In Finland’s surface waters, the average level of nitrateis generally low, <10 mg l-1
NO3 (Fig. 1). This is mainly due to the northern location of the country, which as a result of cold climatic conditions, restricts agricultural activities. Elevated concentrations of nitrate were detected in southern and southwestern parts of the country. Owing to milder climate and more fertile soils, crop production is concentrated in these areas. In the southwestern areas, agriculture is the primary source of anthropogenic nitrogen load into the surface waters, and the field
percentages (24–44%) are the highest in the country (Rekolainen 1993,Pitkänen 1994; Ekholm et al. 1999). In the northern coastal areas, the nitrate concentrations are lower as forestry is the dominating land-use activity.
Maximum NO3 concentration exceeded 25 mg l-1 in 93 sampling sites, and 20
mg l-1 in 130 sampling sites of the 2,376 sites studied altogether in 2000–2002. After
careful examination of those 130 sites, 35 sites were left in the group of significant waters where agriculture has considerable influence on water quality (Table 18, Fig. 1). These sites are situated in 20 rivers, streams or ditches, in one lake and in one bay in the Gulf of Finland. They are all located in southern Finland where agriculture is most intensive and also point-source loading is heaviest. In 2000– 2002, total N load originating from agriculture was estimated to form 18–68% of all nitrogen sources, including natural discharge (Table 18). In those waters where agriculture formed <50% of total N load, point-source loading formed 17–69% of total N load. The River Rakkolanjoki and Lake Haapajärvi receive municipal wastewaters which form over 60% of the total N load. Lake Haapajärvi, located on the course of the River Rakkolanjoki, is a eutrophied lake with high internal loading. The Rivers Porvoonjoki, Vantaanjoki, Risupakanjoki and Myllyoja also receive municipal wastewaters. The Rivers Vantaanjoki and Mäyränoja were also heavily loaded by scattered settlements.
The River Koskenkylänjoki, which discharges into Pernajanlahti Bay, is mainly loaded by agriculture (63% of total N load). The high NO3 values on the shallow site of Pernajanlahti 51 (only 1 m deep) in 2000–2002 (Table 18) were probably due to resuspension and did not reflect changes in land use, because the concentrations at the other sites in close proximity to the river mouth did not peak during the reporting period.
NO3 concentration was typically high (max > 25 mg l-1 NO
3) in the upper
reaches of the rivers, and lower in the sampling sites downstream, for example in the River Yläneenjoki. In the lower reaches, waters discharging from forested areas have a diluting effect. The River Yläneenjoki is situated in the drainage basin of Lake Pyhäjärvi, discussed in more detail in chapter 2.4.1.
According to the Finnish Lake Survey 1995 (Mannio et al. 1999), NO3 concentrations were clearly below 25 mg l-1. The median NO
3-N concentration was
11 µg l-1 (= 0.05 mg l-1 NO
3) and the 90 percentile was 54 µg l
-1 (=0.24 mg l-1 NO 3). This
survey was carried out in autumn 1995 and covered 978 lakes which were selected from the national lake database using stratified random sampling from the entire lake population with unequal sampling fractions. Lake waters of the southern coast showed the highest ammonium and nitrate values. This was expected since the region is subjected to high loads of these forms of nitrogen, both airborne loads and loads from drainage basins (Rekolainen 1989; Syri et al.1998). In autumn, the comparability of the concentrations of these forms might be biased due to differences in sampling temperature; concentrations increased as biological activity and temperature decreased. Seasonal changes in NO3 concentrations are shown in Chap. 2.4.
Mapping of stream water quality was carried out by the Geological Survey of Finland in August-September 1990 in 1,166 small headwater catchments (ca. 30 km2) all
over Finland (Lahermo et al. 1996). Typical NO3concentrations were 0.2–3.0 mg l-1
(median 0.5 mg l-1, mean 0.86 mg l-1, the 98 percentile 5.48 mg l-1). Highest concentrations
were detected in coastal areas where population density is highest and agriculture intensive.
Figure 1. Maximum concentrations of nitrate (mg l-1 NO
3) in Finnish lakes, rivers and coastal
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The Finnish Environment 741 Table 18. Agriculturally loaded Finnish surface waters in 2000–2002 with maximum nitrate concentrations > 20 mg l
-1. The area and field percentage of each river basin and estimation
of total N load by sources are shown for downstream sites of each river. River basins are shown in Fig. 2. wwtp= wastewater treatment plant.
Sampling site Water NO3 (mg l-1) River basin Total N sources (%) Remarks
type
n mean std max No.
Area
km2 Name (Ekholm 1993)
Field
% Agriculture Natural discharge Scattered settlement Point-source loading Other
Rakkolanjoki 003 river 12 8.2 8.4 22.1 06.021 156 Rakkolanjoki lower part 17.9 21 13 2 63 1 Below a wwtp Haapajärvi 006 lake 12 9.7 9.5 23.2 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Haapajärvi 014 lake 4 12.3 18.5 39.6 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Haapajärvi 015 lake 12 10.4 10.2 26.1 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Rakkolanjoki 005 river 12 9.2 10.0 27.4 06.022 105 Rakkolanjoki upper part 23.2 18 10 2 69 1 Below a wwtp Rakkolanjoki 016 river 11 39.4 16.8 66.4 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Rakkolanjoki 019 river 13 55.3 22.2 97.4 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Rakkolanjoki 029 river 12 18.3 16.2 57.5 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Rakkolanjoki 131 river 12 68.2 29.1 111 06.022 Rakkolanjoki upper part 23.2 Below a wwtp Taasianjoki 8,2 river 12 6.0 6.9 21.2 15.001 530 Taasianjoki lower part 30.5 63 31 2 2 2
Taasianjoki 9,2 river 12 6.0 6.9 21.2 15.001 Taasianjoki lower part 30.5 Taasianjoki 9,9 river 12 5.9 7.1 21.7 15.001 Taasianjoki lower part 30.5
Koskenkylänjoki 3,0 6030 river 78 5.4 3.9 25.2 16.001 895 Koskenkylänjoki lower part 30.6 63 28 3 1 5 Koskenkylänjoki River Ilolanjoki 1,3 river 23 6.0 7.6 37.2 17.001 309 Ilolanjoki lower part 26.2 59 31 4 0 6
Porvoonjoki 11,5 6022 river 70 14.1 6.2 37.6 18.012 1138 Strömberg area 28.5 43 20 3 31 3 Porvoonjoki River Mustijoki 4,2 6010 river 70 8.3 5.8 29.2 19.001 783 Mustijoki lower part 30.7 60 26 4 6 4
Vantaa 1,3 river 50 8.0 4.7 27 21.011 1686 Vantaa river mouth area 23.4 44 23 12 17 4 Vantaa River Vantaa 4,2 6040 river 73 7.8 3.4 23.9 21.011 1686 Vantaa river mouth area 23.4 Vantaa River
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34 The F innish Environment 741Table 18 continues.
Sampling site Water NO3 (mg l-1) River basin Total N sources (%) Remarks
type
n mean std max No.
Area
km2 Name (Ekholm 1993)
Field
% Agriculture Natural discharge Scattered settlement Point-source loading Other
Lepsämänjoki 2,6 river 54 6.3 5.3 29.4 21.041 214 Lepsämänki lower part 24.8 53 29 9 4 5 Monitoring of results of EU agri- envi- ronmental support scheme
Sarsalanoja 1,6 stream 36 5.8 6.5 22.3 21.084 19 Sarsalanoja 43.4 68 25 5 0 2 Monitoring of drainage area of Lake Tuusulanjärvi
Mäyränoja 0,4 stream 36 8.8 8.5 34.8 21.085 16 Mäyränoja 35.8 58 24 16 0 2 Monitoring of drainage area of Lake Tuusulanjärvi
Ohkolanjoki 0,6 river 9 4.3 7.5 23.5 21.096 79 Ohkolanjoki 27.1 59 30 8 0 3
Kirkkojoki 1,2 stream 24 8.1 5.9 22.1 22.006 142 Kyrkån 34.2 68 24 6 0 2 Monitoring of results of EU agri- environmental support scheme Kirkkojoki 7,7 stream 24 8.2 6.8 30.1 22.006 142 Kyrkån 34.2 68 24 6 0 2 Monitoring of results of EU agri- environmental support scheme Risupakanjoki 0,5 stream 24 25.4 18.8 88.6 22.007 42 Risubacka ån 21.6 23 12 3 61 1 Below a wwtp
Sirp 22 Lla-Uki va6600 river 19 8.6 5.2 20.4 32.001 378 Sirppujoki lower part 25.7 62 27 4 5 2
Yläneenjoki P2 Vanhakart river 91 3.6 3.3 22.1 34.042 197 Vanhan kartano 30.1 64 30 2 0 4 Monitoring of drainage area of Lake Pyhäjärvi
Yläneenj S13 Vuotavanoja ditch 33 8.4 9.8 53.1 34.045 13 Latvanjoki 47.3 Monitoring of drainage area of Lake Pyhäjärvi
Sammaljoki Nanhiansuo ap river 10 5.5 7.5 25.2 35.181 303 Sammaljoki lower part 25.8 63 31 2 0 4
Vanajav. Hiidenjokisuu river 5 6.2 10.7 25.2 35.233 2279 Hämeenlinna area 21.1 54 28 4 6 8 Hiidenjoki River, below wwtp Tarpianjoki keskusta mts river 10 3.9 6.0 20.4 35.281 597 Tarpianjoki lower part 20.8 50 34 3 3 10
Kirkkojärven Myllyoja river 50 7.6 6.0 32.8 35.729 42 Myllyoja 30.9 45 21 5 22 7 Below a wwtp
Hiidenjoki 7,4 (7310) river 19 4.7 7.4 26.6 35.811 2192 Hiidenjoki mouth area 21.6 55 28 3 <6 14 Hiidenjoki River , upstream of a wwtp Vaasa-Pori mts vp. 9300 river 20 7.4 4.5 20.8 41.001 507 Laihianjoki lower part 28.0 52 34 4 6 4 Laihianjoki River