Eutrophication caused by excessive input of nutrients both from point and diffuse sources is one of the most severe problems in the Baltic Sea. In the open sea areas nitrogen is limiting nutrient for primary production due to low nitrogen/ phosphorus ratio. It has been assumed that in coastal areas where N and P ratios are higher phosphorus is limiting nutrient to the primary production. Due to remarkable improvements in phosphorus removal from municipal and industrial point sources during the last decades the HELCOM focused very much on decreasing by the load of nitrogen from land based sources having a goal of 50% reduction. Unfortunately results are not very good and several countries have not reached the reduction target. According to § 1 of Article 6 of the Helsinki Convention, the contracting parties undertake to prevent and reduce pollution of the marine environment of the Baltic Sea Area from land based sources by using inter alia, Best Environmental Practice for diffuse sources and Best Available Technology for point sources.
The fall of the Iron Curtain resulted in dramatic changes in Eastern Europe, including substantial reductions in the use of fertilisers and livestock production, as well as a marked decrease in water consumption by both the general population and industries. Agriculture in the Baltic Sea Region contributes a significant portion of the nitrogen and phosphorus load to the environment. In 2000 the losses from diffuse sources into inland surface waters within the entire Baltic Sea catchment area amounted to approximately 484 000 tonnes of total nitrogen and 22 040 tonnes of total phosphorus, respectively (HELCOM, 2004). The major part of the nitrogen (79%) and phosphorus (78%) losses from diffuse sources originated from agricultural activities or managed forestry. After comprehensive economic, technical and social changes in Estonia in the 1990s the nutrient load from agricultural landscapes decreased considerably but agriculture remains as the main source of diffuse load to the inland surface waters in Estonia, and comprised 72% of total nitrogen and 71% of total phosphorus losses in 2000 (HELCOM, 2004). The calculated nitrogen load from agriculture comprises about 50% of the overall diffuse load at the catchment area of Lake Peipsi.
Inorganic fertilisers were widely used in Estonian agriculture during the Soviet period (i.e., before 1991), and application reached a peak of 270 thousand tons (300 kg ha-1) annually in 1987-1988. However, comprehensive economic, technical and social changes took place in Estonia after the country regained its independence. For example, the use of fertilisers has decreased considerably over the last 15 years, and the levels observed in 2001 constituted only about 11% (29,700 tons) of the peak in 1987-1988 that correspond to applications of less than 100 kg ha-1 for mineral fertilisers (63 kg N ha-1 and 13 kg P2O5 ha-1) and less than 30 tons ha-1 for organic fertilisers. Furthermore, the number of livestock units decreased from 800,000 (0.82 LU ha-1 of arable land) in 1988 to less than 390,000 (0.34 LU ha-1 of arable land) in 1994, and the level today is approximately the same as in 1994. Slaughtering of livestock reduced correspondingly the amount of manure.
Point source emissions to surface waters in Estonia have decreased even more sharply as a result of the economic recession in the early 1990s, in combination with modernisation of industrial production, as well as construction of new and improved existing wastewater treatment plants in major towns.
Studies carried out in some smaller agricultural watersheds in Estonia have shown relatively low levels of N and P, as compared to the concentrations found in, for example, the Nordic countries. The low levels of N and P detected in Estonia might be explained by the following:
(i) decreased rates of fertiliser application;
(ii) lower livestock density;
(iii) differences in land use (more natural and cultural grasslands);
(iv) hydrological conditions that entail longer water residence time and higher buffering capacity, and substantial retention of these substances within catchments.
The majority of the field drainage ditches used today were established in the 1970s and 1980s, and maintenance of the main ditches in these land improvement systems has been insufficient in recent years. Due to this situation, many watercourses are now overgrown with bushes and macrophytes, which has enhanced the retention potential, denitrification and biological uptake.
It is widely accepted that nutrient concentrations in streams and rivers may respond differently to changes in physical-geographical conditions, agricultural production, rates of fertiliser application, and intensity of land use. Plot experiments carried out in small watersheds in Estonia during the late 1990s have shown that substantial amounts of nitrogen can be lost via the root zone after the harvesting of crops, resulting in nitrate concentrations as high as 60 mg N l-1 in soil-water. At the same time, the nitrate concentrations in groundwater and small agricultural streams are low, only about 2-4 mg N l-1. Several authors pointed out that denitrification, presumably in smaller streams and channels, plays an important role in nitrogen reduction in the Baltic countries. The pH in stream water is usually high (> 8) during summer, which promotes the volatilisation of ammonia. In addition, the water residence times in Estonian river catchments are much longer than in many other areas with similar geographical condition.
Considering the area of arable land (crop fields and cultural grasslands) in Estonia, about 34% was unused in 2001, and the share of wintergreen area was quite remarkable, being as high as 80% in some small agricultural catchments. Moreover, it is possible that the particular hydrogeological conditions influence the rate at which nutrient concentrations in streams and rivers respond to changes in land use and nutrient emissions. The Estonian bedrock consists largely of Silurian and Ordovician limestone. Hence, there can be rapid exchange of water between upper and lower water tables through existing cracks. Many rivers, especially in the Pandivere Upland, are fed by groundwater that is rich in nitrate-nitrogen. After the fall of the Soviet Union, the nitrate content in groundwater aquifers decreased substantially (Tamm, 2003). Several field studies have shown that at locations where oxygen present at very low concentrations and where for example organic carbon is available significant denitrification has been found. Due to insufficiently maintained amelioration systems, the groundwater level rose in Estonia in 1990s and the soil is maintained at saturated or almost saturated conditions for longer time periods leading to anaerobic environments in soils, possible increase in denitrification rates and to lower TN concentrations in open streams.
Despite the improved performance of WWTPs the overall phosphorus load to the rivers is still fairly high. The reason for this is that the number of households and industries connected to the sewerage systems in larger municipalities has grown, which has increased the amount of nutrient-laden wastewater delivered to the treatment plants. Furthermore, rises in the price of drinking water and wastewater treatment have led to a substantial drop in the overall consumption of potable water by the general population, which has resulted in increased concentrations of nutrients in both the wastewaters delivered to WWTPs and the effluents released from those facilities. A typical example of this can be seen in the city of Tartu, which represents the largest single source of pollution in the study area. The efficiency of phosphorus removal is about 80% at the sewage treatment plant in Tartu. However, due to high levels of this element in inlet waters (on average 12.5 mg P l-1 in 2002), the concentrations in the effluent have exceeded the maximum permissible level of 1 mg P l-1. Highly concentrated sewage waters needed totally new approach on how to reduce the pollution load and to fulfill existing standards. Conventional treatment plants are not able to achieve set up targets or need totally new treatment technology.