Nitrate fluctuations

Fluctuations in nitrate levels – whether in soil or waterbodies – are caused by a variety of factors.

Seasonal weather events

Heavy rainfall and storm events: After the soil’s water-holding capacity is reached, excess water washes nitrate from the soil surface and root zone down into the groundwater, causing rapid spikes in concentration.

Drought periods: During dry weather, water’s not moving downward, and microbial activity slows down, allowing organic nitrogen to accumulate in the soil. After dry periods, accumulated nitrate in the soil can flush rapidly into groundwater during intense rain.

Land use practices

Changes in land use can cause long-term trends and short-term fluctuations in nitrate.

Agriculture: Application of synthetic nitrogen fertilisers and concentrated animal waste (urine patches) in grazed pastures can lead to a surplus of nitrate in the soil that can be leached.

Catch crops: Oats, ryecorn, triticale, wheat and barley are sown after winter grazing to mop up excess nitrogen that’s been left in the soil, reducing the amount of nitrate that can be leached during one of the highest-risk times of year.

Irrigation: Like rainfall, intense irrigation can accelerate the leaching of nitrate from soil to the groundwater.

Wastewater disposal: Septic tanks, sewage treatment system, and industrial wastewater also contribute to the nitrate load.

Hydrogeological conditions

The physical nature of the subsurface influences how quickly and how much nitrate moves.

Soil type: Coarse-textured soils (sand and gravel) with high permeability allow water and nitrates to move rapidly to the groundwater. Finer textured and poorly drained soils (clay) slow nitrate migration.

Aquifer depth and type: Shallow, unconfined aquifers are generally more susceptible to rapid, seasonal changes in nitrate concentration than deeper aquifers with longer lag times for water to travel to groundwater.

Oxygen: In low oxygen groundwater environments, denitrification can occur. This is a microbial process that converts nitrate into nitrogen gas, which reduces nitrate concentration.

The interaction of these factors can lead to clear seasonal cycles (for example, higher concentrations of nitrate in winter/spring) and substantial year-to-year variability based on weather patterns.