Environ. Sci. Technol. 2006, 40, 6697-6702
Hydrodynamic and Microbial Processes Controlling Nitrate in a Fissured-Porous Karst Aquifer of the Franconian Alb, Southern Germany F L O R I A N E I N S I E D L * ,† A N D BERNHARD MAYER‡ GSF-National Research Center for Environment and Health, Institute of Groundwater Ecology, Ingolsta¨dter Landstrasse 1, D-85764 Neuherberg, Germany, and Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4
Concentrations and stable isotope compositions of nitrate from 11 karst springs in the Franconian Alb (Southern Germany) were determined during low flow and high flow conditions to assess sources and processes affecting groundwater nitrate. During low flow, nitrate concentrations in groundwater were around 0.10 mM in springs draining forested catchments, whereas in agricultural areas nitrate concentrations were typically higher reaching up to 0.93 mM. The isotopic composition of groundwater nitrate during low flow (δ15N values of -3.1 to 6.7 ‰, δ18O values of +2.1 to 4.0 ‰) in concert with concentration data suggests that nitrate is formed by nitrification in forest and agricultural soils. In addition, synthetic fertilizer N that has undergone immobilization and subsequent remineralization likely constitutes an additional nitrate source in agriculturally used catchments. During recharge conditions, concentrations and δ15N values of groundwater nitrate changed little, but δ18O values were significantly elevated (up to 24.5‰) suggesting that around 25% of the nitrate was directly derived from atmospheric deposition. Groundwater dating revealed that low nitrate concentrations in groundwater (g70 years) are consistent with a mixture of old low nitrate-containing and young water, the latter being affected by anthropogenic N inputs predominantly in the agriculturally used catchment areas during the last few decades. Thermodynamic and hydrogeological evidence also suggests that denitrification may have occurred in the porous rock matrix of the karst aquifer. This study demonstrates that a combination of hydrodynamic, chemical, and isotopic approaches provides unique insights into the sources and the biogeochemical history of nitrate in karst aquifers, and therefore constitutes a valuable tool for assessing the vulnerability of karst aquifers to nitrate pollution in dependence on land use and assessing their selfpurification capacity.
Introduction During the last 40 years, human activity has strongly altered the quality of groundwater in numerous porous aquifers and * Corresponding author phone: +49 89 318 72567; fax: +49 89 31873361; e-mail:
[email protected]. † Institute of Groundwater Ecology. ‡ University of Calgary. 10.1021/es061129x CCC: $33.50 Published on Web 09/23/2006
2006 American Chemical Society
karst terrains (e.g., refs 1-4). A particular problem is increasing nitrate concentrations that have been observed in many aquifers in Europe, Northern America, and elsewhere (5-7), because of the detrimental health effects of elevated nitrate concentrations in drinking water (8). Karst aquifers are particularly vulnerable to nitrate contamination from anthropogenic sources because of their particular flow characteristics consisting of conduit networks that allow rapid water movement along preferential flow paths. During precipitation events, the preferential flow transports pollutants to the deeper groundwater system, and nitrate concentrations of more than 0.7 mM have been observed in different karst areas (9, 10). Since 25% of the world’s population uses karst water as a drinking water resource (11), it is important to implement strategies for monitoring nitrate pollution in karst aquifers, to develop new management and groundwater protection strategies (12, 13), and to identify their self-purification capacity to preserve karst terrains as important drinking water resources. Nitrate in groundwater can be from natural and anthropogenic sources. The stable isotope ratios of nitrogen and oxygen (15N/14N, 18O/16Onitrate and 17O/16Onitrate) have been successfully used to elucidate sources of nitrate (e.g., refs 14-17), particularly if both δ15N and δ18O values of nitrate are determined. Nitrate derived from manure or sewage is usually characterized by δ15N values between +7 and more than +20‰ (18, 19). It is therefore distinct from nitrate in atmospheric deposition (-10 to +8‰), nitrate in synthetic fertilizers (near 0‰), and nitrate generated via nitrification processes in soils (20, and references therein). Usually, the latter three sources cannot be differentiated by nitrogen isotope analyses alone because of their overlapping ranges of δ15N values. However, it has been demonstrated that nitrate in atmospheric deposition has quite positive δ18O values ranging from ca. +30‰ to +80‰ (14, 20-22). Nitratecontaining synthetic fertilizers have δ18Onitrate values near +22‰ ( 3‰ (2, 21, 23). Nitrate derived from nitrification processes in soils typically has δ18O values of less than +15‰ (14, 22, 24, 25), and nitrate in manure and sewage has similarly low δ18O values (2, 19). Hence, the combined determination of δ15N and δ18O values of dissolved nitrate provides a tool for distinguishing between four major nitrate sources: (1) nitrate derived from nitrification, e.g., in soils, (2) nitrate in manure and sewage (see Figure 1), (3) nitrate-containing synthetic fertilizers, and (4) atmospheric nitrate deposition. Denitrification has been shown to cause trends of increasing δ15N and δ18Onitrate values as nitrate concentration decreases (26, 27). The increase in δ15N values due to microbial denitrification appears to be about twice that of δ18Onitrate (28-33). Hence, the remaining nitrate eventually assumes elevated δ15N and δ18O values, which are unique for nitrate that has undergone denitrification under closed system conditions (see Figure 1). So far, only a few studies on nitrate contamination and its sources have been performed in karst terrains (1, 34). Moreover, little is known about the occurrence and extent of natural attenuation processes capable of reducing nitrate concentrations in karst aquifers with matrix diffusion. Pauwels et al. (34) showed in the fractured part of an aquifer in France that denitrification occurred with ferrous iron, pyrite, and organic matter as possible electron donors. Mariotti et al. (35) suggested that nitrate reduction occurred in a chalk aquifer of northern France with matrix porosity. Moncaster et al. (36) found evidence that in a Jurassic limestone aquifer lithotrophic denitrification was controlled by pyrite oxidation. However, none of these studies used a VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic diagram of the isotopic composition of groundwater nitrate from various sources and nitrogen and oxygen isotope composition of karst groundwater nitrate in 2002 during groundwater recharge and during low flow (open symbols) 2003/4 including typical nitrate sources (cluster 1: nitrate from nitrification, cluster 2: manure and sewage, cluster 3: fertilizer nitrate, cluster 4: nitrate derived from atmospheric deposition). dual isotope approach utilizing both the δ15N and δ18O values of nitrate. The objective of this study was to determine the sources and processes affecting nitrate in groundwater in different catchments of the Franconian Alb karst aquifer in southern Germany using chemical and isotopic techniques. Earlier work in this karst area examined hydrogeologic processes and biogeochemical reactions controlling sulfate concentrations (37, 38). A specific goal of this study was to evaluate whether the extent of nitrate contamination in karst groundwater is a function of land use, and whether denitrification is an active amelioration process capable of decreasing nitrate concentrations in the karst aquifer.
Materials and Methods Study Site. The southern Franconian Alb is located in southern Germany and stretches from the No¨rdlinger Ries in the west to the town of Kehlheim in the east (Figure S1, Supporting Information). The study region has a catchment area of about 2000 km2. About 40% of the catchment area is currently used by agriculture, whereas forestry accounts for the majority of the remaining land use. The thickness of the soil zone varies from 0.5 to 2 m. The aquifer bedrock is composed of Upper Jurassic carbonates, and the thickness of the aquifer varies from tens of meters up to 100 m (10). Marls from the base of the karst aquifer are overlain by wellbedded limestones. The latter contain reef complexes, which have been dolomitisized. In comparison to the bedded limestone facies with a rock matrix porosity of less than 2%, the reef facies has high rock matrix porosities ranging from 4% to more than 20%, with an arithmetic mean of 7%. During base flow groundwater flows in the fissure network of the porous aquifer affected by matrix diffusion, whereas during high flow approximately 15% of the total runoff is generated via conduits to the springs (37). Using hydrogeological parameters (discharge, groundwater recharge) and tracer tests the catchment areas of some of the investigated springs in the Franconian Alb (nos. 3, 4, and 8, Gro¨ssdorf, Bo¨hming) were estimated to range between 2 and 30 km2. Pfaff (39) and Glaser (10) determined a general groundwater flow direction from NW to SE north of the river Altmu ¨ hl (Rieshofen, Bo¨hming, Gro¨ssdorf) and from SE to NW for springs situated south of the river Altmu ¨ hl (nos. 1, 5, 6). Precipitation is approximately 750 mm/year. Water balance estimates suggest evapotranspiration of ca. 500-600 mm/year, and the groundwater recharge for the Franconian Alb ranges between 130 and 250 mm/year for forested and agriculturally used areas, respectively (10). Sampling. To investigate the sources and processes affecting nitrate in the karst groundwater of the Franconian 6698
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Alb, 11 springs were sampled repeatedly (n ) 35) between 2002 and 2004 (Figure S1, Supporting Information). Nine sampling sites were located in areas with agricultural land use (n ) 29), whereas the remaining two sampling sites were located in mainly forested catchment areas (Gro ¨ ssdorf village, Rieshofen) (n ) 6). Four springs (site nos. 1, 5-7) are located in the reef facies, while all other springs emerge from the bedded facies. At each sampling event, pH and redox potentials (Eh) were determined in the field. Subsequently, 0.5 L of 0.45 µm filtered groundwater was collected and transported to the laboratory for chemical and isotope measurements. Methods. Field measurements included pH value and redox potentials (Eh). Water samples from the springs were measured by ion chromatography for concentrations of major cations and anions (Dionix DX 100) with analytical uncertainties of (3% estimated from duplicate analyses. Prior to analysis of the isotopic composition of nitrate, DOC was removed from the water samples using XAD-8 resin. Nitrogen and oxygen isotope ratios of nitrate in groundwater were determined using a technique similar to that described by Silva et al. (41). Sulfate was removed from the water as described by Einsiedl and Mayer (37, 38). The remaining solution, containing HNO3 and HCl, was neutralized by adding approximately 8 g Ag2O (Merck, Germany). The AgCl precipitate was removed by membrane filtration, and pure AgNO3 for isotope ratio mass spectrometry was produced subsequently by freeze-drying the remaining solution. For nitrogen isotope analysis, about 300 µg of AgNO3 was weighed into high purity tin cups. These cups were thermally decomposed in an elemental analyzer, and the resultant N2 was analyzed by isotope ratio mass spectrometry in continuous-flow mode. Oxygen isotope ratios of NO3- were determined on CO generated by pyrolysis of high purity silver cups containing 1000 µg of AgNO3 using a TC/EA reactor (1350 °C) coupled to a delta plus XL isotope ratio mass spectrometer in continuous flow mode. Accuracy and precision of the measurements were assured by repeated analyses of lab internal and international references materials (IAEAN1 and N2) and by calibrating all measured oxygen isotope ratios using IAEA-NO-3 with an assigned δ18O value of 25.6‰. Results are reported as parts per thousand (‰) using the conventional delta notation:
δsample(‰) ) [(Rsample - Rstandard)/Rstandard] × 1000 (1) where R is the 15N/14N or the 18O/16O ratio of the sample and the standard, respectively. δ15N values are reported with respect to AIR and δ18O values of nitrate with respect to V-SMOW. δ15N-NO3- values are reported with an overall analytical precision of (0.4‰. δ18O-NO3- determinations have an overall analytical precision of 0.8‰. Tritium (3H) contents were determined for groundwater sampled from the springs in 2003 during base flow conditions. Tritium measurements were conducted by liquid scintillation counting of water after electrolytic enrichment of 3H (40). The detection limit of this method is 0.7 TU.
Results Water Chemistry. Table S1 and S2 (Supporting Information) summarize all chemical data and physical parameters (pH, Eh) for groundwater from up to four sampling events between 2002 and 2004. Groundwater obtained in 2002 was sampled during groundwater recharge conditions whereas all other sampling events occurred during low flow conditions (2003/ 4). Eh values varied between +166 and +310 mV, and pH values ranged between 6.9 and 7.8. All groundwater samples are of the Ca/Mg bicarbonate type characteristic for carbonate aquifers. Table 1 summarizes the nitrate concentrations and isotope ratios for different sampling campaigns. During
TABLE 1. NO3- Concentrations, δ15N and δ18O-NO3- Measurements, and Mean Transit Times [MTTs] in Springwater During 2002 and 2003/4 spring MTT [a]: land use:
#1 150
#2 23
#3
#4
#5 120
#6 70
#7 80
#8 10
Bo1 hming 62
Gro1 ssdorf 60
Rieshofen
predom agr use
predom agr use
predom agr use
predom agr use
predom agr use
predom agr use
predom agr use
predom agr use
forested/ agr use
forested
forested
0.13 1.4/20.5
0.08 1.4/24.5 0.08 0.4/24.5
Groundwater Recharge (2002)
NO3- [mM] δ15N/ δ18O [‰] nd
nd
nd
0.35 0.4/13.1
nd
nd
nd
0.77 5.1/9.7
0.24 3.3/14.5 0.26 4.5/12.6 0.25 3.8/12.5 0.32 4.7/12.3
Low Flow (2003/4) 8/2003 NO3- [mM] δ15N/δ18O [‰] 1/2004 NO3- [mM] δ15N/δ18O [‰] 2/2004 NO3- [mM] δ15N/δ18O [‰]
nd nd
0.66 4.2/3.7
0.65 4.2/3.0
0.40 -0.2/3.9
0.19 6.7/3.1
0.48 4.3/3.0
0.77 3.6/2.2
nd nd
nd nd
0.10 -1.2/-
nd nd
0.22 6.6/3.1
0.89 4.4/2.5
0.76 2.2/3.1
0.41 -0.7/3.8
0.04 nd
0.48 4.5/2.7
nd nd
0.93 3.7/ 3.5
0.29 1.6/4.0
0.10 -3.1/3.6
nd nd
0.15 nd
0.88 4.7/3.1
0.81 2.2/3.1
0.42 -0.6/3.5
3 ‰ increase in δ15N values of nitrate in older groundwater was caused by denitrification, an increase in the δ18O values of at least 1.5‰ would have been expected. This is not evident from our data (Figure 1), but we caution that the expected increase in δ18O of nitrate due to denitrification is not far outside of the analytical uncertainty of this parameter. On the basis of the chemical and isotopic measurements of nitrate and age structure of the groundwater, our data indicate that the lower nitrate concentrations in the old groundwater are primarily a result of lower anthropogenic N inputs prior to 1920. However, slightly elevated δ15N values in old groundwater (>100 years) during base flow and low nitrate concentrations occasionally below the detection limit (no. 5, 2/2004) coupled with reported evidence for bacterial sulfate reduction in the porous rock matrix of spring no. 1 and nos. 5-7 (38) do also suggest possible denitrification in the fissured-porous aquifer. We hypothesize that some denitrification may have occurred in old groundwater of the fissuredporous karst aquifer with H2S and dissolved organic carbon as possible electron donors for denitrification and O2 consumption, respectively (e.g., refs 49-51). If denitrification occurs predominantly in the porous rock matrix, then the rate-limiting step is the diffusion of nitrate from the preferential flow paths in the fractured aquifer to the more immobile matrix water. The diffusion of nitrate into the porous matrix likely proceeds with negligible isotope frac-
tionation (52), and hence, there will be little isotopic evidence in the remaining nitrate in the fissure water indicating that denitrification has occurred. Additionally, increasing δ15N and δ18O values accompanied by decreasing nitrate concentrations in the more immobile matrix water resulting from denitrification may be masked by high nitrate concentrations in the mobile fracture water. Furthermore, complete reduction of nitrate in the rock matrix results in no detectable isotope effects. Therefore, we hypothesize that nitrate in old groundwater contains little isotopic evidence for denitrification, but there is sufficient thermodynamic and hydrodynamic (matrix diffusion) evidence suggesting that denitrification may have occurred. This study demonstrates that a combination of hydrodynamic, chemical, and isotopic approaches provides unique insights into the sources and the biogeochemical history of nitrate in karst aquifers. Moreover, the data may be incorrectly interpreted to indicate a lack of denitrification without a thorough understanding of the hydrogeological setting (matrix diffusion) and the age structure of the groundwater. Combined hydrodynamic, chemical, and isotopic investigations can yield important information regarding the vulnerability of karst aquifers to nitrate pollution and their selfpurification capacity via the process of denitrification. Such information is essential for assessing future trends in drinking water quality of karst catchments.
Acknowledgments The authors thank V. Mu ¨ller for his support during field work. We are also grateful for the thoughtful comments of three anonymous reviewers that contributed significantly to improve this manuscript.
Supporting Information Available Additional information on the catchment area (Figure S1) and all chemical (Table S1) and mean physical parameters (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 11, 2006. Revised manuscript received August 9, 2006. Accepted August 17, 2006. ES061129X
