title: Biogeochemical Characterisation of an Alum Dosed Stream: Implications for Phosphate Cycling in Lake Rotoehu

authors: Eager, Christopher Andrews

subjects: Alum

abstract: Lakes are highly valued freshwater ecosystems which provide many goods and services upon which humans are reliant. Eutrophication of waterways, driven by the over-enrichment of nutrients such as phosphorus and nitrogen, is a threat to the future quality of water resources globally. Lake restoration methods are increasingly being employed to improve eutrophic waterways, via numerous catchment derived and in-lake approaches. However, spatio-temporal heterogeneity in physicochemical and biogeochemical conditions within lakes may restrict the efficacy of restoration methods.  This thesis explores in-stream alum dosing as a lake remediation strategy for the purpose of dissolved phosphorus inactivation, and its physicochemical and biogeochemical interaction and fate within Lake Rotoehu, New Zealand.

 Laboratory and field measurements were undertaken at Lake Rotoehu, on New Zealand’s North Island. The shallow, polymictic lake resides in an active volcanic area with geothermal inputs, and is subject to several management issues including elevated nutrient concentrations, invasive macrophytes, and frequent cyanobacteria blooms. The geothermal Waitangi Springs, which discharge into Lake Rotoehu, contribute ~69 % of the lake’s total ionic content, and are responsible for enhanced concentrations of biologically limiting nutrients including phosphorus, silicon, nitrate, ammonium, and iron. In an effort to curb in-lake phosphorus levels, in-stream alum-dosing has been employed in order to floc out dissolved reactive phosphorus (DRP), through chemical adsorption and sedimentation.  

The results presented here, culminate from an investigation of the physicochemical and geochemical dynamics across the mixing zone from the Waitangi Springs geothermal stream outlet across Te Wairoa Bay to the main lake body.  A combination of approaches was used: two field experiments with fixed location and transect measurements, laboratory analysis and geochemical speciation modelling with PHREEQC. 

Results show sharp changes in physicochemical water properties across the mixing zone within the bay: pH, O2 and dissolved reactive phosphorus values increased with distance from the stream outlet, whereas major ion concentrations, temperature and conductivity values decreased.  Initial in-stream phosphorus stripping through alum dosing is effective in reducing the DRP load by ~50 % of background concentration. As alum is introduced to the stream water (~pH 6) it precipitates to form amorphous aluminium hydroxide (Al(OH)3(am)) and adsorbs phosphorus via hydroxyl ligand exchange. However, elevated levels of iron in amorphous hydrous ferric hydroxides Fe(OH)3(am) are also likely to be contributing to natural phosphorus binding capacity. Sediment core data also indicated that settled Al(OH)3(am) floc and Fe(OH)3(am) particulates were primarily concentrated within the inner portion of Te Wairoa Bay near the Waitangi Springs outlet. Surface water physicochemical and geochemical concentrations were spatially resolved and indicated distinct mixing boundaries (pH, DO, temp) and patchiness (Al, Fe, DRP) within Te Wairoa Bay associated with the locations and configuration of dense submerged macrophytes (Ceratophyllum demersum). Geochemical speciation modelling also indicated that the primary dissolved Al species was Al(OH)4- under the observed daytime conditions, and that Fe was primarily in colloidal form Fe(OH)3(am), which was confirmed through diffusive gradients in thin films (DGT) measurements. 

A diel sampling experiment also confirmed that as alum-dosed water enters the lake, daytime biogeochemical conditions driven by C. demersum alter physicochemical water properties from ~pH 6 to ~pH 9, and DO (supersaturation) via increased photosynthesis.  Diel-fluxes in geochemical solubility (Al, Fe, and DRP) responded to physicochemical shifts (pH, O2,) and demonstrate that C. demersum have the capacity to influence Al solubility and DRP availability within Te Wairoa Bay.
 
This work highlights the complexity of biogeochemical processes within aquatic freshwater ecosystems. Moreover, the results emphasise the need to account for the significant spatial and temporal heterogeneity of physicochemical parameters in the development of effective lake remediation strategies.

text: Biogeochemical Characterisation of an Alum Dosed Stream: Implications for Phosphate Cycling in Lake Rotoehu Lakes are highly valued freshwater ecosystems which provide many goods and services upon which humans are reliant. Eutrophication of waterways, driven by the over-enrichment of nutrients such as phosphorus and nitrogen, is a threat to the future quality of water resources globally. Lake restoration methods are increasingly being employed to improve eutrophic waterways, via numerous catchment derived and in-lake approaches. However, spatio-temporal heterogeneity in physicochemical and biogeochemical conditions within lakes may restrict the efficacy of restoration methods.  This thesis explores in-stream alum dosing as a lake remediation strategy for the purpose of dissolved phosphorus inactivation, and its physicochemical and biogeochemical interaction and fate within Lake Rotoehu, New Zealand.

 Laboratory and field measurements were undertaken at Lake Rotoehu, on New Zealand’s North Island. The shallow, polymictic lake resides in an active volcanic area with geothermal inputs, and is subject to several management issues including elevated nutrient concentrations, invasive macrophytes, and frequent cyanobacteria blooms. The geothermal Waitangi Springs, which discharge into Lake Rotoehu, contribute ~69 % of the lake’s total ionic content, and are responsible for enhanced concentrations of biologically limiting nutrients including phosphorus, silicon, nitrate, ammonium, and iron. In an effort to curb in-lake phosphorus levels, in-stream alum-dosing has been employed in order to floc out dissolved reactive phosphorus (DRP), through chemical adsorption and sedimentation.  

The results presented here, culminate from an investigation of the physicochemical and geochemical dynamics across the mixing zone from the Waitangi Springs geothermal stream outlet across Te Wairoa Bay to the main lake body.  A combination of approaches was used: two field experiments with fixed location and transect measurements, laboratory analysis and geochemical speciation modelling with PHREEQC. 

Results show sharp changes in physicochemical water properties across the mixing zone within the bay: pH, O2 and dissolved reactive phosphorus values increased with distance from the stream outlet, whereas major ion concentrations, temperature and conductivity values decreased.  Initial in-stream phosphorus stripping through alum dosing is effective in reducing the DRP load by ~50 % of background concentration. As alum is introduced to the stream water (~pH 6) it precipitates to form amorphous aluminium hydroxide (Al(OH)3(am)) and adsorbs phosphorus via hydroxyl ligand exchange. However, elevated levels of iron in amorphous hydrous ferric hydroxides Fe(OH)3(am) are also likely to be contributing to natural phosphorus binding capacity. Sediment core data also indicated that settled Al(OH)3(am) floc and Fe(OH)3(am) particulates were primarily concentrated within the inner portion of Te Wairoa Bay near the Waitangi Springs outlet. Surface water physicochemical and geochemical concentrations were spatially resolved and indicated distinct mixing boundaries (pH, DO, temp) and patchiness (Al, Fe, DRP) within Te Wairoa Bay associated with the locations and configuration of dense submerged macrophytes (Ceratophyllum demersum). Geochemical speciation modelling also indicated that the primary dissolved Al species was Al(OH)4- under the observed daytime conditions, and that Fe was primarily in colloidal form Fe(OH)3(am), which was confirmed through diffusive gradients in thin films (DGT) measurements. 

A diel sampling experiment also confirmed that as alum-dosed water enters the lake, daytime biogeochemical conditions driven by C. demersum alter physicochemical water properties from ~pH 6 to ~pH 9, and DO (supersaturation) via increased photosynthesis.  Diel-fluxes in geochemical solubility (Al, Fe, and DRP) responded to physicochemical shifts (pH, O2,) and demonstrate that C. demersum have the capacity to influence Al solubility and DRP availability within Te Wairoa Bay.
 
This work highlights the complexity of biogeochemical processes within aquatic freshwater ecosystems. Moreover, the results emphasise the need to account for the significant spatial and temporal heterogeneity of physicochemical parameters in the development of effective lake remediation strategies.

year: 2017

title: Geology, History and Stratigraphy of Hydrothermal Eruptions in the Rotorua Geothermal Field

authors: Cody, Ashley Douglas

subjects: Geothermal Hydrothermal eruptions Rotorua

abstract: The study area is the Rotorua geothermal field (RGF), which is located in the southern end of the Rotorua caldera. Distribution of surface geothermal activity, hydrothermal eruption (HE) deposits and craters have been mapped. HE deposits formed in geological, historical and modern times were studied to record and describe their distribution, composition, petrography, lithology and stratigraphy.

Locations of all identified HE sites in the RGF have been plotted and shown on accompanying maps. At Ohinemutu and Sulphur Bay, HE breccias have been preserved only where deposits were subsequently silicified by hot spring outflows. At Whakarewarewa two HE breccias outcrop over about 0.5 km² and are up to 23 m thick. Study of stratigraphic relationships with tephras of known ages has determined one as Holocene (c. 20 ka) and one of Late Pleistocene age (c. 65 ka).

Four HE breccias were studied in detail. The mineral composition, physical description, field characteristics and spatial distribution of HE breccias are described and mapped. Clasts and matrix were examined by XRD analyses of whole (67) and clay fraction separates (16). Silica crystallinity indices (14) were determined by XRD to identify devitrification rates of opaline mineral species. The petrography of 67 breccia clasts and matrix samples were studied, and polished sections showed the only sulphide to be pyrite. Geothermal alteration of all prehistorical HE deposits was of advanced argillic to argillic and one modern day deposit was of sub propylitic epithermal alteration.

Study of a HE deposit formed in Kuirau Park on 26 January 2001 allowed properties and mineral composition to be examined and recorded before rapid oxidation and weathering destroyed its physical and mineral composition. Study of this and other twentieth century HE events confirms that these rapidly decompose to leave little or no recognisable deposit in the geological record.

Descriptions and all known historical records of HEs in the RGF have been collated to provide a comprehensive inventory of maps and appendices of these events. This information will greatly assist statutory authorities with mitigation of risk to lives and property from geothermal hazards as well as aid appropriate land use and emergency services planning.

Hydrothermal eruptions are ongoing in the RGF through time and produce craters 2 - 110m diameter with erupted deposit volumes of 1 - 500,000 m³ that have distributed over <5 m² to >0.5 km². These have occurred throughout at least Late Pleistocene to Holocene times and appear to have been triggered by such events as volcanic eruptions from the adjacent Okataina Volcanic Centre and related abrupt changes to the waterlevels of Lake Rotorua. Frequency and spatial distribution of HEs in historical times suggest human interference with the geothermal system is also a significant influence.

This study has brought together all records of known hydrothermal eruptions and other unusual geothermal disturbances in the Rotorua geothermal field. The information has plotted into GIS for use by statutory authorities and others. Two prehistorical HE deposits in Rotorua city have also been recognised and described for the first time, together with identification of their likely vent locations and possible eruption initiating events.

text: Geology, History and Stratigraphy of Hydrothermal Eruptions in the Rotorua Geothermal Field The study area is the Rotorua geothermal field (RGF), which is located in the southern end of the Rotorua caldera. Distribution of surface geothermal activity, hydrothermal eruption (HE) deposits and craters have been mapped. HE deposits formed in geological, historical and modern times were studied to record and describe their distribution, composition, petrography, lithology and stratigraphy.

Locations of all identified HE sites in the RGF have been plotted and shown on accompanying maps. At Ohinemutu and Sulphur Bay, HE breccias have been preserved only where deposits were subsequently silicified by hot spring outflows. At Whakarewarewa two HE breccias outcrop over about 0.5 km² and are up to 23 m thick. Study of stratigraphic relationships with tephras of known ages has determined one as Holocene (c. 20 ka) and one of Late Pleistocene age (c. 65 ka).

Four HE breccias were studied in detail. The mineral composition, physical description, field characteristics and spatial distribution of HE breccias are described and mapped. Clasts and matrix were examined by XRD analyses of whole (67) and clay fraction separates (16). Silica crystallinity indices (14) were determined by XRD to identify devitrification rates of opaline mineral species. The petrography of 67 breccia clasts and matrix samples were studied, and polished sections showed the only sulphide to be pyrite. Geothermal alteration of all prehistorical HE deposits was of advanced argillic to argillic and one modern day deposit was of sub propylitic epithermal alteration.

Study of a HE deposit formed in Kuirau Park on 26 January 2001 allowed properties and mineral composition to be examined and recorded before rapid oxidation and weathering destroyed its physical and mineral composition. Study of this and other twentieth century HE events confirms that these rapidly decompose to leave little or no recognisable deposit in the geological record.

Descriptions and all known historical records of HEs in the RGF have been collated to provide a comprehensive inventory of maps and appendices of these events. This information will greatly assist statutory authorities with mitigation of risk to lives and property from geothermal hazards as well as aid appropriate land use and emergency services planning.

Hydrothermal eruptions are ongoing in the RGF through time and produce craters 2 - 110m diameter with erupted deposit volumes of 1 - 500,000 m³ that have distributed over <5 m² to >0.5 km². These have occurred throughout at least Late Pleistocene to Holocene times and appear to have been triggered by such events as volcanic eruptions from the adjacent Okataina Volcanic Centre and related abrupt changes to the waterlevels of Lake Rotorua. Frequency and spatial distribution of HEs in historical times suggest human interference with the geothermal system is also a significant influence.

This study has brought together all records of known hydrothermal eruptions and other unusual geothermal disturbances in the Rotorua geothermal field. The information has plotted into GIS for use by statutory authorities and others. Two prehistorical HE deposits in Rotorua city have also been recognised and described for the first time, together with identification of their likely vent locations and possible eruption initiating events.

year: 2003

title: Indigenous psychologies globally – a perspective from Aotearoa/New Zealand

authors: Nikora, Linda Waimarie

subjects: Maori

abstract: Discusses indigenous psychology from a global perspective and then from the perspective of Aotearoa/New Zealand.

text: Indigenous psychologies globally – a perspective from Aotearoa/New Zealand Discusses indigenous psychology from a global perspective and then from the perspective of Aotearoa/New Zealand.

year: 2004-06