Determination of nitrogen reduction levels necessary to reach groundwater quality targets in Slovenia.

J O U R NA L OF EN V I RO N M EN T A L S CI EN C ES 2 6 ( 20 1 4 ) 1 8 06–1 8 1 7

Available online at www.sciencedirect.com

ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences

Determination of nitrogen reduction levels necessary to reach groundwater quality targets in Slovenia Miso Andelov1 , Ralf Kunkel2,⁎, Jože Uhan1 , Frank Wendland2 1. Slovenian Environment Agency, Hydrogeological Analysis Division, Vojkova 1b, SI-1000 Ljubljana, Slovenia. E-mail: [email protected] 2. Forschungszentrum Jülich (FZJ), Institute of Bio- and Geosciences (IBG), Institute 3: Agrosphere, 52425 Juelich, Germany

AR TIC LE I N FO

ABS TR ACT

Article history:

Within a collaborative project between Slovenian Environment Agency (ARSO) and

Received 2 November 2013

Research Center Jülich (FZJ), nitrogen reduction levels necessary to reach groundwater

Revised 5 February 2014

quality targets in Slovenia were assessed. For this purpose the hydrological model GROWA–

Accepted 8 February 2014

DENUZ was coupled with agricultural N balances and applied consistently to the whole

Available online 9 July 2014

territory of Slovenia in a spatial resolution of 100 × 100 m. GROWA was used to determine the water balance in Slovenia for the hydrologic period 1971–2000. Simultaneously, the

Keywords:

displaceable N load in soil was assessed from agricultural Slovenian N surpluses for 2011

EU Water Framework Directive

and the atmospheric N deposition. Subsequently, the DENUZ model was used to assess the

Groundwater quality targets

nitrate degradation in soil and, in combination with the percolation water rates from the

Nitrate pollution

GROWA model, to determine nitrate concentration in the leachate. The areas showing

Reactive nitrate transport modeling

predicted nitrate concentrations in the leachate above the EU groundwater quality standard of 50 mg NO−3/L have been identified as priority areas for implementing nitrogen reduction measures. For these “hot spot” areas DENUZ was used in a backward mode to quantify the maximal permissible nitrogen surplus levels in agriculture to guarantee a nitrate concentration in percolation water below 50 mg NO−3/L. Model results indicate that additional N reduction measures should be implemented in priority areas rather than area-covering. Research work will directly support the implementation of the European Union Water Framework Directive in Slovenia, e.g., by using the maximal permissible nitrogen surplus levels as a framework for the derivation of regionally adapted and hence effective nitrogen reduction measures. © 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Introduction Slovenia is one of the smaller European countries (20,273 km2), with a total population of about 2 million inhabitants. It is located at the meeting point of four major European geographical regions: the Alps, the Dinaric Mountains, the Pannonian Basin and the Mediterranean (Adriatic Sea). Whereas only around 3% of Slovenian territory

comprises settlement areas, ca. one third of the country is used for agriculture (mainly in the north-east). More than 58% of the country, however, is covered with forests and in some areas woodland scrub (Fig. 1), ranking Slovenia amongst the most forested countries in Europe. Nevertheless, the input of nitrates into groundwater from point pollution (unregulated livestock manure storage and sewage system) and non-point pollution due to the use of livestock manure

⁎ Corresponding author. E-mail: [email protected] (Ralf Kunkel).

http://dx.doi.org/10.1016/j.jes.2014.06.027 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

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JO U R N A L OF EN V I RO N M EN T A L S CI E NC ES 2 6 ( 20 1 4 ) 1 8 0 6–1 8 1 7

Land use-Corine Continuous urban fabric Discontinuous urban fabric Non-irrgated arable land Mixed agriculture with natural vegetation Specialised crops Grassland Complex cultivation patterns Broad-leaved forest Coniferous forest Mixed forest Bare lands Water Groundwater body

Source: ARSO, GURS Author: Dr. M. Andjelov Model DENUZ

Fig. 1 – Land cover in Slovenia.

and mineral fertilizers may lead to significant nutrient problems in the groundwater and surface waters of Slovenia. The fundamental objectives of the European Union Water Framework Directive (EU-WFD) (WFD, 2000) and the European Union Groundwater Directive (EU-GWD) (GWD, 2006) are to attain a good status of all surface waters and groundwater resources in the EU by 2015. Following the WFD, the chemical status of the Slovenian groundwater had to be determined by 2004 and remediation measures to reduce groundwater pollution had to be implemented for groundwater bodies at risk until 2015 (Republic of Slovenia, 2011). The main source of diffuse nitrogen inputs on agricultural land in Slovenia has been identified as nitrogen from mineral fertilizers and livestock manures, contributing to about 84% to the overall input of nitrogen. Contributions from other input sources like atmospheric deposition or biological fixation by legume crops are less important. The balance of nitrogen, thus the difference between the input and the output of nitrogen, indicates a balance surplus of nitrogen. In the time period 1992– 2008, it amounted to be between 23 and 94 kg N/ha/year with a decreasing trend over time (ARSO, 2009). Because the delineation of groundwater bodies was still in progress and no chemical groundwater status was assessed at 2004, compliance with quality standards was stated. Later investigations showed that groundwater quality problems occur mainly in the alluvial aquifers, especially in the northeastern part of Slovenia, mainly due to nitrates, pesticides and hydrocarbons (Krajnc et al., 2007). Uhan et al. (2012) pointed out that nitrate in Slovenian waters has been a major concern for decades and about 37% of the groundwater in alluvial aquifers has poor chemical status according to EU-WFD criteria, most frequently due to a high concentration of nitrate. In this context inputs from diffuse sources and most of all nitrate losses from agriculturally used

land have been identified as one of the main reasons for probably failing the “good qualitative status” of water resources. Except for the Murska kotlina and Savinjska groundwater bodies which show a clear downward trend of the nitrate concentrations from 1998 to 2011, no statistically significant trends are present in the groundwater bodies of Slovenia (Mihorko and Gacin, 2011). Groundwater in karst and fractured aquifers is less burdened with nitrates due to its geographical conditions, low population density and scarce agricultural land. Presently, the assessment of the chemical status of groundwater resources in Slovenia is assessed annually from a monitoring of the drinking water supply system and on a general monitoring of the groundwater quality alluvial aquifer and major springs. In total 928 supply areas of the drinking water supply system were investigated in 2011 (Lapajne and Sovič, 2012). National groundwater quality monitoring, which is carried out since 1987, is the systematic monitoring of the total 150 various physical and chemical parameters in groundwater bodies (Cvitanič et al., 2011). The monitoring sites in alluvial aquifers are wells and boreholes. In aquifers with karst and fissure porosity monitoring sites are springs and wells. In 2011, the national monitoring network included 130 monitoring sites, which were more concentrated in alluvial aquifers. It is evident that the drafting and implementation of catchment wide measurement programs according to the overall goals of the EU-WFD have to include measures against diffuse nitrate pollution. Consequently, on the part of the water resources management in Slovenia there are increasing demands on the agricultural sector to contribute adequately to the lowering of nitrate inputs into groundwater and surface waters by introducing effective nitrate reduction measures. Agro-environmental measures to reduce nitrate pollution of surface and ground waters will however differently affect the historically evolved

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and established agro-economic structures in Slovenia so that the impacts on the regional N balance and hence the displaceable N surplus in the individual natural landscape units in Slovenia will be uneven. Hence the efficiency of such measures has to be evaluated area-differentiated. Model applications are important tools to quantify the different input pathways, the transport of diffuse nitrate surpluses into groundwater and surface waters and the potential effects of agro-environmental measures. For Slovenia, model based assessments of nitrate fluxes are available only for selected groundwater bodies, but not for the entire area of Slovenia (Uhan, 2012). In this article we show the methodology and the application of the integrated agro-economic-hydrologic model system GROWA/DENUZ for the assessment of the nitrate contamination of groundwater for all of Slovenia. This model system was used because it has been applied successfully to a number of supra-regional areas within and outside of Germany (e.g., Turkey, Greece) and allows an area-covering consideration of Slovenia. The study was carried out in five consecutive steps: (1) analysis of the actual displaceable nitrogen load in soil from diffuse sources across Slovenia, (2) determination of Slovenian water balance with a focus on leachate rate in a high spatial resolution, (3) assessment of denitrification rates in soil and calculation of the related nitrate concentration in the leachate respectively, (4) delineation of priority areas for the implementation of nitrogen reduction measures, and (5) dimensioning of agricultural N reduction levels necessary in order to guarantee nitrate concentrations in the leachate below 50 mg NO−3/L.

1. Methods A review by Heinz et al. (2007) has shown that different model approaches have been developed in the EU member states to support the implementation of the EU-WFD. Approaches to assess water quantity have been developed by Assimacopoulos (2006) and Bazzani et al. (2004) for South European countries like Greece and Italy, whereas approaches to analyze water quality related issues have predominantly been developed in North-western European countries (Groenendijk, 2008; Hirt et al., 2008; van der Veeren and Tol, 2001). Special investigations about the macro-economic effects of the improvement of water quality under the conditions of the WFD have been carried out (Aftab et al., 2007; Brouwer et al., 2008; Fezzi et al., 2010; Moss, 2004; Mysiak and Sigel, 2005; Pulido-Velazquez et al., 2006). Model tools aiming at an evaluation of the impact of measures meant to implement the EU-WFD are still underrepresented (Lindenschmidt et al., 2007). First steps in this direction on the scale of subbasins and/or States in Germany were already done by various authors (Gebel et al., 2010; Hirt et al., 2008; Kreins et al., 2010; Kunkel et al., 2008; Wendland et al., 2007, 2009). For the prognosis of the nitrogen inputs into groundwater and surface waters N surpluses from agriculture were coupled with the water balance model GROWA (Kunkel and Wendland, 2002) in order to determine the relevant input pathways. The assessment of denitrification rates in soil and groundwater was carried out based on the reactive nitrate transport models DENUZ and WEKU (Kunkel et al., 2006; Wendland et al., 2004). These models are described in detail in previous publications (Kreins et al., 2007; Kunkel and Wendland, 2002; Tetzlaff et al., 2009; Wendland et al., 2007, 2009) and will be introduced here only in brief.

1.1. Environmental target The EU Water Framework Directive requires the nitrate concentration in all groundwater monitoring wells to be lower than 50 mg NO3/L. The nitrate concentration in groundwater, however, is very difficult to assess by large-scale models because of lack of data. Therefore the influence of nutrient inputs and in particular the effects of nutrient reduction measures on groundwater quality need to be measured against another environmental target value. In the AGRUM project a mean long-term nitrate concentration in percolation water of 50 mg NO3/L was defined as a suitable environmental target for protecting groundwater against an exceeding of the EU quality standard for nitrate. This value is without any doubt appropriate for oxidization, i.e., not nitrate degrading aquifers as it guarantees a nitrate concentration in groundwater below the EU quality standard for drinking water. In reduced aquifers often low nitrate concentrations in groundwater have been observed, even in the case of high inputs by percolation water. This is due to denitrification processes in the aquifer, taking place in the absence of oxygen and the presence of pyrite and/or organic carbon material. Most of the aquifers in the northern part of the Weser basin show a high denitrification potential (Wendland et al., 2005). Denitrification in groundwater, however, is associated with the irreversible consumption of substances in the groundwater, such as pyrite and organic carbon. Once these substances are exhausted nitrate cannot be denitrified in groundwater any more. As a consequence, nitrate concentrations would start to rise, like it is described for many sites since many years (Rohmann and Sontheimer, 1985). Consequently, the denitrification buffer of groundwater systems has to be prevented from damages. Defining an environmental target value of 50 mg NO3/L in the leachate would ensure a “good groundwater quality status” even in the case of missing or exhausted nitrate degradation capacities in the aquifer.

1.2. Nitrogen concentration in the leachate The nitrogen concentration in the leachate is considered as an indicator to detect and classify “hot-spot” regions with regard to nitrate pollution. The mean long-term nitrate concentration in percolation water (cNO3−, mg NO−3/L) can be calculated using the long-term percolation water rate (Qp, mm/year) and the nitrogen surpluses (Ns, kg N/ha/year) from after denitrification during the travel time (tsoil, year) in the soil: cNO− 3 ¼

443 Ns t soil : Qp

ð1Þ

1.3. Denitrification in the soil Nitrogen surpluses in the soil represent a risk potential since they indicate the amount of nitrogen potentially leaching into groundwater and surface water. As a rule however, not all of the mineral N surpluses in soils are displaced to surface waters via the different transport pathways. A certain amount is degraded in soils to molecular nitrogen by denitrification processes. It is sufficiently known that denitrification losses in soils occur mainly in the root zone in the case of low oxygen

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and high water contents as well as high contents of organic substances (Bremner and Shaw, 1958; Burford and Bremner, 1975; Mosier et al., 2002). In contrast, low denitrification rates can be expected in soils displaying high oxygen contents and low contents of water and organic substances and in soils, which are vulnerable to acidification. In the DENUZ model (Kunkel and Wendland, 2006) denitrification losses in soils are calculated according to a Michaelis–Menten kinetics: dN s t soil Ns t soil þ Dmax ¼0 dt soil k þ Ns t soil

ð2Þ

where, Ns (kg N/ha/year) denotes the remaining nitrate surplus in soils after travel time tsoil (years) in the soil, k (kg N/ha/year) the Michaelis constant and Dmax (kg N/ha/year) the maximal denitrification rate. Within a one-dimensional approach, denitrification in the soil is modeled raster based for each individual raster cell. As input parameters, raster data on the nitrogen surpluses, the denitrification conditions in the soil and the travel times in the soil are required. The data used for modeling are listed in Table 1. Using observed denitrification rates in German soils as a reference, Dmax was assessed from a soil map of Slovenia based on a ranking of the occurring soil types according to their geological substrate, the influence of groundwater and perching water and the average residence time of perching water in the soil as differentiation criteria (Wienhaus et al., 2008). In this way the soil types were assigned into four classes ranging from 10 N/ha/year ≤ Dmax ≤ 150 kg N/ha·year. High maximum denitrification rates can be expected for the carbon rich and water saturated soils in the flood plains near the rivers and above all for all areas where fens and bogs occur. In contrast, low denitrification rates can be expected in areas where well-aerated soils, e.g., Podzol soils, predominate. The Michaelis constant (k) determines the decrease of denitrification rates in the case of small (remaining) N-surpluses. This parameter has been set to values between 2.5 and 18 kg N/ha·year.

1.4. Assessment of percolation water rates and travel times Percolation water rates and travel times of nitrogen in the soil have been assessed based on the water balance model GROWA (Kunkel and Wendland, 2002), which has been developed to support practical water resources management issues of large river basins and has already been applied in different regions of different sizes with different perspectives (Bogena et al., 2005; Kunkel et al., 2005; Tetzlaff et al., 2007; Wendland et al., 2003, 2005, 2007). It employs a raster-based empirical approach with a temporal resolution of one or more years. Annual averages of the main water balance components, e.g., total runoff, percolation water rate, direct runoff and groundwater recharge, are quantified as a function of climate, soil, geology, topography and land use conditions (Table 1). The effective extent of denitrification in soils depends on, last but not the least, the travel time of percolation water through the denitrifying soil horizons (Green et al., 2009; Groenendijk, 2008; Seitzinger et al., 2006), i.e., the rooting zone predominantly. On a long-term average, the travel time of percolation water can be estimated by the integral effective field capacity of the soil in the root zone and the percolation water rate (Müller, 2004). Actual mean annual denitrification

Table 1 – Slovenian database used for modeling. Data type

Scale/ resolution

Agrarian statistical data on N fertilizer input, manure per animal, crop withdrawal etc., Atmospheric deposition of oxidized and reduced nitrogen

GERK (Graphical Unit of Land Use) 50 × 50 km grids

Precipitation data summer/winter (1971– 2000), annual potential evapotranspiration (1971–2000) Land cover Soil types, soil texture, effective field capacities for arable land

100 × 100 m grids

25 ha 1:25.000

Effective field capacities, 1:25.000 influence of perching water, rooting depth Depth to groundwater 1:25.000

Artificially drained areas Digital elevation model (DMR 100)

1:25.000

Geological and hydrogeological map Catchments areas, daily runoff data (1971–2000) River network, political boundaries, cities etc.,

1:100.000, 1:500.000 1:25.000

100 × 100 m

1:25.000

Data source Statistical Office of the Republic of Slovenia, Agricultural institute of Slovenia European Monitoring and Evaluation Programme (EMEP), Agencija Republike Slovenije za okolje ARSO ARSO, Meteorology Office

CORINE data base Ministry of Agriculture, Forestry and Food (MKGP), University of Ljubljana, Biotechnical Faculty, Centre for Soil and Environmental Science Derived based on pedotransfer functions ARSO, Hydrology and State of the Environment Office Ministry of Agriculture, Forestry and Food (MKGP) Surveying and Mapping Authority of the Republic of Slovenia (GURS) Geological Survey of Slovenia (GeoZS) ARSO, Hydrology and State of the Environment Office Surveying and Mapping Authority of the Republic of Slovenia (GURS)

rates are determined as a linear relationship between the realizable maximum annual denitrification rates (assigned by the denitrification conditions) and the effective residence times in the root zone. In this way it is assumed, that e.g., in case the residence time amounts to six months, the extent of denitrification is half of the maximum denitrification rate per year. For the calculation of the travel time of percolation water through the denitrifying zone of the soils it is assumed that the depth of the denitrifying zone corresponds to the effective root zone of the soils. On a long-term average, the travel time of percolation water tsoil (year) can be estimated by the effective field capacity (FCeff, mm/dm) of the soil in the denitrifying zone with thickness (drz, dm) and the percolation water rate QP (mm/year): t soil ¼

FCeff drz : Qp

ð3Þ

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1.5. Nitrogen surpluses

2. Data base preparation

Agricultural nitrogen surpluses are quantified using the methodology of OECD (OECD and EUROSTAT, 2007). Agricultural statistics with data, e.g., on crop yields, livestock farming and land use, were used to balance the nitrogen supplies and extractions for the agricultural area. Nitrogen supply from manure is derived from the number of farm animals, the estimated quantities of excreted organic nitrogen by the different farm animals and the loss of organic ammonium-N from livestock buildings and manure facilities (Döhler et al., 2002; EEA, 2002; Menzi et al., 1997). Because there were no official records available about the usage of mineral nitrogen in Slovenia, data have been taken from the Statistical Office of the Republic of Slovenia (SURS) and field survey data from 11,084 family farms about mineral fertilizer usage (SURS, 2009). It was presumed that the nitrogen input from mineral fertilizers to different cultures is equally distributed and that the amount of N input from organic fertilizers is equally distributed over all the land at the individual farm. The available agricultural data were collected into a database, which is used in expert decision-making models and implemented in spatial algorithms to calculate estimates of total N input to Slovenians Graphical Units of Land Use (GERK, 2010). As a rule, the difference between nitrogen supplies, primarily by mineral fertilizers and farm manure, and nitrogen extractions, primarily by field crops, leads to a positive N-balance (Sušin and Verbič, 2011). Thus, nitrogen surpluses represent a risk potential since they indicate the amount of nitrogen potentially leaching into groundwater and surface water.

A lot of effort was given to the set-up of a comprehensive, thematically consistent and geographically uniform database for the whole Slovenian territory. This included the compilation of spatially distributed climatic, hydrological, pedological, topographical and hydrogeological basic data. A list of the used data sets is given in Table 1. All data sets were embedded in a Geographic Information System (ArcGIS) and a relational data base system (Microsoft ACCESS). Data storage as well as the analysis and the evaluation of results took place in ArcGIS. Independent of the here presented study these data bases can be used for further hydrologic investigations in Slovenia. For the derivation of agricultural N surpluses agricultural statistical data available on Graphical Units of Land Use (GERK, 2010) and on a yearly basis were compiled in order to balance the nitrogen supplies and extractions for the agricultural used areas (Sušin and Verbič, 2011). For atmospheric deposition EMAP values were taken into account (Klein et al., 2011). All hydrologic databases used in this study originate from state authorities and Universities. The Meteorology Office of the Environmental Agency of the Republic of Slovenia (ARSO) processed the climatic parameters (ARSO, 2010b). The soilphysical and topographic parameters for the unconsolidated rock regions were taken from the Digital Soil Information System (Vrščaj et al., 2010) and calculated on the basis of the DMR 100 digital terrain model (GURS, 2000) respectively. The pedological parameters were derived by the University of Ljubljana, Biotechnical Faculty, Centre for Soil and Environmental Science and by the Ministry of Agriculture, Forestry and Food.

N inputs into the soil after retention Less than 5 kg N/(ha.year) 5-10 kg N/(ha.year) 10-25 kg N/(ha.year) 25-50 kg N/(ha.year) 50-75 kg N/(ha.year) 75-100 kg N/(ha.year) 100-150 kg N/(ha.year) More than 150 kg N/(ha.year) Groundwater body

Source: ARSO, KIS, GURS Author: Dr. M. Andjelov Model DENUZ

Fig. 2 – Displaceable N loads in Slovenia 2011.

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Hydrogeological maps were made available by the Geological Survey of Slovenia (Prestor et al., 2006). 26 petrographic features were taken from hydrogeological maps provided by GEOZS (Buser, 2010; Kos et al., 1993; OGK, 1998). The digital elevation model (DMR) used originates from the Surveying and Mapping Authority of the Republic of Slovenia (GURS, 2000). Data about artificially drained areas were provided by the Ministry of Agriculture, Forestry and Food (HMO, 2007). Observed runoff data at gauging stations were made available by Hydrology and State of the Environment Office (ARSO, 2010a). More details about the individual databases can be found in the ARSO Information System (ARSO, 2011). The spatial resolution of the agro-economic calculations and the hydrologic models is significantly different. Whereas N surplus values are determined for administrative units (GERK), the models GROWA–DENUZ calculate hydro(geo)logic parameters for individual plots or grids. For the assessment of the diffuse nitrate fluxes in Slovenia a spatial resolution of 100 × 100 m have been chosen. In this way the Slovenian territory was subdivided into 2.027.300 elementary computation units.

3. Model results 3.1. Nitrogen surpluses Fig. 2 shows the displaceable N surpluses in the soil for the year 2011 determined from the N surpluses by agriculture and

complemented by the atmospheric deposition in Slovenia (EEA, 2002). The displaceable N loads in soil indicate the amount of nitrogen potentially leaching into groundwater and surface waters. In the forested mountainous areas of Slovenia, N-surpluses predominantly below 10 kg N/ha/year are determined by atmospheric deposition only. It becomes clear that small displaceable N loads in soil occur in the mountain ranges in the Southern and Northern part of Slovenia, whereas the N surplus levels above 50 kg N/ha/year occur in few areas only. These areas indicate fertile basin regions in high agricultural intensity. Especially in regions with area-independent animal processing (intensive animal production) nitrogen surpluses result from both animal excretions and mineral fertilizers and exceed 100 kg N/ha/year.

3.2. Leachate rate Fig. 3 shows the leachate rates for the time period 1971–2000 calculated with the GROWA model. A considerable variation occurs within the country, which refers to the heterogeneity of the prevailing site conditions. The regional differentiation is strongly determined by the precipitation patterns, showing a significant decrease from the alpine part to the sub-continental Pannonian basin. The central parts show a wide range of leachate rates between 400 and 1200 mm/year, whereas low leachate rates can be found in the Pannonian basin (below 300 mm/year). High values above 900 mm/year are modeled for the alpine regions in the northern and especially northwestern parts of Slovenia.

Leachate rate 1150 mm/year Groundwater body


Fig. 3 – Calculated leachate rate for the time period 1971–2000.

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a

b

Residence of water in the soil

Denitrification conditions in soil

Less than 0.1 year 0.1-0.25 year 0.25-0.5 year 0.5-0.75 year 0.75-1.00 year 1.00-2.5 year 2.5-5.0 year More than 5 year

Very bad Bad Moderate Good Very good Groundwater body

Groundwater body


Source: ARSO, GURS Author: Dr. M. Andjelov Model DENUZ

Fig. 4 – Denitrification conditions (a) and travel times of percolation water (b) in the soil.

3.3. Denitrification conditions and nitrogen residence times in the soil Denitrification conditions in the soil have been assessed by an evaluation of existing soil data for Slovenia with respect to the occurring soil types, their geological substrate and the influence of groundwater and perching water as differentiation criteria. The result, shown in Fig. 4a, illustrates that in most regions of Slovenia unfavorable denitrification conditions occur. In general, these regions are also characterized by short residence times of the percolation water in the soil (Fig. 4b). Therefore, it can be expected that only very limited amounts of nitrate will be denitrified in the soil. In unconsolidated rock areas, especially in some areas of the northeastern

part of Slovenia, good denitrification conditions coincide with long residence times in the soil. Here, higher amounts of nitrogen may be denitrified. However because these regions are used agriculturally, high N-surpluses in the soil may lead to high N-outputs from the soil despite the effective denitrification.

3.4. Nitrate concentration in the leachate The N output from soil, i.e., the remaining N load in the soil after denitrification is diluted by percolation water (Fig. 3). Fig. 5 shows that the calculated potential nitrate concentration in the leachate (Eq. (1)) ranges between 10 mg NO3/L and more than 100 mg NO3/L. Low nitrate concentrations in the leachate were modeled for the mountain ranges, where

Nitrate concentration in the leachate Less than 5 mg NO3/L 5-10 mg NO3/L 10-25 mg NO3/L 25-50 mg NO3/L 50-100 mg NO3/L More than 100 mg NO3/L Groundwater body


Fig. 5 – Mean annual nitrate concentration in percolation water in Slovenia.

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N reduction requirements to reach 50 mg NO3/L in the leachate 50 kg/(ha.year) Groundwater body


Fig. 6 – Required N-reduction to reach 50 mg NO3/L in the percolation water.

extensive agriculture and high percolation water rates predominate. In contrast, for areas with intensive agriculture and high N-surplus levels nitrate concentrations above 100 mg NO3/L may be present even in the case of low percolation water rates and good denitrification conditions.

3.5. N reduction requirements As mentioned in Section 1.1 a mean long-term nitrate concentration in percolation water of 50 mg NO3/L was defined as an environmental target value for protecting groundwater against an exceeding of the EU groundwater quality standard for nitrate. For those areas currently exceeding this target value, environmental reduction measures to reduce the nitrogen surpluses from agriculture have to be applied. An estimate of the amount of nitrogen surpluses to be reduced can be gathered by a backward calculation of the DENUZ model. By this backward calculation the N-surplus in the soil is determined in an iterative way, which leads after denitrification and dilution by the percolation water to a concentration of 50 mg NO3/L. All other model parameters (percolation water rate, denitrification rate) remain unchanged. Subtraction from the actual N-surplus leads to the N-reduction requirement. As can be seen from Fig. 6 the N-reduction requirement vary between 0 (white areas) and more than 70 kg N/ha·year especially in those regions, which are dominated by are-independent animal production. There, the necessary N surplus reduction would affect vast areas. Appropriate measures to achieve this extent of N reduction in these areas may be the prohibition to apply manure fertilizers after harvesting, improved techniques to apply liquid manure and (to a certain extent) the introduction of catch cropping systems.

However, the adequate use of such model results from the implementation of agro-environmental measures on the State level requires expert knowledge and prudence. Without doubt the use of model results for decision-making is subjected to several constraints. On one hand, the quality of model results depends on the consideration of the underlying hydrological and/or economic processes. On the other hand, there are limitations in the local representativeness of the model input parameters, especially on the superregional (state-wide) scale, which is dealt with in this study. Therefore we are not suggesting implementing far reaching agro-environmental decisions based on the model calculations exclusively. It is self-evident that such model results on a State level can never replace an effective regionally adapted monitoring network and regional expert knowledge. It has the potential however to complement the regional monitoring and decision-making.

4. Application of model results for the management of groundwater resources The model results presented here can be used for an enhanced classification of groundwater bodies with respect to the risk for nitrate pollution. Although the observed concentrations only allow the assessment of the current status of groundwater, it does not necessarily reflect the potential risk of nitrate intakes into groundwater. Pressures to groundwater by nitrates may be present but not observed due to missing monitoring wells or significant denitrification in the aquifer, which may result in low nitrate concentrations despite high nitrate inputs in the groundwater (Bremner and

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a


b


c


Fig. 7 – Quality status of groundwater under consideration of at least 1 groundwater monitoring station with nitrate concentrations above 50 mg NO3/L or nitrate concentrations in the leachate above 50 mg NO3/L on average for the whole groundwater body (a), for more than 25 km2 of the groundwater body (b) and for more than 20% of the groundwater body area (c).


Shaw, 1958; Kunkel and Wendland, 2000; Postma et al., 1991; Seitzinger et al., 2006). A solution to overcome this is the consideration of the calculated nitrate concentration in the leachate as an additional indicator. As examples, the status of a groundwater body can be defined from the condition that more than one groundwater monitoring station in a groundwater body display nitrate concentrations above 50 mg NO3/L or (1) the average nitrate concentration in the leachate of a groundwater body is larger than 50 mg NO3/L; (2) the area of a groundwater body displaying nitrate concentrations in the leachate above 50 mg NO3/L is larger than 25 km2; and (3) the area of a groundwater body displaying nitrate concentrations in the leachate above 50 mg NO3/L is larger than 20% of the groundwater body area. The first condition is the most restrictive, since it requires calculated nitrate concentrations in leachate above 50 mg NO3/L for a large area. Therefore, the groundwater bodies classified as in bad status are mainly those, which are classified as in bad status from the assessment of the monitoring data. However, the bad status of the groundwater body in the central part of Slovenia (Fig. 7a) results not from the monitoring data but from the model results. In this region, denitrification in the aquifer may be the reason why high nitrogen inputs into groundwater do not result in high concentrations in the groundwater monitoring wells. Fig. 7b and c indicate that the number of endangered groundwater bodies increases when the weight of the nitrate concentration in the leachate as an additional indicator is increased. This shows that the role of integrated modeling for the management of environmental relevant pollution fluxes will surely become more important in the future.

5. Discussion and conclusions Coupling the regional diffuse N surpluses with the hydrological model GROWA–DENUZ allows assessing the nitrogen pollution of groundwater in a consistent and spatially highly resolved way for Slovenia. The validity of the model results was checked using measured runoff data at gauging stations and accessible point results concerning nitrate concentrations in leachate. Comparison of the results of the water balance model with observed runoff data for 95 catchments areas in Slovenia showed a good correlation (Tetzlaff et al., 2014). A comparison of the DENUZ model results with measured nitrogen loads from the soil at individual sites is, in general, very problematic, since site- and point-in-time specific observations are compared to model results for long-term averages based on generalized data bases. A very limited amount of observed data from hop-growing areas is available for Slovenia. These data, however, indicate a satisfying derivation between modeled and observed nitrogen loads in a range of ± 9 kg N/ha·year (Uhan, 2012). As all relevant diffuse source inputs are considered simultaneously in the model it is possible to analyze the need for action and the options for action related to both, the regional natural (hydrologic) site conditions and the regional agro-economic site conditions. Such model features are a pre-requisite to assess the actual N pollution inputs into

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groundwater, identify hot spots and provide regional reference values for the dimensioning of N reduction measures. The latter is important for the appropriate allocation of subsidies available for agro-environmental reduction measures according to clear and comprehensible criteria. The model analysis indicates that only a limited number of groundwater bodies are at risk of failing good status due to high nitrate concentrations and that reduction measures would not have to be implemented on the whole area of a groundwater body classified as “at risk”. Instead, subareas for a regional efficient application of measure development and allocation of funds are identified, i.e., the subareas displaying the highest potential for a further implementation of groundwater protecting nitrogen reduction measures.

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