial measures, which is cost effective and sufficient to prevent serious degradation of groundwater quality (Rudraiah and Vinay, 2005). Thus, mathematical models are important tool, which plays a vital role in understanding the mechanisms of groundwater pollution problems.

OBJECTIVE OF THE STUDY In this study a one dimensional analytical model has been developed for multi component transport (nitrates and phosphates) through unsaturated zone and to check the validity of the laboratory experimental leachate data for nitrates and phosphates with the analytical model equations. For this, physicochemical characteristics of agricultural soil are determined and also one-dimensional leaching characteristics of pollutants, (nitrates and phosphates) through column and channel studies were assessed.

MATHEMATICAL FORMULATIONS OF THE MODEL The advective-dispersive equation provides a comprehensive framework for quantitatively describe mass transport with and without accompanying chemical reactions (Schwartz and Zhang, 2004). The general one-dimensional equation, a non-reactive compound, in a steady-state condition, homogeneous media, with constant dispersion coefficient is described by Schwartz and Zhang, (2003);Pachepsky, (2000) and Metry (1976) as: (∂C / ∂t) = Ds (∂2C /∂z2) – V (∂C / ∂z)

(1)

Where, C is the solute concentration (ML-3), Ds is the solute dispersion coefficient (L2 T-1), V is the average pore water velocity (LT-1), z is the distance (L) and t is the time (T). The first term on the right hand side, describes mass transport by dispersion, and the second term describes mass transport by advection. The governing one-dimensional transport equation for the transient transport of contaminants with advection, adsorption, and dispersion under a steady state condition is given by Domenico and Schwartz, (1998). (∂C / ∂t) = D [(∂2C /∂z2)/ Rd] –[V (∂C / ∂z)/ Rd] – K C

(2)

Where, D = Hydraulic Dispersion Coefficient (m2/d) V = Average Linear Velocity, (m/d), K = Total Elimination Rate, (d-1) Rd = RetardationFactor The Retardation factor is estimated as follows, Rd = [1+ (ρb/n) Kd]

(3)

Where, ρb = Bulk mass density of the porous medium (g/ cm3). Kd = the distribution Coefficient (cm3/g). n = Effective porosity. The total elimination rate (K) in the governing equation includes the rate constants of the four transformation reactions namely, degradation, volatilization, leaching and plant uptake. Mathematically these processes are incorporated as shown, K = kd + kv + kl + ku (4) Where, kd = Degradation Rate Constant, (d-1) kv = Volatilization rate constant, (d-1) kl = Rate of leaching, (d-1) ku = Rate of plant uptake, (d-1) Analytical solutions can be obtained only under restrictive assumptions and simplifications as follows:  The porous medium is assumed to be homogeneous, isotropic and steady-state flow conditions are assumed.

 The flow is considered to be unidirectional with constant water content, water density and water viscosity.

 The hydrodynamic dispersion coefficient is assumed to be constant throughout, first order reaction rate, linear sorption along with a set of uniform transport properties, simple flow domain geometry, and simplified pattern of sink and source distributions are considered.

SOLUTION FOR PULSE (INSTANTANEOUS) MODEL In practical terms, the migration of pollutants (agro-chemicals such as nutrients and pesticides) through agricultural sub-soil environment is more of a pulse/ instantaneous source. The solution for the equation (2) is as follows (Schwartz and Zhang, 1998): C (z, 0) = 0

for z > 0

B.C. 1: C (0, t) = Co for t > 0 B.C. 2:

∂c/ ∂z = 0

at z = ∞

C (z, t) = {M/ (4 π D’ t) 0.5} exp (-{(z- V’ t)/ (4 D’ t)}) exp (-K’ t)

(5)

Where, M = Mass spilled per cross sectional area (g / m2), T = Time (Days), D/ = D/ Rd V/ = V/ Rd ,K/ = K/ Rd. SOLUTION FOR CONTINUOUS MODEL The analytical solution for a continuous source pollutant (Equation (2) is as follows (Metry, 1976) : I.C.: C (z, 0) = 0 for z > 0 B.C.1: C (0, t) = Co for t > 0 B.C.2: ∂c/ ∂z = 0 at z = ∞ C/C0

= 0.5 [exp {z /2D/ (v/ – (v/2 + 4D/K/)} {erfc ((z – t ( v/2 + 4D/K// (4D/t) 0.5} + exp {z /2D/ (v/ + (v/2 + 4D/K/) } erfc

( (z + t ( v/2 + 4D/K// (4D/t) 0.5}]

(6)

Where, erfc = complementary error function. The complementary error function occurs frequently in solutions to the advective-dispersion equations. Usually complementary error function is well tabulated in the form of graphs, tables or mathematical equations providing numerical values. The complementary error functions of an integerβ, i.e., erfc (β) is derived from the error function erf (β) by the following relations: erfc (-β) = 1 + erf (β) erfc ( β) = 1 - erf (β)

PROCESSES INFLUENCING CONTAMINANT MIGRATION BIODEGRADATION Biodegradation rate is at which both the carbonaceous as well as nitrogenous materials get utilized by microbes in the soil. The rate of decomposition is assumed to be first order reaction and is mathematically expressed as (Schwartz and Zhang, 2004)

C = C0 e-kd t

(7)

C is the concentration, and k is the rate constant for the reaction and has the units of reciprocal time. A plot of ln (C/C0) versus t will yield a straight line. The rate constant k d can be determined directly from the slope of this line. VOLATILIZATION Volatilization model is based on the chemical mass balance for surface vegetation layer. The volatilization rate constant is given by (Haith et al., 2002): V = kV R C

(8)

Where, V = Chemical vaporized from surface vegetation during hour t (g ha-1), R = Relative volatility of the chemical and water during hour t (d-1), C = Chemical available for volatilization on vegetation at the beginning of hour t (g /ha) kV = Volatilization constant (d-1). LEACHING Water flow in the unsaturated zone of agricultural soil is of great importance in determining the fate of surface applied soluble nutrients and pesticides. Leaching of chemicals from the soil is more likely, and can be estimated from Equation 9 (Haith et al., 2002):

kl = ( PLt - BC ) / FCt

(9)

Where, PLt = Chemicals leached from vegetation during hour t (g ha-1), BC = Background concentration of the chemicals (g/ha-1),FCt = Actual amount of chemicals applied in field (g/ha-1 d-1). PLANT ROOT UPTAKE By assuming passive root uptake, the rate of chemical uptake by plants in the root zone [1/T] is expressed as: (Chu and Marino, 2004). kU = FS

(10)

Where, F = Transpiration stream concentration factor, S = Actual rate of water uptake by the crop [1/T]. In the present section various complex biogeochemical processes, in agricultural sub-soil environment are represented in the form of mathematical equations. These governing equations, narrate the rate constants ultimately affecting the contaminant transport in sub-soil.

VALIDATION OF THE MODEL DEVELOPED The topics of validation stand at the point of transition between model development and application of the model to problem solving. The validation process is described as, given the model structure and parameter estimates, determine behavior under different (observed) input conditions for comparison of the output response with observed behavior. Thus, validation plays an important role in reducing the uncertainty in model utility. In this study, to check the validity of the current model developed, an attempt has been made to compare laboratory experimental column and channel leachate data with the developed model output for nitrates and phosphates.

HYDRODYNAMIC DISPERSION CO-EFFICIENT FOR THE COMBINED LEACHING CHARACTERISTICS OF NITRATES AND PHOSPHATES From Fried and Cumbernous (1971), equation: D = (1/8) [{(z – u t 0.84)/ (t 0.84)0.5} - {(z – u t 0.16)/ (t 0.16)0.5}]2 where, D = Hydrodynamic dispersion coefficient in m2/d, Z = Depth of the soil layer in m, U = Velocity of the tracer solution in the soil layer in m/d, t 0.84, t0.16 = Time required for the relative concentrations of the concentration ratio at 0.160 and 0.840 to reach a particular depth Z.

DATA ANALYSIS AND INTERPRETATION For the analysis of physico-chemical properties of the sub-soil, standard procedures as per IS: 2720 Part IV (1965) was adopted. The results of the index properties of soil are specific Gravity- 2.55, bulk density- 1.7135, permeability – 1.44, water content – 20.66%, Particle size analysis – sandy soil. The chemical properties of soil are – pH -7.26, electrical conductivity (meq / 100g soil) – 1.9135, Nitrates (Initial value, mg/l) - 4.741, Phosphates (Initial value, mg/l) - 0.273.

In the present study, the transport of nitrates and phosphates in the agricultural sub-soil was ascertained by conducting laboratory soil column and channel leachate studies with an initial concentration of 11 mg / l of simulated potassium nitrate solution and 0.3 mg/l of simulated potassium dihydrogen orthophosphate under steady state conditions with a flooding depth maintained at 50 – 90 mm.

Fig. Fig. Fig. Fig. Fig. Fig. 43625187B.T.C B.T.C B.T.C B.T.C B.T.C B.T.C for for for for for for Concentration Concentration Concentration Concentration Concentration Concentration Versus Versus Versus Versus Versus Versus Depth Time Depth Time Time Depth Timefor for for for for for Phosphate Phosphate Nitrates Nitrates Phosphate Phosphate 1.0 through through through through throughChannel Column Column Channel Column Channel Channel and and and and and and Model Model Model Model Model Model output, output, output, output, output, output, 0.9 Time (Days) Total Total Total Total TotalElimination Elimination Elimination Elimination Elimination Rate, Rate, Rate, Rate, Rate, K K K K =0) K =0) =0) =0) =0) 0.8 1.0 0.9 0.8

5 Exp. Time, (Days) 10 Exp. 15 Exp. 4 Model 20 Exp. 8 Model 12 Model 25 Exp. 16 Model 30 Exp. 20 Model 5 Model 4 Exp. 10 Model 8 Exp. 12 Exp. 15 Model 16 Exp. 20 Model 20 Exp. 25 Model 30 Model 1.8

0.7

0.7 0.6

C /CC0/ C 0

0.6

0.5

0.5

0.4

0.4 0.3 0.3 0.2 0.1 0.2 0.0 0.1

0.0

0.3

0.6

0.9

1.2

1.5

Depth, (m)

0.0 0.0

0.2

0.4

0.6

Depth, (m)

0.8

1.0

1.2

The concentration profiles in terms of breakthrough curves are presented for laboratory column (Figure 1, 2, 3, and.4) and channel studies (Figure 5, 6, 7 and 8) for nitrates and phosphates. From Figure 1 and Figure 3, it is observed that the concentration of nitrates and phosphates in the column increases steadily during first few days (around 5 days), which further increases rapidly between 8 to 10 days and continued till 20 days. However, after 20 days it attains its equilibrium. This indicates that the subsoil gets saturated after 20 days of column run. The comparative study of model output and experimental studies (column and channel) are presented in the Figure1, 2, 3, 4, 5, 6, 7 and 8. The comparative study reveals that, model output data for nitrate (soil column studies) is greater (By 30 % to 50 %) than the experimental column studies. This is obvious as the total elimination rate (K) is neglected in the model analysis and thus concentration ratio is higher. However the experimental vertical soil column studies (phosphate) and horizontal flow channel studies (for nitrate and phosphate) are greater (almost by 30 % to 50 %) than the model output. The concentration profile in the form of concentration ratio for the nitrates and phosphates (Horizontal flow channel studies) at various time intervals are presented in Figure 5 and Figure 6, respectively. From Figure 5 and Figure 6, it is observed that the concentration of nitrates and phosphates is increasing steadily in the first few days (1 to 2 days) and then it is increasing rapidly after 10 to 15 days and there after it attains equilibrium. From the Figures 2 and 4 (column studies), and Figures 6 and 8 (channel studies) it is evident that there is decrease in concentration of nutrients with increase in depth. It is apparent that concentration of nutrients (nitrates and phosphates) decreases with increase in depth and the decreasing trend could be attributed to the accumulation of nitrates in the top soil layer and transformation reactions occurring in the deep porous medium.

CONCLUSIONS Based on the extensive laboratory studies and analytical review, the following conclusions are drawn: The laboratory column studies of nitrates and phosphates revealed that the nitrate and phosphate concentrations in the effluent increased steadily in the first few days (around 5 days) and thereafter it increased rapidly upto 10 days. However, it attained equilibrium after 20 days. The results of channel studies revealed that the concentration of nitrates and phosphates in the effluent increased steadily during first few days (1 to 2 days) and thereafter increased rapidly upto 10 to 15 days. It attained equilibrium after 18 days. The hydrodynamic dispersion co-efficient was estimated to be 0.262 m2/d and 0.114 m2/d, for nitrates and phosphates, respectively. From the breakthrough curves obtained from the channel leachate results, the hydrodynamic dispersion co-efficient was found to be 0.321 m2/d and 0.231 m2/d, for nitrates and phosphates, respectively The comparative study of model output and experimental studies (column and channel studies)

revealed that the variation in the leaching characteristics of pollutants (nitrate and phosphate) was found to be about 30 to 50 per cent.

ACKNOWLEDGEMENTS The authors convey their grateful thanks to the Ministry of Environment and Forests (MOEF), Government of India for funding the research project and Department of Environmental Engineering, Sri Jayachamarajendra College of Engineering, Mysore for giving us an opportunity for conducting project.

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