Going native – The role of native vegetation in managing wastewater
Public opinion is increasingly indicating that it is unacceptable to discharge wastewater directly into waterways. In determining how to manage this paradigm shift, several District Councils are examining the potential for irrigating wastewater to land. While this approach offers benefits, nutrients from wastewater are difficult to remove and can enter waterways via ground and surface flows. Given the pathogens and contaminants present in untreated wastewater there may also be a perception of risk to public health. Native vegetation potentially has a role to play. Native vegetation can provide a barrier to odours and aerial dispersal and in riparian areas act as a buffer to nutrients while improving connectivity with other habitats. Native planting can also engage the community in wastewater planning. Key challenges with this approach are change in nutrient removal as buffers age and ensuring effective community engagement. Carterton District Council’s (CDC) purchase of land to discharge wastewater from the Carterton Wastewater Treatment Plant (CWTP) is presented as a case study to consider these issues.
Casestudy: Carterton Wastewater Treatment Plant
Water quality monitoring has identified discharges from the CWTP as impacting the health of the Mangatarere Stream (GWRC, 1999). In 2012, CDC purchased Daleton Farm, a 66 hectare site adjacent to the CWTP as a first step towards no longer discharging wastewater into the Stream (CDC, 2013).
|Figure 1: Aerial photograph of Daleton Farm (not to scale) (Source: Googlemaps)|
|Figure 2:Piezometric surface and velocity vectors for the Wairarapa Valley (Source: GWRC, 2010).|
The Mangatarere runs through the north corner and along the north the farm. A residential area is to the north-east and rural properties are dotted around the perimeter. An ephemeral stream drains the area and land between here and the stream is poorly drained (see Figure 1). The existing land treatment system includes sixteen wetland plots. Excess water that could be irrigated to land is discharged into the stream during high rainfall (Clark, 2010). Groundwater flow at the site is in a south-west direction (Figure 2).
Wastewater irrigation and risk
As NZWWA (2003) notes, irrigating wastewater to land requires management of environmental, social and economic matters.
Nutrients are difficult to remove from wastewater and irrigation can accumulate in waterways, impacting organisms through toxicity and triggering excessive growth of oxygen depleting plants (PCE, 2012). Sub-surface flow can transport soluble nutrients rapidly, sometimes even directly into streams. Surface runoff can transport both particulate and soluble nutrients (Tanner et al, 2005). Nutrients are a key issue for irrigating to land at the CWTP. There is significant interaction between surface and groundwater in the area (GWRC, 2009).
Pathogens and contaminants are usually removed prior to irrigation and odour generation and aerial dispersal can be effectively managed by application design and screening (Magesan and Wang, 2003). Nonetheless, convincing the public that these processes are appropriate can be difficult. There is usually goodwill if the community is well informed but public support can be challenging when bureaucratic structures are perceived as untrustworthy (Mermet et al, 2007).
CDC’s vision for wastewater treatment, highlights the constraints surrounding rate increases and that CDC will partner with farmers rather than purchase the remaining 150 Hectares needed to manage all municipal wastewater. Opportunities to offset water and fertiliser costs of agriculture and forestry and allow an economic return are highly regarded and are also options that can assist nutrient removal (Personal communication: Greg Boyle, 26/3/2014). Crop production and forestry instead of grazing will avoid urine inputs and stock removal increases infiltration as trampling compacts soils (Parkyn et al, 2004). Taupo District Council grows and sells a cut and carry haylage crop (Treeweek et al, 2010). Rotorua irrigates commercial stands of Pinus radiata (Magesan and Wang, 2003).
What role can native vegetation play?
While commercial opportunities will be a priority focus at CWTP, a role for native vegetation should still be considered. Vegetation stands along borders will shield neighbouring properties (USDA, 2007) and in riparian areas will improve stream health. Quinn et al (1997) compared stream health in riparian areas of pasture, pine plantation and native forest. Nutrient levels were more favourable in native forest. Connectivity between native vegetation patches is also improved. Ultimately, native planting activities can attract community participation in wastewater planning.
Nutrient removal differs according to hydrology, soils and vegetation. Surface pollutant transport of soluble nutrients is reduced by increased infiltration caused by root channels and soil structure changes. Particulate nutrients are filtered and slowed by dense vegetation. Soluble nutrients transported through sub-surface flows can be removed by vegetation uptake and denitrification. Denitrification, whereby nitrate is removed as N₂ gas, is common in wetlands with anoxic soils rich in organic matter. Cooper (1990) in Parkyn et al (2004) found nitrate loss was concentrated in wetland areas that occupied only 12% of the border of a stream. Planning at the CWTP will need to consider the ephemeral stream and the poorly drained soils to the west as well as the banks of the stream. As Gillian (1994) notes, these area receive runoff and have shallow groundwater seeps.
Stream health and connectivity
Effects of native vegetation on streams include increased shade and terrestrial food sources and reduced water and air temperature in areas where native fish lay eggs (Parkyn et al, 2004). Death and Collier (2010) emphasise restoration of segments is optimised in areas with significant catchment cover suggesting that connectivity between patches is important. As Akçakaya et al (2007) notes increased connectivity aids in the dispersal of biodiversity but needs to be managed as pest species can also benefit. The local streamcare group in Carterton, the Mangatarere Restoration Society (MRS), is supported by both the District and Regional Councils and has released an action plan based on community consultation. The action plan outlines a strategy to links restored areas to the Tararua Forest Park. Opportunity exists for the CDC to work at the CWTP to support this vision, though a pest management strategy will be needed to ensure biodiversity values at the Tararua Forest Park are protected.
Public participation in ecological restoration is common place in New Zealand (Galbraith, 2013). Native planting offers an opportunity to engage the public on wastewater options. In Carterton, for example, wastewater consultation and native planting could be held at the same time. Community consultation has already indicated native planting as an activity the community wants to participate in and the MRS can harness this interest (personal communication: Jill Greathead, 15/6/2012). As Mermet et al (2008) notes replacing management by government with management by community can assist in resolving contentious issues and could assure the community on public health and odour generation issues. Galbraith (2013) notes a role for community participation in changing attitudes, suggesting that environmental education on household and farming inputs to wastewater could also be a useful focus of planting days.
|Plate 1: MRS members participating in native planting at a local landowners property. Photograph by A.J. Hunter|
Challenges with this approach
In advocating for a role for native vegetation, a number of challenges should be considered. The effectiveness of nutrient removal of native vegetation changes as buffers age. Engaging with communities is challenging.
Nutrient response as buffers age
Howard-Williams and Pickmere (1994) studied a native riparian vegetation stretch for 17 years and found that after twelve years nutrient response decreased but wildlife capacity increased. Pores of soils can become blocked and shading limits the role of aquatic and smaller terrestrial plants in nutrient removal. Parkyn et al (2003) suggest maintenance to allow light entry and grass filter strips to remove more particulate nutrients before they enters riparian areas. Howard-Williams and Pickmere (1994) takes a longer view advocating wildlife values to be embraced and that riparian retirement should be accompanied by improved landuse. Implementing such an approach in Carterton, for example, will require working with downstream landusers to improve their impacts. To some extent this is occurring through Greater Wellington Regional Council’s work with local farmers implementing Land and Environment Plans (personal communication: Richard Parkes, 6/2/2014).
Silvertown et al (2007) notes recruiting broad volunteer participation in conservation as challenging. Attitudes towards environmental issues vary and many organisations report age and gender bias in the small portions of the population they manage to engage. While the participation of MRS members will be a useful addition to wastewater consultation, CDC will need to ensure this occurs as part of a broader strategy or the approach may be perceived as being hijacked to appease a conservation orientated elite. As Mermet et al (2007) emphasise governance and coordination approaches to conservation, while useful in bringing together stakeholders can also be seen as reinforcing defacto power balances and be vulnerable to political manipulation.
Native vegetation has a role to play in managing the environmental, social and economic issues involved with irrigating wastewater to land. Native riparian vegetation can assist in removing nutrients while improving stream health and connectivity. Perhaps more importantly, native vegetation planting can also act as a focal point for attracting community participation in wastewater planning. CDC’s plan to irrigate wastewater to land is an opportunity to implement such an approach. Wastewater management will benefit from native vegetation stands along border areas between neighbouring properties, in riparian areas and along the ephemeral stream. Planting activities will need maintenance as buffers age and should be part of a wider strategy to work with householders and neighbouring landowners in reducing nutrient inputs to the stream. Involving the public in native planting activities as part of the wastewater consultation process, provides an opportunity to discuss and resolve contentious planning issues providing it is part of a package of activities involving the wider community.
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Stuart McKay has over 15 years experience working for governments in Australia, the UK and New Zealand across health, agriculture, housing and environment portfolios. His experiences range from representing Australia at Commission on Sustainable Development meetings at the UN headquarters in New York to providing project management expertise to health, housing and social care projects in the UK. With the New Zealand government at the Ministry for Primary Industries Stuart was responsible for the 2011 independent audit of the Dairying and Clean Streams Accord, the Natural Resource Sector Briefing for Incoming Ministers and part of the cross portfolio team that implemented the National Policy Statement on Freshwater Management. Stuart was also pivotal in securing funding for the establishment of the Mangatarere Restoration Society a community restoration group in the Wairarapa. Stuart is sustainable development consultant at Sustainable Communities and is currently completing a post graduate certificate in ecological restoration at Victoria University, Wellington to supplement and specialise his expertise. He currently holds a Bachelor of Science in Environmental Science and Social Ecology with Honours in Sustainable Development from Murdoch University in Perth, Western Australia.