Water runoff management usually focuses on stormwater at the surface: drains, sediment ponds and discharge points. But much of the rain that falls on a site soaks into the ground instead, and that water carries soluble pollutants down towards the water table.
If your groundwater monitoring results were collected before a period of heavy rain, they may no longer describe the aquifer beneath your site. This article explains how rainfall affects groundwater levels and quality, what that looks like in your data and when to sample again.
How rainfall reaches the water table
Rain that does not run off, evaporate or get taken up by plants infiltrates the soil and moves down through the unsaturated zone to the water table. This is known as groundwater recharge.
On the sandy soils we have here on the Swan Coastal Plain, where the superficial aquifer is unconfined and often shallow, infiltration can be rapid and the water table can respond within days of a large rainfall event. Deeper or confined aquifers elsewhere in the state respond more slowly, sometimes over weeks or months.
As the water moves down, it dissolves and carries whatever substances are in the soil profile. On a clean site that is mostly natural salts and organic matter. On an industrial, mining, agricultural or contaminated site it can include fuels and hydrocarbons, metals, nutrients from fertiliser or wastewater, pesticides, leachate from stockpiles and landfills, and acidity from disturbed acid sulfate soils. This leaching process is how surface activities become groundwater problems.
What heavy rain changes in groundwater results
Heavy rainfall can shift results in several directions at once, so there is no exact handbook as to interpreting differences after a storm. However, these four effects are common.
Rising water levels
This is the most obvious consequence: recharge raises the water table. Standing water levels in shallow bores can rise noticeably and alter flow directions and gradients across the site. A bore that sat upgradient of a source in summer may not be upgradient in winter, so level data from a dry-season round should not be used to interpret wet-season quality results.
Mobilised contaminants
When the water table rises into soil that has been dry, it can pick up contaminants held in that zone. Infiltrating water also flushes soluble contaminants from the soil profile into the water table. Both effects can produce a short-term spike in concentrations in nearby bores, even where the source itself is unchanged.
Dilution
Large volumes of fresh recharge can lower concentrations of salts and dissolved contaminants. Lower results after rain can therefore reflect dilution, not improvement. Once conditions settle, concentrations may return to their previous levels or higher.
Changes to field parameters
Electrical conductivity, pH, dissolved oxygen, redox potential and turbidity all respond to recharge. Fresh, oxygenated rainwater can change redox conditions in a shallow aquifer, which in turn affects how metals and nutrients behave. Turbidity often rises in shallow bores after heavy rain, which can affect metals results unless samples are filtered appropriately.
Reading monitoring data after a wet period
Interpreting post-storm data starts with knowing when the rain fell. We recommend reviewing each data set against these questions:
- How much rain fell in the weeks before sampling and how does that compare with the same period in previous rounds?
- Did standing water levels rise and has the inferred flow direction changed?
- Is the shift consistent across bores, which suggests a site-wide recharge effect, or limited to bores near a known source, which suggests mobilisation?
- Do field parameters (EC, pH, dissolved oxygen, redox) show a fresh-recharge signature?
- Is the new value within the range recorded at background reference wells?
For streamlined data analysis, pressure transducer loggers record water level at set intervals, often hourly, so you can see exactly when and how far each bore responded to a rainfall event. Matched against rainfall records, they separate rainfall effects from pumping and seasonal trends, which a monthly or quarterly dipper reading cannot do.
It’s also important to conduct baseline monitoring that covers at least one full hydrological year, including the wet season. That gives you the reference point to judge future results or reports and assess whether a post-rain result is unusual or just part of the site’s normal seasonal range.
When to sample again after heavy rain
Your results may be out of date if any of the following apply:
- The most recent sampling round was collected in the dry season and there has since been a significant rainfall event or a wet winter
- Standing water levels have risen enough to change flow directions or bring the water table into contact with known contamination
- A previous round showed a spike or a drop that has not been confirmed by a follow-up sample
- Your licence or management plan sets trigger values and you need to show whether a change is either a seasonal effect or a genuine exceedance
- You are relying on pre-rain data for a decision about remediation, dewatering, a closure report or a regulatory submission.
In these cases, an additional sampling round timed after recharge has peaked, or event-based sampling soon after heavy rain, gives a more accurate picture. Our guide on when to repeat groundwater monitoring covers the wider set of triggers.
Good water runoff management protects your groundwater
Stormwater controls on the surface still matter, because they decide where infiltration happens and how much polluted runoff reaches the soil. Keeping clean and contaminated stormwater runoff separate reduces the load that can leach to groundwater.
Infiltration basins and swales should be sited away from known contamination, because they concentrate recharge in one place. Altered site drainage is one of the early warning signs of groundwater contamination worth checking against your monitoring results.
Our approach to wet-season groundwater monitoring
We design and run groundwater monitoring programs across Western Australia for mining, construction, industrial, government and contaminated land projects. For sites where rainfall affects results, our systems usually involve:
- Pressure transducer loggers with barometric compensation, so every rainfall response is recorded between site visits
- Manual level measurements and field parameter readings (pH, EC, temperature, dissolved oxygen, redox and turbidity)
- Sampling timed around the wet season and after significant rainfall events, rather than on a fixed calendar
- Duplicate samples, field blanks and chain-of-custody records, with analysis by NATA-accredited laboratories
- Reports that interpret results against rainfall, water levels and background wells, ready-made for DWER licence reporting and management plans.
Arrange a consultation today
We mobilise quickly across metro and regional WA, so a follow-up round can be in the field soon after a major rainfall event.
If your groundwater data pre-dates this year’s wet season, contact us to arrange an updated monitoring round.