A study on possible use of Urtica dioica (common nettle) plant as polonium 210Po and lead 210Pb contamination biomonitor in the area of phosphogypsum stockpile

The aim of this study was to test a possible use of Urtica dioica (common nettle) plant as a biomonitor of polonium 210Po and lead 210Pb contamination near phosphogypsum stacks by determining concentrations of these radionuclides in samples collected from the area of phosphogypsum stockpile in Wiślinka (northern Poland). The 210Po and 210Pb contents in roots depended on their concentrations in soils. Bioconcentration factor values from soil to root of the plant did not depend on 210Po and 210Pb contents in soils that leads to the conclusion that different polonium and lead species have different affinities to U. dioica plants. The main sources of both analyzed radionuclides in green parts of plants are wet and dry air deposition and transportation from soil. The values of 210Po/210Pb activity ratio indicate natural origin of these radioisotopes in analyzed plants. 210Po and 210Pb concentration in U. dioica roots is negatively weakly correlated with distance from phosphogypsum stockpile.


Introduction
Phosphogypsum consists mainly of CaSO 4 ·2H 2 O and is a byproduct of phosphoric acid production from phosphate rocks (Hull and Burnett 1996). It is usually stored on stacks in specially designated areas. The phosphogypsum stockpile in Wiślinka (northern Poland) contains about 16 million tons of phosphogypsum (Boryło et al. 2013). It is located between the Martwa Wisła river and farm fields, close to the Gdańsk agglomeration (Fig. 1). Phosphate rocks are the starting material for the production of all phosphate products and the main source of phosphorus for fertilizers. They are characterized by high content of natural alpha radioactive elements, especially from 238 U decay chain ( 226 Ra, 222 Rn, 210 Po) and beta emitter 210 Pb. The essence of radiotoxicity of the phosphogypsum is gamma radioactivity and high content of natural radioactive elements which could be leached by rain and bioaccumulated in plants, animals, and humans. In the process of phosphoric acid production, about 80 % of uranium is associated with the phosphoric acid fraction, while about 90 % of the 210 Po and 210 Pb is bound to the phosphogypsum fraction (Azouazi et al. 2001;Baxter 1996;Hull and Burnett 1996). Phoshphogypsum stockpile in Wiślinka is considered to be one of the main contaminators of the Martwa Wisła river. Our previous researches indicate that it might have serious radiological impact on the local environment. Phosphogypsum can be moved by the wind, and radionuclides might be leached by wet precipitation and transported through groundwaters to plants where they are accumulated (Bem 2005;Boryło et al. 2009Boryło et al. , 2012Boryło et al. , 2013Skwarzec et al. 2010;Boryło and Skwarzec 2011;Olszewski et al. 2015).
Both 210 Po and 210 Pb are natural radionuclides, daughters of 238 U decay series. Their half-lives are 138.38 days for 210 Po and 22.3 years for 210 Pb (Boryło et al. 2013). These natural radionuclides are found in varying concentrations in soil, sand, sediment, and natural water and constitute an important component of the natural background radiation. They are known to significantly contribute to the radiation dose of the population (Rajashekara and Narayana 2010). The main source of 210 Po and 210 Pb in the atmosphere is 222 Rn emanation from the ground. 210 Po and 210 Pb return to the earth as dry fallout or are washed out in the rain. Important anthropogenic sources of these radionuclides are burning of fossil fuels, tetraethyl lead in petrol, dust storms, refineries, superphosphate fertilizers, the sintering of ores in steelworks, and the burning of coal in coal-powered power stations (Boryło et al. 2012). 210 Po is highly toxic, and its presence in soils may be traced to the decay of radionuclides of the 238 U chain in the soil (Aslani et al. 2005). Lead is widely distributed in the earth's crust, but the main ore is galena, PbS, PbO, and PbO 2 . C a r b o n a t e s , chlorophosphates, sulfates, sulfatocarbonates, and uosilicates are all less abundant than galena (Jia and Torri 2007). The possible different chemical behavior of both 210 Po and 210 Pb in the water column is characterized by a stronger affinity of 210 Po for particles than its precursor, 210 Pb (Gasco et al. 2002). The usual atmospheric input by rain has 210 Po/ 210 Pb activity ratio of 0.1-0.2 (Jia et al. 2003). The value of activity ratio higher than 1.0 must be affiliated with biogeochemical processes that can control the distribution of the two radionuclides (Jia et al. 2003).
The main aim of this work was to establish a possible use of Urtica dioica (common nettle) plants as 210 Po and 210 Pb contamination bioindicator in the area of phosphogypsum stockpile and to examine the impact of phosphogypsum stockpile on the surrounding environment. Additionally, the values of the 210 Po /210 Pb activity ratio and bioconcentration factor (BCF) and translocation factor (TF) were calculated in order to define both possible 210 Po and 210 Pb sources and level of their accumulation in plants.

Sample collection and analysis
The U. dioica plant samples along with corresponding soils were collected from multiple locations in the area of phosphogypsum stack in Wiślinka (northern Poland)  Fig. 1) in September and October 2014. Over ground shoots of U. dioica plant die during winter, and it spends the winter in the form of underground rhizomes. In this case, we decided to collect samples in autumn when shoots are old and long enough exposed to air deposition. All of the collected samples had similar height (about 1.5 m). Control samples were collected in Malbork in Pomeranian Voivodeship, Poland. U. dioica was chosen for this research due to its commonness in Polish environment. Collected plants were divided into green parts and roots. Roots were washed with double deionized water in order to remove soil particles. Green parts were not washed due to examination of possible dry deposition impact. Only soil particles were removed. Before analysis, each plant sample was air-dried, homogenized using mortar, and dried in 60°C. Soil samples were homogenized and passed through 0.25-mm sieve. From homogenized sample, three subsamples were weighted and enriched with approximately 20 mBq of 209 Po yield tracer. Samples were mineralized using HCl, HNO 3 , and H 2 O 2 mixture. Polonium was electrodeposited on silver discs according to the procedure presented by Skwarzec (1997Skwarzec ( , 2009. 210 Pb was analyzed indirectly through its daughter's 210 Po activity measurement after 6-month storage. After this time, polonium was again electrodeposited on silver disc and 210 Pb activity was calculated through 210 Po activity (Skwarzec 1997(Skwarzec , 2009).

Measurement technique
The activities of 210 Po were measured using an alpha spectrometer (Alpha Analyst S470) equipped in a surface barrier PIPS detector with an active surface of 300-450 mm 2 placed in a vacuum chamber connected to a 1024 multichannel analyzer (Canberra-Packard, USA). Detector yield ranged from 0.30 to 0.40. In most of the used detectors, the resolution was 17-18 keV. Minimum detectable activity (MDA) for 210 Po and 210 Pb were 0.05 and 0.06 mBq g −1 . The accuracy and precision of the radiochemical method were estimated to be less than 7 % by participation in international intercomparison exercises and analyses of IAEA materials. The precision between subsamples was estimated to be less than 3 % for all analyzed radioisotopes. 210 Po activities were corrected for decay between deposition on silver discs and counting on alpha spectrometer. 210 Po and 210 Pb activities were calculated for sampling date. The value of BCF and TF were calculated as (Boryło et al. 2012) The values of 210 Po/ 210 Pb activity ratio in analyzed common nettle U. dioica plants and soils are presented in Table 2. The values ranged from 0.50±0.17 to 0.90±0.08 in green parts, from 1.03±0.13 to 1.53±0.10 in roots, and from 0.91±0.11 to 1.09±0.09 in soils. In green parts, roots, and soils of control samples collected in Malbork, the calculated values of 210 Po/ 210 Pb activity ratio were 0.60±0.09, 1.13±0.11, and 1.00±0.04, respectively, while in wild berries, roots and green parts collected in Finland were higher than 1. This is probably connected with the fact that 210 Po is more available to plants roots than 210 Pb. The authors suggest further research in this matter (Vaaramaa et al. 2009). The 210 Po excess in green parts of Finnish wild berries suggests another source of this radioisotope than wet and dry deposition, probably connected with transfer from soil. Similar results were observed in U. dioica roots in this study. In all analyzed common nettle samples, the values of 210 Po/ 210 Pb activity ratio were higher than 1 that suggests that 210 Po is more mobile in soil where the values of these ratios are close to 1 (Table 2). In green parts of U. dioica plants, values of 210 Po/ 210 Pb activity ratio were lower than 1 suggesting that wet and dry deposition is significant source of 210 Po and 210 Pb radioisotopes in common nettle stems and leaves. Values that are close to unity could suggest either higher rate of metal transfer from soils or impact of phosphogypsum stockpile. The value of 210 Po/ 210 Pb activity ratio in air deposition ranges from 0.03 to 0.05 (Vaaramaa et al. 2010). It is assumed that up to 80 % of natural 210 Po and 210 Pb radioactivity in wild plants is connected with wet and dry deposition of 222 Rn decay products (Persson and Holm 2011). In analyzed U. dioica plants, we received very high Spearman's correlation factors between 210 Po and 210 Pb activities: r s =0.97 in green parts, r s =1.00 in roots, and r s = 1.00 in corresponding soils that confirm natural origin of these radionuclides. Spearman's rank correlation is a nonparametrical alternative for Pearson's correlation. It can be used to calculate the correlation between two variables that do not have normal distribution and are not linear. Moreover, Spearman's rank correlation is resistant for outlier results (Corder and Foreman 2014).
The values of 210 Po and 210 Pb BCF and TF in U. dioica TF, TF green part/soil and BCF, BCF plant/soil common nettle samples were calculated according to Eqs. (1) to (4). The values of

Characteristics affecting 210 Po and 210 Pb bioavailability
Uptake of Po and Pb by plants can occur both through the root system and from atmospheric deposition through activity interception by external plant surfaces (Vandenhove et al. 2009). Large number of factors is known to control metal bioavailability in soils and their accumulation level in plants. These are soil and climatic conditions, plant genotype, and agronomic management, including active/passive transfer processes, sequestration and speciation, redox states, the type of plant root system, and the response of plants to elements in relation to seasonal cycles (Kabata-Pendias and Pendias 1984;Malik et al. 2010). One of the major factors that contribute to extent of the metals taken up by the plants is also the structure and type of the soil. Also, such factors as clay particles, metal solubility controlled by pH, amount of metal cation exchange capacity, organic carbon content, and oxidation state of the system are important in metal availability (Malik et al. 2010). Narayana et al. (2006) reported higher 210 Po and 210 Pb sorption in soils with increasing organic matter content that was confirmed by high correlation factors (0.62 for 210 Pb and 0.70 for 210 Po). Vaaramaa et al. (2010) conducted research of soil cores divided into litter, organic, illuvial, and eluvial horizons and showed that 210 Pb activity is correlated with organic matter content in soil. In all analyzed soil horizons, high 210 Pb activity correlations between organic matter content were proved (the highest 0.85 for eluvium). No relevant correlations between organic matter content and 210 Po concentrations in soils were reported. Similar results were obtained for relationships between Mn, Fe, Al, and Pb contents and 210 Po and 210 Pb activities in soils. Radiolead is strongly correlated with these metal concentrations, while no significant correlations were received for 210 Po activities (Vaaramaa et al. 2010). Berger et al. (1965) indicated that organic soils contain on average three times more 210 Po than mineral soils. These differences might be associated with higher 210 Po sorption on clay and organic matter. In case of 210 Po and 210 Pb, the bioavailability is mainly dependent on their content in soils, plant morphology, and the level of wet and dry precipitation. 210 Po and 210 Pb activity concentrations in root and tuber crops, cereals, and legumes, where the edible portion is protected by inedible plant parts, should not be affected by air deposition containing both 210 Po and 210 Pb (Vandenhove et al. 2009). Po and 210 Pb concentrations in analyzed U. dioica samples are lower than their contents in corresponding soils. It is clearly seen that both 210 Po and 210 Pb activities in common nettle's roots are dependent on their content in soils (r s =0.72 for 210 Po and 0.65 for 210 Pb) (Fig. 2), while the value of BCF does not exhibit this correlation (r s =0.17 for 210 Po and 0.18 for 210 Pb) (Fig. 3). BCF plant/soil values are strongly and negatively correlated with 210 Po and 210 Pb concentrations in soils (r s =−0.92 for 210 Po and −0.93 for 210 Pb) (Fig. 4). According to Chen et al. (2005). there are considerable differences in the uptake and translocation of long-lived radionuclides among different plant species. Perianez and Martinez-Aguirre (1997) reported that BCF plant/soil factor in relation with 210 Po contents in soil can be described using function This phenomenon is confirmed in our study on Fig. 4 where BCF plant/soil factor is not linearly related with 210 Po concentration in soil and can be described by function It is probably connected with the fact that plants readily uptake elements essential for their growth when substrate concentrations are low (Mengel and Kirkby 1979). Usually, plant uptake of non-essential elements is constant in this substrate concentration. Only in high substrate concentrations for both essential and non-essential elements it can decrease, leading to toxicity or death of a plant . Even though, some non-essential elements can mimic an essential element resulting in non-linear relationship between soil concentration and BCF plant/soil at low substrate concentrations (Sheppard and Sheppard 1985). This phenomenon in lesser extent can be also noticed in case of relation between BCF values and 210 Po and 210 Pb concentrations in soils (Fig. 3). Plants take up radionuclides of similar chemical forms as the essential nutrient and transport them to specific tissues based on the function of the element in plant metabolism that is reflected in its higher concentration in a particular part compared to others (Al-Kharouf et al. 2008). Another explanation could be connected with the level of wet and dry deposition on green parts of the plants and with differences in previously mentioned soil and plant characteristics that may affect bioavailability of metals (Persson and Holm 2011). According to Vandenhove et al. (2009) Po and 210 Pb activities in green parts. Probable explanation is that the exposure to precipitation varies with the degree of covering of leaves. Roots serve as a natural barrier preventing the transport of many trace metals including radionuclides to the upper plant parts. The radionuclide translocation from roots to shoots is probably dependent on the species (Shtangeeva 2010). Relations between 210 Po and 210 Pb content in soils and in green parts of analyzed U. dioica plants is also negatively correlated (r s =−0.67 for 210 Po and −0.60 for 210 Pb) (Fig. 5) that confirms differentiated impact of air deposition on these radioisotope contents in plant leaves. As a confirmation, we plotted TF and TF green part/soil values. Received linear function and high r s values (r s =0.90 for both radionuclides) suggest similar source of both 210 Po and 210 Pb in green parts of U. dioica (Fig. 6). According to these results, it is impossible to use common nettle's leaves and stems as a biomonitor of possible phosphogypsum particle deposition from air as their impact is probably irrelevant when compared to air particles or metal transfer from soil. BCF and TF values received for control samples (Tables 3 and 4) indicate that soil characteristics and air deposition are mainly responsible for 210 Po and 210 Pb uptake.

Impact of the phosphogypsum stack
Possible impact of 210 Po and 210 Pb from phosphogypsum stockpile on green parts, roots, and whole plants of common nettle U. dioica in respect with distance from the stack was evaluated. The obtained results indicate that concentrations of both radionuclides in roots of analyzed plants are weakly but negatively correlated with distance from phosphogypsum stockpile (r s =−0.43) (Fig. 7). Stronger correlations were obtained between 210 Po and 210 Pb concentrations in soils and distance from the stack (r s =−0.61 and −0.78, respectively). In case of green parts of U. dioica, no relevant correlations were calculated (r s =0.37) (Fig. 8). Moreover, no significant correlations were observed for 210 Po and 210 Pb activities in

Conclusions
Polonium 210 Po and lead 210 Pb concentrations in analyzed plants and soils allow us to conclude that U. dioica roots can be used as a biomonitor of both 210 Po and 210 Pb soil contamination especially during exploratory surveys. We noticed that both radionuclides activities in roots were related to their concentrations in soils although BCF values did not present similar dependence. The relation between 210 Po and 210 Pb soil concentrations and BCF plant/soil values is non-linear what confirms that some forms of Po and Pb can mimic essential elements for plant growth, and depending on soil characteristics, they can be more easily absorbed by plants. Another explanation could be connected with the level of wet and dry deposition on green parts of the plants. These facts prevent using common nettle's leaves and stems as a biomonitor of possible phosphogypsum particles deposition. The decrease of 210 Po and 210 Pb concentrations in common nettle's root was noticed with increasing distance from phosphogypsum stockpile. This relation was not confirmed for analyzed green parts that is probably connected with the impact of the air deposition. The problem of phosphogypsum stockpile is limited to the zone of maximum 400 m. The highest contents of both 210 Po and 210 Pb were measured in samples collected from the slopes of the stack. The values of 210 Po/ 210 Pb activity ratio confirm natural sources of these radionuclides in U. dioica plants.