7.4 Associated Risk and Mitigation: Environmental Risks

Environmental risks, including soil, plant, groundwater, and ecosystem impacts, often stem from salinity levels, specific plant toxicity, and nutrient pollution. 

Salinity

Effluent can contain elevated salinity levels, which tend to rise through evaporation in treatment ponds, particularly in hot climates. Additionally, flood and furrow irrigation practices can exacerbate salinity as water evaporates, accumulating salt on plant ridges. Secondary salinisation may occur through capillary rise in regions with high groundwater tables, necessitating adequate drainage and leaching measures. 

Salinity is commonly quantified by electrical conductivity (EC). Electrical conductivity can be determined in the laboratory or in situ using salinity sensors. The result is a value expressed in deciSiemens per meter (dS/m) or milli-mhos per centimetre (mmho/cm). Electrical conductivity is temperature-dependent and increases at a rate of ca 1.9% per °C increase. For purposes of comparison, EC is commonly expressed at a reference temperature of 25°C.

High salinity can impede crop water uptake due to osmotic pressure, but different crops exhibit varying sensitivity to salinity. To address this, the cultivation of salt-tolerant crops is an option. The following diagram exhibits the relative reduction in crop yield for crops of different salt tolerance levels as a function of the electrical conductivity of the irrigation water. 

Examples of crops and their sensitivity to salt are presented in the following chart: 

Irrigation should closely match plant requirements to prevent groundwater salinity increase, minimising leaching and inefficiency. Precise methods for calculating irrigation and fertigation needs under drip systems have been developed for various crops and are determined in consultation with agronomists.

Nutrient management evaluates the alignment of target yields and crop nutrient requirements with reclaimed wastewater, fertiliser, and soil nutrient content. The sufficiency concept accounts for nutrient availability in the soil, influenced by soil structure and crop growth stage. 

Apart from total salinity, the sodium-calcium-magnesium ratio, expressed as the Sodium Adsorption Ratio (SAR), is a crucial indicator of irrigation water quality. High sodium levels can induce dispersion and swelling of clay minerals, leading to reduced soil permeability, slower infiltration rates, and the formation of rigid clay crusts.

The table below provides an overview for interpreting water quality for irrigation concerning the Sodium Adsorption Ratio (SAR):

Potential Irrigation Problem	Unit	Degree of Restriction on Use: None	Degree of Restriction on Use: Slight to Moderate	Degree of Restriction on Use: Severe
EC	dS/m	<0.7	0.7 – 3.0	>3.0
TDS	mg/L	<450	450 – 2000	>2000
Na Surface Irrigation	meq/l	<3	3 – 9	>9
Na Sprinkler Irrigation	meq/l	<3	>3
Cl Surface Irrigation	meq/l	<4	4 – 10	>10
Cl Sprinkler Irrigation	meq/l	<3	>3
Boron (B)	mg/L	<0.7	0.7 – 3.0	>3.0
Nitrogen (NO₃ -N)	mg/L	<5	5 – 30	>30

Nutrient Pollution

Certain plants, such as fruit trees, are susceptible to elevated sodium, chloride, or boron levels. Boron, which can originate from detergent bleach, often exhibits low removal rates. While nutrients are essential for achieving high crop yields, excessive nitrogen can reduce yield and disease susceptibility due to overly lush growth. Typically, grassy, and leafy crops display better nutrient uptake.

Plant Toxicity

As detailed in the following table, elevated levels of trace elements can induce plant toxicity. When bioaccumulation is significant, metals, such as cadmium, can also be transferred to consumers through plant consumption.

Trace Element	Long term application (mg/L)	Short term application (mg/L)	Remark
Aluminium (Al)	5.0	20	It can hinder plant growth in acidic soils (pH<5.5) but in more alkaline soils (pH >7), it binds to other elements in the soil, reducing its harmful effects.
Arsenic (As)	0.10	2	The level of toxicity to plants varies significantly, with Sudan grass showing tolerance up to 12 mg/L, while rice is highly sensitive, with toxicity occurring at levels below 0.05 mg/L.
Boron (B)	0.75	2.0	Essential for plant growth at concentrations as low as a few tenths of a mg/L. However, it can become toxic to many sensitive plants, like citrus, at just 1 mg/L, while most grasses can tolerate levels ranging from 2 to 10 mg/L.
Beryllium (Be)	0.10	0.5	The toxicity levels for plants vary significantly, with kale able to tolerate up to 5 mg/L, while bush beans are more sensitive, showing toxicity at levels as low as 0.5 mg/L.
Cadmium (Cd)	0.01	0.05	At concentrations as low as 0.1 mg/L in nutrient solutions, cadmium is toxic to plants like beans, beets, and turnips. Establishing conservative limits is recommended because of its capacity to accumulate in plants and soils, potentially reaching harmful concentrations.
Cobalt(Co)	0.05	5.0	Toxicity occurs in tomatoes at just 0.1 mg/L when exposed to this element in nutrient solutions. It’s worth noting that it tends to become less harmful in neutral and alkaline soils.
Copper (Cu)	0.20	10	Toxicity affects several plant species when concentrations range from 0.1 to 1.0 mg/L in nutrient solutions.
Fluorine (F)	1.0	15	Inactivated by neutral and alkaline soils.
Iron (Fe)	5.0	20	It is not inherently toxic to plants in well-aerated soils, but it can lead to soil acidification and reduce the availability of essential elements like phosphorus and molybdenum. When used in overhead sprinkling, it may also cause unsightly deposits on plant equipment and buildings.
Lithium (Li)	2.5	2.5	Most crops can tolerate it at levels up to 5 mg/L, and it exhibits mobility in soil. However, even at low concentrations < 0.075 mg/L, it can be toxic to citrus plants and behaves similarly to boron.
Manganese (Mn)	0.20	2.0	Toxic to several crops at concentrations ranging from a few tenths to a few milligrams per litre, with relevance in acidic soils.
Molybdenum	0.01	0.05	Harmless to plants at normal concentrations in soil and water. it can be toxic to livestock if the plants they consume have grown in soil with high levels of available molybdenum.
Nickel (Ni)	0.2	2.0	Toxicity to various plants occurs within a range of 0.5 – 1.0 mg/L, but this toxicity is diminished under neutral or alkaline pH conditions.
Lead (Pb)	5.0	10	Plant cell growth can be inhibited at extremely high concentrations.
Selenium (Se)	0.02	0.02	Selenium can be toxic to plants at concentrations as low as 0.025 mg/L and harmful to livestock if forage grows in soils with relatively high levels of added selenium. While selenium is an essential element for animals, it’s required in very low concentrations.
Vanadium (V)	0.10	1	Vanadium can be toxic to many plants at relatively low concentrations.
Zinc (Zn)	2.0	10	Zinc can be toxic to a variety of plants at varying concentrations, but its toxicity is reduced in soils with a pH > 6 and in fine-textured organic soils.

Addressing issues tied to heavy metals and organic contaminants can be mitigated by reducing turbidity, as these pollutants are predominantly associated with solid particles. 

During irrigation, some effluent can percolate into groundwater or run off overland into surface waters, potentially reaching drinking water supplies. As water percolates through the soil, attenuation processes typically reduce the concentrations of nutrients, heavy metals, and organic trace contaminants. 

In areas especially susceptible to direct infiltration, such as karst regions, which have limited capacity to attenuate contaminants, it is crucial to prioritise the establishment of buffer strips (a minimum of 30 meters) around sinkholes and along streams.  

Reducing the risk of groundwater contamination involves careful selection of fields with suitable soil thickness and properties, restricting the application of reclaimed water in groundwater protection zones and near streams, and improving the quality of reused water. The potential consequences of effluent reuse on groundwater quality, particularly in karst regions, should receive special attention.

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