Redox interfaces and reduced compounds at the Lillebæk and Norsminde study areas

Transcription

1 Redox interfaces and reduced compounds at the Lillebæk and Norsminde study areas NiCA Technical Note October 2013 Vibeke Ernstsen Geological Survey of Denmark and Greenland G E U S

2 Referencing this report: Ernstsen V (2013) Redox interfacess and reduced compounds at the Lillebæk and Nors- minde study areas. NiCA Technicall Note, October Available A att NiCA (Nitrate reduction in geologically heterogeneous catchments) is supported by the Danish Strategic Research Council. The NiCA project is led by GEUS (contact: Jens Christian Refsgaard,, mail: and comprise the following partners: Geological Survey off Denmark and Greenland (GEUS) Department of Geography and Geology, University of CopenhagenC n Institute of Food and Resourcess Economics, University of Copenhagen Department of Earth Sciences, Aarhus University Knowledge Centre for Agriculture Laval University, Quebec, Canada Aarhus Geophysics Alectia A/S DHI SkyTEM Municipality of Aarhus Municipality of Odder Read more about the project and seee its outputs at G E U S

3 Content 1. Introduction 2 2. Methods 3 3. Results The Lillebæk study area Depth to redox interface large scale Depth to redox interface local scale Reduced compounds in sediments The Norsminde study area Depth to redox interface large scale Depth to redox interface local scale Reduced compounds in sediments Acknowledgement References 14 1

4 1. Introduction The distribution of nitrate is closely correlated to the geochemical environment of the sediments. Korom (1992) describes nitrate as being unstable under oxygen-free, reduced conditions where it may undergo a transformation to e.g., molecular nitrogen (denitrification), which is released into the atmosphere. Studies of young sediments of Quaternary age of Denmark have demonstrated a close relation between the distribution of nitrate and the colours of the sediments. Nitrate is present in measureable concentrations in oxidized sediments only. Oxidized sediments are shaded in e.g., brown, orange, and yellow (Ernstsen et al, 1991; Ernstsen, 1996; Ernstsen et al., 1998, Ernstsen et al., 2001, Ernstsen, 2005). The linkage between distribution of nitrate, geochemical environment, and sediment colour make the interface between the oxidized and the reduced sediments the redox interface extremely important in our understanding of the occurrence of nitrate. Another very important parameter is the availability of reductants (electron donors) that may contribute to reduction of nitrate and control the progression of nitrate in the sediments. This NiCA technical note focus on these two key parameters in two different geological settings in Denmark, the study area at Lillebæk, Funen, and the study area at Norsminde, South of Aarhus, Jutland. A comprehensive description of the whole project model, including the two parameters mentioned above, is given in Refsgaard et al. (2013) 2

5 2. Methods The drilling places were located on cultivated land. Using auger a drillingg the sediment sam- ples (see Figure 1) weree collected inn well-defined depths for either geological characterisa- tion or for chemical analyses. Figure 1 Auger drilling was used to get information about thee geology (lithology) and the geo- is chemical environment (colour of the sediments). The picture showing s the e drilling equipment from drilling place N12 in the t Lillebækk area. The two small pictures show the redox interface at drilling place N8 and drilling place N122 at Norsminde area. The samples for geological description were collected whenn major changes in the lithology occurred. Within the Lillebæk area, sediment samples for the chemical analyses were colintervals lected with intervals of 0.5 meter above the redox interface and with one meter below the redox interface. Within thee Norsminde area, the first sample was taken just below the surface and then with intervals off one meter. At both study areas the drilling was down to about ten meters below surface for the central wells (see Figure 5) and for the additional wells down to about one meter below the redox interface. During the field work, detailed descriptions of the lithological units and sedimentt colours were carried out. After the t field work, all sediment samples for detailed geological de- reports scribed were handled in the Geological Well Sample Laboratory at GEUS and well (in Danish) can be found in the Jupiter database at GEUS ( Table 1 3

6 sum up the DGU numbers for the central wells from the Lillebæk and the Norsminde study areas. Sediment samples for analysis were kept cool in the field and as soon as possible stored at about 5 C or at about -18 C. Analyses of reduced compounds were performed according to the method by Ernstsen et al (2005). The field work in the Lillebæk study area took place from 6 th September 2010 to 16 th September 2010, and the fieldwork in the Norsminde study area took place from 12 th September 2011 to 23 th September LILLEBÆK STUDY AREA NORSMINDE STUDY AREA Well no. DGU nr. Well no. DGU nr. N N N N N N N N N N N N N N N N N N N N N N N N N N N N N Table 1 Well numbers and corresponding DGU numbers (DGU nr.) for the wells in the Lillebæk and Norsminde study areas. 4

7 3. Results 3.1 The Lillebæk study area Depth to redox interfacee large scale The lithology of the twelve central wells and the position of the redox interface are given in Figure 3. A sandy clayeyy till is the most common type of sediment below an upperr layer of organic rich sediment. Within the sandy till layers thin layerss or lenses of sand may be preor gravel sent. At the drilling places N1, N2 and N9 a one meter or thicker layers of sand was present and at N1, N2, N7, N8, and N9 a more clay-rich till was present. The redox 5 The study area Lillebæk refers to the catchment of the Lillebæk creek, which is located in the south eastern part of Funen, Figure 2. In the central part of the Lillebæk catchment area, twelve wells w were drilled along a 1.4 km NE-SW oriented line, Figure 2. Around five of these central wells (drilling places) ), four or eight additional wells were constructed at different distance (2.5, 5 or r 10 meter) from the central well using a geostatistical sampling design (see Figure 5). Figure 2 Twelve wells along the yelloww line within the catchment area of Lillebæk given by a red line. The total number of wells at each drilling place is indicated by the colours of the dots. A black dot indicates one well, a blue dot indicates one central well w and four r additional wells, and a green dot indicates one central well and eight additional wells.

8 interface was found at 1.8 meter below surface at N1 below post-glacial organic-rich freshwater sediment and in 3.0 to 8.6 meter below in the clayey till dominated sites. The distribution of the depths to the redox interface is shown on a map given in Figure Depth (m) Redox interface Toplayer rich in organicmatter Clayeytill sandy and oxidized Clayeytill sandy and reduced Vibeke Ernstsen, GEUS, 2013 Clayey till clayrich Clay with linses or thin layers of sand Sand or gravel Fillingwith e.g., bricks Figure 3 Lithology and redox interface at the twelve wells places shown in figure 2. See Figure 2 for the location of the wells Depth to redox interface local scale At five out of the twelve drilling places additional wells were carried out after a geostatistical sampling design. Around N1, N3 and N5 additional wells were placed in a distance of 5 meter from the central well in 4 directions with a spacing of 90, starting at 0, figure 5. Around N6 eight additional wells were carried and arranged as just described for N6 and in distances of 5 m and 10 m from the central well. At N11 the additional wells were organised as for N6 but with distances were here 2.5 m and 5 m. The spatial distribution of wells and depths to redox interface at these drilling places are given in figure 5. The key data for the depths to the redox interface are summarized in table 2. 6

9 Figure 4 Depths in meters to redox interface at the twelve drilling places. Figure 5 Central wells (written in bold) ) and additional wells withh depths to the interface (m). 7

10 Drilling places N1 N3 N6 N6 N6 N8 N11 N11 N11 Well observations Total area (m 2 ) Depth to redox interface (m) 1,4 3,4 4,4 4,4 4,3 4,5 3,5 3,5 3,5 1,5 3,6 4,5 4,5 4,4 7,0 3,6 3,5 3,5 1,5 4,0 5,4 5,4 4,5 7,0 3,6 4,0 3,6 1,8 4,0 5,7 5,7 4,9 7,0 3,6 4,0 3,6 2,1 4,1 7,8 7,8 5,4 7,1 3,9 4,4 3,6 5,5 3,9 5,7 3,6 7,8 4,0 8,1 4,4 Mean (m) 1,7 3,8 5,7 5,6 5,6 6,5 3,6 3,9 3,8 Min (m) 1,4 3,4 4,3 4,4 4,3 4,5 3,5 3,5 3,5 Max (m) 2,1 4,1 8,1 7,8 8,3 7,1 3,9 4,4 4,5 σ 0,3 0,3 1,2 1,2 1,3 1,0 0,1 0,3 0,3 Table 2 Mean -, minimum -, maximum values and standard deviation (σ) for depths to the redox interface for central wells and additional wells made up for areas of 25 m 2 to 400 m 2. The depth of the redox interface for the central well is written in bold Reduced compounds in sediments Total amount of reduced compounds in different types of sediments collected from the twelve central wells is given in table 3. The highest mean value (520 e-meq kg -1 ) was measured for of the organic-rich layer (A-horizon), followed by the reduced till (405 e-meq kg -1 ) and reduced sand (105 e-meq kg -1 ). The mean value for oxidized sand was 25 e-meq kg -1 and for oxidized till 57 e-meq kg -1. The pools of reduced compounds in the oxidized sediments are either only slowly or not available for the nitrate reduction processes. Type of sediment Mean value Min value Max value Σ e-meq kg -1 n Organic-rich layer (A-horizon) Till oxidized Till reduced Sand oxidized Sand reduced Table 3 Mean -, minimum -, maximum values and standard deviation (σ) for the amount of reductants in different types of sediments collected within the Lillebæk study area. The number of samples (n) in each category is given in the last column. 8

11 3.2 The Norsminde studyy area Within the Norsminde study area 177 wells were drilled. The wells, except for N11, were located along an approximately 6.6 km NW-SE oriented line. At well place N9 four additional wells were drilled at a distancee of 5 meter from the central well (see Figure 9) ). Figure 6 The Norsminde study area corresponds to the catchment area of Odder stream and Rævså stream which is indicated by a red line at the map. Thee blue line and the blue dot (N11) indicate the location of the wells within the catchment. The lower figure shows in more details the location of the wells in the western part of the catchment area Depth to redox interfacee large scale Sandy clayeyy till was the most common type of sediment att most of thee drilling places, Fig- In N1, ure 7. Only in the N5, N6 and N11 well places clay of Oligocene age was present. N2 and N4 sand and gravel were thee dominating type of sediment. The depths to the redox interface at the 17 drilling places are a also given in figure 7. At N1, N2 and N4, dominated by sand and gravel, no redox interface was located. At N3 a redox sequence of oxidized-reduced-oxidized layers of sedimentss was present with redox intermapped. faces in 4.3 and 6.3 meter. At all other drilling places only one redox interface was 9

12 At N5 and N6, the redox interface was shallow, 2.4 and 1.4 meter below surface respectively. The depth here is most likely controlled by the distribution of clay of Oligocene age. At the other wells the depth to the redox interface varied from 2.7 to 6.2 meter. The depths to the redox interface are also given in figure Depth (m) Redox interface Toplayer rich in organicmatter Clayeytill sandyand oxidized Clayeytill sandyand reduced Clayey till clayrich Clay with linses or thin layers of sand Sand or gravel Gyttja Clay of Oligocene age Vibeke Ernstsen, GEUS, 2013 Figure 7 Lithology and redox interface at the 17 drilling places with locations as shown in figure 6. 10

13 Figure 8 Depths to the redox interfacee at the 17 drilling places in the Norsminde study area Depth to redox interfacee local scale Around N9 four additional wells weree drilled at a distance of 5 meter fromm the central well. The organisation of the wells is givenn in figure 9 and the keyy parameterss for this place is given in table 4. Figure 9 Central well and additional four wells with depths the interface (m)) at N9. 11

14 Drilling place N9 Well observations 5 Total area (m 2 ) 100 Depth to redox interface (m) 3,9 4,1 4,2 4,5 5,4 Mean (m) 4,4 Min (m) 3,9 Max (m) 5,4 σ 0,5 Table 4 Mean -, minimum -, maximum values and standard deviation (σ) for depths to the redox interface for the central well and four additional wells calculated for 100 m 2. The depth of the redox interface for the central well is written in bold Reduced compounds in sediments The content of reduced compounds in samples from the 17 drilling places is given in table 5. The highest content was measured in the reduced Oligocene clay (2850 e-meq kg-1), the organic rich layer (528 e-meq kg -1 ), reduced meltwater clay (474 e-meq kg -1 ), and reduced till (459 e-meq kg -1 ). The content of reduced compounds in the oxidized sediments was markedly lower; for sand 21 e-meq kg -1, for meltwater clay 39 e-meq kg -1 and for till 41 e-meq kg -1. The pools of reductants in the oxidized sediments are either only slowly or not available for the nitrate reduction processes. Type of sediment Mean value Min value Max value Σ e-meq kg -1 n Organic-rich layer (A-horizon) Till oxidized Till reduced Sand oxidized Oligocene clay reduced Meltwater clay oxidized Meltwater clay reduced Table 5 Mean -, minimum -, maximum values and standard deviation (σ) for the amount of reduced compounds in different types of sediments collected in the Norsminde study area. The number of samples (n) in each category is given in the last column. 12

15 4. Acknowledgement We thank Christina Rosenberg Lynge and Pernille Stockmarr, Inorganic Laboratory, GEUS, for the handling and analysis of the sediment samples. Also thanks to Elias Hestbech for his help during the fieldwork including drilling, sediment sampling and - description and valuable pictures of redox interfaces. 13

16 5. References Ernstsen, V Reduction of nitrate by Fe2+ in clay minerals. Clays and Clay Minerals, 44: Ernstsen, V Nitrate reduction in the unsaturated zone. Danish Environmental Protection Agency, Ministry of Environment. Report pp.37. (in Danish). Ernstsen, V., Binnerup, S.J. and Sørensen, J Reduction of nitrate in clayey subsoils controlled by geochemical and microbial barriers. Geomicrobiology Journal, 15: Ernstsen, V., Gravesen, P., Nilsson, B., Brüsch, W., Fredericia, J. and Genders, S Transport and transformation of N and P in the catchment area of Langvad river. I. The NPo Research Progamme from the National Agency of Environmental Protection. B abstracts. pp Ernstsen, V., Henriksen, H.J. and von Platen, F Principles for calculating the reduction of nitrate in soil layers below the root zone. Danish Environmental Protection Agency, Ministry of Environment. Repport no. 24.pp.54 (in Danish) Ernstsen, V. Jørgensen, N. and Lynge, C.R A method for analysis of reduced compounds in sediments. Danish Environmental Protection Agency, Ministry of Environment. Report pp. 57.(in Danish). Korom, S.F Natural denitrification in the saturated zone: a review. Water Resources Research 28: Refsgaard et al Nitrate reduction in geologically heterogeneous catchments A framework for assessing the scale of predictive capability of hydrological models. Science of the Total Environment (Accepted for publication 13th July 2013). 14