Volcanic ash leachate compositions and assessment of health and agricultural hazards from 2012 hydrothermal eruptions, Tongariro, New Zealand

Journal of Volcanology and Geothermal Research 286 (2014) 233–247

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Volcanic ash leachate compositions and assessment of health and agricultural hazards from 2012 hydrothermal eruptions, Tongariro, New Zealand S.J. Cronin a,⁎, C. Stewart b, A.V. Zernack a, M. Brenna a, J.N. Procter a, N. Pardo a, B. Christenson c, T. Wilson d, R.B. Stewart a, M. Irwin a a

Volcanic Risk Solutions, Institute of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 442, New Zealand Joint Centre for Disaster Research, GNS Science/Massey University, Wellington, New Zealand c GNS Science, Lower Hutt, New Zealand d University of Canterbury, Christchurch, New Zealand b

a r t i c l e

i n f o

Article history: Received 24 March 2014 Accepted 3 July 2014 Available online 11 July 2014 Keywords: Fluoride Volcanic gas Hydrothermal Tongariro Volcanic ash Ash leachate Agricultural hazards Volcanic health hazards

a b s t r a c t After almost 80 years of quiescence, the upper Te Maari vent on Mt. Tongariro erupted suddenly at 2352 h (NZ time) on 6 August 2012. The short-lived hydrothermal eruption distributed a fine ash of minor volume (~ 5 × 105 m3) over 200 km from source. The threat of further eruptions prompted an investigation of the possible health and agricultural impacts of any future eruptions from this volcano, particularly since the most recent large-scale ash falls in New Zealand in 1995–1996 had generated significant agricultural problems, including livestock deaths. Deposited ash was sampled between 5 and 200 km from the volcano as soon as possible after the eruption. Two sub-lobes of ash were identified from different vent areas and displayed subtly different leaching properties. The first was an initial small lobe directed NNE, likely formed from drifting low-level clouds associated with the initial lateral explosive blast and surges. The main fall lobe, directed eastward, was sourced from a short-lived vertical plume that rose up to c. 8 km. Ash from the initial fall lobe had higher concentrations of F and Al, in single-step leaches as well as in the totals of three, sequential extractions. Further, the initial lobe showed a higher proportion of soluble F and Al extracted in the first leach, compared to totals. A linear relationship between concentrations of Al and F in single leaches from the 6 August eruption was highly significant (Pearson correlation coefficient r = 0.987 for 1:20 leaching ratios and r = 0.971 for 1:100), suggesting the presence of soluble alumino-fluoride com3 − x ). An even more significant 1:1 ratio is displayed for the largest concentration leached plexes (AlF+ x ions of Ca and SO4, which correspond to the presence of crystalline gypsum throughout the newly excavated hydrothermal system. Although no fresh magma was erupted in this event, a shallow intrusion prior to the hydrothermal explosion apparently provided significant contents of volcanic gas that was dissolved within the hydrothermal fluids and adhering to ejected particles. This and the ubiquitous presence of gypsum dominated the soluble components of these ash deposits leading to a complex leaching profile. The leaching study carried out here showed that agricultural and human health hazard assessment (particularly of F and S) is not straightforward, particularly because F solubility may be complex and not well characterised by simple leaching studies. In the case of S, which is agriculturally important, saturation effects are apparent using normal leaching protocols and also imply a need for modification of standard methods. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Small volume, but widespread ash falls were produced by two eruptions in August and November 2012 from the Upper Te Maari Crater area of Mt. Tongariro, New Zealand. The ash was distributed over several rural houses, a prison and grazed pastures, prompting concerns about possible health impacts associated with future ash falls from this vent. ⁎ Corresponding author. Tel.: +64 63569099; fax: +64 63505632. E-mail address: [email protected] (S.J. Cronin).

http://dx.doi.org/10.1016/j.jvolgeores.2014.07.002 0377-0273/© 2014 Elsevier B.V. All rights reserved.

The eruption was sourced from an active hydrothermal system, and, although substantial volcanic gas was produced, no primary magma was erupted (Pardo et al., 2014). Past experience on a neighbouring volcano (Cronin et al., 1998, 2003) showed that ash eruptions through hydrothermal systems generated significant chemical responses in soils, pastures and waterways, leading to major animal health impacts. This study aimed to evaluate the potential impacts of ash from the northern vents of Mt. Tongariro, as well as to investigate the role that hydrothermal systems may play in influencing the chemical characteristics of tephras. A further aim was to test the appropriateness of various methods

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of health hazard estimation from volcanic ash, through a comparison of single and multiple-leaching studies, as well as evaluation of gastric vs. water leaching methods on this complex ash. Volcanic ash (tephra b 2 mm) is the most widespread product of explosive eruptions and thus the most common environmental hazard or irritant faced by populations in volcanic areas. It has long been recognised that, in addition to the physical impacts of volcanic ash on life and infrastructure, the soluble and reactive surface compounds associated with ash may be harmful to people, grazing animals and a range of crops (Ayris and Delmelle, 2012). Chief amongst the hazardous chemical features of volcanic ash are its acidity and toxic concentrations of ions such as fluoride (Óskarsson, 1980; Cronin et al., 2003). For agricultural systems, F, S and Se cause significant impacts to animal health, pasture and soil compositions, with effects that may last up to a year after ash deposition (Cronin et al., 1997, 1998, 2003). Volcanic ash may have a low pH due to the condensation of strong mineral acids (primarily H2SO4 and HCl) in the cooling plume. This may cause damage on direct contact, for example acidic irritation of skin, eyes and mucous membranes (Weinstein et al., 2013) and acidic damage to crops (Cook et al., 1981). Strongly acidic surface coatings may also cause dissolution of materials that ash is deposited onto, bringing potentially toxic elements such as Cr and As into solution and facilitating their release into the environment. The soluble salts and aerosols associated with volcanic plumes in magmatic eruptions are the result of a complex series of interactions of primary gas with fragmenting magma and tephra particles in the upper conduit and various regions of the volcanic plume (Rose, 1977; Óskarsson, 1980; Halmer et al., 2002; Oppenheimer et al., 2003; Delmelle et al., 2007). Primary volcanic gases (H2O, CO2, SO2, HCl, NH3, H2S, HF and others; Giggenbach, 1996), adhere to particles in the form of salts and sublimates at N700 °C (Naughton et al., 1974), whereas acids may adhere directly onto volcanic glass surfaces between 340 and 700 °C, and within aerosol or fluid droplets, starting with H2SO4 below 340 °C and halogen acids b120 °C (Óskarsson, 1980). Ongoing interaction between aerosol acids and glass surfaces also occurs high in the ash column and as it is transported by wind away from the eruption centre (Delmelle et al., 2007; Bagnato et al., 2013). The chemical impact of ash once it lands depends not only on its own chemical cargo, but also on the substrate reactivity (e.g., acidsusceptible metals, copper-chrome-arsenate treated timber) and the amount of rainfall. Generally, most salts adhering to volcanic ash are rapidly released upon contact with water (e.g., Jones and Gíslason, 2008; Olsson et al., 2013). In some cases, more complex leaching behaviours are seen, particularly if eruptions occur through major hydrothermal systems or crater lakes (e.g. Armienta et al., 1998). In these cases, hydrothermal fluids and sub-vent or vent-filling hydrothermally altered materials are also erupted and distributed together with “juvenile” particles (i.e., fresh magma fragmented during an eruption). These additional fluids and particles include a range of sulphides and metal salts that are not normally associated with fresh magmas. Ashfall impacts on health and agriculture have also occurred when eruptions solely contain materials erupted from hydrothermal systems, without a juvenile component (e.g., Le Guern et al., 1980; Feuillard et al., 1983). During phreatomagmatic eruptions of Mt. Ruapehu, New Zealand, Cronin et al. (2003) noted that fluids from the Crater Lake and sub-lake hydrothermal system (including a pool of molten elemental S; Christenson and Wood, 1993) were erupted along with juvenile ash. In this case F and S had complex leaching behaviours, with a proportion immediately soluble, but a further proportion released over longer periods in a series of successive leaching experiments. F solubility was attributed to a mixture of highly soluble compounds formed in the high-temperature plume (e.g., NaF) as well as more slowly soluble −x components (e.g., AlF+3 complexes) formed before the eruption in x the hydrothermal system. Long-term soluble S production was attributed to high contents of insoluble elemental S within the ash that gradually oxidised, leading to long-term acid-generation and SO2− production. 4

These unusual ash-leachate features during phreatomagmatic Ruapehu eruptions contrast with the properties of open-vent magmatic eruptions (c.f., Jones and Gíslason, 2008), where adhering aerosols or dry salts are normally highly soluble. Eruptions through hydrothermal systems are common in the onset phases of volcanic activity in many andesitic and rhyolitic volcanoes around the world. Cases of purely hydrothermal eruptions, or those ejecting no juvenile magma component commonly do not produce widespread tephra fall. In the 1976–1977 eruption of Soufriere of Guadeloupe significant agricultural and health impacts were reported (Le Guern et al., 1980) in relation to gas emissions and ash contamination of crops and impounded water supplies. In this case, no juvenile magma was erupted, although juvenile magmatic gases were. Here a parallel situation is described, where a hydrothermal eruption, driven by shallow-level magmatic intrusion produced a widespread, but very low-volume ash fall. Leaching experiments on this ash revealed a complex set of properties that raise concerns when assessing health and agricultural impacts from eruptions involving hydrothermal systems. 1.1. Upper Te Maari crater and the 2012 eruptions The formation of the Upper Te Maari crater was associated with the final stages of emplacement of a large lava flow, around AD1500 (Topping, 1974). Subsequently the area has hosted a number of eruptions (Scott and Potter, 2014), with the largest known recent events in 1892 and 1896–97 (Hill, 1893, 1897; Friedlaender, 1898). Both the 1890s events were associated with explosions, high ash plumes, and the latter deposited ash up to 110 km to the east. Similar to the 2012 eruption, both of these eruptions were hydrothermal-system explosions, with little or no magmatic component. Subsequently, a broad area of hydrothermal activity has developed in this area, with a 3 km2 surface manifestation of steaming ground with several vents N90 °C mapped in 1976 (Hochstein, 1985; Walsh et al., 1998). Whilst not the largest area of hydrothermal alteration on Mt. Tongariro, the pyroclastic deposits and breccias excavated by the eruption are in places completely replaced by kaolin-dominated clay (Breard et al., 2014). Around three months before the 6 August 2012 Te Maari eruption, intrusion of magma into shallow levels occurred (Hurst et al., 2014), charging the hydrothermal system with gases and possibly additional heat. In addition to the magmatic gas dissolving into hydrothermal fluids and reacting with a range of hydrothermal minerals, overpressures built, leading to a sudden, shallow-level hydrothermal blast at 2352 h (all times NZ daylight time) (Breard et al., 2014; Lube et al.,). This eruption had two distinctive explosive pulses from separate vent areas. Firstly, initial laterally directed hydrothermal blasts occurred in the slopes above the Upper Te Maari crater, in association with a landslide. These explosions propelled large ballistic clasts east and west of the vent, and produced pyroclastic surges (Breard et al., 2014; Lube et al., 2014). An elutriated ash cloud associated with the surges rose to b1 km above the vent, with ash drifting NNW and being deposited in proximal areas (Pardo et al., 2014). Within seconds of the initial phase explosions, a vertically-directed explosion from the Upper Te Maari crater itself produced an 8– 10 km-high tephra plume (Crouch et al., 2014; Turner et al., 2014) that was rapidly dispersed by strong winds to the east. This high plume resulted in fine, thin ash fall visible on surfaces over hundreds of km2 (Pardo et al., 2014). After the brief 6 August eruption, degassing was intense in and around the vent areas. Measurements made under conditions where the gas plume could be driven under, using Flyspec mini-UV spectrometer system (http://www.flyspec-inc.com) following the eruption yielded estimates of between 1500 and 800 Mg/day of SO2 gas release during August–September. These corroborated with airborne gas measurements, showing a sudden rise from near non-detectable release to N2200 Mg SO2/day (Christenson et al., 2013). Following the degassing period, a small explosive eruptive burst occurred on 21 November

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from the Upper Te Maari crater and was associated with an additional degassing pulse of ~ 700 Mg SO2/day, reduced to b200 Mg SO2/day a week later. This second eruption produced a small vertical eruption plume, with minor ash drifting to the north and barely detectable off the volcano slopes. Degassing slowly decreased in the months following, but with far greater vigorous steam output than before the eruption.

2. Methodology Fresh ash from the 6 August 2012 eruption was sampled along state highways to the north (SH46) and east (SH1) of Te Maari vent, within 4–9 h of the eruption (Fig. 1, Table 1). Bulk samples and measuredarea samples (mass/area) were taken from available clean, flat surfaces. Intermittent rainfall occurred during and after the eruption, with raindrop marks in the ash deposit noted at some locations (Pardo et al., 2014). Bulk samples were also collected further afield on 7 and 8 August. Following the 21 Nov 2012 eruption, a very small amount of ash was collected five hours later from three sites (Table 1). In addition to these samples, streams on the northern flanks of Mt. Tongariro, downstream of Te Maari were sampled on 7, 8 and 12 August (the 12th after heavy rain and small rain-triggered lahars). For characterisation of ash leachate properties, samples were analysed in their field state, with moisture content determinations made on separate subsamples. Care was taken to avoid contact between

235

field-moist samples and metal implements. Samples were air-dried for subsequent archival storage. Extractions of the ash with Milli-Q grade deionised water (18 MΩ) were carried out at ratios of both 1:20 and 1:100 (g ash to mL water), as recommended by a recently-available protocol on analysis of volcanic ash samples for assessment of hazards from leachable elements (Stewart et al., 2013; available from www.ivhhn. org). Following an extraction time of 1 h on a table shaker, samples were centrifuged at 3000 rpm for 3 min, then immediately filtered through 0.45 μm nitrocellulose filters. Samples were filtered into separate containers for determination of anions and cations. For each sample, three repeated extractions were carried out in sequence to examine for more slowly soluble components (c.f., Cronin et al., 2003) and to test whether this provides a better health hazard estimate. Conductivity and pH measurements were made on the filtered samples. Fluoride determinations were made by both F ion selective electrode (ISE) at Massey University and ion chromatography (IC) at the National Isotope Centre, GNS Science. Sulphate and chloride in the leachates were measured by IC. Cation analyses were carried out by Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) at Hill Laboratories, Hamilton and the University of Canterbury, Christchurch, as well as Optical Emission Spectrometry (ICP-OES) Lincoln University, Canterbury. To estimate gastrically-available fractions of fluoride and other potentially toxic elements in the ash, extractions were carried out using the same equipment and general methods as for the deionised water leach, but using a solution of 0.032 M HCl (pH 1.5 ± 0.05) as

Fig. 1. Sampling locations for ashfall produced by 6 August 2012 eruption of Upper Te Maari.

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Table 1 Sample collection and location details. Sample ID

Collection date

Collection time

km from vent

Sa

E

Surface collected from

Ash samples JP-46 TM003 AZ-K3 TM002 TM001 AZ-K1 NP-K2 NP-K1 HK-K1 GIS VEN-K5 MBB1 MBB2 MBB3

7/08/2012 7/08/2012 7/08/2012 7/08/2012 7/08/2012 7/08/2012 7/08/2012 7/08/2012 8/06/2012 7/08/2012 8/08/2012 21/11/2012 21/11/2012 21/11/2012

10 h00 04 h50 09 h00 04 h40 04 h30 08 h50 09 h00 08 h55 ~15 h00 07 h00 13 h00 17 h30 18 h00 18 h30

5.7 6.3 7.0 7.2 8.2 10.9 11.4 12.1 24 200 153 5.2 5 4.5

39 04 58.353 39 06 10.927 39 04 48.243 39 05 29.191 39 04 51.307 39 04 59.041 39 06 52.982 39 05 42.171 39 07 26.310 38 59 25.910 39 02 30.420 39 04 48.719 39 04 25.175 39 05 42.143

175 43 39.201 175 44 37.834 175 44 31.218 175 45 06.281 175 45 31.000 175 47 32.884 175 48 09.978 175 48 33.743 175 57 29.650 177 47 19.050 177 25 05.520 175 43 04.763 175 42 20.916 175 43 6.8514

Wood (bark) Roadside CCA treated wood Roadside Roadside CCA treated wood Concrete Galvanised iron Plastic water tank cover, coated roofing metal Car roof Stainless steel (playground slide) Rock, leaves, wood Plastic tarpaulin Rubber pond liner

7/08/2012 12/08/2012 12/08/2012 7/08/2012 12/08/2012 8/08/2012

12 15 15 12 15 16

39 03 53.340

175 39 21.191

39 04 13.913 39 04 58.531

175 41 26.574 175 43 38.104

39 04 58.586

175 47 28.642

Streamwater samples Mangetetipua Stream Unnamed stream Tahurangi stream Mangahouhounui stream a

h00 h00 h00 h00 h00 h00

Degrees, minutes, seconds (WGS84).

the gastric-leaching solution (Stewart et al., 2013). Extraction ratios of 1:100 were used. Fluoride was determined by ISE, and cations by ICP-OES. Method blanks were run by omitting the ash from centrifuge tubes but subjecting the extracting solution to all other processing steps. Approximately one sample in every ten was run in duplicate. Duplicate samples produced results consistent to within ~5%. In addition to soluble SO2− measurement, elemental sulphur was 4 measured in ash residues from the H2O leaching process after mixing with weak HCl to remove any residual water soluble sulphur species. Elemental sulphur was dissolved in acetone by shaking 0.5 g of ash with 10 g of acetone in glass tubs over 24 h. After the acetone dissolution, the mixture was filtered and the residual dried. Ash sub-samples of 150 mg were taken before and after the acetone dissolution were measured for total S% concentration by combustion in an Elementar vario macro CUBE apparatus (http://www.elementar.de). Elemental S was determined as the difference between the two sub-samples. A selection of samples underwent X-ray Diffraction analysis, including fresh dried ash, ash residues containing insoluble S from the sulphur-speciation tests, as well as fine-grained pore materials from the hydrothermal vent area excavated during the eruption. Standard powder X-ray Diffraction was carried out using a GBC MMA with Co K-alpha radiation with scans from 3 to 50° 2 σ at 2° of 2σ/min. 3. Results 3.1. Major water-extractable species in ash leachates Levels of major water-extractable species Cl, F, SO4, Ca, Mg, Na, K and pH of extracts for single leaches and totals of three sequential extractions are shown in Table 2. Data are also shown graphically for all three sequential leaches at extraction ratios of both 1:20 and 1:100 for Cl (Fig. 2A), SO4 (Fig. 2B), F (Fig. 2C), and the four major cations (Fig. 3A–D). Samples are ordered with increasing distance from vent. Total levels of water-extractable chloride (Fig. 2A) ranged from 39 to 872 mg/kg (1:100) and 50 to 753 mg/kg (1:20). The low values recorded at site HK-K1 are almost certainly an artefact of postdepositional leaching as this sample was collected after almost two days had elapsed, and rainfall had occurred. The single leach efficiencies (proportion of total extracted by a single leach) were generally high (74–98%), for both 1:20 and 1:100 extractions. These findings

suggest that there are no saturation effects occurring for chloridebearing salts. There were no clear differences with respect to distance from vent. Total water-extractable S (as SO4) concentrations (Fig. 2B) ranged from 1806 to 14,363 mg/kg (1:100) and 1926 to 12,947 (1:20). Similar to the chloride analyses, the lowest concentrations were recorded at site HK-K1 where the ash was more strongly affected by rain. Single leach efficiencies were much lower than for chloride, particularly for the 1:20 extraction (34–73%, omitting HK-K1) compared to 55–91% for the 1:100 extractions. For all samples except HK-K1, there was a strong and consistent trend for more SO4 to be extracted in a single leach by a 1:100 extraction compared to a 1:20 ratio. This trend was much less apparent for totals of three sequential extractions. This result suggests saturation effects are seen in this case for sulphate-bearing salts. The highest concentrations occurred in the sample closest to the vent but there is only a weak decrease away from the vent. Total water-extractable F concentrations (Fig. 2C) ranged from 51 to 112 mg/kg (1:100) and 26 to 83 mg/kg (1:20). Concentrations of F at site HK-K1 did not show the depletion relative to other sites as seen in the Cl and SO4 results, suggesting that F may be present in less soluble forms than the latter two. As for sulphate, there appears to be a weak overall decrease in both single leach and total concentrations away from the vent. A subset of samples (‘NE’, referring to those in an ash lobe distributed to the north east of the vent) can be differentiated from the other samples on the basis of higher single leach F concentrations. These samples are from the ash elutriated from the pyroclastic surges of the eruption (JP-46, AZ-K3 and TM001) that drifted at a low level to the NNE. In these surge-cloud samples, there is little difference between amounts extracted by 1:20 and 1:100 leaches, and higher single-leach efficiencies. Water-extractable fluoride concentrations were substantially higher in the three 21 November 2012 ash samples (single leach 123–151 mg/kg; totals 223–246 mg/kg; see Table 2). Ratios between S, Cl and F on a molar (rather than mg/kg) basis are shown in Table 2, for water-extractable totals. Ash leachates are strongly dominated by S with S/F ratios ranging from 7.4 to 42, and S/Cl ratios from 3.7 to 22. For the major cations (Fig. 3A–D), water-extractable concentrations in single leaches decrease in the order Ca NN Mg,Na N K. Lowest concentrations are observed at site HK-K1, suggesting losses due to leaching in

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 286 (2014) 233–247

237

Table 2 Major water-extractable species on ash (reported as mg/kg) for samples collected during 6 August 2012 and 21 November 2012 eruption of Tongariro volcano. Sample

Distance (km)

pH

Ratio

F

SO4

Cl

Ca (mg/kg)

Mg

SL

K

Na

S/F

S/Cl

Cl/F

Molar ratiosc SLa 6 August eruption JP46

5.7

4.7

TM003

6.3

4.2

AZK3

7.0

4.4

TM002

7.2

4.3

TM001

8.2

4.3

AZK1

10.9

4.9

NPK2

10.5

4.7

NPK1

12.1

4.9

HK-K1

25

4.5

VENK5

153

GIS

200

5.2 5.8

21 November eruption MBB3

4.5

4.5

MBB2

5.0

4.7

MBB1 a b c

5.2

4.8

Totalb

1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100 1:20 1:100

66 68 13 28 41 43 15 30 70 71 23 38 14 29 15 20 17 27

79 91 32 59 67 65 37 57 83 86 66 75 43 81 30 51 26 48

13 9 25

36 112

1:20 1:100 1:20 1:100 1:20 1:100

89 123

148 223

131 107 151

243 173 246

SL

Total

SL

Total

SL

9063 12,384 5321 8406 5687 7828 5381 9690 9369 10,790 3838 7036 4564 7694 3977 6612 1616 1577

12,460 14,363 10,108 10,610 11,299 11,842 11,023 11,972 12,947 11,826 11,420 12,881 10,543 12,388 7334 8783 1926 1806

723 734 675 557 250 240 651 621 558 559 181 196 588 748 681 647 16 19

753 766 706 628 297 256 686 645 587 580 224 220 678 765 716 872 50 39

4564 5986 2181 3602 1565 2931 2244 4113 3598 5033 1936 2903 1778 3798 3448 2997 680 743

5203 8750

497 12,165

628

Total

4254 5274 5282 5600 4979 5247 4279 5903

2574

245 318 195 262 94 134 192 278 236 288 97 156 83 141 286 232 29 32

Total

267 314 280 306 191 214 162 192

215

SL 110 110 76 83 56 70 78 96 101 108 51 60 66 112 129 78 26 27

Total

112 122 132 127 113 103 118 152

206

SL 186 260 279 193 64 83 157 212 178 195 77 97 74 113 227 157 27 65

Total

31

6.9

4.5

36

6.2

5.7

36 194 275 213 252 132 145 123 167

17

2.1

42

6.8

6.1

27

7.5

3.6

34

22

1.6

30

6.0

5.1

34

3.7

9.1

7.4

17

0.4

318

668

21

4386

6985

336

380

106

123

393

458

4482

7566

349

395

113

133

413

486

6733

8590

456

486

105

116

482

552

6.7

3.2

Single leach (first extraction). Total of three sequential extractions on same sample. Molar ratios calculated for total of three sequential extractions at 1:100.

this sample. There are no consistent trends with respect to distance from vent. For all elements at most sites, higher concentrations were extracted at ratios of 1:100, especially for Ca, suggesting saturation effects. Full analyses of sequential extraction leachates were carried out on a subset of samples showing least effect from rain (TM001, TM002, AZ-K1 and NP-K2). Single leach efficiencies generally resembled those for sulphate.

Fe N Al NMn N Zn N Cu,Co, Ni. The highest levels of Al occurred in the high-F subset of samples. Interestingly, levels of Fe were markedly higher in all five samples located closest to the vent. Sample TM001 also contained higher concentrations of Co and Ni.

3.3. Sulphur speciation 3.2. Minor water-extractable elements in ash leachates Total concentrations (from three sequential leaches) of Al, As, B, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni and Zn were carried out on a subset of samples, with single leaches on all others (Table 3). The multiple leaches showed no strong systematic trends. For the other elements, most sites had concentrations below detection limits. For some elements, highly anomalous results at some sites indicate contamination from the sampling surface. A Cu concentration of 52 mg/kg was recorded in sample AZ-K1, collected from a wooden fence post. Concentrations of As and Cr were also elevated in this sample, suggesting contamination by CCA (copper-chrome-arsenate) preservative. Similarly, a level of 437 mg/kg Zn was recorded in sample NP-K1, collected from a galvanised steel roadside barrier. Whilst standard methods (such as ash collection procedures housed on http:// www.ivhhn.org/guidelines.html) recommend that a small margin is left at the base of an ashfall deposit to preclude contamination problems, in practice this is difficult for thin ashfalls and locations where there are a lack of flat surfaces. These results show that it is very important that the collection surface is recorded. Omitting samples suspected to be contaminated, waterextractable concentrations of minor elements decrease in the order

Soluble sulphur, elemental sulphur, and insoluble inorganic sulphur are 15, 15, and 70% of total S, respectively and show little variation amongst sites (Table 4). All proximal and medial samples contained between 0.24 and 0.58% S0, which appeared also in surge deposits and ballistics as yellow and grey vein-fills (Breard et al., 2014; Lube et al., 2014). Pyrite and rare other sulphide minerals were also noted in the proximal surge deposits and ballistics, and made up often N3 wt.% of the ash (Table 4).

3.4. X-ray Diffraction analyses The powder XRD analysis showed that crystalline gypsum was ubiquitous in the Te Maari ash as well as pyrite. All residual ash from the sulphur speciation experiments was shown by XRD to contain pyrite. Analysis of the clays and matrix samples from the Te Maari hydrothermal system revealed that the highly altered zones are dominated by kaolin, with small amounts of smectite. In lessaltered matrix samples, XRD patterns showed typical igneous mineral components (feldspar, pyroxene) as well as quartz, cristobalite and the ubiquitous sulphur minerals gypsum and pyrite.

238 S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 286 (2014) 233–247

Fig. 2. Concentrations of water-extractable Cl (A), SO4 (B) and F (C) in first, second and third sequential leaches at extraction ratios of 1:20 and 1:100.

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 286 (2014) 233–247 239

Fig. 3. Concentrations of water-extractable Ca (A), Mg (B), Na (C) and K (D) in first, second and third sequential leaches at extraction ratios of 1:20 and 1:100.

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Table 3 Total concentrations of water extractable minor elements on ash samples collected during 6 August and 21 November 2012 eruption of Tongariro volcano. Ratioa

Sample

Surfaceb

Al

As

B

Cd

Cr

Co

Cu

Fe

Pb

Mn

Mo

Ni

Zn

All as mg/kg dry weight ash 6 August 2012 samples TM002 1:20 TM001 1:20 AZK1 1:20 NPK2 1:20 21 November 2012 samples MBB3 1:20 MBB2 1:20 MBB1 1:100 Global mediansd a b c d

Roadside Roadside Wooden post Concrete

32 119 83 28

0.025 0.025 0.043c bdl

0.80 1.0 0.80 1.4

0.029 0.044 0.012 0.018

0.015 bdl 0.23c bdl

0.44 1.9 0.72 0.29

0.28 1.2 52c 0.23

403 349 35 100

0.033 0.044 0.0092 0.014

22 22 16 7.4

0.0072 0.0081 bdl bdl

0.81 2.4 1.0 0.46

1.8 3.8 16.0 1.2

Rubber cover Plastic tarpaulin Leaves, rock, timber

98 88 130 58

bdl bdl bdl 0.131

nd nd nd 2.6

bdl bdl bdl 0.053

bdl bdl bdl 0.096

bdl bdl bdl 0.186

0.45 0.40 1.25 5.0

31 13 70 21

bdl bdl bdl 0.114

34 36 38 20

bdl bdl bdl 0.063

0.19 0.21 0.23 0.50

4.8 4.7 7.3 3.6

Ratio of grammes ash to mL deionised water. Surface that sample was collected from. Suspected contamination from surface. Median concentrations reported for trace metals in volcanic ash (Ayris and Delmelle, 2012).

3.5. Inter-element correlations

3.7. Normalisation to Mg

Strong positive correlations were found between the following pairs of elements in 1:100 single leach solutions: Ca and SO4; F and Al; K and Cl (Fig. 4 A and B; Table 5). The Ca vs SO 4 trend of Te Maari samples is closer to the anhydrite line, compared to the more S-rich Ruapehu 1995 ash samples (Fig. 4A; Cronin et al., 1998, 2003). In the Ruapehu case, the Crater Lake hydrothermal system was much longer lived with higher inputs of volcanic gas, leading to an accumulation of elemental S in the system (Christenson, 2000). For Na and Cl, the value of r increased from 0.754 (‘significant’) to 0.983 (‘highly significant’) when data for sites NP-K1 and NP-K2 were removed from the correlation. All other correlations between elements were weaker (r b 0.898 for nine data pairs).

Concentrations of Ca, K, Na, Cl, F, SO4, Al, Mn, Fe, Co and Ni on a molar basis were normalised relative to concentrations of Mg (Fig. 6A and B). Mg was found to be a good indicator of fresh volcanic gas input in the gas-fed hydrothermal Crater Lake system at Ruapehu

3.6. Trends in single-leach efficiency The proportions of each anion and cation extracted by a single leach compared to the total of three sequential leaches show a generally decreasing trend with distance, but with a high degree of inter-site variability (Fig. 5A–B). The leaching efficiency decreased in the order Cl N SO 4 N F for the anions. For major cations, Mg had the highest leaching efficiency and all showed minor decreases with distance. For minor elements, the leaching efficiencies decrease in the approximate order of Mn N Al N Fe, Co, Ni, Zn N Cu. The trends are variable, but the overall rate of decrease is highest for Fe, Ni and Co.

Table 4 Sulphur species in ash samples collected after 6 August 2012 eruption of Tongariro volcano. Sample

Compass bearing from vent

Distance from vent (km)

S0 (%)

Insoluble S (%)

Pyrite (%)a

S as SO4 (%)

Total S (%)b

JP46 TM003 AZK3 TM002 TM001 AZK1 NPK1 NPK2 HK-K1 GIS

60 84 63 82 68 75 85 95 97 79

5.7 6.3 7.0 7.2 8.2 10.9 12.1 10.5 25 200

0.34 0.41 0.58 0.45 0.34 0.31 0.24 0.53 0.32 0.069

1.97 1.76 1.77 1.80 1.92 1.99 1.25 1.77 1.15 1.76

3.69 3.30 3.32 3.36 3.59 3.72 2.35 3.30 2.16 3.29

0.48 0.36 0.39 0.40 0.40 0.42 0.30 0.41 0.07 0.41

2.79 2.53 2.74 2.65 2.66 2.72 1.79 2.71 1.54 2.23

a b

Calculated from the Insoluble S, based on the S content of pyrite of 53.45%. Total of elemental sulphur, pyrite-S and sulphate-S.

Fig. 4. (A) Water extractable Ca vs. S for ash from 6 August 2012 eruption of Tongariro and 1995–1996 eruptions of Ruapehu (1:1 line shown). (B) Water extractable F vs. Al for 6 August 2012 Tongariro ash.

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 286 (2014) 233–247 Table 5 Highly significant (p b 0.001) interelement associations in Tongariro ash leachate (single leach, 1:100 extraction ratio only). Element pair

Number of data pairs

Pearsons correlation coefficient r

Inferred mineral phase

Ca, SO4 Al, F K, Cl Na, Cl Na, Cla

9 10 9 9 7

0.978 0.970 0.912 0.754 0.983

Gypsum/anhydrite AlxF3 − x complexes Karnelite Halite Halite

a

Excluding data for sites NP-K1 and NP-K2 which were found to lie off straight line.

(Christenson and Wood, 1993). SO4:Mg ratios were 10:1, whilst Cl: Mg was around 1:1 and F:Mg generally much lower. Of the cations, Ca tracked closely with SO4 at tenfold the Mg concentration, whilst Na:Mg was around 1:1, K:Mg steady at ~ 0.2:1, Al:Mg more variable around 0.1–0.2:1 and the other trace metals considerably lower.

241

Few trends are discernible with increasing distance, other than Fe: Mg ratios starting at ~ 0.3:1 in proximal samples and b 0.1:1 in medial to distal samples. 3.8. Streamwater samples The composition of the few water samples collected from streams draining Mt Tongariro (Fig. 1) during the week following the 6 August 2012 (Table 6) is shown for the same range of elements determined in ash leachates. For Tahurangi stream and Mangetetipua Stream, samples were collected on both 7 August and 12 August, the latter following heavy rain and a small landslide-lake breakout lahar. Enrichment factors (EF) were calculated by dividing values recorded on 12 August by those recorded on 7 August. Where they could be calculated, EFs for all elements were systematically higher in Tahurangi Stream compared to Mangetetipua Stream by factors of approximately two to four. The highest EFs were recorded for the elements Al, Fe and Mn. The Tahurangi

Fig. 5. Trends in single-leach efficiency (% extracted in a single leach in comparison to the total extracted by three successive leaches) for: (A) Anions, Cl, SO4, and F, and (B), cations Ca, Mg, Na, K.

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3.9. Comparison of extractions using deionised water versus gastric leach solution In addition to pure-water leaching, and to assess more broadly the potential agriculture and health impacts of the Te Maari ash, an additional simulated gastric leach (cf., Morman et al., 2009; Plumlee and Morman, 2011) was carried out for three ash samples (AZK3, TM001 and NPK2) from the 6 August eruption and one sample (MBB2) from the 21 November eruption (Table 7, Fig. 7). The gastric leach extracted greater quantities of elements for all elements and in almost all samples compare to the pure water extractions, with the gastric leach:deionised water extraction ratio highest at 19–93 for Fe, followed by 13–19 for Al. For F, the gastric leach: deionised water extraction ratios are 2.2–2.8, and for Ca, Mg, Na, K, Cu and Mn, ratios are generally b2. 4. Discussion 4.1. Spatial and temporal variation in data and volcanic ash source

Fig. 6. Trends in elemental concentrations in 1:100 single leaches normalised to Mg with distance from vent for: (A) anions and (B) cations.

catchment was affected only by ashfall, whereas the Mangetetipua had less ash deposited, but instead a large landslide (Procter et al., 2014), which collapsed to form lahars on August 12th.

The characteristic feature of the Te Maari ash is that it contained no fresh magmatic lithologies. Two sub-lobes of ash were recognised from the short-lived main August 6th eruption (Pardo et al., 2014). The first was most likely generated from elutriated ash from the initial blast and surges, and drifted at low altitude (b2 km) toward the NNE whilst depositing thin, fine ash. The second, from the subsequent crater-centred explosion, was distributed eastward from a N8 km high plume (Crouch et al., 2014; Turner et al., 2014). The vent areas of these two eruptions differed, although no juvenile magmatic component was seen in either of the lobes (Pardo et al., 2014). Ash in the first pulses of this eruption was derived from shallow (b50 m) volcaniclastic deposits that were explosively fragmented and excavated by hydrothermal blasts, released from an over-pressurised and sealed hydrothermal system (Breard et al., 2014). Investigations of the area collapsed in the earliest blasts and surgeproducing events showed that the deposits comprised coarsely bedded 2–5 m-thick pyroclastic breccias and mass-flow deposits associated with the construction of the Upper Te Maari crater, along with older pyroclastics and spatter of the main Tongariro edifice. These deposits are

Table 6 Surface water composition in depositional area of 6 August 2012 eruption of Tongariro volcano. Bold values exceed Maximum Acceptable Values and shaded values exceed Guideline Values for aesthetic properties (Ministry of Health, 2008). pH

F

SO4

Ca

Mg

Na

K

Al

As

Co

<0.001

Cu

Fe

Ni

Mn

Zn

<0.001

All as mg/La,b Mangatetipua Stream (NNW of vent) 7/08/2012

7.05

12/08/2012

0.03

15.9

4.67

5.03

1.59

0.086

<0.001

0.078

38.5

6.7

4.23

2.04

2.47

<0.001

0.96

306.3

16.6

7.57

3.83

0.0019

0.029

0.11

<0.01

0.0025

0.0086

0.54

0.0045

0.33

0.02

0.0451

0.02

0.5

0.022

2.52

0.052

<0.001

Unnamed stream (NNE of vent) 12/08/2012

11.9

0.0018

Tahurangi Stream (NE of vent) 7/08/2012

6.85

12/08/2012

0.053

22.3

0.274

131.6

2.89 19.2

3.44

1.05

0.014

<0.001

9.38

5.11

1.12

<0.001

<0.001 0.0088

<0.01

<0.001

0.032

0.0137

0.17

0.016

1.06

1

0.2

<0.01 0.057

Mangahouhounui Stream (ENE of vent)c 8/08/2012

7.4

MAVsd Guideline valuesd

0.05

7.05

<0.001

1.5 7.0–8.5

0.01 250

200e

200

0.08

0.10

0.4 0.04

1.5

Enrichment factorsf

a

Tahurangi Stream

5.2

5.9

6.6

2.7

4.9

80

>8.8

>14

>17

>16

Mangatetipua Stream

2.6

2.4

1.4

0.8

1.3

29

>2.5

4.5

19

>4.5

Dissolved fractions of metals determined. Cr, Cd and Pb were also determined but concentrations were all b0.001 mg/L and are not shown. c Data courtesy of Annaka Davis, Health Protection Officer, Toi Te Ora Public Health. d Maximum Acceptable Values for determinands of health significance and Guideline Values for aesthetic properties (Ministry of Health, 2008). e Guideline value for water hardness is 200 mg/L (Ca + Mg). f [X]12Aug/[X]7Aug. b

33 3.0

>5.7 >2.0

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243

Table 7 Extractions of four ash samples with deionised water and a simulated gastric leach. F

AZK3

TM001

NPK2

MBB2

11 2 3 Total 1 2 3 Total 1 2 3 Total 1 2 3 Total

Ca

Mg

Na

K

Al

Cu

Fe

Mn

W2

G3

W

G

W

G

W

G

W

G

W

G

W

G

W

G

W

G

31.2 5.9 4.5 42 34.1 5.9 4.9 44.9 45.0 9.8 6.6 61 131 63 49 243

73.3 20.4 19.9 114 72.3 24.1 11.8 108 82.4 27.5 26.7 137 429 212 48 689

2210 131 55 2396 4531 189 55 4774 4443 171 30 4644 4482 2064 1020 7566

4143 275 167 4584 6095 326 101 6523 5607 336 168 6112 7339 726 89 8154

160 4.5 3.0 168 336 15.1 3.5 354 219 7.5 0.2 227 349 37.0 8.0 394

176.9 26.5 50.8 254 344.1 40.3 26.5 411 251.8 34.6 44.1 331 507 49.2

124 54.7 40.1 219 167 60.4 36.0 263 139 41.0 38.2 218 413 53.3 19.5 486

92.1 23.5 45.1 161 191 75 65 331 126 3.7 8.8 139 457 91

130 14.4 5.3 150 32.8 14.7 9.2 57 91.4 9.9 7.4 109 113 14.7 5.1 133

107.4 29.7 37.2 174 72.6 32.7 19.5 125 147 22.8 17.2 187 153 35.6

45.8 1.3 4.1 51 77.2 3.1 1.3 81.6 59.4 3.1 0.9 63.3 59.2 18.5 10.0 88

433 205 315 954 453 352 260 1066 415 204 27 896 1124 528 25.4 1678

5.9 0.5 0.4 6.8 6.2 0.9 0.5 7.6 3.2 1.5 0.5 5.2 0.1 0.2 0.1 0.4

7.5 0.5 1.8 9.8 15.9 1.9 0.7 18.5 7.2 0.6 2.4 10.1 6.4 4.4

107 4.2 3.3 114 42.2 7.7 4.0 54.0 40.8 4.6 0.8 46.2 12.1 0.5 0.1 12.7

597 486 1069 2152 656 1159 939 2754 482 463 904 1849 405 721 51 1178

10.4 0.4 0.2 11 24.1 1.2 0.4 25.7 10.2 0.6 0.1 10.9 32.0 3.0 0.7 35.7

13.2 1.4 5.2 20 39.6 2.4 0.9 43.0 15.6 1.6 2.0 19.2 40.5 3.8 0.5 44.8

556

548

188.5

10.8

1 Extraction 1 (etc.) 2 1:100 extraction of sample with deionised water. 3 1:100 extraction of sample with simulated gastric fluid (HCl, ph 1.5 ± 0.05) as per Stewart et al. (2013) (data in italics).

poorly sorted, with some containing large coherent lava clasts within fine-grained matrixes that are often strongly hydrothermally altered. Pyroclastic breccias are generally the least altered of the exposed deposit suite and contain large block-sized scoriaceous clasts within an ash or scoria lapilli matrix. Several m-thick and tens of m-long regions are completely pseudomorphed by kaolin and smectite clay of white, red and yellow colouration. Several ballistic clasts sourced from the main eruption fissure displayed solidified rivulets of elemental sulphur, pyrite and chalcopyrite. Ash lithlogy (Pardo et al., 2014), is similar, i.e., fragmented equivalents of the source vent lithologies seen, including fragmented lava and spatter lithics of varying degrees of alteration, free crystals, along with hydrothermally altered materials. The second, and main, fall-producing phase of this eruption may have involved deeper excavation of materials due to being focussed in the pre-existing Upper Te Maari Crater. The eruption excavated material only from the central part of this crater (Procter et al., 2014), presumably conduit fill breccias collapsed in from earlier episodes of volcanism. The ash componentry differed little from the NE lobe (Pardo et al., 2014). XRD analysis showed that the initial ash contained highest quantities of gypsum and pyrite, with the second lobe containing less gypsum, consistent with the eruption mining deeper into the hydrothermal system.

Fig. 7. Comparison between water (‘w’ left columns) and gastric leach-extractable (‘g’ right columns) fluoride concentrations for three successive leaches in four ash samples.

Differences between the two different vent areas sourcing ash for this eruption were not strongly manifest in ash componentry, but the ash from the two lobes showed slightly different leaching characteristics. Firstly, there was a different F behaviour in ‘NE’ group of samples, particularly with a higher leaching efficiency from a single leach in JP46, AZK3, and TM001 (Fig. 2C). This likely indicates greater contents of HF or NaF associated with these initial samples, which may indicate an area of localised magmatic gas input under the site of the initial hydrothermal blasts. In addition, there is also a higher leachable Fe content in the five most proximal samples (also seen in Fe/Mg ratios) (Fig. 6A), which also have the lowest pH. The acidity or oxidation state of the proximal ash may have maintained Fe in a soluble form in these samples, or it could have been sourced from acid-dissolution of the substrate ash (cf., Bagnato et al., 2013). The clear association of Ca and SO4 in a 1:1 ratio (Fig. 4), along with the ubiquitous detection of gypsum in the ash and hydrothermal system shows that the soluble sulphur in the Te Maari ash was not formed by high-temperature gas/ash interactions in the plume (as per normal magmatic eruptions), but was inherent from the materials blasted out from the shallow volcanic flanks. In Ruapehu 1995/1996 ash, which also had significant contributions from the hydrothermal system (Cronin et al., 1998), much greater elemental S content was associated with spherical S0 particles found in the ash. This and other features were derived from a deeper, higher-temperature hydrothermal system than was the case at Te Maari. Whilst leachable S in the Te Maari ash may have been derived to a great degree from the hydrothermal system, high levels of degassing (two-three orders of magnitude greater than pre-eruption degassing) measured by flyspec surveys following the hydrothermal eruption either shows that there was considerable S-rich gas trapped in the deeper hydrothermal system, or the eruption occurred in response to new magmatic gas. Seismic precursory signals indicate possible magma movement within a few km of the surface prior to the eruption (Hurst et al., 2014), and isotopic ratios of magmatic gases show a magmatic signature (Christenson et al., 2013). Thus, whilst the eruption physically ejected no new magma, magmatic gas was likely erupted and experienced interaction with low-temperature particles blasted from shallow country rock deposits. The erupted hydrothermal system components were most likely b200 and N115 °C, indicated by the presence of molten sulphur in ballistically ejected particles and also measurements made by thermal infrared camera from the fresh eruption scars (Breard et al., 2014). The slow F leaching behaviour in the Te Maari ash was similar to that of Ruapehu 1995/1996 (Cronin et al., 2003). In Te Maari, the slowly-

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soluble forms of F present appear mostly to be within Al-F complexes (c.f. Fig. 4). There is a general decrease in single leach efficiencies with increasing distance (Fig. 5) even over the small range of distance sampled (7–12 km). This could indicate ongoing chemical interaction of the soluble chemical cargo on the ash surface during transport, which would be unlikely given the low-temperature, non-magmatic plume. The trend could also reflect the different substrate conditions of the more distal and lower-elevation samples. In these the pH measured is lower, possibly indicating that buffering interactions have already occurred with the substrate, with an accompanying greater complexation of surface ions. 4.2. Fate of leachable elements Rapid loss of readily-soluble elements must have occurred in the heavy rains that occurred several days after the ashfall, leading to transient elevations in dissolved constituents in local streams. Four streams draining the northern and northeastern flanks of Mt Tongariro, were sampled on 7 and 8 August (prior to heavy rainfall) and 12 August (during heavy rainfall) (Table 6). Where possible, enrichment factors (EFs) were calculated for each constituent X as [X]12Aug/[X]7Aug. Almost all constituents were higher in the 12 August samples leading to EFs N 1. The elements exhibiting the greatest enrichments were Al, Fe, Mn, Cu, Ni and Co, followed by Zn, Ca, Mg, K, F and Na. Dramatic post-eruption increases in concentrations of dissolved Fe, Al and Mn have also been noted for streams affected by the 1980 Mt St Helens, USA, eruption (Klein, 1984), the 2000 Mt Hekla, Iceland, eruption (Flaathen and Gislason, 2007) and the subglacial eruption beneath the Vatnajökull glacier, Iceland in 1996 (Gíslason et al., 2002). Enrichments of streamwaters appear to be more strongly controlled by pre-existing stream composition than by the surface composition of the ash; for example, the 6 August ashfall described here is strongly dominated by Ca but this element is only modestly enriched in streams draining the depositional area. One of the streams draining Mt Tongariro (Mangahouhounui Stream, Fig., 1) is used as a raw water source for a drinking water treatment plant serving 220 people. Sampling carried out by public health officials on 8 August for this stream showed that pH, fluoride, sulphate and dissolved arsenic all remained within their normal ranges (Leonard et al., 2014–in this issue) (Table 6). The water treatment plant, however, experienced other impacts such as increased turbidity due to the eruption. 4.3. Indicative calculations of hazards for drinking-water supplies due to leachable elements In general, leachable elements from ashfalls are likely to pose few hazards for consumers of water supplies. Levels of dissolved elements such as Fe, Al and Mn may be elevated above chronic exposure guideline values, particularly during the first heavy rainfall event following an ashfall, but this is likely to be a short-lived effect, due to the small amount of ash in this case. Physical impacts of suspended ash on operation of water treatment plants, and water shortages, are likely to be much more significant consequences of an ashfall (Stewart et al., 2006; Wilson et al., 2011; Stewart et al., 2009). The case of roof catchment rainwater tanks needs to be considered separately; these have increased vulnerability to the effects of ashfall due to their high surface area to volume ratio (Stewart et al., 2006). Most rural New Zealand households (and 10% of New Zealand households overall) rely on rainwater tanks. The depositional area of the 6 August Mt Tongariro eruption was sparsely populated, and only b12 households received measurable ashfall. Due to the small scale of the event, local emergency services cleaned ash from roofs and water tanks and refilled them with potable water on an individual basis. This approach would not have been practicable in the event of a more

widespread ashfall; hence we present indicative calculations for predicting the effects of the ashfall on rainwater tank composition. A simple model to predict concentrations in roof catchment rainwater tanks (Stewart et al., 2013) is as follows: Cwater ¼ Cash  L  A=V

ð1Þ

where Cwater is the predicted concentration in tank water in mg/L, Cash is mg soluble element per kg ash from leachate characterisation, L is ash loading in kg/m2 and is determined from measured-area sampling, A is the roof catchment plan area in m2 and V is the volume of water in the tank in litres. Indicative calculations for the elements noted already for high values: F, Al, Mn and Fe, as well as As (a typical element of interest to public health authorities) for two contrasting scenarios are shown in Table 8. The roof area and tank volume dimensions are based on a typical rural household rainwater collection system in New Zealand. Values of Cash are from this study, and values of L based on measured-area sampling are reported by Pardo et al. (2014). For the worst-case scenario, the water tank is assumed to be at its minimum storage volume (roof area = 92 m2 with storage volume of 1.3 m3); the maximum end of the concentration ranges recorded; and the maximum loading for this event is recorded by Pardo et al. (2014). For the realistic scenario, we assume that the water tank is 50% full and we have used measured data for loadings and Cash values for site AZ-K3, located nearest to households at the SE end of Lake Rotoaira. For the worst-case scenario, all five elements are predicted to equal or exceed health-based Maximum Acceptable Values (MAVs) and Guideline Values (GVs) set for elements that can affect the appearance, taste or odour of drinking water (Drinking-Water Standards for New Zealand (revised 2008): http://www.health.govt.nz/publication/ drinking-water-standards-new-zealand-2005-revised-2008-0). However, it is important to note that (1) MAV levels are set on the basis of a lifetime exposure at a daily drinking-water consumption rate of 2.1 L/day for adults and 1.2 L/day for children, and are not relevant for assessing the effects of short-term exposure to high concentration and (2) concentrations of Fe, Al and Mn exceed potability-based GVs by large ratios, and therefore it is unlikely that consumers would find the water potable. For the realistic scenario, F and As are well below MAVs, and levels of Al and Fe are slightly above GVs. As MAVs for drinking-water are not satisfactory for assessing the health risks of short-term consumption of high levels of toxic constituents, a different approach was also employed whereby available toxicological threshold data (ATSDR, 1998, 2003; Robjohns, 2008) were used to calculate acutely-toxic concentrations relevant to short-term exposure (Table 9). Acute mechanisms of toxicity refer to physiological responses to a single (or short term), high dose of a toxicant. The lowest lethal dose for F (19 mg/kg body mass/day) corresponds to a drinking-water concentration of 633 mg/L for a 70 kg adult, at a consumption rate of 2.1 L/day, and 258 mg/L for a 20 kg child (1.2 L/day). Worst-case predictions of F concentrations (20 mg/L) are well below these levels. Acute exposure to doses of 1 mg/kg bw/day can result in the onset of mild F intoxication (manifested as nausea, vomiting and gastric pain). This is thought to be due to the formation of hydrofluoric acid HF in the stomach (ATSDR, 2003). This dose corresponds to concentrations of 33 mg/L for adults and 8.3 mg/L for children, indicating some risk of mild F intoxication to children under the worst-case scenario. In practice, this risk is likely to be substantially mitigated by the potability issues caused by the predicted concentrations of Fe, Al and Mn also associated with ash-leachate. For arsenic, even worst-case predicted concentrations are well below acutely toxic concentration for both lethal and non-lethal intoxication. Overall the health risks to consumers from ashfall of the compositions similar to the 2012 Te Maari ash via contamination of roof catchment household rainwater tanks are low. Even if the loading of ash

S.J. Cronin et al. / Journal of Volcanology and Geothermal Research 286 (2014) 233–247

245

Table 8 Predicted effects of ash from 6 August 2012 eruption of Tongariro volcano on composition of rainwater tanks typical of NZ rural households. Drinking-water standardsa MAVs Worst case scenario Elements of health significance Elements affecting potability of water

Realistic scenario Elements of health significance Elements affecting potability of water

F As Al Fe Mne

1.5 0.01

F As Al Fe Mn

1.5 0.01

Cash b (mg/kg)

Lc (kg/m2)

A/Vd (m2/L)

Cwater (mg/L)

GVs

0.4

0.4

Exceedance ratios Cwater/MAV

0.1 0.2 0.04

246 0.123 119 403 24

1.1 1.1 1.1 1.1 1.1

0.073 0.073 0.073 0.073 0.073

0.1 0.2 0.04

65 0.025 119 349 22

0.2 0.2 0.2 0.2 0.2

0.0074 0.0074 0.0074 0.0074 0.0074

20 0.01 9.6 32.4 1.9

0.1 0.000037 0.18 0.52 0.033

Cwater/GV

13 1.0 96 162 48

5

0.06 0.004

0.08

1.8 2.6 0.8

a

Maximum Acceptable Values (MAVs) and Guideline Values (GVs) for aesthetic properties from the Drinking-Water Standards for New Zealand (Ministry of Health, 2008). b Water-extractable concentrations of element as mg/kg ash (dry weight basis). Worst-case values are the maximum concentrations recorded for either the 6 August or 21 November 2012 eruptions. Realistic values were those recorded at sites AZ-K3 which was the closest to a household at the SE end of Lake Rotoaira, or site TM001 if data was not available for site AZK3. c Loading (mass of ash per unit area). Worst-case values were the highest recorded for the 6 August 2012 eruption (Pardo et al.,). Realistic values were those recorded at the SE end of Lake Rotoaira. d A/V refers to the total plan area of a typical NZ roof catchment (92 m2) divided by the volume of water in a typical tank. Worst case values were based on the tank being at its minimum volume (1.3 m3), and realistic values were based on the tank being 50% full (12.5 m3). e Both MAVs and GVs are set for manganese, but it is grouped with elements that affect potability of water because the GV is less than the MAV.

soluble forms of some elements. A new ‘Protocol for analysis of volcanic ash samples for assessment of hazards from leachable elements’ has been developed under the auspices of the International Volcanic Health Hazard Network (www.ivhhn.org). Our recommendation, supporting that of the protocol, is that leaching be carried out at ratios of both 1:20 and 1:100, and if possible to carry out re-extractions. It is also evident that water extraction may underestimate hazards for ash ingestion by grazing animals. Livestock may ingest substantial quantities of ash along with their food, with close-grazing animals such as sheep being particularly susceptible. For volcanic ash, fluoride is generally the most important toxicant to evaluate for gastric bioaccessibility. For example, analyses of ash from the 1995–1996 Mt Ruapehu eruptions contained only 30–90 mg/kg water-extractable F, yet several thousand sheep died of fluorosis poisoning following the eruptions (Cronin et al., 2003). These authors proposed that the hydrothermal system contributed material to the ash that led to the formation of calcium and aluminium fluoride and phosphate adsorbed phases. These phases are sparingly soluble in water but more soluble in the digestive

was scaled up considerably (in the event of larger eruptions and greater fall thicknesses), potability effects (due to increases in Fe, Al and Mn concentrations) are likely to dominate toxicity effects from F or As. However, it is important in the event of any eruption to characterise ash surface composition, to carry out simple hazard calculations as demonstrated here, and to communicate these results rapidly to health agencies. 4.4. Implications of leaching behaviours for hazard assessment Witham et al. (2005) reviewed available ash leachate characterisation methods and recommended a single leach at a ratio of 1:25 (g ash: mL extractant). This method has been widely adopted (e.g. Armienta et al., 2010, 2011; Durant et al, 2011). The US Geological Survey recommends a ratio of 1:20 for its field leach test (Hageman, 2007). However, the results of the current study imply that a 1:20 single leach may underestimate water-extractable elements such as S and F in some types of ash, due to saturation effects and/or the presence of more slowly-

Table 9 Calculations of acutely toxic concentrations of inorganic arsenic and fluoride in drinking-water. Dose

Body weighta (kg)

Daily volume ingestedb (L)

Acutely toxic concentration (mg/L)

mg/kg body weight/day Fluoride Lethalc Severe poisoning

d

Onset of symptomsd

Arsenic Lethale Onset of symptomse a b c d e

Adult Child Adult Child Adult Child

Adult Child Adult Child

19 19 7.2 7.2 1 1

0.6 0.6 0.02 0.02

70 10 70 10 70 10

2.1 1.2 2.1 1.2 2.1 1.2

70 10 70 10

2.1 1.2 2.1 1.2

633 258 240 60 33 8.3

20 5.0 0.67 0.17

Body weights from Ministry of Health (2008). Russell et al. (1999) 95% percentile value. Threshold toxicity values from (Robjohns, 2008). Lower end of reported range for fatal dose (as NaF) used as conservative input to calculation. From ATSDR (2003). From ATSDR (1989); ‘onset of symptom’ range (0.02–0.06 mg/kg/day) used as conservative input to calculation.

Predicted concentrations in rainwater tanks (mg/L) from Table 8 Worst-case

Realistic

20

0.1

0.01

0.000037

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systems of grazing animals. In the current study, approximately two to three fold more F was released from the four samples using a gastric leach (HCl, pH 1.5) compared to a water leach. We therefore recommend the use of this method for the more accurate determination of hazards of ash ingestion to grazing animals. No safety thresholds have been established for agricultural hazard and the range of impacts is likely to depend on animal stress levels and climatic conditions. However, collective leachable concentrations approaching 100 mg F/kg ash over multiple leaches is a strong signal for potential impact (c.f., Cronin et al., 2003) and to implement mitigation measures such as sourcing alternative water and feed supplies or temporarily relocating livestock. 5. Conclusions The 2012 Te Maari eruption was an unusual, but not unique, case where ashfall without a juvenile magmatic component affected a broad area. Despite no fresh magma being erupted, magmatic gases modified the hydrothermal system before the event, and trapped magmatic gases were released during the explosive blasts. Subtle variations in leaching behaviour between ash derived from two different vent locations may have indicated different depths of pre-eruptive gas storage, or the uneven supply of magmatic gas to the vent areas. In general, abundances of surface water-soluble elements on ash from the 2012 eruptions of Mt Tongariro are strongly dominated by Ca and SO4, with a 1:1 association suggesting their origin was gypsum present as a hydrothermal material in the vent. Compared to global median levels (Ayris and Delmelle, 2012), Ca, SO4 and Fe are higher in the Te Maari ash, but other elements are lower. These hybrid properties are likely to be observed in similar eruptions through hydrothermal systems, especially in onset phases of eruptions when vent-clearing explosions occur. Conducting single leach studies on volcanic ash with deionised water is a widely used method for characterising ash surface composition, particularly for the purpose of assessing hazards to public health and agriculture. However, we show here that the single water leach is unlikely to be adequate for characterising agriculturally important elements such as S and F and possibly other minor trace elements. In the case of S, single-leach extractions may underestimate soluble S concentrations due to solution saturation effects. These concentrated brines may also impact on the solubility of other species in these multicompo− x nent mixtures. For F, slowly soluble phases, including AlF+3 comx plexes and CaF2 formed in hydrothermal systems hinder its accurate estimation in rapid one-off leaching tests. In terms of human health impacts from leachable elements, the greatest threat in rural areas of New Zealand is the contamination of roof-fed rainwater tank water supplies. Using a range of typical tank dimensions and evaluating a worst-case impact scenario, we conclude that health threats are unlikely with the Te Maari ash compositions. In any event, the taste and turbidity of water would likely deter consumers before acute toxic concentrations were reached. To gauge accurately the potential animal health implications of ash ingestion for pastoral livestock, we conclude that the use of a leach that simulates gastric conditions is more appropriate than simply water leaching. Through this method, a gastric leach involving 0.032 M HCl (pH 1.5 ± 0.05) showed a doubling to trebling of estimated F availability to grazing animals from the Te Maari ash, with other elements such as Al estimated at levels N20 times as high in simulated gastric fluid extracts. This could explain why tephras with relatively low soluble F concentrations have led to strong animal fluorosis symptoms (Flueck and Smith-Flueck, 2013). In hydrothermal systems that are encountered by rising magmatic gases, complex pH, temperature and oxidation state variations may exist. These will cause a range of elemental interactions that differ from high-temperature magma–gas interactions in a volcanic plume. Thus eruptions produced from hydrothermal areas will have a different cocktail of leachable elements. Assessing the health and agricultural impacts of such eruptions thus requires great care in the choice of

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