The site screening is carried out by completing a Risk Screening System for Contaminated Sites Template (see Appendix A), which mirrors the risk equation for the three exposure pathways considered. To make the template easy to use it is presented as a spreadsheet, which is available on the attached CD-ROM or from the Ministry for the Environment's website, www.mfe.govt.nz. The template can also be used in hard copy format by manually calculating the ranking (ie, multiplying each parameter value together along each pathway).
Do not alter the RSS template without clearly noting this in the assessment. Altering the template may render the results incompatible with other regional or national site rankings.
The standard version of the RSS is intended to be used as a qualitative tool for the risk categorisation and general prioritisation of sites on a nationally consistent basis. It:
- is not considered suitable as a quantitative risk assessment tool
- may not always be able to distinguish between sites for the purposes of district or regional prioritisation (eg, all sites of a particular type within a region may return the same risk category, in which case other factors should be considered when assigning priorities).
The special case mode of the RSS, for comparing like sites, returns a numerical score rather than a high, low or medium risk ranking. This numerical score:
- can be used to compare sites of similar type in a qualitative way in order to assign priorities, but cannot be compared with scores from the standard version
- cannot be used for quantitative risk assessment.
A site is normally assessed by considering the risk from contaminants that are assumed to be in the ground at the time of the assessment, either from the current use or resulting from residues from past use(s). The relevant pathways are those existing at the time of the assessment. The system can also be used to predict past or future risks, assuming conditions applicable for the time. A new ranking should be completed for each scenario.
The spreadsheet format facilitates rapid 'what if' analyses to be carried out with alternative parameter values to assess the critical risk scenario for multiple hazards present at a site. Multiple hazards may result from:
- different hazardous substances
- different current or historical uses or management practices.
The overall site ranking should reflect the hazard that presents the greatest risk. This requires you to estimate the current degree of hazard a particular substance presents, whether from a past or present use. A particular substance may in fact have accumulated from a number of site uses, all of which you must consider before selecting the worst case for reporting.
Given that the pathway and receptors will be constant for a particular time of assessment, multiple analyses should only involve altering the hazard parameters (toxicity, extent/quantity and mobility) to reflect the different hazard being considered.
If multiple assessments are carried out for a site, each assessment can be identified using the Site Type and Assessment Type fields, with additional comments in the Comments box, at the top of the assessment template. As a minimum, the chemical(s) being considered should be entered into the Comments box. Each scenario can be saved as a separate electronic file or printed out after each trial for filing. [Trialing of the system has shown that it is efficient to resave the file with a different name without clearing the parameters from the previous scenario. The new scenario analysis can then be very quickly performed by replacing the scenario description at the top and overwriting only those hazard parameters that need changing, before saving once again.]
An alternative use of the system is to assess how the risk might change if the site use were to change (in the case of redevelopment), or if the surrounding use were to change, particularly to a more sensitive use. In this case the hazard parameters may not change (unless attenuation or remediation is anticipated), but redevelopment might change pathway barriers and a site use change could change the receptor sensitivity.
You don't need detailed knowledge of contaminants and concentrations at the site. In fact such knowledge would suggest that site investigation and assessment have gone beyond the purpose of the screening system. Rather, you will need to exercise judgement as to the likely nature of the hazard, based on the type of industry and current or historical operational practices. Where some degree of investigation has been carried out at a site (providing quantitative analytical data), the results of that investigation will not be used in the assessment, but will increase the confidence of the final ranking by providing more specific site information.
What-if analysis can also be used to gauge the effect on the overall risk if you are uncertain of the best value to enter for a particular parameter. In general, altering a single parameter will only have a small effect.
The site risk ranking is obtained by examining three main exposure pathways (surface water, groundwater and direct contact) in turn, obtaining a score for each. The site ranking is taken as the worst-case (highest) risk ranking of the three pathways.
3.2 Assessment template
The template has space for entering the site name, the assessment type, the site type (see Ministry for the Environment, 2004), comments for the particular assessment, the assessor's name, aquifer type, parameter values, confidence flags, comments for each parameter, and a guide to the valuerange for the parameter.
The Assessment Type field enables you to enter whether the assessment is for a historical use or activity, the current use or some proposed use. The Comments field at the top left should be used to describe the particular scenario being analysed, if the assessment is part of a what-if analysis. Comments against each parameter allow, for example, the source of the information to be noted or the reasons why standard values have not been used.
The assessment template is sized to fit on a single page, with the aim of visually and logistically simplifying the assessment process. The template is prepared in spreadsheet format, but can be printed out and used or stored as hard copy. The spreadsheet has been prepared using Microsoft Excel 97 and Excel 2000. It will also function if saved as a Corel Quattro Pro 8 file. The template's suitability for other spreadsheet packages has not been tested and should be verified before use.
Before opening the electronic assessment template, ensure that your spreadsheet programme (eg, Microsoft Excel) has macros enabled; otherwise the template will not function properly.
In electronic form, the template has some additional functions that are not available in hard copy.
- There is a Help button that turns basic instructions on and off (see Screen shot 1).
Screen Shot 1: Help button
- A site name must be entered before the system will return a risk ranking (seen at the bottom of each pathway and in the centre-top of the template). Without the site name the ranking boxes will remain blank.
- Leaving one or more of the parameter values blank (or failure to select an aquifer type in the case of the groundwater pathway) will result in a risk ranking of 'no score', to signify that a value is missing.
- There are drop-down lists for many fields, including Assessment Type, Site Type, Aquifer Type and all the parameter value boxes. Trying to manually enter a value not in the list will produce an error message giving the valid values. Guidance on how to enter data is given for these fields.
- Parameter values less than or equal to zero, greater than 1, or to more than one decimal place will not be accepted. No more than one decimal place is allowed, as the risk screening system is incapable of providing greater accuracy than one decimal place. An error message will be displayed giving valid entries (see Screen shot 2).
Screen Shot 2: Error box
- A parameter value entry of 'na' is accepted in any of the Toxicity, Extent / Quantity or Mobility fields to enable the special case mode of assessment where sites of the same type are to be compared. This has the effect of bypassing all three fields for the pathway in which 'na' is entered. It is only necessary to enter 'na' in one of Toxicity, Extent / Quantity or Mobility for each pathway for the bypass mode to be enabled for that pathway. However, make sure 'na' is entered in each pathway separately. Special case is signalled by 'Special Case' appearing in the Risk field for the particular pathway (see Screen shot 3). A numerical value will appear as a special case score at the bottom of each pathway when the remainder of the parameter boxes are validly filled. This numerical value is not displayed (the box will read 'na') when the special case mode is not enabled.
Screen Shot 3: Special case
- User guide information for each of the parameter entry boxes can be accessed by hovering the mouse pointer over the input box (see Screen shot 4).
Screen Shot 4: User guide information
- Site ranking is calculated automatically, although a pathway's ranking will not be calculated until the site name has been entered and a value has been entered into each parameter box for that pathway.
- A summary compilation of the site's information is provided on a separate worksheet (labelled 'Results'), to enable the data to be easily copied to a separate database.
- The parameter boxes may be cleared by using the Reset button. This will not clear the information fields at the top left of the template.
3.3 Site ranking
The three exposure pathways considered (surface water, groundwater and direct contact) are independently assessed. A site's risk ranking is simply that of the 'critical' pathway (the pathway returning the highest risk).
While the overall site ranking is based on the worst-case pathway, all three pathways are reported, in the format 'surface water rank - groundwater rank - direct contact rank' at the top of the template, with the worst case (highest ranking) reported as the Site Risk immediately below (see Screen shot 5).
Screen Shot 5: Pathway risks
For example, if a site is assessed as having a high risk for both the ground and surface water pathways, and a medium risk for the direct contact pathway, the pathway risk is reported as 'High-High-Medium' and the overall site risk is 'High'.
An exposure pathway is considered to have one of the following levels of risk based on the overall calculated score (ie, the product of the individual parameter values) for that pathway:
- 0.4 to 1: high risk
- 0.02 to < 0.4: medium risk
- 0 to < 0.02: low risk.
3.4 Information gaps
The RSS is entirely multiplicative, and all parameter values need to be input before a site score can be calculated. Given that the RSS has been developed as an easy-to-use method based on the application of generalised assessment parameters, assigning arbitrary values to any one of these parameters may cause gross errors and is not recommended. Sufficient information should be available from readily accessible sources to enable valid entries to be made. However, if you consider the information to be questionable, note this fact on the template by marking the tick box beside the relevant parameter value, together with an explanation in the Comments box. As outlined above, a what-if analysis can be rapidly carried out to determine the degree of uncertainty caused by the parameter in question.