Release of nickel by pyrite oxidation caused by barometric pumping
This report presents an investigation with the aim of evaluating the importance of the phenomena "barometric pumping" or "barometric breathing" on the release of nickel to the groundwater in the Limestone aquifers of the eastern part of Zeeland, Denmark. In this area, elevated nickel concentrations in the groundwater have proven to be an increasing water quality problem. Nickel can cause allergic reactions and is therefore undesirable in groundwater in even rather small concentrations. In Denmark, the drinking water limit for nickel is at the moment 20 µg/l.
Barometric pumping can occur where atmospheric air is in contact with the vadose zone gas phase, e.g. where boreholes are screened in the vadose zone. Under such conditions it has been observed that atmospheric pressure fluctuations can cause subsurface gas flow from regions of high pressure towards those with lower pressure.
In more detail, barometric pumping can occur provided that:
The atmospheric pressure is constantly changing. During periods of increasing atmospheric pressure, advective transport of gas into the vadose zone will take place, while when the atmospheric pressure decreases gas flows out of the vadose zone.
Atmospheric oxygen is supplied to the vadose zone during periods of increasing atmospheric pressure. If sulphide minerals (e.g. pyrite) are present in the vadose zone, these will react with the atmospheric oxygen. Oxidation of the sulphide minerals causes the release of iron, sulphate, and a number of trace elements, including nickel to the groundwater.
The importance of barometric pumping in relation to changes in the natural groundwater quality has thus far not been documented.
In the present project, measurements were first carried out in five water abstraction wells, which were chosen in order to investigate different water abstraction situations. Furthermore, where it was possible, the wells were chosen in order also to represent different geological settings. Following this, a detailed study was carried out at Tune (south-east of Roskilde) where the barometric pumping had shown to be largest. The purpose of the detailed study at Tune was to obtain detailed information regarding the chemical and physical processes taking place due to barometric pumping. Furthermore, the purpose was to collect data in order to be able to quantitatively evaluate the importance of barometric pumping on changes in the regional water quality. Finally, the knowledge obtained from the study at Tune was used to evaluate the importance of barometric pumping in relation to the present nickel problems in groundwater in the eastern part of Zeeland.
The subsurface gas transport
Data collection was carried out systematically in five selected water abstraction wells located at Tårnby, Tune, Hvidovre, Brøndby, and Tommestrup, respectively. Barometric pumping was observed in three out of the five wells. The three wells where barometric pumping was observed are completed in fine-grained, almost chalk-like deposits of Danian age and Greensand deposits of Selandian age.
The magnitude of the gas flow caused by barometric pumping is dependent on the effective gas permeability and the volume of the gas phase in the vadose zone. In the limestone aquifers investigated in this project, advective gas flow occurs exclusively in high permeability fractures, as opposed to the low permeability matrix, which is nearly totally water saturated due to capillary forces. For a given change in barometric pressure, there is a linear relationship between the volume of vadose zone affected and the magnitude of gas flow.
The average measured gas flow for the five abstraction wells was between 0.5 m3/h and 17 m3/h, and the cumulative volume of gas exchanged between atmosphere and vadose zone was up to 1,500 m3. At extreme pressure changes of up to 70 mbar, up to 5,000 m3 of gas may be exchanged between atmosphere and vadose zone at Tune, where the maximum gas flow was measured to be 60 m3/h, and the annual exchange of gas between the atmosphere and the vadose zone gas phase amounts to 200,000 m3. Likewise at Tune, the vadose zone gas volume affected by barometric pumping is calculated to be in the order of 70,000 m3. The smallest barometric pumping was observed at Brøndby, where the affected vadose zone gas volume is 2,000 m3 and the annual exchange of gas between the atmosphere and the vadose zone amounts to 5,000 m3.
The chemical processes
Measurements of the chemical composition of the gas entering or leaving the vadose zone document together with the flow measurements that chemical reactions in the vadose zone change the chemical composition of the gas with an atmospheric origin.
The oxygen consumption, in the three wells where barometric pumping was observed, amounts annually to between 230 m3 and 4,300 m3 (equal to 10,000 and 190,000 mol/yr, respectively). Assuming that all oxygen is used for pyrite oxidation, this is equal to an annual oxidation of between 320 kg and 6,000 kg pyrite. Furthermore, assuming the nickel content in the pyrite to be between 0.2 and 3 g/kg pyrite (equal to 0.02-0.3 % (w/w) or 0.4-6 mmol Ni/mol pyrite (Knudsen, 1999; this study)), the pyrite oxidation will result in an annual release of between 0.06 and 18 kg nickel. Apparently, the oxidation of pyrite takes place locally at the wells, and therefore the amount of pyrite present in the vadose zone will decrease with time. Estimates of the time required to remove pyrite totally from the affected part of the vadose zone by barometric pumping provides remaining lifetimes for the nickel release in the order of up to 100 years.
In the detailed study at Tune, data were collected from sediment, water, and gas phase. Three new boreholes were drilled at Tune at distances of 10, 40, and 100 m from the water abstraction well, where considerable barometric pumping was observed in the first part of the study. Sediment cores from the vadose zone were taken in all three boreholes and sediment analyses were carried out in order to determine pyrite and nickel content and other sediment related parameters. The chemical composition of the groundwater and the porewater in the vadose zone was also determined. Gas flow in the abstraction well and gas composition with respect to oxygen and carbon dioxide in the three new boreholes and the abstraction well were measured every hour during a period of one month in the spring 2002.
The results from Tune show high sulphate and nickel concentrations in the porewater at a distance of at least 100 meters from the abstraction well. Sulphate is present at concentrations up to 2,000 mg/l and the nickel concentration is up to 40 µg/l in the vadose zone. At the location of the groundwater table, the nickel concentration is as high as 300 µg/l. Chemical speciation calculations show sulphate concentrations in the porewater of the vadose zone to be controlled by equilibrium with gypsum (CaSO4· 2H2O). Sediment extractions show that a large part of the nickel, which originally has been released by pyrite oxidation, is retained in the sediments. In addition, preliminary results from laboratory experiments have shown a considerable binding of nickel by the sediments. Thus, at the aqueous concentrations observed at Tune, approximately 95 % of the nickel in a sediment/water batch is present in the solid phase. The binding agents in the sediment are presumably glauconite, calcite and/or iron oxides.
The mean pyrite content of the sediments in the investigated wells is at present between 2 and 20 mg pyrite/kg sediment (15 and 160 µmol pyrite/kg sediment), and as a consequence of more frequent gas exchange occurrences close to the abstraction well, the pyrite content increases with increasing distance from the abstraction well. The average nickel content of pyrite grains from Tune was measured to be between 0.02 and 0.05 % (w/w), with maximum concentrations of up to 0.45 % (w/w). In comparison, Knudsen (1999) observed average nickel contents in pyrite in the range of 0.04-0.3 % (w/w) in a study of pyrites from the area around Copenhagen.
The chemical composition of the pore-/groundwater, the pyrite distribution of the sediments, and the observed oxygen consumption in the vadose zone indicate that considerable pyrite oxidation takes place in the vicinity of the abstraction well at Tune. Furthermore, this oxidation causes a considerable release of nickel, resulting in high concentrations of nickel in sediment and pore water. The pyrite source present in the affected part of the vadose zone will be used up over time, because the pyrite oxidation apparently only takes place locally around the air-entry well.
At Tune, it has been shown that the sediment associated nickel and sulphate is released in large amounts to the groundwater when the level of the groundwater table increases. Sulphate is released as a consequence of the dissolution of gypsum. The process releasing nickel in this case is not known yet, but is probably the result of either dissolution of carbonate minerals or desorption from carbonate minerals, clay minerals or oxides. However, more detailed investigations are required in order to obtain a good description of the nickel releasing process in the case of an increasing groundwater table.
On the basis of data from the councils, previously published reports, and database information, thematic maps have been constructed with the purpose of evaluating the regional importance of barometric pumping in relation to the release of nickel to groundwater. In line with the conclusions of other studies in the area around Copenhagen, the thematic maps show close correlation between areas with unconfined water table and areas with both high sulphate and nickel concentrations. Often, the sulphate concentration exceeds 100-200 mg/l in these areas. In many of the high sulphate areas, infiltration of nitrate to the groundwater is considered negligible, hence indicating that pyrite oxidation takes place as a consequence of supply of oxygen to the vadose zone by gas transport. The transport mechanism can be either diffusion or advective flow caused by barometric pumping.
Generally, a good correlation between high sulphate and high nickel concentrations exists. However, deviations may occur locally because nickel is retarded in the sediments while sulphate is transported conservatively at the same velocity as the groundwater. At some places, high nickel concentrations may occur as a consequence of a rising groundwater table, and if gypsum is present in the vadose zone, high sulphate concentrations will also be present.
Previous investigations have documented the presence of certain zones (stratigraphical horizons) with sulphides especially high in nickel content (Knudsen & Nygård, 1996; Knudsen, 1997; 1999). Locally, this can result in extraordinarily high nickel concentrations per amount of oxygen supplied, when sulphides in these horizons of the saturated zone are oxidized. However, this is only possible in the case where dissolved oxygen in the saturated zone reaches these horizons as the first sulphide containing zone. Additional stratigraphic information for the area than is currently available is necessary in order to provide a more quantitative description of the relative importance of sulphide oxidation in the high nickel content horizons.
Under certain circumstances, nickel may locally be released from peat deposits. However, nickel released in these settings is probably adsorbed by organic matter in the peat or clay minerals in underlying clay deposits and therefore does not reach abstraction wells.
In summary, the important new findings and conclusions regarding the problem of high nickel concentrations in the groundwater of eastern Zeeland are:
Barometric pumping through wells is a consequence of traditions regarding the construction of water abstraction wells in areas where water abstraction takes place from limestone aquifers. Typically, the casing is completed about 1-2 meters down into the limestone formations. Below this level, the borehole comprises an open hole in the limestone. If the limestone aquifer is unconfined and at the same time the casing is finalised above the groundwater table, an exchange of gas between the atmosphere and the vadose zone can take place due barometric pumping resulting in the release of nickel and other water quality problems.
In areas where high nickel concentrations occur due to barometric pumping, the following precautions are recommended:
Several additional details regarding the construction and management of abstraction wells in areas where nickel problems occur or might occur are provided in the report.