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Summary
The main conclusion of the Danish Aquatic Monitoring and Assessment Programme (NOVA-2003) in 2002 is that there have been marked reductions in nutrients discharged with wastewater and discharges from cultivated areas since 1989. These reductions have improved the natural and environmental conditions in lakes and marine waters. A beginning reduction in the nitrate content in the upper, newly formed groundwater in sandy areas has been observed. The environmental quality in streams is determined mainly by the hydromorphological conditions and the input of organic matter, and the environmental conditions in streams have improved slightly during the recent 4 years.
In spite of the improvements, only a minority of the water bodies complied with the quality objectives and the most comprehensive oxygen deficiency hitherto observed in inner Danish marine waters was measured in 2002.
Pollution sources: Organic matter, nitrogen and phosphorus
Compared to a climatically normal year, pollution from most sources was higher in 2002 due to high precipitation. The result of high precipitation is not only increased leaching of nitrogen and phosphorus from farmland, but also increased atmospheric deposition and higher discharges from urban areas.
Table 1 shows a total list of the Danish sources of pollution of water bodies, and the atmospheric input to the Danish marine territory. The main nitrogen sources are cultivation of land and atmospheric deposition. The greatest phosphorus load derives from cultivation of the land although the wastewater load as a whole was at the same level.
Contributions of organic matter from the various sources of pollution are not directly comparable with the background loss because the quality of the organic matter in wastewater is different from that of naturally occurring substances. The polluting effect is therefore relatively higher.
Wastewater treatment plants
Organic matter (BOD5) and the nutrients nitrogen (N) and phosphorus (P) are generally removed efficiently in the wastewater treatment plants. Since the mid-1980s, i.e. before the implementation of the Danish Action Plan on the Aquatic Environment and until 2002, there has been a reduction in the discharges of BOD5, N and P of 96%, 77% and 91%, respectively. Particularly discharges of BOD5 and phosphorus as a whole are now considerably below the requirements of the Action Plan on the Aquatic Environment. Most plants encompassed by the general requirements of the Action Plan on the Aquatic Environment consequently achieve BOD5 concentrations of 2-4 mg/l and phosphorus concentrations of 0.2-0.5 mg/l. According to the Action Plan on the Aquatic Environment, the general requirements to BOD5 and phosphorus are 15 and 1.5 mg/l, respectively, for plants with more than 5,000 persons connected to the plant.
Enterprises
Separate industrial dischargers have reduced their pollution to the same extent as the wastewater plants. Pollution from freshwater fish farms and marine fish farms was also reduced somewhat although the relative reduction is far less significant than that from treatment plants and industry.
Leaching from cultivated areas
Nutrient leaching from cultivated areas is influenced by cultivation practices, fertilizer consumption and the character of the areas. The amount of nitrogen in the applied commercial fertilizer was reduced from 395,000 tonnes in 1985 to 206,000 tonnes in 2002. This has contributed to a reduction in nitrogen runoff from cultivated areas during the period 1989-2002. The measured mean reductions in nitrate leaching from the root zone constituted 32% in clayey areas and 47% in sandy areas (to 14 and 18 mg N/l, respectively), but with large deviations of the results. Nationwide model calculations show a reduction in nitrogen leaching from 41% to 70 kg N/hectares per year.
Phosphorus in commercial fertilizer was reduced from 47,800 tonnes in 1985 to 13,800 tonnes in 2002, and livestock manure is generally the predominant type of phosphorus fertilizer in Denmark. The phosphorus input from livestock farms still exceeds the amount removed from the fields with the crops (figure 1).
There are marked interannual variations in phosphorus leaching from Danish agricultural land depending on the precipitation. No general efforts have been made to reduce phosphorus leaching and the monitoring results do not indicate a trend in the losses of phosphorus from cultivated land.
Atmospheric nitrogen deposition
Nitrogen deposition on land areas varies typically between 10 and 25 kg N/ha per year with the highest deposition in areas with large livestock herds and high precipitation. Deposition on the sea is somewhat lower (7-15 kg N/ha per year) because of the generally wider distance to the pollution sources, and lower precipitation. The most important pollution sources are nitrogen oxides deriving from combustion processes and ammonia volatilization from livestock manure. During the period 1989-2002 there was an estimated reduction of approx. 17% in the total atmospheric deposition of nitrogen compounds on Danish marine waters.
Water bodies
Groundwater
The total groundwater abstraction in 2002 constituted 653 million m3. Since 1989 there has been a marked decline in the amount of abstracted water, mainly due to a reduction in the consumption of the public common waterworks from 640 million m3 in 1989 to 410 million m3 in 2002.
Nitrate concentrations are highest in the upper groundwater formed within the recent decades. Under cultivated fields in the agricultural monitoring catchments, 29% of the water intakes contained more than 50 mg NO3-/l in 2002. There is an apparent reduction in nitrate concentrations in the upper groundwater. Figure 2 illustrates the trend in nitrate concentrations in the oxic upper groundwater in the groundwater monitoring areas. The figures reveal large deviations with an apparently slight increase throughout most of the 1990s and a slight decrease since the late 1990s.
Pesticide pollution in groundwater is primarily found in the subsurface groundwater, but the depth distribution of pesticide findings shows that pesticides are also found at depths exceeding 30 metres. Triazines and their metabolites are detected frequently in both groundwater and streams. Atrazine, which has been banned since 1984, contributes to some of these findings. A pool of the substance has probably been accumulated in the root zone and is slowly being released.
Lakes
The environmental state of the monitoring programme lakes as a whole has improved since 1989. This is reflected in, for instance, the average Secchi depth in the lakes. Improvements are seen in lakes, where measures against phosphorus inputs from wastewater have been introduced before or after 1989. Improvements are generally not observed in lakes that do not receive wastewater. In these lakes the predominant source of pollution is phosphorus lost from cultivated areas in the catchment and this loss has not declined. In order to comply with the quality objectives of those lakes where the majority of the catchment is cultivated, a reduction in phosphorus leaching from the lake catchments is required. This applies to most Danish lakes.
Streams
Danish streams are especially impacted by the physical changes in the natural course of the streams in form of weirs, straightening and stream maintenance. Many streams were previously also contaminated by organic matter from wastewater, but this pollution was largely relieved by wastewater treatment initiated in the 1970s.
The biological quality of streams has gradually and slowly improved over the recent decades. The same station network and assessment method have been used since 1999. Investigations reveal that the proportion of streams with unaffected or slightly affected macroinvertebrate fauna has increased from 35% to just over 44% during this period. Conditions are generally worst in small streams and streams on the islands east of the Great Belt.
The biological conditions in Danish streams are only slightly affected by the nutrient concentrations, but the streams transport the nutrients to lakes and marine waters, where the input of nutrients is a major pollution source.
Nitrogen and phosphorus concentrations in Danish streams have generally declined since 1989. Nitrogen concentrations have fallen with an average 2 mg N/l or approx. 30%, mainly as a consequence of reduced leaching from cultivated fields (figure 3). The concentrations started declining in the early 1990s. Phosphorus concentrations were reduced with just over 40% since 1989, but the reduction probably started earlier as a result of phosphorus removal from wastewater, which was initiated before 1989 (figure 4).
Transport from land to the sea
The level of pollution in Danish coastal waters is largely determined by nutrient inputs from the land. Figure 5 shows the trend in the annual input of water, nitrogen and phosphorus from Danish land areas to the Danish coastal waters. For each year the inputs of nitrogen and phosphorus are divided into diffuse sources (leaching from the soil), point sources to freshwater (wastewater) and direct wastewater discharges to coastal waters. In spite of the general discharge reductions, figure 5 shows that there has been considerable nutrient transport to marine waters during the recent 5 years. This is related to the levels of precipitation and runoff from Denmark being higher than normal in all 5 years.
Marine waters
The major, general source of pollution of Danish marine waters is the input of nitrogen and phosphorus from land and air. The shallow Danish marine waters are more vulnerable towards eutrophication than most other marine waters because the water exchange with the open sea is often limited, and because the stratification of the water column often restricts the supply of oxygen to the water close to the bottom.
In 2002, the inner Danish marine waters were hit by the worst case ever of oxygen depletion culminating in late September. Figure 6 shows that the most severely affected areas were the areas along the east coast of Jutland and south of Funen. Large areas in the southern Kattegat were also affected. The oxygen depletion resulted in dead benthic invertebrates and fish in the affected areas. The oxygen depletion was attributable to a combination of several factors: high precipitation and high nutrient inputs, water temperatures above the normal and a late summer with no strong winds.
In spite of the comprehensive oxygen depletion in 2002 there are some indications of improvements in the state of the marine waters. Nutrient concentrations in fjords and coastal waters are now on the decrease and the algal production is increasingly being limited by the lack of nitrogen and phosphorus. Since the 1980s there are also clear indications of improvement in the Secchi depth in fjords and coastal waters and of a decrease in the algal abundance and production. These improvements have, however, not yet resulted in improved conditions for macrophytes (including eelgrass) or benthic invertebrates. Similarly, there are no signs of improvements in the oxygen content in the water close to the bottom, neither in fjords and coastal waters nor in the open marine waters.
The concentration of heavy metals found in fish varies. In fish from the Sound the mercury concentrations are generally high. Exeedence of the consummation limit values was observed in one flounder filet.
Quality objective compliance
The various examples of compliance with the current quality objectives in water bodies described below do not fully reflect the level of anthropogenic impact because the specific requirements of the county councils to each water body may vary.
Wastewater treatment plants
Out of the 266 plants encompassed by the requirement of Danish Action Plan on the Aquatic Environment, only one plant did not comply with the general phosphorus requirement, and only two plants did not comply with the nitrogen requirement, while all plants complied with the BOD5 requirement. Out of all 1,060 treatment plants in Denmark, exceedences of one or more of the stipulated specific discharge requirements were only recorded in 71 plants in 2002.
Losses from cultivated fields
The quality objective of the action plans for the aquatic environment is an approx. 50% reduction in nitrogen leaching from the root zone in cultivated areas. On the basis of the monitoring results the reduction in leaching is calculated at 41% in relation to leaching in the 1980s. In addition, there is a minor reduction in nitrogen transport via streams because of the nitrogen removal that takes place in the re-established wetlands introduced as a part of the Action Plan on the Aquatic Environment II. The evaluation report on the Action Plan on the Aquatic Environment II concludes that the prognosis of the total effect of the action plans on the aquatic environment shows a total reduction of almost 150,000 tonnes N/year, corresponding to approx. 48% of the leaching of 310,000 tonnes N/year in the 1980s (Grant & Waagepetersen, 2003).
Groundwater
Groundwater is most importantly used for water consumption. Only approx. 1% of the water supply abstraction wells did not comply with the limit values for nitrate in drinking water (50 mg NO3-/l). This result was, however, achieved by closing down wells with high nitrate concentrations and does therefore not reflect the general groundwater quality. In the groundwater monitoring areas more than 50 mg NO3-/l were detected in 16% of the wells.
37% of the groundwater used for drinking water in 2002 contained pesticides, of which 4% had pesticide concentrations exceeding the limit values for drinking water.
Lakes
The counties have estimated that only 4 of the 31 examined lakes complied with the quality objectives in 2002, which is a decrease of 3 lakes since 2000. Some of the lakes will experience improved conditions when the internal phosphorus release deriving from the wastewater discharges of the past has ceased. The majority of lakes will, however, only be able to comply with the quality objectives if phosphorus inputs from agricultural catchments and scattered dwellings are also reduced.
Streams
The environmental condition of streams is best in Jutland, on Funen and Bornholm, where approx. 55% of the quality objectives of streams are met, in contrast to only a third of the streams on the islands east of the Great Belt. Quality objective compliance on a national level was 50% in total. In order for the rest of the streams to comply with the current objectives, it is necessary to change the stream morphology to resemble the natural conditions with varied types of stream bed. Furthermore, many small streams are still polluted by insufficiently treated wastewater, particularly from scattered dwellings. However, clear-water fauna is prevented in many streams by a natural lack of slopes and by summer drought.
Marine waters
The assessment of the councils and NERI is that only a few of the Danish marine waters comply with the quality objectives. Only one of the investigated shallow coastal areas (Dybsø Fjord) is assessed as complying with the quality objectives. Beyond this the open marine waters of the North Sea and Skagerrak are also assessed as having complied with the quality objectives. Reasons for non-compliance with the quality objectives is serious impact on plant and animal community due to increased nutrient concentrations, and in some places also high concentrations of tributyltin (TBT).
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