# Ambient Background Values of Selected Chemical Substances in Four Groundwater Bodies in the Pannonian Region of Croatia

^{*}

## Abstract

**:**

## 1. Introduction

_{4}), chloride (Cl), and nitrate (NO

_{3}) were considered, which are stipulated by EU and Croatian regulations as key substances for the assessment of the chemical status of groundwater bodies and should be taken into account when determining background concentrations. These substances occasionally occur in higher concentrations than reference values prescribed by EU and Croatian regulations. Following the recommendations of the EU research project BRIDGE, iron (Fe) was also included in the analysis. Iron is particularly sensitive to change in redox conditions due to human influence that can lead to an enrichment of dissolved iron in groundwater.

## 2. Materials and Methods

#### 2.1. Study area Description

#### 2.2. Available Data Set

_{4}), chloride (Cl), nitrate (NO

_{3}), and iron (Fe). The UL were estimated for substances that may be due to natural and anthropogenic conditions. The EU Groundwater Directive (2006/118/EC), in Annex II, specifically lists substances that occur naturally and under the human influence, as well as pollution indicators, which need to be considered when setting national groundwater quality standards. This group includes arsenic, sulphate and chloride, as well as nitrate, for which the Groundwater Directive sets Community criteria for the assessment of the chemical status of bodies of groundwater (50 mg NO

_{3}/L). The EU research project “Background Criteria for Identification of Groundwater Thresholds (BRIDGE)”, funded under the 6th Framework Program of the European Union, proposed that the determination of background concentrations of substances is carried out for important pollutants that occur as a result of natural conditions and for certain characteristic substances, such as iron, which may occur in elevated concentrations due to human activity. Hence, all selected substances are to be considered for the assessment of groundwater body chemical status, according to EU and Croatian regulations and guidelines.

#### 2.3. Description of Methods

- Sort measured data $\left\{{X}_{1},\dots ,{X}_{n}\right\}$ from the smallest to the largest. The label for sorted data is $\left\{{X}_{\left(1\right)},\dots ,{X}_{\left(n\right)}\right\}$;
- Calculate ${p}_{i}=\left(i-0.5\right)/n$, where n is the number of data, and i denotes the index of the data in the sorted sequence from the step 1;
- Calculate ${t}_{i}=\sqrt{-\mathrm{ln}\left(4{p}_{i}\left(1-{p}_{i}\right)\right)}$;
- Calculate ${z}_{i}=sign\left({p}_{i}-0.5\right)\cdot 1,238\cdot {t}_{i}\left(1+0.0262\cdot {t}_{i}\right)$, where $sign\left({p}_{i}-0.5\right)=1$ if ${p}_{i}-0.5>0$, and $sign\left({p}_{i}-0.5\right)=\text{}-1$, if ${p}_{i}-0.5<0$;
- Take a natural logarithm of sorted data, i.e., to calculate $\left\{\mathrm{ln}\left({X}_{\left(1\right)}\right),\dots ,\mathrm{ln}\left({X}_{\left(n\right)}\right)\right\}$;
- The probability plot is obtained by displaying logarithmic values (from the step 5) on the x-axis and the corresponding values of z
_{i}on the y-axis.

_{3}> 50 mg/L or active substances in pesticides > 0.1 µg/L (>0.5 µg/L for total pesticides) or sum trichloroethylene and tetrachloroethylene > 10 µg/L, in line with the provision of the EU Groundwater Directive (2006/118/EC); and (d) anaerobic samples (DO < 1 mg/L), following the approach described in a previous study [18]. From the resulting data set per groundwater body (Table 2), the upper limit of background concentrations can be expressed as a 70th, 90th, or 95th percentile of the remaining data range, indicating appropriate confidence level, using the following procedure: (a) 95th percentile if N > 30; (b) 90th percentile if 20 < N < 30; and (c) 70th percentile if 10 < N < 20.

## 3. Results and Discussion

_{3}(groundwater bodies CDGI_23 and CSGI_29), the proportion of <LOQ values was higher than 50%, hence UL were not estimated by either method.

#### 3.1. Arsenic

#### 3.2. Iron

#### 3.3. Sulphate

#### 3.4. Chloride

#### 3.5. Nitrate

_{3}/L), were attributed to agricultural activities, sewage leaks and household discharges not connected to sewerage systems [26]. From Table 2 it is clear that the mean value of nitrate in the body CDGI_19 is significantly higher than mean values recorded in other bodies.

_{3}/L), significantly deviate from the majority of data characterized by the high percentage of <LOQ values in data set, thus having a strong influence on the calculation of the UL estimate.

#### 3.6. Comparison of UL Estimates and Methods

## 4. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Abbreviation

Full Term Name | Abbreviation |

European Union | EU |

Natural Baseline Quality in European Aquifers: A Basis for Aquifer Management | BaSeLiNe |

Probability plot | PP |

Limit of quantification | LOQ |

European Commission | EC |

Median absolute deviation | MAD |

Standard deviation | SD |

Background Criteria for Identification of Groundwater Thresholds | BRIDGE |

Upper limit of the range of ambient background values | UL |

Number of data | N |

Dissolved Oxygen | DO |

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**Figure 2.**Conceptual model of the groundwater body CDGI_19: (

**a**) Planar extent of the groundwater body; (

**b**) Schematic 3D hydrogeological cross-section.

**Figure 3.**Conceptual model of the groundwater body CDGI_23: (

**a**) Planar extent of the groundwater body; (

**b**) Schematic 3D hydrogeological cross-section.

**Figure 4.**Conceptual model of the groundwater body CSGI_29: (

**a**) Planar extent of the groundwater body; (

**b**) Schematic 3D hydrogeological cross-section.

**Figure 5.**Conceptual model of the groundwater body CSGN_25: (

**a**) Planar extent of the groundwater body; (

**b**) Schematic 3D hydrogeological cross-section.

**Figure 6.**Cumulative relative frequencies of arsenic (As) for the groundwater body CDGI_23. Arrow indicates value

**x**considered for the calculation of Mean + 2SD and Median + 2MAD ranges.

**Figure 7.**Cumulative relative frequencies of iron (Fe) for analysed groundwater bodies. Arrow indicates value

**x**considered for the calculation of Mean + 2SD and Median + 2MAD ranges for each groundwater body.

**Figure 8.**Lognormal probability plot of iron (Fe) for analysed groundwater bodies. Arrow indicates inflection point that separate ambient and non-ambient populations for each groundwater body.

**Figure 9.**Cumulative relative frequencies of sulphate (SO

_{4}) for groundwater bodies CDGI_19 and CSGI_29. Arrow indicates value

**x**considered for the calculation of Mean + 2SD and Median + 2MAD ranges for each groundwater body.

**Figure 10.**Lognormal probability plot of sulphate (SO

_{4}) for the groundwater body CDGI_19. Arrow indicates inflection point that separate ambient and non-ambient populations.

**Figure 11.**Cumulative relative frequencies of chloride (Cl) for analysed groundwater bodies. Arrow indicates value

**x**considered for the calculation of Mean + 2SD and Median + 2MAD ranges for each groundwater body.

**Figure 12.**Lognormal probability plot of chloride (Cl) for analysed groundwater bodies. Arrow indicates inflection point that separates ambient and non-ambient populations for each groundwater body.

**Figure 13.**Cumulative relative frequencies of nitrate (NO

_{3}) for the groundwater body CDGI_19. Arrow indicates value

**x**considered for the calculation of Mean + 2SD and Median + 2MAD ranges.

**Figure 14.**Lognormal probability plot of nitrate (NO

_{3}) for the groundwater body CDGI_19. Arrow indicates inflection point that separates ambient and non-ambient populations.

**Table 1.**Groundwater body characteristics. Thickness and hydraulic conductivity data refer to gravel and sand aquifers in groundwater bodies.

Groundwater Body | Area (km^{2}) | Total Thickness of Permeable Layers (m) | Average Hydraulic Conductivity (K) (m/day) |
---|---|---|---|

CDGI_19 | 402.1 | 80 | 210 |

CDGI_23 | 5010.9 | 120 | 30 |

CSGI_29 | 3329.4 | 50 | 110 |

CSGN_25 | 5188.1 | 40 | 50 |

Chemical Substance | Statistics | Groundwater Body | |||
---|---|---|---|---|---|

CDGI_19 | CDGI_23 | CSGI_29 | CSGN_25 | ||

Arsenic (µg/L) | Arithmetic mean | - | 37.3 (37.2 *) | 14.3 (14.0 *) | 5.5 (4.5 *) |

Standard deviation | - | 67.3 (68.3 *) | 19.8 (21.3 *) | 6.5 (5.4 *) | |

Coefficient of variation | - | 1.8 (1.8 *) | 1.4 (1.5 *) | 1.2 (1.2 *) | |

N of samples | 342 | 400 (293 *) | 227 (161 *) | 135 (108 *) | |

<LOQ | 342 | 109 (93 *) | 102 (87 *) | 62 (54 *) | |

Percentage of <LOQ | 100.00 | 27.3 (31.7 *) | 44.9 (54.0 *) | 45.9 (50.0 *) | |

Iron (µg/L) | Arithmetic mean | 21.6 (23.2 *) | 1822.4 | 3402.2 | 219.4 |

Standard deviation | 67.1 (83.6 *) | 1873.7 | 6101.2 | 382.6 | |

Coefficient of variation | 3.1 (3.6 *) | 1.0 | 1.8 | 1.7 | |

N of samples | 284 (165 *) | 404 | 227 | 135 | |

<LOQ | 79 (52 *) | 7 | 10 | 11 | |

Percentage of <LOQ | 27.8 (31.5 *) | 1.7 | 4.4 | 8.1 | |

Sulphate (mg/L) | Arithmetic mean | 34.8 | 14.8 (17.3 *) | 7.7 (8.2 *) | 15.9 (15.2 *) |

Standard deviation | 31.7 | 35.2 (37.6 *) | 10.5 (11.8 *) | 28.7 (28.4 *) | |

Coefficient of variation | 0.9 | 2.4 (2.2 *) | 1.4 (1.4 *) | 1.8 (1.9 *) | |

N of samples | 342 | 454 (332 *) | 255 (180 *) | 151 (122 *) | |

<LOQ | 0 | 190 (128 *) | 59 (35 *) | 49 (34 *) | |

Percentage of <LOQ | 0.0 | 41.9 (38.6 *) | 23.1 (19.4 *) | 32.5 (27.9 *) | |

Chloride (mg/L) | Arithmetic mean | 12.6 | 13.7 | 6.4 | 16.4 |

Standard deviation | 5.2 | 19.7 | 7.5 | 13.7 | |

Coefficient of variation | 0.4 | 1.4 | 1.2 | 0.8 | |

N of samples | 342 | 454 | 255 | 151 | |

<LOQ | 0 | 1 | 0 | 0 | |

Percentage of <LOQ | 0.0 | 0.2 | 0.0 | 0.0 | |

Nitrate (mg/L) | Arithmetic mean | 33.4 | 3.9 (3.2 *) | 5.4 (6.2 *) | 7.9 (8.0 *) |

Standard deviation | 35.6 | 10.9 (4.7 *) | 9.6 (9.2 *) | 9.9 (10.0 *) | |

Coefficient of variation | 1.1 | 2.8 (1.5 *) | 1.8 (1.5 *) | 1.3 (1.2 *) | |

N of samples | 342 | 454 (332 *) | 255 (180 *) | 151 (122 *) | |

<LOQ | 13 | 349 (240 *) | 167 (102 *) | 74 (61 *) | |

Percentage of <LOQ | 3.8 | 76.9 (72.3 *) | 65.5 (56.7 *) | 49.0 (50.0 *) |

**Table 3.**Methods selected for calculating the upper limits of ranges of ambient background concentrations of analysed substances based on criteria described in Section 2.3.

Chemical Substance | GW Body | |||
---|---|---|---|---|

CDGI_19 | CDGI_23 | CSGI_29 | CSGN_25 | |

Arsenic (µg/L) | none | modified Lepeltier method pre-selection method * | none | pre-selection method |

Iron (µg/L) | modified Lepeltier method pre-selection method * | probability plot, modified Lepeltier method | probability plot, modified Lepeltier method | probability plot, modified Lepeltier method |

Sulphate (mg/L) | probability plot, modified Lepeltier method | pre-selection method | modified Lepeltier method pre-selection method * | pre-selection method |

Chloride (mg/L) | probability plot, modified Lepeltier method | probability plot, modified Lepeltier method | probability plot, modified Lepeltier method | probability plot, modified Lepeltier method |

Nitrate (mg/L) | probability plot, modified Lepeltier method | none | none | pre-selection method |

**Table 4.**Estimated Mean + 2SD and Median + 2MAD ranges (for modified Lepeltier method) and the upper limits of ranges of ambient background concentrations (UL) of arsenic (As) obtained by selected methods. EU Drinking Water Standard for As is 10 µg/L.

GW Body | Method | Mean + 2SD (µg/L) | Median + 2MAD (µg/L) | UL (µg/L) | |
---|---|---|---|---|---|

CDGI_23 | modified Lepeltier method | 29.5 + 96.6 | 6.1 + 10.2 | 126.1 (Mean + 2SD) | 16.3 (Median + 2MAD) |

pre-selection method * | - | 174.9 | |||

CSGN_25 | pre-selection method * | - | 15.5 |

**Table 5.**Estimated Mean + 2SD and Median + 2MAD ranges (for modified Lepeltier method) and the upper limits of ranges of ambient background concentrations (UL) of iron (Fe) obtained by selected methods. EU Drinking Water Standard for Fe is 200 µg/L.

GW Body | Method | Mean + 2SD (µg/L) | Median + 2MAD (µg/L) | UL (µg/L) | |
---|---|---|---|---|---|

CDGI_19 | modified Lepeltier method | 9.5 + 16.2 | 5.6 + 6.6 | 25.7 (Mean + 2SD) | 12.2 (Median + 2MAD) |

pre-selection method * | - | 47.5 | |||

CDGI_23 | modified Lepeltier method | 1156.4 + 2239.8 | 712.0 + 1421.4 | 3396.2 (Mean + 2SD) | 2133.4 (Median + 2MAD) |

probability plot | - | 1950.0 | |||

CSGI_29 | modified Lepeltier method | 301.2 + 1141.8 | 39.0 + 74.0 | 1443.1 (Mean + 2SD) | 113.0 (Median + 2MAD) |

probability plot | - | 4270.0 | |||

CSGN_25 | modified Lepeltier method | 83.2 + 178.8 | 30.1 + 56.2 | 262.0 (Mean + 2SD) | 86.3 (Median + 2MAD) |

probability plot | - | 292 |

**Table 6.**Estimated Mean + 2SD and Median + 2MAD ranges (for modified Lepeltier method) and the upper limits of ranges of ambient background concentrations (UL) of sulphate (SO

_{4}) obtained by selected methods. EU Drinking Water Standard for SO

_{4}is 250 mg/L.

GW Body | Method | Mean + 2SD (mg/L) | Median + 2MAD (mg/L) | UL (mg/L) | |
---|---|---|---|---|---|

CDGI_19 | modified Lepeltier method | 24.8 + 14.8 | 27.9 + 5.6 | 39.6 (Mean + 2SD) | 33.5 (Median + 2MAD) |

probability plot | - | 33.6 | |||

CDGI_23 | pre-selection method * | - | 121.4 | ||

CSGI_29 | modified Lepeltier method | 5.8 + 11.0 | 2.9 + 5.4 | 16.8 (Mean + 2SD) | 8.3 (Median + 2MAD) |

pre-selection method * | - | 44.5 | |||

CSGN_25 | pre-selection method * | - | 87.3 |

**Table 7.**Estimated Mean + 2SD and Median + 2MAD ranges (for modified Lepeltier method) and the upper limits of ranges of ambient background concentrations (UL) of chloride (Cl) obtained by selected methods. EU Drinking Water Standard for Cl is 250 mg/L.

GW Body | Method | Mean + 2SD (mg/L) | Median + 2MAD (mg/L) | UL (mg/L) | |
---|---|---|---|---|---|

CDGI_19 | modified Lepeltier method | 3.2 + 1.5 | 3.0 + 1.3 | 4.7 (Mean + 2SD) | 4.3 (Median + 2MAD) |

probability plot | - | 5.0 | |||

CDGI_23 | modified Lepeltier method | 4.9 + 3.1 | 5.1 + 2.4 | 8.0 (Mean + 2SD) | 7.5 (Median + 2MAD) |

probability plot | - | 7.7 | |||

CSGI_29 | modified Lepeltier method | 3.6 + 3.5 | 3.7 + 3.5 | 7.1 (Mean + 2SD) | 7.2 (Median + 2MAD) |

probability plot | - | 6.4 | |||

CSGN_25 | modified Lepeltier method | 4.9 + 7.3 | 4.0 + 5.4 | 12.2 (Mean + 2SD) | 9.4 (Median + 2MAD) |

probability plot | - | 15.3 |

**Table 8.**Estimated Mean + 2SD and Median + 2MAD ranges (for modified Lepeltier method) and the upper limits of ranges of ambient background concentrations (UL) of nitrates (NO

_{3}) obtained by selected methods. EU Drinking Water Standard for NO

_{3}is 50 mg NO

_{3}/L.

GW Body | Method | Mean + 2SD (mg/L) | Median + 2MAD (mg/L) | UL (mg/L) | |
---|---|---|---|---|---|

CDGI_19 | modified Lepeltier method | 8.6 + 9.6 | 8.7 + 15.6 | 18.2 | 24.3 |

probability plot | - | 19.0 | |||

CSGN_25 | pre-selection method * | - | 29.1 |

**Table 9.**Comparison of 90th and 95th percentile UL estimates for pre-selected data sets of arsenic (As), iron (Fe), and sulphate (SO

_{4}) for three groundwater bodies.

GW Body | Substance | UL Estimate | |
---|---|---|---|

90th Percentile | 95th Percentile | ||

CDGI_19 | iron (µg/L) | 29.7 | 47.5 |

CDGI_23 | arsenic (µg/L) | 133.6 | 174.9 |

CSGI_29 | sulphate (mg/L) | 16.4 | 44.5 |

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**MDPI and ACS Style**

Nakić, Z.; Kovač, Z.; Parlov, J.; Perković, D. Ambient Background Values of Selected Chemical Substances in Four Groundwater Bodies in the Pannonian Region of Croatia. *Water* **2020**, *12*, 2671.
https://doi.org/10.3390/w12102671

**AMA Style**

Nakić Z, Kovač Z, Parlov J, Perković D. Ambient Background Values of Selected Chemical Substances in Four Groundwater Bodies in the Pannonian Region of Croatia. *Water*. 2020; 12(10):2671.
https://doi.org/10.3390/w12102671

**Chicago/Turabian Style**

Nakić, Zoran, Zoran Kovač, Jelena Parlov, and Dario Perković. 2020. "Ambient Background Values of Selected Chemical Substances in Four Groundwater Bodies in the Pannonian Region of Croatia" *Water* 12, no. 10: 2671.
https://doi.org/10.3390/w12102671