3. Results

## 3.1. Vegetation growth of the Rastorf landfill

The recultivated layer of the temporary capped area is used as pasture with a grass and clover mixture of flat-rooted, densely growing, and perennial grasses. The seed mixture used in 2008/ 2009 was composed as follows: 20% perennial ryegrass (Lolium perenne), 20% cocksfoot (Dactylis glomerata), 21% red fescue (Festuca rubra), 21% sheep fescue (Festuca ovina), 10% meadow grass (Poa pratensis), 8% white clover (Trifolium repens), and a biannual mowing is carried out. Nowadays, the total coverage of the grass and clover mixture varies between 85 and 100% across the landfill surface (Figure 3).

The species composition is significantly different from the initial seed mixture after several years of growth: 70–80% cocksfoot (Dactylis glomerata), red and sheep fescue (Festuca rubra, ovina), and meadow grass (Poa pratensis), respectively (Figure 3).

The white clover (Trifolium repens) was characterised by an area fraction of about 5% and perennial ryegrass (Lolium perenne) with an area fraction of about 10%, predominantly on the areas (1000 m2 ) subsequently secured in 2013 because of the reduced vegetation growth with locally available compost made out of tree and shrub cutting (Figure 4).

> The landscape-ecological inventories and pedological excavations during 2013 and 2015 resulted in fine roots that were able to reach a maximum depth of 25–30 cm (flat rooting) and

> the upper part of the recultivated layer (Figure 4). The subsequently secured areas showed deeper and pronounced rooting with depths of 35–40 cm (medium rooting) and a medium to

> The evapotranspiration parameters for the HELP model are summarised in Table 2. The average annual wind speed varied between 4.35 m s<sup>1</sup> in 2015 and 4.91 m s<sup>1</sup> in 2013 and the average relative humidity (%) between 70.6 and 87.3% in the spring and summer months and between 82.5 and 95.2% in the autumn and winter months (Table 2). Additionally, the maximum leaf area indices with values between 1.0 and 3.5 were chosen on the basis of average

> The porosities of the boulder marl differ between 0.292 and 0.307 m3 m<sup>3</sup> in the barrier soil layer and 0.317 and 0.356 m3 m<sup>3</sup> in the drainage layer as well as the percolation layer. The FC

> 0.167 m<sup>3</sup> m<sup>3</sup> (Table 3). The highest Ks values were identified in the drainage layer between

).

Figure 4. Vegetation growth before (left) and after (right) compost application June 2013.

annual LAI measurements in March, May, and July and October, respectively.

3.2. Weather data, vegetative period and leaf are index

3.3. Landfill design and soil physical parameters

values range between 0.175 and 0.213 m3 m<sup>3</sup>

), mainly along smaller hair or shrinkage cracks in

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, while the WP values varied between 0.117 and

a weak to medium intensity (< 10 roots dm<sup>2</sup>

strong intensity (11–20 roots dm<sup>2</sup>

Figure 3. Vegetation growth of the Rastorf landfill between 2008 (left) and 2015 (right).

Effectiveness of Grassland Vegetation on a Temporary Capped Landfill Site http://dx.doi.org/10.5772/intechopen.80324 9

Figure 4. Vegetation growth before (left) and after (right) compost application June 2013.

The landscape-ecological inventories and pedological excavations during 2013 and 2015 resulted in fine roots that were able to reach a maximum depth of 25–30 cm (flat rooting) and a weak to medium intensity (< 10 roots dm<sup>2</sup> ), mainly along smaller hair or shrinkage cracks in the upper part of the recultivated layer (Figure 4). The subsequently secured areas showed deeper and pronounced rooting with depths of 35–40 cm (medium rooting) and a medium to strong intensity (11–20 roots dm<sup>2</sup> ).

#### 3.2. Weather data, vegetative period and leaf are index

3. Results

8 Forage Groups

areas (1000 m2

3.1. Vegetation growth of the Rastorf landfill

and 100% across the landfill surface (Figure 3).

ovina), and meadow grass (Poa pratensis), respectively (Figure 3).

locally available compost made out of tree and shrub cutting (Figure 4).

Figure 3. Vegetation growth of the Rastorf landfill between 2008 (left) and 2015 (right).

The recultivated layer of the temporary capped area is used as pasture with a grass and clover mixture of flat-rooted, densely growing, and perennial grasses. The seed mixture used in 2008/ 2009 was composed as follows: 20% perennial ryegrass (Lolium perenne), 20% cocksfoot (Dactylis glomerata), 21% red fescue (Festuca rubra), 21% sheep fescue (Festuca ovina), 10% meadow grass (Poa pratensis), 8% white clover (Trifolium repens), and a biannual mowing is carried out. Nowadays, the total coverage of the grass and clover mixture varies between 85

The species composition is significantly different from the initial seed mixture after several years of growth: 70–80% cocksfoot (Dactylis glomerata), red and sheep fescue (Festuca rubra,

The white clover (Trifolium repens) was characterised by an area fraction of about 5% and perennial ryegrass (Lolium perenne) with an area fraction of about 10%, predominantly on the

) subsequently secured in 2013 because of the reduced vegetation growth with

The evapotranspiration parameters for the HELP model are summarised in Table 2. The average annual wind speed varied between 4.35 m s<sup>1</sup> in 2015 and 4.91 m s<sup>1</sup> in 2013 and the average relative humidity (%) between 70.6 and 87.3% in the spring and summer months and between 82.5 and 95.2% in the autumn and winter months (Table 2). Additionally, the maximum leaf area indices with values between 1.0 and 3.5 were chosen on the basis of average annual LAI measurements in March, May, and July and October, respectively.

#### 3.3. Landfill design and soil physical parameters

The porosities of the boulder marl differ between 0.292 and 0.307 m3 m<sup>3</sup> in the barrier soil layer and 0.317 and 0.356 m3 m<sup>3</sup> in the drainage layer as well as the percolation layer. The FC values range between 0.175 and 0.213 m3 m<sup>3</sup> , while the WP values varied between 0.117 and 0.167 m<sup>3</sup> m<sup>3</sup> (Table 3). The highest Ks values were identified in the drainage layer between


Table 2. Input data for the HELP model: Evapotranspiration parameters (latitude 54.2).


precipitation rates with 655 and 669 mm, respectively, compared to the average annual precipitation rate of 728 mm. The winters of 2008–2015 were mostly mild and only had

Table 4. Average annual water balance components for the period between 2008 and 2015.

Precipitation 757 726 852 760 655 669 753 767 Potential evapotranspiration\* 602 619 555 557 526 556 571 534 Actual evapotranspiration\* 277 284 280 383 390 332 362 364 Outflow\*\* 351 297 457 262 179 270 285 300 Δ soil moisture content 0 0.5 0.5 1.5 0.2 0.2 0.3 0.3 Leachate 149 137 116 103 84 70 109 105

The modelled average annual ETa values ranged between 33% in 2010 and 60% in 2012, and the outflow rates between 39% in 2009 and 54% in 2010 of the annual precipitation. The changes in soil moisture content with 0.3 and 1.5 mm year<sup>1</sup> were moderate and the modelled leachate rates ranged between 14 and 18% in 2008–2010, and between 11 and 15%

These drier phases are characterised by higher discrepancies between ETp and ETa up to

other side, the early warming phase during March to May showed moderate discrepancies of

The ETa values ranged between 46 and 50%, and since 2011 between 60 and 69% of the ETp with the increasing depth of the evaporative zone (20, 30–50 cm) and the maximum leaf area

Figure 5. Average modelled potential and actual evapotranspiration rates (ETp, ETa) between 2008 and 2015 for the areas

, especially in the warmer months between June and September (Figure 5). On the

, and the period October to February of the following year indicated mostly no

2008 2009 2010 2011 2012 2013 2014 2015

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some snow.

Water balance [mm year<sup>1</sup>

]

\*Including interception.

\*\*Surface runoff and lateral drainage.

4.9 mm d<sup>1</sup>

I–III.

0.6–2.7 mm d<sup>1</sup>

in 2011–2015 of the annual precipitation (Table 4).

discrepancies between the potential and actual evapotranspiration.

Data of the three subcatchment areas (I–III), n = 7–10 undisturbed soil cores per layer for the average values of porosity, field capacity (FC), wilting point (WP) and saturated hydraulic conductivity (Ks), including initial water content (WC) and slope length and gradient.

\*Field capacity (33 kPa), \*\*Wilting point, \*\*\*Water content at the beginning of the year 2012.

Table 3. Input data for the HELP model: landfill design and soil physical parameters.

5.6e-04 and 6.3e-04 m s<sup>1</sup> , lower values were determined in the percolation layer between 4.5e-06 and 5.9e-06 m s<sup>1</sup> and the barrier soil layer had values ≤6.1e-09 m s<sup>1</sup> .

#### 3.4. Modelled water balance components of the Rastorf landfill between 2008 and 2015

In the study period between 2008 and 2015, the climatic water balance was positive (precipitation > evapotranspiration) and with regard to the German weather conditions, the outflow (2008–2010) and the actual evapotranspiration (2011–2015) were the greatest output values of the water balance (Table 4). The years 2012 and 2013 showed lower annual


\*Including interception.

5.6e-04 and 6.3e-04 m s<sup>1</sup>

and slope length and gradient.

Average annual wind speed (m s<sup>1</sup>

10 Forage Groups

Average relative humidity (%)

, lower values were determined in the percolation layer between

.

4.5e-06 and 5.9e-06 m s<sup>1</sup> and the barrier soil layer had values ≤6.1e-09 m s<sup>1</sup>

\*Field capacity (33 kPa), \*\*Wilting point, \*\*\*Water content at the beginning of the year 2012.

Table 3. Input data for the HELP model: landfill design and soil physical parameters.

3.4. Modelled water balance components of the Rastorf landfill between 2008 and 2015

In the study period between 2008 and 2015, the climatic water balance was positive (precipitation > evapotranspiration) and with regard to the German weather conditions, the outflow (2008–2010) and the actual evapotranspiration (2011–2015) were the greatest output values of the water balance (Table 4). The years 2012 and 2013 showed lower annual

Data of the three subcatchment areas (I–III), n = 7–10 undisturbed soil cores per layer for the average values of porosity, field capacity (FC), wilting point (WP) and saturated hydraulic conductivity (Ks), including initial water content (WC)

Year 2008 2009 2010 2011 2012 2013 2014 2015

Evaporative zone depth (cm) 20 20 30 50 50 50 50 50 Maximum leaf area index () 1.0 2.0 2.0 3.5 3.5 3.5 3.5 3.5 Vegetative period (days) 262 345 219 231 230 220 266 255

1. Quarter 82.5 88.2 87.6 87.7 88.5 89.7 89.2 90.8 2. Quarter 70.6 71.2 77.5 73.8 78.1 79.7 80.8 79.0 3. Quarter 81.0 76.3 80.5 87.3 82.3 81.6 82.0 82.6 4. Quarter 91.1 89.4 93.1 93.9 94.6 92.8 95.2 93.5

Study area and layer Porosity FC\* WP\*\* Ks WC\*\*\* Slope length and gradient

) (m s<sup>1</sup>

) (m<sup>3</sup> m<sup>3</sup>

) (m)/(%)

) (m<sup>3</sup> m<sup>3</sup>

Table 2. Input data for the HELP model: Evapotranspiration parameters (latitude 54.2).

) (m3 m<sup>3</sup>

I Percolation layer 0.356 0.184 0.127 4.5E-06 0.212 62/12 Drainage layer 0.317 0.206 0.136 5.6E-04 0.244 Barrier soil layer 0.292 0.175 0.121 3.7E-09 0.292 II Percolation layer 0.352 0.191 0.117 5.8E-06 0.259 44/28 Drainage layer 0.327 0.213 0.147 6.3E-04 0.226 Barrier soil layer 0.302 0.196 0.143 6.1E-09 0.302 III Percolation layer 0.332 0.207 0.167 5.9E-06 0.215 52/30 Drainage layer 0.325 0.196 0.139 5.8E-04 0.217 Barrier soil layer 0.307 0.213 0.149 3.6E-09 0.307

(m3 m<sup>3</sup>

) 4.76 4.56 4.73 4.75 4.78 4.91 4.58 4.35

\*\*Surface runoff and lateral drainage.

Table 4. Average annual water balance components for the period between 2008 and 2015.

precipitation rates with 655 and 669 mm, respectively, compared to the average annual precipitation rate of 728 mm. The winters of 2008–2015 were mostly mild and only had some snow.

The modelled average annual ETa values ranged between 33% in 2010 and 60% in 2012, and the outflow rates between 39% in 2009 and 54% in 2010 of the annual precipitation. The changes in soil moisture content with 0.3 and 1.5 mm year<sup>1</sup> were moderate and the modelled leachate rates ranged between 14 and 18% in 2008–2010, and between 11 and 15% in 2011–2015 of the annual precipitation (Table 4).

These drier phases are characterised by higher discrepancies between ETp and ETa up to 4.9 mm d<sup>1</sup> , especially in the warmer months between June and September (Figure 5). On the other side, the early warming phase during March to May showed moderate discrepancies of 0.6–2.7 mm d<sup>1</sup> , and the period October to February of the following year indicated mostly no discrepancies between the potential and actual evapotranspiration.

The ETa values ranged between 46 and 50%, and since 2011 between 60 and 69% of the ETp with the increasing depth of the evaporative zone (20, 30–50 cm) and the maximum leaf area

Figure 5. Average modelled potential and actual evapotranspiration rates (ETp, ETa) between 2008 and 2015 for the areas I–III.

of an entire day [21] and the evaporative capacity of the wind-exposed Rastorf landfill must

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Longer phases during the vegetative period (July–September) nearly reached the wilting point of 65 mm, so, the evaporative zone (0.5 m) dried out more strongly and the transpiration capacity and thereby the ETa values of the grassland were restricted by (a) the inadequate water availability in the evaporative zone and (b) the limited water storage capacity, and (c) the limited capillary rise from deeper soil layers due to the compacted construction of the temporary capping system [5, 6]. Thus, phases with water contents below the critical field capacity of 95 mm should be as short as possible to prevent desiccation in the deeper layer, thus, the modelled water content is a

Tree species or shrub vegetation (i.e., Salix caprea and Ligustrum vulgare) have a higher transpiration potential with ETa values of 600–700 mm year<sup>1</sup> and are more effective than grassland to minimise the leachate generation of landfill capping systems [13]. However, more deeprooted plants require thicker recultivation layers (2.0–3.0 m) to prevent shrinkage-induced crack formation in soil barriers due to desiccation and plant root penetration [9, 10]. Thus, the conflict of interest with regard to the choice of vegetation mainly depends on the local weather conditions, where robust grassland species should be preferred for locations with low precip-

The modelled leachate rates were at a consistent level of 11–18% of the annual precipitation rate without significant deviations but exceeded the requirements of [2] at most 60 mm year<sup>1</sup>

Otherwise, the modelled leachate rates indicate a sufficient percolation of water into the waste body to support the microbial processes [4]; between 2008 and 2017, the settlements of the

The slightly varying annual leachate rates indicate the functionality of the temporary capping system; continuously rising leachate rates would be an indicator for shrinkage crack formation or root penetration in the sealing layer [10], thus, the capping system would be ineffective. So, the hydraulic stability of the temporary capping system and especially the barrier soil layer can

In summary, the water balance model is not suitable to estimate more specific soil physical problems (i.e., preferential flow through cracks or root holes) of recultivation or sealing layers [5]. For an approved process description due to the model limitations, the numerical-based FEFLOW could be a more precise two-dimensional process description of the water fluxes of

The HELP model is one of the most commonly used statistical-empirical approaches and is an useful option to successfully determine the leachate quantity of landfill capping systems and to

, so, the temporary system fulfils its purpose.

itation [34], while more transpiring plant species are useful in more humid locations.

first indicator to describe the hydraulic stability of the capping system.

also be regarded as underestimated.

5 years after construction at the latest.

be assumed as ensured.

5. Conclusion

waste body decreased from >20 to <4 cm year<sup>1</sup>

the Rastorf landfill in the saturated and unsaturated soil [14].

Figure 6. Average water content in the evaporative zone between 2008 and 2015 for the areas I–III. The dashed lines indicate the area between the HELP correlated field capacity (FC) and wilting point (WP).

indices (1, 2–3.5), respectively (Figure 5). The maximum depth complied with the part of the recultivation layer, in which the water content fluctuated intensely during the study period. The modelled water content in this evaporative zone appeared mostly above the field capacity of 95 mm, while longer phases during the vegetative period (July–September) nearly reached the wilting point of 65 mm, resulting in a decreased ETa capacity (Figure 6).
