Reading groundwater bore logs: 7 common mistakes

A single standing water level on a bore log is one reading, at one depth, on one day, and treating it as the design groundwater level is the mistake that trips up most people reading these records. Bore logs are among the most useful items in a desktop review, but they are also among the easiest to misread, because the numbers look precise and the context that makes them meaningful is usually somewhere other than the number itself.

Here are the seven mistakes people make reading groundwater bore logs and standing water levels, why each goes wrong, and what to do instead. If you need the basics first, start with how to read bore logs and groundwater records.

1. Treating one standing water level as the design level

The standing water level printed on a log is the level measured at that bore, at that time, under whatever weather preceded it. Groundwater moves with season and rainfall. Boronina et al. (2015) set out a methodology for design groundwater levels precisely because a single measurement tells you little: their Perth work drew on Department of Water bores with records back to 1952, and separated seasonal minimum, seasonal maximum and historical maximum surfaces. A design level is derived from a record over time, not read off one log.

What to do instead: treat any single reading as indicative only, look for bores with repeat measurements, and where the level matters for footings, basements or dewatering, commission monitoring over a season.

2. Confusing a perched water table with the real one

Water struck at 2 metres in weathered shale is often perched, meaning it sits in an isolated saturated zone above the regional water table, not the water table itself. McNally (2004) described the shale country of Western Sydney as a stack of sporadic, poorly connected ephemeral perched systems, and questioned whether a single continuous water table exists there at all. Read one perched level as the regional level and you will over- or under-estimate everything downstream of it.

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

3. Assuming a dry log means a dry site

A bore logged as dry at the time of drilling is not proof of a dry site. McNally (2004) noted that boreholes and piezometers in Western Sydney shale can appear dry when first drilled and then fill slowly over several weeks, because the fractures feeding them are tight and recovery is slow. A rig moves off the same day the hole is drilled, so a slow-recovering hole reads dry on the log.

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

Water inflows recorded during drilling only mean something if you know how the hole was drilled. Griffioen and McDowell (2006), investigating salinity around Launceston, observed inflows at 13 of their 24 boreholes but recorded no inflow for one hole specifically because it was cored using drilling fluid, which masks natural water strikes. A mud-rotary or fluid-flush hole cannot give you a reliable water strike.

What to do instead: read the method column alongside the water observations. Auger and air-flush holes show inflows; fluid-drilled holes hide them.

5. Reading different piezometer levels as multiple water tables

Two piezometers in the same borehole at different depths can show different water levels without there being any perched aquifer at all. Pells and Pells (2012) pointed out that in uniform ground, piezometers screened at different depths register different heads simply because of vertical flow direction, which has nothing to do with perched water. People misread this as evidence of separate aquifers.

What to do instead: before invoking a perched system, ask whether a vertical gradient in 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 water level quoted as metres below ground level is only comparable between bores once you know the surface elevation of each. Griffioen and McDowell's (2006) Launceston bores ranged in surface reduced level from around 150 to 230 metres AHD, so a level 6 metres below ground at one bore and 6 metres below ground at another describe completely different water surfaces. Compare depths across a sloping site and you will draw a groundwater surface that does not exist.

What to do instead: convert every reading to a reduced level in metres AHD before comparing bores, and check each bore has a surveyed collar elevation rather than an estimated one.

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

The nearest registered bore might be hundreds of metres away, in a different geological unit, screened at a different depth. Boronina et al. (2015) verified surface-water-to-aquifer connection by measurement rather than assumption, because low-permeability sediments can decouple a nearby water body from the aquifer under a site. A bore in alluvium tells you little about a lot on residual clay 300 metres away.

What to do instead: record each reference bore's distance, geology and screen depth, and weight it accordingly. A desktop review of nearby bores scopes the field investigation and flags whether shallow or perched water is likely; it does not replace on-site piezometers for a specific lot.

Where a desktop review of bore logs fits

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

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 or Victoria.

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.

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