Summary

The main conclusion of the Danish Aquatic Monitoring and Assessment Programme (NOVA-2003) for the year 2003 is that wastewater discharges of organic matter, nitrogen and phosphorus and nitrogen loss from cultivated land have decreased considerably since 1989 following implementation of the 1987 Action Plan on the Aquatic Environment.

 These reductions have led to moderate improvements in ecological and environmental conditions in the lakes and marine waters.

 As regards groundwater, a minor decrease in nitrate concentration has been detected in the youngest groundwater.

 In watercourses, where environmental quality is mainly determined by the physical conditions and input of organic matter, slight improvements in state have been observed over the past five years.

 Due to the low winter precipitation in 2003, nutrient leaching from the soil was particularly low in 2003. Discharges from point sources were also generally lower than in the preceding years.

 Despite the improvements, less than half of all water bodies complied with the current quality objectives in 2003.

Sources of nutrient and organic matter pollution

With most sources, the nutrient and organic matter load in 2003 was low compared to a climatically normal year because the precipitation was low. Due to the low precipitation in the winter period, leaching of nitrogen and phosphorus from cultivated land decreased in 2003 because most leaching occurs during periods of high runoff in winter. A further consequence of the low precipitation was that the discharge of water from wastewater treatment plants and urban stormwater outfalls was less than normal.

From the figures for nutrient and organic matter inputs to the aquatic environment (Table 1) it is apparent that the dominant sources of nitrogen loading are leaching from cultivated land and deposition from the atmosphere. The atmospheric nitrogen load derives from combustion processes and from ammonia volatilization from manure in Denmark and abroad. The atmospheric nutrient load is distributed over all the Danish marine waters and is thus of minor significance for the state of the fjords and coastal waters.

In 2003, the phosphorus load to the aquatic environment mainly derived from wastewater, even though good treatment has reduced these discharges to the lowest level yet.

The organic matter load from the various sources of pollution is not directly comparable with the natural background load as the organic matter in wastewater differs in character from that of naturally occurring organic matter. Thus its polluting effect is relatively greater.

 

Wastewater treatment plants

Removal of organic matter (BOD5) and the nutrients nitrogen (N) and phosphorus (P) at the wastewater treatment plants is generally very effective. In 2003, 90% of all wastewater underwent organic matter, nitrogen and phosphorus treatment. Since the mid 1980s, discharges of BOD5, N and P have been reduced by 96%, 81% and 93%, respectively. The wastewater treatment efficiencies for BOD5 and phosphorus are generally much better than the outlet criteria in the Action Plan on the Aquatic Environment I (APAE I). The majority of the wastewater treatment plants encompassed by the Action Plan’s general treatment requirements thus clean the wastewater down to 2–4 mg BOD5/l and 0.2–0.5 mg P/l. The Action Plan’s general treatment requirements for wastewater treatment plants with a capacity exceeding 5,000 PE are a BOD5 concentration of 15 mg/l and a phosphorus concentration of 1.5 mg P/l. The nitrogen concentration is also generally lower than the general discharge criterion of 8 mg N/l. In 2003, all the wastewater treatment plants encompassed by the requirements of Action Plan on the Aquatic Environment I met the discharge criteria for BOD5 and phosphorus, while five of the 199 plants subject to the discharge criterion for nitrogen failed to meet it.

Enterprises

Industrial enterprises with their own wastewater outfall have generally reduced their proportion of the total discharges to the same extent as the wastewater treatment plants. Nutrient and organic matter loading from freshwater fish farms and marine fish farms have also decreased slightly, although the reduction is relatively smaller than for the wastewater treatment plants and industry.

 

Leaching from cultivated land

Leaching of nutrients from cultivated land is determined by the agricultural practice, the amount of fertilizer applied and the nature of the land. The amount of nitrogen applied in the form of commercial fertilizer has decreased from 395,000 tonnes in 1985 to 196,000 tonnes in 2003, while the amount of nitrogen applied in the form of manure and sewage sludge has remained largely unchanged. This has led to a reduction in nitrogen leaching from cultivated land over the period 1989–2003. The measured mean reduction in the nitrate concentration in root zone water is 38% in clayey soils and 50% in sandy soils, although the results are subject to considerable variation.

The amount of phosphorus applied in the form of commercial fertilizer has decreased from approx. 40,000 tonnes in 1990 to approx. 13,000 tonnes in 2003, and manure is now the dominant form of phosphorus fertilizer in Denmark. On livestock holdings, considerably more phosphorus is still applied than is removed in the crops. The amount of phosphorus leaching from cultivated land varies considerably from area to area and from year to year depending on the precipitation. No trend has been detected in the losses of total phosphorus from cultivated land.

 

Atmospheric deposition of nitrogen

Nitrogen deposition on the land typically varies from 12 to 24 kg N/ha/yr and is greatest in areas with large livestock herds and high precipitation. Deposition on marine waters is lower, i.e. 7–17 kg/N/yr, among other reasons because of the greater distance to the sources of pollution and the lower precipitation. The main sources of the nitrogen are nitrogen oxide formation in combustion processes and ammonia volatilization from manure. The majority of the nitrogen deposited on the marine waters derives from foreign sources, while Danish sources account for a greater proportion of that deposited on the landmass. The Danish proportion is greatest in Jutland (38%), where it mainly derives from ammonia volatilization from agriculture. It is estimated that total atmospheric deposition of nitrogen on the Danish marine waters and landmass has decreased by approx. 21% over the period 1989–2003.

 

Groundwater

In 2003, total groundwater abstraction amounted to 634 million m3, corresponding to a 39% reduction in abstraction since 1989. Of the total, 64% was abstracted for the public water supply. The nitrate concentration is highest in the uppermost groundwater formed within the past few decades. The data for 2003 show that the nitrate concentration in the youngest groundwater has been decreasing since 1989. In 2003, the mean nitrate concentration in the water percolating down towards the aquifers from cultivated land was close to 50 mg nitrate/l, which is the limit value for nitrate in drinking water. In oxic aquifers the nitrate concentration can still be at this level, whereas the phosphorus concentration in such aquifers is low. In anoxic and usually deeper aquifers, in contrast, the nitrate has been converted to atmospheric nitrogen, and the nitrate concentration is therefore very low. Moreover, the phosphorus concentration is high because part of the naturally occurring phosphorus in the soil dissolves under anoxic conditions.

The frequency with which pesticides are detected in connection with groundwater monitoring has remained at the same level in recent years, while the proportion of samples exceeding the limit value for drinking water has been increasing slightly. As regards waterworks wells, in contrast, the proportion of samples exceeding the limit value for drinking water has decreased, probably due to the cessation of abstraction from wells with high pesticide concentrations.

 

Quality objective compliance

At the groundwater monitoring sites the nitrate concentration exceeds the limit value for drinking water in 16% of the filters. The corresponding figure for the waterworks wells was only 1% because wells with a nitrate concentration exceeding 50 mg/l are not used for abstracting water for the drinking water supply.

In 2003, the pesticide concentration exceeded the limit value for drinking water in approximately 10% of the filters analysed at the groundwater monitoring sites. The proportion of samples exceeding the limit value has been increasing slightly in recent years. As regards waterworks wells, in contrast, the proportion has decreased from approximately 10% in 1998 to -approximately 5% in 2003.

 

Lakes

In general, the environmental state of the monitoring lakes that receive wastewater was better in 2003 than in 1989. Among other things, this is reflected in a moderate increase in the mean Secchi depth in the lakes and a corresponding reduction in algal biomass in the water. Improvement has taken place in lakes where phosphorus input from wastewater has been reduced. In the other lakes, no improvement has generally been seen. In these lakes the main source of phosphorus input is normally leaching from cultivated land in the catchment, which has not been reduced. With many lakes, wastewater discharges from rural properties outside the sewerage catchment (sparsely built-up areas) also comprise a major source of pollution.

The occurrence of pesticides and other hazardous substances in the eight lakes investigated is generally minor.

 

Quality objective compliance

Of the 31 lakes investigated, seven to eight met their quality objectives in 2003. The state of some of the lakes will improve when internal release of phosphorus from previous wastewater inputs ceases. With the majority of the lakes, however, it will only be possible to meet the politically determined quality objectives if phosphorus inputs from cultivated land and sparsely built-up areas are also reduced.

 

Watercourses

The environmental state of Danish watercourses is particularly affected by the physical changes in their natural courses that have occurred as a result of damming and channelization, and which still occur as a result of watercourse maintenance. In earlier times, many watercourses were also polluted with organic matter from wastewater, but this pollution has been reduced to a much lower level through wastewater treatment since the 1970s.

The biological quality of the watercourses has improved over the last decades. The present station network and assessment method have remained unchanged since 1999. The measurements show that the proportion of the watercourses in which the macroinvertebrate fauna is unaffected or only slightly affected has increased from just under 35% in 1999 to just over 44% in 2003. Biological quality is generally lowest in the small watercourses and in watercourses east of the Great Belt, and best on Funen and in Jutland.

The biological conditions in Danish watercourses are only slightly dependent on the nutrient concentration in the water, but the watercourses transport the nutrients to lakes and marine waters, where nutrients are the main pollutant. The concentrations of nitrogen and phosphorus in Danish watercourses have generally decreased since 1989. Thus the watercourse nitrogen concentration in 2003 was approx. 2 mg N/l or approx. 30% less than in 1989, mainly due to reduced leaching from cultivated land. The decrease began early in the 1990s. The phosphorus concentration has decreased by just over 40% since 1989, but the reduction probably started earlier as a result of the introduction of phosphorus removal from wastewater prior to 1989.

A number of pesticides and their degradation products have been detected in watercourses. The substances most frequently detected are the active ingredient of Roundup, glyphosate, and its degradation product AMPA. Another frequently detected substance is BAM, a degradation product of the active ingredient of pesticides such as Casoron, which is presently prohibited. Other hazardous substances occur so infrequently in the investigated watercourses that no general picture can be formed.

 

Quality objective compliance

Of the watercourses investigated, 51% met their quality objective in 2003. In order for the other watercourses to be able to meet their politically determined quality objectives, physical conditions will have to be changed to make them more resemble natural conditions with varying types of streambed. In addition, many small watercourses are still polluted by inadequately treated wastewater, especially from rural properties outside the sewerage catchment. A naturally small slope and dry-out in the summer often limit the possibilities for a clean water fauna, however, especially in eastern Denmark.

 

Nutrient inputs to the sea

Pollution pressure on the Danish coastal waters is largely attributable to nutrient loading from land-based sources. Phosphorus loading has decreased considerably due to effective treatment of wastewater (Figure 1). The total nitrogen and phosphorus inputs via diffuse loading are highly correlated to freshwater runoff via the watercourses. As diffuse loading is the dominant source of nitrogen, the total nitrogen input varies markedly with precipitation and freshwater runoff in the individual year. A statistically reliable reduction in nitrogen input to the sea can only be demonstrated by correcting the measured nitrogen inputs for interannual variation in freshwater runoff. After correction for variation in freshwater runoff, the reduction in total nitrogen input to the sea over the period 1989–2003 is 43%. The corresponding reduction in total phosphorus input is 81%.

In the absence of correction for annual runoff, no statistically significant reduction in nitrogen input to Danish marine waters can be discerned. This is due to the fact that freshwater runoff and hence nitrogen leaching were higher than normal in each of the years 1998–2002. A statistically significant reduction in phosphorus input can be discerned, however, also without correction for runoff.

 

Marine waters

The main pollution pressure on Danish marine waters results from inputs of nitrogen and phosphorus to the sea from land-based sources and via the atmosphere. The shallow Danish marine waters are more vulnerable to eutrophication than the majority of other marine waters elsewhere in the world because water exchange with the open sea is often limited, and because stratification of the water masses often limits the input of oxygen to the bottom water. The most strongly polluted of the marine waters are those with a high freshwater input and little water exchange with adjoining marine waters.

There are initial signs of improvement in the state of the marine waters. Nutrient concentrations in the fjords and coastal waters have begun to decrease, and algal production is increasingly being limited by a lack of nitrogen and phosphorus. Secchi depth is also tending to increase in the fjords and coastal waters, and algal biomass and production have decreased since the 1980s. These improvements have not yet led to increases in the distribution of submerged aquatic vegetation (including eelgrass) or benthic invertebrates. Neither are there any signs of general improvements in oxygen content in the bottom water in the fjords and coastal waters or in the open marine waters.

A number of hazardous substances were detected in the sediment of the fjords and inner marine waters investigated in 2003. These include tributyl tin (TBT), which has been used as an antifouling agent in hull paints. In the marine environment, TBT affects the reproductive characteristics of gastropod molluscs and in the worst case causes sterility. In 2003, the effects of TBT on gastropod molluscs were found to be widespread in the marine waters, even in open seas such as the North Sea and the Skagerrak.

 

Quality objective compliance

The politically determined objective that marine flora and fauna may only be slightly affected by pollutant inputs is generally considered to be fulfilled in the open parts of the North Sea and the Skagerrak. In the northern part of the Kattegat, the objective is considered close to being met. In the other marine waters the objective has not yet been met, primarily because high nutrient inputs have enhanced algal biomass. In some fjords and coastal waters the lack of quality objective compliance is also attributable to the presence of hazardous substances. Fulfilment of the objectives requires further reductions in nutrient inputs and in some cases, also of inputs of hazardous substances and heavy -metals.

 

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