| Table B-1: Identification of issues/gaps | |||
|---|---|---|---|
| Issue/Gap | Description | Implication | Recommended Action |
| Definition of geology beyond the site | There is no indication of the geology outside the site area, particularly downgradient of the site. It is implied in the reports reviewed that similar conditions to those at the site exist outside the site but this is not substantiated. | The implications of this relate to the extent of the aquifer and the impact of aquifer geometry on the flux and direction of contaminants. The assumption of similar conditions outside the site is likely to be conservative as the most likely alternative would be the pinching out or truncation of the aquifer which would reduce the extent of contaminant migration off-site. This gap in information is therefore of low risk. | None required |
| Uncertainty in groundwater levels and flow direction | See discussion in text | ||
| Definition of aquifer base on-site | No details of the variation in the base of the aquifer (e.g. contours) are provided in the reports reviewed other than one value each for the eastern and western parts of the site and a single cross-section. | Whilst the groundwater levels will indicate the direction of groundwater flow (although this is uncertain as discussed below), the aquifer transmissivity (a product of the thickness and permeability) will determine the flux. Variation in the depth of the aquifer appears to have been considered in the calculations provided by CH2M HILL through the adoption of ranges of saturated aquifer thickness values for the beaches to the east and west of the site. The risk of significant variation from the values shown is unlikely and therefore the risk is low. | Produce saturated thickness isopachs for transmissivity determination and confirmation of fluxes. |
| Moutere Gravel hydraulic properties | No hydrogeological properties of the Moutere Gravel are presented in the reports reviewed to allow confirmation as an aquitard. However, based on the soil description the designation seems reasonable. | If the gravels were considered as part of the aquifer then the implications would not be significant. The risk associated with this is considered to be low. | None required. |
| Bund and cut-off wall construction | Only limited information has been provided on the various clay bunds / cut-off walls on the site i.e. along the eastern foreshore and various boundaries of FCC Landfill. There is little discussion of the impact of these features on the groundwater flow directions and therefore discharges to the estuary or Mapua Channel. | The potential impact of these features is to reduce flow across them to an extent that would depend on their construction. This could result in the diversion of groundwater around them and, potentially, divert water toward receptors to the south. Similarly, the gravel backfill in the centre of the eastern bund could result in preferential groundwater at that point. However, the interpreted groundwater flow directions have been based on the observed groundwater levels which are the most direct indicators of groundwater flow directions. There will be a lag between any changes to the groundwater system and the appearance of effects as contamination will take time to migrate under the new conditions. If these features had been in place prior to the contamination of the site then there would be no doubt that the contamination plume was equilibrated to their presence; however more recent installation raises the issue of time lag. Consideration of the effects of these features is required although it is considered likely that the effects of the time lag for contamination effects are minimal and the gap in information is considered low risk. | Confirm of bund/cut off wall construction and hydraulic properties and/or installation of more monitoring wells, and reassess impact on groundwater flow. |
| Preferential flow zones | Whilst it is understood all service trenches on-site were removed (but not under Tahi Street), no comment is made on their presence off-site and their potential to influence contaminant migration. | Service trenches and other similar man-made excavations have the potential to act as high permeability pathways that could allow relatively more rapid migration of contaminants locally. Most service trenches would be expected to be no deeper than 1.5 m and based on the site groundwater levels, groundwater over most of the developed parts of the site might be expected to be below this. In addition the effects of such high permeability pathways would influence groundwater levels. The implications of this are that contaminants may be transported to areas of the site more distant and rapidly than expected. The level of risk associated with this gap is not quantifiable. | Confirm depth to underground services and compare with depth to groundwater. If service trenches are below groundwater level, assess potential for migration pathways. |
| Effect of fill on aquifer properties | There is little assessment on the potential impact of emplacement of fill of different hydraulic properties from the original ground on the groundwater flow and contaminant migration. | Where the base of the fill is above the groundwater level no direct impact on groundwater flow would be expected aside from a localised alteration to recharge; however, where the fill extends down into the saturated zone, the difference in hydrogeological properties will impact the groundwater flow pattern. The degree of this impact will be dependent on the proportion of saturated aquifer that has been replaced with fill and the difference between fill and natural ground properties. This could alter the groundwater flow pattern and divert contaminated groundwater to receptors to the south. Groundwater levels recorded prior to remediation and during remediation appear to be broadly similar and therefore this does not appear to have had a significant effect and is therefore considered low risk. | None required |
| Discharge rate estimation | The discharge calculations presented by CH2M HILL correctly incorporate porosity in the calculation of groundwater velocities but incorrectly incorporate it in the calculation of flux. Determination of groundwater velocity (specifically “average linear velocity”) requires hydraulic gradient, permeability, and porosity; determination of flux requires hydraulic gradient, permeability and cross-sectional area. | The fluxes calculated by CH2MHill should be reduced by between a factor of three and five based on the porosity values that they have used. There is no risk in this as use of the correct calculation would be less conservative. | Recalculated in text. |
| Methodology for calculation of discharge dilution | There is considerable variation in the estimates of dilution that have been put forward by T&T and CH2M HILL which suggests a high level of uncertainty. | CH2M HILL calculate dilution of 80,000 to 1.5 million in the Mapua Channel and 1,000 to 25,000 in the Waimea Inlet. These are several orders of magnitude higher than the dilution factors of 100 and five respectively for the Channel and Inlet calculated by T&T in their AEE. This reflects the different methodologies with T&T basing their estimates on flow rates past the discharge zones and a 20 m mixing zone and CH2M HILL basing theirs on complete mixing within the volume of water emptied during each tidal cycle. The discharge dilution rate has a direct effect on potential effects on the marine ecosystem. The T&T estimate is very conservative and the CH2M HILL estimate probably extreme. With respect to the marine ecosystem within the sea there is therefore very low risk in using the two dilution rates (five and 20) for the basis of the PETCs. What is not addressed in either estimate however is that there will be periods where there is undiluted groundwater seepage during low tide which will impact on the ecosystems based on the surface and within the beach sediments within the discharge zones. This has already been noted in the enhanced algal growths in these areas (presumably because of increased nutrient concentrations in the groundwater from use of urea and diammonium phosphate). This risk is considered to be high. | Maintain T&T dilution factors. Review acceptability of undiluted groundwater discharge concentrations on local surface ecosystems. Carry out marine biota sampling and compare with existing biota (snail) sampling elsewhere. |
| Uncertainties in parameters used for discharge estimation | See next issue below | ||
| No site water balance | There is very little discussion of recharge across the site, how it has changed during and after remediation and what it could be under future site use, and no attempt to calculate a water balance in any of the material reviewed. | Whilst there will be uncertainties in the precise volumes of runoff, evapotranspiration, and recharge across the site, they are likely to be less than those associated with estimation of aquifer transmissivity and hydraulic gradient for which uncertainty can often be in the range of orders of magnitude. The level of uncertainty in the current throughflow calculations is considered to be medium to high and therefore the uncertainty over the selection of the appropriate PETCs is also medium to high. It should be noted that the impact of the change in throughflow is only of relevance to the marine environment; protection of groundwater for future use such as for drinking is unaffected. | Develop a water balance for the site under existing and potential future conditions and review PETCs with respect to marine ecosystem protection. |
| Groundwater abstraction | There is no discussion relating to the effects of groundwater abstraction that may occur from nearby private wells (current or future). In addition there is no confirmation of the construction and the aquifer which is being exploited by these wells. | Unaccounted for abstraction from these wells could result in the drawing in of contaminated groundwater and influence groundwater patterns if the wells are in the same aquifer. Conversely, wells drawing from deeper aquifers are unlikely to influence groundwater flow patterns or be at risk of drawing in contaminated groundwater. In general the takes are expected to be low and therefore the risk of changing flow directions low. | Confirm well construction and abstraction rates and assess potential impacts on groundwater flow patterns. |
| Time of travel | Calculations have been made for the groundwater velocities based on the CH2M HILL groundwater contours; however, no calculation has been made based on the alternative contours presented by PDP. | The alternative contours presented by PDP indicate that there is a component of groundwater flow to the south; however, without an estimation of the time of travel it is uncertain how long it will take for any effects of remediation to reach the groundwater receptors to the south. | Travel time to groundwater receptors is calculated based on PDP’s alternative groundwater contours. |
| Attenuation | There is very little discussion of attenuation processes in the soil which may serve to reduce groundwater concentrations. This is not an issue for areas where direct observation is possible but is for those where it is not (e.g. receptors to the south) | Taking no account of attenuation is conservative as it will lead to prediction of lower concentrations at distance. The current assessment is therefore conservative and there is therefore no risk. | None required. |
| Declining versus constant source | In general there is little discussion regarding the nature of the contaminant source within the soil in terms of its persistence. It is generally implied that the source is constant. | The approach that has been taken is a conservative one and therefore there is no risk associated with this. | None required. |
| Remaining contamination in source soils/saturation of source soils | This relates to calculations of contaminants remaining on site and in fill and the differing impact of contamination sources above and below the water table. | Most data indicate that any increases in the concentration of contaminants of concern have now peaked. This means that for current conditions the effects of the remediation on groundwater have been observed and are known and issues relating to the contamination source (which is assumed to not change in future – see previous comment on constant source ) are not of great relevance. The exception to this would be changes in site use that increase the rate of infiltration through the soil and/or raise the watertable and result in greater volumes of fill below the watertable. In this case increases in the concentration of contaminants of concern could occur. However, given the current site land use (grassed) future site use is unlikely to have this effect. The risk associated with this is therefore low. | Ensure that infiltration is not greater than current for future site use or assess effects if future development has the potential to increase infiltration (e.g. disposing of stormwater by infiltration). |
Appendix C – Hydrogeological Uncertainties
June 2009
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