Catchment Restoration - Reducing external nutrient load
History
Forest and wetlands both act as filters, absorbing nutrients and helping prevent eutrophication in lakes and streams. As there were 90% more wetlands than there are today, and 50% more forested areas, historically nutrient input to streams and lakes was relatively low.
Following the influx of people to New Zealand, large areas of forest were felled and wetlands drained to provide areas for urban, agricultural and industrial development to support New Zealand's growing population. These developments produced waste products, many of which were discarded directly into rivers, lakes and the sea, before people realised the consequence of these actions. Following large declines in water quality globally, with many water sources unfit to drink, swim in or use for irrigation, the necessity of conserving these water sources has become apparent. Widespread efforts to restore lakes and rivers to conserve conservation, recreation and use values are underway in New Zealand and overseas.
Background science
Activities in a catchment such as pastural, urban or industrial land developments can result in nutrient, stormwater or wastewater run-off entering freshwater environments. These sources of pollution are classified as point source or diffuse source pollution.
A point source of pollution is discharged directly into the environment .Direct discharge of sewage treatment ponds into a lake or stream is an example of a point source of pollution. Fertiliser application to pasture is an example of a diffuse source of pollution. Eventually, nutrients from diffuse sources will enter lakes and streams through ground or surface water. A diffuse source of pollution has usually been diluted and often takes a long period of time to enter the freshwater environment.
When we try to reduce the external nutrient load in a lake both point and diffuse sources of nutrients and pollutants must be addressed.
Methods
Point sources of pollution are easier to target than diffuse sources, as it is usually obvious where they are coming from. Point sources can be diverted or may undergo wastewater treatment until they reach a high enough standard to be discharged.
Some commonly used methods to reduce external nutrient loading are listed below:
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Artificial/enhancement of wetlands
Purpose:
To slow stream inflows, thereby encouraging the absorption of nutrients into wetland vegetation and the denitrification of ammonia.
Effective for:
Point discharges from farmland e.g. dairy shed or piggery effluent.
Removes nitrogen effectively and a limited amount of phosphate.
Diffuse nutrient runoff.
Iron making slag or clay based minerals can be used as substrate for artificial wetlands to enhance phosphorus or nitrogen binding. A carbon source e.g. untreated sawdust, can be used to enhance bacterial activity and nitrogen.
Limitations:
Limited phosphorus binding.
Cost:
Expensive to build from scratch. However, the enhancement of an existing wetland or swampy area via stock exclusion and some native plantings may be very cost effective.
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Riparian plantings
Purpose:
To reduce both phosphorus and nitrogen input to lakes. This is done by using a buffer of vegetation around the lake or stream inflows to absorb nutrients from the ground or surface water run-off into plant matter. Riparian zones also help reduce erosion, as they have a stablising effect on the soil. This in turn helps prevent sediment bound nutrients from entering the lake.
Effective for:
Preventing diffuse nutrient discharges from surrounding farmland entering the target lake.
Settling area for suspended sediment.
Removing organic nutrients and faecal material by bacterial action, settling out and adsorbing nutrients.
Limitations:
This method will not work if the water moves to quickly through this area (i.e. flood events).
The buffer needs to be proportional to the catchment area it is meant to protect. Therefore it should be large/wide enough to adsorb the desired amount of nutrients.
Generally riparian plantings around lakes should be complimented with silt traps or riparian buffers on the major stream and river inflows.
Livestock should be prevented from entering the riparian buffer zone.
If not designed or maintained correctly riparian buffer zones may have a limited life span. For example, a riparian buffer of only grass species may become phosphate saturated relatively quickly if large amounts of phosphate enter the riparian zone regularly.
Limited removal of dissolved inorganic nitrogen. Nitrate from urine patches easily leaches into the groundwater and then has the ability to bypass below the riparian zone (Parkyn, 2004).
Trying it at home:
- Generally young growing plants arranged in a thick band using a variety of species will work best. If the plants are growing they will have a greater nutrient requirement (thus absorbing more nutrients).
- Using a variety of species is more effective as some species require more of one nutrient (e.g. nitrogen) than others. Additionally, try and use close ground growing species such as flax, Baumea and other native rushes and sedges. These have thick bases and will effectively slow down water and sediment movement through the area. Sediment and nutrient runoff has actually been shown to increase in riparian strips planted with pines and lacking ground cover vegetation (Elliot & Sorrell, 2002).
- The slope of the buffer zone should be considered as water movement should be slow enough to allow uptake into plant tissue. A combination of deep and shallow rooted plants would help to remove nutrients from the top soil surface layers and deep soil layers.
- Native plants to encourage native fauna and for aesthetic reasons may be the most desirable. If riparian plantings are targeted at a catchment scale, for example the entire river system entering the lake has riparian buffers as does the lake, benefits such as increased nutrient removal and colonisation of native flora and fauna will be enhanced.
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Silt traps
Purpose:
To reduce the amount of suspended sediment entering the lake. This is important, as nutrients can be bound to sediments and suspended sediment increases light attenuation, thus decreasing macrophyte survival or likelihood of recolonisation.
Effective for:
Point sources such as drains entering the lake that are surrounded by farmland. Riparian vegetation and fencing along these drains will also help reduce sediment load entering the lake.
Limitations:
The design of the sediment trap must slow down water movement sufficiently to allow suspended sediment to drop out of the water column.
Sediment traps must be maintained and deepened once they become full.
Fencing to exclude stock
Purpose:
To prevent the addition of nutrients and faecal coliforms directly to the lake via stock excretion and to prevent erosion resulting from trampling of vegetation and pugging of the lake margins. Additionally, if the lake does have poor water quality fencing will prevent stock from ingesting toxins during a cyanobacterial bloom.
Effective for:
Reducing point sources of nutrients from stock excretion and erosion and diffuse sources of nutrients from the surrounding farmland, when used in conjunction will a riparian buffer strip (see below).
Problems:
May result in the retirement of some previously stocked area and prevent stock access to a drinking water source. However, if the lake does suffer from cyanobacterial blooms the latter 'problem' can be considered an advantage (Steffenson et al., 1999; Hammil, 2001; Falconer & Humpage, 2005).
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References:
Elliot S. & Sorrell B. (2002). Lake Managers Handbook: Land-Water Interactions. Prepared for the Ministry of the Environment.
Falconer I.R. & Humpage A.R. (2005). Health risk assessment of cyanobacterial (blue-green algal) toxins in drinking water. International Journal of Environmental Research and Public Health, 2: 43-50.
Grove E., Paris B. & Clarkson B. (2006). Waiwhakareke Restoration Plantings: Establishment of Monitoring Plots 2005-06. CBER Report No. 44.
Hammil K. (2001). Toxicity in benthic freshwater cyanobacteria (blue-green algae): first observations in New Zealand. New Zealand Journal of Marine and Freshwater Research Abstracts, 35: 1057-1059.
Parkyn S. (2004). Review of Riparian Buffer Zone Effectiveness. MAF Technical Paper No. 2004/05.
