Reading groundwater bore logs: 7 common mistakes

The most common mistake in reading a groundwater bore log is treating its standing water level as the design groundwater level. It is one reading, at one depth, on one day. In south-west Western Australia, about 230 km of main roads were damaged when groundwater rose higher, season to season or over years, than their design allowed for (McRobert and Foley 1999, cited by Boronina et al. 2015). Bore logs are easy to misread because the numbers look precise and the context that gives them meaning sits elsewhere on the page.

For where to find the records in each state, start with how to find and read bore logs and groundwater records.

1. Treating one standing water level as the design level

The printed level reflects whatever weather came before it. Boronina et al. (2015) set out how to derive a design groundwater level for the Gateway WA freight road project around Perth Airport. The longest-running government bore nearby had records back to 1952, yet most state monitoring bores are now dipped by hand only twice a year, aimed at the May low and the October high, so the true peak can fall between visits and never be recorded. Their method starts from a measured seasonal peak and adjusts it to the design year, so it needs at least one wet-season peak on record, and more is better.

What to do instead: treat any single reading as indicative, look for bores with repeat measurements, and where the level matters for footings, basements or dewatering, monitor through at least one wet season.

2. Confusing a perched water table with the real one

Water struck at 2 metres in weathered shale is often perched: an isolated saturated zone sitting above the regional water table. McNally (2004) described the shale country of Western Sydney as a stack of scattered, poorly connected perched water bodies, and doubted that a single continuous water table exists there at all.

What to do instead: check the geology. In the Wianamatta Group shales and similar low-permeability units, expect perched water and patchy saturation rather than a clean water table.

3. Assuming a dry log means a dry site

McNally (2004) noted that boreholes and piezometers (monitoring pipes that measure water pressure at a set depth) in Western Sydney shale can look dry when first drilled and then fill slowly over several weeks, because the fractures feeding them are tight. Griffioen and McDowell (2006) noted anecdotal evidence of the same in Launceston: shallow bores ending in clay at 5 to 6 m, dry on the day, making water about a week later. The rig leaves the day the hole is drilled, so the log says dry.

What to do instead: note the drilling method and whether the hole was left open long enough to recover. In tight clays and shales, a dry reading on the day means little.

4. Ignoring the drilling fluid

A water strike recorded during drilling only means something if you know how the hole was drilled. In Griffioen and McDowell's (2006) Launceston study, water came in during drilling at 13 of 24 holes. One hole recorded none because it was cored using drilling fluid, which masks a natural water strike.

What to do instead: read the method column alongside the water observations. Holes drilled without fluid, such as auger holes, are more likely to show inflows; fluid-drilled holes hide them.

5. Reading different piezometer levels as multiple water tables

Two piezometers at different depths in the same borehole can show different water levels with no perched aquifer involved. Pells and Pells (2012) pointed out that in uniform ground, piezometers at different levels register different heads simply because water is flowing up or down. It is easily misread as separate aquifers.

What to do instead: before invoking a perched system, ask whether vertical flow through uniform material would explain the difference. Confined water below a low-permeability layer can also sit higher than the water table above it, so head and depth are not the same thing.

6. Reading the level without the ground surface elevation

A depth below ground cannot be compared between bores until you know each bore's surface elevation. Griffioen and McDowell's (2006) Launceston bores ranged from about 150 to 230 metres AHD at the surface, so 6 metres below ground at one bore and 6 metres below ground at another describe quite different water surfaces. Even there, the collar levels were estimated from 5 m council contours, not surveyed. Compare raw depths across a slope and you draw a water surface that does not exist.

What to do instead: convert every reading to a reduced level in metres AHD before comparing bores, and check whether each collar elevation was surveyed or estimated.

7. Using an off-site bore without checking distance and geology

The nearest registered bore might be hundreds of metres away, in different geology, screened at a different depth. Even close bores disagree: McNally (2004) reported standing levels in Western Sydney piezometers 100 to 200 m apart differing by 1 to 3 m on the same day, with seasonal swings of 3 to 4 m. A bore in alluvium tells you little about a lot on residual clay 300 metres away, and a nearby lake is no safer a guide: Boronina et al. (2015) advise checking by measurement that it connects to the aquifer below, because low-permeability sediments can separate the two.

What to do instead: record each reference bore's distance, geology and screen depth, and weight it accordingly. Nearby bores scope the field investigation and flag whether shallow or perched water is likely; they do not replace piezometers on the lot.

Where does a desktop review of bore logs fit?

Used well, nearby bore records tell you what to expect before a rig is booked: likely depth to water, whether the geology favours perched water, and how much monitoring the site will need. Used carelessly, one number on one log becomes a design assumption that the excavation later proves wrong.

To pull the registered bores, geology and groundwater context for a site into one place, generate a desktop report for the address or see a sample report first.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria and South Australia.

Sources

• S. E. Pells and P. J. N. Pells (2012). Impacts of longwall mining and coal seam gas extraction on groundwater regimes in the Sydney Basin: Part 2 – Practical applications. Australian Geomechanics, Volume 47, Number 3 (Sep 2012). geomechanics.org.au
• A. Boronina, R. Clayton and C. Gwynne (2015). Recommended methodology for determination of design groundwater levels. Australian Geomechanics, Volume 50, Number 3 (Sep 2015). geomechanics.org.au
• Greg McNally (2004). Shale, salinity and groundwater in Western Sydney. Australian Geomechanics, Volume 39, Number 3 (Sep 2004). geomechanics.org.au
• J. W. Griffioen and B. McDowell (2006). Urban salinity scoping study for Greater Launceston area: Part 2 Investigation work. Australian Geomechanics, Volume 41, Number 1 (Mar 2006). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

← All articles