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Management of salinity and sodicity in a land FILTER system, for treating saline wastewater on a saline-sodic soil.


Conventional systems of land application of wastewater on soils with restricted internal drainage, which occur extensively around urban areas in south-eastern Australia, could lead to soil degradation through waterlogging and salinisation. Existing systems for land treatment of wastewater using cropping and forestry are sometimes less economical than wastewater treatment plants. This is mainly due to the need for expensive winter and wet weather storage, when crop irrigation requirements are low.

In order to overcome problems associated with traditional wastewater treatment techniques, the land FILTER (Filtration and Irrigated Cropping for Land Treatment and Effluent Reuse) technique was developed at CSIRO, Griffith (Jayawardane 1995).

The land FILTER technique combines the use of nutrient-rich wastewater for intensive cropping with filtration through the soil to a subsurface drainage system. This technique is thus capable of handling high volumes of wastewater during the periods of low cropping activity or periods of high rainfall. Wastewater application and subsurface drainage in the FILTER system can be regulated to ensure adequate removal of pollutants, thereby producing minimum-pollutant drainage water which meets general environmental criteria for discharge to surface water bodies throughout the year. The specific design and operation of the FILTER system at a given site needs to take into account site features such as soil and hydrological conditions, rainfall, and potential evapotranspiration rates. Preliminary testing of the FILTER technique on 1-ha plots showed that the FILTER system met its objectives of reducing nutrients in drainage waters below environmental limits, while maintaining adequate drainage rates (Jayawardane et al. 1997a, 1997b). In addition, crop yields comparable to district average yields were obtained, which could be used to offset costs in a commercial system. The other beneficial effects were reduced suspended solids, E. coli, oil, and grease, and an increased N:P ratio.

However, the long-term sustainability of FILTER plots requires careful management of salinity and sodicity of the soils. Previous studies have shown that a reduction in electrolyte concentration or an increase in sodium adsorption ratio (SAR) of a percolating solution results in an increase in clay swelling (McNeal et al. 1966), a change in pore size distribution (Jayawardane and Beattie 1979), and a decrease in saturated conductivity of soils (Quirk and Schofield 1955; McNeal and Coleman 1966). Quirk and Schofield (1955) defined the threshold electrolyte concentration as the concentration at which a 20% reduction in the soil hydraulic conductivity occurs, at any given exchangeable sodium percentage (ESP). McNeal and Coleman (1966) showed that the threshold concentration varied according to the soil properties. Jayawardane (1977, 1979) introduced the `equivalent salt solutions' concept, and used it to predict the changes in saturated and unsaturated hydraulic conductivity of soils in the presence of different salt solutions (Jayawardane 1979, 1983, 1992; Jayawardane and Blackwell 1991). Rengasamy et al. (1984) developed a scheme for predicting the dispersive behaviour of Red-brown Earth soils based on the relationships between spontaneous and mechanical dispersion, SAR, and the total cation concentration (TCC). They also suggested soil ameliorative practices which need to be adopted to prevent soil structural deterioration in the 6 groups of soils identified in this scheme. Rengasamy et al. (1984) proposed the use of Eqn 1 for calculating the threshold electrolyte concentration (TEC) for spontaneous dispersion in surface and subsurface layers of a Red-brown Earth. They also proposed Eqn 2 to calculate the TEC for mechanical dispersion in the clayey subsurface layers of the Red-brown Earths:

(1) TEC = 0.016 SAR + 0.014

(2) TEC = 0.319 SAR - 0.17

where SAR is measured in 1:5 soil: water extracts. SAR is a measure of potential sodicity hazard and is expressed as:

SAR = [Na]/[square root of [Ca]+[Mg]]

where [Na], [Ca], and [Mg] are the concentrations in mmol/L. However, it should be emphasised that the measurement procedure to evaluate mechanical dispersion used by Rengasamy et al. (1984) involved shaking the soil to obtain suspensions, and this procedure destroys the structure of the soil, which is involved in controlling permeability. Hence, Eqn 2 is likely to provide an estimate of the TEC required to maintain soil stability that is higher than what is required where the external mechanical force applied is less. Rowell et al. (1969) showed in a laboratory study that the type of experimental procedure used to estimate the extent of clay dispersion could affect the relationship between SAR and TCC in any given soil. They found that at any given SAR, the threshold electrolyte concentration for reducing permeability was lower than the electrolyte concentration required for flocculation of a soil suspension.

The relationship between the subsurface drainage flow rate and the water table height at the mid-point between subsurface drains can be used to monitor the changes in the whole plot soil hydraulic properties (Jayawardane et al. 1997a, 1997b) of the FILTER plots. For subsurface drains running normally without back pressure, Youngs (1985) found the Hooghoudt's equation could be simplified to:

(3) [H.sub.m]/D = [([q.sub.t]/K).sup.1/a]

where [q.sub.t] is the drainage flow rate (m/s); [H.sub.m] is the water table height at the midway point between the drains (m); K is the saturated hydraulic conductivity of the soil (m/s); and 2D is the spacing between drains (m); a is a coefficient given by:

(4) a = 2[(d/D).sup.d/D]

where d is the depth below the drain to the impermeable layer (m) and 0 [less than or equal to] d/D [less than or equal to] 0.35. For the situation where d/D [right arrow] [varies], the value of a was found by Youngs (1985) to be approximately equal to 1.36 and where d/D = 0, the value of a is equal to 2.0.

Rearranging Eqn 3 and substituting the value of the water table height at a given time ([H.sub.t]) for [H.sub.m], we obtain:

(5) [q.sub.t] = K[[H.sub.t]/D].sup.a]

By plotting log [q.sub.t] against log [H.sub.t] we can derive the value of the slope a and by substituting in the bracketed term of Eqn 6, the value of K can be calculated.

This paper examines the changes in salinity and sodicity in FILTER plots during 3 cropping seasons, and their potential effects on soil structural stability.

Materials and methods

This experiment was conducted at the Griffith City Council Sewage Treatment Works effluent disposal site. The experimental site is located on Chequers Road, off the Griffith-Hillston Road, approximately 5 km west of Griffith. The soil at the site is a transitional Red-brown Earth (Stace et al. 1968) or a Typic Chromexert in the Soil Taxonomy (Soil Survey Staff 1975). The site has a high saline groundwater table. The site was previously used for land application of saline sewage effluent for irrigation during summer months, without provision of subsurface drainage.

Two FILTER experimental blocks were established within a 12-ha area, which was laser levelled nominally to a 1:4000 slope. Field data are presented from experimental Block B consisting of plots 1, 2, 3, and 4, which were used for winter cropping during the period from May 1995 to November 1995 (i.e. the second cropping season at the trial site), followed by 2 cropping seasons (Jayawardane et al. 1997a). Each FILTER plot used for this study was 40 m wide by 250 m long (Fig. 1), was surrounded by a 0.4-m-high bank, and was prepared as follows. To increase porosity and hydraulic conductivity, the plot was ripped to 0.9 m depth using a D6 tractor. The ripper tines were spaced 1.2 m apart. Two passes were made over the plots. During the second pass the ripper blades were aligned between the rip lines of the first pass. Mined gypsum was applied at 8 t/ha, to reduce dispersion on this sodic soil surface that would otherwise occur after heavy rainfall.


The installation of the subsurface drainpipe system in each plot was started by digging a 1.5-m-deep trench at the bottom ends of the plots. A commercial drainpipe layer was then used to install 7 parallel drainpipes at a spacing of 5 m, at right angles to the trench. The 250-m-long pipes of 0.10 m diameter were laid at a slope of 0.1% and a mean drain depth of 1.2 m. The diameter and slopes of pipes were selected to avoid any restrictions to flow within the pipes. The drainpipes were surrounded by a layer of fine gravel (<7 mm diam.). The drainpipes were closed at the top end. At the bottom end, the 4 drainpipes at the 10-m spacing were connected to a collector pipe which opened into a 1.6-m-deep sump. The 3 remaining drainpipes were used to measure the water table height at the mid-point between the 10-m spaced drains. They were connected to a second collector pipe which also opened into the sump. Inside the sump, a vertical tube was attached to the end of the second collector pipe. The water table height inside this vertical tube indicates the water table height in the plots midway between the open drains. Refilling of the trenches above the drainpipes was done by deep ripping the soil to 0.9 m depth, as described above. Each sump was fitted with a pump. The drainage water from the sumps was pumped into a drainage channel, which takes the drainage water from the site.

Effluent treatment by FILTER operates on approximately a 2-weekly cycle, where about 1-1.5 ML of effluent is applied to each 1-ha plot, followed by a 1-2-day post-irrigation equilibration period. This is followed by an 8-10-day pumping period, when the drainage pump is turned on and the effluent slowly passes through the soil and the subsurface drainpipes to a collection sump. After the pumping period, the pump is turned off for 1-2 days, allowing the water table to reach equilibrium. The cycle is then repeated. The subsurface drainage system provides adequate drainage and soil aeration conditions for crop growth even during heavy rainfall and low evapotranspiration periods. During this regulated flow of effluent, nutrients and other pollutants are adsorbed on the surface of soil particles, taken up by the crop and weeds, or lost through volatilisation and denitrification, thereby reducing nutrients in the drainage outflow. The quality of treated effluent ([less than or equal to] 10 mg N/L and [less than or equal to] 0.4 mg P/L) meets current NSW EPA nutrient limits for discharge to surface water bodies (Jayawardane et al. 1997a).

The cropping sequences in plots 1-4 are given in Table 1. The crops were subjected to the following agronomic practices. In plots 2 and 3, pasture grasses [ryegrass multimix (var. Concord, Richmond, Midmar) at 25 kg/ha, ryegrass (var. Victorian) at 5 kg/ha, and tall fescue (var. Demeter) at 3.3 kg/ha] were sown on 9 May 1995. Due to a breakdown of harvest equipment only one harvest was carried out on 20 November 1995 at the end of the winter 1995 filtration season. The pasture crop was continued into the third and fourth cropping seasons. The pasture was harvested at the end of the third cropping season on 6 March 1996, and during the fourth cropping season on 5 September 1996 and 22 November 1996. In plots 1 and 4, oats (var. Enterprise and Cooba at seed rates of 150 and 50 kg/ha, respectively) were planted on 20 May 1995. The oats were cut for fodder on 20 November 1995. At the start of the third cropping season, plots 1 and 4 were split lengthwise to establish maize (var. Highcorn 52) and sorghum (var. Superdan) crops on 18 December 1995. A fodder harvest was taken on 10 March 1996. At the start of fourth cropping season, a wheat crop (var. Currowang at a seed rate of 150 kg/ha) was sown on 16 May 1996. The crop was harvested for grain and dry fodder on 17 December 1996.

During the second, third, and fourth cropping seasons, a total of 12, 8, and 12 filter events were carried out. During each filter event, the following measurements were taken on each plot. The volume of effluent applied to each plot was measured. The rate of drainage water running into the sumps was measured at regular intervals from the electricity consumption of the pumps used for emptying the sumps. The changes in height of the water table in the plots midway between subsurface drains were also measured periodically. These water table height measurements were made using the vertical tube in the sump connected to the second collector pipe from the 3 drains at intermediate spacing which were not used for drainage.

Soil samples were taken before effluent application commenced at the start of the winter (second) cropping season 1996, and at the end of the second, third, and fourth cropping seasons. At the start of the experiment, in each plot soil cores were extracted and cut up into 0.10-m segments at soil depths 0-0.4 m, and into 0.20-m segments below 0.4 m depth, and retained for soil analysis after drying. Composite soil samples from selected depth intervals were prepared by mixing the soil samples from the 4 plots. These composite soil samples were analysed to characterise the initial soil condition before the filtration phase. A similar soil sampling procedure was used at the end of the second, third, and fourth cropping seasons. Soil measurements included pH and exchangeable cations (Ca, Mg, Na, K). SAR and electrical conductivity ([EC.sub.1:5]) were measured in 1:5 soil: water extracts.

Continuous irrigation and drainage water samples were collected using a GAMET auto sampler and a sample bleeding-tube arrangement, respectively. Samples were stored at 4 [degrees] C before analysis for pH, EC, and dissolved Na, Ca, and Mg.

Results and discussion

The initial mean soil [EC.sub.1:5] of the 4 plots (Fig. 2), shows that the site was extensively salinised by previous application of sewage effluent with EC 1.3 dS/m and SAR 7.2, as subsurface drainage was not provided. The initial ESP of the soil was also high (21-29%) at all depths (Fig. 3). Falkiner and Smith (1997) found that 4 years of irrigation with slight to moderately saline and sodic effluent resulted in marked increases in soil salinity and increases in soil sodicity to around 20-25%. Smith et al. (1996) showed that excess water needed to be applied at this effluent-irrigated plantation to promote the leaching of excess salt accumulating in the soil at the site.


With application of moderately saline sewage effluent after establishment of the FILTER plots, initially the salinity of the subsurface drainage water was high (Fig. 4) due to leaching of the stored salt in the soil profile, as well as the concentration of salts in drainage water through crop evapotranspiration. Thus, while the ratio of the volume of effluent applied to drainage water was around 3, the EC of the drainage water increased by more than 10-fold, indicating marked leaching of the saline soils at the FILTER site. The drainage water salinity progressively decreased during the second, third, and fourth cropping season due to progressive reductions in soil profile salinity, through gradual leaching of the salt in the soil. Figure 2 shows that during the second cropping season, the reduction in soil salinity mainly occurred in the layers above the drain depth. During the third and fourth cropping seasons, the decrease in the soil salinity occurred above and below the drain depth. Research studies are currently in progress to model the flow of water and salts through FILTER plots, to explain these progressive changes in soil salinity and drainage water salinity.


The SAR of the composite drainage water sample during the second cropping season was higher than the SAR of the effluent irrigation sample (Table 2). The high SAR of the drainage water could be related to the high initial soil ESP and the associated high SAR of the soil solution which leaches into the drainage water. Due to relatively lower SAR in the effluent applied, the calcium and magnesium in the effluent waters are likely to replace the sodium on the soil exchange complex, reducing the soil ESP. Consequently the SAR of the soil solution in equilibrium with the cations on the soil exchange complex is likely to decrease. Thus, the SAR of the composite drainage water sample during the fourth cropping season was slightly lower than in the composite drainage water sample collected during the second cropping season. The soil ESP (Fig. 3) and the SAR of the 1:5 soil: water preparations (Table 3) also decreased at the end of the fourth cropping season compared with the corresponding values before filtration. These changes could thus be attributed to the use of relatively low-value SAR wastewater for irrigation and the provision of subsurface drainage to leach the excess salts to prevent their accumulation in the soil. In addition to these changes, the periodic rise in the shallow saline groundwater tables in the area will bring in additional salts into the soil profile, thereby slowing down the rate of soil salinity reduction in the FILTER soils through leaching.

The field data thus show that the provision of subsurface drainage in the FILTER system not only prevented a build-up of salinity and sodicity in the soil at this saline-sewage effluent application site, but also reversed these adverse soil degradation effects caused by previous pre-FILTER effluent applications.

The calculated TEC values for spontaneous and mechanical dispersion of soils collected before establishment of the FILTER system and after the fourth cropping seasons using Eqns 1 and 2 are shown in Table 3. The measured soil [EC.sub.1:5] values are greater than the TEC values for spontaneous dispersion, indicating that the pre-FILTER soils will be structurally stable during flow of effluent through the soil to the subsurface drains. Although the soil EC is reduced due to leaching during the FILTER cycles, the associated reduction in SAR results in a reduction of TEC for spontaneous dispersion. Therefore, the soils will remain stable during effluent flow through the soil, even after 3 cropping seasons of the field trial. However, as the EC of the soil sampled before cropping and after 3 cropping seasons is less than the TEC for mechanical dispersion, care is necessary to prevent structural breakdown due to factors such as rainfall impact on bare soil, mechanical disturbance, and trafficking damage when the soil is wet.

The relationship of the measured SAR of the 1:5 soil:water extracts to the soil ESP is shown in Fig. 5. This relationship, combined with the routine measurement of SAR, could be used as an alternative method for monitoring the soil ESP. The linear relationship is unlikely to be valid outside the SAR range of the measurements, and differs markedly from the relationship obtained by Rengasamy et al. (1984) at low SAR/ESP. The power relationship is likely to provide a better fit in extrapolating the relationship to small values of ESP/SAR.


The shift in the relationship between the subsurface drainage flow rate and the water table height at the mid-point between subsurface drains (Fig. 6) between the second, third, and fourth cropping season was small, and statistically not significant (P > 0.01). The calculated values of K and a for the second and fourth cropping seasons were not significantly different in each of the 4 plots (Jayawardane et al. 1997b). The small shift in this relationship could, at least partly, be attributed to the natural settling of this soil after deep ripping. The data thus indicate that the soil hydraulic properties of the subsurface layers were not markedly affected by the changes in soil salinity and sodicity during the 3 cropping seasons. This observation is consistent with the soil EC remaining above the TEC for spontaneous dispersion during the field trial. However, we noticed a tendency for surface ponding of water at the bottom end of the plots, especially during periods of prolonged rainfall during the third and fourth cropping seasons. This could be attributed to the low salinity of rain waters and the reduced salinity of the leached surface soil layers, causing soil dispersion near the soil surface during heavy rainfall when the crop cover is low. This loss of surface soil structural stability may have been aggravated by trafficking of the plots to carry out the agronomic practices.


Table 4 shows the water balance and the mass salt balance in plot 4 during the winter 1995 cropping season. The measured values of water and salt contents and their changes could include errors involved in making these measurements on large 1-ha plots. The subsurface drains removed 42% of the effluent applied and rainfall, the balance being largely removed through evapotranspiration. The smaller amount of unaccounted water losses represents lateral outward seepage across the plot borders and deep percolation losses. The salt balance shows a much greater salt removal in the drainage waters compared with additions in effluent irrigation. This is largely due to the leaching of the salts stored in the soil profile prior to establishment of the FILTER plots, resulting in reduction in soil profile salinity during FILTER operations (Fig. 2). The unaccounted salt gains largely represent the upward movement of the salts from the underlying groundwater into the measured soil profile, as the saline water table is raised to the drain depth during the first few filter events and maintained at this depth during the successive filter events. Lateral movement of salt into the drained plots from the surrounding area without subsurface drains could have also provided a contribution.

Careful long-term planning of FILTER plot design and management, and use of optimised routine FILTER operations, could minimise the risks of surface soil structural breakdown due to mechanical factors. For instance surface soil dispersion problems could be countered by building up the surface soil organic matter, by putting the area to be used for FILTER under a pasture crop for a few years before and after FILTER installation. Application of chemical amendments, such as gypsum, during and subsequent to this period will help in increasing surface soil stability during periods of application of mechanical forces. Several FILTER management options are available to reduce the TEC of the surface and subsurface soils and to increase the soil EC at different critical times in the FILTER operations, such as during exposure of bare soil to rainfall, periods of land preparation, and trafficking. These could include cutting off the drainage during summer to increase surface soil salinity, re-applying the highly saline drainage water to the soil surface as irrigation, and application of chemical amendments, such as gypsum, to the soil surface during critical periods. Another approach could be to ensure that the surface soil layers are dry during the land preparation and trafficking periods. An alternative approach is to minimise the adverse effects of surface soil structural breakdown on effluent infiltration and crop growth by using gypsum-slotting in the bottom sections of the FILTER plots. The highly calcium-enriched gypsum slots could provide routing of excess surface waters to the drains during high rainfall periods and provide increased protection of the soil during trafficking. The optimum combination of such practices required will depend on specific site conditions.


Provision of a subsurface drainage system in FILTER plots at the Griffith land application site with impeded drainage reversed the pre-FILTER accumulation of salinity and sodicity and will hence minimise future long-term soil degradation. The threshold electrolyte concentrations for spontaneous dispersion calculated using the method proposed by Rengasamy et al. (1984) was much lower than the [EC.sub.1:5] measured in the soil before filtration and after the 3 cropping seasons of the trial, indicating that subsurface soil structural stability is likely to be maintained during effluent flow through the soil to the subsurface drains. The relatively small shift in the relationship between the subsurface drainage rate and the water table height midway between drains during the 3 cropping seasons indicates maintenance of subsurface soil structural stability. Modelling of sodicity and salinity changes in FILTER soils can be used to predict the longer term soil stability effects, and the needs for any remedial measures such as periodic application of gypsum to the soil surface.

The threshold electrolyte concentrations for mechanical dispersion were higher than the EC measured in the soil before filtration and after 3 cropping seasons of the trial, indicating that soil structural deterioration could potentially occur in these soils, if subjected to application of mechanical forces. However, careful long-term planning of FILTER plot management and routine operations could minimise the risks of surface soil structural breakdown, and their adverse effects on effluent infiltration and crop growth. The optimum combination of such practices required will depend on site conditions.
Table 1. The cropping sequence in the FILTER plots 1-4

Plot Cropping season

 2 3 4

1 Oats Sorghum / maize Wheat
2 Pasture Pasture Pasture
3 Pasture Pasture Pasture
4 Oats Sorghum / maize Wheat
Table 2. The mean SAR of composite effluent irrigation and composite
subsurface drainage water samples

Sample type Filtration Na Mg Ca SAR
 season ([mmol.sub.c]/L)

Irrigation 2 7.6 1.1 1.0 7.4
 4 11.2 1.8 1.6 8.7

Drainage 2 112.6 40.6 9.5 22.5
 4 73.3 23.6 6.2 19.0
Table 3. Measured SAR and EC in 1:5 soil:water extract, and
calculated TEC from Eqns 1 and 2 (Rengasamy et al. 1984) for
pre-FILTER soil and at the end of the fourth cropping season

Depth SAR E[C.sub.1:5] TEC
(m) (dS/m) Spontaneous Mechanical


 0-0.2 13.9 1.46 0.24 4.28
0.2-0.4 14.3 1.6 0.24 4.4
0.4-0.8 15.2 1.52 0.26 4.68
0.8-1.2 16.6 1.4 0.28 5.12
1.2-1.6 14.1 1.43 0.24 4.32
1.6-2.0 16.2 1.34 0.27 4.99

 End of cropping season 4

 0-0.2 6.2 0.39 0.11 1.82
0.2-0.4 8.7 0.42 0.15 2.62
0.4-0.8 11.0 0.49 0.19 3.33
0.8-1.2 11.3 0.68 0.19 3.43
1.2-1.6 13.5 0.79 0.23 4.14
1.6-2.0 12.8 0.83 0.22 3.90
Table 4. Water balance and mass salt balance in plot 4
during 1995 winter cropping season

Inputs and outputs Water (mm) Salt (t/ha)

Irrigation and rainfall 1251 9
Subsurface drainage 529 66
Crop uptake 569 0
Changes within soil profile 49 -46
Unknown additions/losses -104 11


We acknowledge funding for these studies from ACIAR, Griffith City Council, NHT and DLWC. We thank G. Nicoll and D. Wallett for assistance in the field work, to establish and operate the FILTER plots.


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Manuscript received 20 July 2000, accepted 12 April 2001

N. S. Jayawardane (A), T. K. Biswas (A), J. Blackwell (A), and F. J. Cook (B)

(A) CSIRO Land & Water, PMB 3, Griffith, NSW 2680, Australia.

(B) CSIRO Land & Water, QDNR, Meiers Road, Indooroopilly, QLD 4075, Australia.
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Author:Jayawardane, N.S.; Biswas, T.K.; Blackwell, J.; Cook, F.J.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:8AUST
Date:Nov 1, 2001
Previous Article:Sodic soil reclamation: Modelling and field study.
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