2. Materials and methods

#### 2.1. Study site and weather conditions

The Rastorf landfill (lat. 54� 16'N, long. 10� 19<sup>0</sup> E) in Schleswig-Holstein (Northern Germany) was actively operated from February 1977 to May 2005 with a total area of 105,000 m<sup>2</sup> and about 2.0 million tons of municipal domestic wastes were deposited in it (Figure 1).

Effectiveness of Grassland Vegetation on a Temporary Capped Landfill Site http://dx.doi.org/10.5772/intechopen.80324 3

Figure 1. Digital elevation model of the Rastorf landfill with the temporary capped area (section I–III) [15].

In case of this study, semipermeable, temporary capping systems intend a specific shutdown of the bioreactor, containing heterogeneous wastes and different amounts of biodegradable material, through controlled infiltration of precipitation into the waste body [4] and also allow

Temporary capping systems regularly consist of a recultivated layer, a drainage layer, and a sealing layer consisting of mineral substrates or in combination with polymers [6]. The major aim of the recultivated layer is to restrain landfill gas migration and to minimise leachate generation (precipitation contaminated with heavy metals or polycyclic hydrocarbons) by a high water storage capacity in combination with a distinct evapotranspiration rate from the

Therefore, the choice of a locally adapted vegetation type (grassland, shrubs, forest) is essential

establishment (erosion protection, slope stability), and avoid deep shrinkage-induced cracking (capillary rise from deeper horizons) and rooting to protect the sealing layer as last barrier

The functional requirements of the vegetation in the nutrient and water availability considering a proper air capacity and plant available water capacity [2], whereby the technical challenges in landfill construction, compacted installation versus loose installation of mineral substrates, can

The effectiveness of the vegetation can be assessed by the water balance or rather the leachate generation under the specific climate and soil conditions [4, 11, 12]. There are several modelling approaches of landfill capping systems, with and without polymers, combining water balance calculations with the predominant statistical-empirical Hydrologic Evaluation of Landfill Performance (HELP) model [13] or numerical models like Finite Element subsurface FLOW system (FEFLOW) [14]. Such predictive models can be used to support the planning of a landfill and/or to optimise the particular system from an economic point of view [12] and to verify the long-

This study presents modelled water balance data and in particular the annual leachate rate of the Rastorf landfill during an 8-year period in the context of (a) grassland vegetation and (b)

was actively operated from February 1977 to May 2005 with a total area of 105,000 m<sup>2</sup> and

about 2.0 million tons of municipal domestic wastes were deposited in it (Figure 1).

above the waste body depending on the thickness of the recultivated layer [4, 8–10].

), a quick vegetation

E) in Schleswig-Holstein (Northern Germany)

to ensure high evapotranspiration rates (grassland: 450–550 mm year�<sup>1</sup>

significantly influence the growth conditions of the vegetation [3].

term hydraulic stability of a final capping system.

local weather conditions.

2. Materials and methods

2.1. Study site and weather conditions

The Rastorf landfill (lat. 54� 16'N, long. 10� 19<sup>0</sup>

biogas extraction [5].

2 Forage Groups

vegetation and soil surface [3, 7].

The temporary capped area of nearly 75,000 m2 with three sections (I: 21,275 m2 , II: 29,961 m2 , III: 22,208 m2 ) consists of three mineral layers (boulder marl) with a partially permeable recultivated layer (humus topsoil: 40 cm, humus-poor subsoil: 30 cm) and, below this layer, is a low permeable, 30 cm thick mineral sealing layer, which serves as a water and root barrier to prevent leachate formation and the groundwater contamination. The bottom layer consists of hardly permeable up to 20 m thick clay. A high-density polymer of 2.5 mm thickness and a drainage system above the bottom layer collects the leachate before the treatment by inverse osmosis (Figure 2).

Figure 2. Schematic cross section through the temporary capped area with water balance components, data logger and measuring devices in 20, 50, 80 and 100 cm depth.


2.4. Estimation of the water balance components of the Rastorf landfill

daily basis. The global solar radiation was calculated on the basis of [21].

) per area with a folding ruler according to [21]:

plant transpiration computed by a simplified approach of [26]:

2015.

sects (1 m2

classified according to [23].

2.5. HELP modules

to [25].

measurements according to [20]. The leachate rate (L) was calculated as follows:

The HELP model was validated with actual landfill data with respect to field and laboratory

Effectiveness of Grassland Vegetation on a Temporary Capped Landfill Site

http://dx.doi.org/10.5772/intechopen.80324

where: L = leachate rate, P = precipitation, ET = actual evapotranspiration (including interception), R = runoff, D = lateral drainage (interflow) and ΔS = change in soil moisture content in mm year-1 and m3 and it is the time step, performed from January 1, 2012 until December 31,

A separate water balance was modelled for each area (I–III) and a weather station located close to the landfill recorded the actual meteorological data such as precipitation (uncorrected), air temperature, wind speed, wind direction, air pressure, air moisture, and relative humidity on

In addition, the wind speed was measured in 10 m height and a logarithmic approximation was used to calculate the wind speed for 2 m height. The leaf area index (LAI) was calculated on the basis of the quarterly measured average vegetation height (h) in 8–10 repetitive tran-

The average root intensity was determined annually on the basis of repetitive soil profile images in the three areas with the colour threshold method using ImageJ software [22] and

The water balance calculations based on analytical and empirical equations, while a detailed description is shown in [19, 24]. With regard to the atmospheric boundary conditions, the method used in the HELP 3.95 D for calculating evapotranspiration was designed according

The potential evapotranspiration consists of (a) evaporation of surface water (primarily evaporation of intercepted water, besides this evaporation of snow), (b) soil evaporation, and (c)

where: Eoi = potential evapotranspiration on day i (mm), PENRi = radiative component of the Penman equation on day i (langleys), PENAi = aerodynamic component of the Penman

ð1Þ

5

ð2Þ

ð3Þ

ð4Þ

Table 1. Average slope gradient, slope length, and exposure of the sections I–III.

The maritime, semi-humid climate in Rastorf is characterised by an annual precipitation rate which is in the long-term average regularly between 6 and 9 months year-1 higher than the potential evapotranspiration rate [16]. The local weather conditions also affect water balances of the landfill capping system with 10-year average precipitation rates of 728 mm and, between 2012 and 2015, an average annual temperature of 9.0�C. The slope gradient varies between 7 and 16� and the slope length between 48 and 99 m (Table 1).

#### 2.2. Laboratory measurements

In 2012, more than 160 undisturbed soil cores (100 cm3 ) were sampled in the capping system in vertical (90�) and horizontal (0�) direction in area I (54�28<sup>0</sup> 20"N, 10�32<sup>0</sup> 6000E), II (54�28<sup>0</sup> 11"N, 10�32<sup>0</sup> 7100E) and III (54�28<sup>0</sup> 08"N, 10�32<sup>0</sup> 7500E) in depths of 0.2, 0.5 and 0.8 m. The saturated hydraulic conductivity (Ks) was measured under instationary conditions (n = 10 per depth) according to [17]. The pore size distribution (n = 7 per depth) was determined by a combined pressure plate (saturated, �6, �30) and � 1500 kPa ceramic vacuum outflow method as well as oven-dried at 105�C, respectively [18].

#### 2.3. Hydraulic Evaluation of Landfill Performance (HELP) model

The Hydraulic Evaluation of Landfill Performance (HELP) model is a quasi two-dimensional hydrologic model which combines one dimensional soil physical and hydrological processes in (a) vertical direction and (b) lateral direction according to [13]. Thus, the model requires data of the landfill design, weather conditions, and material properties such as porosity, field capacity, wilting point and saturated hydraulic conductivity as input parameters [19]. In addition, the evaporative zone corresponds to the root depth of the vegetative cover and was calculated to quantify the maximum soil depth from which water can be removed through evapotranspiration [12].

With respect to the landfill design data, the upper part of the recultivation layer (0–0.4 m) was classified as vertical percolation layer, the bottom part (0.4–0.7 m) was conducted as lateral drainage layer to take into account the lateral saturated hydraulic conductivity. The sealing layer was classified as barrier soil liner.

The HELP model was validated with actual landfill data with respect to field and laboratory measurements according to [20].

#### 2.4. Estimation of the water balance components of the Rastorf landfill

The HELP model was validated with actual landfill data with respect to field and laboratory measurements according to [20]. The leachate rate (L) was calculated as follows:

$$\mathbf{L(t\_i)} = \mathbf{P(t\_i)} - \mathbf{ET(t\_i)} - \mathbf{R(t\_i)} - \mathbf{D(t\_i)} \pm \Delta \mathbf{S(t\_i)} \tag{1}$$

where: L = leachate rate, P = precipitation, ET = actual evapotranspiration (including interception), R = runoff, D = lateral drainage (interflow) and ΔS = change in soil moisture content in mm year-1 and m3 and it is the time step, performed from January 1, 2012 until December 31, 2015.

A separate water balance was modelled for each area (I–III) and a weather station located close to the landfill recorded the actual meteorological data such as precipitation (uncorrected), air temperature, wind speed, wind direction, air pressure, air moisture, and relative humidity on daily basis. The global solar radiation was calculated on the basis of [21].

In addition, the wind speed was measured in 10 m height and a logarithmic approximation was used to calculate the wind speed for 2 m height. The leaf area index (LAI) was calculated on the basis of the quarterly measured average vegetation height (h) in 8–10 repetitive transects (1 m2 ) per area with a folding ruler according to [21]:

$$\mathbf{LAI} = \mathbf{24} \cdot \mathbf{h} \tag{2}$$

The average root intensity was determined annually on the basis of repetitive soil profile images in the three areas with the colour threshold method using ImageJ software [22] and classified according to [23].

#### 2.5. HELP modules

The maritime, semi-humid climate in Rastorf is characterised by an annual precipitation rate which is in the long-term average regularly between 6 and 9 months year-1 higher than the potential evapotranspiration rate [16]. The local weather conditions also affect water balances of the landfill capping system with 10-year average precipitation rates of 728 mm and, between 2012 and 2015, an average annual temperature of 9.0�C. The slope gradient varies between 7

Area I II III Average slope gradient (�) 7 � 3 14 � 3 16 � 4 Average slope length (m) 99 � 65 48 � 23 69 � 4 Exposure N/NE SE SW

hydraulic conductivity (Ks) was measured under instationary conditions (n = 10 per depth) according to [17]. The pore size distribution (n = 7 per depth) was determined by a combined pressure plate (saturated, �6, �30) and � 1500 kPa ceramic vacuum outflow method as well as

The Hydraulic Evaluation of Landfill Performance (HELP) model is a quasi two-dimensional hydrologic model which combines one dimensional soil physical and hydrological processes in (a) vertical direction and (b) lateral direction according to [13]. Thus, the model requires data of the landfill design, weather conditions, and material properties such as porosity, field capacity, wilting point and saturated hydraulic conductivity as input parameters [19]. In addition, the evaporative zone corresponds to the root depth of the vegetative cover and was calculated to quantify the maximum soil depth from which water can be removed through

With respect to the landfill design data, the upper part of the recultivation layer (0–0.4 m) was classified as vertical percolation layer, the bottom part (0.4–0.7 m) was conducted as lateral drainage layer to take into account the lateral saturated hydraulic conductivity. The sealing

The HELP model was validated with actual landfill data with respect to field and laboratory

) were sampled in the capping system in

6000E), II (54�28<sup>0</sup>

11"N,

20"N, 10�32<sup>0</sup>

7500E) in depths of 0.2, 0.5 and 0.8 m. The saturated

and 16� and the slope length between 48 and 99 m (Table 1).

Table 1. Average slope gradient, slope length, and exposure of the sections I–III.

In 2012, more than 160 undisturbed soil cores (100 cm3

vertical (90�) and horizontal (0�) direction in area I (54�28<sup>0</sup>

08"N, 10�32<sup>0</sup>

2.3. Hydraulic Evaluation of Landfill Performance (HELP) model

2.2. Laboratory measurements

The symbol � corresponds to the standard deviation.

7100E) and III (54�28<sup>0</sup>

evapotranspiration [12].

layer was classified as barrier soil liner.

measurements according to [20].

oven-dried at 105�C, respectively [18].

10�32<sup>0</sup>

4 Forage Groups

The water balance calculations based on analytical and empirical equations, while a detailed description is shown in [19, 24]. With regard to the atmospheric boundary conditions, the method used in the HELP 3.95 D for calculating evapotranspiration was designed according to [25].

The potential evapotranspiration consists of (a) evaporation of surface water (primarily evaporation of intercepted water, besides this evaporation of snow), (b) soil evaporation, and (c) plant transpiration computed by a simplified approach of [26]:

$$\mathbf{E}\_{\mathbf{o}\_{l}} = \frac{\mathbf{PENR\_{l}} + \mathbf{PENA\_{l}}}{\mathbf{L\_{v}}} \tag{3}$$

$$\mathbf{L\_{v}} = \begin{cases} \mathbf{59.7} - \mathbf{0.0564} \mathbf{T\_{c\_l}} & \text{for water} \\ \mathbf{67.67} - \mathbf{0.0564} \mathbf{T\_{c\_l}} & \text{for snow} \end{cases} \tag{4}$$

where: Eoi = potential evapotranspiration on day i (mm), PENRi = radiative component of the Penman equation on day i (langleys), PENAi = aerodynamic component of the Penman equation on day i (langleys), Lv = latent heat for vaporisation (for evaporating water) or latent heat of fusion (for evaporating snow) in langleys per mm and Ts = snow temperature (�C).

The actual evapotranspiration (ETa) was mainly calculated by an approach of [25] using a model of vegetation growth and decay by [27]. Thus, the vegetative growth and decay sub-model included in HELP was taken from the model SWRRB [27]. The ETa is limited by the water availability at the landfill surface and the maximum depth of the evaporative zone according to [20]. Therefore, the plant available water capacity inside the evaporative zone (field capacity– wilting point) can only be removed by evapotranspiration, while the field capacity (US: �330 hPa) is the lowest soil water content to allow unsaturated vertical flow (drainage) within the evaporative zone [28]. The capacity of the interception storage and the interception height were calculated following Hoyningen-Huene (1983), modified and adapted to German standards by [28].

The area factor v was implemented in the modelling approach and corresponds to the ratio of the monthly sums of the global solar radiation (Rs) on inclined and horizontal reception areas consider the exposure and the inclination angle (�) and a corrected albedo of 0.23 in the summer-half (05/01-10/31) and in the winter-half (11/01-04/30) under climatic conditions in Germany [29].

The vertical percolation (drainage) is estimated using the equation for the unsaturated hydraulic conductivity (Eq. 4) which is based on [30]. The saturated lateral drainage is modelled by a steady-state solution of the Boussinesq equation in combination with the Dupuit-Forchheimer (Forchheimer, 1930) assumptions, which take into account the Ks value of the drainage layer. The unsaturated conductivity for each soil layer was calculated as follows:

$$\mathbf{K}\_{\mathbf{u}} = \mathbf{K}\_{\mathbf{u}} \left[ \frac{\boldsymbol{\theta} - \boldsymbol{\theta}\_{\mathbf{r}}}{\boldsymbol{\Phi} - \boldsymbol{\theta}\_{\mathbf{r}}} \right]^{\mathbf{a} + \left( \frac{\mathbf{r}}{\mathbf{x}} \right)} \tag{5}$$

The curve numbers for the areas I–III were obtained under the terms of the surface slope, the slope length, and the vegetation cover and also modified according to the previous sensitivity

The lateral drainage layer required information about the maximum drainage length as length of the horizontal projection of a representative flow path and the drain slope for the areas I–III

where: x\* = x/L (nondimensional horizontal distance), y\* = y/L (nondimensional depth of saturation above liner), qD\*=qD/KD (nondimensional lateral drainage rate) with KD = saturated hydraulic conductivity of the drain layer (cm/s) and α = inclination angle of the liner surface.

The validity of the data used as input and output values for the comparison of observed and modelled data is of major importance [20]. Therefore, the sensitivity analysis, calibration, and validation for the period from 2008 to 2015 were performed in a previous study on the basis of

Therefore, an increasing evaporative zone depth from 10 to 100 cm can increase the actual

The associated calibration study made it necessary to implement a lateral drainage layer instead of a vertical percolation layer in 0.4–0.7 m depth to take into account the basic concept of the landfill capping system due to anisotropic Ks values of the compacted layer (see Section 2.4).

ð7Þ

7

ð8Þ

ð9Þ

), S = potential maximum soil moisture retention

Effectiveness of Grassland Vegetation on a Temporary Capped Landfill Site

http://dx.doi.org/10.5772/intechopen.80324

. The retention parameter S is transformed into a

; an increasing LAI from 1 to 5 can increase the ETa

. Additionally, an increasing slope of the drainage layer from 2–30%

) is an index of goodness of fit between the observed and mode-

) and Ia = initial abstractions (sum of interception + evapotranspira-

analysis. The SCS-CN method based on the following basic form [32]:

), P = precipitation (m3

[30]. The lateral drainage equation can be described as follows [19]:

where: R = runoff (m<sup>3</sup>

auf the runoff begins (m<sup>3</sup>

tion + infiltration + depression storage) in m<sup>3</sup>

2.6. Model calibration and sensitivity analysis

input and output values of the HELP model [28].

can reduce the annual leachate rate of about 25%.

evapotranspiration up to 100 mm year�<sup>1</sup>

values up to 85 mm year�<sup>1</sup>

The correlation coefficient (r<sup>2</sup>

lled data according to [33].

curve number (CN) with following relationship [24]:

where: Ku = unsaturated hydraulic conductivity (cm s�<sup>1</sup> ), Ks = saturated hydraulic conductivity (cm s�<sup>1</sup> ), θ = actual volumetric water content (m<sup>3</sup> m�<sup>3</sup> ), θ<sup>r</sup> = residual volumetric water content (m<sup>3</sup> m�<sup>3</sup> ), Φ = total porosity (m<sup>3</sup> m�<sup>3</sup> ) and λ = pore-size distribution index (�).

Therefore, θ<sup>r</sup> is the amount of water remaining in a layer under infinite capillary suction and was estimated as follows [24]:

$$\Theta\_r = \begin{cases} \text{0.6 WP} & WP < \text{0.04} \\ \text{0.014} + \text{0.25 WP} & WP \ge \text{0.04} \end{cases} \tag{6}$$

where: WP = volumetric wilting point (m3 m�<sup>3</sup> ).

The leakage rate depends upon the depth of the water-saturated soil (head) above the base of the layer, the liner thickness and the Ks value of the barrier soil. So, the leakage occurs whenever the moisture content of the layer above the liner is greater than the field capacity of the layer [19, 24].

In addition, the rainfall-runoff process is modelled using the SCS curve-number method with values above 0 up to 100, as presented in Section 4 of the National Engineering Handbook [31]. The curve numbers for the areas I–III were obtained under the terms of the surface slope, the slope length, and the vegetation cover and also modified according to the previous sensitivity analysis. The SCS-CN method based on the following basic form [32]:

$$\mathbb{R} = \begin{cases} \frac{\left(\mathbb{P} - \mathbb{I}\_{\mathfrak{d}}\right)^{2}}{\mathbb{P} - \mathbb{I}\_{\mathfrak{d}} + \mathbb{S}} & \mathbb{P} > \mathbb{I}\_{\mathfrak{d}}\\ 0 & \mathbb{P} \le \mathbb{I}\_{\mathfrak{d}} \end{cases} \tag{7}$$

where: R = runoff (m<sup>3</sup> ), P = precipitation (m3 ), S = potential maximum soil moisture retention auf the runoff begins (m<sup>3</sup> ) and Ia = initial abstractions (sum of interception + evapotranspiration + infiltration + depression storage) in m<sup>3</sup> . The retention parameter S is transformed into a curve number (CN) with following relationship [24]:

$$\text{CN} = \frac{\textbf{1000}}{\textbf{S} + \textbf{10}} \tag{8}$$

The lateral drainage layer required information about the maximum drainage length as length of the horizontal projection of a representative flow path and the drain slope for the areas I–III [30]. The lateral drainage equation can be described as follows [19]:

$$\mathbf{y}^\* = \frac{\mathbf{d}^2 \mathbf{y}^\*}{\mathbf{d} \mathbf{x}^{\*\mathbf{t}}} + \left(\frac{\mathbf{d} \mathbf{y}^\*}{\mathbf{d} \mathbf{x}^\*}\right)^2 + (\tan \alpha) \frac{\mathbf{d} \mathbf{y}^\*}{\mathbf{d} \mathbf{x}^\*} = \frac{\mathbf{q}\_\mathbf{D}}{\cos^2 \alpha} \tag{9}$$

where: x\* = x/L (nondimensional horizontal distance), y\* = y/L (nondimensional depth of saturation above liner), qD\*=qD/KD (nondimensional lateral drainage rate) with KD = saturated hydraulic conductivity of the drain layer (cm/s) and α = inclination angle of the liner surface.

#### 2.6. Model calibration and sensitivity analysis

equation on day i (langleys), Lv = latent heat for vaporisation (for evaporating water) or latent heat of fusion (for evaporating snow) in langleys per mm and Ts = snow temperature (�C).

The actual evapotranspiration (ETa) was mainly calculated by an approach of [25] using a model of vegetation growth and decay by [27]. Thus, the vegetative growth and decay sub-model included in HELP was taken from the model SWRRB [27]. The ETa is limited by the water availability at the landfill surface and the maximum depth of the evaporative zone according to [20]. Therefore, the plant available water capacity inside the evaporative zone (field capacity– wilting point) can only be removed by evapotranspiration, while the field capacity (US: �330 hPa) is the lowest soil water content to allow unsaturated vertical flow (drainage) within the evaporative zone [28]. The capacity of the interception storage and the interception height were calculated

following Hoyningen-Huene (1983), modified and adapted to German standards by [28].

Germany [29].

6 Forage Groups

ity (cm s�<sup>1</sup>

content (m<sup>3</sup> m�<sup>3</sup>

was estimated as follows [24]:

The area factor v was implemented in the modelling approach and corresponds to the ratio of the monthly sums of the global solar radiation (Rs) on inclined and horizontal reception areas consider the exposure and the inclination angle (�) and a corrected albedo of 0.23 in the summer-half (05/01-10/31) and in the winter-half (11/01-04/30) under climatic conditions in

The vertical percolation (drainage) is estimated using the equation for the unsaturated hydraulic conductivity (Eq. 4) which is based on [30]. The saturated lateral drainage is modelled by a steady-state solution of the Boussinesq equation in combination with the Dupuit-Forchheimer (Forchheimer, 1930) assumptions, which take into account the Ks value of the drainage layer.

Therefore, θ<sup>r</sup> is the amount of water remaining in a layer under infinite capillary suction and

). The leakage rate depends upon the depth of the water-saturated soil (head) above the base of the layer, the liner thickness and the Ks value of the barrier soil. So, the leakage occurs whenever the moisture content of the layer above the liner is greater than the field capacity of the layer [19, 24]. In addition, the rainfall-runoff process is modelled using the SCS curve-number method with values above 0 up to 100, as presented in Section 4 of the National Engineering Handbook [31].

ð5Þ

ð6Þ

), Ks = saturated hydraulic conductiv-

) and λ = pore-size distribution index (�).

), θ<sup>r</sup> = residual volumetric water

The unsaturated conductivity for each soil layer was calculated as follows:

), θ = actual volumetric water content (m<sup>3</sup> m�<sup>3</sup>

where: Ku = unsaturated hydraulic conductivity (cm s�<sup>1</sup>

where: WP = volumetric wilting point (m3 m�<sup>3</sup>

), Φ = total porosity (m<sup>3</sup> m�<sup>3</sup>

The validity of the data used as input and output values for the comparison of observed and modelled data is of major importance [20]. Therefore, the sensitivity analysis, calibration, and validation for the period from 2008 to 2015 were performed in a previous study on the basis of input and output values of the HELP model [28].

Therefore, an increasing evaporative zone depth from 10 to 100 cm can increase the actual evapotranspiration up to 100 mm year�<sup>1</sup> ; an increasing LAI from 1 to 5 can increase the ETa values up to 85 mm year�<sup>1</sup> . Additionally, an increasing slope of the drainage layer from 2–30% can reduce the annual leachate rate of about 25%.

The associated calibration study made it necessary to implement a lateral drainage layer instead of a vertical percolation layer in 0.4–0.7 m depth to take into account the basic concept of the landfill capping system due to anisotropic Ks values of the compacted layer (see Section 2.4).

The correlation coefficient (r<sup>2</sup> ) is an index of goodness of fit between the observed and modelled data according to [33].
