Thirteen-Year Cesium-137 Distribution Environmental Analysis in an Undisturbed Area

Abstract

¹³⁷Cs activity concentration in soil was analyzed yearly for 13 years (2010–2022) in an undisturbed area in Mexico. The temporal variation at 17 sampling points is consistent with the natural radioactive decrease of ¹³⁷Cs, excluding increased activity concentration detected after the Fukushima accident at 4 sampling points. Geospatial analysis was permitted to obtain ¹³⁷Cs activity concentration distributions for each year. The highest ¹³⁷Cs activity concentration was found at higher topography levels and close to intermittent water streams: 87.1 ± 5.5 Bq kg⁻¹ for the year 2011, which increased to 135.5 ± 14.5 Bq kg⁻¹ for the year 2013, and then ¹³⁷Cs decreased down to 46.4 ± 4.0 Bq kg⁻¹ by the year 2022. The lowest ¹³⁷Cs activity concentration was in the range of 0.6 ± 0.1 Bq kg⁻¹ to 13.5 ± 1.0 Bq kg⁻¹ at the points far from the undisturbed area, probably due to anthropogenic activities.

1. Introduction

Uranium fission from nuclear power plants and nuclear weapon tests produces radioactive cesium isotopes ¹³⁴Cs and ¹³⁷Cs, which are transformed into stable elements by radioactive decay. The isotope ¹³⁴Cs decays with a half life of 2.06 years to the non-radioactive isotope ¹³⁴Xe, while the radioactive isotope ¹³⁷Cs decays with a halflife of 30.08 years to the stable nuclei ¹³⁷Ba [1].

¹³⁷Cs is deposited in soil, water and vegetation in different proportions in the environment. The so-called radioactive fallout from nuclear weapon testing and accidents defines its distribution into the environment. These radioactive concentrations increase potential damage to public health and economic activities, as they can contaminate the food chain and risk long-term radiation exposure [2].

It is estimated that 948 PBq of ¹³⁷Cs was globally emitted into the atmosphere during the period 1945–1993 by nuclear weapon tests [3]. These tests were both on land and underground in the deep atolls of sea, especially the Pacific Ocean. The nuclear accident that occurred at the Chernobyl nuclear power plant in Ukraine on 26 April 1986 locally emitted 70 PBq of ¹³⁷Cs into the atmosphere and 11 PBq of ¹³⁷Cs was locally emitted from the nuclear power accident in Fukushima Daiichi, Japan, on 11 March 2011.

Table 1. ¹³⁷Cs emissions from nuclear weapon tests, accidents and reprocessing plants.

Source	Years	PBq	References
Nuclear weapon tests (world deposition)	1945–1993	948	UNSCEAR, 2000 [3]
Chernobyl accident (local emission)	1986	70	UNSCEAR, 1993 [4]
Reprocessing plants (local emission)	1950–1997	42	UNSCEAR, 2000 [3]
Fukushima Daiichi accident (local emission)	2011	11	UNSCEAR, 2013 [5]

summarizes these emissions.

The highest ¹³⁷Cs deposition in the environment has been towards the Northern Hemisphere, with the peak deposition occurring around 1963 [6,7].

In the days following the Chernobyl accident, a radioactive cloud transferred ¹³⁷Cs to the northern European countries, generating contamination to grasslands and to faunae that consumed those grasslands [8,9]. Ireland, a milk exporter, was one of the most affected countries because a significant amount of ¹³⁷Cs was transferred to cow’s milk [10]. Mexico detected and prevented the consumption of 39,000 tons of powdered milk imported from the Republic of Ireland in 1987 [11].

International regulations levels on ¹³⁷Cs radioactive contamination in food, acceptable for human consumption, began by the “Codex Alimentarius” at the end of 1986 at Rome, Italy. The Mexican Ministry of Health published the strictest limit of 50 Bq kg⁻¹ for ¹³⁷Cs radioactivity in milk as technical standard 316 in the Official Gazette in August 1988 [12]; in the November of same year, a new “Codex Alimentarius” meeting agreed a limit of 150 Bq kg⁻¹ [11].

¹³⁷C is considered the leading source of long-term man-made radiation to the public, scattered in the environment and it has a relatively long half-life of 30.08 years [13,14,15,16,17]. Therefore, its evolution in the environment is of interest to assess the risks to the population and ecosystems [18,19,20,21]. Because radionuclides can be incorporated into the food chain [2], it is important to identify the origin and presence of radioactive elements in order to anticipate their possible impacts.

¹³⁷Cs radionuclide released into the atmosphere is transported to the stratosphere, gradually descending to the Earth’s surface. ¹³⁷Cs then meets the soil and can only move with the soil’s constituent particles, meaning that any change in ¹³⁷Cs readings indicates soil redistribution processes by physical agents (erosion). Given this use of ¹³⁷Cs, the concept of reference sites emerged, which was developed to quantitatively express the processes of loss and accumulation. According to the FAO/IAEA, from the year 2017, no sedimentation has occurred in addition to the original ¹³⁷Cs radioactive fallout/deposition and the reductions are due to radioactive decay. The method is based on a comparison of the studied and reference sites. If the studied site contains less ¹³⁷Cs than the reference site, it implies that the studied site is eroded. If the number of ¹³⁷Cs recorded is higher, it indicates accumulation due to sedimentation. This simple relationship is interpreted using conversion models to convert the differentiated ¹³⁷Cs records into soil erosion and/or sedimentation rates. For a correct interpretation of the ¹³⁷Cs method, the correct selection of the reference site is crucial [22].

Radioactive ¹³⁷Cs fallout is reported to have greater deposition in the Northern Hemisphere, while very little is known about the spatial distribution of its deposition in the Southern Hemisphere. However, recent studies indicate a possible underestimated distribution in the latitudinal range (10–20°) N and (50–70°) S. The limited ¹³⁷Cs studies on the Southern Hemisphere suggest uncertainties on ¹³⁷Cs distribution and the factors governing it [23].

Mexico is located in the latitude range (14° to 32°) N; there are only two ¹³⁷Cs reports in Mexico on activity concentration in soils by Garay et al., 2003 [24] and Hernández, 2018 [25], at Zacatecas and Jalisco, respectively. This paper covers a broader study of ¹³⁷Cs environmental fallout analysis over thirteen years in an undisturbed area.

2. Materials and Methods

2.1. Site Description

The study site shown in Figure 1 is a rural area towards the south of Jalisco and Zacatecas and north México City. The site location is geo-referenced, and contour lines are displayed in black in Figure 1a,b. The elevation file is downloaded from the WebGis site, SRMT3 (Shuttle Radar Topography Mission); the file for Mexico has a resolution of up to 90 × 90 m [26]. The file is subsequently incorporated into the module AERMAP for obtaining a georeferenced map. Hydrology layers are obtained from the National Commission for the Knowledge and Use of Biodiversity from Mexico [27] to view the water flows in the study area. Figure 1b shows the site, far from anthropogenic activities, where thirteen points (the blue points) are located. During the rainy season, the area has intermittent water streams (shown with a light blue color) that flow towards the descending topography direction of the site. One of the water streams flows from the southeast to the central north, descending from a higher elevation; the maximum elevation difference is 53 m at the northwest and southeast regions; the others move from the southeast to the southwest.

2.2. Sampling and Measurements

The sampling strategy identifies undisturbed anthropogenic sites with ¹³⁷Cs potentially present, which could be a risk to humans, including wetlands and grazing areas. Sampling points were selected along a 765 m linear transect parallel to the intermittent water streams at accessible locations for safety sampling conditions paths. Additionally, four sampling points were placed in a selected zone to the south, which are considered unaffected by ¹³⁷Cs overflow transport, being influenced only by atmospheric deposition and the initially suspected pedestrian circulation. Soil from seventeen sampling points is collected yearly from 2010 to 2022 with some exceptions due to COVID-19 pandemic conditions. Each sample consists of 1.0 kg of soil taken from the first 0.2 m top surface. Each sample was placed in pre-labeled polyethylene bags and sealed to prevent mixing and contamination. The bags were then taken to the laboratory where they were dried at 40 °C for 24 h, and then crushed and sieved to 100 μm mesh. Then samples were dried again at 100 °C for 24 h to remove undesirable moisture traces. Each sample was placed in a Marinelli beaker of approximately 0.5 L in volume to introduce it to a gamma spectroscopy detector (AMETEK ORTEC, Oak Ridge, TN, USA) for analysis. The detector is a high-resolution Ortec HPGe (high-purity germanium) with a 30% relative efficiency [28]. Calibration and efficiency of the Ge(Hp) detector was performed following ANSI Standard N.42.14–1999 [29], employing certified sources which allow multi-analysis isotopes in the energy ranges from 46 keV to 1836 keV. The resulting multi-elemental analysis of soil samples included the presence of ¹³⁷Cs. Presence of ¹³⁷Cs photopeak is at 661.87 keV energy as a result of the multi-elemental analysis with the Ge(Hp) detector.

3. Results and Discussion

3.1. ¹³⁷Cs Activity Concentration

Table 2. Average annual ¹³⁷Cs activity concentration.

YEAR
Point	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021	2022
	Bq kg−1
4U	13.5	87.1	14.7	135.5	17.5	17.1	87.9	LI	LI	LI	LI	12.2	29.3
13U	80.2	29.6	66.2	52.2	28.2	88.2	LI	LI	LI	LI	LI	LI	46.4
2D	13.0	27.6	9.2	16.6	12.7	15.7	64.5	13.3	16.6	14.5	1.7	23.1	15.1
12D	43.3	41.8	37.5	30.3	31.9	26.4	LI	45.6	50.3	76.7	27.9	18.1	29.7
11D	39.8	32.1	30.4	27.9	34.2	26.7	LI	LI	LI	LI	10.7	23.0	30.1
10D	68.1	49.5	48.3	31.9	45.0	38.5	LI	LI	LI	LI	LI	2.9	42.5
1	17.2	18.1	10.7	12.1	19.5	20.3	12.8	LI	LI	LI	5.8	11.2	6.8
3	6.5	25.0	4.1	3.5	5.2	2.5	4.1	7.2	4.1	8.2	LI	7.6	6.2
5	.5	18.2	0.6	1.2	LI	LI	13.1	9.6	2.9	3.7	2.5	3.8	2.2
6	11.0	3.2	7.2	6.3	12.0	7.1	1.7	1.1	0.7	6.9	1.7	16.4	6.5
7	1.8	5.9	2.1	2.2	9.7	1.6	1.5	LI	LI	LI	1.7	1.9	1.2
8	7.5	17.5	5.5	8.4	10.3	9.7	LI	LI	LI	LI	2.6	4.6	1.8
9	5.6	5.4	2.7	3.5	10.4	9.5	LI	4.6	4.8	11.9	10.5	7.6	14.9
14	5.4	2.1	1.7	1.6	1.2	1.7	1.4	LI	LI	LI	LI	LI	0.8
15	1.6	LI	0.8	0.7	0.9	0.7	2.7	3.5	1.0	0.8	LI	LI	1.3
16	1.1	2.1	0.9	0.7	1.5	1.7	LI	13.5	LI	LI	LI	LI	LI
17	LI	1.2	4.5	3.0	1.4	1.5	1.6	1.9	0.9	LI	LI	1.8	LI

Errors in measurements are around 10% for two digits figures and around 50% for one digit figure. LI means lack of information.

indicate the 211 samples collected from 2021 to 2022 at seventeen sampling points (first column) 52 samples were not suitable for analysis and have been marked as lack of information (LI) the activity concentration of the remaining 169 samples is given in Bq kg⁻¹. Errors in measurements are around 10% for two digits figures and around 50% for one digit. The capital letter U attached to the number of the point means that the point is located upstream, whereas the capital letter D means that the point is located downstream (see Figure 1).

At the beginning of the sapling years (2010 and 2011), there were six relative anomalous ¹³⁷Cs concentrations higher than 40 Bq kg⁻¹, which show the right selection of undisturbed sites for continuing the present study. Point 4U was 87.1 Bq kg⁻¹, the highest ¹³⁷Cs value of those six points that, for being up the hill, looks “normal” to us.

The first four points in Table 2, 4U, 13U, 2D and 12D (showed in green) show peaks of ¹³⁷Cs activity concentration from the Fukushima accident in 2011. This ¹³⁷Cs peak is shown delayed in time (point 4U and 13U) and reveals that the intermittent water stream (see Figure 1) has an influence in this delayed time, because the ¹³⁷Cs peak is shown much later in the points 13U and 12D downstream as one would expect. This ¹³⁷Cs peak effect is either not observed at other points due to the lack of experimental information (yellow points) because points are not close to water stream or because points are located far into an anthropogenic area where the ¹³⁷Cs activity concentration is lower (shown in gray).

Sampling points 14, 15, 16 and 17 in Table 2 (shown in gray) are located 1.1 km to 5.8 km away from the center area (see Figure 1a). Their range of ¹³⁷Cs activity concentration for all years is (0.7 ± 0.1–13.5 ± 1.0) Bq kg⁻¹, which is similar to or less than the values of ¹³⁷Cs found in other studies, that took place in Zacatecas [25] and Jalisco [24] and much lower concentrations, than those in the United States of America [30] and Canada [6], as shown in

Table 3. ¹³⁷Cs activity concentration in soil in Mexico, USA and Canada.

Other Sites			This Study
Place	Reference	137Cs Bq kg−1	Out points from undisturbed site	137Cs Bq kg−1
Zacatecas, Mexico.	(Hernández, 2018) [25]	0.49–5.15	1.0 km south	0.8–5.4
Jalisco, Mexico.	(Garay et al., 2003) [24]	0.5–20.4	5.8 km southeast	0.7–4.3
Canada.	(Blagoeva, Zikovsky, 1995) [6]	0.2–212	2.0 km southeast	0.6–13.5
USA.	(OPPD, 2018) [30]	11.1–111	1.7 km northwest	0.8–6.9

.

3.2. Projected ¹³⁷Cs Decay for Reported Data

A ¹³⁷Cs decay curve construction is carried out to graphically compare these reported results to published results, which help to compare ¹³⁷Cs activity concentration reported in different years. Reference [3] mentions a ¹³⁷Cs world deposition of 948 PBq from 1945 to 1993, 48 years. This is an integral value in those 48 years. We chose zero (1945) as the lower limit in the integral and the upper limit of the integral as 48 (corresponding to 1993). The curve in Figure 2 is constructed by integrating the decay in Equation (1). Once we have the equation, it is possible to extend the graph up to 2012 (for the period 2022–1945 = 77 years), the year we have evaluated ¹³⁷Cs in. Curve A(t) was use for ¹³⁷Cs as a function of time in those 48 years from 1945 up to 1993 and to extend the curve up to 2022.

$$At={\int }_0^{48}A0 e^{-\lambda t}dt$$

(1)

where λ decay constant and t_1/2 is the half-life of ¹³⁷Cs given by

$$\lambda ={\displaystyle \frac{0.693}{t_{1/2}}}={\displaystyle \frac{0.693}{30.08}}$$

$$948 PBq={\int }_0^{48}{A}_{0 }{e}^{-\left[{\displaystyle \frac{0.693}{30.08}}\right]t}dt$$

$$948 PBq=-{\displaystyle \frac{A_0}{0.023}}[0.3315-1]$$

$$A_0\left(1945\right)=32.911 \mathrm{P}\mathrm{B}\mathrm{q}$$

Then

$$A\left(t\right)=32.911 e^{-0.023\left(t\right)} PBq$$

The corresponding plot is displayed in Figure 2. The red line shows the expected ¹³⁷Cs decay curve from nuclear tests globally released to the environment in the period 1945–2022. Two local ¹³⁷Cs emissions, not globally reported, from the Chernobyl and Fukushima accidents, are shown on the curve.

3.3. ¹³⁷Cs Decay Behavior for Experimental Data

Similar decay-curve calculations were applied for ¹³⁷Cs activity concentration measurements shown in Table 2. Figure 3 shows the experimental points taken from Table 2 as blue bars for four sampling points 4U, 13U, 2D and 12D. Each extrapolated ¹³⁷C decay-curve (shown as blue triangles) is normalized to the last ¹³⁷Cs activity concentration obtained in 2012 (column three in

Table 4. Main characteristics of the calculated ¹³⁷Cs decay curves for point 4U, 13U, 2D and 12D.

Data Point	Intersection at Year 1986	Normalize Point Year 2012	137Cs peak Contribution from Fukushima Accident
	Bq kg−1	Bq kg−1	Year Peak	Bq kg−1
4U	67.0	29.3	2013	135.5 ± 14.5
13U	108.7	46.4	2015	88.2 ± 5.5
2D	35.3	15.1	2016	64.5 ± 1.7
12D	69.5	29.7	2019	76.7 ± 2.7

) and are extrapolated back to the expected ¹³⁷Cs decay activity concentration at the time of the Chernobyl accident (column two in Table 4).

The maximum ¹³⁷Cs activity concentration from Table 3 is used to construct the decay curves for the United States of America (red rhombic), Canada (red triangles), Zacatecas (blue squares) and Jalisco (blue rhombic). Note that the ¹³⁷Cs concentration displayed in blue figures in Figure 3 corresponds to the blue y-axis on the right-hand side, whereas red figures correspond to the y-axis on the left-hand side. The ¹³⁷Cs value at the reported year was used as the normalization point for building the corresponding decay curves extrapolating them back to Chernobyl and to the year 2023.

¹³⁷Cs activity concentration data from Table 2 for points 4U, 13U, 2D and 12D are shown as blue bars in Figure 3. Several of the 13 remaining measuring points do not follow similar decay tendency either due to the lack of information (LI, see Table 2) or because they are 1.1 km to 5.8 km away from the site, where the normal anthropogenic activities avoided natural accumulation of fallout ¹³⁷Cs.

Two of those six points, 4U and 13U, are located at the southeast, the other two points, 2D and 12D, are placed to the north; all four points are situated close to an intermittent water stream running from southeast to north (see the light-blue line in Figure 1). The thick blue bars on curves at curves 4U, 13U, 2D and 12D show that the ¹³⁷Cs activity concentration increases due to the Fukushima Daichii accident that occurred in 2011. This ¹³⁷Cs increment has a delay time of more than two years for point 4U and point 13U, situated at the highest altitude of the study site (see Table 4). There is a five-year delay to point 2D and an eight-years delay to point 12D (see Table 4), situated at the lower elevation of the study site, which denote a ¹³⁷Cs slow natural transport, from southeast to north, due to the difference in height of the land and because the four points are close to the intermittent water stream. By the year 2022, all 17 points have reduced their ¹³⁷Cs activity concentration in the range of (46.4 ± 4.0–0.8 ± 0.2) Bq kg⁻¹ as indicated in Table 2.

The ¹³⁷Cs activity concentration obtained in this study is lower than that for the northern countries Canada and USA. Although the extrapolation of the curves for the study site is slightly higher than for the states of Jalisco and Zacatecas, this is due to the fact that the site is undisturbed, whereas in those states, the studies reported them as having anthropogenic activity.

3.4. Spatial Distribution of ¹³⁷Cs

All ¹³⁷Cs activity concentration distribution in soil was spatially analyzed for each year; Figure 4 shows the results of ¹³⁷Cs interpolation for the year 2011, which is the year with the highest activity concentration at the study site before Fukushima effect. The points of highest activity concentration are most found to the south, decreasing towards the north of the study site; see the activity concentration values in the table inserted at the left-hand side of Figure 4. Several sampling points are located near an intermittent water stream (blue line) running from southeast to north.

Figure 5 shows the ¹³⁷Cs activity concentration distributions for the years 2011, 2013 and 2022. A general pattern is identified for the three distributions; the maximum activity concentration (red vertical bar guiding the eye) is located in the south, where the corresponding sampling points are at the highest altitude of the site. A decrease of ¹³⁷Cs is identified in the north area (green vertical bar) at the lowest altitude of the site, close to intermittent water streams. The distribution of ¹³⁷Cs activity concentration for the year 2011 has a maximum range of (40–87.1) Bq kg⁻¹; this range did not reduce two years after in 2013, but the range increased to (46.2–135.5) Bq kg⁻¹ due to ¹³⁷Cs fallout from the Fukushima accident and its deposition on soil. Nine years later, in 2022, the maximum ¹³⁷Cs interval is (31.8–46.4) Bq kg⁻¹, showing the ¹³⁷Cs decay effect and natural movement of ¹³⁷Cs due to environmental conditions.

The highest ¹³⁷Cs activity concentration is found at point 4U, 87.1 ± 5.5 Bq kg⁻¹ and 135.5 ± 14.5 Bq kg⁻¹ for the years 2011 and 2013, respectively; those activity concentrations reduce to 29.3 ± 1.5 Bq kg⁻¹ by the year 2022. The decreasing activity concentration rate for the two points 2D and 12D are lower than expected due to the topography condition, because those two points are at the lowest elevation zone at the north of the site, whose difference is 53 m from south, and the intermittent water stream plays an important effect in the long-time transport of ¹³⁷Cs.

These reported results are consistent with other researchers [31,32,33,34,35], who determined that ¹³⁷Cs deposition in soils from nuclear accidents, specifically Chernobyl in 1986 and Fukushima in 2011, is transported to topographic places of lower elevation due to natural temperature gradients and rainfall. At the study site, the highest rainfall occurs in July and the lowest in February; however, it was not possible to find a possible correlation to annual rainfall to ¹³⁷Cs.

4. Conclusions

During the thirteen years of the sampling period, it was corroborated that the study area had no human activity that was possible to use to analyze the temporal ¹³⁷Cs activity concentration.

The ¹³⁷Cs peak contribution resulting from the Fukushima accident was noticeably detected in the constructed decay curves at four sampling points, 4U, 13U, 2D and 12D. This suggests a post-event deposition by rainfall over the study area and slow ¹³⁷Cs transport by intermittent water stream running from south to north.

The distribution of ¹³⁷Cs activity concentration for the year 2013 clearly represents the maximum ¹³⁷Cs contribution from the Fukushima accident whose concentrations are lower or within environmental concentrations in Canada and USA.

The four sampling points 14, 15, 16 and 17 are outside the undisturbed zone where there is pedestrian circulation; nearly 50% of the samples had ¹³⁷Cs activity concentration below detection (LI) and the rest of the samples had low ¹³⁷Cs activity concertation due to anthropogenic disturbance of the zone. A study [36] reports that the ¹³⁷Cs inventories differ due to the existence of different landforms and deposition environments, ranging from geographic depressions to areas with anthropogenic activities.

This first long-term ¹³⁷Cs deposition study of 17 sampling points at latitude 19N corroborates the statement of previous studies that major ¹³⁷Cs deposition has occurred towards the Northern Hemisphere. The maximum ¹³⁷Cs activity concentration of this work is lower than the maximum ¹³⁷Cs activity concentrations found in Canada and the USA.

Although the non-disturbance anthropogenic characteristics result from our site of study, this site cannot be used as a reference site for erosion studies [22] because there is disturbance in the site, due to environmental characteristics of the area, specifically the low ¹³⁷Cs transportation by intermittent water streams.