Evaluation of Element Mobility in River Sediment Using Different Single Extraction Procedures and Assessment of Probabilistic Ecological Risk

: In this manuscript, samples of Kupa River sediments were examined using three different extraction agents. The aim of this study was to evaluate the applicability of single extraction procedures to investigate the bioavailability and mobility of major and trace elements (Al, As, Ba, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Pb, S, Si, Sr, Ti, V, and Zn) from river sediment. Two forms of studied elements were evaluated: mobile, the most toxic element form (extraction with 1 M CH 3 COONH 4 and 0.01 M CaCl 2 ) and potentially mobilized form (2 M HNO 3 extraction). The estimation of the ecological risk, with the application of the probability distribution of RI (potential ecological risk index) values, is yielded with the help of the Monte Carlo simulation (MCS). Ammonium acetate is proved to be a better extraction agent than calcium chloride. A positive correlation between the content of all extracted elements with nitric acid and the total element content indicates that 2 M HNO 3 efﬁciently extracts all studied elements. Results showed anthropogenic sources of cadmium and copper and high barium mobility. The MCS suggests that risk of Cr, Cu, Ni, Pb, and Zn was low; As and Cd posed a lower and median ecological risk in the studied areas. correlation was observed between the of elements extracted with nitric acid and calcium chloride (Ba, K, Na, P, S, Si, and Sr) and those extracted with ammonium acetate (Al, Ba, Cd, Co, Fe, K, Mn, Na, Ni, S, Si, and Sr). The observed correlations show that ammonium acetate is a more efﬁcient means of extracting the most mobile fraction of elements. Correlations mobile


Introduction
Sediments play an important role in the transport of nutrients, metals, and other contaminants through river systems to the world's oceans and seas [1]. River sediments are reservoirs of materials derived from both anthropogenic and natural weathering processes and have been used as an important tool to assess the health status of aquatic ecosystems and are an integral component for the functioning of ecological integrity [2].
Petrographic, mineralogical, and geochemical composition of watercourse sediment samples, if sampled in an uninhabited area, reflects lithology upstream of the sampling site, if the anthropogenic impact is small or absent. However, if a large lithological diversity is present, it causes difficulties in interpreting the origin of the source material, as a result of the large mixing of eroded material and its downstream transport. Some authors, such as [3], also point out the problem of the opposite effect in cases of long and narrow valleys without tributaries, when samples of watercourse sediments taken along the valley are only replicas of the same material from the same source, without new geochemical information. According to the same authors, active watercourse sediments are recent deposits, originating from a limited number of currently active material sources. anomaly in sediments in the Kupica and Kupa rivers was discovered during work on the Ph.D. thesis of Frančišković-Bilinski and was published in 2006 [16], it is very important to examine the mobility of barium and other trace elements in more detail. Several studies investigated the contamination of the Kupa River [16][17][18][19], but in the current manuscript, it is the first time the use of different types of extractions as a tool to assess the potential element availability in river sediments was evaluated. A single extraction method was carried out to determine mobility and bioavailability of elements from sediments and, for this reason, different single extraction procedures were evaluated. The extraction was performed using three extraction agents: calcium chloride (0.01 M CaCl 2 ), ammonium acetate (1 M CH 3 COONH 4 ), and nitric acid (2 M HNO 3 ). Obtained results were compared with total element content, after BCR sequential extraction. In addition, for the first time, evaluation of probabilistic ecological risk of PTEs in these river sediments was computed using a Monte Carlo simulation.

Study Area
The Kupa River basin occupies the west-central part of Croatia and is shared by two neighboring countries (Slovenia, Bosnia and Herzegovina). Details about the Kupa River can be found in [18], who investigated and described for the first time its geomorphology, tectonic setting, lithological framework, granulometric properties, and pollution status of this transboundary river basin. The Kupa itself is a tributary to the Sava River and meets the latter at Sisak after traversing a distance of 294 km. The Sava River belongs to the Danube River watershed and enters the Danube River at Belgrade (Serbia). The Kupa River drainage basin is situated at the very south of the Danube drainage basin.
The map of Croatia, with a rectangle indicating the position of the Kupa drainage basin, the course of the Kupa River, and its catchment area showing sediment sampling locations are presented in Figure 1. The total area of 10,605 km 2 of the Kupa River drainage basin is divisible into several sub-basins as per its countrywide distributions: 79.32% belongs to Croatia, 18.32% to Slovenia, and 2.36% to Bosnia and Herzegovina. The river basin is one of the most significant water resources in Croatia. Although shared by other adjoining countries, about 85% of the river water, being chiefly derived from carbonate karst springs, river springs, precipitation, and run off, discharges on the Croatian side. The karst aquifiers of the Dinarides are highly vulnerable because of the rapid water exchange with the groundwater through numerous shallow holes. The availability of about 3.5 m 3 /s of very good quality spring water has given a strategic importance to the area based on the fact that the whole Adriatic coast and numerous settlements in the continental area have come into existence [20].

Sampling and Sample Preparation
Positions of sampling locations are presented in Figure 1 and in Table 1. Two locations (IŠ and 51) are on the Kupica River; one location (52) is on the Kupa River upstream Kupica inflow, while all other locations are in Kupa River downstream from the Kupica River inflow. Sample DN-2 is taken from the upper flow of Dobra River between villages Gornja and Donja Dobra. This location is located very close to the Brod na Kupi andČedanj locations on the Kupica and Kupa rivers (5-7 km air distance), but it does not have any direct connections with those locations. Therefore, despite its relative vicinity to sampling locations IŠ, 51, 52,ČD, and 50 and similar geological composition of surrounding areas, on sampling point DN-2, there is no influence of waste from the abundant barite mine in Homer.
Locations where fine-grained sediment accumulates along the river bank were chosen. On each sampling site, at least three grab samples of active fine-grained surface sediment (0-5 cm deep) were collected from different places in an area of 5 m 2 . From this material, a composite sample was taken weighing up to 1.5 kg. This procedure decreased the possible bias caused by local variability.  After sampling, the sediments were dried in air at room temperature and then sieved through 2000 µm and 63 µm sieves (Fritsch, Weimar, Germany) to obtain two sediment fractions: fine fraction containing clay and silt (<63 µm) and coarser fraction containing sand (63-2000 µm). Obtained sediment fractions were used for further analysis.

Sample Extractions and Measurement Using ICP-OES
Sediment samples were crushed and homogenized and after measuring certain sample masses, extracted with three extraction agents: 0.01 M CaCl 2 , 1 M CH 3 COONH 4 , and 2 M HNO 3 .
Calcium chloride extractable About 2 g of the sediment sample was weighed into a 50 mL centrifuge tube, and 20 mL 0.01 M CaCl 2 was added. The solution thus prepared was shaken for 3 h on a rotary shaker (Heidolph) [21][22][23].
Ammonium acetate extractable About 1 g of the soil sample was weighed into a 50 mL centrifuge tube, and 40 mL of 1 M CH 3 COONH 4 was added into each sediment sample. The solution thus prepared was shaken for 2 h on a rotary shaker (Heidolph) [23].
Extraction with HNO 3 About 2 g of sediment sample was weighed into a centrifuge tube. A 20 mL of 2 M HNO 3 was added into each sediment sample. The solution thus prepared was shaken for 1 h on a rotary shaker (Heidolph) at room temperature [24,25].
After the extraction process, all samples were centrifuged at 3000 rpm for 10 min. The supernatant was filtered, and the filtrate was filled up to 50 mL with 1 M HNO 3 and stored in a polyethylene bottle at 4 • C until needed for analysis.
The total amounts of elements in this manuscript are defined as the sum of extracted elements in the four binding fractions (BCR extractions). A detailed description of this method is shown in [19].

Measurement Using ICP-OES
The content of elements in the extracts was determined using ICP-OES (inductively coupled plasma optical emission spectrometer) devices (iCAP-6500Duo, ThermoScientific, Paisley, UK). The detector was a RACID86 Charge injector device (CID). This instrument operates sequentially with both radial and axial torch configurations. The analytical performance of the iCAP 6000 Series is demonstrated by its improved detection limits, enhanced linearity, superior long-term stability, and high-resolution images [20].

Pollution Risk Assessment and Monte Carlo Simulation
As it could be seen from Qu et al. [26] and Wu et al. [27], the Monte Carlo method is very applicable in PTEs' pollution risk assessment. Based on their work, we developed our software, which is written in Qt, and a proven pseudo random number generator produces a normal distribution with long-term repeatability. The program used was tested on several models, and as a final test, a reproduction of the results from Qu et al. [26] and Wu et al. [27] was conducted in its entirety, based on the input data, and calculated with the help of our software. The first time our software was applied occurred in Sakan et al. 2020 [19].
In the presented research, instead of Håkanson's RI, the probabilistic distribution of RI was calculated using the Monte Carlo simulation. The potential ecological risk index (RI) in sediments can be calculated using the following equation [28,29]: RI = ∑ E r i , where E r i = T r i C i f, Tr i is the toxic-response factor for a given substance (for Hg, Cd, As, Cr, and Zn, they are 40, 30, 10, 2, and 1, respectively; and five for Pb, Cu, and Ni) [29,30], and C i f is the contamination factor [28]. E r i i is the potential ecological risk for single factor, and RI is calculated as the sum of all risk factors for heavy metals in sediments.

Determination of Magnetic Susceptibility
Magnetic susceptibility was measured using SM30, a small magnetic susceptibility meter, which can assess the high sensitivity measure sediments and rocks with an extremely low level of magnetic susceptibility and, in addition, can distinctly measure diamagnetic materials such as limestone, quartz, and also water. Sensitivity of SM30 is 1 × 10 −7 SI units, what is about ten times better than the sensitivity of most of the competitive instruments. The operating frequency is 8 kHz, measurement time less than 5 s, and operating temperature −20 • C to 50 • C. The SM30 has an 8 kHz LC (inductor-capacitor) oscillator with a large-size pick-up coil as a sensor. The oscillation frequency is measured when the coil is put to the surface of the measured sample and when the coil is removed tens of cm away. Each sample was measured three times, and the mean value was taken as final result of measurement to assure as precise data as possible.

Quality Control and Assurance
To check the quality of the element analyses, the certified reference material BCR 483 (Sewage Sludge Amended Soil) was analyzed for extraction with CaCl 2 . In Table 2 is shown results of comparisons of the obtained calcium chloride extractable content and indicative values for BCR 483. The recoveries for Cd, Cr, Cu, Ni, Pb, and Zn were between 97.6 and 117.1%.

Discussion about Extracted Elements Contents by Different Extraction Agents
The extraction was performed using three extraction agents: calcium chloride (0.01 M CaCl 2 ), ammonium acetate (1 M CH 3 COONH 4 ), and nitric acid (2 M HNO 3 ). CaCl 2 and CH 3 COONH 4 are classified in groups of unbuffered salts, called "soft" or "mild" extractants. Obtained results are presented in Tables 3 and 4 and Figures 2 and 3. The relationship between the contents of the elements extracted by different extraction agents is also considered, and the relationship between the extracted contents is shown as a percentage (Supplementary Material, Tables S1-S3). Ratios were calculated only in samples in which element content is greater than the detection limits. From the calculated concentration ratios, it can be concluded which extraction agent is more efficient for the extraction of a certain element, depending on whether the obtained value is less than or greater than one, or calculated as a percentage less than or greater than 100.
Nitric acid extraction in comparison with CaCl 2 extraction gave better results during the extraction of the following elements: Al, As, Ba, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, S, Si, Sr, Ti, V, and Zn, where the observed relationship is noticeable for the elements: Al, Ba, Cu, K, Mg, Mn, Na, S, and Si. This result may indicate that Al, Ba, Cu, K, Mg, Mn, Na, S, and Si do not have high mobility in the examined sediments. During the extraction of beryllium, lithium, and lead, values below the detection limit in all samples were obtained, so these ratios could not be calculated. It is possible to conclude that nitric acid is a more efficient extraction agent in relation to calcium chloride. These results are expected given that the extract after extraction with HNO 3 contains elements bounded to sulphide and phosphates (released forms) and represents maximum contents of potentially available fraction. The low content of elements extracted using CaCl 2 indicates that studied elements in the examined river sediments do not have high mobility, since this solution simulates the natural soil solution, and element contents approximately correspond to their water soluble and exchangeable contents [30].    Nitric acid extraction in comparison with CH 3 COONH 4 gave better results when extracting the following elements: Al, Be, Ca, Cd, Co, Cu, Fe, K, Li, Mg, Mn, Ni, P, S, Sb, Si, Sr, Ti, V, and Zn, where the observed ratio is noticeable for the elements: Al, Co, Cu, Fe, Li, Mg, Mn, P, Ni, Si, V, and Zn (Table S2). During the extraction of arsenic, chromium, and lead, values below the detection limit in all samples were obtained, so these ratios could not be calculated. When determining barium, the observed ratio was calculated in 25 samples, in which in 20 samples, a better result was obtained during extraction with ammonium acetate. When determining sodium, the observed ratio was calculated in 25 samples, in which in 12 samples, a better result was obtained during extraction with ammonium acetate. When determining sulphur, the observed ratio was calculated in 25 samples, in which in 13 samples, a better result was obtained during extraction with ammonium acetate. These results can be explained by the high heterogeneity of the studied sediments with significant differences in the geochemical composition of the substrates. In most samples, better results were obtained after nitric acid extraction, but it should be noted that a significant amount of barium and sodium was extracted using ammonium acetate. This result is consistent with [23] and [31] that extraction with ammonium acetate may be used to assess the amount of available K, Na, Li, Ba, Mg, and Ca.
Extraction with ammonium acetate in comparison with CaCl 2 gave better results when extracting the following elements: Al, Ba, Cd, Co, Cu, Fe, K, Mg, Mn, Na, Ni, P, S, Si, Sr, Ti, V, and Zn, where the largest differences were observed in the following elements: Ba, Al, K, Mg, Mn, Na, and Si (Tables 3 and 4). During the extraction of arsenic, beryllium, chromium, lithium, and lead, values below the detection limit in all samples were obtained, so these ratios could not be calculated for these elements. Since that better results were obtained during extraction with ammonium acetate, the conclusion is that ammonium acetate is a more efficient extraction agent than calcium chloride.

Discussion of Concentration Ratios of Studied Elements Using Different Extraction Agents with Results of Amounts of Elements Extracted by the BCR Sequential Extraction Procedure (Total Element Content)
Concentration ratios of studied elements are presented separately for each extraction agent (calcium chloride, ammonium acetate, and nitric acid) in Supplementary Materials, Tables S4-S6. Total element contents represent the sum of elements extracted during the BCR extraction [19].
Ratios between concentrations obtained by calcium chloride, which is the mildest of used extraction agents, and concentrations obtained by total extraction showed that values for the majority of elements are extremely low. Only a few elements, which will be mentioned, show slightly higher values. Barium has values >1 on several locations only in the coarser fraction, with the highest value reaching a bit above 6%. This finding has significant implications, as it could be a sign of increased bioavailability of Ba in Kupa River sediments. Concentrations of Ba are extremely high in the upper and middle flow of the river due to the Ba-anomaly originating from uncareful disposal of waste from a barite mine in the Homer mine, Lokve, Gorski Kotar. An especially high Ba-concentration is in the Kupica River spring, to which it penetrated through vulnerable karstic underground, and this spring is used as the main water supply for the Delnice town, which is the central settlement of the whole Gorski Kotar area. Taking in account that Ba is being dissolved with an extraction agent as weak as calcium chloride is, it may imply that its concentration might get elevated, and what could cause problems with tap-water quality. It is known that some forms of Ba are toxic, so it could lead to health problems of local inhabitants. Unfortunately, Ba is not measured in the routine monitoring of water quality in Croatia. Therefore, it would be important to initiate some additional research on this topic in the affected area, as up to now only one preliminary study dealing with Ba's influence on health was performed [17]. In that study, authors applied geochemical and medical methods to investigate the possible impact of disposal of waste from the barite mine on human health in Lokve. The necessity of such measurements in future studies has been highlighted. Their preliminary study of diseases diagnosed in Lokve shows that about 18% of the total inhabitants have serious medical problems. Diseases of the circulatory system, as well as endocrine, nutritional, and metabolic diseases, neoplasms, and respiratory diseases predominate. They called for further multidisciplinary research on the health effects of barium and trace elements, as well as for bioremediation of contaminated gardens and for watershed management of vulnerable karstic aquifers. From other studied elements, only sulphur has several elevated percentages, with the highest value of about 12.5% in the coarser fraction. This probably could be explained with the fact that the barite (BaSO 4 ) mineral from the abundant mine is being dissolved, so together with Ba itself, S is also being released from this compound. All other elements show very small percentages.
Ratios between concentrations obtained by ammonium acetate, which is a slightly stronger agent, and concentrations obtained by total extraction, showed that ratios for the majority of elements are higher than when using calcium chloride. Similarly, as with the previous extraction agent, the highest values are observed for Ba and S, confirming everything mentioned in the previous paragraph. From other elements, excluding natural lithogenic elements such as Ca, Si, etc. originating from nearby carbonate rocks, the following elements have rather high percentages: Cd, Cu, K, Mn, and Na. This indicates their potential bioavailability.
Ratios between concentrations obtained by nitric acid, which is the strongest of all three used agents within the current research, and total element content showed the highest values among all three of them. This observation can be explained as follows: During the extraction with HNO 3 , maximum contents of potentially available fraction were released. Fractions obtained during extraction with CaCl 2 and ammonium acetate, so-called mobile forms, contain mainly elements in their ion-changing form. Table 5 shows a statistical analysis of the data about the relationship between content of elements extracted with 2 M HNO 3 and total extracted element contents (with BCR extraction). When it comes to the finer fraction (<63 µm), it is possible to notice that Ca and Cu were extracted in a high percentage using 2 M HNO 3 , which indicates that this extraction agent is very efficient for extraction of these two elements from the fine sediment fraction. These results indicate that calcium is predominantly present as carbonate at the examined localities. When it comes to copper, it is possible to conclude that this element is not significantly bound to silicates, but is probably bounded to manganese and iron oxides, which are very efficiently destroyed by the use of 2 M HNO 3 . When the maximum values of the extracted elements are observed, it is possible to notice that a high content of magnesium and strontium was extracted at certain localities, which is probably a consequence of the significant carbonate content. Additionally, a high percentage of extracted zinc using 2 M HNO 3 (up to 85.30%) was observed at some localities, which indicates high mobility and possible local contamination with this element. Chromium should also be pointed out, since it is a lithophilic and very immobile element in nature, which is confirmed by the results for the average percentage of extracted chromium using 2 M HNO 3 (about 11% in both fractions). An increased percentage of the extracted element was observed at some localities (up to 41.22% in the fraction <63 µm, or 49.85% in the fraction 63-2000 µm), which may indicate increased mobility of this element in some localities.
When the fraction 63-2000 µm is observed, 2 M HNO 3 proved to be an extremely efficient means for extraction of Ca and Cu, but also Mg and Sr, while in some localities lead was also extracted up to 100%. Considering that the average value of extracted lead in this fraction is 46.10%, the high efficiency of extraction at certain localities can be explained by the existence of anthropogenic sources of lead, as a result of which lead is present in more mobile fractions.
At some sites, it was observed that a higher content of Cu was obtained by extraction with 2 M HNO 3 than by destruction using BCR extraction. Given that the measurements were not made in the same time period, as well as that the ICP OES technique is a sensitive technique, and sediment is a complex matrix, it should be noted that this is a problem of a technical nature and can be seen only in a small number of samples. A similar situation was observed with magnesium. Many elements have similar concentrations when extracted with nitric acid as well as when BCR extraction was performed on them. This means that this type of extraction, which is much easier than total sequential extraction, could be enough to get reasonable results for total content of some elements (Ca, Cu, Mg, and Sr) in sediments. Additionally, it should be noted that this acid can be used for rapid screening of sediment and soil contamination, given that high extraction efficiency was shown for Zn, Pb, and Cd in some localities. In Ref. [30], it is shown that the distribution of Zn is controlled by a similar mechanism as Pb.
In Table 6 is shown results of comparisons of extracted elements' content in this research with similar investigations. content of Cu and Cd was higher in soils [11] than in studied sediments in this manuscript.
Obtained higher values for Cu and Cd in soils are due to treatment of soils by fertilizer and pesticides. Total content of extracted Zn from river sediments was higher than in soil [11] because of possible different sources of zinc pollution in the river basin. The total content of the other examined elements (Pb, Ni, and Cr) in sediments (this study) and soils [11] is fairly uniform. Higher content of CaCl 2 in extractable Pb, Cu, Cd, and Zn content in [12] can be explained by the fact that the investigated soil was largely contaminated ( Table 6). The low extractability observed for Pb while using CaCl 2 as reagent was observed in our research, but also in [11] and [12]. An explanation for this is that lead concentrations in contaminated soil extracts are controlled by precipitation processes (such as carbonates, hydroxides, sulphates, and phosphates), limiting the use of un-buffered salt solutions for the estimation of lead availability in soils [12]. The higher content of elements extracted using ammonium acetate in the yield of other extraction agents (in our case, it is calcium chloride, Table 6) is a consequence of the fact that 1 M ammonium acetate (pH 7) is perhaps the most preferred reagent for exchangeable metals because of its relatively high concentration and the metal complexing power of the acetate ion, both of which prevent readsorption or precipitation of released metal ions [32]. This reagent released bigger amounts of heavy metals than did ammonium nitrate [33].

Correlation Analysis
The Pearson correlation coefficients (r) measure the strength and direction of linear relationships between two or more random variables. In the present study, r is used to describe the interrelationships between the analyzed elements, and the results of correlation analysis are shown in Table 7. Table 7. Correlation analysis of extracted element contents.

E_AA E_NA E_BCR
of the elements with the total content indicate the existence of anthropogenic sources of cadmium and copper at the examined localities. CaCl 2 is recommended in [11] and [12] as a suitable reagent for extraction in the mobile form of Cu. In Ref. [30], it is shown that mobilization of Cu is mainly controlled by soil reaction. A positive correlation between CaCl 2 -extractable and total content of Cu in this manuscript indicated that in parts of the Kupa basin, there are vineyards that have been treated by fungicide copper (II)-sulphate. Additionally, positive correlations of mobile barium fractions using CaC and AA indicate the high mobility of this element. The positive correlation between the content of mobile contents of macroelements and the total content of elements indicates that the application of weaker extraction agents leads to complete or partial decomposition of carbonates, sulfates, phosphates, and even manganese oxides, and to the release of toxic elements related to them. We recommend extraction with ammonium acetate to assess mobile fraction elements that are equivalent to the "actually available" metal fraction, while nitric acid can be used to assess mobilizable fraction, i.e., the potentially available forms of trace elements in soils and sediments.

Ecological Risk Assessment of Potentially Toxic Elements Using Monte Carlo Simulation
A distribution curve on E i r and HRI (Total ecological risk comprehensive index) values is shown in Figure 4. The probability that ecological risk appeared at different risk levels with reference to a risk level classification standard was analyzed, as shown in Table 8. The Monte Carlo simulation suggests that risk of Cr, Cu, Ni, Pb, and Zn was low, and As and Cd posed a lower ecological risk in the studied areas. Cd is the most important factor in the Kupa River basin.  As shown in Table 9, the probability of the HRI values being at a low risk level was 100%, i.e., the total ecological risk level of PTEs pollutants in the sediments of the Kupa river.

Magnetic Susceptibility (MS) Measurements vs. Element Concentrations in Different Dissolution
Correlations between MS and elements contents obtained using three extraction agents were performed, and only one significant correlation is detected. For dissolution with 2 M HNO 3 , the only significant correlation was found for Mg (0.50). From all three used agents in this paper, HNO 3 is the strongest one, dissolving a significant part of the sediment, including both elements of anthropogenic and natural origin. Mg in Kupa River sediments is an element of natural origin, deriving from carbonate rocks, mostly dolomites.

Conclusions
Based on presented results for the Kupa river, it is possible to conclude that differences in amounts of a single element extracted from sediment by different procedures varied from location to location and from element to element, which is the result of high heterogenity of the studied river sediments in the geochemical composition. These results indicated the significance of the application of different extractions reagents on the assessment of mobility of trace elements. Nitric acid is a more efficient extraction agent in relation to calcium chloride and ammonium-acetate, and ammonium acetate is a more efficient extraction agent than calcium chloride. The results of the single extraction methods indicate increased bioavailability of Ba, Cd, Cu, K, Mn, and Na and low bioavailability for chromium, since it is a lithophilic and very immobile element in nature. Numerous elements were extracted in similar contents when sediments were extracted with nitric acid as well as when performing BCR extraction (total element content). Extraction with 2 M HNO 3 , which is much easier than sequential extraction and total element content determination, could be enough to get reasonable results for the determination of total content of Ca, Cu, Mg, and Sr in sediments. High extraction efficiency was shown also for Zn, Pb, and Cd in some localities. Additionally, it should be noted that 2 M HNO 3 can be used for rapid screening of sediment and soil contamination. We recommend extraction with ammonium acetate to assess mobile fraction elements that are equivalent to the "actually available" metal fraction, while nitric acid can be used to assess mobilizable fraction, i.e., the potentially available forms of trace elements in soils and sediments. CaCl 2 extraction is recommended for the determination of Cu mobile forms in sediments.
Based on a Monte Carlo simulation, it was found that the lower risk probabilities of Cd were 78.93% and 1.09% for median risk, which indicate that Cd was the most important toxic element in the Kupa River. The probability of ecological risk for all factors indicated that the potential ecological risk of toxic elements in the Kupa River is low at present. However, despite the low risk at present, there are indications that contents of some toxic metals are increasing at some locations, especially in the Kupa River lower flow, which could increase the ecological risk in the future. Therefore, we suggest the need for future systematic monitoring of the Kupa River and its drainage basin with respect to toxic element and ecological risk estimations.
Supplementary Materials: The following are available online: https://www.mdpi.com/article/ 10.3390/w13101411/s1, Table S1: Relationship between the contents of elements extracted with CaCl 2 and HNO 3 (%); Table S2: Relationship between the contents of elements extracted with CH 3 COONH 4 and HNO 3 ; Table S3: Relationship between the contents of elements extracted with CaCl 2 and CH 3 COONH 4 ; Table S4: Relationship between the contents of elements extracted with CaCl 2 and total element content-BCR extraction (%); Table S5: Relationship between the contents of elements extracted with CH 3 COONH 4 and total element content-BCR extraction (%); Table S6: Relationship between the contents of elements extracted with HNO 3 and total element content-BCR extraction (%).