Waikato Regional Council Technical Report 2011/18 Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) www.waikatoregion.govt.nz ISSN 2230-4339 (Print) ISSN 2230-4347 (Online) Prepared by: Pattle Delamore Partners Ltd For: Waikato Regional Council Private Bag 3038, Waikato Mail Centre Hamilton 3240 June 2011 Document #: 1994182 Approved for release by: Dominique Noiton Date June 2011 Disclaimer This technical report has been prepared for the use of Waikato Regional Council as a reference document and as such does not constitute Council’s policy. Council requests that if excerpts or inferences are drawn from this document for further use by individuals or organisations, due care should be taken to ensure that the appropriate context has been preserved, and is accurately reflected and referenced in any subsequent spoken or written communication. While Waikato Regional Council has exercised all reasonable skill and care in controlling the contents of this report, Council accepts no liability in contract, tort or otherwise, for any loss, damage, injury or expense (whether direct, indirect or consequential) arising out of the provision of this information or its use by you or any other party. Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) π Prepared for Environment Waikato π March 2008 PATTLE DELAMORE PARTNERS LTD vi Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) Quality Control Sheet TITLE Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) CLIENT Environment Waikato VERSION Final DATE March 2008 JOB REFERENCE A02029101 S O U R C E F I L E ( S ) AO2029101R001 Prepared by SIGNATURE Andrew Rumsby Directed, reviewed and approved by SIGNATURE Keith Delamore Limitations: This report has been prepared on the basis of visual observations of the lake, and limited sampling of the lake bed sediments and the lake water. This information has been used to describe the lake conditions in the vicinity of sample locations but the interpreted conditions cannot be guaranteed. Part of the assessment relies on information supplied by others. This information is believed to be correct but has not been independently verified by Pattle Delamore Partners Limited. The information contained within this report applies to the date of the preparation of this report (January 2008). With time, the site conditions and environmental guidelines could change so that the reported assessment and conclusions are no longer valid. Thus, in future, the report should not be used without confirming the validity of the report’s information at that time. The report has been prepared for Environment Waikato, according to their instructions, for the particular objectives described in the report. The information contained in this report should not be used by anyone else or for any other purposes. EWDOCS_n1994182_v1_Significance_of_Arsenic_in_Sediments_of_Lake_Rotoroa_(Hamilton_Lake) PATTLE DELAMORE PARTNERS LTD vii Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) Executive Summary Background Environment Waikato has commissioned Pattle Delamore Partners Limited (PDP) to assist with the evaluation of the significance of arsenic in the sediment of Lake Rotoroa (Hamilton Lake), in Hamilton. Although this report focuses mainly on the significance of arsenic within the sediments of the lake, the concentrations of a number of other elements (Cd, Cr, Cu, Fe, Hg, Ni, P, Pb and Zn) were also measured in the sediment and overlaying waters. The results of all of these elements are presented in this report together with an assessment of the environmental significance of the elements, the method of deposition of the elements and the relationship of the lake chemistry to the chemistry of other lakes in the Waikato region. Using a motor boat for access to the sampling locations, 34 grab samples of the top 10cm of the lake bed were obtained by PDP on a distorted grid of 130m spacing, together with 5 core samples with a maximum depth of 0.45m, and 4 water samples. These samples, together with 4 composite samples prepared from some of the lake bed samples were analysed by Hill Laboratory in Hamilton for a suite of metal and organic parameters. Findings: Comparison to Environmental Guidelines π Five elements (arsenic, copper, lead, mercury and zinc) exceeded the ANZECC (2000) guideline value at some of the surficial sediment sampling sites. The multi-element analysis of 33 metals on four composite samples revealed that antimony and polycyclic aromatic hydrocarbons (PAHs) exceed the ANZECC (2000) ISQG-low guideline value at some locations. The concentration of some PAHs exceeded the ANZECC (2000) ISQG-high guideline value at one location after organic carbon normalisation procedures where used. π The average concentration of arsenic within Hamilton Lake surficial sediments was 170mg/kg. Arsenic concentrations exceeded the ANZECC (2000) ISQG-low guideline value in all 34 surficial sediment sampling sites and the ANZECC (2000) ISQG-high concentrations in 73% of the surficial sediment sampling sites. π The average concentration of copper within Hamilton Lake surficial sediments was 32mg/kg. Copper concentrations exceeded the ANZECC (2000) ISQG-low guideline value at one sampling site and there were no exceedances of the ANZECC (2000) ISQG-high guideline value. π The average concentration of lead within surficial sediments was 77.5mg/kg. Lead concentrations in the surficial sediments exceeded the ANZECC (2000) ISQG-low guideline at 19 sampling sites and the ANZECC (2000) ISQG-high guideline value at one sampling site. EWDOCS_n1994182_v1_Significance_of_Arsenic_in_Sediments_of_Lake_Rotoroa_(Hamilton_Lake) PATTLE DELAMORE PARTNERS LTD viii Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) π The average concentration of mercury within the surficial sediments was 0.13mg/kg. Mercury concentrations in surficial sediments exceeded the ANZECC (2000) ISQG-low guideline at five sampling locations but did not exceed the ANZECC (2000) ISQG-high guideline value at any of the sampling sites. π The average concentration of zinc within surficial sediment samples was 182mg/kg. Zinc concentrations exceeded the ANZECC (2000) ISQG-low guideline value at 12 sampling sites and exceeded the ANZECC (2000) ISQG- high guideline value at 2 sampling sites. π The concentration of all elements is generally greater in sediments collected near the top of the sediments cores and decreases in sediments collected at greater depth. The exception to this trend is arsenic in sediment core CS3, the highest concentrations of arsenic in this core occurs at 9-10cm below the water-sediment interface. It is believed that this sampling site may be impacted by high sediments rates caused by stormwater discharges. π The concentration of both dissolved and total metals within the lake water are below the relevant ANZECC (2000) fresh water trigger values for 95% ecosystem protection and NZ Drinking Water Standards (2006). A majority of the metals present in the lake are associated with the dissolved phase. Findings: Relative Enrichment To determine if the elements in the sediments of Hamilton Lake have been significantly enriched above typical background concentrations, the concentration of elements in this survey were compared with the concentrations of elements measured in 9 rural lakes in the Waikato region. The enrichment ratios calculated for the elements examined in the surficial sediments of Hamilton Lake reveal that: π The concentration of arsenic within Hamilton Lake is significantly enriched, with enrichment factors ranging from 4.5 to 105 times higher than those typically found in Waikato rural lakes. π The concentration of antimony is significantly enriched within Hamilton Lake surficial sediments, with enrichment ratios ranging from 5.2 to 7.3 times higher than those typically found in Waikato rural lakes. π The concentration of tin is significantly enriched within Hamilton Lake surficial sediments, with enrichment ratios ranging from 3 to 4.8 times higher than those typically found in Waikato rural lakes. π The concentration of lead in surficial sediments of Hamilton Lake is significantly enriched in most sampling locations. Lead appears to be mostly enriched in sediment sampling locations in the southern portion of the lake, where a number of stormwater discharges occur into the lake. π The concentration of silver in surficial sediments of Hamilton Lake is significantly enriched in some sampling locations. EWDOCS_n1994182_v1_Significance_of_Arsenic_in_Sediments_of_Lake_Rotoroa_(Hamilton_Lake) PATTLE DELAMORE PARTNERS LTD ix Significance of Arsenic in Sediments of Lake Rotoroa (Hamilton Lake) π The concentration of aluminium in surficial sediments of Hamilton Lake is significantly enriched above background concentrations found in Waikato rural lakes, but is lower than the typical concentration within Waikato soils. π The concentrations of chromium, copper and zinc are significantly enriched in some surficial sediment sampling locations (mainly around the southern portion of the lake). π The average concentrations of mercury in the surficial sediments of the lake do not appear to be significantly enriched when compared to the average concentration of Waikato rural lakes. However, there is one sampling location which does appear to be enriched in mercury with respect to Waikato rural lakes. The source of the mercury is unknown, and the elevated mercury concentrations measured at this location may be due to sample inhomogeneity.