4.1.6 Water Quality

Key Knowledge: issues of water quality related to food and fibre production: levels of nitrogen, phosphorus and dissolved oxygen; acidity or alkalinity (pH); electrical conductivity (EC); turbidity

If you have lived in Australia for more than one Summer, I don't have to tell you how important water is.

A reliable supply of water is essential to agricultural and horticultural systems, but its quality directly impacts, and is impacted by, food and fibre production.

Managing water quality requires a precise understanding of chemical, physical, and biological indicators.

When water interacts with agricultural land, changes in its properties can signal environmental degradation or threaten the health of crops and livestock.

Nitrogen (N) and Phosphorus (P)

Nitrogen and phosphorus are essential plant nutrients, but in aquatic ecosystems, excess amounts lead to severe problems.

Source: Runoff of synthetic fertilisers, animal manure, or effluent into waterways.
Impact: High nutrient levels trigger rapid growth of algae which can become algal blooms. When this algae dies and decomposes, microbes consume the available oxygen, leading to a condition called eutrophication that can suffocate aquatic life.

Dissolved Oxygen (DO)

Dissolved oxygen is the amount of gaseous oxygen dissolved in water, which is vital for the survival of aquatic organisms and healthy root systems.

Source/Influence: DO levels are reduced by high water temperatures, slow-moving water, and the decomposition of organic matter (such as dying algae).
Impact: Low DO levels create hypoxic zones where fish and beneficial aquatic microbes die, severely damaging the health of the local river ecosystem.

Acidity or Alkalinity (pH)

The pH scale measures how acidic or alkaline the water is, operating on a scale from 1 to 14, with 7 being neutral, 1 being highly acidic and 14 being highly alkaline.

Source: Industrial runoff, acid sulfate soils exposed by drainage, or intensive fertiliser use can lower water pH.
Impact: Highly acidic or highly alkaline water can corrode irrigation infrastructure, burn plant roots, and alter the availability of nutrients in the soil when irrigated. Most aquatic life prefers a stable pH between 6.5 and 8.0.

Electrical Conductivity (EC)

Electrical conductivity measures the ability of water to pass an electrical current, which directly indicates the concentration of dissolved salts. The higher the salinity, the higher the conductivity.

Source: Rising water tables bringing underground salts to surface streams, or over-extraction of fresh water from rivers.
Impact: High EC water can cause salt burn on crop leaves, stunt plant growth, and lead to toxic salt accumulation in the soil profile over time.

Turbidity

Turbidity is the measure of water clarity, indicating the amount of suspended solids (like clay, silt, or algae) trapped in the water column.

Source: Soil erosion from bare paddocks washing into streams during heavy rain, or livestock trampling riverbanks.
Impact: High turbidity blocks sunlight from reaching submerged aquatic plants, reducing photosynthesis. It also clogs irrigation filters and deposits unwanted sediment over crops.

Soil Nutrient Availability/pH Chart

This chart from the Agriculture Victoria page shows, by the thickness of the line, how much of each nutrient is available to plants at a particular pH level.

The following information is directly from the site:

Soil pH will influence both the availability of soil nutrients to plants and how the nutrients react with each other.

For example:

At a low pH, many elements become less available to plants, while others such as iron, aluminium and manganese become toxic to plants. Aluminium, iron and phosphorus also combine to form insoluble compounds.
At a high pH, calcium ties up phosphorus, making it unavailable to plants, and molybdenum becomes toxic in some soils. Boron may also be toxic in some soils.

PRACTICE Use the marking scheme to evaluate your work

Skill Check: The Water Quality Data Lab

Look at the water quality data collected from a creek running through a dairy farm below and solve the puzzle.

The Farm Data Log:

Upstream (Before entering the farm): pH: 7.2 | EC: 150 micro Siemens/cm | Turbidity: Low | DO: High
Downstream (After leaving the farm): pH: 6.8 | EC: 160 micro Siemens/cm | Turbidity: High | DO: Low

The Task: Based on this data, write down the two most likely management issues occurring on this dairy farm that caused the downstream changes.

Photo by Hans Reniers on Unsplash

PRACTICE Use the marking scheme to evaluate your work

Exam-Style Questions

Question 1 (2 Marks)

Explain how a farmer using high amounts of phosphorus fertiliser on a hillside can cause a drop in dissolved oxygen levels in a creek at the bottom of the hill.

Question 2 (3 Marks)

An orchardist tests their irrigation water and finds it has a very high electrical conductivity reading. Analyse the biological impact this water will have on the fruit trees if used continuously.

PRACTICE

Interactive Task: The Water Quality Continuum

To map out how these indicators connect, create a cause-and-effect flowchart in your workbook using these exact five terms: High Nitrogen Runoff, Algal Bloom, Microbial Decomposition, Low Dissolved Oxygen, and Fish Kill.

Draw arrows to show the sequence of events. Once finished, write a one-sentence caption explaining where a manager should intervene to break the chain.

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