A sediment budget showing the sources of suspended sediment to a small stream in eastern Jutland, Denmark, reveals that although bank erosion is the main source, soil erosion and tile drains also deliver a substantial proportion of sediment to the stream. The sediment budget will show great year-to-year variations due to the tight relationship between climate and ero- sion. For example, rill erosion on arable fi elds in Denmark has been shown to vary 10–20-fold between wet and dry years.
Suspended sediment yields in Danish streams are low when compared to other regions of the world. The suspended sediment transported in Danish streams is very fi ne-grained and is therefore of importance as carrier of environmentally harmful adsorbed substances, such as phosphorus, heavy metals, certain pesticides, PAHs, etc. The transport of suspended sediments in Danish streams varies considerably from year to year, and the majority of the annual suspended sediment yield is often transported during a few days with rain or snow melt.
The movement of coaser sediment on the stream bed (bed load) is seldom measured in Danish streams because it is extremely diffi cult to obtain precise measurements [7]. However, the few existing quantitative measurements show that the bed load is considerably higher in the dominantly groundwa- ter fed streams in western Jutland as opposed to streams in eastern Jutland and on the islands.
May 1993 to April 1994 May 1994 to April 1995
Subsurface tile drainage water 15% 11% Soil erosion and surface runoff 19% 0% Stream bank and bed erosion 66% 89%
Estimated contribution of subsurface drainage water, surface runoff and stream bank and bed erosion to total suspended sediment export from the 11 km2Gelbæk Stream catchment in central Jutland, Denmark (adopted from [8]).
Stream Catchment area Measurement period Suspended sediment load Sediment bed load (km2) (tonnes/km2 per year) (tonnes/km2 per year) Vorgod Stream 455 1976,1977 7.1 < 5.0 River Skjern 2,220 1976,1977 6.4 12.4 River Skjern 2,220 1994,1995 7.9 10.2 Grindsted Stream 238 1970,1974,1975 18.7 22.8 Ansager Stream 138 1970,1974,1975 9.5 19.7 Varde Stream 598 1974,1975 9.4 < 1 Brede Stream 292 1968 6.2 14.7 Grønå Stream 603 1968,1981 6.8 10.3 River Gudenå 1,787 1974 3.4 < 1 Mattrup Stream 88 1974 2.2 < 1 Århus Stream 120 1986–1987 14.3 < 1 Lyngbygaard Stream 126 1986–1987 28.4 < 1 Tude Stream 146 1970,1971 5.4 < 1 Suså Stream (upstream) 42 1978,1979 7.7 < 1 Suså Stream 601 1978,1979 5.6 < 1
Average annual suspended sediment load and sediment bed load in different Danish streams [7, 9]. 0 50 100 150 200 0 100 200 300 400
Suspended sediment loss
(kg/ha)
programmes (Box 2.2). Such accurate knowledge on the transport of sus- pended sediment in streams is valuable as fi ne sediment is an important carrier of phosphorus, heavy metals and micro-pollutants.
The bed load of sediment in Danish streams has been less intensely stud- ied and the majority of information available is related to large streams in western Denmark with a stable hydrological regime and thus a high sediment transport capacity during the entire year. Large differences in the bed load of sediment in Danish streams have been measured for the different stream types (Box 2.2).
Even though sediment transport is low in Denmark compared to most other countries, high delivery of sedi- ment to streams from unstable stream banks causes ecological problems because of accumulation of sand and mud on the stream bed during low-fl ow periods. Straightening and channelisa- tion of most Danish streams have trig- gered both a higher sediment input to the streams and a lowering of the trans- port capacity, thereby forcing stream managers to excavate streambed sedi- ment as part of stream maintenance. The natural physical conditions in Danish streams are therefore severely impacted.
Acidifi cation
Acidifi cation because of acid rain or enhanced nitrifi cation caused by agri- cultural practice affects sensitive water bodies located in carbonate-poor soils with limited buffer capacity to neutral- ise acidic compounds. In areas of low buffer capacity, acid rain and drainage water also release aluminium from soils into lakes and streams, generating toxic conditions for aquatic species.
High alkalinity and pH in streams are associated with calcareous soils in the catchments in eastern Jutland and on the islands where glacial till deposits from the last glaciation period prevail (Weichsel 14,000 years ago). Such soil types have a natural buffering capac- ity, reducing the risk of acidifi cation. Western Jutland in contrast, has mineral weathered 140,000 years old moraines from the Saale glaciation, and coarse textural soils on the outwash plains from the Weichsel period. Both soil types are low in carbonates and vulnerable to acidifi cation. Moreover, smaller areas with windblown deposits along the west coast of Denmark and in some central parts of Jutland are also low in carbon- ates and vulnerable to acidifi cation.
Most Danish streams generally have a high or very high buffering
capacity (>0.1 mmol HCO3–/l) against
acidifi cation. A buffering capacity of
0.02–0.1 mmol HCO3–/l can be found
in smaller Danish streams in vulner- able areas, whereas a low buffering
capacity of 0.01–0.02 mmol HCO3–/l
is rarely found [10]. Thus, the major- ity of Danish streams have, on aver- age, an annual pH exceeding 6.5 and should not suffer any biological damage owing to acidity (Figure 2.8). However, streams and lakes of low or very low buffer capacity have been documented to show a downward trend in pH during the period 1977– 1989. The strong recent decline in the emission of acidifying gases (mainly sulphure oxides) to the atmosphere has decreased the potential problem with acidifi cation of Danish streams. How- ever, the large emission and deposition of ammonia from Danish agriculture is currently as important as acid precipi- tation and may locally acidify Danish streams of low buffering capacity.
Ochre
Ochre (Fe(OH)3) and ferrous iron (Fe2+)
can damage the fl ora and fauna in streams. High concentrations of fer- rous iron (>0.5 mg/l) are toxic for fi sh and macroinvertebrates, and precipi- tated ochre on spawning gravel, plant surfaces and animal gills can physi- cally inhibit their organisms. Ochre
is ferric (Fe3+) oxy-hydroxides that
precipitate under aerobic conditions at pH higher than 4 in surface water because of the input of soluble ferrous iron and ferric iron ( pH<3) from soil water and groundwater. Ferrous iron is found naturally in some deep anaero- bic groundwater aquifers, but the main source of ferrous iron in Danish streams is from oxidation of natural
deposits of pyrite (FeS2) in waterlogged
soils. The water level in these soils has in many cases been lowered due to reclamation of agricultural land. Land drainage has reduced the groundwater
<7.0 7.0–7.5 7.5–8.0 8.0> No data Acidity pH
Figure 2.8 pH in Danish streams illu- strated as annual means for streams in 67 larger river catchments during the period 1989–1998.
table and caused oxidation of pyrite to ferrous iron, sulphate and protons that escape with the drainage water to surface waters (see Box 2.1). Deposits of pyrite in Danish soils are mainly found along coastal areas and fl ood- plains in Jutland where old brackish or marine deposits exist. A survey of low-lying areas in south-western and northern Jutland showed that 25% of almost 500,000 ha low-lying soils have a potential for releasing ferrous iron and cause ochre problems in surface waters. The concentration of total iron in Danish streams refl ects these condi- tions as the concentrations are highest in western Jutland and lowest on the islands (Figure 2.9).
Nutrients
The quantity of nitrogen and phospho- rus exported to the streams and lakes in Denmark and their sources has been calculated for the period 1989–2002 (Box 2.3). The amount of nitrogen and phosphorus discharged to water from point sources has been reduced by 70% for nitrogen and 84% for phosphorus during the same period. Thus, nitrogen and phosphorus losses from diffuse sources and the losses from agriculture in particular, have become increasingly important.
Important indicators of the pollu- tion potential from agricultural produc- tion are the consumption of chemical fertiliser and the production of animal manure on arable land. Several sci- entists have documented a positive relationship between the consumption of chemical fertiliser in agriculture and the resulting concentration or transport of nutrients in streams [11]. The nitro- gen and phosphorus input to Danish
arable land reached a maximum in the early 1980s, following a strong increase in the consumption of chemical fertilis- ers during the 1960s and 1970s (Box 2.4). The consumption of nitrogen and phosphorus in chemical fertiliser has decreased in Denmark since the mid-1980s due to the implementation of four major Danish Action Plans on the Aquatic Environment. The Action Plans have posed several restrictions on Danish farmers, especially regard- ing their storage facilities for liquid manure, application times of liquid manure on the soils, demands for uti- lisation of nitrogen in animal manure, reductions of livestock densities and reductions of quota for allowed use of nitrogen for crops [12].
Nitrogen and phosphorus cycling in Danish agriculture has been monitored since 1989 in fi ve small agricultural catchments by use of questionnaire surveys at the fi eld level and measure- ments of nitrogen and phosphorus in
<0.5 0.5–1.0 1.0–1.5 1.5> No data Total iron Fe (mg/l)
Figure 2.9 Concentrations of total iron in Danish streams as annual means for streams in 67 larger river basins during the period 1989–1998.
Application of slurry from an increasing pig production is now adays done in the spring due to the increase in storage facilities (9 months) demanded by the 1st Action Plan on the Aquatic Environment.
soil water, drainage water, groundwa- ter and stream water [2]. The fi ve catch- ments represent typical Danish farming systems ranging from crop production on the predominantly loamy soils on the islands to animal production on the sandy soils in Jutland. The data col- lected can be used to establish overall nitrogen and phosphorus budgets for small catchments, revealing the major input and outputs of nutrients (Box 2.4).
Dissolved inorganic nitrogen is responsible for the main transport of nitrogen in Danish streams that drain catchments dominated by agricultural
land use (80–90%). Nitrate-N (NO3–-N)
is the main constituent (78–85%). The average annual loss of total nitrogen from agricultural land in small catch- ments was nearby two times higher from loamy catchments (23 kg N/ha) than sandy catchments (12 kg N/ha) during the monitoring period 1998– 2003 (Box 2.4). In comparison, the aver- age annual loss from predominantly forested catchments was 2.3 kg N/ha during the same period.
Dissolved reactive phosphorus (inorganic) constitutes on average 38% of the total phosphorus in streams draining loamy and sandy catchments in Denmark. Particulate phosphorus associated with inorganic and organic matter is of great importance in Danish streams. The speciation of the phos- phorus export from small agricultural catchments implies that the pathways are linked partly to the movement of water in the soil column and partly to erosional processes. The average annual export of total phosphorus from loamy (0.53 kg P/ha) and sandy agricultural catchments (0.51 kg P/ha) was almost identical during the moni- toring period 1989–1998 (Box 2.4). In comparison, the average annual export