Nitrate exposure from drinking water in Denmark over the last 35 years

The Danish water supply is solely based on simple treated groundwater, and protection of groundwater has therefore a high priority. At the same time, Danish farming is among the most intensive in the world. Numerous waterworks and wells have been closed due to nitrate pollution (Environmental Agency 1998), and approximately 15% of the area of Denmark has been classified as nitrate vulnerable groundwater abstraction areas (Hansen and Thorling 2008). Since the 1980s, regulations implemented by Danish farmers have succeeded in optimizing the N (nitrogen) management at farm level, and the overall national upward trend of the nitrate concentrations in oxic groundwater was reversed around 1980. On a national level, the N-surplus has been reduced by approximately 40% from 1980 to 2007 and the nitrate concentrations in oxic groundwater have reduced by also approximately 40% from 1980 to 2000 (Hansen et al 2011). Locally, nitrate trend analyses in monitoring wells have shown a more complex pattern with both upward and downward nitrate trends depending on the age of the groundwater and local agro-hydro-geochemical conditions (Hansen et al 2012). Therefore site-specific groundwater monitoring and protection plans are being carried out in order to further protect drinking water resources from nitrate pollution.

The water supply structure is highly decentralized: approximately 2,600 public waterworks supply five million inhabitants, while the remaining approximately 0.4 million inhabitants use water from approximately 70 000 private wells (Nature Agency 2012). Public water supply is the main source of drinking water, bottled water consumption being amongst the lowest in the EU, with only one fifth of the EU average (UNESDA 2014).

Danish drinking water quality is regulated and monitored routinely. Historically, the yearly frequency of the nitrate analyses at each waterworks has been variable. Currently, the nitrate content of drinking water is monitored routinely for waterworks with abstraction higher than 3,000 m³ per year. The frequency of the nitrate analyses varies from 1 every second year to 37 analyses per year depending on the abstraction volume (MoE 2014). For the smaller waterworks the frequency of the nitrate analyses depends on the policies of the local municipality. While some municipalities request analyses every fifth year, others leave it up to the owners to monitor their water quality. All nitrate analyses of the drinking water are performed by certified Danish laboratories and registered in the publicly available geo-database JUPITER.

The registered waterworks were divided into public supplies, supplying 10 or more households and private wells, supplying less than 10 households (WHO 2011). All registered nitrate concentrations were extracted from JUPITER in October 2013. Annual mean concentrations were calculated for each well field. Measurements below the detection limit were set to half its value (European Commission 2009). The detection limit varied from 0.001 mg L⁻¹ to 1 mg L⁻¹. For 10% of the public supplies and 61% of the private wells no coordinates were registered in JUPITER. For these, addresses were geocoded using ESRIʼs ArcGIS World Geocode Service, resulting in a total of 95% and 93% of public supplies and private wells, respectively, with both coordinates and nitrate measurements.

Four methods with increasing work effort were applied and compared to estimate the exposure to nitrate from drinking water over time. For all methods it was assumed that the ratio of publicly and privately supplied residents remained constant over time (5 : 0.4, see 1.1):

• Step 1: By analyses (public supplies & private wells). In a first step, annual mean concentrations for each waterworks with registered nitrate measurements for the given year were analyzed. This yielded the largest available dataset of the applied methods. Each waterworks was assumed to supply the same share of the population. It is assumed that the data for each year are representative of the whole population for publicly and privately supplied residents, respectively.
• Step 2: By abstraction volume (public supplies). In the above method, all waterworks are weighted equally. As a second step, concentrations were weighted by the annual abstraction volume of the respective waterworks. However, as registrations of abstraction volumes in JUPITER are incomplete, this leads to a smaller dataset. Only 3% of the private wells have abstraction volumes registered. Therefore, only the first step was applied on the private wells.
• Step 3: By Thiessen polygons (public supplies). Thiessen polygons were created for each year, based on the public supplies. The whole country was divided into water supply areas with assigned nitrate concentrations, assuming every point in the country is supplied by its nearest public supply.
• Step 4: By water supply areas (public supplies). To achieve highest possible spatial precision of exposure, Denmark was divided into public water supply areas as shown in figure 1. Information on the areas that are supplied by each waterworks is not registered centrally in JUPITER, but locally in the 98 municipalities. Most municipalities have an administrative tool called water supply plan, which shows water supply areas with varying degree of precision. While some municipalities divide their whole area into public water supply areas, others show the actual supplied areas and leave unpopulated or non-supplied areas blank. It is important to notice that even though a resident lives in a water supply area, they might not be connected to the public supply, but get their drinking water from a privately owned well. Where a water supply plan was missing, municipalities or waterworks were contacted directly. Table 1 summarizes the sources. In total 2,852 water supply areas were assigned the water quality measurements of the public supplies by which they are supplied. 74% of the water supply areas are supplied by only one waterworks. 22% have more than one waterworks supplying the area and 4% are supplied by large waterworks that distribute their water into several areas (typically the large waterworks of the main cities).

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For steps 3 and 4, a population density map was created using population counts in the 2,323 Danish parishes in 2008. Parishes are the smallest census units, leading to the highest spatial resolution. The population density was calculated and the polygon data converted into a raster map with a resolution of 500 m by 500 m. The total number of inhabitants in each Thiessen polygon and water supply area was calculated in GIS.

The change in fraction of population exposed to low nitrate concentrations ($\lt$ 10 mg L⁻¹) and elevated nitrate concentrations ($\gt$ 25 mg L⁻¹) was analyzed using a standard linear regression model. 95%-confidence intervals of the trends were calculated and the null hypothesis (no trend over time) was tested on statistical significance ($\alpha =0.05$). Trend analyses were computed for both public supplies and private wells for the period 1978–2012. For the public supplies, the results from step 4 were used; for the private wells, the results from step 1 were used. This was done for the whole of Denmark, as well as for the 10 so-called geo-regions (Kronvang et al 2008, Hansen et al 2012), see figure 1.

The following analysis is based on the results from step 4, where the exposed population to different nitrate concentrations is calculated by using the water supply areas and a population density map. Figure 4(a) shows the percentage of consumers using public water supplies exposed to different nitrate concentration classes, normalized to the total available data per year. The solid line depicts the percentage of the number of consumers that were assigned a nitrate exposure from public supplies that year in relation to the total population in the density map (5.4 million). The missing values to 100% correspond to the population in the 'empty' areas in figure 3. During the 1970s, less than 25% of the total population could be assigned a nitrate exposure from drinking water. The coverage increases in the 1980s to approximately 70%; and from 1991, more than 90% of the total population could be classified in a nitrate exposure class.

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From figures 3 and 4(a), a general downward tendency in concentrations can be observed. Concentrations above the drinking water standard of 50 mg L⁻¹ are getting less frequent over time. While the standard was exceeded for up to 7% of the publicly supplied consumers in the 1970s, this is the case for only 3 in 10 000 in the latest complete dataset (2012). The percentage exposed to low nitrate concentrations ($\lt$10 mg L⁻¹) is increasing over time, from approximately 70% in 1980 to 94% from 2006.

Table 2 shows the percentage of consumers supplied by public supplies exposed to low ($\lt$ 10 mg L⁻¹) and elevated $(\gt 25\;{\rm mg}\;{{{\rm L}}^{-1}})$ nitrate concentrations in 2012, as well as the change since 1978. For the whole of Denmark, 2.5% of the population connected to public supplies was exposed to elevated nitrate concentrations in 2012. However, there is a large spread between the geo-regions. Noticeable is especially the northern part of Jylland: in Thy (III), Nordjylland (IV) and Himmerland (V) between 11% and 16% were exposed to elevated concentrations. Contrarily, in the eastern part of Denmark (geo-regions VII-X) less than 0.5% of the population was exposed to elevated nitrate concentrations in public drinking water in 2012.

For the whole of Denmark, a statistically significant increase in the percentage of publicly supplied consumers exposed to low concentrations of 0.85 (0.75; 0.94) percent-points per year can be observed. For all geo-regions, except Bornholm (X), such a statistically significant increase is found. For the population exposed to elevated concentrations ($\gt$ 25 mg L⁻¹), the statistically significant national downward trend of 0.29 (0.23; 0.36) percent-points per year is reflected in all of the geo-regions, except Nordjylland (IV). Here, the population exposed to elevated concentrations has been statistically significant increasing and was in 2012 on the highest level of all geo-regions (16%).

Table 4 shows the estimated percentage of publicly supplied consumers exposed to elevated nitrate concentrations ($\gt$ 25 mg L⁻¹) for each method (step 1 to 4). The two first steps estimated this group with 4.8% and 3.7% slightly higher than the spatially referenced steps 3 and 4 with 3.1% and 2.5% in 2012. Weighing all analyses equally as in the first method, overestimates the population exposed to elevated nitrate concentrations, as the very large water supply companies in the main cities supply large water volumes with low nitrate concentrations to a large number of urban residents. An important exception is the city of Aalborg (pop. 110 000), located in geo-region Himmerland (V). Here, in the 1970s and 1980s annual averages above the drinking water standard of 50 mg L⁻¹ were common and reached up to 175 mg L⁻¹ for some water supply areas. In 2012, most of the water supply areas in the city of Aalborg are classified in the 25–50 mg L⁻¹ class.

Figure 4 (b) shows the percentage of private wells in different nitrate concentration classes, normed to the total number of registered private wells per year. The solid line depicts the percentage of private wells with registered nitrate concentrations per year in relation to the total 70 000. As only the method in step 1 was carried out on the private well data, it is assumed that each private well is supplying the same number of residents. This implies that the percentage of private wells in a nitrate concentration class is equal to the percentage of exposed consumers in this class. As shown above, the data on private wells is much sparser compared to public supplies. However, looking at the distribution of population in the exposure classes, no obvious change can be observed during the increase from 2% in 2006 to 7% in 2008. Therefore, the private wells registered before 2007, even though accounting for only approximately 1% of the total number of private wells, are assumed to be representative of the private wells registered after 2007. However, it has to be kept in mind, that only a small fraction of the estimated total 70 000 private wells is registered. Whether there is registration bias (i.e. owners of poorly installed wells avoid registration), cannot be answered here.

A clear difference to the development of public water supplies can be seen in figure 4(b): the percentage of private well users exposed to nitrate in drinking water above the standard of 50 mg L⁻¹ is substantial, between 14% and 30% over the last 30 years. As shown in table 3, the nationwide trend is opposite to the one of the public supplies: the group exposed to low concentrations is statistically significant decreasing by 0.22 (0.10; 0.35) percent-points per year, while the group exposed to elevated concentrations is statistically significant increasing by 0.13 (0.00; 0.25) percent-points per year.

In 2012, 37% of all private well users were exposed to elevated nitrate concentrations ($\gt$ 25 mg L⁻¹) in drinking water. Nordsjælland (VIII) was the only geo-region with less than 25%. As in public supplies, the geo-regions on Jylland have the highest levels.

At the geo-region-level, the trends show no overall pattern and are mostly not statistically significant. Significant trends can be observed in the group exposed to low concentrations, which decreased in Vestjylland (I), Østdanmark (VII) and Sydsjælland (IX), while there was an increase in Djursland (VI). The group exposed to elevated concentrations decreased in Djursland (VI) and increased in Sydsjælland (IX).