Arsenic behavior in newly drilled wells (original) (raw)
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Environmental Geochemistry and Health, 2009
Arsenic concentrations exceeding 10 lg/l, the United States maximum contaminant level and the World Health Organization guideline value, are frequently reported in groundwater from bedrock and unconsolidated aquifers of southeastern Michigan. Although arsenic-bearing minerals (including arsenian pyrite and oxide/hydroxide phases) have been identified in Marshall Sandstone bedrock of the Mississippian aquifer system and in tills of the unconsolidated aquifer system, mechanisms responsible for arsenic mobilization and subsequent transport in groundwater are equivocal. Recent evidence has begun to suggest that groundwater recharge and characteristics of well construction may affect arsenic mobilization and transport. Therefore, we investigated the relationship between dissolved arsenic concentrations, reported groundwater recharge rates, well construction characteristics, and geology in unconsolidated and bedrock aquifers. Results of multiple linear regression analyses indicate that arsenic contamination is more prevalent in bedrock wells that are cased in proximity to the bedrock-unconsolidated interface; no other factors were associated with arsenic contamination in water drawn from bedrock or unconsolidated aquifers. Conditions appropriate for arsenic mobilization may be found along the bedrockunconsolidated interface, including changes in reduction/oxidation potential and enhanced biogeochemical activity because of differences between geologic strata. These results are valuable for understanding arsenic mobilization and guiding well construction practices in southeastern Michigan, and may also provide insights for other regions faced with groundwater arsenic contamination.
The Source and Transport of Arsenic in Northeastern Ohio Groundwaters
1980
Groundwaters in the Northwest School System area of Canal Fulton, Ohio were examined for their hydrologic and chemical properties. In general, the groundwaters in the study area were interconnected by a complex system of aquifer and aquitards. Arsenic concentrations were above EPA limits in two wells, but were elevated above the background value throughout much of the area. Two major aquifer systems exist within the study area: The Sharon Sandstone of the upland areas; and the outwash sand and gravel deposits of the buried valleys. Flow is generally from the north, but local variations are caused by the Tuscarawas River Valley on the south and west of the study area. Recharge to the local aquifer system may be occurring in the central portion of the study area from the chloride-con taminated Tuscarawas River. The areal distribution of arsenic within the study area indicates that the arsenic is not sourced from the Tuscarawas River and gives no indication of an outside source of arsenic. Within the study area, there is no evidence for an anthropogenic source of arsenic to the groundwaters. Agricultural soils, abandoned underground coal mines, industrial impoundments to the north, and an abandoned industrial dump site within the study area were all eliminated as possible sources for the arsenic. Theoretical and laboratory studies of arsenic in these groundwaters demonstrates that it is entirely of inorganic make-up, and consistsof about equal parts of arsenate and arsenite. Redox considerations suggest that arsenic is controlled by an adsorption equi1ibrium with ferric hydroxides, and that the reduction of the ferric hydroxides by a recent lowering of Eh and/or pH in the aquifer has liberated both iron and arsenic to solution. A high correlation between ferrous iron and total dissolved arsenic supports this model. ACKNOWLEDGEMENTS This study was performed as a Master's thesis at Case Western Reserve University by Christopher John Khourey, District Geologist, Ohio EPA. Additional information, interpretation, and recommendations may be found in the thesis (in preparation).
A review of the source, behaviour and distribution of arsenic in natural waters Contents
The range of As concentrations found in natural waters is large, ranging from less than 0.5 mg l À1 to more than 5000 mg l À1 . Typical concentrations in freshwater are less than 10 mg l À1 and frequently less than 1 mg l À1 . Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be 'affected' (defined as those exceeding 50 mg l À1 ) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing 'excessive' As. These large-scale 'natural' As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale, in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1-20 mg kg À1 . There appear to be two distinct 'triggers' that can lead to the release of As on a large scale. The first is the development of high pH (> 8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO 4 concentrations are small (typically 1 mg l À1 or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe. #
Study of arsenic content in mine groundwater commonly used for human consumption in Utah
Faculty of Built Environment and Engineering ??? or Environ. Technol.?, 2008
Of the various sources of arsenic released in to the environment, the presence of arsenic in water probably poses the greatest threat to human health. Arsenic is released in to the environment through water by dissolution of minerals and ores. Natural release is slow, but in some areas the concentration of arsenic in groundwater (commonly referred to as Acid Mine Drainage (or AMD)) is accelerated by mining activity. In fact the presence of arsenic may last a long time even after the mining activity has ceased. Hence it is imperative to study the quality of water (especially for those areas in the vicinity of mines) used for different purposes to identify an appropriate remediation technique for effective pollution control. In this paper, contents of arsenic and other metals in the water were quantified from three different sources: (1) groundwater from the mining tunnel (Judge tunnel), (2) drinking water, and (3) water used in the hydrant-flushed distribution system (Park City) in Utah (USA). The results showed the content of arsenic from the mining tunnel, after chlorination, and in tap water were below 10 microgl(-1). However, significant amounts of arsenic, lead, zinc, iron, manganese and antimony have been found in water samples taken from the distribution systems. In the consideration of the further use of mine groundwater for drinking purposes and the distribution system, Park City should regularly be maintained by a flushing program in the distribution system.
Archives of Environmental Contamination and Toxicology, 2021
The presence of elevated arsenic concentrations (≥ 10 µg L−1) in groundwaters has been widely reported in areas of South-East Asia with recent studies showing its detection in fractured bedrock aquifers is occurring mainly in regions of north-eastern USA. However, data within Europe remain limited; therefore, the objective of this work was to understand the geochemical mobilisation mechanism of arsenic in this geologic setting using a study site in Ireland as a case study. Physicochemical (pH, Eh, d-O2), trace metals, major ion and arsenic speciation samples were collected and analysed using a variety of field and laboratory-based techniques and evaluated using statistical analysis. Groundwaters containing elevated dissolved arsenic concentrations (up to 73.95 µg L−1) were characterised as oxic-alkali groundwaters with the co-occurrence of other oxyanions (including Mo, Se, Sb and U), low dissolved concentrations of Fe and Mn, and low Na/Ca ratios indicated that arsenic was mobilise...
Correlates of Arsenic Mobilization into the Groundwater in El Paso, Texas
Air, Soil and Water …
This paper addresses the contamination of groundwater by arsenic, a naturally occurring phenomenon that has caused serious cases of arsenic poisoning around the world. While a number of chemical processes are known to be capable of mobilizing arsenic, the extent to which different processes are active in actual geological settings is much less clear. In this work, the El Paso, Texas region is analyzed as a case study to better understand the factors associated with high arsenic levels in groundwater. This study includes two basins that supply drinking water to approximately 2.5 million people. The average arsenic was 8.5 ppb, which is below the current American and WHO Maximum Contaminant Level of 10 ppb. However, arsenic concentrations reached approximately 80 ppb in three different locations. Governmental archival information was combined with field water sampling, and with leaching and analysis of solid phase materials from well cuttings (sediments of the aquifers). The study identifies evidence for both competitive desorption and reductive dissolution operating to mobilize arsenic, with the importance of different mechanisms likely varying throughout the aquifers. A mean of 21% of the solid arsenic content was leached out to solution at pH 9, and mean solid phase arsenic concentration was 4.3 ppm, solid phase iron 7000 ppm, and solid carbon 0.6%, consistent with arsenic desorption out of sediments into the aqueous phase. A potential role of geothermal waters was also identified at a southern hot spot. This information is important to better understand the basic science of the arsenic geochemical cycle and may also provide a rough guide as to where low arsenic waters may be found: groundwater with high potentiometric head and short flow paths, groundwater under the influence of surface water, and lower pH groundwater.
Seasonal fluctuations and mobility of arsenic in groundwater resources, Anchorage, Alaska
2011
There are approximately 12,000 private wells in the Municipality of Anchorage, Alaska, USA, that supply drinking water to thousands of homeowners. The results presented in this paper are from a study conducted to understand the speciation and seasonal fluctuations of As in the groundwater of Anchorage. A total of eight private drinking water wells were sampled from May to October, 2007, to determine inorganic species of As (III/V) and other physiochemical parameters of the groundwater. Arsenic concentrations above Environmental Protection Agency (EPA) drinking water standard of 10 lg/L had been previously measured in all of these eight wells by the Municipality of Anchorage. Seven of the wells draw water from glaciofluvial aquifers and one well taps into the older bedrock aquifer on the mountain-side. In-situ measurements as well as groundwater samples were collected from each well and inorganic species of As (III/V) were separated in the field using anion exchange columns. Elemental analysis was conducted by inductively coupled plasma mass spectrometry (ICP-MS), major anions by ion chromatography (IC), and Fe (II) and total Fe were measured in the field with a portable spectrophotometer. The groundwater was neutral to basic ranging in pH from 7.6 to 8.8, dissolved O 2 was generally 0.1-1.0 mg/L and specific conductance ranged from 300 to 1000 lS/cm. Most of the groundwater is classified as Ca-Mg-HCO 3 and dissolved As concentrations ranged from below detection ($0.5 lg/L) to 117 lg/L with arsenite as the dominant species. Filtered and unfiltered water samples had less than 1% difference in As concentrations suggesting that most As occurs in the dissolved form. Arsenic concentrations were positively correlated with water levels indicating that the highest As concentrations occur in the aquifer during recharge events. Positive relationships with dissolved Fe and supersaturation with respect to secondary Fe oxides indicates that the As is likely associated with the Fe oxides that are partially dissociated under the dominating reducing conditions of the aquifers. There is also evidence of a positive relationship between As and SO 2À 4 which indicates that some of the As may be associated with the oxidation of Febearing sulfides such as pyrite or arsenopyrite, however, this is thought to be a less important process for this system compared to Fe reduction. Most of the groundwater samples indicate supersaturation with respect to carbonates such as calcite, dolomite and siderite and there is a positive relation between As and HCO À 3 indicating that carbonate buffering is an important process in the groundwater geochemistry of As. In some cases homeowners filter their tap water with household water treatment systems which have a range of As removal effectiveness from about 25-100%.
Can Arsenic Occurrence Rates in Bedrock Aquifers Be Predicted?
Environmental Science & Technology, 2012
A high percentage (31%) of groundwater samples from bedrock aquifers in the greater Augusta area, Maine was found to contain greater than 10 µg L −1 of arsenic. Elevated arsenic concentrations are associated with bedrock geology, and more frequently observed in samples with high pH, low dissolved oxygen, and low nitrate. These associations were quantitatively compared by statistical analysis. Stepwise logistic regression models using bedrock geology and/ or water chemistry parameters are developed and tested with external data sets to explore the feasibility of predicting groundwater arsenic occurrence rates (the percentages of arsenic concentrations higher than 10 µg L −1 ) in bedrock aquifers. Despite the under-prediction of high arsenic occurrence rates, models including groundwater geochemistry parameters predict arsenic occurrence rates better than those with bedrock geology only. Such simple models with very few parameters can be applied to obtain a preliminary arsenic risk assessment in bedrock aquifers at local to intermediate scales at other localities with similar geology. .