The spread of nitrogen compounds in an active groundwater exchange zone within a valuable natural ecosystem

Ecological Engineering 146 (2020) 105746

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The spread of nitrogen compounds in an active groundwater exchange zone within a valuable natural ecosystem

T

Anna Podlaseka, , Filip Bujakowskib, Eugeniusz Kodaa ⁎

a b

Institute of Civil Engineering, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776 Warsaw, Poland Institute of Environmental Engineering, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776 Warsaw, Poland

ARTICLE INFO

ABSTRACT

Keywords: Modelling Fertilisers Nature reserves Ammonium Nitrate Groundwater

The aim of the paper is to present a methodical approach for developing a hydrodynamic model of groundwater flow and transport of nitrogen compounds in the area covered by the precision farming system. The study area covers the part of the Vistula River floodplain terrace near Warsaw (Poland), in the vicinity of nature reserves: Wyspy Zawadowskie, Wyspy Świderskie and Łęgi Oborskie. The components of the water balance in the analysed area were determined using the HELP (Hydrologic Evaluation of Landfill Performance) model. In reference to the doses of mineral nitrogen used during precise fertilisation, the scenarios of the spread of nitrogen ions in the soil-water environment were detailed. The results were presented graphically in the form of hydroisohypses and isolines of nitrate and ammonium concentrations. The activity of overbank flows and the effect of the geological settings on the course of aquifer layers in the active exchange zone were taken into consideration when describing groundwater flow. This allowed us to indicate potential paths of intense nitrate migration resulting from the evolution of the fluvial environment. Moreover, “hydrogeological windows” have been identified, where contaminants can freely migrate to usable aquifer in the major groundwater basin.

1. Introduction For many years, the increased concentration of inorganic nitrogen forms in groundwater and surface water has been clearly observed worldwide (Russo, 1985; Nixon, 1995; Smil, 2001; Constable et al., 2003; Bian et al., 2016; Koda et al., 2016). The main problems related to the occurrence of inorganic nitrogen is due to the acidification of aquatic ecosystems and toxicity (World Health Organization (WHO), 2004; Lang et al., 2019). Many researchers (Smith et al., 1999; Philips et al., 2002; Savci, 2012; Kiedrzyńska et al., 2014; Withers et al., 2014; Andersen et al., 2015; Xu et al., 2018), reported the significant impact of elevated concentrations of inorganic nitrogen forms in water on eutrophication which is considered as a one of the most serious problems related to environmental pollution. Elbl et al. (2014) and Kringel et al. (2016) reported that the application of fertilisers exceeding plant uptake and removal by harvest cause a large nitrogen export with rivers and contaminating of groundwater reservoirs. An important approach to reduce nitrogen losses in the environment is to optimise the doses of fertilisation, considering the timely manner of the fertiliser application, adapted to the current needs of plants. As a result, significant losses of fertiliser components from agriculture can be limited by using modern fertilisation techniques. Precision agriculture, ⁎

in which the doses of fertilisers are adjusted to field productivity, resulting mainly from the physicochemical properties of the topsoil and plant demands, is currently particularly important. What is more, the application of the principles of precision agriculture is usually associated with limiting the excessive use of mineral fertilisers and reducing the risk of the loss of unused fertiliser components. Due to a comprehensive assessment of the mechanisms occurring in aquifer systems, hydrogeological modelling is increasingly becoming the basic tool to support the assessment of the extent of soil and water contamination, particularly in agricultural areas or landfill sites (Ehteshami et al., 2013; Koda et al., 2016; Sieczka et al., 2018a). Apart from the hydrogeological modelling, the presence of contaminants should be controlled by traditional methods of field monitoring and experimental findings to assess the impact of anthropogenic activities on natural environment (Brtnický et al., 2019; Vaverková et al., 2019). Hariharan and Uma Shankar (2017) and Beegum et al. (2018) reported that hydrogeological software has found applications in a variety of cases of groundwater flow simulation. According to Filipović (2013), the modelling represents the future of environmental protection and can be used for better understanding the processes occurring in the soil-water environment. Fouépé Takounjou et al. (2013) and Gusyev

Corresponding author. E-mail address: [email protected] (A. Podlasek).

https://doi.org/10.1016/j.ecoleng.2020.105746 Received 29 March 2019; Received in revised form 24 January 2020; Accepted 5 February 2020 0925-8574/ © 2020 Elsevier B.V. All rights reserved.

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et al. (2015) stated that the application of groundwater modelling gives excellent perspectives for monitoring and protecting aquifer system from spatial and temporal pollutant migration. What is extremely significant, is the ability to predict the spread and concentration levels of specific nitrogen forms in any location from the source of application using numerical modelling techniques (Kłonowski et al., 2001). However, this issue is related to the risk resulting from the lack of knowledge concerning the basic parameters which occurred in the mathematical equations shown in the contaminant transport (Dąbrowski et al., 2010; Koda et al., 2013). Kaczmarek et al. (2008) stated that the field of hydrogeological modelling is currently the most limited due to insufficient knowledge about the values of parameters occurring in the equations describing groundwater flow and contaminant transport. Bearing in mind the above explanations, the main objective of this paper is to present a methodical approach for the development of a hydrodynamic model of groundwater flow and the migration of ammonium and nitrate ions from fertilisers (ammonium nitrate) in the area covered by the precision farming system. Due to the location of the study site, in the vicinity of valuable natural areas (Natura 2000 site – the Middle Vistula River Valley as well as nature reserves), the application of presented approach can be significant for estimating the possible effects of short and long-term groundwater contamination by nitrogen compounds, planning the nitrogen doses or indicating spots that are the most vulnerable to pollution in any location of the modelled area. The authors' intention is to present an approach that can be implemented in a crop planning as well as in predictive monitoring, based on which, even before the occurrence of exceeding levels of pollution, it will be possible to indicate the areas which are the most likely to be contaminated.

a part of the Chojnowski Landscape Park, is a forest nature reserve and was established to protect the remains of riparian forests in the Vistula Valley which have not been significantly transformed by human activities. It contains marshy depressions with peat soils as well as the alder buckthorn and willow communities. Close to the modelling area, Skarpa Oborska and Obory reserves are located as parts of the Chojnowski Landscape Park. Skarpa Oborska is a landscape nature reserve and was established to protect a richly carved scarp of the Vistula Valley, overgrown with multispecies deciduous forest. Obory reserve is a forest nature reserve located in the south region of the Konstancin-Jeziorna commune and is characterised by species-rich undergrowth and complex of oak-hornbeam forests. 2.2. Geological and hydrogeological conditions It is known that the dense network of river valleys in the European Lowland is largely used for agricultural purposes. The reason for this is, in particular, the availability of groundwater and surface water resources and the good quality of the soils which are there. In the regional groundwater flow schemes, river valleys are generally treated as drainage to the uplands zones (Freeze and Whitherspoon, 1967). Filling the valleys with permeable deposits (channel facies), in combination with the occurrence of significant hydraulic gradients in the upland scarp zones (Bujakowski et al., 2014), induces a significant velocity of groundwater flow in the alluvial layer. The short filtration path, the neighbourhood of recharge (aquifer levels of adjacent uplands as well as infiltration supply) and drainage areas (river channel) combined with the high conductivity of the aquifer deposits make the river valleys active groundwater recharge zones. Alluvial aquifer belongs to the hypergenesis zone, where the chemistry of groundwater is determined by exogenous processes (Macioszczyk, 2006), particularly by human impact on the environment. The current arrangement of aquifers filling the valley is the result of the evolution of the fluvial environment, initiated after the last glaciation. Originally, under the conditions of the optimum Holocene climate, the rivers of the Polish Lowlands were characterised by a balance of erosion and accumulation processes. In such conditions, the meandering character of the river channel dominates. Lowland meandering rivers sediment the considerable thickness of packets of low permeable alluvial facies. Currently, as a result of climate change in the Holocene and the intensification of agriculture, especially expressed in the cultivation of root crops, a significant increase in the supply of clastic material to rivers is shown. During the overbank flows, rivers overloaded by the bedload, transform the floodplain with their dynamic flows. In many parts of the research area, the package of low permeable alluvial soils was eroded and replaced by the facies of the overbank flow, overloaded by the bedload of the braided river. It was also reported that there are significant differences between overbank flow facies of braided and meandering rivers. Facies of the braided river were formed as medium and coarse sands, whereas facies of the meandering river were formed as clays and loams. (Bujakowski and Falkowski, 2019). What is important, in such zones, the susceptibility to water pollution in the first main exploitable aquifer increases significantly and is particularly visible during intensive rainfall. Secondary filled erosion channels and zones of transformed terrace are in such conditions treated as drainage collectors of the valley area. The filtration velocity in these zones can be up to ten times higher than in the untransformed terrace zone (Sieczka et al., 2018a). These described factors increase the intensity and velocity of water exchange in the alluvial aquifer, rising its sensitivity and susceptibility to anthropopressure. The impact of the occurrence of individual landforms on the conditions of supply and groundwater drainage was tested based on the results of hydrogeological modelling. A detailed hydrogeological analysis was possible due to the data available from the Airborne Laser Scanning (ALS) and multispectral and high resolution satellite images.

2. Materials and methods 2.1. Characteristics of the study site and its surroundings The study site covers an area of about 17.5 km2 and lies within the contemporary floodplain of the Vistula river near Warsaw, Poland (Fig. 1). The agricultural fields covered by the precision agriculture system are positioned in the southern area. Agricultural fields were fertilised with the use of ammonium nitrate (averagely 80 kg N/ha). According to Kondracki (2002), the research area lies in the mesoregion of the Middle Vistula River Valley, protected under the Natura 2000 program. From the north, the model boundary is set by the Jeziorka river channel and is surrounded by Wyspy Zawadowskie reserve. The faunistic nature reserve - Wyspy Zawadowskie covers an area of approx. 530 ha and includes mainly sandbanks, shoals and waters of the Vistula river. The original purpose of its construction was to protect the aquatic ecosystems in the central Vistula channel. Moreover, it is a nesting place for rare species of birds and a habitat of animals associated with the aquatic environment. It is also worth noting that this area has great landscape values and belongs to the national ecological ECONETPoland network. From the east, the study site is located in the vicinity of Wyspy Świderskie reserve. Wyspy Świderskie is a faunistic nature reserve, established to protect water ecosystems in the central part of the Vistula channel. It includes numerous sandbanks, shoals in the area of the Świder river mouth as well as the waters of the Vistula river. Along with the Wyspy Zawadowskie reserve, it belongs to the ECONET-Poland network. On the south side, the boundary of the modelled area is marked by the Gwoździe Lake and Cieciszewskie Lake. From the west, Łegi Oborskie reserve and Olszyna Łyczyńska reserve set the boundary of the study area. Łęgi Oborskie is a forest nature reserve, located in the Konstancin-Jeziorna commune. Ash-elm riparian forest, as the remains of the natural vegetation of the Vistula valley, is under environmental protection within this area. Olszyna Łyczyńska, as 2

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Fig. 1. Location of the study site on the background of protected areas.

Trautwein apparatus in saturated conditions, following the constant head method with regards to the ASTM D5084-00 (2001) procedure. Sorption parameters were determined in batch studies that were carried out with reference to guidelines presented by the United States Environmental Protection Agency (1992). The determination of migration parameters of ammonium and nitrate was conducted using the solution of ammonium nitrate, as an example of fertiliser commonly applied in agricultural practice. For the purpose of modelling the spread of nitrogen ions in groundwater, we used an input concentration of ammonium nitrate corresponding to the rate of nitrogen dose applied in precise fertilisation, equal to 80 kg N/ha. Moreover, the quality of the groundwater samples was tested in laboratory conditions for the purpose of the calibration of the contaminant migration model. Groundwater samples were analysed for their ammonium content by the Nessler method (PN-C-04576-4, 1994). The concentrations of nitrate were determined with reference to the 8171 HACH method (Water Analysis Handbook, 2012), using a spectrophotometer UV-VIS DR 6000 (Hach Lange, Düsseldorf, Germany). Every single one of the reagents used in chemical analyses were of analytical reagent grade.

2.3. Field tests Field studies basically concerned site inspections and collection of soil and water samples for further analysis in the laboratory. Moreover, a monitoring network of groundwater level measurements and its quality was set up, including piezometers and dug wells. Monitoring points were levelled and connected to the National Geodetic Network. Coordinates of monitoring points were set according to the Polish PUWG 1992 geodetic system using GPS-RTK equipment. In this method, the vertical accuracy of measurements was equal to 0.05 m and its horizontal accuracy was 0.03 m. Soil samples were collected in accordance with PN-EN ISO 22475-1 (2006). Groundwater samples were collected following standards presented in PN-ISO 5667-11 (2004) and PN-EN ISO 5667-3 (2013). The depth to the groundwater table was measured using a hydrogeological whistle. Drops in water level were marked in surface watercourses within the analysed area and on its boundaries. 2.4. Laboratory tests Particular attention has been paid to the determination of the physicochemical features of collected soils and contaminant migration parameters, i.e., parameters of advection, dispersion and sorption. Parameters obtained from laboratory studies were then applied to the hydrogeological model. Following McCarthy and Zachara (1989), it was assumed that the prediction of contaminant migration can succeed only when major transport mechanisms are well defined. Physical properties of tested soils were determined in accordance with European standard (PN-EN ISO 14688-1, 2006), as well as Polish standards (PN-B-02480, 1986 and PN-B-04481, 1988). The parameters responsible for the migration of nitrogen compounds (hydraulic conductivity, flow velocity, dispersion coefficient, and retardation factor) were determined using a

2.5. Hydrogeological modelling The main idea of creating the fate and transport model was to compute the concentrations of nitrogen forms at any specified time and place within the study site. Particularly, we have focused on simulating the spatial and temporal distributions of nitrate and ammonium ions in the groundwater system. The main goal of modelling in the vadose (unsaturated) zone was to determine the recharge characteristics (infiltration rate) of the model area. In the HELP model (Fig. 2), the profile of the vadose zone was recreated based on shallow drillings which were performed as part of field studies and the analysis of archival materials from the resources of 3

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A similar approach was adopted by Fouépé Takounjou et al. (2013) to assess the groundwater nitrate pollution in Yaoundé, Cameroon and by Lalehzari et al. (2013) to study the influence of agricultural land use on nitrate leaching from soils towards deeper zones. The three-dimensional groundwater flow was described using the following equation (Schwartz and Zhang, 2003):

x

kx

h h h h + ky + kz + W = Ss x y y z z t

(1)

where: kx, ky, kz – values of hydraulic conductivity along the x, y and z coordinate axes [L/T], h – the potentiometric head [L], W – volumetric flux per unit volume concerning sources or sinks of water (W < 0 represents flow out from the system, W > 0 represents flow into the system) [T−1], Ss – the specific storage of the porous material [L−1], t – time [T]. The MT3DMS of a Visual Modflow software, was applied to create a three-dimensional transport model that simulates advective-dispersive transport of nitrogen ions in groundwater. Following Zheng and Wang (1999), the transport of selected pollutants was described using the equation below:

Fig. 2. Conceptualisation of modelling approach with HELP (Sieczka, 2018).

the National Geological Archives (NGA) of Poland. The diversification of the structure of the top layer of the modelling area was considered when constructing the model body. Several analyses were carried out to the detailed recognition of the vadose zone (Fig. 3), especially in the area of agricultural fields fertilised with ammonium nitrate. For the implementation of the vadose zone model, meteorological parameters were also adopted, i.e. monthly values of precipitation and average air temperatures. These data were obtained from the Institute of Meteorology and Water Management for the Warsaw-Okęcie meteorological station and from the HELP model of the UnSat Suite Plus program, which has a built-in weather generator. The water-balance was calculated for a 5-year simulation period. Modelling in the saturated zone (Fig. 4) was performed using the MODFLOW 2005 and MT3DMS engines. MODFLOW 2005 was applied for groundwater flow modelling, whereas MT3DMS was used for predicting nitrogen ions spreading.

C C = Dij t xi xj

xi

(vi C ) +

qs n

Cs +

Rn −3

(2)

where: C – contaminant concentration [ML ], t – time [T], xi – distance along the respective Cartesian coordinate axis [L], Dij – hydrodynamic dispersion coefficient [L2 T−1]; vi – pore water velocity [L/ T], qs – volumetric flow rate per unit volume of aquifer [L3 T−1], Cs – concentration of the solute in the source or sink fluid [ML−3], n – porosity of the porous medium [−], Rn – chemical reaction term [−]. The application of Visual Modflow and its MT3DMS engine in presented research was scientifically justified as several studies have been successfully carried out in order to predict the transport of nitrogen forms in groundwater (Jovanovic et al., 2009; Shi et al., 2010). Hydrogeological modelling was performed based on data obtained

Fig. 3. Particle size distribution curves of soils collected from the vadose zone. 4

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A. Podlasek, et al. L

(4)

= 0.1 x

where: αL - longitudinal dispersivity [L], x – the spatial scale of the hydrogeological system [L]. The horizontal longitudinal dispersivity and the vertical longitudinal dispersivity were adopted in the model according to the following expressions (Patil and Chore, 2014):

from the laboratory, in situ tests and migration parameters determined in laboratory conditions. In addition, the information about the contaminants was applied, i.e. nitrogen load inputs used in fertilisation, concentrations of nitrogen forms measured in piezometers, and decay rates. The half-life t1/2 of nitrogen decomposition in groundwater was taken into consideration, following the literature data and our previous studies. The half-life t1/2 of nitrate was adopted from Sieczka et al. (2018b) at a level of 3.7 years. According to Witczak et al. (2013) the half-life t1/2 of ammonium in groundwater is thought to be equal to 1–5 years. Buss et al. (2004) stated that the range of t1/2 for ammonium is from 1 to 6 years. Bumb et al. (1987) proved that t1/2 for ammonium is around 2.85 years. During the model preparation a “trial and error” method was applied to adjust the proper value of a half-life t1/2 of nitrogen forms. The most reliable value of a half-life t1/2 was considered the one for which the best fit of real data compared to the model data was obtained. It was recognised that such a methodological approach of taking half-time based on literature is more justified than completely omitting it, especially when many scientific publications document the denitrification in groundwater (Lee et al., 2006; Ashok and Hait, 2015; Wang and Chu, 2016). Lasagna et al. (2013) stated that nitrate transformation in the aquifer can be associated with denitrification, dilution and dissimilatory nitrate reduction to ammonium. Moreover, Bednarek et al. (2014) stated that the denitrification process prevents the migration of unused nitrate into groundwater, and consequently reducing its potential to contamination. First-order decay coefficient was calculated based on the formula presented by Almasri and Kaluarachchi (2007):

0.693 t1/2

= 0.1

TV

= 0.01

(5)

L

L

(6)

where: αTH – horizontal longitudinal dispersivity [L], αTV – vertical longitudinal dispersivity [L]. The dispersion parameters applied in the created model are summarised in Table 1. The morphology of the first layer was set using the Digital Elevation Model (DEM) from the resources of the Geodetic and Cartographic Documentation Centre. The impermeable boundary of the model (sublayer) was interpolated using data from cards of deep boreholes, collected from the Central Geological Archive of the Polish Geological Institute - National Research Institute. The remaining layers were mapped assigning them an average thickness of river sediments in a given study area. The following layers were separated for the purpose of preparation for the model: 1st layer characterised by medium sands, 2nd layer representing Holocene deposits in the form of sandy clayey loams, 3rd layer representing Holocene medium sands and sandy gravels, 4th layer characterised by Pleistocene deposits of pebbles and gravels, 5th layer representing Pleistocene sandy gravels, medium sands and gravels and 6th layer formed as clay materials. The model uses the values of hydraulic conductivity (k), primarily obtained from laboratory tests carried out in the Trautwein apparatus. For deeper parts of soil profile, values of hydraulic conductivity (k) were set based on literature research. Values of specific storage (Ss) were taken from Batu (1998), whereas specific yield (Sy) values were calculated according to Bieciński (1960). The total porosity (n) was adopted in the model according to laboratory studies. The effective porosity (ne) was calculated based on column studies, following the equation presented by Marciniak et al. (2006). The values of retardation factors (R) of ammonium (NH4+) and nitrate (NO3−) were adopted on the outcome of laboratory tests results. The summary of hydrogeological parameters applied in a six-layered model are presented in Table 2. Three types of conditions were taken into consideration when setting the model boundary: Dirichlet's, Neumann's and Cauchy's, respectively. The Dirichlet's condition (1st type) was set on the south site of modelled area using Constant Head variant, regarding the groundwater level in landlocked depressions. The Neumann's condition (2nd type) was used to simulate recharge to the groundwater, resulting from the infiltration of water from precipitation into the deeper part of the soil system. This type of condition was applied as the Recharge variant of Visual Modflow. The Cauchy's condition (3rd type) was set in the northern, eastern and western boundaries of the modelled system using the River variant in Visual Modflow. In this manner, the impact of surface water on groundwater was modelled. A sensitivity analysis was performed to investigate the impact of changes made in model input parameters on the sensivity of the model. The procedure was carried out by changing only one input parameter while keeping all others fixed. An analogical procedure of sensivity analysis was presented by Lalehzari et al. (2013). The main objective of the model calibration was to obtain the highest consistency between the hydrodynamic state of groundwater flow recorded during field tests and the state obtained in modelling studies. Similarly, for the contaminant transport model, the aim was to adjust the values of ammonium and nitrate concentration determined in the laboratory and estimated in the model as accurately as possible. An error analysis was used to assess the conformity of the modelled hydraulic heads with values obtained from field measurements and

Fig. 4. Conceptualisation of modelling approach with Visual Modflow (Sieczka, 2018).

=

TH

(3)

where: λ – first order decay coefficient [T−1], t1/2 – half-life of contaminant [T]. A decay was considered only as one of the processes affecting the transformation of nitrogen in groundwater. Similarly, Shi et al. (2010) in their model, adopted the degradation of nitrogen as a first-order dynamic reaction. For a more accurate representation of occurring processes, also the advection, dispersion and sorption parameters, obtained in laboratory tests, were applied in the model. The concept of the model system was mapped from interpretation of data from borehole cards and the hydrogeological cross-section created for the research region (Fig. 5). The conceptual model was set up using a horizontal plane with a mesh of 200 m. According to Schulze-Makuch (2009) it was assumed that the longitudinal dispersivity is larger than transverse dispersivity as the local variation in the velocity prevails in the direction of flow, rather than in the direction perpendicular to it. Following Appelo and Postma (1999), the longitudinal dispersivity was set at 10% of a spatial scale of the hydrogeological system, following the equation: 5

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Fig. 5. Generalised hydrogeological cross-section. Table 1 Dispersion parameters adopted in the hydrogeological model. Dispersion parameter

Value [m]

Longitudinal dispersivity Horizontal longitudinal dispersivity Vertical longitudinal dispersivity

20 2 0.2

Table 3 Values of retardation factors of ammonium and nitrate based on laboratory studies.

Table 2 Hydrogeological parameters of model layers. Layer

k [m/ s]

1 2 3 4 5 6

1.0 3.6 1.0 3.8 2.2 1.9

× × × × × ×

10−5 10−6 10−4 10−3 10−3 10−8

Sy [−]

Ss [m−1]

0.115 0.099 0.159 0.268 0.248 0.047

1.28 2.55 1.65 7.56 1.65 1.92

× × × × × ×

10−4 10−3 10−4 10−4 10−4 10−3

n [−]

ne [−]

0.43 0.40 0.39 0.34 0.39 0.40

0.33 0.14 0.30 0.28 0.30 0.04

Soil

RNH4

RNO3

Loam Medium sand Silty loam Coarse sand

10.5 3.5 131.4 1.6

2.6 1.0 11.2 1.0

where: RNH4 – retardation factor of ammonium, RNO3 – retardation factor of nitrate.

capacity of nitrate ions (R = 11.2). In addition, it was noted that nitrate ions can be sorbed on loam, but the intensity of the sorption of these ions should be considered as medium (R = 2.6). The test results showed that both medium sand and coarse sand do not have the ability to sorb nitrate ions (R = 1). In comparison, Fouépé Takounjou et al. (2013) assumed that nitrate is non-reactive and the retardation processes are negligible for this ion. Regarding the sorption of nitrate ions, the same conclusion can be derived based on our previous study (Sieczka and Koda, 2016). The sorption of ammonium onto silty loam can be characterised as very large (R = 131.4), whereas the retardation of this ion onto loam is large (R = 10.5). The retardation of ammonium onto medium sand is considered as medium (R = 3.5). Ammonium ions are sorbed by coarse sand with a small intensity. The interpretation of sorption intensity was performed in accordance with the classification presented by Osmęda-Ernst and Witczak (1991) (Table 4).

concentrations of nitrogen forms measured in the laboratory with concentrations predicted in the model. 3. Results and discussion 3.1. Outcomes from field and laboratory tests and numerical modelling Several researches confirmed that negatively charged nitrate ions are not or are only slightly sorbed by non-cohesive soils what leads to their leaching to the deeper parts of soil profiles and groundwater (Buss et al., 2005; Wakida and Lerner, 2005). Delle Site (2001) stated that sandy soils, due to the presence of larger and well-connected pore spaces in their structure, are more susceptible to leaching of pollutants into groundwater, comparing to clayey ones. Based on laboratory tests performed for the purpose of presented research, it was also revealed that the velocity of ammonium migration compared to nitrate is many times smaller, which is related to sorption processes, responsible for retardation of ammonium onto negatively charged soil particles (Table 3). The results of the obtained retardation factors (R) of nitrate ions indicate that silty loam is characterised by the large retardation

Table 4 Classification of sorption intensity (Osmęda-Ernst and Witczak, 1991).

6

Retardation factor R [−]

Sorption intensity

1–2 2–10 10–100 100–1000 > 1000

Small Medium Large Very large Unlimited

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breaking through the zone of embankments (proximal part of the floodplain). These landforms are usually visible on aerial and satellite photographs. During the subsequent overbank flow these landforms were filled with alluvial deposits of channel facies (medium and coarse sands) of a contemporary braided river. 2. Secondary filled flow channels and oxbows. As the activity of flood waters caused the local removal of the top part of the clay series (alluvium) of the meandering river, the facies of the braided river (sands) were deposited in their place. The contemporary overbank flows also caused the transformation of oxbows, involving the blurring of organic deposits (peats and muds) and then refilling them with mineral deposits. In contrast to typical oxbows (Miall, 1996; Fryis and Brigley, 2013) which are filled with low permeable organic deposits, their bottoms are mostly permeable.

Table 5 Values of distribution coefficient Kd with reference to ammonium retardation. Distribution coefficient Kd (ml/g)

Retardation factor R (−)

0.25 0.5 1 2 10 100

2–3.5 3–6 5–11 9–21 41–106 401–1050

With reference to the values of the distribution coefficient obtained from “batch” tests, it was also possible to assess the range of retardation factors. Deutsch and Sieghel (1997) demonstrated that as the value of distribution coefficient increases, the retardation factor also increases (Table 5). United States Environmental Protection Agency (2004) reported that the distribution coefficient Kd should be used for site-specific contaminant and risk assessment calculations. The distribution coefficients of ammonium sorption obtained in this study in comparison with literature studies were presented in Table 6. Modelling studies revealed that the river regime is shaped by infiltration (71%), groundwater recharge (2%) and recharge from the surface water system forming the western boundary of the model (27%). Moreover, it was proved that in conditions of average precipitation (110 mm per year), the major part of the study site is drained by the Vistula river (Fig. 6). As a result of works carried out, the occurrence of geological landforms which are important for the conditions of the spread of nitrogen compounds has been documented. The entire site covered by the research is in the Holocene floodplain area. Its construction is not homogeneous as the initial layout of geological layers was disturbed due to the occurrence of overbank flows, both before the construction of embankments and later during incidents related to the damage of these structures. The nature of these processes is influenced, particularly, by the culmination of the erosional socle along the Vistula river's course. It was observed that overbank flows led to the transformation of the lithological profile in many places of the surface zone. The erosion of the layers originally forming the floodplain as well as the alluviation of fluvial deposits led to the development of two types of environments that are important for the specificity of hydrogeological conditions. These are:

Crevasses and the zone of oxbows running in the central part are treated as collectors for groundwater and surface waters, draining the area covered by the research. Moreover, the occurrence of two Quaternary aquifer horizons was found within the floodplain, comprising of: 1. First - subsoil aquifer (not exploitable) built of sands with silty and clayey interbeds being sediments of overbank flows of the contemporary braided Vistula river. The values of hydraulic conductivity of these deposits are in a range of 10−4 m/s. These deposits do not form a continuous cover over the entire floodplain of the research area and are underlain by the package of low permeable flood deposits. Zones of crevasses and transformed oxbows, filled with permeable deposits, contribute to hydrogeological windows, providing a hydraulic connection between the subsurface and lower lying aquifers. In conditions of a low and high precipitation, these landforms are treated as zones of water exchange between shallow and deeper aquifer. Pollutants from agriculture can easily infiltrate through hydrogeological windows to the deeper, exploitable aquifer. Hydrogeological windows within the alluvial layers may therefore contribute a point or linear pollution acceleration to the major groundwater basin no. 222 of the research area. The free water table of the first aquifer in the conditions of average precipitation is made stable at the depth of 2.83 m below surface level. 2. Second (main) aquifer built of gravels, unsorted and sediment sands of the meandering and braided Vistula river (channel facies). The values of hydraulic conductivity of these deposits are in a range of 10−3 m/s.

1. Crevasses. Erosional channels created as a result of overbank flows, Table 6 Values of the distribution coefficient Kd for ammonium retardation. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Adsorbent

Adsorbate

Adsorbate concentration (mg NH4+/l)

Kd (ml/g)

Source

Clay Siltstone Siltstone Clay Clay Limestone Sand and gravel Sand Montmorillonite Clay Sand and gravel Bentonite Sand Medium sand Sandy clayey loam Sandy gravel Gravel Medium sand Clay

NH4+ NH4+

10–40 10–40 4.24 10–40 2–260 2–260 3.6 50 862 no data no data 1430 20.3 52 52 52 52 52 52

1.47–2.50 5.21–8.53 4.46–5.72 6.09–7.03 1.23–8.95 0.84–9.92 0.23–0.57 1.4 0.24–2.00 2.00–4.00 0.4–0.9 4.9–6.6 0.47–0.82 2.2 2.2 1.5 1.1 2.1 5.2

Environment Agency (2000) Environment Agency (2000) Environment Agency (2000) Environment Agency (2000) British Geological Survey (2002) British Geological Survey (2002) Ceazan et al. (1989) Erskine (2000) Griffin et al. (1976) Buss et al. (2003) Buss et al. (2003) Pivato and Raga (2006) Jellali et al. (2010) This study This study This study This study This study This study

solution solution Leachate NH4+ solution Leachate Leachate Groundwater NH4+ solution Leachate NH4+ solution NH4+ solution Leachate NH4+ solution NH4+ solution NH4+ solution NH4+ solution NH4+ solution NH4+ solution NH4+ solution

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Fig. 6. Directions of groundwater flow in the research area.

Based on modelling studies, it was stated that groundwater flow is from the south to the northern and north-eastern direction. Groundwater levels are observed at a level of 86.5 m above sea level (a.s.l.) in the south site of the modelled area, 84.5 m a.s.l. in the centre and 83.0 m a.s.l. in the north site. The velocity of the groundwater flow estimated in the model were in the range 8.5·10−5 m/s – 1.28·10−4 m/ s. According to Witczak et al. (2013), it was assumed in presented research that the velocity of conservative pollutant migration (nitrate) can be associated with the velocity of groundwater flow. The assessment of the chemical status of groundwater quality was made by comparing the values of the concentration of nitrogen ion compounds in piezometers with limit values for individual groundwater quality classes. According to the Regulation of the Minister of the Environment of Poland (2015), the first, second and third class of groundwater quality indicate a good chemical status. The fourth and fifth class are assigned to bad chemical status. Concerning the concentrations of nitrogen forms observed in groundwater samples collected from piezometers (Fig. 7) and concentrations predicted in the model within the 20 year time span of simulation (Figs. 8 and 9), it can be said that both nitrate and ammonium concentrations indicate good chemical statuses. It has been shown that the mean concentrations of nitrates measured in groundwater samples collected from piezometers located within the agricultural areas do not exceed the limit value of 10 mg/l,

which can be assigned to the first class of groundwater quality. Moreover, due to the precise fertilisation with ammonium nitrate, average concentrations of ammonium in groundwater do not exceed the limit value of 0.5 mg/l, characteristic for the first class of groundwater quality. With the current fertilisation schedule, when the fertiliser is applied precisely (at specific dates and doses adapted to the current demand of plants), modelling variants indicate that the quality of groundwater meets the standards required by Polish law. As shown in Fig. 8, the nitrate plume migrates with groundwater in a north-easterly direction. The decrease of nitrate concentration occurs along the groundwater flow, which can be explained by the dilution of contaminant plume (Molénat and Gascuel-Odoux, 2002). Lasagna et al. (2013) reported that this may be the most important mechanism of attenuation of the non-reactive and non-sorbed solutes in groundwater. It was also stated (Lasagna et al., 2013) that the higher the dilution capability of groundwater, the higher the decrease in nitrate concentration is observed. In the analysed case, the reduction of nitrate in groundwater can be also related to the denitrification process, which is regarded to be an important mechanism for the removal of excess nitrate before leaching to groundwater, transport in saturated zone, or discharge to surface aquifers through the subsurface drainage (Fenton et al., 2009). Nitrate plume generally migrates following the path of groundwater flow which can be assigned to the advection process responsible for its movement. The movement of nitrate is also controlled 8

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Fig. 7. Statistical summary of nitrate and ammonium concentration in piezometers located within the study site.

by the dispersion which is assigned to the spreading of contaminant constituents in groundwater due to variations in velocities within pore spaces. The dispersion is in this case the effect of advective groundwater velocity and the dispersivity, referred to longitudinal dispersivity and the dispersivity perpendicular to the flow direction. The travel time of nitrate ions from the source of contamination (agricultural field fertilised with ammonium nitrate) to the Vistula river was predicted to take between 378 and 586 days, depending on meteorological and soil conditions. The travel time of ammonium ions from the agricultural field to the Vistula river was predicted assuming that the migration of the ions susceptible to sorption (reactive) is Rtimes longer that the migration time of conservative ions, following the

formula:

t0 = R t

(7)

where: t0⁎ - travel time of reactive component [T], R – retardation factor [−], t – travel time of conservative ion [T]. It was also revealed for the purpose of this study that the best model fitting can be obtained for a half-life of ammonium equal to around 3 years. Moreover, it has been shown that the extent and velocity of the ammonium ion migration compared to nitrate ions is many times smaller (Fig. 9), which is related to the sorption process, during which positive charge ammonium ions are retained on negatively charged soil particles. 9

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Fig. 8. Simulated patterns of nitrate migration in groundwater after: a) 1 year, b) 5 years, c) 10 years and d) 20 years of precise fertilisation.

Concentrations obtained from modelling are convergent with concentrations obtained from laboratory tests of water samples taken from piezometers installed within agricultural fields. The results of model calibration (Table 7) show that the model can realistically reflect the groundwater system of the study area. A good match between modelled results (hydraulic heads and concentrations) and observed data showed that the model is well calibrated and can be used in further studies predicting the spread of other contaminants. What is also important is the conducted research proves that the

process of differentiation of the filtration parameters in the alluvial layer is related to the specificity of the evolution of the fluvial environment in the research area. The river flowing in conditions of constant flow was able to dissect “humps” made of Pliocene clays covered by patches of preglacial sediments and other glacial deposits, forming the erosional socle of the valley. The main evidence of the evolution in the fluvial environment is the occurrence of the following zones within the contemporary valleys: 1. The terrace zone of meandering river, formed in the Holocene Atlantic period. The profile of its deposits includes, most importantly, 10

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Fig. 9. Simulated patterns of ammonium migration in groundwater after: a) 1 year, b) 5 years, c) 10 years and d) 20 years of precise fertilisation.

flood deposits. According to lithology, these are loams and sandy loams. On the surface of this form, the zones of contemporary flood flows are encountered in the shape of crevasse splays (Sieczka et al., 2018a), made of fine sands and silts. Under the flood sediments, there are channel deposits of the meandering river, formed as medium and coarse sands and medium sands with gravels. 2. The terrace zone of braided river. This zone concerns the proximal part of the floodplain (the terrace of the contemporary river). The surface of this form is made of sands and gravels of channel facies, covered by a layered package of flood deposits.

Table 7 Results of model calibration. Measured error

Groundwater flow model

Transport model

Mean error Absolute residual mean error Root mean square error

−0.03 m 0.17 m 0.18 m

−0.02 mg/l 0.19 mg/l 0.26 mg/l

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Fig. 10. Multi-stage approach for predicting the spread of nitrogen compounds in the soil-water environment.

model derived from aerial laser scanning is also noteworthy. Additionally, it was reported by Wierzbicki et al. (2013) that the use of aerial imaging and LIDAR data helps in the determination of various geological landforms. The remote sensing tools are especially useful in recognition of environmental conditions in areas used for agriculture and they also allow for the estimation of zones differentiated in terms of infiltration parameters (hydrogeological windows). It is worth noting that due to the genesis associated with the erosive activity of flood flows, the occurrence of such zones is rather limited to the areas of the valleys of large lowland rivers and may be associated with the glacitectonics and poligenesis of these valleys (Falkowska, 2003). Moreover, remote sensing techniques can be efficient in recognising and precise determination of the boundaries of the outcrops of geological landforms (and the shape of these landforms), which may be regarded as zones of privileged filtration paths, affecting the flow dynamics and the conditions of transport of nitrates. Recognition of such zones and then taking into consideration their occurrences in the digital model, may have an impact on the results of the model calibration. The preparation of the numerical model of groundwater flow and contaminant transport should be considered as the basic step for the subsequent stages of the procedure of management in agricultural practice, including ‘forecasts’ and ‘assessments’. Forecasts based on numerical modelling results using Visual Modflow software should include multi-variant simulations illustrating scenarios of groundwater flow and the transport of selected pollutants in the area under consideration. The impact of external factors (infiltration rate, type, concentration and application of contaminant) is also important when preparing scenarios of nitrogen ions spreading. The presented methodology is applicable in particular in areas within the Pleistocene glaciation zones. It was also noted by Falkowska (2009), that the glacial genesis determines the way in which other pollutants (not only nitrogen forms), e.g. heavy metals, are spread. Our approach presents in particular a scheme enabling for identification of zones characterised by the occurrence of deposits with increased hydraulic conductivity (hydrological windows) in the vicinity of low permeable deposits. The universal factor that can determine the spread of pollutant and self-purification of the soil-water environment is the presence of an elevation of a low permeable subgrade. In the valley zones, it controls the course and nature of floods and the presence of hydrogeological windows in the low-permeable floodplain formations. In the presented work, the elevation of the sub-alluvial protrusion consisted of Pliocene clays, but mentioned formations can be found in the bases of glacial deposits of north-eastern Europe (also documented in many related works: Bujakowski et al., 2014; Huuse and Lykke-Andersen, 2000;

3. The zone of meandering river transformed by contemporary overbank flows. This form covers a significant part of the research area. The surface of this zone is made of medium sands. Above them, a layer of flood sediments (light alluvial deposits built of silts and silt loams) occurs. Over the light alluvial soils, especially in local depressions, loams and clays (heavy alluvial soils) are observed. The course of the evolution in the fluvial environment of the Vistula valley and the contemporary activity of the overbank flows led to differentiation of groundwater flow conditions in the alluvial layer. Crucially, the valley's morphogenesis was the creation of erosional divisions connecting waters of the first and second aquifer. Because these divisions are formed as crevasses, oxbows and broader erosional areas (filled with sands), in the zone of their occurrence, rainwater can freely infiltrate deeper in the soil profile, feeding an alluvial usable aquifer. 3.2. Proposed procedure for predicting the spread of nitrogen compounds The identification of factors responsible for a migration of nitrogen compounds in the soil-water environment for the needs of proper management in agricultural practice should be performed as a multistage procedure (Fig. 10). Firstly, the analysis of archival materials including the interpretation of borehole cards specific for the study site should be taken into consideration. As presented in this study, the data from deep boreholes should be adopted to specify the depth to impermeable layers, which at the stage of numerical modelling is crucial for determining the bottom boundary of the model. It is also necessary to collect the meteorological data relevant to the research area, mainly the precipitation and air temperature, in order to estimate water balance components. What was revealed, an effective tool for determination the water balance components is HELP model associated with UnSat Suite Plus software. To determine the parameters of processes affecting the contaminant transport in the groundwater stream, the column experiment method and the static “batch” method should be used, following the ASTM and EPA guidelines, respectively. As presented here, in our previous studies (Sieczka and Koda, 2016) and reported by other researchers (Satake and Tang, 2016), the outputs from the column experiment can be treated as the characteristics of advective-dispersive transport of contaminants. Batch experiments provide crucial characteristics of sorption phenomena, especially the distribution coefficients that can be adopted in the preparation of the hydrogeological model. In the procedure related to the recognition of contaminant transport paths, the usefulness of applying methods of morphogenetic analysis based on high resolution satellite imagery and the numerical terrain 12

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Falkowski et al., 2017). Presented model refers only to the analysed area of Central Poland, and therefore for the purposes of predicting and analyzing processes occurring in other valley areas it would be necessary to construct a new model. Nevertheless, for other areas located in valleys, presented methodology could be applied, with a special attention given to the conceptualization of the aquifer, the evolution of the fluvial environment occurring from the end of the Pleistocene, and the sculptural activity of flood flows. The presented methodology can be reproduced in the entire catchment area of the Baltic and North Sea as well as northern Russia, Canada and the northern part of the USA.

phosphorus in materials from the Throckmorton foot and mouth mass burial site. In: BGS Internal Report CR/02/165. Environment Agency, 2000. CEC and Kd determination in landfill performance evaluation: a review of methodologies and preparation of standard materials for laboratory anal000ysis. In: R&D Technical Report P340, Bristol. PN-B-02480, 1986. Grunty budowlane - Określenia, symbole, podział i opis gruntów. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). PN-B-04481, 1988. Grunty budowlane - Badania próbek gruntu. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). PN-EN ISO 5667-3, 2013. Jakość wody - Pobieranie próbek - Część 3: Utrwalanie i postępowanie z próbkami wody. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). PN-EN ISO 14688-1, 2006. Badania geotechniczne - Oznaczanie i klasyfikowanie gruntów - Część 1: Oznaczanie i opis. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). PN-EN ISO 22475-1, 2006. Rozpoznanie i badania geotechniczne - Pobieranie próbek metodą wiercenia i odkrywek oraz pomiary wód gruntowych - Część 1: Techniczne zasady wykonania. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). PN-C-04576-4, 1994. Woda i ścieki. Badania zawartości związków azotu. Oznaczanie azotu amonowego w wodzie metodą bezpośredniej nessleryzacji. (In Polish). PN-ISO 5667-11, 2004. Jakość wody - pobieranie próbek - część 11: Wytyczne dotyczące pobierania próbek wód podziemnych. Polski Komitet Normalizacyjny, Warszawa, Poland (In Polish). World Health Organization (WHO), 2004. Guidelines for Drinking Water Quality: Volume 1 - Recommendations 3rd ed., Geneva. Almasri, M.N., Kaluarachchi, J.J., 2007. Modeling nitrate contamination of groundwater in agricultural watersheds. J. Hydrol. 343, 211–229. https://doi.org/10.1016/j. jhydrol.2007.06.016. Andersen, J.H., Carstensen, J., Conley, D.J., Dromph, K., Fleming-Lehtinen, V., Gustafsson, B.G., Josefson, A.B., Norkko, A., Villnäs, A., Murray, C., 2015. Long-term temporal and spatial trends in eutrophication status of the Baltic Sea. Biol. Rev. 135149. https://doi.org/10.1111/brv.12221. Appelo, C.A.J., Postma, D., 1999. Geochemistry, Groundwater and Pollution. A.A. Balkema, Rottermdam, Brookfield. Ashok, V., Hait, S., 2015. Remediation of nitrate-contaminated water by solid-phase denitrification process - a review. Environ. Sci. Pollut. Res. 22, 8075. https://doi.org/ 10.1007/s11356-015-4334-9. Batu, V., 1998. Aquifer Hydraulics: A Comprehensive Guide to Hydrogeologic Data Analysis. John Wiley & Sons, New York, USA. Bednarek, A., Szklarek, S., Zalewski, M., 2014. Nitrogen pollution removal from areas of intensive farming - comparison of various denitrification biotechnologies. Ecohydrology & Hydrobiology 14 (2), 132–141. https://doi.org/10.1016/j.ecohyd. 2014.01.005. Beegum, S., Šimůnek, J., Szymkiewicz, A., Sudheer, K.P., Nambi, I.M., 2018. Updating the coupling algorithm between HYDRUS and MODFLOW in the HYDRUS Package for MODFLOW. Vadose Zone J. 17. https://doi.org/10.2136/vzj2018.02.0034. Bian, J., Liu, C., Zhang, Z., Wang, R., Gao, Y., 2016. Hydro-Geochemical Characteristics and Health Risk Evaluation of Nitrate in Groundwater. Pol J Environ Stud 25 (2), 521–527. https://doi.org/10.15244/pjoes/61113. Bieciński, P.A., 1960. Nowyj metod opredelenija koefficienta wodootdaczi wodonosnych płastow [a new method for determining the storage coefficient of aquifers]. Gidrotehnika i Melioracija 6, 15–20. Brtnický, M., Pecina, V., Hladký, J., Radziemska, M., Koudelková, Z., Klimánek, M., Richtera, L., Adamcová, D., Elbl, J., Vašinová Galiová, M., Baláková, L., Kynický, J., Smolíková, V., Houška, J., Vaverková, M.D., 2019. Assessment of phytotoxicity, environmental and health risks of historical urban park soils. Chemosphere 220, 678–686. https://doi.org/10.1016/j.chemosphere.2018.12.188. Bujakowski, F., Falkowski, T., 2019. Hydrogeological analysis supported by remote sensing methods as a tool for assessing the safety of embankments (case study from Vistula River Valley, Poland). Water 11, 266. https://doi.org/10.3390/w11020266. Bujakowski, F., Ostrowski, P., Sopel, Ł., Zlotoszewska-Niedziałek, H., 2014. Glacitektonika krawędziowa w dolinie Wisły a dynamika wód podziemnych [connection between glacitectonic forms and groundwater flow in the Vistula River Valley]. Landf. Anal. 26, 61–69. https://doi.org/10.12657/landfana.026.005. (In Polish). Bumb, A.C., McKee, C.R., Way, S.C., Drever, J.T., Halepaska, J.C., 1987. Ammonia and Nitrate Migration from the Vadose Zone to the Ground Water System: Containment, Recovery and Natural Restoration. Proceedings of the First National Outdoor Action Conference on Aquifer Restoration. National Water Well Association, Dublin, Ohio, pp. 95–123. 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4. Conclusions From the example of the presented case study of the agricultural area located in the Vistula river valley, it can be concluded that prepared (and then verified) methodology could be adopted in programmes related to agricultural management. Even though the created model of groundwater flow could be applied only for the presented specific area of Central Poland, it should be highlighted that the methodology is universal, especially when recognising the migration processes of nitrogen compounds in the soil-water system in other agricultural areas located in a valley. Our idea was to highlight that river valleys are zones of intensive groundwater exchange and generally constitute the drainage to the neighboring areas cut by them. In our study, due to the close location of the channel zone constituting the collector draining water from aquifers, and the shallow deposition of these layers in the floodplain zone, it was indicated that the pollution induced in this area can move to surface waters relatively quickly, posing a potential threat, in particular to the sensitive ecosystem of the Baltic Sea. The presented methodological approach, with particular attention paid to remote sensing tools, can be adopted as the basic stage of the procedure aiming at identification zones which are susceptible to pollution. This study has revealed that the application of hydrogeological modelling techniques gives great perspectives for monitoring and protecting the aquifer system from spatial and temporal migration of nitrogen forms. In practical terms, results of the afore presented research may have significant implications in agroecology and agroecological engineering, especially in aspects related to agricultural management, planning the fertilisation schedules and predicting the short and longterm impact of fertilisation on groundwater quality. Declaration of Competing Interest None. Acknowledgements This research was performed within the project entitled “Analysis of contaminant migration processes in the soil-water environment using laboratory tests and numerical modeling techniques” supported by the National Science Centre, Poland, under a grant No. 2017/25/N/ST10/ 00909. Preliminary studies of the presented research, concerning the recognition of the vadose zone in agricultural fields, were supported by the European Regional Development Fund under the Innovative Economy Operational Programme: BIOPRODUCTS, innovative production technologies of pro-healthy bakery products and pasta with reduced caloric value (POIG.01.03.01-14-041/12). References ASTM D5084-00, 2001. Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter. ASTM International, West Conshohocken. British Geological Survey, 2002. Measurement of Kd values for ammonia, potassium and

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