Wednesday, 13 July 2016

Groundwater finds its way to surface water – but where?

Heather Martindale sampling
in the Hutt River.
Heather Martindale (GNS Science Water Dating Laboratory Snr Technican) recently graduated with a Masters in Environmental Management from Massey University in Palmerston North. Her project looked at “The use of radon and complementary hydrochemistry tracers for the identification of groundwater – surface water interaction in New Zealand”.

Knowing the interaction between groundwater and surface water is an essential part of understanding the movement of harmful nutrients in groundwater to surface water.

Why is Radon used as a tracer to find out where groundwater flows into surface water?

Radon-222, a colourless gas with a half-life of 3.8 days, is generated by the decay of uranium present in rocks and soil and is abundant in groundwater. While, radon has a very low concentration in surface water (rivers, streams) due to degassing (water releasing the gas to the atmosphere), the concentrations are slightly higher at, and/or immediately downstream of groundwater leakage to surface water. This concentration difference makes radon an ideal tracer to identify locations of groundwater leakage to surface water.
Heather’s Master’s project involved the collection of water samples at 500m – 1000m intervals in a 14km stretch of the Hutt River. She mixed the samples with a scintillation cocktail. When radon radioactively decays, it emits energy in the form of an alpha particle. This energy is absorbed by the scintillation cocktail and then released as a measurable light pulse. Heather’s study clearly demonstrated the usefulness of radon as a tool for identifying groundwater leakage into surface water, as shown in Fig. below.

Shows the measured radon concentrations in the Hutt River, with green (no radon) indicating areas of  no groundwater discharging into the river and red (high radon) indicating areas of high groundwater discharge into the river.
 For further reading, Heather’s full thesis is published here.

Friday, 15 April 2016

Small but powerful XRF-device

Figure 1: Adam Martin is using the 
hand-held XRF in the field. This method 
has the advantage of giving a result
straight away and helps deciding which
samples should be send to a Laboratory to
 get a more precise answer.
Adam Martin is a Research Geologist based at GNS Science’s Dunedin office. In Figure 1 he is using a state of the art hand-held X-ray fluorescent (XRF) device to obtain in-situ geochemical information for elements between Magnesium and Uranium in the Periodic Table. This device can determine elemental compositions for bedrock (outcrop), rock pieces (hand specimen), sliced and polished rock (thin sections), cylindrical depth sections (drill cores), rock chips, soil, fluid and liquid samples.

Figure 2: Diagram showing the principle of the XRF device.
 The XRF device sends out a continuous x-ray which hits an atom and ejects an electron from the atom’s orbital shell. This gap gets filled by an electron dropping from a higher energy orbital shell to a lower energy orbital shell releasing a fluorescent x-ray (Figure 2). It is used to analyse a substance to determine the chemical composition of the sample of interest. Nearly every element has a unique fluorescent x-ray energy signature.

Figure 3: Lead (Pb) versus arsenic (As) plot for hand-held and lab XRF.

This instrument was used to analyse soil compositions at various locations around Dunedin City as part of an urban geochemical baseline survey to help assess the state of the environment in our cities. This provides a chemical snapshot that can be used to assess human and geological influences. The measurements so far of lead (Pb) and arsenic (As) concentration by hand-held XRF are comparable to Pb and As concentrations measured by lab XRF (taken from a more regional soil study as shown in the graphic opposite in Figure 3). These results show that Dunedin City soil has safe levels of Pb and As and that hand-held XRF results can be used to supplement laboratory XRF data for certain elements.

Wednesday, 17 February 2016

Iron nano-particle magnetic sensor – a new device to measure magnetic fields

Magnetic field measurements are found in many applications around us. This ranges from the compass system in our smartphones, vehicle detection for traffic lights, metal detection at airport security to non-contact measurements in power supplies. While many existing technologies are available, our industrial partners have identified gaps where existing technologies are insufficient in their current form. For instance, there is a need for sensors that can measure low magnetic fields reliably even after exposure to large magnetic fields. This means overcoming the magnetic remanence naturally occurring in most magnetic materials used in sensors. Another challenge that was presented to us was to be able to measure both low and large magnetic fields with a single compact system. Our approach, considering the use of nanostructured materials, is aimed at solving these challenges and provide our partners with useable and more efficient alternatives to existing solutions.

Jérôme Leveneur, John Futter and John Kennedy 
(from left) setting up the ion implanter.
Dr John Kennedy and his team of scientists at GNS Science developed a new process using ion implantation and thermal processing in order to create a new material in which electrical resistance varies with a magnetic field. Currently magnetometer devices measuring large magnetic fields are based on the Hall effect. However, they exhibit a large temperature dependency which is not favorable for easy measurements. The materials developed by the team use a different mechanism which is more resilient to temperature changes. It relies on having iron nanoparticles, with dimensions of the order of a thousand times smaller than the diameter of an average human hair, on the surface of an insulator 1.

Ion implantation is a technique used to modify the composition and properties of the  surface of materials. It uses a particle accelerator which first strips electrons from some atoms to create ions which are then accelerated and funneled towards the sample to modify it. The bombarding of ions through this method progressively modifies the material’s surface. This technique is commonly used in microelectronics to precisely tune the properties of electronic components on a chip (electronic circuit on a small plate of silicon). In fact, most consumer electronics have now had components treated with such a technique!

Schematics of magnetic sensor
Sensor are made by implanting iron atoms into the first 30 nanometres of a 400 nanometre thick substrate of SiO2 [Silicon dioxide], (1 nanometre = 0.000000001m). The ion implanted target is then heat treated for one hour at 1000 °C to form iron nano-particles.

Uniformly formed iron nano-particles measured by AFM.

Uniformity of the produced iron nano-particle is determined by measuring the surface with an Atomic Force Microscope (AFM). The AFM moves over the sample surface line-by-line and the up and down movements get recorded and stitched together to give a 3D topography image of the surface at very small scale.

1  United States Patent number: US 8,872,615 B2