Appendix 22 Assessment of Beca s Report on Effluent Discharge to Sand Dunes

Transcription

1 Appendix 22 Assessment of Beca s Report on Effluent Discharge to Sand Dunes

2 Page 1 From: To: Selva Selvarajah, ENVIROKNOWLEDGE Ltd Wolfgang Kanz, Gisborne District Council Date: 30 June 2017 Topic: Assessment of CH2M Beca s report on effluent discharge to sand dunes Summary Sand dunes with subsurface effluent disposal have the potential to be used as a land discharge option. Such an option could be applied in lieu of wetland. CH2M Beca sand dune memo provides some initial guidance on land area requirements and potential issues. Based on the above memo, with suitable site hydraulic conductivity, the identified sand dune area may be adequate to discharge effluent hydraulically under annual average and wet weather flows. However, in order to consider the sand dunes option, substantial amount of field data, information on environmental impact and mitigation (both contaminants and hydraulic) and community/iwi support are required. Information assessed: CH2M Beca s April 2017 memo to GDC on Gisborne Dune Effluent Discharge Concept Assessment. Assessment: My assessment of the memo is on the practical applications of the information provided by CH2M Beca and whether effluent discharge to sand dunes can be an option for GDC.

3 Page 2 Hydraulic conductivity The memo provided by CH2M Beca was based on a desktop study as intended for providing requested information to GDC. Consequently, such a study is expected to be based on many assumptions. Of the assumptions, the most critical was hydraulic conductivity. The hydraulic conductivity range used was between 1x10-4 1x10-6 m/s which is within the potential range for typical sand dunes with varying hydraulic conductivities. The above factor is critical in determining land area required for the application of effluent. The total available land area was estimated as 30 ha. The estimation showed that at the daily effluent application rates of mm, the required land area varied between 1.6 and 160 ha to dispose average daily effluent volume of 13,800 m 3. However, at the peak daily discharge rate of 38,800 m 3, the required land area was ha. Clearly, a homogenous hydraulic conductivity cannot be expected from natural sand dunes. Therefore, whilst the estimated values may not apply for practical purposes, they will provide guidance on worst case scenarios. It was clear if hydraulic conductivity of 1x10-4 m/s existed uniformly over the entire area, disposal is possible throughout the year, using only 5-15% of the available dunes area under average and wet weather flow conditions, provided breakouts are avoided. Given under the above hydraulic conductivity regime sufficient extra land area is available, breakouts can be avoided/minimised by spreading effluent over the entire available area. I presume the hydraulic conductivity values used are not saturated hydraulic conductivities. Saturated hydraulic conductivities are greater than hydraulic conductivities. Application of saturated conductivity values may be more appropriate in a regularly used land disposal system where the medium is always wet. The other factors which could influence the required application area are rainfall (RF) and evapotranspiration (ET) values. The above should be incorporated into the model for any future studies.

4 Page 3 Contaminant discharges Obviously, the purpose of the CH2M Beca memo was to consider hydraulic loading options. Any such land disposal studies must also consider effluent contaminants behaviour and fate. The key contaminants are N, P and TSS. TSS levels in applied effluent are critical to assess any long term clogging issues, whilst N and P levels and behaviour are critical in assessing any environmental impacts on the ocean and groundwater quality. Judging by the GDC proposals, the effluent will be clarified and UV treated before applying onto sand dunes. On the above basis and as shown in Table 1, the winter discharge from single BTF will have greater total-n of 13 mg/l (with a high proportion of ammoniacal-n of 10 mg/l), whilst the double BTF discharge will have 5.5 mg/l of TN with a high proportion of nitrate-n (2.5 mg/l). Table 1. Predicted BTF effluent quality following clarification (adopted from NIWA GDC wetland report) Single BTF Double BTFs Summer Winter Summer Winter Temp. ( C) BOD 5 (mg/l) TSS (mg/l) NH 4-N (mg/l) NO x-n (mg/l) TKN (mg/l) TN (mg/l) TP (mg/l) Using the average effluent hydraulic loading of 13,800 m 3 /d and the total available sand dune area for effluent disposal of 30 ha, the winter loading of total-n from single BTF effluent disposal is estimated as 2.2 t/ha/year, whilst the double BTF effluent winter total- N loading is estimated as 0.9 t/ha/year.

5 Page 4 To put the above into context, a typical cut and carry pasture system can receive an annual N loading of 0.6 t/ha. Much of the N is taken up by pasture which is removed by harvest. However, in the case of effluent disposal to sand dunes much of the applied-n is expected to leach into groundwater and sea water. This is not different to the approach taken to dispose SBR and UV treated Wanaka-Albert Town sewage into land trenches in Wanaka. Presumably, the effluent will be applied as subsurface disposal hence very little ammonia loss can be expected from the applied effluent. Sand medium is known to mineralise organic-n into ammoniacal-n and nitrify ammoniacal-n to nitrate-n. Unless there are anaerobic/anoxic conditions caused by effluent mounding, any N losses by denitrification will be limited. For the above reasons, the total-n entering groundwater and sea water will be the applied-n with minor N transformation between organic-n, ammoniacal-n and nitrate-n. Any impacts of the discharged-n or P on the ocean is dependent on mixing. The daily N loading over winter is 179 kg from single BTF discharge and 75 kg from double BTF discharge. Such a loading is spread along 5 km long sand dunes and sea water-groundwater interface. Provided breakout of applied effluent does not occur, particularly during low tide periods, any elevated sea water algal activity may be low. However, this has to be verified by a marine biologist. Regulatory requirements Under the Resource Management Act (RMA), under s15 of the Act, discharge of contaminant onto or into land in circumstances which may result in that contaminant (or any other contaminant emanating as a result of natural processes from that contaminant) entering water (s15(1)b); or discharge of contaminant from any industrial or trade premises onto or into land (s15(1)d) will require resource consent unless allowed by NES, other regulations or regional rule. Since the above activity is not a permitted activity under the GDC rules, the activity would require resource consents, potentially under s15(1)b and s15(1)d because effluent could be

6 Page 5 considered as from an industrial and trade premises under the RMA interpretation in s2 of the Act. Provided the disposal and disturbance area are not within the coastal marine area, there may not be any Coastal Permits required. However, there may be land use consents required under the GDC district plans. Any resource consent application to discharge effluent to sand dunes must contain adequate AEE (which will determine whether adverse effects are more than minor) and consultation with the local community and iwi. The adverse effects must address issues related to public access during installation and disposal, hydraulic impacts of the discharge, effects of contaminants on groundwater and sea water quality, any potential long-term accumulation of contaminants including heavy metals and any cultural issues. Given the significance of public access of the site, the consent may be publicly notified in accordance with council rules and policies. Is sand dune option useful for the purpose of GDC effluent disposal? Given the local surface water receiving environment does not have the capacity to assimilate treated effluent discharges of this scale, GDC has the following options to treat and discharge the effluent from either single or double BTF: Option 1. BTF Clarification UV Wetland Ocean outfall Option 2. BTF Clarification UV Wetland Sand dunes Option 3. BTF Clarification UV Sand dunes Option 4. BTF Clarification UV Wetland (a) Sand dunes (up to 20,000 m 3 ) and (b) Ocean outfall (>20,000 m 3 ) UV treatment appears to be inevitable under both land discharge options. Clarification is compulsory for UV treatment and to reduce contaminant levels and loadings. One of the key factors is satisfying cultural needs by passing effluent through land, by using sand dunes or wetland.

7 Page 6 The use of sand dunes is not necessary under Option 1. All remaining options require sand dunes. Of the remaining options, Option 3 may be more cost effective given the absence of wetland. In short, sand dunes can be used in lieu of wetlands provided the option is technically feasible and has the community/iwi support. Options 2 and 4 could be costly and excessive to the cultural requirements. If Option 3 is considered, the existing information on sand dune option is inadequate to progress on the sand dune option. Additional information required to consider sand dunes option If the sand dune option is assessed, in addition to the information required under the Recommendations section of the CH2M Beca s sand dunes memo, the following information is also required: Suitable subsurface depth and effluent disposal system to apply effluent Community and iwi support Contaminant accumulation (including heavy metals) and migration in sand dunes and impacts on the receiving environment including groundwater and sea water under single and double BTF summer and winter discharges Hydraulic loading impacts Any mitigations used to minimise the hydraulic and contaminants impacts Field measurements of saturated hydraulic conductivities Potential for clogging by 30 mg/l TSS level in effluent Cost of installation and operation