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Groundwater Site Risk Assessment Tool

The factors used in the DRASTIC vulnerability mapping system have been adapted and extended to provide a more relevant system to determine the risk of the transport of contaminants through the sub-soils and underlying groundwater aquifers. The factors are then combined in a unique way which bears minimal relationship with DRASTIC for instance, instead of being added after weighting, the factors in the new model are multiplied together to reflect the more dramatic effects that specific circumstances such as preferential flow paths caused by construction activities, drains etc - can have on the potential for transport of contaminants. Additionally, added factors cannot allow a null result, when for instance, the aquifer at risk is confined under the site and so there is no risk. Multiplied factors allow very small or zero results.

This form allows the user to enter a variety of site conditions or parameters and then calculates the likely risk of groundwater contamination for the contaminant in question.

To calculate the likely risks to YOUR site, fill in each of the parameters or questions and then hit the calculate button on the bottom of the page to determine if your site has a high or low risk of contamination of groundwater.  

The "Rationale" statements highlighted in blue indicate the nature of any simplification or importance of that particular factor.

Site name or identifier:     

1. General

1.1 Annual rainfall at the site:

  • Heavy (1+ metres per year)
  • Medium (0.5 - 1 metre per year)
  • Low ( < 0.5 metre per year)  


If the rainfall is high then there is more likelihood of the contaminant reaching the surface waters or the groundwater.

1.2 Does stormwater and other surface water discharge directly into an open drain, stream, river or seawater?

  • Yes
  • No
  • Rationale

    This is a significant factor - if the contaminant is directly discharged into the receiving environment then there is obviously a high chance of contamination.

    1.3 Special factors 

    Is there any significant factor other than those associated with the properties of the subsoils and aquifer materials and properties which would enhance the mobility of the contaminant(s) in the subsoil and aquifer. Such factors are highly acidic, heavily flushed with water or detergent, long duration spillage or a long time since large volumes of contaminated fluid were deposited on or in the ground.  This list may not be exhaustive.

  • Yes
  • No
  • Rationale

    This factor can be especially important as the modification to chemical conditions, or the time or volume will greatly increase the risk of movement of the contaminant.

    1.4 Quantity - Is the estimated spillage on the site ....

  • Small ( less than 1 m3 = 1000 L)
  • Medium (Between 1 and 5 m3)
  • Large (greater than 5 m3)
  • Unknown
  • Rationale

    This factor is used only to calculate the volume of soil likely to be impregnated with the contaminant liquid.

    1.5 Area - what is the extent of the spill on the site ...

  • Small (less than 1 m2)
  • Medium (Between 1 and 4 m2)
  • Large (greater than 4 m2)
  • Can't tell
  • Rationale

    This factor is used only to calculate the volume of soil likely to be impregnated with the contaminant liquid.

    1.6 Substance of concern:                

    2. Site Characteristics 

    2.1 Minumum seasonal depth to water table

  • Deep (greater than 10 m)
  • Intermediate (Between 2 and 10 m)
  • Shallow (less than 2 m)
  • Confined aquifer only
  • Water table is above ground
  • Don't know
  • Rationale

    Our analysis has shown that this is the most significant factor in determining if a contaminant will reach the groundwater in the topmost aquifer.  Unless the spillage has been very recent and there is no reason to expect any significant change in the water table level, enter minimum seasonal depth.

    2.2 Soil media physical integrity 

    2.2.1 Is the dominant material between the surface and the top of the water table?

  • Very impermeable/tight?
  • Permeable?
  • Very permeable?
  • Rationale

    If the rainfall is high then there is more likelihood of the contaminant reaching the surface waters or the groundwater.  


    2.2.2 Are there obvious or any known preferential flow paths in the soils above the water table?

  • Yes
  • No
  • Don't know
  • Rationale

    These preferential flow paths do not include constructed drains unless they are to sumps which discharge directly to groundwater aquifers.  Preferential flow paths could be infilled trenches around services, or foundations, or tree roots.  Buried tanks provide such flowpaths if they are sited within the groundwater table.

    2.3 Aquifer media, hydraulic properties 

    2.3.1 Is the dominant material in the uppermost underlying aquifer

  • Rather impermeable? e.g. silts and mudstones
  • Permeable? e.g. sands and gravels
  • Very permeable? e.g. fractured basalt, stones and boulders
  • Extremely permeable? e.g. limestone channels
  • Rationale

    A simplification has taken place here to relate permeability not only to flow rate but to tortuosity, which will maximise the opportunity for absorption, precipitation or degradation of the contaminating species - and the converse, where channels will minimise opportunities for such processes.  Dilution processes are also relevant but are accounted for in the "distance factor" (3.1).

    2.3.2 Are there any obvious or known preferential flow paths in the aquifer through trenches, permeable faults, wells, foundations, excavations, which are likely to provide continuous or near continuous flowpaths to a receiving surface water ecosystem?

    • Yes
    • No
    • Don't know


    This is a critical factor, as such pathways can allow rapid transport of contaminants through what might otherwise be a tight aquifer. 

    2.4 Impact of the vadose zone

    Which of the soil-types listed here best or most nearly describes the chemical absorption properties of the subsoil in the vadose zone under the site? Keep in mind that generally chemical absorption by a soil is most affected by particle size, organic matter and clay content. Choose the most absorbent material present which is > 10% of the total. 

    Soil type:            

    2.5 Aquifer materials

    Which soil type would approximate the chemical absorption properties of the aquifer materials between the site and any surface receiving water?  The chemical absorption by a material is most affected by particle size and clay content.  If the materials are variable, choose the one most likely to be the finest-grained or the one with the highest organic or clay content.

  • Fractured or channel eroded rock
  • Contains fine-grained sands and silts
  • Contains clays and weathered rock
  • Comprises fresh volcanic ash
  • Has high organic content
  • Rationale

    Much the same as for 2.3.1.

    3. Other factors

    3.1 Distance to potential receiving waters or springs:

  • less than 10 m
  • Between 10 and 30 m
  • Between 30 and 100 m
  • greater than 100m
  • Rationale

    As well as the opportunity that distance provides for the absorption of the contaminant, this factor takes into account the dilution/dispersion potential within the aquifer, which, in typical high velocity New Zealand aquifers, is very relevant.  If the aquifer is tight and flow rates are slow, choose the next higher distance range.

    3.2 Sensitivity of receiving waters

    This relates to the sensitivity of the surface receiving water - and relates to both the dilution potential and the ecological significance.  If there is likely to be more than one surface receiving water, re-run the model for each to assess the risk for each.

  • Receiving water has large volume flow
  • Receiving water is an open sea shore
  • Receiving water sometimes has limited flow
  • Receiving water is a small pool
  • Receiving water is a large pool/lake
  • Receiving water is a wetland
  • Receiving water has special ecological significance
  • There is no apparent surface receiving water
  • Receiving water is an intermittent stream or drainage ditch
  • Rationale

    Give that the protection of sensitive ecosystems is the intention of the routine, this factor is of prime importance.


    To run the risk model, press   

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    Page last updated: 01 May 2007

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