(C) PLOS One
This story was originally published by PLOS One and is unaltered.
. . . . . . . . . .



Arsenic in groundwater in the Grand Canyon region and an evaluation of potential pathways for arsenic contamination of groundwater from breccia pipe uranium mining [1]

['Fred D. Tillman', 'U.S. Geological Survey', 'Arizona Water Science Center', 'Tucson', 'Arizona', 'United States Of America', 'Kimberly R. Beisner', 'New Mexico Water Science Center', 'Albuquerque', 'New Mexico']

Date: 2023-07

Eighty-seven pairs of environmental and replicate sample results for arsenic were available for the study area. Seven of these replicate pairs had arsenic values that exceeded the USEPA Maximum Contaminant Level (MCL) of 10 μg/L; these pairs had less than 2% difference between the environmental and replicate samples. The difference between all environmental and replicate samples ranged from 0 μg/L to a maximum difference of 1.2 μg/L (Fig A in S1 Fig , Table A in S1 Table ). Dissolved arsenic results were available for 42 blank analyses. All but two were below the reporting limits; the two blank samples with recorded arsenic were both 0.02 μg/L. Based on this evaluation of replicate and blank quality assurance results, there is high confidence in environmental sample results >1 μg/L and expected sampling and laboratory variability is ≤1.2 μg/L. Available replicate and blank results for other parameters and elements discussed in this manuscript are provided in Table A in S1 Table . To summarize these quality-control data, there is high confidence in sulfate and uranium results at similar levels as arsenic (i.e., >1 μg/L), total dissolved solids at >1 mg/L, tritium at >0.5 pCi/L (based on 2 replicates), pH at 0.1 standard units (based on 2 replicates), and dissolved oxygen at 0.1 mg/L (based on 2 replicates). Iron quality-assurance results periodically indicate the presence of sample contamination, with iron detections in blank samples as high as 5.7 μg/L and substantial discrepancies between replicates at <100 μg/L iron concentrations (e.g., replicates of 32 and 17 μg/L, 4 and 10 μg/L, 7.8 and 0.9 μg/L, 7 and 40 μg/L). Therefore, there is low confidence in accuracy or precision of iron concentrations below about 100 μg/L.

Site-wide observations on arsenic concentrations.

Analytical results for arsenic in groundwater in the Grand Canyon region were available for 652 samples collected from 230 sites between June 1, 1977, and September 30, 2022. Site information and analytical results for all samples are available in Tables B and C in S1 Table, and mapped results are presented in Fig 4 [43]. Additionally, arsenic concentration results in this study may be further explored in the USGS online interactive map “Uranium and Arsenic in Groundwater in the Grand Canyon Region” (https://webapps.usgs.gov/uraniummap/). Of the 230 groundwater sites sampled, 48 are wells and 182 are springs. Springs primarily discharge along canyon walls and floors, located above, within, and below the Permian strata that host uranium ore in breccia pipes in the area. Wells are located on the plateaus above the Grand Canyon and have depths ranging from 24 m to more than 1,100 m (Table B in S1 Table).

Maximum observed arsenic concentrations at groundwater sites in the Grand Canyon region ranged from <1 μg/L at 52 sites to 875 μg/L at Pumpkin Spring (USGS Site ID 355459113195900, S1 in Fig 4). About 88% (202) of the 230 groundwater sites had a maximum arsenic concentration at or below the USEPA MCL of 10 μg/L (Figs 4 and 5), substantially less than the 95% of groundwater sites with uranium concentrations below the uranium MCL reported by Tillman et al. [8]. Of the remaining 12% of sites, all but three had maximum concentrations <40 μg/L. The sites with a maximum arsenic concentration above the MCL are scattered throughout the study area, but some general observations can be made. Of the 22 spring sites with a maximum arsenic concentration above the MCL that discharge along the Colorado River or its tributaries, all but Saddle Canyon spring (USGS Site ID 362135111541801, S2 in Fig 4) and Fence Fault Left spring (USGS Site ID 363100111504701, S3 in Fig 4) are located on the South Rim of the Grand Canyon. The springs with the highest maximum arsenic concentration are in the southwestern region of the study area on or near the Hualapai Indian Reservation.

Previous studies have indicated likely pH and redox controls on arsenic mobilization and transport in groundwater, and possible relations between arsenic and other water-chemistry constituents and properties including concentrations of sulfate, iron, total dissolved solids (TDS), and groundwater age, among others. Paired results for pH and maximum arsenic concentration at a groundwater site were available for 173 sites in the study area. For these samples, pH values ranged from 3.8 to 8.8, with a median of 7.7 (Fig B in S1 Fig), and arsenic was significantly (p-value = 0.003) but weakly (0.15) correlated with pH (Kendall’s tau; Fig B in S1 Fig). Paired dissolved oxygen (DO) and maximum arsenic concentrations were available for 152 groundwater sites. DO at these sites ranged from <1 mg/L to 11.5 mg/L, with a median of 7.2 mg/L. Based on the definition by Thiros et al. [52] of oxic waters having DO concentrations ≥ 0.5 mg/L, 10 sites (2.3%) in the study area are considered anoxic. DO was not correlated with arsenic concentrations at study area sites (Kendall’s tau; p-value > 0.05; Fig B in S1 Fig). Oxidation of sulfide minerals, especially in mining or mineralized areas, may influence arsenic levels in groundwater [33]. Maximum arsenic concentrations and available paired sulfate concentrations were evaluated for 185 sites in the study area, with sulfate ranging from 1.5 mg/L to 3,450 mg/L with a median of 73.0 mg/L (Fig B in S1 Fig). Sulfate was not correlated with arsenic concentrations at study area groundwater sites (Kendall’s tau; p-value > 0.05). Iron and arsenic concentrations may be correlated in sediments and reductive dissolution of iron oxides may be important for mobilizing arsenic in groundwater [33]. Maximum arsenic concentrations and available paired iron concentrations were evaluated for 187 sites in the study area, with iron ranging from < 0.2 μg/L to 10,300 μg/L with a median of 4 μg/L (Fig B in S1 Fig). Iron was not correlated with arsenic concentrations at study area groundwater sites (Kendall’s tau; p-value > 0.05), although further investigation of iron in different oxidation states in water may provide more insight about the relationship. In an investigation of principal aquifers in the western United States, Rosecrans and Musgrove [41] found high arsenic concentrations to be associated with TDS concentrations greater than 500 mg/L. Paired arsenic and TDS sample results were available for 183 groundwater sites, with TDS concentrations ranging from 69 mg/L to 9,630 mg/L, with a median of 459 mg/L (Fig B in S1 Fig). TDS was not correlated with arsenic concentrations at study area groundwater sites (Kendall’s tau; p-value > 0.05). Elevated arsenic has commonly been found in premodern or mixed-age groundwater in the western United States [41]. As a broad estimation of groundwater age in the study area, paired tritium and maximum arsenic concentrations were compared for 138 sites (Fig B in S1 Fig). Tritium samples at these sites ranged from -1 pCi/L to 33 pCi/L, with a median of 2.2 pCi/L. Elevated arsenic concentrations were not associated with either premodern (< 1.3 pCi/L) [25] or modern groundwater in the study area. Although not known to be a control on, or commonly related to, arsenic in groundwater, uranium concentrations also were evaluated with arsenic at groundwater sites, owing to concern about uranium mining effects on groundwater-quality in the region (Fig C in S1 Fig). Paired results for uranium and maximum arsenic concentration at groundwater sites were available for 205 sites in the study area. For these samples, uranium values ranged from 0.114 μg/L to 231 μg/L, with a median of 3.7 μg/L. Uranium was not correlated with arsenic concentrations at study-area sites (Kendall’s tau; p-value > 0.05).

The specific geology and stratigraphic unit that spring sites discharge from is not always known for samples in this study, but some of the high arsenic values were found in travertine springs. Modern travertine-depositing springs and inactive travertine deposits are often located along faults in the Grand Canyon [26]. Pumpkin Spring (S1 in Fig 4), Travertine Falls Spring (maximum arsenic concentration = 243 μg/L, USGS Site ID 354522113264800, S4 in Fig 4) and Travertine Canyon above the mouth at River Mile 229 (maximum arsenic concentration = 100 μg/L, USGS Site ID 354503113252600, S5 in Fig 4) are active travertine-depositing springs on the Hualapai Indian Reservation in the western part of the Grand Canyon region. Pumpkin Spring is fault-controlled, and although it discharges from the saline Tapeats Sandstone, its warm temperature may indicate a deeper, geothermal source [26, 53]. Fence Spring (maximum arsenic concentration = 16.4 μg/L, USGS Site ID 363123111503101, S6 in Fig 4), located in the eastern portion of the Grand Canyon region and along the Fence fault zone, also is a travertine-depositing spring. High arsenic concentrations at these sites may be influenced by deep mantle mixing [26] and carbonate concentrations in groundwater [54]. Relationships between high arsenic and travertine have been found throughout the western United States, including Mammoth Hot Springs in Yellowstone National Park of Wyoming [55] and Montezuma Well located in the Verde Valley of Arizona [56, 57], and likely represent groundwater mixing with deep-earth hydrothermal fluids containing elevated arsenic concentrations [41].

Other groundwater sites with elevated arsenic concentrations do not have a clear source. Near Travertine Falls spring and Travertine Canyon spring, springs B-27-11 10 UNSURV (USGS Site ID 354406113263400, S7 in Fig 4, about 3 km south of Travertine Falls spring) and B-28-12 35 UNSURV (USGS Site ID 354550113313400, S8 in Fig 4, about 9 km west of Travertine Falls spring), have maximum arsenic values of 38 μg/L and 35 μg/L, respectively. Six mineralized breccia pipes with above-background levels of uranium are present on the west rim of Travertine Canyon. Billingsley et al. [58], however, found no obvious relation between locations or alignments of faults and breccia pipes in this area. Wooley Spring (USGS Site ID 365308112472301, S9 in Fig 4) is located north of the Grand Canyon on the Kaibab Indian Reservation and has a maximum arsenic value of 35.8 μg/L. Wooley Spring discharges from the Kayenta Formation of the Glen Canyon Group of Jurassic age, which is present in limited northern parts of the study area [59]. Havasu Spring (USGS Site ID 361303112411200, S10 in Fig 4), the primary source of water for the Havasupai Tribe in Supai Village, is a travertine spring with a maximum observed arsenic concentration of 17 μg/L. Supai Well No. 3 (USGS Site ID 361352112413201, W1 in Fig 4), a 46-m-deep alluvium well located two kilometers downstream of Havasu Spring, has been observed with arsenic values of 5.2 μg/L (2016) and 12 μg/L (1994).

Groundwater in Arizona, like much of the southwestern United States, has elevated arsenic concentrations when compared to the rest of the United States [14]. In drinking-water wells in the southwestern United States, arsenic was found to exceed the MCL more than twice as frequently than in drinking-water wells in the United States as a whole [52]. Primary sources of arsenic in Arizona groundwater include geothermal water and release from volcanic rocks [12, 14]. Water quality data through September 30, 2022 for 2,343 groundwater sites in Arizona outside of the Grand Canyon study area available in NWIS [43] (Fig D in S1 Fig) have a maximum arsenic concentration ranging from < 1 μg/L to 1,400 μg/L, with a median of 4 μg/L. 25% (653) of the Arizona groundwater sites not located in the study area have a maximum arsenic concentration that exceeds the USEPA MCL. The maximum arsenic concentration for all Arizona springs was from the Verde Hot Springs (USGS Site ID 342118111423201; Fig D in S1 Fig) sampled in 1979 (1,400 μg/L), which is classified by Foust et al. [56] as geothermally influenced. Verde Hot Springs is the only spring sampled in Arizona with a higher arsenic concentration than Pumpkin Spring. The maximum arsenic concentration in well samples for all Arizona was taken in 1978 at a 500-ft deep well, C-19-01 28ADC (USGS Site ID 314439112222201), located near the border of Mexico (830 μg/L; Fig D in S1 Fig). Although the Grand Canyon region contains groundwater discharge locations with high levels of arsenic, maximum arsenic concentrations at groundwater sites in the Grand Canyon study area are significantly less than maximum arsenic concentrations at other groundwater sites in Arizona (nonparametric Wilcoxon test; p-value < 4.0 × 10−12; Fig E in S1 Fig).

[END]
---
[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000109

Published and (C) by PLOS One
Content appears here under this condition or license: Creative Commons - Attribution BY 4.0.

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/plosone/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.