Elizabeth MacPhee and Gabriel Wilks
Yarrangobilly Caves is a tourist destination within Kosciusko National Park (KNP), New South Wales. The Yarrangobilly Caves Wastewater Treatment Plant (WTP) has been established to treat greywater produced at the tourist centre, to stop nitrogen moving into the limestone karst system of the caves.
To optimise benefits from the WTP, the Rehabilitation team undertook the planting of locally native grass species in the discharge area, with a view to producing seed and weed-free mulch for use in the KNP Former Snowy Sites restoration program.
Effluent is initially treated using a bacterial blivet and then undergoes an ultra-violet treatment process so that it is within a “greywater” classification. It is then stored in a 200,000 litre tank and released under pressure to a discharge area. Prior to being discharged the effluent is diluted with fresh water to an average ratio of 7:3 (effluent:fresh water) in order to reduce the total nitrogen in the irrigated water to around 10 mg/L, which has been used as a threshold figure for nutrient loading. Once at the right concentration, the effluent is discharged in a large flat sedimentary rock area of about 1 ha in size. The irrigation area in which the plant species are grown is approximately 0.5 ha.
Vegetation treatments. From 2006 to 2010, some 20,000 plants of a number of species of the grass genus Poa were planted in the discharge area of the WTP, at 50cm spacings (Fig 1). The four main species were: Poa costiniana; P. fawcettiae, P. sieberiana and P. ensiformis; all native to KNP. Over the last 6 years, more than 300 kilos of highly viable Poa spp. seed has been collected and used in restoration works across the Park. The thatch (seed heads and cut off straw) has also been harvested and used as mulch on some of the sites.
Other species needed for rehabilitation in KNP have also been planted in the site over the last two years. Bossiaea foliosa and Lomandra longifolia have been grown for seed production and a variety of difficult to germinate shrubs have been grown to provide cutting material for propagation.
Soil sampling and soil treatments. Sampling was conducted prior to and after plant harvest to gauge the soil’s physical and nutrient status. The samples (10cm cores of topsoil and subsoil) were sent to the Environmental and Analytical Laboratories at Charles Sturt University for analysis of Total Phosphorus and Total Nitrogen. (ammonia and nitrates as Nitrogen and phosphorus as Phosphorus (Bray)).
As early soil tests showed that pH reduced, Lime was applied to the discharge area in 2010 at 1 – 1.5 tonnes to to raise topsoil pH approximately 1 unit.
Seed and mulch production: Within the first 18 month period, nearly 100 kilos of seed was collected. To date over 300 kilos of highly viable Poa spp. seed has been collected and used in rehabilitation across the park, with the 2011/2012 harvest producing approximately 58 kilograms of seed. In the 2012-12 harvest, an estimated 288 kilograms of thatch was removed for use as mulch in restoration areas in the Park.
Soil fertility. More nitrogen and phosphorus was discharged during the 2011/2012 season than could be removed by plants season, with the native species having naturally low nutrient removal rates. Annual soil monitoring and peizometer monitoring of the ground water is keeping track of the use and movement of nitrogen in this landscape and to monitor any changes in soil chemistry.
Suggestions for improvements:
- Review irrigation scheduling to ensure the bulk of irrigation is occurring from November to March when nutrient uptake will be at its highest (rather than in the cooler months).
- De-thatch the grass species at the start of spring to encourage fresh re-growth and therefore improve nutrient uptake over the spring and summer months
- Test effluent on a regular basis to assess salt load;
- Further treat effluent to reduce the nitrogen, phosphorous and sodium load;
- Monitor and adjust pH as required; and
- Reseed bare patches to maximise nutrient uptake by plants.
In 2012 a progressive replacement planting program commenced, where sections of the oldest plants were poisoned and replaced with young plants. This continual renewal replanting will ensure the plantation remains actively growing, taking up maximum levels of nutrient and producing high quality seed and mulch.
Acknowledgements. Funding for this project came from The Former Snowy Sites Rehabilitation project with soil and plant nutrient data provided by D.M McMahon (2008, 2012): Environmental Monitoring Use of Effluent for Irrigation, Yarrangobilly Caves, NSW. Environmental Consultants (agronomy) Wagga, Wagga.