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4 Parameter Descriptions and Input Values

4.1 Overview

This section describes each parameter and provides guidance on the value to assign to each parameter (without being overly prescriptive). The guidance values are not intended to be absolute. Within the range of values given you can apply any value you consider appropriate, since there may be particular situations that you consider are not reflected by the suggested parameter values.

In such situations, you can assign any valid value (> 0,  1, in 0.1 increments) you choose, but you will need to note the reasons for any values departing from the recommended values given in this guide, and on the Risk Screening System (RSS) template, as a comment against the value on the template. We recommend a cautious approach to deviating too far from the standard values. Note also that the guidance values indicate whether a parameter risk is considered to be linear from best to worst, or whether the risk increase (or decrease) is skewed (is weighted towards best or worst).

For each exposure pathway (surface water, groundwater and direct contact) the template requires input for each of the components of the risk equation - the hazard, the pathway and the receptor. The first two components consist of several parameters, while the receptor component comprises a single parameter.

  • Hazard component - the parameters include toxicity, extent/quantity, and mobility. A greater extent or larger quantity, a more toxic substance or a more mobile contaminant presents a greater hazard than small quantities, low toxicities, or immobile contamination.
  • Pathway component - the parameters are intended to represent the probability and extent to which the pathway between the hazard and receptor will be completed. The pathway parameters affect whether a receptor will come in direct contact with an immobile contaminant or whether a potentially mobile contaminant is likely to be transported to a receptor. Typical parameters are the distance between the hazard and receptor, and factors affecting transport such as potential for run-off and ground permeability.

The parameters are described in more detail below. In all cases, values are assigned to the parameters - whether for the hazard, the pathway or the receptor - for the current condition of the site, unless the assessment is being used to predict the risk for some future changed condition.

4.2 Hazard component

The hazard component has three parameters, each of which must be considered relative to the receptor pathway being considered:

  • the toxicity of the contaminant
  • the extent or quantity of the contaminant
  • the mobility of the contaminant along a pathway when released into the environment.

These parameters are purely a measure of the hazard potential - not a measure of whether the hazard potential is realised as a risk. The potential for realising the risk is dealt with by the pathway and receptor parameters.

4.2.1 Toxicity

The toxicity parameter is a measure of the ability of the contaminants to cause adverse human health and environmental effects. This document does not present a definitive list of relative toxicities of various substances. If you are using the screening system you should have sufficient training and experience in hazardous substances to know the relative toxicities of common substances. A list of common types of contaminants is presented below for guidance on assigning appropriate values, and a list of some of the more commonly encountered substances is provided in Contaminated Land Management Guideline Schedule B. The most up-to-date version of the Schedule can be found at the Ministry's website, A number of Internet resources are also listed in 'Additional Information'.

In judging relative toxicities it is useful to compare the World Health Organisation (WHO) acceptable or tolerable daily intake (ADI, TDI), or equivalent values such as the United States Environmental Protection Agency's (US EPA) reference dose (RfD) or the Agency for Toxic Substances and Disease Registry (ATSDR) chronic minimum reference level (MRL). The WHO's ADIs are used in deriving the Drinking Water Standards for New Zealand, with many ADI values given in the Ministry of Health's drinking water quality management guideline document (Ministry of Health, 1995).

ADIs (TDIs or RfDs and MRLs) for threshold contaminants are given in terms of the amount of substance per kilogram of body weight per day (mg/ kg bw/day) that will not cause an observable health effect in a sensitive individual. For the purposes of the RSS, a high-concern substance is defined as one which has an ADI of  0.02 mg/kg bw/day, a medium concern substance has an ADI that falls between 0.02 and 0.2 mg/kg bw/day and a low concern substance has an ADI > 0.2 mg/kg bw/day. ADIs can be used to calculate guideline values for soil contaminants assuming particular exposure scenarios. Using the assumptions provided in the Timber Treatment Guidelines (Ministry for the Environment and Ministry of Health, 1997) for residential scenarios, a high concern substance is one which that creates a potential risk at concentrations less than approximately 3,000 mg/kg for exposure via soil ingestion. This is an indicative concentration and does not include other pathways of exposure (eg, produce consumption, dermal absorption), which are likely to result in a potential risk occurring at lower concentrations.

Parameter range:

0.2 Low-concern contaminants

ADI > 0.2 mg/kg/day

0.6 Medium-concern contaminants

ADI  0.2 mg/kg/day

1 High-concern contaminants

ADI  0.02 mg/kg/day

Carcinogenic substances (non-threshold contaminants) cannot be treated in this way. If a substance has been determined as a potential human carcinogen then it should be considered a high-concern contaminant.

A generic list of high-, medium- and low-concern contaminants is given below and a more detailed list for some specific hazardous substances, compiled using ADI values, is given in Appendix C. Many substances are more toxic through the inhalation pathway than the oral pathway, and you should consider this when assessing toxicities. Also, some substances are far more toxic to plant life or the aquatic environment than they are to humans. Therefore human toxicity should be used with caution when considering risks to the wider environment.

High-concern waste/substance types are:

  • materials that are persistent, bio-accumulative and toxic
  • heavy metals (eg, mercury, arsenic, lead)
  • industrial waste (eg, pesticides, herbicides, paint sludge, acid and alkaline solutions, petroleum hydrocarbons)
  • institutional waste (eg, laboratories, hospitals)
  • pathological waste
  • radioactive waste.

Medium-concern waste/substance types are:

  • liquid waste not covered above, including non-volatile hydrocarbons (eg, heavy oils), septic tank pumpings, agricultural and chemical containers
  • food-processing wastes
  • non-hazardous incinerator or boiler residues (eg, ash)
  • municipal solid wastes (domestic)
  • organic and vegetable wastes
  • mining residues.

Low concern waste/substance types are:

  • industrial and commercial solid wastes (eg, construction materials such as wood, metal, sand/silt piles, foundry sands).

4.2.2 Extent/quantity

The extent/quantity parameter is a measure of the amount of the potentially hazardous substances on the site being assessed at the time of the assessment. This must be treated independently of the toxicity, but the extent/quantity combined with the toxicity gives a measure of the hazard at the source. Thus, the combination of a small quantity of a highly toxic material may present a similar hazard to a large quantity of a substance with a lower toxicity.

Parameter range:

0.4 Small quantity or proportion of site affected

0.7 Medium quantity or proportion of site affected

1 Large quantity or proportion of site affected

Quantity/extent is distinct from the actual quantities of a hazardous substance in the material that may have been released or deposited. Also, the quantity of hazardous material is distinct from the potential for escape, which is covered by the storage conditions (containment) of the hazard (see section 4.3.1).

It is difficult to entirely separate the extent/quantity from the site use for the direct contact pathway. You will need to consider the proportion of the site affected when determining the value for this parameter. Clearly, a small quantity of affected soil on a relatively small site (where occupancy is likely to be more intense) is more significant for direct contact than the same quantity on a much larger site.

Particular difficulties can be encountered for residential sites where small quantities can present a high risk if readily accessible to children (eg, lead paint flakes in garden soil). In that case, it is reasonable to assign a high value to extent/quantity. Another example is a small sheep dip site with a few cubic metres of affected soil, which may not present a particular risk on a farm but could present a significant risk if there are nearby residential properties from which children might gain access. A substantial risk would be presented if the same sheep dip ended up on a residential property following subdivision of the farm.

Historical contamination results in particular difficulties for assigning a value to this parameter, because past activities, storage conditions and management practices will affect the likelihood of the hazardous substances getting into the ground, and therefore will affect the current extent/quantity estimate.

For simple sites it is possible to assign a value to extent/quantity based on the quantity of the substance historically stored, and then deal with the likelihood of the substance being in the ground using the containment pathway parameter (see the discussion in the note at the end of section 4.3.1). However, for complex historical sites this simplified approach cannot be used and the historical containment conditions must be factored into the extent/quantity parameter. The containment parameter would then be assigned a value of 1 to indicate the substance is already in the ground.

Thus, for historical sites the extent/quantity parameter must consider:

  • the activities and management practices that were typical of the time - poor storage and disposal practices will mean ground effects are more likely, and more severe
  • the period of operation, which will affect the extent of effects if practices were not satisfactory
  • whether a number of different activities might have resulted in the accumulation of the same or similar substance(s)
  • the degree of natural attenuation of the substance that might have occurred since the historical activity or land use ceased - this is particularly relevant for substances that are volatile, soluble (where leaching is possible) or readily biodegradable
  • the amount of soil removal that might have occurred if there had been any ground modification - this is relevant for sites that may have undergone redevelopment.

The following quantities of liquids currently or historically stored or used and contaminated soil are suggested as a rough guide:

  • small quantities - tens to hundreds of litres or tens of cubic metres of soil; or, for the direct contact pathway, less than 10% of the site affected (noting that a residential site with a small quantity of readily accessible soil may present a high risk)
  • medium quantities - hundreds to thousands of litres or hundreds of cubic metres of soil; or, for the direct contact pathway, 10-50% of the site affected
  • large quantities - thousands of litres or hundreds to thousands of cubic metres of soil; or, for the direct contact pathway, greater than 50% of the site affected.

4.2.3 Mobility

The mobility parameter assesses the ability of the hazardous substance to migrate or be transported along a pathway once released from containment (eg, a measure of properties such as volatility, water solubility, likelihood of partitioning to soil or water (log Koc and log Kow values). Mobility is distinct from the storage conditions (containment) of the hazard (see section 4.3.1), which is a site factor rather than a property of the substance.

Parameter range:

0.3 Low mobility for the pathway

0.7 Medium mobility for the pathway

1 High mobility for the pathway

The mobility parameter is affected by the pathway. For example, a compound that migrates rapidly towards an aqueous receptor is also migrating away from a point where direct contact may be made. A substance that is immobilised on the surface, and therefore unlikely to affect groundwater, is available for direct contact. In addition, migration of the substance may not just be as the pure substance (whether as a gas or liquid), but also transported by another medium (eg, dissolved in water or attached to sediment or dust).

A substance would normally be considered highly mobile if it is:

  • a liquid, particularly of low viscosity, facilitating migration through the ground
  • a gas or a volatile liquid, facilitating migration through the ground in vapour phase
  • soluble, facilitating leaching and/or transport by surface or groundwater
  • conservative in solution (doesn't tend to partition to soil or degrade), facilitating transport in surface or groundwater.

The converse would apply for substances with the opposite properties. However, there are two exceptions where normally immobile substances with a tendency to partition to soil (eg, many metals and semi-volatile organic compounds) should be assigned high mobility so as not to spuriously reduce the risk:

  • where adsorption to dust or sediment is likely, and the dust or sediment is readily available for transport to a receptor by wind or run-off (see section 4.3.2)
  • for the direct contact pathway, where partitioning to soil ensures the substance remains on the surface available for contact.

There are also exceptions where highly mobile organic substances may in fact present a low risk for historical contamination, because the properties that suggest greater mobility can also increase susceptibility to more rapid attenuation. This must be factored into the quantity/extent parameter, as mentioned previously. Thus highly mobile substances may also:

  • attenuate rapidly because of their high volatility
  • degrade rapidly because their high solubility makes them available for bio-degradation.

As a result, some care and experience is required in assigning values to this parameter. As a guide, light and volatile organic compounds are typically more mobile (soluble) than heavier organic compounds. Semi-volatile organic compounds (eg, chlorinated pesticides and polyaromatic hydrocarbons) tend to have both a low solubility and a strong tendency to partition to soil. Many heavy metals and metalloids (eg, lead, copper, arsenic) have a low mobility because of their affinity to adsorb to soil, but this can be species- and soil pH-dependent.

Consult chemical references or Internet databases if necessary. A list of useful Internet sites is presented at the back in 'Additional Information'.

4.3 Pathway component

The pathway component defines the likelihood of contact with, or transport to, a receptor. The pathway to consider is normally the current pathway, not the pathway when some historical contamination may have occurred, because it is the current risk based on current site conditions that is being assessed (but see note under containment below). Historical site conditions or layout cannot affect whether a contaminant is now likely to come into contact with a receptor. Any history (including past pathways that might have facilitated spread of the contaminant) is factored into the hazard parameter of extent/quantity (see section 4.2.2). However, if predictions of future or past risk are required, then the pathway conditions applying at the time should be used.

The pathway component and associated parameters are functions of the site and surroundings, not of the hazardous substance itself. One parameter is common to each of the three exposure pathways considered: the containment parameter, which defines the security of the contaminant containment at the site. Otherwise, each of the exposure pathways has a different set of parameters, defining barriers to transport or contact.

There are either two or three pathway component parameters, depending on the pathway being considered. A consequence of the multiplicative nature of the assessment is that, for a given numerical value, three parameters will result in a lower apparent risk than two values. To avoid this bias, the recommended values have been adjusted so that, for example, applying medium values across all parameters produces a similar result for each exposure pathway when all the values are multiplied together. If you decide the suggested values are not appropriate, you will need to consider the potential for multiplicative bias between pathways when choosing alternative parameter values.

4.3.1 Containment

The contaminant parameter is an indicator of the current potential for a stored hazard to be released into the environment (see the note below for a discussion on historical storage). This parameter is intended to apply to engineered structures and does not include natural ground conditions providing containment, although it could include engineered soil linings. Containment provided by low ground permeability is considered as part of other parameters and should not be considered here.

Parameter range:

0.2 Fully contained

0.4 Fully contained, but with the potential for the containment to be compromised

0.7 Medium containment

1 No containment, or contaminant already present in the environment

Where the hazard is shown to be present in the environment (eg, from knowledge of a spill or leak into the ground, from knowledge of deliberate disposal, or from observations or measurements of contamination), the site is considered to have no containment. Conversely, evidence of a double-skinned fuel storage container in a bunded compound would suggest a small potential for escape, and therefore the contents can be considered to be fully contained. Single-skinned underground storage tanks made of steel or poorly bunded storage areas might be considered to have the potential for the containment to be compromised.

Note on storage and containment

For historical sites where it is known that hazardous substances were stored in some form of engineered structure (eg, storage tanks, dangerous goods stores, warehouses), it is difficult to separate out the conditions of storage (the containment parameter) from the extent/quantity parameter in the hazard component. There are two ways of determining the probability of that substance now being in the ground and available for contact with, or transport to, a receptor.


  • factor the past containment conditions in the estimate of extent/quantity and then assume the contaminant exists in the ground by assigning the containment parameter a value of 1; or
  • assign a value to the extent/quantity parameter based on the amount of historical storage of the substance and then determine the probability of its being in the ground by using the containment parameter for the historical storage conditions.

In the first method, the historical containment is implicitly factored into the assessment of the quantity of affected ground that may now exist. The second method is rather more transparent, and attempts to separately account for the quantity stored and the historical storage conditions. Both methods are appropriate for simple sites with a single period of historical storage, and should give the same answer. However, the first method should be used where contamination is known to exist in the ground and for complex sites where multiple periods of use must be accounted for. Multiple uses will most likely have multiple historical containment conditions that must be considered in arriving at estimated amounts of hazardous substances that may now be in the ground.

4.3.2 Direct or sediment run-off and flood potential (surface water)

The remaining surface-water pathway parameter assesses the risk to a surface water receptor from:

  • direct run-off of a leak or spill
  • contaminated sediment transported by storm water run-off
  • the surface waterway flooding the site.

Parameter range:

0.2 Low potential for run-off or flood

0.6 Medium potential for run-off or flood

1 Preferential path, or water body within tens of metres

This is a hybrid parameter that must take into account a number of site and topographical considerations. The choice of value will depend on a combination of surface cover on the site and between the site and waterway, topography (slope and a viable drainage path) and distance to the waterway. Run-off potential will be relatively higher for steep, unvegetated or impermeable slopes with a nearby surface water body, than for flat slopes, vegetated or permeable ground and/or a distant water body. A water body running through the site, or within a few tens of metres, would generally suggest a high potential for effects on that water body, whereas one more than a few tens of metres and up to a hundred metres away would suggest a medium potential. Distances greater than 100 metres would suggest a low potential for run-off effects unless there are factors that make run-off more likely, such as impermeable surfaces, steep slopes or preferential pathways.

Where there is the potential for preferential migration (eg, through service trenches or stormwater culverts in an urban setting, or surface drains in a rural setting), the run-off potential should be modified to compensate for the more direct flow paths. For example, in the extreme case, run-off going into a stormwater drain that discharges directly to a surface water body would be assigned a value of 1, but if there was a potential for dilution on the way then a lower value might be assigned.

Estimates of slope, distance and whether there is a viable drainage path may be obtained from 1:50,000 topographical maps, smaller-scale contour mapping (try district or city councils), estimates from city or district council service plans, or by site inspection.

Flood potential will depend on the topography and distance - whether a flood is likely to reach the site - and whether there is any protective cover on the site. Flood hazard analysis has been carried out for many rivers and may be available in the form of local authority flood hazard maps or by consulting regional council hydrologists. Where flood hazard maps are not available, an assessment of height above the nearest waterway, distance from the waterway and anecdotal evidence of floods will need to be used to estimate the potential for flooding at the site.

4.3.3 Groundwater pathway

For there to be a risk to a groundwater pathway receptor, the contaminant must migrate to an aquifer and travel through the aquifer to the point of contact. This has been assessed by including parameters for:

  • the thickness of any low-permeability (silt, or clay equivalent) protective layer overlying aquifers of concern
  • the distance to the nearest user.

(a) Thickness of protective layer

The thickness of overlying low-permeability layers, including surface paving where relevant, is a measure of the ability of contaminants to migrate down to an aquifer. The top of any low-permeable layers must be taken as the lowest point at which a hazard is released. A good-quality pavement in the case of a surface release can be considered to be the equivalent of greater than 15 m of low-permeability material overlying an aquifer.

Parameter range:

0.4 > 15 m of low-permeability material overlying aquifer

0.7 5 m of low-permeability material overlying aquifer

1 Unconfined

(b) Distance to user/aquifer type

The distance to user/aquifer type parameter is used to assess the ability of a hazardous substance to migrate through an aquifer to a point of use. 'Use' is broadly defined as a point where a receptor could come in contact with the contaminant, and includes a point of abstraction (a well) or a point of discharge to the surface (a spring) or surface water body.

Parameter range

Parameter value Distance to receptor for aquifer type (typical permeability)
Clay, silt (low) Fine sand, silty gravel (low-moderate) Coarse sand, sandy gravel (moderate) Gravel (high) Fractured rock (moderate)

0.3 (low risk)

100 m

300 m

1000 m

2000 m

1500 m

0.6 (medium risk)

50 m

100 m

500 m

1000 m

800 m

1 (high risk)

< 20 m

< 50 m

< 300 m

800 m

600 m

The parameter is a measure of risk for different distances to the point of use as a function of broad aquifer types used to define permeability ranges (ranging from 10-8 m/s or lower for silts and clays, to 10-2 m/s or higher for gravels; see Freeze and Cherry, 1979, Table 2.2, or similar groundwater text). The greater the distance to the point of use for a particular aquifer type, the lower the risk value. Mobility of the substance is accounted for separately in the mobility parameter. Where the user is aware of preferential pathways (eg, gravel lenses over short distances within an otherwise low-permeability sediment), judgement must be exercised as to what average aquifer type should apply over the distance between the contaminant source and the particular groundwater use location. Where the bulk properties of the underlying aquifer are known (from pump tests), then the most suitable aquifer type should be used.

Five aquifer types have been selected to represent aquifers typically encountered in New Zealand. Distances for low-, medium- and high-risk values (values of 0.3, 0.6 and 1, respectively) have been determined from a combination of experience and 1-D dispersion calculations for a conservative contaminant over a period of 10 years. Degradation of the contaminant (as distinct from retardation due to adsorption, which is accounted for in the mobility parameter) has not been considered. Where degradation is likely to occur (eg, for biodegradable and volatile substances), the travel distances for each risk value should be adjusted down accordingly, meaning that for a given distance to a point of use, a substance that does not degrade will result in a higher risk of exposure than a substance that does degrade.

The fractured-rock aquifer type is intended to represent aquifers similar to the Auckland basalts, and perhaps closely jointed greywacke. Fractured rock is inherently variable and the values given must be used with caution. There is no attempt to represent cavernous limestone, where special consideration must be given. If you have any doubt on what aquifer type to select, consult a hydrogeologist or the relevant regional council.

Where one of the standard aquifer types does not fit the particular situation, there is provision in the electronic template to select 'Other' and then assign whatever value between 0 and 1 that is appropriate to the situation. Make sure you provide comments justifying the selected value.

4.3.4 Direct contact pathway

The direct contact exposure pathways considered are dermal and inhalation mechanisms. The inhalation pathway includes exposure to both volatile substances (eg, hydrocarbons) and particulate matter (eg, contaminated dust and asbestos). The two direct contact mechanisms are independent, although in both cases the likelihood of a complete pathway is dependent on whether there are barriers to the pathway (eg, the pathway length, surface cover or ground permeability). The pathway score reports the mechanism with the greatest assessed risk. The direct contact pathway is influenced by:

  • the depth to the hazard, and either:
  • the surface cover of the ground surface, or
  • the permeability of the soil, in the event of a volatile hazard.

(a) Depth to hazard

The depth-to-hazard parameter is used to assess the risk to direct contact receptors from a sub-surface hazard. The hazard may have been released underground (eg, via storage tanks), may have been buried (eg, in landfills) or may have migrated through permeable materials to the water table. The risk presented by the hazard will lessen as the depth to the release point increases.

Parameter range:

0.5 Greater than 4 m below the ground surface

0.8 1-4 m below the ground surface

1 Within 1 m of the ground surface

(b) Surface cover

The surface cover parameter assesses the risk to human health from direct dermal contact with the hazard. This risk reduces with the increase in effectiveness of the ground surface cover. Effective cover includes paving and adequate earth-cover material over the affected ground (eg, cover over landfills), such that contact cannot reasonably occur during normal site use. For semi-volatile contaminants, a thick, well-maintained grass cover provides some barrier to contact relative to bare earth. The likelihood of excavation or other disturbance to the soil, with subsequent soil contact, should be considered for the particular site use.

Parameter range:

0.3 No access, or paved

0.8 Limited access or paving

1 No restraint to access

(c) Soil permeability

The soil permeability parameter assesses the risk relating to the inhalation of a sub-surface volatile hazard. The presence of low-permeability ground may reduce this risk.

Parameter range:

0.3 Low-permeability soils (eg, clay)

0.8 Medium-permeability soils

1 High-permeability soils (eg, silty sand soils, gravel)

4.4 Receptor component

The risk to receptors is dependent on contact with contaminated material, whether soil or water. This may depend on the type of site use, in the case of the direct contact pathway, or the likelihood of a person or ecological receptor coming into contact with, or using, contaminated water.

These parameters assess only the physical criteria relating to a site. There may be other concerns that affect the perceived site ranking, such as cultural values, but these cannot be considered in a generic fashion as part of the Risk Screening System. Where this occurs, you will need to use your best judgement and factor the value accordingly. In such cases it is imperative to provide a comment to justify the selected value.

4.4.1 Water use

The water use parameter applies to both groundwater and surface water receptors, where appropriate. The risk may be to human health through use of water as drinking water or for recreation, to crops or stock in an agricultural setting, or to ecological values where ground- or surface water discharges to a significant waterway.

Parameter range:

0.2 Not used/industrial

0.7 Irrigation

0.7 Stockwater

0.7 Ecologically significant waterway

1 Contact recreation

1 Domestic/potable

Where the discharge of groundwater to surface water is being considered, dilution in the receiving waterway must be taken into account. In general, the risk to all but the smallest of waterways from groundwater discharge will be low (see section 4.5).

A value for industrial water use has also been given. A low risk value has been assigned to this use, because concerns regarding industrial water use are generally not health-based.

4.4.2 Land use

The land-use parameter defines the risk to receptors for the direct contact pathway in proportion to a number of exposure factors that are unique to the receptor's environment. These include the exposure frequency (days per year) and exposure rate (rate at which hazard is ingested, inhaled or contacted). Land-use scenarios and relevant exposure rates broadly match those used in health-based soil guideline documents developed for timber treatment (Ministry for the Environment and Ministry of Health, 1997), petroleum hydrocarbon (Ministry for the Environment, 1999) and gasworks (Ministry for the Environment, 1997) sites.

Parameter range:

0.2 Parks, recreation

0.4 Maintenance work

0.5 Industrial/commercial

0.5 Secondary schools and higher educational establishments

1 Agricultural

1 Pre- and primary schools

1 Residential

The values recognise, for example, the relatively higher exposure to soil a residential receptor has relative to a receptor on an industrial site. A residential occupant is considered to spend more time at home and be more likely to have contact with bare soil than a worker at an industrial site. Preschools and primary schools are given a high value because, although the receptors spend only a small part of the day at the site, there is the likelihood with young children of increased contact with the hazard (eg, through direct soil ingestion).

4.5 Pathway interaction

Some interaction may occur between different pathways, such as groundwater in hydraulic contact with surface water presenting a risk to the surface water, or groundwater seeping to the surface and presenting a direct contact risk. There is no simple way of presenting these scenarios on the template without jumping from one pathway to another, which has the potential to create confusion. In such cases, therefore, the risk via one pathway should be assessed separately, off the template, on the basis of the contribution by the other pathway, and the input parameters modified on the template accordingly. A note should be made to this effect on the template.

For example, in the case of groundwater discharging to surface water, the surface water receptor (whether water use, ecological, stockwater, etc.) would in effect become the water use value of the groundwater pathway. The distance to the receptor and ground permeability as they affect the likelihood of reaching the receptor are already included in the groundwater pathway. But, as discussed in section 4.4.1, effects of dilution in the surface waterway and whether there is likely to be any surface water receptor (given that different receptors will have a different tolerance to contaminants, as shown by the suggested values for the surface-water water use parameter) must be considered separately.

A judgement must then be made as to what value to give the groundwater water use parameter. For a remote surface waterway with a large flow, the value would be small - probably 0.1. However, for a small but ecologically significant waterway receiving seepage through gravels from an immediately adjacent site, the value could be higher, perhaps 0.7, similar to that given for the surface-water water use parameter for a significant waterway.