Improving water quality through nitrogen removal
Biological denitrification is the process by which nitrate (NO3-) is converted into nitrogen (N). This means that less nitrate ends up in ground and surface water, resulting in less eutrophication. (Eutrophication is a strong increase in nutrients in water that causes strong growth and multiplication of certain species, usually resulting in a sharp reduction in species richness or biodiversity.) Biological denitrification takes place in most ecosystems. Denitrification occurs in poorly drained soils of forests, grasslands and agricultural lands, in partially to completely water-saturated soils, in seepage areas and riparian zones, in sediments of rivers, lakes and estuaries, etc.
In addition, conversion of agricultural areas to nature reserves can ensure that the area is no longer fertilized. Since the nitrate dose on agricultural land often exceeds the needs of the plants, leaching into the groundwater will occur. If fertilization is stopped or a switch is made to precision fertilization, this leaching will no longer take place. This can also be taken as a benefit.
This ecosystem service contributes to better water quality.
Required information:
- Soil texture (https://dov.vlaanderen.be/dovweb/html/bottomloketten.html#bodemkaarten)
- N load in mg N/l of the incoming groundwater: to be determined via various methods (see quantitative valuation)
- Average highest groundwater level (GHG) is the average of the 25% highest groundwater levels in the area this year.
- Average lowest groundwater level (GLG) is the average of the 25% lowest groundwater levels in the area this year.
- If groundwater levels are not available, you can also try to derive the groundwater levels from the following geodesk: https://dov.vlaanderen.be/page/bepalen-average-grondwaterstand-op-een-plaats. OR use the drainage class that you can look up at https://www.dov.vlaanderen.be/page/grondkaart by clicking (2nd letter)
Qualitative valuation​
ECOPLAN calculated this service for Flanders in 2016. Based on the distribution in Flanders, a score table was drawn up ranging from 1 (no denitrification) to 10 (very important for denitrification).
Quantitative valuation​
In terrestrial ecosystems, absolute nitrate removal is mainly determined by the combination of a sufficiently shallow groundwater level and the supply of shallow nitrate-containing groundwater. To calculate the percentage removal rate, the potential maximum denitrification rate must be multiplied by the groundwater supply rate.
The zones where denitrification can take place potentially are determined by the soil hydrology (GHG/GLG or drainage class). The soil moisture content has a major influence on oxygen diffusion, which is a determining factor for the occurrence of a boundary between oxygen-rich environment (nitrification) and oxygen-poor environment (denitrification). In general, denitrification only occurs if the soil is more than 60% water saturated.
First, the potential maximum denitrification rate is derived based on the average groundwater levels (see table).
In a second step, the supply rate of the shallow groundwater is calculated. The supply rate determines the extent to which denitrification can occur over a given period of time and is dependent on the soil texture and topography which determines how quickly and how much groundwater flows to the area. The faster groundwater is supplied to the denitrification zone, the more denitrification can take place. A more permeable soil such as sand will have a greater supply rate and therefore a higher denitrification potential than a poorly permeable soil such as clay. If no data on the supply rate are available, assumptions can be made based on hydrology (GHG/GLG) and texture, with the maximum supply rate estimated based on the groundwater levels (see table and table) and this is then corrected for soil permeability (see table).
The third step is to determine the nitrogen load of the supplied water. If no local data on the nitrogen concentrations in the supplied shallow groundwater are available, the following options are proposed to estimate the nitrate load on the groundwater:
- A general assumption can be made of an average nitrate load based on the VMM annual Water report.
- The measurement data from the MAP measurement network (http://www.vmm.be/geoview/). The MAP measuring network provides an image of the nitrate concentrations in surface water, but these measuring points are exclusively located in agricultural areas and are therefore less representative of non-agricultural areas.
- An estimate can be made based on agricultural use and the soil texture within the drainage area to which the study area belongs. We do this by estimating the average leaching of nitrate into the shallow groundwater for the agricultural crops within the selected area. This depends on the amount of nitrogen from fertilization that remains on a plot after harvest (autumn residue) and on the relative leaching of the residue between autumn and spring, which depends on the soil texture. The nitrate residues (see table) are based on the average nitrogen residue values in the autumn per crop according to the Nitrate Residue Report 2014 and Coppens et al. 2007. It is noted that these values are only valid for plots without a management agreement. If a certain management applies, a correction factor is required for the reduced nitrogen input via fertilization.
To determine the zone within which the nitrogen residue values should be included, we first determine which watercourse segments are within the study area (VHA atlas version September 2018). We then determine which areas supply water to these watercourse segments based on VHA drainage areas and the upstream-downstream relationship determined on the basis of the watercourse network. We then make a cross-section of these runoff areas with a buffer of 2 km around the study area (the distance from which we can say that the nitrate reaches the study area). Within this cross-section we determine the land use and the nitrogen risk linked to that particular land use. The nitrogen content arriving in the study area is then a weighted average of this leached residue plus the leached nitrogen within the study area itself.
The latter method is applied in the Nature Value Explorer tool.
Avoided washout​
In agricultural land, the nitrate dose often exceeds the needs of the plants and there will be a leaching of nitrogen into groundwater or surface water. If agricultural land is converted into nature, leaching is avoided. This can be taken as a benefit. Based on table below, the avoided or additional leaching within the study area can be calculated with changing land use.
Intertidal areas​
Based on the OMES model, simulations were made for nitrogen removal for some example areas along the Sea Scheldt. The figures are very zone-specific (denitrification in freshwater is higher than in brackish water and saltwater), but on average 3 mmol N/d.m2 is removed over the entire Sea Scheldt. A reduced tidal area removes an average of 153 kg N/ha.year through denitrification. Depoldering leads to slightly higher denitrification gains according to model simulations.
The figures used in this manual distinguish between freshwater areas and brackish/salty areas. Because no data are available for the saline zones, we assume that the figures for the brackish zone from the model are representative of saline zones (Liekens et al. 2004).
Monetary valuation​
For the denitrification service the avoided reduction costs method is used. The key figures are based on the costs that various sectors (households, industry, agriculture) have to incur to achieve the eutrophication target in freshwater and marine waters in the context of the implementation of the European Water Framework Directive. It builds on calculations of the MKM water for Flanders (Broekx et al. 2008).
Measures were considered for industry, households and agriculture. The costs and effects of these measures were collected in various preparatory studies. With the MKM Water, rankings can be drawn up quantitatively between measures based on their cost-effectiveness (€/kg reduction). The cost of the last (most expensive) measures included in the approved 2009 program of measures is an approximation of how much society is willing to pay for a further reduction of nitrogen and provides an estimate of the value of this ecosystem service.
For nitrogen, this marginal reduction cost is €74/kg N. This is a high value compared to international literature. We therefore use this value as a high value. As a low value we use an average of the lowest values from the literature (5€/kgN). We would like to point out that the high value in Flanders is more likely than the low value.
Assumptions​
- For terrestrial ecosystems, we start from the leaching of nitrogen to groundwater to calculate the supply of N to the area.
- We assume that the surface water runoff area is a proxy for the flow direction of shallow groundwater
- The costs of the packages of measures provide an approximation of society's willingness to pay.
- If there is no watercourse segment in the study area, then there is no supply zone outside the study area, so then supply zone = study area
- Less reliable in polder & coastal area, tidal area, port area and near canals. The key figures from the Sigma Plan can be used for tidal areas.
Numbers to use​
Table: Qualitative valuation score
| lower limit kg/ha.year | score |
|---|---|
| 0 | 1 |
| 7,2 | 2 |
| 25,2 | 3 |
| 54 | 4 |
| 79,2 | 5 |
| 104,4 | 6 |
| 136,8 | 7 |
| 176,4 | 8 |
| 223,2 | 9 |
| 280,8 | 10 |
(ECOPLAN, 2016)
Table: potential maximum denitrification based on GHG-GLG %
| lower bound | GHG | 50 | 45 | 40 | 35 | 30 | 25 | 20 | 15 | 10 | 5 | 0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| GLG | ||||||||||||
| 50 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 | 60 | 60 | |
| 45 | 30 | 35 | 40 | 45 | 50 | 55 | 60 | 65 | 70 | 70 | ||
| 40 | 45 | 50 | 55 | 60 | 65 | 70 | 75 | 80 | 80 | |||
| 35 | 60 | 65 | 70 | 75 | 80 | 85 | 90 | 90 | ||||
| 30 | 75 | 80 | 85 | 90 | 95 | 95 | 95 | |||||
| 25 | 90 | 95 | 100 | 100 | 100 | 100 | ||||||
| 20 | 100 | 100 | 100 | 95 | 95 | |||||||
| 15 | 100 | 95 | 90 | 90 | ||||||||
| 10 | 85 | 80 | 80 | |||||||||
| 5 | 70 | 70 | ||||||||||
| 0 | 70 |
Table: maximum supply rate based on groundwater levels (heavy soils: A, L, E, U, G, V, W combinations, mm/day)
| GLG | < 0 | 0-10 | 10-20 | 20-30 | 30-40 | 40-50 | 50-60 | 60-70 | 70-80 | 80-90 | 90-100 | 100-110 | 110-120 | >120 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| GHG | |||||||||||||||
| 0-0 | 10 | 10 | 10 | 10 | 10 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | |
| 0-10 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | ||
| 10-20 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | |||
| 20-30 | 8 | 8 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | ||||
| 30-40 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||
| 40-50 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | ||||||
| 50-60 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||
| 60-70 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||
| 70-80 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 80-90 | 1 | 1 | 1 | 1 | 1 | ||||||||||
| 90-100 | 1 | 1 | 1 | 1 | |||||||||||
| 100-110 | 1 | 1 | 1 | ||||||||||||
| 110-120 | 1 | 1 | |||||||||||||
| > 120 | 1 |
Pinay et al. 2007
Table: maximum supply rate based on groundwater levels (light soils: Z, S, P, X, in mm/day)
| GLG | 0 | 0-10 | 10-20 | 20-30 | 30-40 | 40-50 | 50-60 | 60-70 | 70-80 | 80-90 | 90-100 | 100-110 | 110-120 | >120 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| GHG | |||||||||||||||
| 0 | 10 | 10 | 10 | 10 | 10 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | |
| 0-10 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | ||
| 10-20 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 2 | 2 | 2 | 2 | 2 | |||
| 20-30 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | ||||
| 30-40 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |||||
| 40-50 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||
| 50-60 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||
| 60-70 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||
| 70-80 | 1 | 1 | 1 | 1 | 1 | 1 | |||||||||
| 80-90 | 1 | 1 | 1 | 1 | 1 | ||||||||||
| 90-100 | 1 | 1 | 1 | 1 | |||||||||||
| 100-110 | 1 | 1 | 1 | ||||||||||||
| 110-120 | 1 | 1 | |||||||||||||
| > 120 | 1 |
Pinay et al. 2007
Table: correction of maximum supply rate (mm/day) as a function of the soil texture
| Soil texture | 1 | 2 | 8 | 10 |
|---|---|---|---|---|
| Sand (Z,X) | 1 | 2 | 8 | 10 |
| Loamy sand (S) | 1 | 2 | 7 | 9 |
| Light sandy loam (P) | 1 | 2 | 7 | 8 |
| Sand loam (L) | 1 | 1 | 5 | 6 |
| Loam (A,G) | 1 | 1 | 4 | 5 |
| Clay (E) | 1 | 1 | 5 | 6 |
| Heavy clay (U) | 0 | 1 | 3 | 4 |
Pinay et al. 2007
Table: nitrogen leaching based on crop and soil texture
| Main crop | Cultivation | Texture | Fertilization (kg N/ha) | N residue (kg N/ha) | % fertilization | % leaching | leaching (kg N/ha.y) |
|---|---|---|---|---|---|---|---|
| fodder | grassland | Z, S | 350 | 60 | 17% | 54% | 32 |
| fodder | grassland | P, L, A | 370 | 67 | 18% | 39% | 26 |
| fodder | grassland | E, U | 380 | 73 | 19% | 32% | 23 |
| fodder | fodder beets | Z, S | 305 | 49 | 16% | 61% | 30 |
| fodder | fodder beets | P, L, A | 330 | 55 | 17% | 43% | 24 |
| fodder | fodder beets | E, U | 330 | 60 | 18% | 35% | 21 |
| maize | maize | Z, S | 205 | 86 | 42% | 66% | 57 |
| maize | maize | P, L, A | 220 | 96 | 44% | 42% | 40 |
| maize | maize | E, U | 220 | 105 | 48% | 39% | 41 |
| grains, seeds and pulse | winter barley or cereal crops | Z, S | 200 | 69 | 35% | 61% | 42 |
| grains, seeds and pulse | winter barley or cereal crops | P, L, A | 215 | 77 | 36% | 43% | 33 |
| grains, seeds and pulse | winter barley or cereal crops | E, U | 215 | 84 | 39% | 35% | 30 |
| grains, seeds and pulse | winter wheat tricital | Z, S | 250 | 80 | 32% | 52% | 42 |
| grains, seeds and pulse | winter wheat tricital | P, L, A | 264 | 89 | 34% | 37% | 33 |
| grains, seeds and pulse | winter wheat tricital | E, U | 265 | 98 | 37% | 30% | 29 |
| vegetables, herbs and ornamental plants | crops with low nitrogen requirements | Z, S | 165 | 69 | 42% | 61% | 42 |
| vegetables, herbs and ornamental plants | crops with low nitrogen requirements | P, L, A | 175 | 76 | 44% | 43% | 33 |
| vegetables, herbs and ornamental plants | crops with low nitrogen requirements | E, U | 175 | 84 | 48% | 35% | 29 |
| vegetables, herbs and ornamental plants | vegetables group 2 | Z, S | 180 | 86 | 48% | 61% | 53 |
| vegetables, herbs and ornamental plants | vegetables group 2 | P, L, A | 180 | 96 | 53% | 43% | 41 |
| vegetables, herbs and ornamental plants | vegetables group 2 | E, U | 180 | 105 | 58% | 35% | 37 |
| Potatoes | potatoes | Z, S | 280 | 111 | 40% | 61% | 68 |
| Potatoes | potatoes | P, L, A | 280 | 124 | 44% | 43% | 53 |
| Potatoes | potatoes | E, U | 280 | 136 | 49% | 35% | 48 |
| Sugar beets | sugar beets | Z, S | 205 | 54 | 26% | 61% | 33 |
| Sugar beets | sugar beets | P, L, A | 220 | 60 | 27% | 43% | 26 |
| Sugar beets | sugar beets | E, U | 220 | 66 | 30% | 35% | 23 |
| vegetables, herbs and ornamental plants | vegetables group 3 | Z, S | 125 | 66 | 53% | 61% | 40 |
| vegetables, herbs and ornamental plants | vegetables group 3 | P, L, A | 125 | 74 | 59% | 43% | 32 |
| vegetables, herbs and ornamental plants | vegetables group 3 | E, U | 125 | 81 | 65% | 35% | 28 |
| vegetables, herbs and ornamental plants | vegetables group 1 | Z, S | 250 | 114 | 45% | 61% | 69 |
| vegetables, herbs and ornamental plants | vegetables group 1 | P, L, A | 250 | 126 | 50% | 43% | 54 |
| vegetables, herbs and ornamental plants | vegetables group 1 | E, U | 250 | 139 | 56% | 35% | 49 |
| other crops | other crops | Z, S | 200 | 90 | 45% | 61% | 55 |
| other crops | other crops | P, L, A | 215 | 100 | 46% | 43% | 43 |
| other crops | other crops | E, U | 215 | 110 | 51% | 35% | 38 |
| Grains, seeds and pulse | pulse other than peas and beans | Z, S | 120 | 39 | 32% | 61% | 24 |
| Grains, seeds and pulse | pulse other than peas and beans | P, L, A | 125 | 43 | 35% | 43% | 19 |
| Grains, seeds and pulse | pulse other than peas and beans | E, U | 125 | 48 | 38% | 35% | 17 |
| Grains, seeds and pulse | Peas and beans | Z, S | 125 | 66 | 53% | 61% | 40 |
| Grains, seeds and pulse | Peas and beans | P, L, A | 125 | 74 | 59% | 43% | 32 |
| Grains, seeds and pulse | Peas and beans | E, U | 125 | 81 | 65% | 35% | 28 |
vegetables group 1: Cauliflower, green celery, Brussels sprouts, white cabbage, kale, pointed cabbage, leek, broccoli, romanesco cabbage, white celery, red cabbage, Savoy cabbage, artichoke, Chinese cabbage, rhubarb or strawberries;
vegetables group 2: Spinach, courgettes, lettuce, early potatoes, celeriac, parsley, chives, basil, gherkins, pumpkins, fennel, kohlrabi, bok choy or other vegetables that are not group 1 vegetables, group 3 vegetables or not grown with low nitrogen requirements are;
vegetables group 3: Carrots, turnips, kohlrabi, beetroot, parsnips, black radish, radish, horseradish, salsify, parsley root, asparagus, peas, beans, dill, chervil, thyme, or other herbs, excluding parsley, chives and basil; vegetables with low nitrogen requirements: chicory, chicory, fruit (except strawberries), shallots, onions and flax
Flat plains and marches brackish/salt: 107 kg N/ha.year Flat plains and marches fresh: 176 kg N/ha.year
Translation to an indicator​
We translate the nitrogen removed from groundwater into the number of population equivalents (the average amount of wastewater produced by one person per day) for which a water treatment plant removes nitrogen every day. Per PE, a water treatment station removes approximately 9.7 g N every day. Per year this means a total of 3.5 kg per PE.
Example​
For the example, we refer to the Dutch version of the manual.