Modelling the feasibility and cost-effectiveness of edge-of-field mitigations for reducing nitrogen and phosphorus loads in the Waituna Lagoon Catchment, Southland

Figures & data

Figure 1. Overview of the spatial location of the Waituna Lagoon Catchment (Southland, New Zealand) and spatial data used to establish EoFM feasibility. Shown are the DEM (A), soil type (B), landuse (C) and contributing creeks (D).

Table 1. Estimated proportion (%) of N and P loads intercepted by different EoFM devices in on-farm catchments with and without tile drainage based on nutrient losses in drainage in a nearby catchment (Monaghan et al. 2016) and using the upper limit of a EoFM efficiency study in Southland (Hughes et al. 2013). In Equation (1), these values are used as a factor, i.e. for wetlands this is 0.9 for tile drained and 0.5 for non-tile drained; and for other EoFM, it is 0 or 0.9.

EoFM device	Nitrogen		Phosphorus
	No artificial drainage	Artificial drainage	No artificial drainage	Artificial drainage
Constructed wetland	50%	90%^a	90%	100%
Woodchip bioreactor and zeolite filter	0%	90%	0%	90%

^a Monaghan et al. (2016)

Table 2. Seasonal nitrogen load reductions (% of influent load removed) by EoFM devices.

EoFM	Source	Winter	Spring	Summer	Autumn
Constructed wetland (3% of catchment)	Tanner et al.(2013)	32%	83%	83%	32%
Constructed wetland (5% of catchment)	Tanner et al.(2013)	48%	92%	92%	48%
Woodchip bioreactor	McKergow et al. (2015)	50%	50%	74%	40%

Table 3. Seasonal phosphorus load reductions (% of influent load removed) by EoFM devices.

EoFM	Source	Winter	Spring	Summer	Autumn
Constructed wetland (3% of catchment)	Tanner et al.(2013)	28%	60%	60%	28%
Constructed wetland (5% of catchment)	Tanner et al.(2013)	40%	75%	75%	40%
Zeolite filter	Hudson et al. (2017)	70%	70%	70%	70%

Table 4. Price of EoFM by size of device. In Equation (3), mitigation cost is used on m² basis, but costs are shown based on the field-trial size as the relative efficiency by size varies greatly between these mitigations. Sources are McKergow et al. (2015) and Tanner et al. (2013) updated to 2017. The difference between these costs has been explained in Section 2.6.

EoFM	Field-trial size	Initial cost	Construction cost	Maintenance cost	Depreciation Time
Unit	m^2	NZD	NZD/size	NZD/size/year	years
Constructed wetland^a	2000	$25,000	$25,000	$120	25
Woodchip bioreactor^b	100	$6200	$13,000	$50	20
Zeolite Filter^b	1	$4800	$5200	$110	12

^a Based on modelled 5% wetland (2000 m², in catchment of 4 ha).

^b Based on a filter depth of 1 m.

Table 5. Price of EoFM as shown in Table 4 translated to price per ha of contributing catchment. Only the costs that vary per EoFM size are shown.

EoFM	Size per contributing catchment	Construction cost	Maintenance cost
Unit	m^2/ha	NZD/size	NZD/size/year
Constructed wetland (3%)	300	$3700	$18
Constructed wetland (5%)	500	$6200	$30
Woodchip bioreactor	11	$1400	$6
Zeolite Filter	0.7	$3500	$73

Figure 2. Presence and characterisation of on-farm-catchments in the Waituna Lagoon Catchment. Per on-farm-catchment larger or equal to 5 hectares (ha), the outflow location, potential wetland area and presence of artificial drainage are shown. Inset 1 focusses on a larger than 5 ha artificially drained on-farm-catchment with a large potential wetland area near the outflow location. Inset 2 focusses on a larger than 5 ha on-farm-catchment area with no artificial drainage and a potential wetland area near the outflow location. The outflow locations are at first or higher order streams.

Figure 3. Boxplots showing the range of sizes for the on-farm-catchment area (catchment, selected for >5 ha) and available potential wetland area located in >5 ha on-farm-catchments (pot. wetland). On the boxplot, the middle line indicates the median, the upper box the 75 percentile, the lower box the 25 percentile, end of the top line the 95 percentile and end of bottom line 5 percentile.

Figure 4. Price per hectare mitigated per EoFM for on-farm-catchments (selected for >5 ha). 3% wetland (orange), 5% wetland (red), woodchip filter (blue) and zeolite filter (black). The dots on the lines are on-farm-catchment sizes where potential EoFM can be applied based on location suitability in the Waituna Lagoon Catchment.

Figure 5. Distribution of cost per kg N reduced for constructed wetland and denitrifying bioreactor EoFM when applied to applicable on-farm-catchments in the Waituna Lagoon Catchment. Outliers are omitted from the figure. On the boxplot, the middle line indicates the median, the upper box the 75 percentile, the lower box the 25 percentile, end of the top line the 95 percentile and end of bottom line 5 percentile.

Figure 6. Distribution of cost per kg P reduced for each when applied to applicable on-farm-catchments in the Waituna Lagoon Catchment. Outliers are omitted from the figure. On the boxplot, the middle line indicates the median, the upper box the 75 percentile, the lower box the 25 percentile, end of the top line the 95 percentile and end of bottom line 5 percentile.

Figure 7. Optimal investment in EoFM based on farm-based cost-effectiveness approach (Farm) and catchment-based cost-effectiveness approach (Watershed) for N and P reduction of the received load for >5 ha on-farm-catchments in the Waituna Lagoon Catchment independent of EoFM used for mitigation. These abatement curves are shown with no preference for mitigating either N or P (A), mitigating with focus on N (B) and mitigating with focus on P (C). The dotted reference lines indicate the approach threshold when the farmer-based abatement curve becomes less cost effective on a catchment scale compared to the catchment-based abatement curve.

Table 6. Overview of abatement curve maximum for EoFM placement in the Waituna Lagoon Catchment. These results are assessed for an N & P, N and P focused mitigation placement strategy and a farm-based and catchment-collective cost-effective mitigation placement strategy. The approach threshold indicates when the farm-based approach becomes less cost-effective compared to the catchment-collective approach.

		Nutrient reduction objective on received load from >5 ha on-farm catchments
Approach		Approach threshold			Maximum mitigation
Farm-based		N & P	N	P	N & P	N	P
	Reduction (%)
	N & P	16%	16%	17%	30%	25%	31%
	N	28%;	33%	3%	41%	42%	8%
	P	4%	0%	30%	20%	7%	53%
	Annualised Cost (millions NZD)	$ .8	$ .9	$2.2	$3.1	$2.5	$6.2
	EoFMs
	3% wetlands	3	0	42	261	99	473
	5% wetlands	0	0	0	0	0	0
	woodchip b.	151	230	0	308	482	0
	zeolite f.	8	0	183	12	0	108
Catchment-collective	Reduction (%)
	N & P	–	–	–	53%	52%	31%
	N	–	–	–	53%	54%	5%
	P	–	–	–	54%	49%	57%
	Annualised cost	–	–	–	$8.1	$7.9	$6.8
	(millions NZD)
	EoFMs
	3% wetlands	–	–	–	74	43	5
	5% wetlands	–	–	–	461	498	111
	woodchip b.	–	–	–	8	40	0
	zeolite f.	–	–	–	38	0	465

Figure 8. EoFM selection is based on a catchment-based approach for N reduction and P reduction in the Waituna Lagoon Catchment with focus on both nutrient loads (A), mitigation with focus on N (B) and mitigation with focus on P (C). The dotted reference lines indicate the approach threshold when the farmer-based abatement curve becomes less cost effective on a catchment scale compared to the catchment-based abatement curve.

Monaghan RM, Smith LC, Muirhead RW. 2016. Pathways of contaminant transfers to water from an artificially-drained soil under intensive grazing by dairy cows. Agriculture, Ecosystems & Environment. 220:76–88. doi:10.1016/j.agee.2015.12.024.

 Web of Science ®Google Scholar

Hughes A, Semadeni-Davies A, Tanner C. 2013. Nutrient and sediment attenuation potential of wetlands in Southland and South Otago dairying areas. NIWA Client Report HAM-2013-016, March 2013 prepared for Pastoral 21 Research Group under contract to AgResearch.

 Google Scholar

Tanner CC, Hughes A, Sukias J. 2013. Assessment of potential constructed wetland sites within the Waituna Catchment. Report prepared for Environment Southland and DairyNZ [Internet]. Hamilton, New Zealand. https://www.dairynz.co.nz/media/5795469/assessment-of-potential-constructed-wetland-sites-within-the-waituna-catchment_niwa_2013.pdf.

 Google Scholar

McKergow L, Tanner C, Baddock E, Pattinson P, Burger D, Scandrett J. 2015. The ‘nitrate catcher’ – design and installation of a bioreactor to monitor nitrate-N removal from drainage water [internet]. Prepared for DairyNZ and living waters. Hamilton, New Zealand: NIWA. https://www.dairynz.co.nz/media/5795471/the-nitrate-catcher-design-and-installation-of-a-bioreactor-to-monitor-nitrate-n-removal-from-drainage-water_niwa_2015.pdf.

 Google Scholar

Hudson N, McKergow L, Baddock E, Scandrett J. 2017. Waituna Wetland – evaluation of nutrient mitigation options : Nitrogen and Phosphorus filters (Draft report 23 May 2017) [Internet]. Report for DairyNZ, NIWA, Hamilton, New Zealand. https://www.dairynz.co.nz/media/5795473/waituna-wetland_evaluatipon-of-nutrient-mitigation-options_nitrogen-and-phosphours-filters_niwa_2017.pdf.

 Google Scholar