Water, Air, and Soil Pollution

, Volume 185, Issue 1, pp 131–148

Characterizing Groundwater Dynamics Based on Impact of Pulp and Paper Mill Effluent Irrigation and Climate Variability

  • Shahbaz Khan
  • Muhammad Nadeem Asghar
  • Tariq Rana
Article

DOI: 10.1007/s11270-007-9437-6

Cite this article as:
Khan, S., Asghar, M.N. & Rana, T. Water Air Soil Pollut (2007) 185: 131. doi:10.1007/s11270-007-9437-6

Abstract

Change in groundwater dynamics (in terms of changes in depth to watertable and its salinity) is a key environmental concern for agricultural production using pulp and paper mill effluent for irrigation purposes. At the study site, the treated effluent is delivered from the mill into a winter storage dam. This storage dam is also meant to provide an opportunity for runoff collection and recycling for irrigated areas. A natural creek also exists along the farm boundary. This paper presents, using field observation data and computer simulation results, the impact of using treated effluent from the pulp and paper mill on groundwater dynamics at the farm (covering areas both under and outside the effluent irrigation paddocks); and on the flows in the adjacent creek. The modeling results show that after 5 years of operations, the change in aquifer storage is more under average climatic conditions (−23.5 mm/year) as compared to −7.1 and −9.0 mm/year under dry and wet climatic conditions, .respectively. Under average climatic conditions, the combined effect of irrigation and rainfall creates more hydraulic gradient towards the creek thereby depleting the aquifer storage more as compared to wet and dry climatic conditions. Resultantly, the subsurface groundwater flows towards the creek becomes around 57.9 mm/year under average scenario as compared to 55.0 and 36.7 mm/year under wet and dry climatic conditions, respectively. During the average climatic condition, 456.6 mm evaporation from shallow groundwater was estimated under the current management practices; which was reduced to 399.1 mm under the best management practices due to better use of all sources of water and capillary upflow from shallow groundwater. Thus, with the adoption of best management practices, there would be less risk of salinisation due to evaporation from shallow groundwater tables.

Keywords

Pulp and paper mill Effluent reuse Groundwater dynamics Climate variability Australia 

1 Introduction

The pulp and paper making industry is one of the major effluent generation industries in the world. The volume of effluent generated and its characteristics are normally governed by factors such as the technology adopted, effectiveness of the treatment process and the amount of treated effluent recycled (Wiseman and Ogden 1996; Robertson and Schwingel 1997; Norris 1998). It is reported that a pulp and paper mill generates wastewater as low as 1.5 m3/tonne of paper produced (Szolosi 2003) to as high as 60 m3/tonne of paper produced (Thompson et al. 2001). In most cases, this effluent (treated or raw) is discharged back into a river, creek, stream or other water body; resulting in negative social and environmental concerns among the downstream users.

In recent years, the use of treated, partially treated or raw effluent for irrigating productive agriculture or forest crops has become a popular alternative to discharge into surface water bodies (Juwarkar and Subrahmanyam 1987; Fazeli et al. 1991, 1998; Kannan and Oblisami 1990; Al-Jamal et al. 2002; Phukan and Bhattacharyya 2003). However, such practice may result in other environmental concerns, if not properly managed (EPR 1988). These include increased recharge (quality and quantity) to the groundwater, accumulation of salts in the soil profile, and risk of runoff of these contaminants into surface water bodies.

Under effluent irrigation areas, the effluent quality, soil characteristics, depth to watertable, quality of the receiving groundwater, and the proximity of the effluent irrigation area to a discharging areas (like a river, creek, stream or other water body) determine the extent to which the effluent irrigation induced recharge impacts the groundwater (Bond 1998; Dominguez-Mariani et al. 2003). If groundwater is already saline, then the extra salts entering the groundwater is unlikely to be of concern. Where the groundwater salinity is lower than the receiving surface water body, the salinity of the receiving body will reduce due to dilution. The level of dilution depends on the rate of recharge, effluent irrigation area, volume of the receiving body and the rate of groundwater flow (due to aquifer permeability and hydraulic gradient) under the effluent irrigation areas.

Thus, while the addition of salt to underlying groundwater is often inevitable, this impact needs to be weighed up in consideration with all risks and benefits relating to effluent reuse. In this context, the paper presents results of assessing groundwater dynamics under an agricultural farm that was located adjacent to a natural creek, and was dependent on treated effluent from the mill to irrigate forage and fodder crops for producing animals feed. In this process of assessing groundwater dynamics, particular emphasis was given to incorporate the impacts of effluent irrigation under a range of climate and water availability conditions.

2 The Study Area

2.1 General Description

The studied pulp and paper mill currently produces 240,000 tonnes of paper per annum and utilises over 800,000 tonnes per annum of local pine plantation pulp wood and sawmill residues. The raw materials are further supplemented with up to 60,000 tonnes per annum of domestic and commercial wastepaper, to produce high quality ‘kraft’ linerboard paper. The freshwater consumption of this mill is 5.5 m3/tonne of paper produced, while its effluent discharge is only 1.5 m3/tonne of paper produced. Before diverting this wastewater for effluent irrigation at the farm, this effluent (clean condensate) is treated in the Waste Water Treatment Plant (WWTP). This WWTP is a sequencing batch reactor with biological nutrient removal activated sludge process.

The sludge from the WWTP, the lime mud from the lime kiln, and the ash from the boiler is used as a combined fertiliser for the soil. The treated effluent is delivered from the mill through a pipeline into the Winter Storage Dam (WSD). Current capacity of the winter storage dam is 490,000 m3/year, which is defined based on the annual effluent generation from the mill and a 90 percentile wet year design inflow. This storage dam was also meant to provide an opportunity for runoff collection and recycling for irrigated areas. The treated effluent the WSD is then used for irrigating pasture (forage and fodder) crops for silage and hay production. The farm also incorporates around 2,000 head of cattle.

For irrigation applications on 110 ha of farm land, which is 42% of the total farm land, the water is pumped from winter storage dam to five individual paddocks with Centre Pivot (CP) irrigators and one rectangular paddock with Soft Hose Travelling (SHT) irrigator. Table 1 describes the main features of effluent reuse paddocks. On these paddocks, mainly irrigated pasture (forage and fodder) crops were grown for silage and hay production. A summary of the wastewater application data starting from October 2001 to June 2006 is given in Table 2. During this period, a total of 2,388 mm of wastewater was applied on different wastewater reuse paddocks at the farm.
Table 1

Salient features of the wastewater reuse paddocks

Paddocks (units)

Per single rotation

Area (ha)

Radius (m)

Flow rate (l/s)

Time (h)

Application quantity (mm)

Central Pivot (CP) paddocks

CP 1

28.27

300

39.40

5.60

2.81

CP 2

12.06

196

16.80

3.50

1.73

CP 3 (high flow)

25.70

286

35.80

5.60

2.81

CP 3 (low flow)

25.70

286

17.30

5.60

1.36

CP 4 (high flow)

16.60

230

23.24

4.48

2.21

CP 4 (low flow)

16.60

230

17.30

4.48

1.65

CP 5

10.18

180

16.90

3.36

1.92

Rectangular paddock for Soft Hose Travelling (SHT) irrigator

SHT single setting

03.24

 

15.50

12.00

20.67

SHT all area

17.50

 

15.50

64.81

20.67

Table 2

Summary of wastewater applications to different irrigation paddocks during October 2001–June 2006

Irrigation season

Volume irrigated per Centre Pivots (CP)/Soft Hose Traveller (SHT) (in mm)

CP 1

CP 2

CP 3

CP 4

CP 5

SHT

Oct 01–Jun 02

122.7

51.4

123.0

101.5

55.2

00.0

Jul 02–Jun 03

90.2

73.1

93.6

117.4

42.7

00.2

Jul 03–Jun 04

143.1

39.6

155.9

38.7

39.4

99.2

Jul 04–Jun 05

182.1

77.4

101.7

60.9

72.2

65.0

Jul 05–Jun 06

164.1

73.7

50.3

94.1

58.3

23.4

Total

702.2

315.2

524.5

412.7

267.7

187.8

The effluent reuse paddocks at the farm are covered with deep and well to imperfectly drained Red and Yellow Podzolic soils on igneous and metamorphic parent rocks. Overall, three main soil types exist at the farm. In the high elevation areas, the soil is mostly sandy clay loam with no root impeding layers and has reasonably well natural drainage. In the low-lying areas, the soil is light to medium clay. This medium clay will act as a root impeding layer and will limit infiltration as well. In the mid elevation areas, the soil is clay loam to light and medium clay. Thus, there will be a risk of interflow from A2 soil horizon1 (if existed in the high elevation areas of the farm) to the mid and lower slops, thereby posing the potential for rising groundwater table and perching, seep development and salt accumulation in the down slope areas (low lying areas).

2.2 Salient Features of the Monitoring Network

The whole wastewater reuse operation is under a strict monitoring system: (1) wastewater irrigation monitoring, (2) soil monitoring, (3) groundwater monitoring, and (4) plant tissue monitoring. Figure 1 shows the study area, and presents an overview of the monitoring network installed at this farm to monitor the water flows and salts status.
Fig. 1

Map of the study area

The effluent from the WWTP is discharged to the winter storage dam. This is an excellent provision in the adopted effluent management strategy to improve the quality of effluent. If effluent is diluted in around 3:1 proportion before irrigation applications, the resulting mix could be successfully used for irrigating crops without increasing the soil exchangeable sodium percentage (Juwarkar and Subrahmanyam 1987). Under the winter storage dam, a series of subsurface drains beneath the clay liner were constructed to manage the potential uplift pressure that would occur under the liner due to groundwater.

From these individual paddocks, runoff from irrigation (if occurs) as well as from rainfall is captured and stored in several runoff dams. The water from these runoff dams is then supplied back to the winter storage dam or let it evaporate. However, rainfall runoff, which is not captured by the runoff dams and winter storage dam, flows directly to adjacent creek due to topography of the farm. Furthermore, rainfall runoff from the neighbouring farm on upstream of winter storage dam also flows directly to the creek.

Irrigation applications and rainfalls at each paddock contribute to the soil moisture storage in the root zone to meet the crop water requirements. To observe soil moisture changes in the root zone, every paddock has one C-probe installed, except for CP 1 where there are two C-probes. These probes provide the estimations of volumetric moisture contents at 10, 30, 50 and 100 cm soil profile depths. The soil moisture deficit estimated from these C-probes can be used to define the wastewater applications on any particular day; otherwise improper wastewater applications would not only endanger the soil and water environment at the farm but may also affect the flows in the creek (WHO 1989). The proper use of C-probes data would also help in maximising the benefits of rainfalls.

Excess water from irrigation applications and rainfall at each paddock also contributes to subsurface groundwater flows. Due to the unique hydrogeology of the farm, these subsurface groundwater flows have two components: contributing vertically to the (shallow and deep) groundwater aquifer, and laterally towards the low lying areas. Thus, the creek is happened to be a water receiving creek: (1) for surface runoff flows due to topography of the farm, and (2) subsurface groundwater flows due to underlying hydraulic gradients towards the creek.

To observe the groundwater dynamics, a number of shallow and deep piezometers had already been installed at the farm. Using the data observed during July 2001–June 2006, the temporal behaviour of depth to watertable in areas outside the effluent reuse paddocks is presented in Fig. 2. Groundwater fluctuates seasonally; increases during summer and falls during winter. Where these fluctuations occur at deeper depth from the soil surface, there is a minimum danger of capillary upward flows coming to the soil surface. However, evaporation from shallow watertable (when occurs within 2 m depth from the soil surface), will leave the salts behind in the soil profile. At present, rainfall is helping to leach these salts down, and groundwater flows take them away from the area. Thus, groundwater dynamics (both in terms of changes in depth to watertable and its salinity), observed from July 2001 till June 2006 does not present any negative environmental impacts on the areas outside the effluent reuse paddocks at the farm.
Fig. 2

Temporal behaviour of depth to watertable in areas outside the effluent reuse paddocks during July 2001–June 2006

3 Surface-groundwater Interaction Modeling

Although, a number of piezometers had already been installed to observe the groundwater dynamics at the farm; but none of them were installed under the irrigation paddocks. As a part of this study, 16 piezometers (8 shallow which are 2 m deep and 8 deep which are 4–8 m deep) were installed to monitoring shallow and deep groundwater dynamics under the irrigation systems. Thus, the data observed from the previously installed and newly installed piezometers was used in surface-groundwater modeling to study the impact of effluent irrigation on groundwater dynamics at the farm, covering areas under and outside the effluent reuse paddocks, under different climatic and water availability situations.

In this study, the modelling framework, the US Geological Survey model MODFLOW (McDonald and Harbaugh 1988) coupled with the MT3D (Zheng 1990) solute transport simulator, under a PMWIN (Chiang and Kinzelbach 1998) environment was used. The spatial domain represented in the model consists of two layers with 32 rows and 26 columns (100 m × 100 m cell size). Figure 3 represents the conceptual model for the whole farm area, illustrating the hydrological flows in, through and out of the modeling domain. Groundwater model layers 1 and 2 are arbitrary sub-divisions of the modeling aquifer to represent shallow and deeper groundwater dynamics. The thickness of first layer is 10 m, and the other layer extends till 50 m depth. A stress period length of 30 days was used to enable simulation of different seasons with a computational time step of 10 days. The model inputs include groundwater recharge, aquifer storage, hydraulic conductivity, reservoir (for winter storage dam) and river (for the creek) packages. Extensive datasets on the aquifer lithology (structural contours, bore logs, and aquifer properties, geophysical electrical array results), piezometric levels and groundwater salinity have been collected and collated in model input file format.
Fig. 3

Conceptual model of the farm, illustrating the hydrological flows in, through and out of the modelling domain

Figure 4 presents depth to watertable and groundwater salinity conditions at the farm, as of November 2005. These conditions serve as a benchmark, and are used in evaluating how November 2005 groundwater conditions would respond under different climatic and management scenarios. In November 2005, the 25% of the area has depth to watertable (DTW) <0.45 m and another 25% area has DTW > 4.92 m. The depth to watertable of 50% of farm is 0.45 – 4.92 m and the average DTW is 3.04 m. The 25% of the area has groundwater salinity <471 μS/cm and another 25% area has groundwater salinity >719 μS/cm. The groundwater salinity of 50% of farm is 471 – 719 μS/cm and the average groundwater salinity 581 μS/cm.
Fig. 4

Depth to watertable and groundwater salinity conditions at the farm, as of November 2005

3.1 Data Collection and Coalition

Structural contour information to derive top and bottom elevations of formations was derived from the digital elevation model. The key hydro-geological parameters for the study area were derived from the bore logs information, geophysical investigations, and in situ slug and bail tests. Layout of the creek was derived from digital elevation and topographic maps of the area. Other hydraulic parameters (bed elevation, width, depth etc.) of the creek were derived from the recently surveyed data. Location and area of irrigated and non-irrigated lands was sourced from land use maps of the farm. Layout of the winter storage dam was derived from GIS maps of the farm. Other hydraulic parameters (bed elevation, width, depth etc) of the dam were sourced from land dam construction reports and data sheets. The seepage from the dam was set equal to nominal rate of 1 mm/day (Lewis 2002) based on the geotechnical studies of the dam, and the seepage loss was linked with the changes in water levels in the winter storage dam.

The last 50 years climatic data for Tumut (Meteorological Station #072044) was downloaded from the SILO Patched Point Dataset, which presents a daily meteorological dataset for 4650 Bureau of Meteorology recording stations around Australia. Using this historical data, statistical analysis was carried out to identify dry, wet and average rainfall years. Based on this analysis the July 2002–June 2003 with annual rainfall of 455 mm represents ‘dry year’ and July 1988–June 1989 with annual rainfall of 1,084 mm represents the ‘wet year’. The July 2004–June 2005 represents the ‘average year’ with annual rainfall year with 770 mm.

Perennial pasture was selected for groundwater recharge estimations. The monthly crop factors for pasture were taken from DEC (2004), which indicates the crop factor for maximum crop cover was 0.70. The planting date was set equal to July 01. The crop cover fraction on a particular day was determined by linear interpretation between the dates of emergence (July 10), 20% cover (July 30), maximum cover (September 18), maturity (January 31) and harvest (June 30). The salinity threshold level of pasture was considered equal to 2,500 μS/cm, and its production was considered to reduce by 10% when root zone salinity becomes 5,000 μS/cm.

The timing of irrigation was defined to irrigate when 30% depletion of the total available water occurs (Allen et al. 1998). Irrigation application depths were defined to refill to 100% of the total available water. Irrigation scheduling started from planting date of each crop. It was also considered to apply additional 30% of irrigation when soil salinity becomes 2,500 μS/cm. The average salinity in the root zone as well as in the groundwater was taken equal to 1,850 μS/cm. It was assumed that the electrical conductivity (EC) of this rainfall was negligible; however the average EC of the wastewater was taken equal to 330 μS/cm.

Based on the observed data, the soils of the farm were defined as: (1) volumetric moisture contents at saturation = 45.3%, (2) volumetric moisture content at field capacity = 32.4%, (3) volumetric moisture content at wilting point = 10.4%, (4) total available soil moisture = 220 mm/m depth of the root zone, (5) drainage coefficient, which indicates drainage when volumetric moisture content is between saturation and field capacity, =0.13%, and (6) maximum infiltration rate = 40 mm/day. All these numbers are average numbers, assuming the same soil under each CP and the SHT.

Groundwater recharge estimations are critical to assess impact of effluent irrigation on soil and groundwater under different climatic and water availability situations. In this study, the WaSim model (Hess and Counsell 2000), which is a Water Simulation model, was used to estimate the groundwater recharges at the farm while considering the respective soil type, water (wastewater and rainfall), crops (perennial pasture), and depth to watertable situations as of November 2005. Table 3 represents WaSim simulation results for monthly groundwater recharge estimates for areas representing different paddocks and outside paddocks. Negative recharge means contribution from shallow watertable to the root zone soil moisture, and the positive recharge means that deep percolation from the root zone is contributing to the groundwater. These monthly groundwater recharge estimations are the indicative values to be used as input data for the representative grids in the surface-groundwater interaction model.
Table 3

WaSim simulation results for monthly groundwater recharge estimates (in mm) for areas representing different paddocks and outside paddocks

 

Under irrigation paddocks

Outside irrigation paddocks

CP 1

CP 2

CP 3

CP 4

CP 5

SHT

CP 1

CP 2

CP 3

CP 4

CP 5

SHT

Average year

July

−7.90

−7.56

−15.54

−9.19

−7.51

−7.90

−2.17

−2.17

−21.93

−2.80

−2.17

−2.17

August

47.89

50.01

−49.16

5.56

50.28

47.89

25.69

25.69

−41.85

−5.53

25.69

25.69

September

28.90

50.83

52.85

24.03

50.57

28.90

55.64

56.80

71.41

44.26

56.80

55.64

October

−27.24

3.67

4.14

−13.31

3.64

−27.24

37.87

43.40

−17.96

−17.96

43.40

37.87

November

13.92

8.06

10.28

17.18

8.04

13.92

−6.01

−3.06

4.06

4.06

−3.06

−6.01

December

−16.67

4.09

3.29

−12.74

4.09

−16.67

11.00

10.38

26.76

26.76

10.38

11.00

January

10.29

3.60

29.90

29.50

3.60

10.29

7.35

6.52

17.67

17.67

6.52

7.35

February

6.65

4.68

−3.27

0.24

4.68

6.65

5.17

5.21

3.93

3.93

5.21

5.17

March

−0.82

−1.28

−10.73

−6.30

−1.28

−0.82

−8.90

−8.84

−8.77

−8.77

−8.84

−8.90

April

−8.44

−7.41

−25.36

−10.60

−7.41

−8.44

3.08

3.07

2.99

2.99

3.06

3.08

May

−5.86

−6.62

5.66

−7.29

−6.62

−5.86

−2.84

−2.84

−2.86

−2.86

−2.83

−2.84

June

6.99

19.42

−73.75

8.15

19.42

6.99

−11.55

−11.57

−11.66

−11.66

−11.57

−11.55

Dry year

July

−8.99

−8.07

−17.43

−11.34

−7.92

−8.99

6.87

6.87

−24.18

6.02

6.87

6.87

August

12.53

13.21

10.27

6.15

13.31

12.53

12.26

12.26

−3.44

10.26

12.26

12.26

September

5.56

5.44

−7.51

5.00

5.43

5.56

19.73

19.73

14.72

16.22

19.73

19.73

October

4.47

4.24

39.05

17.21

4.24

4.47

26.36

26.36

40.79

27.83

26.36

26.36

November

6.47

6.55

12.27

−6.40

6.55

6.47

−1.29

−1.29

2.18

−0.84

−1.29

−1.29

December

−4.17

−4.17

−25.35

6.36

−4.49

−4.17

5.62

5.62

4.84

5.62

5.62

5.62

January

−0.27

−0.27

12.77

1.13

−0.52

−0.27

8.46

8.47

7.49

8.32

8.47

8.46

February

−1.11

−1.23

−40.14

−22.56

−1.79

−1.11

14.75

14.75

15.11

14.74

14.75

14.75

March

9.55

9.54

36.86

29.00

9.29

9.55

−15.26

−15.26

−15.27

−15.26

−15.26

−15.26

April

−4.99

−4.98

−20.76

−16.27

−5.88

−4.99

3.23

3.23

3.23

3.19

3.23

3.23

May

−8.89

−8.89

0.16

−6.49

−7.48

−8.89

6.87

6.87

6.87

6.87

6.87

6.87

June

16.35

16.23

−50.10

−8.65

12.22

16.35

−19.03

−19.03

−18.64

−19.03

−19.03

−19.03

Wet year

July

30.33

32.05

−69.45

9.82

32.33

30.33

35.44

35.44

−70.30

6.38

35.44

35.44

August

26.53

34.19

−2.41

−11.33

34.35

26.53

26.66

26.66

41.58

1.33

26.66

26.66

September

13.62

33.58

−1.35

−39.47

33.21

13.62

11.31

15.63

28.37

24.39

15.63

11.31

October

−10.59

3.78

109.87

76.89

3.76

−10.59

42.06

47.68

−4.91

−6.98

47.68

42.06

November

−6.11

0.18

−65.76

−34.22

0.18

−6.11

31.74

33.63

45.08

44.96

33.62

31.74

December

37.33

31.80

65.12

66.49

55.35

37.33

−28.10

−26.58

−23.05

−23.07

−26.57

−28.10

January

−19.36

8.44

62.97

18.21

10.29

−19.36

8.45

6.74

16.06

16.07

6.75

8.45

February

3.12

−9.40

−17.84

−18.86

6.18

3.12

−4.83

−4.92

−4.11

−4.12

−4.92

−4.83

March

64.66

54.02

−40.75

5.11

21.08

64.66

−17.62

−17.54

−18.33

−18.30

−17.55

−17.62

April

−8.80

38.83

−6.42

−6.42

63.01

−8.80

−33.89

−33.51

−33.64

−33.71

−33.50

−33.89

May

−98.74

−98.74

−98.74

−98.74

−98.74

−98.74

−8.40

−8.20

−8.19

−8.18

−8.20

−8.40

June

−9.11

−9.11

−9.11

−9.11

−9.11

−9.11

55.16

54.14

80.56

80.56

54.14

55.16

3.2 Impacts of Management Scenarios

The current management practices are based on water supply; there is a tendency of reusing effluent as much as possible before winter. However, demand based irrigation applications, using the local meteorological and root zone soil moisture observations to define the irrigation schedule, would present the best management practices.

Figure 5 presents water balance for the period starting from July 2004 till June 2005 under current management practices at the farm, whereas Fig. 6 presents water balance for average climatic condition (i.e., during July 2004–June 2005) under best management practices at the farm. Therefore, these figures help comparing water and mass balances for the period starting from July 2004 till June 2005 under current and best management practices at the farm. The water balance results have shown discrepancies of less than 0.01%, which is generally considered an acceptable error. In these water balance diagrams, storage changes are referred as ΔS with plus or minus sign. A minus sign refers to the water released from the aquifer storage and plus sign refers to the water added to the aquifer storage.
Fig. 5

Annual water balance for the period starting from July 2004 till June 2005 under current management practices at the farm. All figures are in mm

Fig. 6

Annual water balance for average climatic condition (i.e., during July 2004–June 2005) under best management practices at the farm. All figures are in mm.

Under both the scenarios, similar hydro-climatic conditions prevail for non-irrigated areas; only variation comes from irrigation applications. Under the current management practices, 620.6 mm recharge and 456.6 mm evaporation from shallow groundwater (ET) was estimated. However, under the best management practices, these recharge and ET estimates became 494.5 and 399.1 mm, respectively. Thus, due to less recharge, improvement comes from reduction in ET as well, which means that there would be less risk of salinisation due to evaporation from shallow groundwater tables. Therefore, for better environmental management of effluent reuse paddocks, it is recommended to follow irrigation schedules using the best irrigation management practices by taking advantage of soil moisture depletion data from C-probes installed in every paddocks and matching it with the crop evapotranspiration (Allen et al. 1998).

3.3 Impacts of Climatic Conditions

As described earlier, dry, wet and average rainfall years were identified using statistical analysis for the last 50 years rainfall data. Based on this analysis, the annual rainfall of 455, 1,084 and 770 mm represents dry, wet and average climatic conditions. Therefore, to evaluate the impact of climatic conditions on groundwater dynamics at the effluent reuse farm, it was assumed that the same rainfall pattern would continue for 5 years under dry, wet and average climatic conditions.

After 5 years of average climatic conditions scenario under the best management practices for effluent reuse operations at the farm (Fig. 7), the 25% of the area has DTW < 1.80 m and another 25% area has DTW > 2.88 m. The depth to watertable of 50% of farm is 1.80–2.88 m and the average DTW is 2.01 m. It means that during 5 years of average climatic conditions, the depth to watertable on 25% of the area has increased from 0.45 to 1.80 m. However, DTW has decreased from 4.92 to 2.88 on another 25% area, and average depth to watertable under 50% of farm area has decreased from 3.04 to 2.01 m, which is still below the root zone. There is an overall decreasing trend of salinity, except for CP 2 and CP 5. In these paddocks, there is increase in salinity levels but the increase is less than 100 μS/cm.
Fig. 7

Depth to watertable and groundwater salinity situations after 5 years under average climatic and water availability scenario. a Likely situation of depth to watertable, b change in groundwater salinity

After 5 years of dry climatic conditions scenario (Fig. 8), the 25% of the area has DTW < 1.85 m and another 25% area has DTW > 3.75 m. The depth to watertable of 50% of farm is 1.85–3.75 m and the average DTW is 2.16 m. It means that during 5 years of dry climatic conditions, the depth to watertable on 25% of the area has increased from 0.45 m to 1.85 m. However, DTW has decreased from 4.92 to 3.75 on another 25% area, and the average depth to watertable under 50% of farm area has decreased from 3.04 to 2.16 m, which is still below the root zone. There is an overall deceasing trend of salinity, except for CP 2, where there is increase in salinity levels but the increase is less than 100 μS/cm.
Fig. 8

Depth to watertable and groundwater salinity situations after 5 years under dry climatic and water availability scenario. a Likely situation of depth to watertable, b Change in groundwater salinity

After 5 years of wet climatic conditions scenario (Fig. 9), the 25% of the area has DTW < 1.87 m and another 25% area has DTW > 3.47 m. The depth to watertable under 50% of farm is 1.87–3.47 m and the average DTW is 2.38 m. It means that during 5 years of wet climatic conditions, the depth to watertable on 25% of the area has increased from 0.45 to 1.87 m. However, DTW has decreased from 4.92 to 3.47 on another 25% area, and the average depth to watertable under 50% of farm area has decreased from 3.04 to 2.38 m, which is still below the root zone. There is an increasing groundwater salinity trend under CP 2, CP 5 and SHT, but the increase is less than 100 μS/cm.
Fig. 9

Depth to watertable and groundwater salinity situations after 5 years under wet climatic and water availability scenario. a Likely situation of depth to watertable, b change in groundwater salinity

After the introduction of wastewater irrigation applications on the farm, aquifer storage tends to show depletion pattern under most climatic conditions (Table  4), however the change in aquifer storage is more under average climatic conditions (−23.5 mm/year) as compared to −7.1 and −9.0 mm/year under dry and wet climatic conditions, respectively. Under average climatic conditions, the combined effect of irrigation and rainfall creates more hydraulic gradient towards the creek thereby depleting the aquifer storage more as compared to wet and dry climatic conditions. Although, the subsurface groundwater flows towards the winter storage dam under dry climatic conditions was 22.5 mm/year compared with 11.6 mm/year and 12.3 mm/year under wet and average climatic conditions, respectively. This difference attributes to more irrigation applications on wastewater irrigation paddocks under dry climatic conditions as compared to wet and average climatic conditions. However, the subsurface groundwater flows towards the creek are around 36.7 mm/year under dry scenario as compared to 55.0 and 57.9 mm/year under wet and average climatic conditions, respectively. Thus, rainfall has significant impact on the subsurface groundwater flows towards the creek.
Table 4

Annual groundwater budgets after 5 years of dry, wet and average climatic conditions under the best management practices for effluent reuse operations at the farm

Components

Dry

Wet

Average

Recharge

282.8

487.7

528.5

ET

−261.5

−435.4

−441.1

Dam

22.5

11.6

12.3

Creek

−36.7

−55.0

−57.9

ΔS

−7.1

−9.0

−23.5

All figures are in mm.

Overall, the effluent irrigation applications are not having any salinity concerns at the farm, which is evident from the average salinity trends observed over the period of time in wastewater, groundwater and the adjacent creek (Fig. 10). The salinity of the wastewater used for irrigation applications is always of very low salinity level, whereas the average salinity of the groundwater is of low level. Root zone salinity trends shows that the soil is still suitable for sensitive crops. Similarly, the average salinity of the surface water flowing in the creek is also of very low salinity level (ANZECC 2000).
Fig. 10

Average salinity (EC, in dS/m) trends in wastewater, groundwater and Sandy Creek (All values are in dS/m; 1 dS/m = 1,000 μS/cm)

4 Conclusions and Recommendations

The effluent irrigation applications and winter storage dam operations were not having any negative environmental impact on the soil and groundwater under the effluent irrigated area and on the adjacent creek. However, social attitudes to the use of crops that have been irrigated with recycled waters and the resulting impact on market value of the produce would likely be a major consideration for the longer term viability of this operation.

The current management practices are based on water supply. Therefore, there is a tendency of reusing effluent as much as possible before winter. However, demand based irrigation applications, using the local meteorological and root zone soil moisture observations to define the irrigation schedule, would present the best management practices.

Under the current management practices during the average climatic condition, 456.6 mm evaporation from shallow groundwater was estimated; which was reduced to 399.1 mm under the best management practices due to better use of all sources of water and capillary upflow from shallow groundwater. Thus, with the adoption of best management practices, there would be less risk of salinisation due to evaporation from shallow groundwater tables.

Under most climatic conditions, aquifer storage tends to show depletion pattern after 5 years of the introduction of effluent reuse using best management practices for irrigation applications at the farm. Under average climatic conditions, the combined effect of irrigation and rainfall creates more hydraulic gradient towards the creek thereby depleting the aquifer storage more as compared to wet and dry climatic conditions. Resultantly, the subsurface groundwater flows towards the creek becomes higher under average scenario as compared to wet and dry climatic conditions. As the salinity of groundwater entering the creek was much lower than the receiving surface water; therefore, there will be a net dilution effect on the quality of flows in the creek.

Footnotes
1

According to the Australian Soil Classification (Isbell 1996), soil horizon is a specific layer in the soil which parallels the land surface and possesses physical characteristics which differ from the layers above and beneath. Among the several other main horizons, A horizon is a surface horizon, and as such is also known as the zone in which most biological activity occurs. It is usually subdivided into several sub-horizons. The A1 is dark colored and high in content of organic matter; the A2 is usually light colored and leached; the A3 is transitional to the B horizon.

 

Copyright information

© Springer Science+Business Media B.V. 2007

Authors and Affiliations

  • Shahbaz Khan
    • 1
  • Muhammad Nadeem Asghar
    • 1
  • Tariq Rana
    • 2
  1. 1.International Centre of Water for Food SecurityCharles Sturt UniversityWagga WaggaAustralia
  2. 2.CSIRO Land and WaterCharles Sturt UniversityWagga WaggaAustralia

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