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SUSTAINABLE MANAGEMENT OF LANDFILL LEACHATE BY IRRIGATION MARK S. BOWMAN∗ , TIMOTHY S. CLUNE and BRUCE G. SUTTON Department of Crop Sciences, University of Sydney, NSW, 2006 Australia (∗ author for correspondence, e-mail: [email protected]; fax: +61 2 9351 4172)

(Received 12 July 2000; accepted 31 January 2001)

Abstract. Leachate from domestic landfills is a significant environmental hazard. In the urban environment, irrigation of recreational turf and parkland with nitrogen-rich landfill leachate provides both low-cost treatment that minimises pollution of surrounding waters and a valuable water resource. Of particular interest is the capacity of the turf-soil system to ameliorate the ammonium-rich leachate. To address this issue, a two-year field trial was completed at the Newington Landfill irrigating with saline, ammonium-rich leachate. The field trial suggested that in situ bioremediation is sustainable provided that management strategies such as dilution of leachate to reduce solution electrical conductivity to 3.6 dS m−1 are adopted. Furthermore, pollution due to leaching of nitrogen can be minimised by managing the soil to enhance in situ denitrification of applied nitrogen. The management regimes adopted during the Newington field trial enabled nitrogen application rates in excess −1 yr−1 . However, the capacity of the system to ameliorate the leachate appears of 1400 kg NH+ 4 ha limited by soil salinity and sodicity rather than the control of nitrogen leaching by denitrification, −1 yr−1 may be viable if the salinity hazard can be suggesting that rates of up to 3500 kg NH+ 4 ha effectively managed. Keywords: ammonium, bioremediation, denitrification, irrigation, landfill leachate, pollution, salinity, sustainable management

1. Introduction The environmental risk associated with landfill leachate is largely attributable to the presence of high ammonium (NH+ 4 ) nitrogen levels that threaten surrounding waters (Lu et al., 1984). Since it is a potential source of ammonia (NH3 ), which poses a significant threat to fresh and marine environments (ANZECC, 1992) removal of NH+ 4 is an essential component of leachate remediation. Commonly used batch treatment processes for the removal of nitrogen (N) from leachate require high energy and capital inputs and can generate large quantities of by-products (Kosson and Ahlert, 1985). In contrast, irrigating with leachate using the plant-soil system to manage excess N, dissolved salts and water appears to offer a cost-effective, low energy alternative (Burford and Bremner, 1975). Consequently, irrigation of recreational areas such as turf and parkland provides a mechanism for the beneficial reuse of leachate in the urban environment (Revel et al., 1999). Irrigation of leachate as a means of in situ bioremediation is limited by the capacity of the system to accommodate added pollutants, particularly N. Where Water, Air, and Soil Pollution 134: 81–96, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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soil conditions are favourable and leachate N is in an appropriate form, bacterial denitrification can provide a significant sink of N (Stevenson, 1982). For in situ denitrification to occur, NH+ 4 applied to the soil must first be oxidised (nitrification) to ). Conversion of NO− form nitrate (NO− 3 3 to gaseous N (denitrification) is favoured by high soil moisture (Stevenson, 1982), temperatures of 15–35 ◦ C (Dawson and Murphy, 1972), neutral pH (Crasswell and Martin, 1974), the presence of suitable bacterial populations (Jacobsen and Alexander, 1980), and by the availability of a soluble carbon source (Leffelaar, 1987). Although leachate irrigation aims to promote denitrification, attention must also be given to other environmental concerns. Variation in leachate composition, soil physical and chemical properties, climatic conditions and the tolerance of target plant species will also affect the volume and sustainability of leachate irrigation (Sapari, 1987). Of particular interest to this study was the affect of leachate salinity and sodicity on growth and survival of the turf grass species Couch (Cynodon dactylon) and Kikuyu (Pennisetum clandestinum) and the concurrent affect on soil physical properties. Since salinity threshold levels for Couch and Kikuyu are reportedly 6.9 and 3.0 dS m−1 , respectively (Shaw, 1999), and salinity levels for leachate have exceeded 15 dS m−1 (Lu et al., 1985; Sapari, 1987), control of salinity and sodicity must therefore form an integral component of any leachateirrigated system. In the current study, initial laboratory investigations suggested that with appropriate management, leachate irrigation could be sustained. Subsequently, a twoyear field experiment incorporating subsurface leachate irrigation of recreational turf was conducted at the Newington Landfill, Homebush Bay, Sydney. This site was selected because it was being converted to recreational parkland for the 2000 Sydney Olympics and it generates saline N-rich landfill leachate. A requirement of the local Environmental Protection Authority was that leachate emissions from −1 −1 and 0.1 mg NH+ the site must not contain greater than 10 mg NO− 3 L 4 L (ANZECC, 1992), to minimise pollution of the adjacent Parramatta River. As such, the ANZECC guidelines were adopted as the primary criteria for the success of the experimental program.

2. Materials and Methods Accumulation of landfill leachate in urban areas is often controlled by removal to sewer, a management technique which is uneconomical and unsustainable. Utilising the dissolved nutrients in the leachate to support turf growth in recreational space provides a valuable resource. A fundamental component of irrigating with Nrich leachate is managing irrigation to prevent the pollution of surrounding surface and groundwater. As such, the primary objective of this study was to identify management techniques to prevent leaching of N by promoting in situ bioremediation while maximising irrigation at the Newington landfill, and to control salinity and

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sodicity. To determine the suitability of each management technique, a stringent monitoring program was adopted which sought to develop an agronomically-based leachate irrigation and monitoring protocol suitable for utilisation by turf managers at other landfill sites in the Sydney basin. The Newington Landfill test site was located on a reclaimed section of the Parramatta River foreshore at Homebush Bay, 10 km west of Sydney, Australia. The site was reclaimed in 1974 by positioning a rock retaining wall on a tidally inundated sand beach, backfilling with compacted clay and builders spoil (thickness 600 mm) and capping the clay with topsoil (thickness 200 mm). The sandy loam topsoil was generally well structured with appreciable aggregation and organic matter content (2.3%). In contrast, the clay subsoil layer appears to have been heavily compacted during the site establishment phase and contained lower organic matter levels (1.1%) than the topsoil. The topsoil was stabilised with turf, predominantly Kikuyu and Couch grass. A typical analysis of the top-soil at the site indicated a moderate effective cation exchange capacity (ECEC) of 10.3 cmol kg−1 , neutral pH (6.9) and low salinity (0.2 dS m−1 ). Weather conditions (maximum and minimum temperature, wind speed, soil temperature, rainfall, and solar radiation) were monitored at 0.25 hr intervals and several important trends were identified. Temperature data were found to be highly correlated (R2 = 0.97) between the two years of operation. Mean temperature maxima were 23.4 and 22.2 ◦ C for 1998 and 1999, respectively and mean minimum temperatures were 14.2 and 13.8 ◦ C for 1998 and 1999, respectively. The rainfall data were characterised by periods of high intensity rainfall, typically within the first six months of the year (Figure 1a). However, reasonable correlation was still observed for the rainfall (R2 = 0.63) and evapotranspiration (R2 = 0.68) data between the two years (Figure 1a). Total rainfall during the experiment was 2830 mm and evapotranspiration was equivalent to 1692 mm. Evapotranspiration losses were highest during summer months (December to March) and were replaced by the irrigation system. Leachate was obtained from an adjacent (within 25 m) sub-surface interception drain with a two-week supply stored on-site. Treatments were enhanced by cycling irrigation to create anaerobic pulses within the system and to maintain available soil water (ASW%) at 60 to 70% (Figure 1b) to promote denitrification (Stevenson, 1982), mowing to encourage turf growth and spraying with selective herbicides to suppress weed growth. Leachate equivalent to 1435 mm of rainfall was applied during the experiment. Low salinity water from the town water mains was used to irrigate the control plot (0% leachate). The second and third treatment plots received leachate diluted to a concentration of 20 and 50% by applying pulses of town water. Diluted leachate treatments were applied by pulsing leachate and then town water through the irrigation system. The 20% treatment was applied by pulsing 2 volumes of leachate followed by 3 volumes of town water and the 50% treatment was applied by pulsing 1 volume of leachate followed by 1 volume of town water. Concentrated leachate (undiluted leachate) was applied to a fourth treat-

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Figure 1. Rainfall and evapotranspiration (ET) and leachate irrigation data (mm).

ment plot. Treatments were applied by sub-surface drip irrigation laterals (Geoflow RootguardTM 16 mm, 2 L hr−1 ) placed at 1 m intervals and at a depth of 150 mm. A submersible solar powered electric pump supplied leachate from the tank to the 20 and 50% and undiluted leachate plots via twin sand-bed filters. Irrigation was applied as rainfall equivalents (mm), scheduled by computer and corrected for soil moisture and evapotranspiration to deliver a precise volume of solution (±1 L) to each treatment plot. The Newington Landfill leachate was found to be saline (EC 14–17 dS m−1 ) −1 and contained 250–330 mg NH+ 4 L . The leachate was also characterised by high − −1 chloride (6700–8000 mg Cl L ) and sodium (3000–4000 mg Na+ L−1 ) levels (Table I). In contrast, the available town water contained low levels of dissolved salts, low salinity and negligible NH+ 4 -N (Table I). Given the ready availability of town water, dilution of the leachate was considered an appropriate management technique. Irrigation treatments were derived using the equation ECI W (mix) = (ECL × XL ) + (ECT W × XT W ),

(1)

where ECI W (mix) is the EC of the applied irrigation treatment, ECL and ECT W are the EC of the irrigation waters and XL and XT W were the fractions of leachate and town water used (Rhoades et al., 1992). Irrigation treatments had salinity values of


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TABLE I Mean analysis of undiluted leachate and town water (± SD) Analyte pH EC NH+ 4 Cl− Na+ SARa

(dS m−1 ) (mg L−1 ) (mg L−1 ) (mg L−1 )

Leachate

Town water

6.8±1.2 17.6±4.6 285.0±31.8 8172.0±563 4012.0±830 33

6.90±0.4 0.10±0.04 0.05±0.01 15.00±2.2 8.00±1.9 1

a Sodium Adsorption Ratio (Rhoades et al., 1992).

0.2, 3.6, 9.1 and 17.6 dS m−1 for the control, 20 and 50% and undiluted leachate additions, respectively. Measurements of shoot biomass (monthly) and mean root distribution (annually) were used to determine turf response to leachate irrigation. Soil cores were − obtained monthly and assayed (1:5 soil 2 M KCl extracts) for NH+ 4 , nitrate (NO3 ) − and nitrite (NO2 ) by ALPKEM colorimetric flow analysis. Soil Cl− , pH and EC were assayed using extracts (1:5 soil water). Soil moisture status was monitored using Soilspec tensiometers (H and TS Electronics), neutron probes (CampbellPacific Nuclear) and gravimetrically following coring. Soil solution chemistry was monitored using porous ceramic cup samplers (Cooinda Ceramics, Melbourne, size 7) installed approximately 230 mm from the emitter, at two depths (300 and 600 mm) as described by Hansen and Harris (1975). Six replicates were installed in each of the four plots. Solution samples were collected and assayed directly − − for NH+ 4 , NO3 , Cl , pH and EC. Results were calculated based on the flux (L m−2 d−1 ) and concentration of solution passing the sampler each month, and were used to determine N and Cl− budgets for each treatment plot. To complement the conventional sampler data, a revised solution sampler design was constructed of 40 mm electrical conduit with a 3-way PVC fitting (diameter 25 mm) used to form a horizontal arm to which the ceramic cup was attached (Figure 2). The vertical arm of the PVC fitting was sealed with a push-on cap (25 mm) to form a reservoir from which solution was collected. The ceramic cup sampler was placed directly below the emitter at a distance of 300 mm. Three microlysimeters were constructed (Figure 3) at the Newington site to collect soil solution from confined, intact soil cores (diameter 300 mm × depth 600 mm) to determine if the unconfined ceramic cup samplers obtained representative solution samples. The space between the intact column and the PVC cylinder was then filled with warm (50 ◦ C) liquefied petroleum jelly to completely surround the sides of the soil monolith (Cameron et al., 1990). Irrigation was scheduled by the main site computer to achieve the same 20% leachate dilution used to irrigate

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Figure 2. Comparison of (a) conventional and (b) modified soil solution sampler designs. The horizontal arm (5◦ gradient) incorporated in the modified sampler facilitated collection of solution samples from undisturbed soil beneath point-source emitters.


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Figure 3. Microlysimeter design schematics: (a) cross-section highlighting solution sampling, intact core placement and structural support, and (b) cut-away view highlighting dimensions and solution drainage from the intact column. Solution was collected by applying vacuum (–200 mm H2 O) to approximate field suction. Periodic solution sampling was conducted via a second line.

the main leachate-irrigated plots. Solution was applied via pressure compensating drippers (4 L hr−1 ). Solution mineral N was monitored on a weekly basis. The relative efficiency of each solution sampling technique was assessed using Cl− in the leachate as a tracer to monitor solution movement. Significant differences (P ≤ 0.05) between means were determined by ANOVA.

3. Results and Discussion The monitoring protocol adopted at the Newington site allowed accurate quantification of turf-soil response to leachate treatments. The Cl− tracer data provided a useful indication of the relative performance and descriptive capability of each soil

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TABLE II Comparison of mean Cl− (kg ha−1 yr−1 ) leaching losses determined by three solution sampling techniques. Data are presented for 20 and 50% and undiluted leachate irrigated plots Sampler

20%a

50%a

Undiluteda

1759±35 1407±303 1677±148b

4512±104 3767±526 4219±292b

8160±215 7052±799 8021±312b

Cl− (kg ha−1 ) Microlysimeters Conventional Modified

Recovery efficiency (%Cl− )c Microlysimeters Conventional Modified

102.8 79.9 95.3

101.7 83.4 93.5

102.3 86.4 98.3

a Data are the mean (± SD) total of 24 monthly observations. b Significant differences (P ≤ 0.05) reported for conventional and

modified samplers relative to the microlysimeter data.

c Recovery efficiencies (%) equivalent to the percentage of Cl−

measured in solution relative to the difference between applied and accumulated soil Cl− levels.

solution sampling technique. The microlysimeters recovered more Cl− than was applied by irrigation (102.8, 101.7 and 102.3% for the 20%, 50% and undiluted leachate treated plots), suggesting some leaching of pre-existing soil Cl− occurred. The conventional solution samplers recovered significantly less Cl− (1407, 3767 and 7052 kg Cl− ha−1 yr−1 for the 20%, 50% and undiluted leachate treatments, respectively) than the modified design (1677, 4219 and 8021 kg Cl− ha−1 yr−1 for the 20%, 50% and undiluted leachate treatments, respectively) (Table II). The rate of conventional sampler Cl− recovery in the 20% leachate treated plot (79.9%) was particularly low compared to the modified samplers (95.3%) and microlysimeters (102.8%). These results suggest that the modified sampler design adequately described soil solution chemistry and could be confidently used to monitor N leaching losses. The modified soil solution sampler most effectively described the movement of applied Cl− because it facilitated continuous sampling of solution passing the cup between monthly sampling events. In contrast, the conventional samplers provided a discontinuous record of changes in solution chemistry because the design only allowed intermittent sample collection, limiting their capacity to detect solution fluctuations associated with high rainfall or irrigation events. Furthermore, the modified samplers were installed directly below the irrigation emitters, minimising potential error associated with preferential flow. In comparison, the conventional


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−1 −1 Figure 4. Nitrate (mg NO− 3 L ) and ammonia (mg NH3 L ) leached in solution from leachate irrigated plots. Data presented are the means of triplicate samples.

samplers were installed adjacent to the emitter, increasing their susceptibility to preferential flow. Consequently, the modified sampler design offers enhanced estimation of soil solution chemistry, particularly for monitoring sub-surface irrigation systems, where appropriate placement of the sampler in relation to the emitter is necessary so as to obtain representative soil solution. Additionally, the modified solution samplers installed in the unconfined plots indicated that a 20% leachate dilution most effectively reduced the risk of excessive −1 N leaching into the nearby Parramatta River. Elevated mineral N (>7 mg NO− 3 L ) leaching rates (Figure 4) in the undiluted leachate treatment during February, April and May 1999 and rates approaching the environmental guidelines of 10 mg NO− 3 −1 L−1 (or 3.5 kg NO− ha month) during October 1999 (ANZECC, 1992). Losses in 3 the 20% treated plot (106 kg N ha−1 yr−1 ) were analogous to losses in the control

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Figure 5. Comparison of nitrate (NO− 3 ) and ammonia (NH3 ) leaching data from microlysimeters and the unconfined 20% leachate treated plot. Data presented are the means of triplicate samples.

plot (80 kg ha−1 yr−1 ). In contrast, N leaching losses were significantly greater (P ≤ 0.05) in the 50% (359 kg ha−1 yr−1 ) and undiluted leachate (624 kg ha−1 yr−1 ) treatments than in the control (Table III). Although the 50% dilution recorded significantly (P ≤ 0.05) higher leaching rates relative to the 20% treatment, −1 measured rates were generally 7 mg NO− 3 L associated with equipment failure indicating that system reliability must form an integral component of any leachate irrigation system. Breakthrough rates of N at the base of the microlysimeter soil columns (depth 600 mm) were comparable to samples obtained using the modified ceramic cup design (Figure 5). Nitrogen leaching rates in the confined columns were also below −1 and 0.01 mg NH3 L−1 (ANZECC, 1992) the guideline limit of 10 mg NO− 3 L

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TABLE III Partitioning of applied N (kg ha−1 ) for the Newington field site (1998 and 1999) Nitrogen sink

Control

20%

50%

Undiluted

1998

Applieda Leachedb Immobilisation Turf uptake Gaseous loss c

121 82 (68) 24 (20) 10 (8) 5 (4)

1216 97 (8) 559 (49) 37 (3) 523 (43)

3337 331 (10) 1401 (42) 171 (5) 1434 (43)

6082 446 (7) 2099 (35) 221 (4) 3316 (55)

1999

Applied a Leachedb Immobilisation Turf uptake Gaseous loss c

139 78 (56) 30 (22) 19 (14) 12 (9)

1431d 115 (8) 386 (27)d 57 (4) 873 (61)d

3576d 386 (11) 866 (24)d 173 (5) 2151(60)d

7156d 781(11) 1973 (28) 292 (4) 4110 (57)d

Mean

Applieda Leachedb Immobilisation Turf uptake Gaseous lossc

130 80 (62) 27 (21) 15 (11) 9 (7)

1324 106 (8) 473 (36) 47 (4) 698 (53)

3457 359 (10) 1134 (33) 172 (5) 1793 (52)

6619 624 (9) 2036 (31) 257 (4) 3713 (56)

Numbers in parentheses are percentages (%) of applied N. ∗ Data are the mean of triplicate samples. a Includes N mineralisation (kg ha−1 ). b Comprises mineral N (NO− NH+ , and NH ) lost below root zone (600 mm). 3 3 4 c Derived from mass balance calculations and assume losses are due to denitrification and volatilisation. d Significant at P ≤ 0.05 for comparison between yearly N partitioning.

and were not significantly (P ≥ 0.05) different from the 20% leachate treated unconfined plot. This validated the assumption that N losses were not attributable to sampling error caused by soil variability. The closed-system provided by the microlysimeters gave an unambiguous measurement of turf-soil influence on leachate solution composition and confirmed that leaching accounted for only a small proportion of added N. Gaseous loss of N, calculated from mass balance equations, has been predominantly associated with denitrification with small contributions assumed from volatilisation. Denitrification was estimated to be the major sink of applied NH+ 4 accounting for around 54% in the 20%, 50% and undiluted leachate treated plots (Table III). Mean applied NH+ 4 in the undiluted leachate treated plot was equiv−1 −1 alent to 6619 kg ha yr , with the highest proportion of losses attributable to denitrification (3713 kg ha−1 yr−1 ). Similarly, a large proportion of applied N was lost to denitrification in the 20% (698 kg ha−1 yr−1 ) and 50% (1793 kg ha−1

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yr−1 ) treated plots. The high rates of applied N directed towards denitrification across all treatments suggest that the soil water management regimes adopted were successful. Furthermore, these rates are consistent with Pratt and Adriano (1973) who reported losses following N fertiliser applications of 53% in an irrigated clay soil, and Lund (1979) who reported gaseous N losses from 45 to 60% of applied N under a range of field crops. Immobilisation, or incorporation of N in the soil matrix, was the second major sink of applied N and was equivalent to a mean total of 34% (Table III). Total quantities of N immobilised during 1998 were 559 kg N ha−1 , 1401 kg N ha−1 and 2099 kg N ha−1 for the 20%, 50% and undiluted leachate treated plots, respectively. In contrast, during the second year of operation, total immobilised N was significantly lower (P ≤ 0.05) at 386 kg N ha−1 , 866 kg N ha−1 and 1973 kg N ha−1 for the 20%, 50% and undiluted leachate treated plots, respectively. The measured decline in the soil’s capacity to immobilise N would appear to suggest that the turf-soil system is approaching saturation. However, while incorporation declined during the second year of irrigation, the percentage of total N leached was not significantly different (P ≥ 0.05) from the previous year. Turf uptake of N was a relatively minor N sink for all treatments, averaging around 5.8% per year during the experiment (Table III). This rate is consistent with the turf uptake level of 7% reported by Broadbent and Carlton (1979). If the capacity of the system to accommodate N was grossly exceeded as the incorporation data appears to indicate, significantly higher N leaching would be expected. Therefore, it appears that the management regimes adopted at the site were instead favouring soil-denitrifying bacteria which is consistent with the significantly higher (P ≤ 0.05) rates of gaseous N losses reported in 1999. Soil salinity was most effectively controlled in the 20% leachate treated plot, which was consistent with an increase in turf shoot biomass production and prevention of soil physical degradation. In contrast, increases in soil salinity in the 50% and undiluted treatments degraded soil structure. During 1998 and 1999, net increases in soil ECe were equivalent to only 0.5 dS m−1 in the 20% treatment at all depths (Figure 6). In contrast, the 50% and undiluted treatments displayed mean increases in ECe levels of approximately 2.4 dS m−1 and 3.1 dS m−1 , respectively. Salt accumulation in the 50% plot was apparent with an ECe in the 0–150 mm zone of 9.6 dS m−1 . In the undiluted leachate treated plot, soil ECe in the 0–150 mm zone exceeded 17.6 dS m−1 , falling to 7 dS m−1 at 600 mm depth (Figure 6). Increases in soil ECe were associated with accumulation of Cl− . Lowest increases were reported in the 20% plot (6870 kg ha−1 yr−1 ) and highest in the undiluted treated plot (36 485 kg ha−1 yr−1 ). In association with increased soil salinity, leachate irrigation also had an affect on soil physical properties (Table IV). In the plot treated with undiluted leachate, bulk density increased from 1.1 to 1.3 g cm−3 , porosity declined from 62 to 55%, and aggregate stability class (Emerson, 1967) declined from 5 to 3. In contrast, the soil physical properties of the 20% treated plot were not significantly different


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Figure 6. Saturated soil extract ECe (dS m−1 ) for each treatment. Data are the mean of 10 samples.

from the control. Changing soil characteristics associated with leachate irrigation were reflected in the turf response (Table V). Total biomass production during the experiment was significantly (P ≤ 0.05) higher in the control and 20% treatment in the second year of operation. Monthly mean biomass production rates in the 20% treatment increased from 291 to 430 kg ha−1 month, respectively, relative to the control which returned rates equivalent to 168 kg ha−1 in 1998 and 186 kg ha−1 in 1999. In contrast, biomass production rates in the 50% and undiluted treatments decreased from 205 and 140 kg ha−1 month in 1998 to 168 and 67 kg ha−1 month during 1999, respectively.

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TABLE IV Changes in soil physical properties under irrigation Controla

20%a

50%a

0.9 0.9

1.1 1.2∗

Soil property

Year

Bulk density (g cm3 )

1998 1999

Porosity (%)

1998 1999

61 57

65 66

Emerson classb

1998 1999

5 5

5 5

1.0 1.1

69 63

Undiluteda 1.1 1.3∗∗ 61 55

5 3∗∗

5 3∗

a Data are the mean (± SD) of triplicate samples. b Aggregation based on the classification system of Emerson (1967). ∗ , ∗∗ Differences between 1998 and 1999 are significant at P ≤ 0.05 and 0.01,

respectively. TABLE V Turf biomass production (kg ha−1 ) and production rates (kg ha−1 month) is response to leachate irrigation treatments

Year Control 20% 50% Undiluted

Biomass (kg ha−1a

Rate (kg ha−1 month)a

1998

1999

1998

1999

2019±46 3488±38 2455±113 1682±74

2228±31∗ 5157±49∗ 2014±78∗ 800±216∗

168±4 291±3 205±9 140±6

186±6∗ 430±4∗ 168±8∗ 67±18∗

a Data are the mean (± SD) of triplicate samples. ∗ Significant at P ≤ 0.05 with respect to production from the previous

corresponding year.

The observed degradation in soil physical properties under 50% diluted and undiluted leachate irrigation was probably associated with the accumulation of Na+ ions relative to Ca+ in the soil. Olsen and Mesri (1970) related the tendency of clay minerals to disperse and compress when irrigated with elevated Na+ solutions, leading to restriction of plant growth. The structural decline observed in the 50% and undiluted Newington leachate treatment plots was consistent with the measured reduction in turf biomass production. The measured decline in shoot biomass production during 1999 in the 50% and undiluted treatments relative to the 20% treatment suggests that the Couch and Kikuyu turf species were becoming increasingly intolerant of undiluted leachate applications. In contrast, shoot biomass production was significantly (P ≤ 0.05) greater relative to the control in the 20%


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leachate treated plot, indicating yield decline for the two species (Shaw, 1999) was not exceeded. Therefore, the sustainability of leachate irrigation at the Newington site appears to be limited by soil salinity and sodicity, rather than control of N leaching by denitrification. However, the use of amendments, such as gypsum, at the time of site re-development or in a dissolved form as part of irrigation management (Davidson and Quirk, 1961) may improve soil response at leachate concentrations >20% at the Newington site. Furthermore, the Newington leachate appears to represent a worst-case salinity scenario, suggesting that other Sydney landfills with a lower leachate salinity hazard may tolerate more concentrated leachate applications.

4. Conclusions and Summary In conclusion, the current investigation has established the following: 1. Scheduling leachate irrigation in response to soil moisture, evapotranspiration losses and rapid soil solution N analysis, enabled real-time, low-cost control of N leaching losses. 2. The greatest limitation of irrigating with the Newington leachate was its high salinity which adversely affected turf growth and soil structure when applied in an undiluted state. 3. Adequate control of soil salinity was only achieved when leachate was blended with town water to achieve a 20% leachate concentration (