PATTLE DELAMORE PARTNERS LTD Level 1, Suite 6 89 Grey Street, Tauranga 3110 PO Box 13274, Tauranga 3141, New Zealand Tel +64 7 213 0858 Fax +64 9 523 6901 Web www.pdp.co.nz Auckland Tauranga Wellington Christchurch memorandum TO Greg Manzano FROM Blair Thornburrow Rotorua Lakes Council DATE 5 August 2016 RE Rotoiti Rotoma WWTP & LDS: Assessment of contaminant migration 1.0 Introduction RLC are currently in the process of preparing the Assessment of Effects on the Environment (AEE) for the proposed Rotoiti-Rotoma wastewater treatment plant (WWTP) and land disposal scheme (LDS). PDP were commissioned by RLC in January 2016 to undertake a hydrogeological investigation of the site 1. The investigation included a conservative assessment of nutrient loading to Lake Rotoiti. In July 2016, RLC has commissioned PDP to prepare an additional assessment of nutrient loading, in particular: To update the loading calculations based on inclusion of the Rotoehu community in the scheme; To consider mixing and dispersion of the treated wastewater (TWW) contaminant plume in estimating potential near-shore nutrient loading rates; and, To assess the potential for phosphorus assimilation in the unsaturated soil zone. The work outlined in this memo is intended to support preparation of the AEE of the proposed LDS. 2.0 Nitrogen Plume Dispersion Assessment The Stage 2 hydrogeological investigation report provided a conservative assessment of nutrient loading to Lake Rotoiti. This assessment considered both the cumulative mass loading to Lake Rotoiti, and the potential near-shore loading rate per metre of shoreline. In terms of cumulative mass loading rates, the previous assessment of nitrogen loading, which assumed full transfer of nitrogen to Lake Rotoiti via the groundwater system is considered valid due to the conservative nature of dissolved nitrogen. The revised TN loading rate for the scheme is 720 kg/yr (up from 640 kg/yr) and the revised Average Daily Flow (ADF) is 432 m 3 /d (up from 391 m 3 /d), due to the addition of Rotoehu to the scheme 2. 2.1 Methodology To provide an indication of the potential lateral dispersion of contaminants in groundwater following discharge into the LDS, an assessment of plume mixing and dispersion was undertaken using an analytical/numerical modelling approach using MODFLOW. The assumptions adopted for this assessment were: 1 PDP (2016) Rotoiti-Rotoma WWTP and LDS: Stage 2 Hydrogeological Investigation. Report prepared for Rotorua Lakes Council, May 2016. 2 Rotoiti/Rotoma WWTP & LDS: Concept Design Addendum 1: Rotoehu Connection. PDP memorandum dated 2 August 2016. T01548205M001.docx
2 A domain of 700 m long by 600 m wide with a constant head along one end to represent discharge to the lake shore; A 10 m thick saturated zone within the base of the aquifer was assumed with a hydraulic conductivity (K) of 5 m/d and porosity of 0.3 K was adjusted to provide a representative approximation to the observed head gradient; A land surface recharge rate of 900 mm/y (50% of rainfall) was assumed; A disposal field of 200 m length by 50 m width, situated 550 m distant from the discharge boundary; A combined hydraulic load of 432 m 3 /d and solute (N) load of 720 kg/y was applied to the disposal field area; and, Longitudinal and transverse dispersivity values of 10 m and 1 m were assumed, respectively. 2.2 Results The results of the assessment in terms of near shore loading rates are presented in Figure 1. Based on a 200 m long disposal footprint running parallel to the lake shore, the nitrogen loading rate per metre of trench is approximately 3.6 kg/m/y. Allowing for mixing and dispersion of the plume between the disposal field and the lake, the plume is estimated to occupy a width of approximately 500 m. The estimated maximum loading rate per metre of shore is 1.8 kg/m/y. This is similar to the previous estimate of the existing nitrogen loading rate density at Gisborne Point from septic tanks (estimated 2 kg/m/y). Figure 2 shows the estimated timing of nutrient break-through following transport via groundwater. This indicates that breakthrough would start approximately 2 years following commencement of discharge and that full breakthrough of the plume would occur after approximately 4 5 years. 4 3.5 Discharge to lake Disposal field N Load Density (kg/m/y) 3 2.5 2 1.5 1 0.5 0 0 100 200 300 400 500 600 Horizontal distance (m) Figure 1: Estimated near shore loading rates of Nitrogen considering plume dispersion
3 800 700 600 N Load Rate (kg/y) 500 400 300 200 100 0 0 2 4 6 8 10 Time (years) Figure 2: Estimated break-through of treated wastewater effluent to Lake Rotoiti 2.3 Sensitivity and Uncertainty While the concept design sizing assumed actual infiltration trench combined lengths of 160 m, a 200 m long disposal field footprint has been assumed for the purposes of this assessment, allowing for the recommended configuration of trenches (i.e. trenches subdivided into shorter sections). To provide additional confidence in the results, the sensitivity of results to a shorter field width was tested, assuming that all TWW discharge would occur over a 50 m long disposal field. The results of this check indicated that maximum near shore loading rates were not sensitive to smaller field lengths, due to the influence of hydraulic loading and mass displacement on plume dispersion. The results presented above are most sensitive to the rate of groundwater throughflow velocity from the LDS site towards the discharge boundary. Using the approach described above, groundwater throughflow velocity is influenced by land surface recharge, upstream catchment area and aquifer thickness. A conservatively high throughflow scenario was established to provide an upper bound for near shore loading rates (i.e. leading to minimal plume dispersion). In this scenario, the model area was extended to 600 m beyond the up-gradient edge of the LDS in order to capture additional rainfall recharge, with a constant recharge rate of 900 mm/y applied. This resulted in a higher rate of groundwater flow past the LDS and enhanced transport of solute mass. Results of this assessment indicated a maximum near shore loading rate of 2.7 kg/m/y and a plume breakthrough occurring at approximately 1 year after commencement of discharge. However, it should be noted that the existing assessment is already conservative in this regard, based on the assignment of an aquifer thickness of only 10 m. Summary Long term near shore loading rate: Maximum expected 1.8 kg N/m/yr Maximum possible 2.7 kg N/m/yr
4 3.0 Phosphorus Attenuation Previously, a conservative assessment of phosphorus (P) loading to Lake Rotoiti was undertaken, based on the assumption that in the long term, the capacity of soils to assimilate P would be depleted, and leaching of dissolved P to groundwater would eventually occur. This approach was taken for the following reasons: Low expected P loading rates associated with treated effluent discharge; and, Lack of information available to undertake a quantitative assessment of P uptake in the large unsaturated zone beneath the LDS site. RLC have subsequently requested that an additional assessment be undertaken to comment on a likely P attenuation scenario. Unlike nitrate, dissolved phosphorus readily binds to solid particles in the soil profile through adsorption. The proposed LDS site has the advantage of being situated above a very deep unsaturated zone (approximately 65 m thickness). It is reasonable to expect that P adsorption could occur over this entire depth. Therefore, a large volume of unsaturated soil particles are available for potentially assimilating dissolved P. Assuming a total field area of 200 m by 50 m (based on 3 parallel rows of trenches and allowing some lateral spreading of TWW) and a depth of 60 m (allowing for potential water table mounding), this equates to an available soil volume of approximately 600,000 m 3. Assuming a bulk density of 840 kg/m 3 (based on S-map 3 data for surface soils the site deeper bulk densities are likely to be higher), this relates to a mass of 520,000 t. The soils at the site (S-map ID: Oropi_2a) is listed as having a medium topsoil P retention (51%). However, this does not characterise the subsoil through which the TWW will ultimately pass. Hence for the purposes of this assessment, a conservatively low phosphorus storage capacity has been assumed. A maximum P storage capacity of 100 mg/kg was assumed 4 for the soils and rocks residing in the unsaturated zone. This is considered a very low and conservative value for P storage capacity. For example, allophanic soils have a P storage capacity in the range of 2,000 3,000 mg/kg. On this basis, the site would provide approximately 50 t of P storage. This is equivalent to the total P discharged from the scheme over a period of approximately 160 years, based on an effluent loading rate of 310 kgp/y. Observations of the geological materials encountered during drilling confirmed that materials were typically characterised as being relatively porous. Therefore, the available contact surface area in the unsaturated zone is considered favourable for P adsorption. Based on consideration of the large depth of unsaturated material within which dissolved P could be adsorbed, it is considered unlikely that any leaching of P to groundwater (and ultimately Lake Rotoiti) over the lifetime of the project would be more than minor, and would be significantly delayed following commencement of scheme operation. Summary Long term near shore loading rate: Expected not detectable Maximum possible negligible 3 http://smap.landcareresearch.co.nz/home 4 Conservative value selected based on work by CPG (2009) - Waipawa Wastewater Project Land Treatment Site Resource Assessment
5 4.0 Limitations This memorandum has been prepared by Pattle Delamore Partners Limited (PDP) on the basis of information provided by Rotorua Lakes Council. PDP has not independently verified the provided information and has relied upon it being accurate and sufficient for use by PDP in preparing the memorandum. PDP accepts no responsibility for errors or omissions in, or the currency or sufficiency of, the provided information. This memorandum has been prepared by PDP on the specific instructions of Rotorua Lakes Council for the limited purposes described in the memorandum. PDP accepts no liability if the memorandum is used for a different purpose or if it is used or relied on by any other person. Any such use or reliance will be solely at their own risk. Prepared by Checked by Blair Thornburrow Senior Water Resources Scientist Robert Docherty Technical Director