Chemically-treated Water Supply: Impacts on Agriculture, Organics, Soil, and Plant Health.
Compiled by: Andy Kirkwood, Justine Flanagan, 2019.
Contact: [firstName] @ islandbooth.com.
This is a working draft. Updated 8 Dec 2021.
As part of the Te Mato Vai Project, the Government of the Cook Islands are proposing to treat Rarotonga’s central water supply using polyaluminum chloride, and calcium hypochlorite (chlorine solution). All water will be chemically-treated, including that used for horticulture and livestock.
Agriculture is estimated to be the largest single sector for usage of water on Rarotonga with approximately 40% of water usage attributed to agricultural and horticultural practice.
Cook Islands National Water Policy (2016)
In subsistence farming, crops are grown for direct consumption by a farmer and their family; or to be fed to livestock.
In cash cropping a short-term crop is produced for market — to earn a living.
Industrialised cash cropping favours mono-cropping: mass-planting of a single crop. Crop rotation is common to manage nutrient levels in the soil. Synthetic compound fertilisers are used to maximise (short-term) plant growth. Inorganic pesticides and herbicides control insects and weeds to maximise crop yields.
In Rarotonga, farmers have been moving to organic farming practices, which favour inter-cropping: growing a combination of crops on a single site. This approach is often out of necessity, given the fragmented nature of land-title which results in small, discontinuous blocks of land for agricultural use. Rather than using synthetic compounds, in organic farming nutritional needs and pest control are achieved through an understanding of soil biology, natural fertilisers, and companion planting, where the companion plant may be of no direct commercial benefit.
…agricultural demand will be met from the distribution network. Although this will mean treating an additional volume of water, this is significantly cheaper than running a separate distribution network for agricultural water. Use of the existing network as a separate distribution network for agricultural water is not feasible due to the poor condition of the network and anticipated high ongoing maintenance costs associated with this.
Te Mato Vai: Water Supply Master Plan for Rarotonga (2014)
The chemical cost has been estimated at $500,000 per annum; for stocks of calcium hypochlorite and polyaluminum chloride.
In plant root systems, beneficial bacteria form a symbiotic relationship. The plant provides sugars to the bacteria, and the bacteria protects the roots against pathogens. (This is similar to the function of beneficial bacteria in the human digestive system, see ‘microbiome’.)
Chlorine kills bacteria, including bacteria in the soil. In the absence of beneficial bacteria, the plant is more susceptable to pathogens and pest insects. Crop growth and productivity is negatively impacted.
In Rarotonga, the common practise with crops such as tomato and watermelon is to use plastic irrigation drip tape to feed water directly to the roots of the plants.
Further research is needed to determine the likely impacts on plant health based on:
Irrigating crops with chlorinated water may impact on the commerical viability of organic farming.
The significant organic crops in Rarotonga for export are noni and vanilla (under establishment). There is also future potential for cacoa beans which are used for making chocolate. However cash crop farmers have also been rehabilitating land and moving to organic production methods to supply the local market with herbs, greens, and seasonal fruit.
Certification of organic produce is not only dependent on direct farming practises (such as use of natural fertilisers), but also the long-term exposure of workers to hazardous chemicals or unsafe working environments in the planting, harvesting, processing or storage of produce.
In a public release (Aug 2019) the Te Mato Vai Project Management Unit assured the public and growers that chlorinated water was safe for organic production.
One key concern has been from Rarotonga’s agricultural community about the impact of using chlorinated water on crops and compost. The organic farming community seem particularly worried about the potential impact on their certification.
The PMU can confidently state that chlorination is safe for use in agriculture, including organics. International standards confirm that water supplies chlorinated to the appropriate standards are approved for use in organic agriculture.
Chlorination confirmed safe for organic farming, TMV PMU, Aug 2019.
The release goes on to provide the specific example of where chlorine can be used:
The Pacific Organic Standard allows for the use of calcium hypochlorite (the type of chlorine used to disinfect the water supply) to disinfect food preparation surfaces with no limitations outlined.
None of these standards state that chlorine is a prohibited chemical, or that a chlorinated water supply would result in the loss of organic certification.
The PMU’s interpretation of the Pacific Organic Standards (POS) is accurate in as much as chlorine is not listed as a prohibited chemical; and calcium hypochlorite is listed under Cleansers and Disinfectants Applied Directly To Food Preparation Surfaces; with no Limitations. Water chlorinated to ‘the appropriate standards’ is approved for use in organic agiculture for washing or disinfecting produce.
…using chemically-treated water to irrigate is ‘anti-organic’.
However washing is not growing: washing is not irrigation; and no mention is made in the Pacific Organic Standard of the impacts of chlorination on crop productivity, soil health, or composting.
In principle, using chlorinated water to irrigate is ‘anti-organic’.
Pacific Organic Principles
Health – organic agriculture sustains and enhances the health of the soil, which enables the production of healthy plants and animals to enhance the lives of people and their environment, as one and indivisible.
Ecology – organic agriculture is based on living ecological systems and cycles, works with them, emulates them and helps to sustain them.
Care – organic agriculture is managed in a precautionary and responsible manner to protect the health and well-being of current and future generations and the environment.
Pacific Organic Standard. Secretariat of the Pacific Community, 2008.
Although a trace amount of chlorine is required for healthy plant growth, this is typically supplied by natural rainfall. Chlorine-supplementation is not needed in locations subject to high levels of salinity; and the majority of growing areas in Rarotonga are within 2-3km of the coast.
Crop Production 4.4.5
Mineral fertilisers shall only be used in a programme to address long-term soil fertility needs together with other techniques such as addition of organic matter, green manures, rotations and nitrogen-fixing plants, e.g. legumes. Fertilisers of mineral origin shall be applied in the form in which they naturally exist and are extracted.…
According to the above standard, chlorinated water (mineral) is not permitted, as in Rarotonga it does not address any long-term soil fertility needs.
See also: Appendix A: A New Organic Standard.
The PMU’s release also disregards the concerns of local growers who practice organic methods.
“We have sprinklers that moisturise our compost because the bacteria needs water to function properly. Chlorinated water will kill these bacteria and we have to start looking at carting water which adds to our cost. When the cost goes up, we will have no choice but to pass it on consumers.
We are down on our knees already and we are choking. And you, the government, are squeezing us again. It’s like getting blood out of a rock. You can’t do it. We are already dead in the water almost and are flat lining, and then they want to strangle us.”
1 Aug 2019: Organic growers chlorine fear: ‘It will put me out of business’. Rashneel Kumar, Cook Islands News.
The standards for organic certification vary from county-to-country; or more imporantly, from market-to-market. Practically, the standards start with formalising commonly-utilised methods and materials. As new methods and materials are introduced, the standards require revision.
Areas of contemporary consideration (in 2020) relate to water quality, soil health, and plant nutrition.
Impacts of chlorinated irrigation water on soil health and the soil microbiome: State of the Science. Produce Marketing Association (United States). June 2019.
Discussion of soil health microbiome begins at 27:00.
Organic standards refer to water quality in relation to the washing of produce – not because chlorinated supply has been approved for irrigating crops – but because research is in its infancy, and growers lack direct methods to assess the impacts of water treatment chemicals on soil and plant health.
A basic system for de-chlorinating water involves aeration in an open tank allowing the chlorine to break down due to exposure to UV/sunlight and off-gas.
“If water is disinfected by Chlorine, and that water enters the soil, it will disinfect the soil as well. That includes the majority of the beneficial organisms as well as the strict minority of the latent pathogenic organisms.
If the government carries out this plan after the risk analysis, I would suggest to create an open (contact with air*) buffer tank with water that is aerated for minimum 12 hours.
Either by mixing the water or literally aerating with an air pump and a line with many small holes before using this water for irrigation purposes.”
*Be careful that Children have no access.
Pius Floris, Managing Director, Plant Health Cure BV
In 2010 the understanding of plant health and nutrician was advanced by the discovery of the rhizophagy cycle. In this process a plant uptakes nutrients through its root system by absorbing and oxidising bacteria from the surrounding soil. The oxidation process removes the outer skin of the bacteria which provide a source of soluble nutrient. The husked bacteria is then ejected along with plant-generated sugars back into the soil (and regenerates).
The oxidation process also stimulates root growth. In the absence of a companion bacterial population — for example when sterilised seeds are grown in sterilised soil — plant growth rates are affected and root structures are underdeveloped.
Webinar: The rhizophagy cycle: How plants harvest nutrients from microbes. James F White, Rutgers University, April 2020.
Water treatment chemicals such as chlorine and aluminium are also oxidising agents and are used to prevent bacteria in water from reproducing. The impacts of oxidising chemicals on rhizophagy has yet to be investigated, however when such chemicals are released close to the root structure, they might reduce the value of soil bacteria as a nutrient source; affecting plant health and nutrient uptake.
In addition to the proposed disinfection with chlorine solution, polyaluminium chloride (PACl) will be used as a coagulant; to remove the fine particles from the water in high-turbidity conditions (heavy rainfall).
The proposed Te Mato Vai system does not automatically vary chemical dose in response to changing water flow conditions. The amount of chemical that will be added to the water can only be adjusted by an operator onsite at the each water treatment facility. In dry conditions, overdosing is possible, which will result in an increase in the levels of dissolved aluminium entering the public supply; also the source of irrigation water.
Toxic levels of aluminium in the soil solution affect root cell division and the ability of the root to elongate. The root tips are deformed and brittle and root growth and branching is reduced . Poor crop and pasture growth, crop yield reduction and smaller grain size occur as a result of inadequate water and nutrition… Roots are unable to effectively grow through acidic subsurface soil, which forms a barrier and restricts access to stored subsoil water for grain filling.
-Effects of soil acidity, Western Australian Government. Accessed 6 Dec 2019.
Aluminium toxicity is an issue for the soils of Rarotonga:
8.7 Analysis of Southern Cook Islands Fertility Capability Soil Classification units
…Low ability to supply potassium, strong to moderate acidity and aluminium toxicity are considered major problems with high phosphorus fixation and low available phosphorus within 50cm of the soil surface are also of significance.
– An Information Manual for Understanding and Managing the Soil Resources of the Southern Cook Islands Group. FAO Field Publication.
The two main soil types prone to aluminum toxicity are the Pouara and Rutaki soil types which appear to be the extensively farmed areas located between the coastal fringes and foothills (see map below).
Much of the research into the disposal of water treatment residuals (WTR) relates to use of the compound aluminum sulfate (alum), and there are commonalities as alum and PACl compounds both have wastewater and sludge high in dissolved aluminium as a by-product.
3.4.6 Land application for agricultural use or disposal purposes
Land application of water treatment sludges is the most commonly practiced beneficial use method used by US utilities. Cornwell (2006) reported at least 25% of responding utilities performed land application of residuals, with the assumption made that the majority of residuals were applied to agricultural soils. … Positive impacts reported included the improvement of soil structure, adjustment of soil pH, addition of trace minerals, increased moisture holding capacity, and soil aeration. Some negative impacts were reported, including a tendency to bind plant-available phosphorus, and the potential for aluminium phytotoxicity in acidic soils. Ippolito, Barbarick et al. (2011) cite several studies which corroborate the association of plant phosphorus concentration decreases, phosphorus deficiency symptoms, and decreased plant yields with soil application of water treatment residuals.…
Smart Water Fund: 10OS-42 Alum Sludge Reuse Investigation. GHD, August 2015.
A cautionary note is that in technical reports ‘agricultural use’ often includes the application of sludge to non-food crop soils such as forestry blocks. The negative impacts of nutrients binding with sludge are less significant if the nutrient is less-critical to the growth of the crop.
In March 2020, landowners visited the Turangi waterworks with a visiting GHD engineer. At that time discussion turned to potential reuse options for the PACl sludge. Following the site visit, confirmation was sought and clarification received July 2020:
TVOM: Can you direct me to the research about PACl sludge being used for gardening/agriculture?
GHD: Upon further review of the research we have found that the information does not demonstrate enough depth to reference and does not relate specifically to the local context of Rarotonga.
-GHD, July 2020
Webinar: The Phosphorus Paradox. Dr Christine Jones, April 2021.
Due to Cook Islands land tenure, small-scale agricultural blocks are piecemeal/dis-continuous. Infrastructure to filter chemical-residuals from irrigation water (such as carbon filtration or de-chlorination tanks) would need to be replicated for each plot, or filtered water carted from a central treatment facility. This is unlikely to be commercially viable, and subsidence growers will be more likely to irrigate without filtering out water treatment chemicals.
Long-term, chlorinated supply affects the salinity (salt content) of soil. Plants in high-salinity soil are less able to uptate moisture than those in lower-salinity soil. More water will be required to irrigate crops if chlorinated supply is used. Changes to weather conditions attributed to climate change are likely to result in greater seasonal variation: wetter wet seasons and longer dry seasons. Soil moisture management will become of increasing importance, given the intention of the Cook Islands government to impose water-use tariffs which will further increase the cost of growing.
The Cook Islands National Water Standards provide little detail regarding appropriate water standards for agricultural use, preferring instead to remind the majority-user not to impact upon the quality of drinking-water.
3.4 Ensure adequate and affordable water for horticultural and agricultural production.
…Horticulture and agriculture users will not threaten the quality or sustainability of water sources for other users. Users of water at each of these [agricultural] sites must ensure that water is protected from contamination. Information will be provided to users to comply with drinking water safety plans and maintain applicable standards where possible.
The New Zealand Policy on Freshwater Management is a more productive source for the consideration of agricultural needs, including the needs of livestock.
Irrigation, cultivation and food production – The freshwater management unit meets irrigation needs for any purpose.
Water quality and quantity would be suitable for irrigation needs, including supporting the cultivation of food crops, the production of food from domesticated animals, non-food crops such as fibre and timber, pasture, sports fields and recreational areas. Attributes will need to be specific to irrigation and food production requirements.
Animal drinking water – The freshwater management unit meets the needs of stock.
Water quality and quantity would meet the needs of stock, including whether it is palatable and safe.
(New Zealand) National Policy Statement for Freshwater Management 2014
In 2015-16 senior officials from the Cook Islands Ministry of Agriculture — including (now) associate minister for agriculture Patrick Arioka — investigated rehabilitating the old water main for use by the agricultural sector. This progressed to a $6m proposal for Green Climate Fund consideration; under ‘Food Security’.
The newsletter where this initiative is listed did not detail why officials felt it might be necessary for a separate water distribution system for agricultural users; or why the water supplied by the new Te Mato Vai system might not be appropriate.
Cook Islands organic grower association, Nautura Kuki Airani, approached The Pacific Organic and Ethical Trade Community (PoetCom) and June 2020, for clarification regarding the interpretation of the Pacific Organic Standard (POS):
The (2008) POS does not include a specific standard on using a chemically treated public water supply for irrigation purposes. However, it should not be interpreted that a chemically treated public water supply is automatically considered safe for organics.…
Organic standards need to be broad enough to work around the world while also specific enough to maintain organic integrity, including the consumer perception of organic products as being produced naturally, without the use of synthetic inputs. A central premise of Organic Growing is sustaining soil health through natural processes and avoiding the use of synthetic chemicals including fertilizers, pesticides and herbicides.
The ‘Pacific Organic Principles’ (specifically Health, Ecology, and Care), imply that the use of irrigation water containing water treatment chemicals including chlorine could pose a risk to soil health and so should be avoided…
PoetCom have also drafted Standards and Certification Committee Guideline on Implementation of the Pacific Organic Standard Guideline No. P1/2020, which recommends an intervention step to preserve microbial diversity where irrigation water has been chemically treated:
Organic farmers should ideally use only natural, uncontaminated water sources such as rainwater or streams etc. When water is sourced from the public supply, organic farmers should investigate the treatment method.
In order to preserve microbial biodiversity where chemical methods have been used to treat the public supply, a producer should include an intervention step such as carbon filtration, an open air holding tank or mist/spray system to remove chemicals before using water for irrigation.
In July 2021, consulting firm Tonkin+Taylor submitted an EIA for a PACl Sludge Disposal site (on behalf of the new water authority).
The EIA report makes reference to the potential impacts of the PACl sludge on land and groundwater values, based on the result of sludge analysis conducted as part of the PACl trial (included in the EIA is the Characterisation of PACl Sludge Samples.
In addition to the impacts of increased aluminium and chloride content, the analysis also showed unexpectely high levels of total sulphur.
The Total Sulphur concentrations measured in the PACl sludge samples were high, at 2,690 to 4,520 mg/kg, compared with a variable range of 142 to 491 mg/kg in the five soil samples. This results indicate the source of the sulphur is the PACl product.
8.5 Impacts of contaminants on land and groundwater
…Total recoverable aluminium, chloride and total sulphur and sulphate were higher than the natural soil samples.…The GHD report noted:
- The total recoverable aluminium does not represent an environmental risk as the extractable (plant available) aluminium content of the sludge samples was no different than the natural soils;
- The chloride concentrations were approaching levels that can cause damage to sensitive crops. However, the chloride concentrations will reduce naturally due to rainfall percolation and dilution and dispersion are not of concern; and
- In terms of total sulphur and sulphate, provided the disposal facility will be founded in natural mineral soils, the sulphate will react with soil cations (magnesium, calcium, sodium, potassium) and produce benign compounds.
Sulphate sulphur could reduce the availability of some trace elements for plant growth, although this effect can be mitigated if the PACl sludge is mixed (ploughed) into the soil. These micronutrients are only needed in very small amounts for plants. Trace elements can be readily added to phosphorus fertiliser, including compost.
Adding a high sulphur content sludge to soil could lead to induced soil deficiency for other trace elements because the sulphate may react with soil cations. For example, sulphur will react with positively charged ions such as calcium, sodium, magnesium and potassium, reducing the availability of these essential micronutrients for plant uptake. If the sludge is mixed in with the native soils this effect will be moderated by the lower sulphur content of the natural soils.
For agronomic applications [ploughing] would have some advantages compared to a broadacre spreading process on the soil surface.… because the sludge lacks structure and PACl sludge has been reported in other studies to develop water repellency when dried (which can inhibit rainfall infiltration).
…The only practical constraint and consideration is the need for trace element fertiliser due to sulphur induced trace element deficiencies. We recommend that an application and fertiliser regime should be developed in conjunction with the Cook Islands Ministry of Agriculture.
The technical evaluation of the sludge is a far cry from the broad statements issued by project managers GHD recommending agricultural reuse, but it still falls short of acknowledging the aversion of growers (both conventional and organic), to the agricultural use of water treatment sludge.
While the author of the characterisation report acknowlegdes ‘practical constraints’ he does not engage with the matter of cultural preference. Given the evident uncertainty regarding sludge composition; the impacts on crop health; and reliance on a vague (and likely costly) application and fertiliser regime; the only likely takers for sludge are those courting political favour.
Classification/subjects: Te Mato Vai, To Tatou Vai, chlorination, de-chlorination, disinfection, water standards, water quality, Rarotonga, Cook Islands, South Pacifc, agriculture, horticulture, livestock, organic standards, regenerative farming, soil health, aluminum/aluminium toxicity, rhizophagy cycle, plant microbiome, soil microbiology, water treatment residual disposal methods, sludge.
Working Draft. Updated: 8 Dec 2021.