Geological controls on locally elevated arsenic in the Glenorchy area, Otago, New Zealand

ABSTRACT

The Glenorchy area of northwest Otago consists of a low-relief Holocene fluvial delta complex below glaciated mountains of Otago Schist basement, and has locally elevated arsenic (As) from two geogenic pathways: Mesozoic hydrothermal processes and Holocene shallow groundwater transport. Soils on the delta complex are primitive and contain abundant sand and silt derived from unoxidised schist basement, with background As contents generally <30 mg/kg. Some soils in wetland areas on the delta have naturally elevated (geogenic) arsenic contents that are locally >300 mg/kg on the cm² scale. Arsenic enrichment has occurred in the upper 30 cm where dissolved As and Fe from low-redox chemical conditions in shallow groundwater has become oxidised, thereby lowering their solubility. Both As and Fe are being mobilised from background sulfide minerals in schist-derived debris, and groundwater As concentrations up to 0.8 mg/L have been recorded. Historic mining of scheelite for tungsten on the surrounding mountains has exposed small volumes (typically tens to hundreds of m³) of arsenopyrite-bearing mine wastes with spot As contents from <500 to >10 000 mg/kg. Water discharging from mine adits have pH∼8 and dissolved As up to 0.3 mg/L, but these waters are rapidly diluted downstream to <0.02 mg/L.

KEYWORDS:

• Geogenic
• arsenic
• mine
• soil
• pyrite
• ferrihydrite
• tungsten

Introduction

Mobilisation and concentration of geological sources of arsenic (As) in the surficial environment is a global issue that is attracting increasing attention, especially where these processes occur in human contexts (Nickson et al. 2000; Morin and Calas 2006; Polizzotto et al. 2008; Podgorski and Berg 2020). There are two principal geological pathways involved in creating localised elevated surficial As concentrations: dissolution of sulfidic mineralised rocks, especially around gold mines; and shallow groundwater mobilisation in sediments (Nickson et al. 2000; Morin and Calas 2006; Polizzotto et al. 2008; Lottermoser 2010; Podgorski and Berg 2020). Mine-related As is a well-known phenomenon and As management is a routine aspect of modern mine management (Lottermoser 2010; Bowell and Craw 2014; Craw and Bowell 2014). Groundwater-related As enrichment situations were initially identified in Bangladesh in the 1990s, where low levels of As affected up to 50 million people (Nickson et al. 2000; Polizzotto et al. 2008; Podgorski and Berg 2020; Shaji et al. 2021). Similar groundwater issues have since been identified around the world, and identification, management, and treatment of As-bearing groundwater is a subject of much on-going research as the numbers of people affected are large (possibly hundreds of millions; Rowland et al. 2011; Smedley and Kinniburgh 2013; Robles et al. 2016; Podgorski and Berg 2020; Stopelli et al. 2020; Shaji et al. 2021).

In New Zealand, mine-related As is well-known because of the widespread occurrences and historic development, of gold mines in basement rocks, especially in Coromandel, Westland, and Otago (Haffert et al. 2010; Craw and Bowell 2014; Craw and Pope 2017; Kerr et al. 2018; Weightman et al. 2022). In contrast, the phenomena of groundwater As mobilisation and surficial concentration have been less studied, although As dissolved in drinking water is receiving more attention with increased scrutiny of domestic water quality (ORC 2021; Shaji et al. 2021), with a recommended drinking water upper limit for As of 0.01 mg/L (WHO 2022). Similarly, more recent attention has been paid to elevated As in soils, especially where those soils are associated with domestic dwellings (e.g. QLDC 2016). This has been done in the context of the recommended upper limit for As in soils, for residential areas with small gardens, that has decreased to only 20 mg/kg (MfE 2011), close to the background level in some ordinary rocks (Blake et al. 2019).

In this study, we investigate the distribution of As in the surficial environment around the town of Glenorchy, at the foot of the Southern Alps mountains of NW Otago (Figure 1A,B), in the context of inferred geological processes. This area is useful for this type of study because there has been an extensive history of mining of As-bearing rocks on the slopes above the town (Figure 1B). In addition, the town itself is located on a river delta (Figure 1B) that is affected by shallow groundwater in fresh sedimentary rocks that contain low but important levels of background As that are available for near-surface mobilisation. Hence, the area provides an ideal setting for comparison of the two principal pathways for As mobilisation in a New Zealand mountain context. This study is also intended to provide some context for a well-defined area of surficial As enrichment at Frankton, a suburb of Queenstown (Figure 1A; Figure 2A–E; QLDC 2016). In the Frankton situation, the area has been extensively modified by urban development so details of the geological setting have been lost. Glenorchy area is underlain by essentially identical basement and sediments and therefore provides a useful proxy for understanding the Frankton situation.

Figure 1. Location maps for this study (A) Location in the Otago Schist belt. (B) Hillshade digital elevation model (DEM) image of the Glenorchy area, showing topography, drainage, and locations of principal mined areas (size exaggerated for clarity). Summary As concentrations in water, rocks and soils are indicated (see text for references).

Figure 2. Summary of features in the Frankton area relevant to As mobility and enrichment. (A) DEM of the Queenstown district, showing general location of Wakatipu shallow groundwater aquifer (Rosen and Jones 1998) and Frankton. (B) As concentrations in Wakatipu aquifer groundwaters, summarised from ORC (2021). (C) As concentrations in Frankton substrates, summarised from QLDC(2016). (D) Photograph of excavated section through substrate near top of Frankton terrace, with summary of fp-XRF As analyses (mg/kg) at different levels. (E) Active deposition of ferric oxyhydroxide, with adsorbed As (mg/kg; fp-XRF analysis summary), from groundwater seep at bottom of Frankton terrace.

General setting

Basement rocks

The Glenorchy area is underlain by Mesozoic Otago Schist basement, mostly of Caples Terrane with Aspiring Terrane rocks exposed on the upper parts of the Richardson Range (Figure 1A,B; Craw 1984; Turnbull 2000). There is a metamorphic transition from sub-greenschist facies greywacke, argillite and weakly foliated schists on the western side of Lake Wakatipu and lower Dart River valley, to lower greenschist facies schists in the Rees River valley and upper greenschist facies rocks near the top of the Richardson Range (Figure 1B; Paterson 1982; Craw 1984; Turnbull 2000). The Forbes Range and nearby Mt Alfred (Figure 1B) are underlain by sub-greenschist facies schists that are mostly pelitic and are locally highly micaceous and foliated but only weakly segregated metamorphically (Craw 1984). In contrast, rocks on the Richardson Range are interlayered psammitic and pelitic schists with extensive metamorphic foliation and segregation. All the metasedimentary schists of the area have similar quartzofeldspathic compositions, although there are subtle geochemical differences between Caples Terrane and Aspiring Terrane rocks (Palmer et al. 1991; Mortimer and Roser 1992). Metabasic schist horizons are rare except in Aspiring Terrane rocks high on the Richardson Range (Figure 1B; Craw 1984; Turnbull 2000).

Background As concentrations in Otago Schists and their precursor metasediments are typically between 1 and 30 mg/kg when measured in bulk hand specimens (Palmer et al. 1991; Pitcairn et al. 2006; 2010). This range of As contents is typical of bulk samples of quartzofeldspathic metasediments in the South Island in general (Hamisi et al. 2017). Similar background levels of As occur in schist-derived loess and colluvium in eastern Otago near the Macraes gold mine, as measured with field-portable X-ray fluorescence (fp-XRF; Figure 3A,B; Blake et al. 2019). Arsenic is inhomogeneosly distributed within the Otago Schist rocks themselves, and it occurs principally within trace amounts of sulfide minerals (Figure 4A; Pitcairn et al. 2010; Large et al. 2012; Hu et al. 2016). In particular, As is present at low levels (∼1 wt%) in solid solution in pyrite, and in ultra-trace amounts of cobaltite (CoAsS; Figure 4A; Pitcairn et al. 2006, 2010; Hu et al. 2016). Similarly, these sulfidic As hosts are typical of South Island quartzofeldspathic metasediments in general (Pitcairn et al. 2010; Hamisi et al. 2017).

Figure 3. Validation and calibration of fp-XRF analysis; see text for references. (A) Background As and Fe concentrations in Otago Schist and schist-derived colluvium near Macraes in east Otago (Figure 1A). (B) Calibration over 5 orders of magnitide of fp-XRF As analyses compared to lab-derived XRF data for same dried samples.

Figure 4. Typical mineralogical and textural distribution of As-bearing sulfides in schist samples relevant to this study. (A) Sketch of pyrite and cobaltite in sub-greenschist Caples Terrane rock (after Pitcairn et al. 2010). (B) Close view of foliation surface of a sub-greenschist facies pelitic schist from an outcrop in Forbes Range (OU 42272). Brown spots indicate incipient oxidation of pyrite within the foliation. (C) Cobble of Forbes Range sub-greenschist facies schist from Earnslaw Burn, showing oxidation spots for pyrite in cross-cutting metamorphic quartz veins. (D) Broken section through a cobble of well-segregated greenschist facies schist from Richardson Range. Brown spots (yellow arrows) indicate pyrite oxidation on foliation, and in foliation-parallel deformed quartz veins (top).

A prominent swarm of hydrothermal vein systems was emplaced in the schists of the Richardson Range in the Cretaceous, and some minor veins occur distributed sparsely farther to the northwest (Figure 1B; Williams 1974; Paterson 1982; Mortensen et al. 2010). Many of these veins systems were mined historically for scheelite (CaWO₄), with some mining continuing to the 1970s (Williams 1974). Gold is locally anomalous in the veins, but gold was mined from only one principal vein system in the lower Rees valley (Williams 1974; Hay and Craw 1993). All the vein systems contain arsenopyrite and pyrite (Paterson 1982; Mortensen et al. 2010). The mineralised rocks are broadly coeval with similar Au-W-As rocks at the Macraes mine in eastern Otago (Figure 1A; Mortensen et al. 2010; MacKenzie et al. 2017), and data from the modern Macraes mine are incorporated in this study for comparison.

Surficial setting

The studied area lies on the eastern side of the Southern Alps mountains that are rising in the hanging wall of the Alpine Fault (Figure 1A). The town of Glenorchy lies on a fluvial delta complex formed by coalescence of outwash from the Dart and Rees Rivers and the Earnslaw Burn (Figure 1B). The area was extensively glaciated during the Pleistocene, with ∼1 km thick ice extending from the mountains southwards down Lake Wakatipu (Figure 1B; Turnbull 2000), and some high cirque glaciers have persisted to the present. Lake Wakatipu was higher than present during early Holocene deglaciation, and left a silt-dominated bench in some places at ∼50 m above present level at ∼12 ka (Turnbull 2000), although that sediment has since been mostly eroded from the Glenorchy delta. Glaciation has ensured that most rock outcrops are either unoxidised or only weakly oxidised, and sediments carried by the rivers are largely unoxidised as well. Steep stream channels carry abundant coarse gravel eroded from valley walls, and these have formed gravel-rich alluvial fans, some of which extend onto the delta to join the main rivers.

The delta is an area of low relief with a water table close to the surface in many places. There are extensive areas of wetlands adjacent to the main river channels, and some streams emanating from valley walls meander across the delta plain (Figure 1B). Gravel channel margins are dominated by fine sediment, especially silt and sand deposits derived from glacial flour. Wind-blown silt from river flats farther upstream is a frequent component of the modern environment, and associated loess deposits cover the surface and are interbedded with underlying fluvial sediments.

The climate of the Glenorchy delta is cool temperate, with mean annual temperature of ∼10°C, some cold winter nights (−5°C) and some hot (>30°C) summer days (NIWA 2021). Annual rainfall is ∼1000 mm spread throughout the year, but rainfall on the delta can be highly variable on the km scale as rain events are localised by the mountain topography, especially via channelling of northwest storms down the Dart valley. Strong winds, rain, and surface flooding associated with these storms in particular are responsible for frequently redistributing loess on parts of the delta.

Methods

Analyses of solid materials in this study were obtained with fp-XRF, with focus on As and Fe contents. An Innov-X Model Omega instrument was used for analysing mine-related As contents in the Buckler Burn catchment (Figure 1B). This instrument had a practical detection limit for As of ∼50 mg/kg. A newer Olympus Vanta M Series fp-XRF instrument was used for all other analyses. Practical detection limit for this instrument is 3–5 mg/kg, and results are reproducible to ±2 mg/kg at 10 mg/kg, as calibrated by Blake et al. (2019). The instruments have been calibrated for a wide range of As concentrations in a range of studies of surface As distribution, and combined calibrations over several orders of magnitude relevant to the present study are presented in Figure 3(B) (Druzbicka and Craw 2015; Malloch and Craw 2017; McLachlan and Craw 2018; Blake et al. 2019). In general, fp-XRF tends to over-estimate As concentrations at high As levels by up to 20% (Figure 3B), but Blake et al. (2019) found a strong agreement between the Olympus fp-XRF and laboratory results at low As levels. On the other hand, analysis of damp materials in situ in the field, as done for some of this study, can lead to under-estimation of As concentrations in relation to the dry materials used in calibration (Figure 3B). The device analyses an area of ∼1 cm² with limited depth penetration (mm scale), so analyses are essentially of surfaces rather than volumes. The schist basement rocks are inherently inhomogeneous at this scale (Figure 4B–D), so multiple analyses were obtained. Because of these uncertainties, we report herein only raw fp-XRF data for As and Fe. For the purposes of this study, it is the relative values of As and Fe determined within the Glenorchy area and elsewhere that are most important.

Two unpublished soil analyses, from Kinloch and Glenorchy areas (Figure 1B; Table 1) provide confidence in the relative differences in results for the fp-XRF instrument. These soil heavy metal analyses were determined from bulked composites of >10 soil subsamples that were dried and <2 mm fractions were digested with nitric and hydrochloric acid (USEPA 200.2) and analysed by ICP-MS at Hill-Labs (Hamilton, NZ), an internationally accredited laboratory. Surface substrate As analyses at Frankton (Figure 2C) have been compiled from Queenstown-Lakes District Council data (QLDC 2016). These are compared in Table 1 to typical schist rock data (Paterson and Rankin 1979; Palmer et al. 1991; Craw 2002).

Table 1. Arsenic and metal concentrations (mg/kg) in soils and rocks relevant to this study.

Location	Sample	As	Cd	Co	Cr	Cu	Pb	Ni	Zn
Glenorchy delta	Soil composite	146	0.11	nd	14	19	24	14	59
Kinloch fan	Soil composite	8	0.14	nd	15	13	11	11	50
Richardson Range	Psammitic schist	15	nd	nd	nd	27	20	16	80
Richardson Range	Psammitic schist	8	nd	nd	38	21	15	16	87
Richardson Range	Pelitic schist	nd	nd	nd	53	28	12	22	99
Richardson Range	Pelitic schist	nd	nd	nd	60	29	12	22	96
Richardson Range	Psammitic schist	nd	nd	nd	30	16	15	13	79
Richardson Range	Pelitic schist	nd	nd	nd	45	39	14	18	96
Forbes Range	Pelitic schist	11	nd	nd	50	28	27	20	98
Caples Terrane	Pelitic schist	7.3	nd	7.9	nd	<1	14	26	44
Macraes	Pelitic schist	30	<0.1	15	54	26	29	18	80

Note: See text for source references. nd = not determined.

Field pH measurements of surface waters were determined with an Oakton PC450 metre calibrated daily with standard solutions (pH 4,7,10). Water samples in the Buckler Burn catchment (Figure 1B; Table 2) were collected in acid-washed plastic bottles and stored with ice until analysis at Hill-Labs in Hamilton. Filtered samples (0.45 µm) for metal analysis were preserved with nitric acid in the field, and analysed by APHA (2005) method 3125 B, with detection limit of 0.02 mg/L for As. Standard methods (APHA 2005) were used for major ions after 0.45 µm filtration in the lab: Method 311 IB was used for Ca, Mg, Na and K, Method 4500C1 used for chloride, and Method 2320B used for dissolved carbonate (alkalinity titration), sulfate was determined by ion chromatography (Method 4110B). Unpublished analyses of Dart River surface and shallow groundwater (Table 2) were obtained from a commercial organisation and are used here for background comparison, along with data from Rosen and Jones (1998) for major ions in the Wakatipu aquifer near Queenstown (Figure 1A; Figure 2A). Arsenic analyses of shallow groundwaters at Glenorchy (Figure 1b) and in the Wakatipu aquifer (Figure 2B) have been compiled from Otago Regional Council data (ORC 2021).

Table 2. Water analyses for the Buckler Burn catchment (Figure 8a) compared to Dart River background.

Location	Water type	pH	EC	Na	K	Ca	Mg	Cl	Alk	SO4	As
			mS/cm	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L	mg/L
Bonnie Jean mine	Adit	7.7	146	1.7	< 1.0	25	1.9	0.5	65	17.3	0.34
Bonnie Jean	Creek above adit	7.8	92	1.1	< 1.0	15.4	1	0.5	51	6.2	0.02
Bonnie Jean	Creek below adit	7.6	90	1	< 1.0	14.5	1	0.5	47	6.3	<0.02
State mine	Adit	8	237	3.5	< 1.0	41	1.9	1	97	35	0.21
Mine	Adit	7.8	121	1.8	< 1.0	21	1.1	0.6	67	5	0.04
Waste pile	Creek below	7.8	126	2.1	< 1.0	21	1.5	0.7	69	7.1	0.03
Waste pile	Creek below	7.8	43	1.9	< 1.0	4.8	1.3	0.6	27	1.7	<0.02
Waste pile	Creek below	7.9	85	1.7	< 1.0	13.4	1.1	0.8	45	4.5	<0.02
Surface water	Creek	7.9	232	2.1	< 1.0	42	1.5	0.8	118	18.9	0.05
Buckler Burn	Main stream	7.9	76	0.9	< 1.0	13.1	0.5	0.5	40	4.7	<0.02
Dart River	Main stream	7.5	73	0.5	0.4	13	0.03	0.6	42	8	<0.001
Dart River	Groundwater	6.8	110	1.9	0.6	18	0.8	1	67	7	<0.001

Note: Alkalinity is excpressed as ΣHCO₃^-.

Results

Arsenic in background rocks and sediments

Silt deposits are exposed in the open river beds of the Glenorchy delta area after flood events and wind-storms. These sediments mostly have background levels of As (<30 mg/kg; Figure 5A). The As concentrations are variable (4–45 mg/kg; Figure 5A), reflecting the inhomogeneity of the source areas and vagaries of sedimentary deposition. However, these As concentrations are typical for bulk analyses of Otago Schist rocks (e.g. Table 1). A selection of rocks from outcrops in the Forbes and Richardson Ranges and from river beds emanating from those ranges, analysed with fp-XRF, shows a much wider range of As concentrations than the sediments, up to 190 mg/kg (Figure 5B). The highest As concentrations (>100 mg/kg) were measured on rocks collected at outcrops in those ranges (Figure 5C,D), and these higher concentrations were obtained on micaceous foliation surfaces (e.g.; Figure 4B). Of these higher concentrations, most were obtained from micaceous (pelitic) schists of the Forbes Range (Figure 5C). Rocks in river beds have been extensively abraded during transport, and have abundant erosion-resistant quartzofelspathic segregations and/or quartz veins exposed on exterior surfaces (e.g. Figure 4C,D). Consequently, the As concentrations measured on the surfaces of these rocks were generally lower (<50 mg/kg) than those from outcrops (Figure 5B).

Figure 5. Arsenic and Fe analyses (fp-XRF) of solid materials on Glenorchy delta and nearby mountains. (A) Sandy silt from exposed portions of active river beds, and soil on fans near Kinloch (Figure 1B). (B). Rocks from Forbes and Richardson Ranges, from outcrop and stream cobbles. Several analyses were done on different surfaces of some rocks. (C) Analyses as in b for rocks from outcrops on Forbes Range. (D) Analyses as in b for rocks from outcrops on Richardson Range. (E) Analyses of shallow soils (upper 15 cm) from Lower Rees valley on Glenorchy delta (Figure 1B). Data for deeper soil profiles (as in f) are also plotted. (F) Variations of As with depth for two soil profiles (Figure 6A–C) in the lower Rees valley, with generalised envelope indicating surface enrichment.

The higher As concentrations generally occur in analyses with higher Fe concentrations, although the data show wide scatter in this regard (Figure 5A–D). Chlorite and pyrite are the two principal Fe-bearing minerals in these rocks and sediments, and both of these occur within the micaceous laminae (e.g. Figure 4B). Since pyrite is one of the main hosts of As, and is commonly accompanied by cobaltite (Figure 4A), the distribution of pyrite has a strong effect on measured As concentrations, and a lesser effect on measured Fe concentrations within a chlorite-rich background. The sulfide minerals are typically fine-grained (micron-scale) in these rocks (Figure 4A), so surface analyses at the scale of the fp-XRF instrument cannot resolve the sulfide distribution. The locations of pyrite grains, or clusters of grains, are indicated on the surfaces of many rocks by oxidation spots on foliation surfaces and in metamorphic veins (Figure 4B–D). However, these spots are too small to analyse specifically with the fp-XRF instrument, and none of the highest As concentrations were obtained on these spots.

Arsenic in soils

Soils in the Glenorchy delta area are primitive, as they have been developing on fresh rocks and sediments only since the last glaciation (<20 ka). Further, the delta is a dynamic fluvial environment, so river channel migrations have eroded older surfaces and left newer ones exposed to revegetation. Hence, most soil profiles have a thin (<10 cm) organic-rich layer at the surface, with plant roots extending down into essentially original silt, sand and gravel sediments. There has been minor oxidation of the near-surface portions of some of the soil profiles, but many soils have retained their original grey colour typical of the fresh sediments on the active delta surfaces. The As concentrations of these soils are generally low (<10 mg/kg), reflecting the As concentrations of the original sediments (e.g. Kinloch soils; Figure 5A).

Soils in the lower Rees valley generally have typical background As concentrations (<30 mg/kg), but there are some areas where the As contents are distinctly higher than these background levels (Figure 1B; Figure 5E; Table 1). These higher As concentrations (>50 mg/kg) occur in and near wetlands and meandering low-gradient streams draining across the delta plain (Figure 1B). Surface waters have pH between 5.6 and 7.2. These soils have higher organic contents than elsewhere, and display greater contrast between an oxidised surface zone and largely unoxidised deeper levels (Figure 6A–C) than is seen in better-drained and less water-logged environments. The sediments on which these soils have developed are fine-grained (silt and sand), with no gravel components (Figure 6A–C). Mineralogically, these fine sediments are dominated by micaceous material (muscovite and chlorite; Figure 6C).

Figure 6. Photographs of soil profiles from lower Rees valley, with As analyses (Figure 5E,F). (A) Profile with transition from oxidised (brown, top) to low-oxygen (grey, bottom) environments, and summaries of As concentrations (mg/kg). (B) Profile with sharp redox boundary (yellow dashed line) between oxidised (top) and low-oxygen environments, with As spot analyses (mg/kg). (C) Close view of central portion of b, showing patches of ferric staining. White sheen shows prevalent mica flakes.

In some of the wetland soil profiles there is a sharp redox boundary between the relatively oxidised surface zone and the unoxidised deeper levels (Figure 6B), but in most of these soil profiles this change is transitional over ∼30 cm (Figure 6A). The highest As concentrations occur in the more oxidised upper zones in these soils, and spot analyses as high as 340 mg/kg As were obtained (Figure 5E; Figure 6A). The spot As analyses were highly variable within the oxidised surficial layers, but most analyses were higher than background levels seen elsewhere on the delta (Figure 5E,F; Figure 6A–C). In contrast, As concentrations of the sediment below the redox boundary or transition were similar to those of the background sediments (Figure 5E,F; Figure 6A,B).

There is a crude positive correlation between higher As concentrations and higher Fe concentrations in the wetland soils (Figure 5E). This correlation is supported by observations of secondary ferric oxyhydroxide staining within the surficial zone (Figure 6A–C). Direct measurements over the brownest spots yielded As analyses elevated above background, but did not yield the highest As concentrations. Some of the best-developed ferric stains occur around plant rootlets and these zones commonly have cavitation in the soil (Figure 6C), limiting the effectiveness of fp-XRF analysis. The ∼1 cm² spatial analytical scale of the fp-XRF instrument is apparently too large to allow better definition of this As-Fe relationship.

Arsenic in mine debris

Scheelite mines in the Glenorchy area (Figure 1B) were developed on a swarm of hydrothermal quartz veins that are typically 1–2 metres thick, and some veins are traceable around the hillsides for >200 m. On the Richardson Range, the veins have a shallow eastward dip, and the mines followed these structures into the hillsides. The mineralised zones are locally strongly enriched in As, up to ∼10 000 mg/kg (wt % levels). An extensive analytical data set from the active Macraes mine in east Otago (Figure 1A), which has similar vein systems to Glenorchy, shows that As concentrations are commonly as high as W concentrations in bulk samples (Figure 7A). However, at more detailed scales of hand specimens and outcrops, scheelite occurs within vein quartz with very low As concentrations (<10 mg/kg), and the As is concentrated on the margins of the veins in arsenopyrite and pyrite (Figure 7B–D). The As-bearing vein selvedges are only mm- to cm-scale on many veins, although a halo of anomalous As can extend for up to a metre. Some thin (mm-cm) As-bearing seams similar to the vein selvedges occur within veins as well, sub-parallel to the vein margins.

Figure 7. Relations between As and W at mine sites. (A) Bulk analyses of As and W at Macraes mine (MacKenzie et al. 2017), paragenetically similar to Glenorchy mineralisation system. (B) Portion of a typical scheelite-bearing vein from Glenorchy, with an As-rich selvedge. (C) Perpendicular view of the As-bearing selvedge in b. A = arsenopyrite. (D) Close view of relatively coarse sulfides in the selvedge in c. A = arsenopyrite; P = pyrite. (E) Glenorchy State mine waste rock pile dominated by unmineralised schist debris. (f) Bonnie Jean mine waste rock pile, with fp-XRFAs analyses. Paler portions are quartz-rich debris.

When the Glenorchy veins were mined, the scheelite-bearing quartz was the principal target, but the As-bearing vein margins and some of the unmineralised host rocks were extracted as well. The quartz-rich ore was separated for further processing to concentrate the scheelite, and the rest of the mined material was dumped at mine entrances as waste rock piles (Figure 7E,F). Waste rock piles remain scattered across the mined landscape, and these are the principal repositories of surface As in the mined areas, especially in the Buckler Burn catchment, where most of the mines were located (Figure 1B; Figure 7E,F; Figure 8A). Some waste rock piles are dominated by unmineralised schist that has only background As concentrations and minor scattered As-bearing cobbles (Figure 7E). Others have abundant As-bearing material, typically with As concentrations between 2000 and 5000 mg/kg as measured with fp-XRF (e.g.; Figure 7F). The As-bearing selvedge material (Figure 7B–D) is friable and has crumbled to fine grained residues (silt, sand, fine gravel) on many of the waste rock piles and locally forms a matrix to the harder unmineralised rocks.

Figure 8. Buckler Burn mine sites. (A) General map of the catchment, showing locations of waste rock piles (summarised from GoogleEarth), and As water concentrations at sampled sites (Table 2). (B) Map and As concentrations of tailings pile at the Glenorchy State mine processing plant. (C) Oblique photograph of map view in b. (D) Excavated depression in tailings to reach As-rich material beneath quartz-rich surface.

Ore from the mines was processed, mainly by crushing and water-washing, to separate scheelite from quartz. Some of that quartz-rich ore inevitably had As-bearing selvedge material attached, so tailings from processing were As-bearing. These tailings were mostly discharged into the nearest creek, and those tailings have since been washed away during floods. One small tailings deposit remains at the Glenorchy State Mine processing site (Figure 8A–C). The surface of this tailings pile has developed a relatively low-As quartz-rich armouring layer 1–5 cm thick as a result of rain-washing after the site was abandoned in 1970s (Figure 8C,D). Arsenic-rich residues persist below this surface layer, with As concentrations up to wt% levels (Figure 8B–D). The mineralogical form of this As is not known because of its fine grain size and intimate mixing with fine grained silicates.

Water compositions

A set of stream waters in the Buckler Burn catchment was analysed for major ions and dissolved As in this study as representatives of typical hill-slope waters above and within the scheelite mining area (Figure 8A; Figure 9A–F; Table 2). Shallow groundwater was analysed from samples taken at mine adit discharge points (Figure 7F; Figure 8A; Figure 9A–F). These water compositions are compared to background Dart River waters sampled on the Glenorchy delta (Figure 1B; Table 2) and typical Southern Alps rain water (Jacobson et al. 2003; Figure 9A). Shallow groundwater on the Glenorchy delta locally has elevated dissolved As concentrations (up to 0.84 mg/kg; Figure 1B; ORC 2021), and these data provide context for comparison to the dissolved As concentrations of mine-related waters in the nearby Buckler Burn (Figure 8A).

Figure 9. Chemistry of Buckler Burn waters (Table 2) compared to other relevant waters (see text for references). (A) Na versus Cl, showing deviation from marine aerosol composition (blue line). (B) Ca versus alkalinity, showing strong relationship between water compositions and calcite dissolution (brown line). (C) Mg versus sulfate, showing deviation to higher sulfatefrom marine aerosol compositions (blue line). (D) Electrical conductivity (EC) versus pH, showing generally higher pH and lower EC in Buckler Burn. (E) Comparison of pH and dissolved As for Buckler Burn and Macraes mine waste rock discharge waters. (F) Comparison of pH and dissolved As for Buckler Burn waters with Macraes mine tailings waters, and waters derived from experimental leaching of arsenopyrite-rich rock (ore from Reefton; Figure 1a; Kerr et al. 2015).

Surface water and shallow groundwater in the Glenorchy area retains remnants of marine aerosols that arrive in rain, so that Na/Cl molar ratio is near to 1:1 (Figure 9A; Table 2). However, minor water-rock interaction and albite dissolution has raised the Na contents relative to Cl, similar to groundwaters in the nearby Wakatipu aquifer (Figure 2A; Figure 9A). Water-rock interaction has also caused calcite dissolution, which controls the Ca:alkalinity ratio close to molar 1:2, as in the Wakatipu aquifer (Figure 9B). Likewise, oxidation of pyrite (e.g. Figure 4B–D) has caused some relatively elevated dissolved sulfate concentrations compared to original marine aerosols (Figure 9C). These water-rock interactions, principally dissolution of calcite, have led to all the Glenorchy waters having neutral-alkaline pH (pH 6.5–8) that is generally higher than most of the waters in the Wakatipu aquifer (Figure 9D).

The dissolved As concentrations of many Buckler Burn waters associated with mines are higher than the analytical detection limit of 0.02 mg/L (Figure 8A; Figure 9E; Table 2). In contrast, stream waters above mines, and the downstream Buckler Burn itself, have dissolved As contents below this detection limit (Figure 8A;Table 2). The highest dissolved As concentrations, up to 0.34 mg/L, are in shallow groundwaters discharging from mine adits (Figure 7F; Figure 8A; Table 2). These mine-related dissolved As concentrations in the Buckler Burn are similar to, or slightly higher than, typical Macraes gold mine waste rock waters (Figure 9E). The Buckler Burn waters are distinctly lower in As concentrations than Macraes mine tailings waters, and experimental leaching waters from arsenopyrite-rich gold ore material (Figure 9F). The experimental waters also have distinctly lower pH (down to pH 4) than the Buckler Burn waters and Macraes mine tailings (Figure 9F), reflecting sufide-related acidification that is countered by calcite dissolution in the Buckler Burn (Figure 9B).

Discussion

Near-surface As mobilisation

The sands and silts that dominate the surficial environment on the Glenorchy delta contain background concentrations of As (typically <30 mg/kg; Figure 5A,E), and rocks in gravels have similar or slightly higher As concentrations on their exposed surfaces (typically <100 mg/kg; Figure 5B). The mountain environment, combined with active and recent glaciation ensures that these sediments are unoxidised or only incipiently oxidised, and the As within them is likely to occur predominantly in sulfide minerals and their incipient weathering products (Figure 4A–D; Figure 10A). This As is therefore available for mobilisation by shallow groundwater, especially under low-oxygen conditions where As is highly soluble as As³⁺ complexes (Figure 10B; Haffert et al. 2010; Nordstrom et al. 2014; Chi et al. 2018). Mobilisation of As in the near-surface environment in this manner is widespread around the world, especially in active orogenic belts where fresh sediments accumulate (Nickson et al. 2000; Zheng et al. 2004; Chi et al. 2018; Stopelli et al. 2020; Shaji et al. 2021). These As mobilisation processes are particularly effective in water-saturated unconsolidated sediments, especially wetlands, in these tectonic settings (Zheng et al. 2004; Polizzotto et al. 2008; Podgorski and Berg 2020; Shaji et al. 2021). In these low-oxygen environments, ferrous iron is also readily mobilised as Fe²⁺ (Figure 10B; Zheng et al. 2004; Haffert et al. 2010; Chi et al. 2018).

Figure 10. Summary of geogenic As mobilisation and concentration processes relevant to Glenorchy area. (A) Comparisons of principal As concentrations in solid materials, and their relative proportions, for regional geogenic processes and mine sites. (B) Sketch section through soil and underlying fresh schist-derived sediment, showing the principal variations in oxygen contents that control As and Fe mobility in shallow groundwater. (C) Sketch composite cross section from Richardson Range to Forbes Range, showing the present geological structure, with mineralised veins cutting a Mesozoic metamorphic transition that was subsequently folded into a synform in the Miocene. (D) Reconstruction of a Mesozoic crustal metamorphic section, showing Cretaceous mineralisation processes. (E) Arsenic, Au and W in sub-greenschist facies pyrite in the Otago Schist belt that is a precursor to the mineralisation system (Large et al. 2012).

Once the As has been mobilised as As³⁺, it can migrate in groundwater through the hosting sediments, and this is the principal form in which elevated As occurs in domestic water supplies around the world (Nickson et al. 2000; Zheng et al. 2004; Smedley and Kinniburgh 2013; Chi et al. 2018; Stopelli et al. 2020; Shaji et al. 2021). This is the probable source of elevated dissolved As in the Glenorchy area (Figure 1B; ORC 2021). However, if the dissolved As rises into more oxidised near-surface zones, including the soil, As³⁺ is rapidly oxidised to form As⁵⁺ complexes, and Fe²⁺ is oxidised to form insoluble ferric oxyhydroxide precipitates (Figure 10B; Nickson et al. 2000; Zheng et al. 2004; Haffert et al. 2010; Campbell and Nordstrom 2014; Fresno et al. 2016). The ferric oxyhydroxides are effective at immobilising As via adsorption, resulting in near-surface localised As enrichment (Figure 10B; Gao et al. 2006; Zhao et al. 2011; Qi and Pichler 2016; Chi et al. 2018).

The generalised positive correlation between Fe and As in As-enriched soils on the Glenorchy delta (Figure 5E,F; Figure 6A–C) reflects this adsorption of mobilised, then oxidised, As in sediments near wetlands (Figure 10B). The As enrichment of the oxidised zone occurs at a small scale (<1 m depth; Figure 10B), and can be as narrow as 20 cm (Figure 6B). At greater depths, water-saturated sediments likely have low-oxygen conditions across the delta. Hence, near-surface As mobilisation, and localised As enrichment of soils on the Glenorchy delta, are results of regional-scale (tens of km²) near-surface dissolution, and minor re-deposition, of trace amounts of As that occurs in all the rocks and sediments (Figure 10A). This As is ultimately derived from the sulfide minerals, especially pyrite, dispersed throughout these rocks (Figure 10A,B).

Arsenic at mine sites

In contrast to the regional scale and diffuse sources of As mobility described above, Glenorchy mine sites have much higher As concentrations in much more focused structures (Figure 1B; Figure 7B–D; Figure 10A,C). The high concentrations of As (up to wt% level) have been focused into relatively small volumes, typically only tens to hundreds of m³, of rocks in surface piles and underground mineralised zones (Figure 1B; Figure 7E,F; Figure 8A; Figure 10A). Most of the As occurs in arsenopyrite (Figure 7C,D; Figure 10A), which readily oxidises in wet surficial environments (Craw and Bowell 2014; Kerr et al. 2015; Blake et al. 2019). Minor As in hydrothermal pyrite can also be released by near-surface oxidation (Craw and Bowell 2014). Potential for dissolution of As from the mineralogically uncharacterised processing tailings (Figure 8B–D) has not been determined, but the volume of these residues is small.

Dissolved As has been mobilised in surface and shallow groundwaters related to mine wastes and underground mine workings in the Buckler Burn (Figure 8A). However, our water analyses indicate that this mobilisation is currently relatively minor, apart from mine waters emanating from underground tunnels (Figure 8A; Table 2). Even the highest dissolved As in analysed water, 0.34 mg/L, discharging from the Bonnie Jean underground mine (Figure 7F; Figure 8A; Table 2) is lower than the highest groundwater As concentration measured in Glenorchy town (0.84 mg/L; Figure 1B; Figure 8A; ORC 2021). Notably, all the waters with elevated dissolved As from the Buckler Burn mines have been diluted by other runoff in the catchment and the lower Buckler Burn waters have dissolved As <0.02 mg/L (Figure 8A; Table 2). Silt and sand on the lower Buckler Burn delta has slightly higher As concentrations than other delta river beds (Figure 5A) but within the range of typical Richardson Range rocks (Figure 5B). It is possible that there is a component of mine-related debris in these Buckler Burn sediments but if so, it is only trace amounts.

The Glenorchy scheelite mines were developed on a swarm of veins that was originally formed by similar processes to the regional-scale As mobility described above for the Glenorchy delta, but at higher temperatures and greater depths (Figure 10C,D). Like the regional-scale near-surface As mobility in groundwater, the scheelite-bearing veins derived their metals and As from the dispersed sulfide minerals, including pyrite and cobaltite, in the schist host rocks (Figure 10D,E; Pitcairn et al. 2006; 2010; Large et al. 2012; Hu et al. 2016). The tungsten pathway was more complex than depicted in Figure 10D,E (Cave et al. 2017; Palmer et al. 2022) but the overall principles were similar. In this mineralisation process, As migration occurred on a vertical scale of kilometres in shears and fault zones, rather than a horizontal scale of kilometres in fractured bedrock and sedimentary aquifers of the modern Glenorchy delta environment.

Comparison of Glenorchy and Frankton

Surface As enrichment at Frankton occurs on a terrace 20–30 m above the Kawarau River where the river drains from Lake Wakatipu (Figure 2A,C). The terrace is underlain by schist basement and has a surface cover, up to 10 m thick, of well-bedded sands, silts and muds (Figure 2D) that were deposited during the early Holocene lake high-stand (Turnbull 2000), and are mineralogically identical to those of the Glenorchy delta, including the high mica content (cf Figure 6C). The sediments were primarily derived from the schist basement in the Wakatipu catchment, although some of the sediment may have been derived from the nearby Shotover River as well (Figure 2A). Minor gold mining in As-bearing vein systems occurred in the Shotover catchment, > 40 km to the north, but the As signal from mining residues is small and detectable only for ∼2 km downstream (Haffert and Craw 2010).

Downcutting by the Kawarau River has left the Frankton terrace unsaturated, with a dry surface, and near-surface sediments are weakly oxidised to a pale tan colour with minor staining by ferric oxyhydroxides (Figure 2D). At depth, the Frankton sediments are still grey and unoxidised, and the redox boundary is locally sharp (Figure 2D; cf Figure 6B) but is mostly transitional over >1 m. The oxidised surficial zone of the sediments on the Frankton terrace has minor elevated As (<200 mg/kg) in an area of ∼0.5 km² (Figure 2C,D; QLDC 2016). The unoxidised sediments below the redox transition have lower As concentrations (e.g.; 30–35 mg/kg; Figure 2D), close to typical background levels for these sediments on the Glenorchy delta (Figure 5A). Most of the present terrace surface sediments have As concentrations below 20 mg/kg (Figure 2C QLDC 2016), but the geological setting of those sediments has now been obscured by urban development.

A groundwater seep at river level below the terrace is actively depositing a mm-thick layer of ferric oxyhydroxide on the surface of an organic-rich wetland, and this ferric oxyhydroxide has adsorbed As up to 400 mg/kg (Figure 2E). Groundwater in the nearby Wakatipu aquifer has dissolved As near to, and locally exceeding, 0.01 mg/L (Figure 2B; ORC 2021). Water of this type flowing beneath the Frankton terrace is the probable source for the elevated adsorbed As in the wetland, as well as the dissolved ferrous iron to make the ferric oxyhydroxide at the discharge point (Figure 2E; cf Zheng et al. 2004; Haffert et al. 2010; Fresno et al. 2016). The As mobilisation and enrichment features at Frankton are essentially the same as those observed on the Glenorchy delta, although the Frankton site has more relief. These chemical processes are probably still active in the near-surface Glenorchy sediments and soils, whereas As enrichment of the Frankton terrace sediments has ceased since the sediments became unsaturated, and the active processes are now confined to lower levels below the Frankton terrace surface.

Conclusions

The Glenorchy area has locally elevated arsenic (As) derived from two geogenic pathways. The most widespread pathway involves Holocene shallow groundwater processes in schist-derived sediments on the low-lying fluvial delta (Figure 10A,B). Trace amounts of As are leached from sulfide minerals, transported in shallow anoxic groundwater as As³⁺ complexes, and some of that As becomes locally enriched in surficial oxidised zones, probably via adsorption as As⁵⁺ complexes to ferric oxyhydroxide. Elevated As concentrations, exceeding drinking water standards (>0.01 mg/L), can arise in the shallow groundwater, especially where that groundwater is slow-moving in low-permeability deltaic silts that contain unoxidised sulfide minerals. Measured As concentrations in boreholes in these sediments in Glenorchy town area are up to 80 times higher than recommended drinking water levels (Figure 1B). Elevated As concentrations in soils can develop above recommended levels (20 mg/kg), principally in the upper few cm of silt-rich sites that are frequently water-saturated.

The other geogenic pathway for As enrichment in the Glenorchy area involved Mesozoic hydrothermal processes to form quartz veins with scheelite (±gold) in schist basement (Figure 10C,D). These features have been disturbed at the surface by historic mining activity, resulting in small surficial deposits with high As concentrations (locally >10 000 mg/kg) in sulfide-rich rocks (Figure 10A). Waters discharging from mine adits have pH∼8 and dissolved As up to 0.3 mg/L, and water downstream of waste rock piles has As up to 0.03 mg/L. All these waters are rapidly diluted downstream to <0.02 mg/L in the principal drainage stream, the Buckler Burn (Figure 8A). The town of Glenorchy receives most of its domestic water supply from boreholes in gravel beside the lower Buckler Burn and this water has As concentrations <0.01 mg/L, compared to the locally high levels in boreholes in the silt-dominated deltaic sediments (Figure 1B; Figure 8A).

The spatial distributions of elevated geogenic As concentrations in water and soils in the Glenorchy delta area are as yet poorly defined, but the key features described above may assist in further spatial characterisiation of these issues. They are distinct from, and more widespread near the surface than, the localised As anomalies around historic W-Au mines. These near-surface geogenic processes are common around the world where relatively unoxidised rocks are rapidly eroded from mountains and deposited on fluvial plains. The processes have significance for human health because of exposure to elevated As concentrations in soils and waters in the Glenorchy area (Figure 1B) and geologically similar mountain areas in northwestern Otago, such as at Frankton (Figure 2A).

Acknowledgements

We thank Department of Conservation for a low-impact research permit for the Buckler Burn catchment. Field assistance by Lauren Farmer is gratefully acknowledged. Discussions with Jo Wakelin on Glenorchy soils were particularly helpful. Helpful reviews by Travis Horton and an anonymous journal referee improved the presentation of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

All data relevant to this study are included within this paper and within cited references.

Additional information

Funding

This research was funded by University of Otago.