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Waikato peat lakes sediment nutrient removal scoping exercise

TR 2006/15

Report: TR06/15

Authors: Carolyn L. Faithfull, David P. Hamilton, David F. Burger and Ian Duggan (Centre for Biodiversity and Ecology Research)

Abstract

This report was commissioned by Environment Waikato to examine the available methods for internal (bottom sediment) nutrient removal and their suitability for application in the Waikato peat lakes. Lakes Ngaroto, Kainui, Rotomanuka and Cameron were chosen as focus lakes for the study, based on existing restoration objectives and recreational and conservation values.

A range of methods designed to reduce internal nutrient loading was reviewed, including hypolimnetic aeration, hypolimnetic withdrawal and fish removal, with sediment removal, and additions of alum, Phoslock, zeolite, iron making slag and carbon examined in detail. Experimental and observational work was carried out to assess the feasibility of these methods in the target lakes:

  • Alum was determined to be the least desirable of the options examined; as there is a high probability soluble toxic forms of alum will be expressed or released due to the low alkalinity and pH of the target lakes.

  • Sediment removal is the most expensive option, but may be appropriate in lakes without high ecological values. However, the external nutrient load must be reduced substantially to prolong benefits of dredging.

  • Phoslock and zeolite both effectively reduced phosphate concentrations in the incubation cores. Zeolite was more effective than Phoslock in preventing nutrient release from the sediments following anoxia. However, Phoslock was more effective than zeolite in removing nutrients from the water column. Both substances should be applied with care and with careful documentation of chemical and biological effects as there may be hitherto undocumented effects on aquatic biota.

  • The use of iron making slag to reduce internal nutrient load is not recommended, as nutrient concentrations were not reduced in the iron treated cores in the incubation study.

  • Dissolved organic carbon addition may be useful in the naturally dystrophic peat lakes, and can be added naturally if measures are taken to conserve the surrounding peat substrate (e.g. riparian vegetation, maintenance of lake levels). Further study is required to assess how humic substances would affect primary productivity and the availability of phosphate for algae growth.

  • Hypolimnetic withdrawal is not a suitable nutrient reduction technique, as none of the Waikato peat lakes possesses a large anoxic hypolimnion, a long period of anoxia or a consistently large lake inflow.

  • Hypolimnetic aeration may be useful in Lake Rotomanuka, as this the only peat lake known to regularly stratify and become anoxic during summer. Hypolimnetic aeration can break down stratification and potentially reduce the abundance of cyanobacteria by disrupting their buoyancy.

Before a pilot study is carried out to reduce internal nutrient load, it is recommended the external nutrient load of the target lake(s) is reduced substantially, to the order of 50 % for most lakes where the catchment is highly modified and effects of restoration measures are not evident. On the basis of the evidence presented in this report, we recommend using fish removal and macrophyte re-establishment as part of an integrated strategy to improve water quality and transparency. Use of a flocculent could also be considered as part of an integrated strategy for reducing internal nutrient loads. The latter strategy should be undertaken under circumstances of mutual cooperation and communication amongst environmental managers, scientists and material suppliers, as optimising the timing and quantity of the flocculent will be an iterative process requiring adjustments for each waterbody application.

Waikato Peat Lakes Sediment Nutrient Removal Scoping Exercise [PDF, 710 KB]

Contents
Executive summary 1
1.0 Introduction 3
1.1 Lake Ngaroto 5
1.2 Lake Kainui 6
1.3 Lake Rotomanuka (North) 6
1.4 Lake Cameron 6
1.5 Outline of report 7
2.0 Sediment removal 9
2.1 Literature review 9
2.1.1 Political issues 10
2.1.2 Case studies 11
2.1.3 Suitability for the Waikato peat lakes 13
2.2 Laboratory study 15
2.2.2 Porewater dissolved nutrients 15
2.2.3 Total phosphorus 16
2.2.4 Heavy metals 16
2.2.5 Sedimentation rate 16
2.2.6 Percentage water content 17
2.2.7 Percentage organic content 17
2.2.8 Settling rate 17
2.2.9 Sediment resuspension 17
2.3 Results 18
2.3.1 Volume of sediment to be removed 18
2.3.2 Toxic substances 23
2.3.3 Longevity of treatment 24
2.3.4 Disposal area design 25
2.3.5 Sediment resuspension 29
2.4 Do the benefits offset the costs? 30
2.5 Conclusion 31
3.0 Phosphorus inactivation using alum 32
3.1 Literature review 32
3.1.1 Political issues 34
3.1.2 Case studies 34
3.2 Suitability of alum treatment for the Waikato peat lakes 36
3.2.1 Alkalinity and pH 36
3.2.2 Tannins 37
3.2.3 Sedimentation rate 38
3.2.4 Mixing regime 38
3.2.5 External nutrient reduction 39
3.3 Conclusion 39
4.0 Sediment capping and phosphorus inactivation techniques - Literature Review 40
4.1 Phoslock 40
4.1.1 Political/ethical issues 42
4.1.2 Case studies 42
4.1.3 Suitability for the Waikato peat lakes 43
4.1.4 Conclusion 44
4.2 Zeolite 44
4.2.1 Suitability for the Waikato peat lakes 45
4.2.2 Conclusion 46
4.3 Iron addition 46
4.3.1 Political/ethical issues 48
4.3.2 Case studies 49
4.3.3 Suitability for the Waikato peat lakes 51
4.4 Organic matter or carbon addition 52
4.4.1 Case studies 54
4.4.2 Suitability for the Waikato peat lakes 55
5.0 Sediment capping and phosphorus inactivation techniques - Core incubation experiment 56
5.1 Methods 56
5.2 Results 58
5.3 Discussion 63
5.3.1 Phoslock 63
5.3.2 Zeolite 64
5.3.3 Carbon 64
5.3.4 Iron making slag 65
5.4 Conclusion 65
6.0 Hypolimnetic withdrawal (Deep water discharge) 67
6.1 Political issues 68
6.2 Case studies 68
6.3 Suitability for the Waikato peat lakes 70
7.0 Hypolimnetic aeration 71
7.1 Political issues 72
7.2 Case studies 72
7.3 Suitability for the Waikato peat lakes 73
8.0 Fish removal (Biomanipulation) 74
8.1 Political issues 75
8.2 Case studies 76
8.3 Suitability for the Waikato peat lakes 76
8.4 Conclusions 77
9.0 Conclusions and recommendations 78
10.0 Pilot study 83
10.1 Candidate lakes 84
10.2 Consent requirements 84
10.3 Equipment requirements 85
10.4 Monitoring 85
10.5 Treatments 86
11.0 References 87
12.0 Appendices 95
12.1 Appendix 1 95
12.2 Appendix 2 96
12.3 Appendix 3 102
12.4 Appendix 4 103
12.5 Appendix 5 104
12.6 Appendix 6 107
12.7 Appendix 7 108