Study of Gross Alpha and Gross Beta Radioactivity in Soils Compared to Airborne RADNET Levels

Transcription

1 Study of Gross Alpha and Gross Beta Radioactivity in Soils Compared to Airborne RADNET Levels L. Tyler, A. Wallace Department of Physics and Geosciences, Angelo State University, Texas Abstract Since radionuclides can be found in soil, vegetation, air, and water we decided to focus on the correlation between the levels of gross alpha and gross beta radioactivity in the soil and to known airborne levels within the same area and time frame using online RadNet data available from the Environmental Protection Agency (EPA). The soil was primarily chosen for testing due to the unique unstudied geology of the area sampled. The Red Arroyo River is unique due to the fact that it is an active watershed yet it is a stagnant system with almost zero movement of water or sediments unless abundant rainfall occurs. The sampled area was a 0.5 mile portion along the Red Arroyo River in San Angelo, Texas in which 20 total samples were collected. The sampling processes elapsed over a 1 month time period. Soil samples had a gross beta average of ± Bq kg -1 and gross alpha average of 4.91 ± 1.12 Bq kg -1 respectively. In comparison, the levels taken from the RadNet (EPA, 2016) system were averaging ± mbq kg -1 for gross alpha and ± mbq kg -1 during this time. The advantage to these gross beta and gross alpha data sets will to help provide useful data in creating a baseline for the San Angelo, Texas area. 1.0 Introduction Radionuclides are found in air, water, soil, and plants (Biswas et al., 2015). Radionuclides are present everywhere and in everything we do in our daily lives from the roads we drive on to the food on our plates at our dinner table. While everywhere, radionuclides also range in radioactivity levels they emit. Some are natural at background levels and others are man-made under various levels of radioactivity. A problem that arises from this is the range in which these radionuclides are projecting harmful radioactivity levels. Although some levels of radioactivity is safe to be around at background levels there are some that can be major health and safety issues from direct and even indirect exposure. While the monitoring of personal exposure is of importance, some radionuclides have the potential to affect crops and vegetation or even physical materials that we humans can come in contact with thus causing personal harm unknowingly.

2 Page 1 One way to cut back the chance of these tragedies happening is by simply monitoring the radioactivity levels in the environment. Conducting research studies such as this presents the chance to gather a large amount of data from our samples to extrapolate a baseline for a given area. For this research it was concluded that the correlation between airborne values and the deposited radionuclides in the surrounding soils would provide this baseline. Soils acts as a source of transfers of radionuclides through the food chain depending on their chemical properties and the uptake process by the roots to plants and animals; hence, it is the basic indicator of the radiological status of the environment (Biswass et al., 2016). Since the soils must obtain their levels from some source that suggests fallout from airborne currents are the answer. In this comparison between soil radioactivity levels and the airborne radioactivity levels a baseline for the Red Arroyo River area will be established. 1.1 Site Description The Red Arroyo River is located in the West Central portion of San Angelo surrounded by numerous residential communities on all sides. The 0.5 mile expanse chosen for this study encompasses the area between Sul Ross Street and College Hills Boulevard as seen in Figure 1. The area studied is relatively level along the stretch with slight dipping in topology running South East. Water flow of the river for the majority of the year is very minimal except during major rain events when it floods. Geologically, it s homogeneous along the sampled stretch consisting of poorly graded sandy clays with only minimal well-rounded rock deposits found along the edges of the floodplain.

3 Page 2 Figure 1: Locations of all samples along Red Arroyo River 2.0 Materials and Methods 2.1 Collection Method of Soils The initial 6 samples were taken in a finite area on November 4 th, The remaining 14 samples were gathered on November 16 th, The initial 6 samples were taken as a mix of dry and saturated mud samples to verify a working testing method was obtained. It was decided that for the remaining samples all should be dry to prevent error in data points and create consistency. The dry soil samples were taken from first 5 cm on the surface at every location that was picked at random running along the Red Arroyo River. Samples were collected within clear high-density polyethylene plastic containers carrying an average of 48 grams of soil. Samples were taken in a fashion to ensure moisture and vegetation would be as minimal as possible. Samples were delivered to lab room same day as taken in a sealed cooler to reduce storage environment differences. 2.2 Testing Methods for Gross Alpha and Gross Beta Activity Testing was performed using an Alpha/Beta Detector Ludlum Model attached to a Ratemeter Ludlum Model Detector calibrations were done using Sr-90 and Po-210 to calculate efficiencies. The gross alpha efficiency using the Po-210 was 2.96 pci cpm -1 and the gross beta efficiency using the Sr-90 was 4.41 pci cpm -1. Before each set a background count was taken using a blank dish 3 times to create an average background count. Subtraction of the background count from the sample count gives the net count of the environmental sample (Biswas et al., 2015). Since the data from the Ratemeter was displayed in counts per minute (cpm), calculations had to be done to convert it to picocuries (pci) and then to Bq kg -1 later. Each sample was weighed to an average of grams and deposited into a dish to have the detector placed over it to begin to take readings. Once the counts were taken, that particular sample was replaced and steps were repeated for each consecutive sample. 2.3 Calculations for Gross Alpha and Gross Beta Activity Converting counts in cpm to pci: Converting pci g -1 to Bq kg -1 : pci = cpm efficiency pppppp gg = BBBB kkkk Calculating Standard Deviation for each sample:

4 Page 3 σσ = ± CC CC 2 nn CC iiii tthee vvvvvvvvvv iiii BBBB kkkk CC iiii tthee mmmmmmmm vvvvvvvvvv iiii BBBB kkkk nn iiii tthee nnnnnnnnnnnn oooo vvvvvvvvvvvv Standard Error: uu = σσ nn 3.0 Results and Discussions 3.1 Gross Alpha activity in Soil Sample The observed gross alpha activity ranged from a peak of ± 1.12 Bq kg -1 to a non-zero low of 2.36 ± 1.12 Bq kg -1. A non-zero notation was added due to some values reading zero counts for the alpha activity which is normal. As shown in Figure 2 and 6, the values are spread out irregularly along the research area. The gross alpha activity of the area had an average of 4.91 ± 1.12 Bq kg Gross Beta Activity in Soil Sample Figure 3 and 6 displays the gross beta activity that illustrates a broader range than the gross alpha activity with the highest being ± Bq kg -1 and the lowest of ± Bq kg -1 of radioactivity. The gross alpha activity average for the set is ± Bq kg Interpreting Map Data of Samples In Figure 4, we see that through using the kriging interpolation tool that uses geostatistical methods, statistical algorithms, and the existing data value points within the ARCGIS program it is possible to create a trend model shown in the color range. This model suggests that as we begin on the west side (orange) and travel down the floodplain east (blue) the values for the gross alpha and gross beta will continue to decrease on average. That suggested trend follows the flow direction that leads to the east. Figure 5 suggests the same trend but with the raw gross activity in pci instead of the gross values. This model also shows a potential build up in levels of activity along the western banks of the two roads of Sul Ross and College Hills Boulevard as seen with the purple colored plumes. This implies that when flooding events happen and mobilize the radionuclides they are settling out along those areas. 3.4 Comparison of Gross Activity in Soil and Air Airborne gross radioactivity values were obtained online through the EPA s RadNet database for the month of November 2016 to correspond with the month the testing of soils took place. The samples were taken twice a week, every week during this time period. It was noted that according to the RadNet data, there were severe jumps in activity counts that seem to be 1 week apart with the highest being on the week of November 8 th, The airborne counts were

5 Page 4 converted to Bq kg -1 using the volume of air passing through the filter and the density of air in order to equivalently compare them to the average soil values in Bq kg -1. Values were predictably small with the gross alpha activity averaging in at ± mbq kg -1 and the gross alpha activity averaging only ± mbq kg -1 (EPA, 2016). The gross alpha activity in the soil compared to the airborne alpha activity had a ratio of 1.13: and a ratio of the average gross beta activity to the airborne activity of 1.00: This also equates out to 1.13 to 12.4 ppm for gross alpha activity and 1.00 to 4.15 ppm for gross beta activity. 3.5 Figures and Maps Figure 2: Gross Beta Radioactivity at sample location

6 Page 5 Figure 3: Gross Alpha Radioactivity at sample location Figure 4: Gross Beta Radioactivity Trend Model using Kriging in ArcGIS

7 Page 6 Figure 5: Beta Radioactivity Trend Model using Kriging in ArcGIS Sample # Alpha Activity (pci) Beta Activity (pci) Gross Alpha (Bq/kg) Gross Beta (Bq/kg) 16F F F F F F F F F F F F F F Figure 6: Measured Alpha and Beta Radioactivity Levels in Soil

8 Page Conclusions This study has shown that there is definitely a presence of gross alpha and beta activity in the sediments along the Red Arroyo River in San Angelo, Texas. The initial randomness of the gross radioactivity values taken was difficult to correlate, but with some modeling through ArcGIS possible trends were able to be visualized. This trend in correlation with the RadNet data collected provides a preliminary baseline yet raises more questions on how the mobilization of airborne radionuclides affect the distribution of radionuclides in the soils. Due to the limited amount of information in this area of gross alpha and gross beta activity an evaluation of the regions geochemistry of the radionuclides would be the next step to identify the active isotopes responsible for these levels and identify the possible origins through air masses. References 1. Ferdous, J., Biswas, S., Begum, A., & Ferdous, N. (2015). Study of Gross Alpha and Gross Beta Radioactivity in Environmental Samples. Journal of Scientific Research, 7(1-2), doi: /jsr.v7i RadNet Air Data From San Angelo, TX. (2016). Retrieved December 11, 2016, from 3. Hernández, F., Hernández-Armas, J., Catalán, A., Fernández-Aldecoa, J., & Karlsson, L. (2005). Gross alpha, gross beta activities and gamma emitting radionuclides composition of airborne particulate samples in an oceanic island. Atmospheric Environment, 39(22), doi: /j.atmosenv Langa, H.B., Auwal, T.M., Ali Auta, & Jimoh, O.H.I. (2016). Determination of Gross Alpha and Beta Activity in Water: (A Case Study of Gombe Metropolis). Journal of Basic and Applied Research, 2(4), Todorović, Nataša, Jovana Nikolov, Branislava Tenjović, Ištvan Bikit, and Miroslav Veskovic. (2012). "Establishment of a Method for Measurement of Gross Alpha/beta Activities in Water from Vojvodina Region." Radiation Measurements 47(11-12), Office of Environmental Policy and Assistance. (1998). Compendium of EPA-Approved Analytical Methods for Measuring Radionuclides in Drinking Water. 1-62