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Volcanic Ash Leachate Analysis Database and Recommended Methods

The information below is summarised from the Witham et al. (2005) paper of similar title. This contains more information on adsorption of species onto ash and sampling and applications of ash-leachates as well as the leachates database. A pdf is available here:
Download Witham et al. (2005)

Introduction
Controls on ash leachates
Leachate database
Database review
Recommendations
References

Introduction

Volcanic ash particles are able to scavenge volatile components of volcanic plumes, resulting in rapid deposition of sulphur, halogens and many other potentially harmful elements. These species may be subsequently leached (e.g., by rainfall), causing heavy loadings to soils and water bodies. The resulting leachate poses a hazard to aquatic, vegetative and soil environments, as well as human health. Several eruptions have resulted in apparent contamination of pasture, sometimes with serious impacts on livestock. Concerns have also been raised over the integrity of drinking-water supplies following tephra fall and have prompted regular sampling of water quality in some active volcanic areas. The main controls on volcanic ash leachate concentrations are summarised below and a database of previous methods used in their sampling and analysis is given. A standard method for sampling volcanic-ash leachates is recommended in the hope that this will enable comparison between future studies.
 

Controls On Ash leachates

The processes by which adsorption of volatile elements onto tephra occurs are poorly understood, but some of the main controls are:

  • Magma type and tephra composition;
  • Mode of eruption;
  • Gas-pyroclast dispersion immediately following fragmentation;
  • Concentration of the plume;
  • Ratio of particles to gas;
  • Particle size-fractions;
  • Particle surface area, porosity and texture;
  • The temperature/chemical history of the particle trajectory through the plume;
  • Environmental conditions, including wind and humidity;
  • Extent of hydrothermal interaction at the volcano.

The concentrations of the different chemical components measured in ash leachate studies depend not only on these adsorption processes and mechanisms, but also on:

  • Sample location - distance from the vent and relation to the wind direction;
  • The pH of the solution used as the leach and the type of acid used, if any;
  • The ash/leach-solution ratio;
  • The time that the ash is left in contact with the leach solution;
  • The ash grain-size fraction used;
  • Whether samples were ground before leaching, exposing greater vesicle surface area;
  • The contribution of dry deposition following the ash deposition;
  • Any rainfall or increase in humidity (such as fog) following deposition;
  • The time to analysis following sample collection.

For example, smaller particle size-fractions usually give higher elemental concentrations than larger particles and acid leach treatments generally remove much larger amounts of material from ash than water leaches. All of these controlling factors, except for the conditions following deposition, can be controlled by methodology. Standardisation of results may also be helped by collection of fresh ash immediately following eruption. This will reduce the risk of loss of water-soluble components through subsequent rainfall, although many eruptions will be accompanied by rain.

Leach compositions may also be influenced by partial dissolution of the ash particles, but comparison of the leach constituents to the bulk composition of the ash will help discriminate between species' sources. Additionally, detected leach concentrations of certain elements may be dependent on the presence of other species under certain pH conditions, e.g. Al may reduce F.

The Database

The database shows that historically leachate studies have been conducted for four main reasons:

  1. To be used as a proxy for volcanic-plume gas concentrations;
  2. Investigation of environmental impacts (including effects on soil, crops, algae, ocean water, snow and human health);
  3. Investigation of the chemistry and processes of adsorption;
  4. To determine the origins of the adsorbed material.

The first two reasons are the most common. Over 55 soluble components have been detected in volcanic-ash leachate studies, of which the most commonly analysed are Cl, Ca, Na, SO42-, Mg and F. These are also the elements with the highest concentrations in volcanic-ash water leachates. The elements of most relevance to the environment and to health depend somewhat on the purpose of the investigation, but Al, As, Cl, F, Fe, Hg, Pb and SO42-, are particularly important (Table 1, below). Some elements, including Fe, are important because they increase the surface acidity or reactivity of the ash, which then increases the respiratory hazard associated with the ash. Others are important because of potential deterioration in water quality, including change in pH, and impacts on vegetation. Aluminium is included, because, as well as impacting on health, it counters the biological availability of fluoride.

The database demonstrates the wide variety of analysis techniques used in previous studies. The main inconsistencies in methodology are:

Choice of particle-size fraction;
Grinding of samples;
Use of water or acid solutions;
Ash-to-water ratio. These vary by over one order of magnitude from 1:2 to 1:80;
Contact times, which vary from 3 minutes to 24 hours;
Sample agitation. Some authors agitate the ash-solute mixture continually, others for a period and some not at all. Agitation times vary from 3 minutes to 24 hours;
Different ion analysis techniques;
Different units of reporting for concentrations, including mg/kg ash, ppm mass, ppm volume, mg l-1 and µeq l-1.
The use of such a wide-range of methods introduces error to direct numerical comparisons between surveys and suggests that a common leachate methodology would be beneficial for future work in this area.

Table 1: Volcanic-ash water-leachate concentration ranges for some of the important health-related ions.

Ion Number of studies Range of concentrations
(mg ion/kg ash)		 Calculated water concentrations (mg/l) WHO drinking water guideline levels (mg/l)
Al 16 2.4 (Galunggung) - 2117 (Gorely) 0.096 - 84.68 -
As 8 0.01 (Popocatepetl) - <4 (Ruapehu) 0.0004 - 0.16 0.01
Cl 42 3.8 (Mt. St. Helens) - 11160 (Irazu) 0.152 - 446.4 250a
F 30 0.1 (Galunggung) - 2043 (Avacha) 0.004 - 81.72 1.5
Fe 22 0.01 (Mt. St. Helens) - 91 (Ruapehu) 0.0004 - 3.64 -
Hg 3 0.0001-0.0087 (Mt. St. Helens only) 4 x 10-6 - 3.48 x 10-4 0.001
Pb 12 0.001 - 17.56 (Popocatepetl spans whole range) 4 x 10-5 - 0.7024 0.01
SO4 33 2.4 (Mt. St. Helens)-21775 (Popocatepetl) 0.096 - 871 500b
  • The calculated water concentrations for each ion are derived using the ash-water dilution ration of 1:25 recommended here. Drinking water quality guidelines (WHO, 1993) for these ions are also provided for reference purposes. Note that the leachate concentrations for different volcanoes are derived using different methods and assume complete sampling of all the adsorbed ion mass.
  1. no guideline value, but concentrations of this level can give rise to a detectable taste in water.
  2. no guideline value, but gastrointestinal effects can result from ingestion of high levels.

Recommendations

To facilitate comparison between the results of leachate studies, we suggest that the following method should be used in addition to any other variations in method that individual workers might like to implement. (Further explanation of the recommended method is provided below.)

  • Use polyethlyene equipment at all stages of the analysis;
  • Ash should be pristine (i.e. not subjected to rainfall or deposited through cloud or fog). If not pristine, then this should be noted in the data report. The times since eruption and sample collection should also be noted;
  • The leach solution should be distilled deionised water;
  • Non-ground ash from all particle size fractions at the sample site should be used in the leach;
  • An ash (g) to water (ml) ratio of 1:25 should be used;
  • Agitate ash-leach mixtures for 90 minutes (ideally by shaking) in a sealed container;
  • Filter mixture using 0.45 micron surfactant-free cellulose acetate membrane filters (commonly used filters are those manufactured by Millipore and Whatman). Centrifuging samples prior to filtration will enable faster filtration and easier ash sample recovery. For analysis of mercury, addition of a preservative to a separate filtered split is required to prevent volatilisation or sorption onto container walls;
  • Analyse filtered leach samples using available equipment, such as ion chromatographs, ICP spectrometers, or ion-selective electrodes, with appropriate standard solutions as calibration;
  • Dry and weigh each ash sample and determine its particle size distribution (for small, 1-2 g, leached samples, sieving a larger mass of a bulk sample will prevent large sieve losses);
  • Ideally, repeat the analysis with similar ash samples to obtain a mean for that sample site;
  • Present results in mg/kg ash, stating the distance from the vent, the particle size distribution and the number of samples analysed.

For other techniques used, the full method, particularly the ash to solute ratio, should be stated.

It is important to note that this recommended method is not based on an exhaustive study to find optimal conditions, but rather on the most-used methodologies and findings from previous work. We have suggested a deionised water leach, due to its frequent past use, availability, ease of use in the field, and comparability to drinking water. A mildly acidic leach is more representative of rainwater, particularly in the vicinity of an active eruption, so a repeated analysis of the ash with an acid solution would also be insightful. Acid concentrations used in previous work have varied from 0.1 to 0.0001M (pH 1 - 4) and the leaches have been composed of either nitric acid (HNO3) or hydrochloric acid (HCl). To facilitate analysis of adsorbed Cl-, we recommend the use of nitric acid at a standard solution of 0.001M (pH 3). This level of acidity is in keeping with rainwater pH measured in active volcanic regions. To examine the leaching effects of rainwater in detail, knowledge of the composition of rainwater in the region of interest could be used to make up a suitable proxy solution.

We recommend using the whole ash sample for the leach, as this prevents contamination that might occur when splitting into size fractions. It also gives the most representative value for the total leachate loadings at each site.

The recommended agitation time and lack of resting time is a departure from the approach based on the Taylor and Stoiber (1973) method used by most workers. The ninety-minute combined agitation and contact time was selected based on studies that have examined changing leachate concentrations with time (Frogner et al., 2001; Oskarsson, 1980; Risacher and Alonso, 2001). In all of these, the majority of leaching occurred within the first 60-100 minutes. Studies of mine-waste leachates have also demonstrated that substantial changes in chemistry are possible when samples are left to sit in the leach solution for any length of time following agitation. For rapid appraisal of leachable ions in the field where a health-hazard is feared, agitation can be replaced by quick shaking of the ash-water mixture by hand. This leach solution can then be analysed for the most important ions using ion electrodes. The results will provide a minimum concentration for the leachate loadings and should be followed up by analysis of ash by the complete method.

The unit for reporting leachate results of mg ion/kg ash is recommended as this allows comparison of adsorption between particle sizes, sample sites and volcanoes, and calculation of plume volatile masses where total erupted ash mass is known. If the leach volume is reported, leachate loadings for different sites can be calculated and then extrapolated to wider areas of deposition. This approach assumes that the majority of the leached species are from adsorption and not the bulk ash. For health and environmental studies, reporting concentrations of species in the leach (mg L-1) would also be beneficial and is recommended.
 

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Witham, C.S., Oppenheimer, C. and Horwell, C.J., 2005, Volcanic ash-leachates: a review and recommendations for sampling methods. Journal of Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2004.11.010