Article pubs.acs.org/est
Quantification of Phosphorus Transport from a Karstic Agricultural Watershed to Emerging Spring Water Per-Erik Mellander,*,† Philip Jordan,‡ Alice R. Melland,†,¶ Paul N. C. Murphy,†,¶ David P. Wall,§ Sarah Mechan,† Robert Meehan,∥ Coran Kelly,⊥ Oliver Shine,† and Ger Shortle† †
Agricultural Catchments Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, County Wexford, Ireland School of Environmental Sciences, University of Ulster, Coleraine, Northern Ireland § Crops, Environment and Land Use Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, County Wexford, Ireland ∥ Talamh Consulting, Navan, County Meath, Ireland ⊥ Tobin & Co. Ltd., Block 10-4, Blanchardstown Corporate Park, Dublin 15, Ireland ‡
S Supporting Information *
ABSTRACT: The degree to which waters in a given watershed will be affected by nutrient export can be defined as that watershed’s nutrient vulnerability. This study applied concepts of specific phosphorus (P) vulnerability to develop intrinsic groundwater vulnerability risk assessments in a 32 km2 karst watershed (spring zone of contribution) in a relatively intensive agricultural landscape. To explain why emergent spring water was below an ecological impairment threshold, concepts of P attenuation potential were investigated along the nutrient transfer continuum based on soil P buffering, depth to bedrock, and retention within the aquifer. Surface karst features, such as enclosed depressions, were reclassified based on P attenuation potential in soil at the base. New techniques of high temporal resolution monitoring of P loads in the emergent spring made it possible to estimate P transfer pathways and retention within the aquifer and indicated small−medium fissure flows to be the dominant pathway, delivering 52−90% of P loads during storm events. Annual total P delivery to the main emerging spring was 92.7 and 138.4 kg total P (and 52.4 and 91.3 kg as total reactive P) for two monitored years, respectively. A revised groundwater vulnerability assessment was used to produce a specific P vulnerability map that used the soil and hydrogeological P buffering potential of the watershed as key assumptions in moderating P export to the emergent spring. Using this map and soil P data, the definition of critical source areas in karst landscapes was demonstrated.
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INTRODUCTION Mitigation of agricultural exports of nutrients to waterbodies, to avert eutrophication risk, generally constrains the use of both nitrogen (N) and phosphorus (P) and is in many countries based in Good Agricultural Practice legislation.1 However, while nutrient mobilization risk is managed through, for example, closed winter periods for nutrient applications to land, the role of nutrient transfer pathways is less well-defined in regulations. Identifying where pollution sources and hydrological connectivity overlap, i.e. critical source areas (CSA),2 is important and may help to improve the effectiveness of measures to mitigate nutrient exports.3 Agriculture can be a widespread source of pollution and is a major concern in groundwater-fed surface waters where the delivery of nutrients to water bodies can occur through the soil−aquifer continuum.4−6 The magnitude of nutrient exports from watersheds and subsequent impacts on receiving water bodies can be defined as a watershed’s nutrient vulnerability. Karst aquifers are seen as particularly vulnerable to contami© 2013 American Chemical Society
nation and in need of special protection as they are important for groundwater-fed surface water ecosystems7 and also important drinking water resources.8 In the European Union, approximately 30% of the land area is underlain by karstified limestone9 and, where chemical monitoring data are absent, the intrinsic vulnerability of karst aquifers to pollution transfer from overlying agricultural land is assumed to be high due to rapid translocation of water from soil surfaces via karst features and conduit networks.10,11 In Ireland, where approximately 19% of the landscape is underlain by karst,10 there is a concern that P in groundwater from karst aquifers may contribute to poor ecology of adjacent surface waters.12 Groundwater in Ireland is considered to be impaired and at “poor status” when an Environmental Quality Standard (EQS) of an annual mean Received: Revised: Accepted: Published: 6111
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concentration of 0.035 mg L−1 of total (molybdate) reactive P is exceeded and when the hydraulic load from that groundwater body exceeds 50% of a receiving surface water body that is at less than “good status”. Specific groundwater vulnerability concepts were developed within the European Union for land use planning and groundwater protection9 and are transferrable to other regions of karst landscape. In contrast to intrinsic vulnerability, specific vulnerability classification takes both the properties of the contaminant and the intrinsic vulnerability of the area into account but is constrained by the nature and depth of data related to the contaminant. This specific classification is useful, for example, when dichotomies arise in terms of intrinsic vulnerability classification and observed water quality.13−15 Using these considerations, this study investigated the reasons why the emergent groundwater in a spring, measured subhourly from a 32 km2 karstic agricultural watershed with a comparatively high soil P source potential (i.e., similar to other regions of intensive agriculture), showed no strong evidence of P pollution associated with agricultural land use. This was despite previous intrinsic vulnerability assessments classifying 97% of the contributing zone as of high to extreme risk of pollution to groundwater.14 Three general physio−chemical precepts were investigated in the current study that were considered to influence the attenuation of P in soils: depth to bedrock,16 soil P buffering,17 and bedrock fissure P buffering.18 These characteristics were compared against 2.5 years of hourly averaged discharge and P concentration data monitored in the emerging spring. The intention was to use insight from these new data to investigate the P transfer potential and develop the intrinsic groundwater vulnerability classification into a specific P vulnerability classification that would take the P attenuation into account.
Figure 1. (a) Cregduff karst watershed with experimental design, (b) County Mayo with rivers and lakes, and (c) location within Ireland (printed under license number 6155 from the Ordnance Survey Ireland).
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Field Monitoring. Water discharge was calculated for the primary spring emergence using an ultrasonic sensor (ThermoFisher time-of-flight area velocity meter) by integrating subhourly water level and velocity measurements within an engineered uniform cross-section. Rainfall was averaged from four ARG100 rain gauges within or close to the watershed (Figure 1a). Two rain gauges were connected to Campbell BSW-200 weather stations and two were connected to Solinst rainloggers. Phosphorus concentrations (TP and TRP) were monitored concurrently using a Hach-Lange SigmataxPhosphax suite of instruments. This type of bankside-analyzer suite21,22 continuously and alternately measures TP and TRP (operationally equivalent to unfiltered molybdate reactive P) by acid digestion and colorimetry using the molybdate−antimony method, with the digestion procedure omitted on the TRP cycle. The detection limit for TP and TRP is 0.01 mg L−1 to 5.00 mg L−1. Hydrochemical data (3−4 measures per hour) were synchronized with spring discharge to hourly averages and totals from which hourly loads were calculated using the WISKI-7 database management system.23 Catchment soils were sampled to assess soil P status as described elsewhere.14 In summary, a composite of 20 subsamples of surface soil (0−10 cm) was taken from 2 ha areas using a soil corer. Each composite sample was analyzed for Morgan P24 which estimates the labile soil P (with soil samples extracted in 1:5 soil solution ratio of 10% sodium acetate buffered at pH 4.8 for 30 min at 20 °C) and is the standard agronomic soil P test in the Republic of Ireland. Soil P attenuation potential (buffering) was characterized for surface soils in a sampling grid with 31 sites by the Mehlich 3-P
EXPERIMENTAL SECTION Study Area. The 32 km2 karst watershed is in Cregduff, County Mayo, western Ireland (Figure 1). The geology consists of a medium to thick bedded pure Carboniferous calcarenite (calcareous sedimentary rock) limestone overlain by relatively thin glacial till deposits (0−5 m) which thin out toward the west.19 The topography is gently undulating and characterized by numerous karst features such as springs, sink holes, ephemeral streams, losing streams, dolines (enclosed depressions of varying morphology and the most frequent feature), epikarst windows (exposed epikarst), superficial solution features, and turlough areas (seasonal groundwater-fed lakes).14 Soils in the area are dominated by shallow Brown Earths (Cambisols) and Rendzinas (Leptosols) with Gleys (Gleysols) and Peats (Histosols) in areas with till and/or which are prone to seasonal standing water. The main adjacent river is the Robe, with a 320 km2 watershed area,20 in which there is considerable interaction with the regional groundwater (see Supporting Information (SI)). The area has a cool, temperate maritime climate with annual mean air temperature of 12.6 °C and mean annual rainfall of 1203 mm (1981−2010 mean, Met Éireann, Irish Meteorological Service). Land use is dominated by permanent pasture (92%) that is typically grazed for 7−8 months by sheep and cattle with an average stocking rate equivalent to ca. 120 kg organic N ha−1 (range 12−250 kg organic N ha−1, equivalent to 2−38 kg TP ha−1). In winter, animals are kept indoors and manure is stored as directed under the EU Nitrates Directive National Action Programme (NAP).1 6112
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Saturation Ratio (M3PSR) method which uses a ratio of P to iron and aluminum.25,26 Additionally, a soil identification and classification survey was conducted within the watershed; five soil catenas (sequences of soil types) were surveyed, comprising 23 soil profiles in total. Phosphorus attenuation potential was characterized for dolines, the most prevalent karst feature, using a detailed classification of the geomorphology in conjunction with depth of soil at the base and the surrounding soil depth. In the present study the depth to bedrock map was refined with a new category of 8 mg L−1 Morgan P for grassland); the index class which is considered unnecessarily high for agronomic production and is, therefore, an environmental risk in terms of P loss to water.27 Twenty six percent of fields were in Index 3 (5.1−8.0 mg L−1 Morgan P for grassland), considered the agronomic optimum range, and 51% were considered below optimum (Index 2; 34%) or deficient (Index 1, 17%) for grassland production. Compared with three other grassland-dominated study watersheds in Ireland, this distribution of fields classified as P index 4 was high28 and indicated a relatively high potential for the soil to release P through leaching. For example, two of the other study grassland watersheds had 6 and 14% of soils in Index 4, while a third had 26%. This latter watershed was dominated by intensive dairy production systems, by Irish standards, with high stocking rates and fertilizer inputs. The area of the watershed with a soil depth of 8 mg L−1) and high vulnerability are identified as high risk, areas of soil P Index 3 (5−8 mg L−1) and high vulnerability are identified as moderate risk, and all other areas are identified as low risk. This CSA map highlights areas of high risk for P transfer to the aquifer and suggests where measures could be targeted for greater effect. Across the watershed, this CSA approach identified 2% of the sampled farmed land at high risk and 4% at moderate risk. It should be noted that, as with the existing intrinsic groundwater vulnerability classification, this proposed specific P vulnerability classification uses simple categories of risk assessment based on conceptual models of nutrient loss and attenuation processes in a karst landscape combined with (at least) data on source pressures. At the watershed or contributing area scale, this is only validated by the magnitude of total spring exports and will require further research on the P buffering of individual soil and hydrogeological components. Importantly, however, the assessment includes all concepts of the P transfer continuum from source to delivery and can be used to modify expectations of risk and focus management efforts in karst landscapes sensitive to nutrient loss and eutrophication.
Figure 6. An example of how (a) soil phosphorus status and (b) specific groundwater phosphorus vulnerability can be used to produce a (c) critical source area map for phosphorus transfer to groundwater over a section of the karst watershed.
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ASSOCIATED CONTENT
S Supporting Information *
Information on the study area, depth to bedrock, and soil profiles (including one figure and one table). This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; phone: +353 (0)53 917 1315. Author Contributions ¶
The manuscript was written through contributions of all authors. A.R.M. and P.N.C.M. contributed equally.
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Notes
artificial dualtracer experiment with caffeine. Water Res. 2012, 46 (16), 5381−5388. (16) White, R. E. Retention and release of phosphate by soil and constituents. In Soils and agriculture: Critical Reports on Applied Chemistry; Tinker, P., Ed.; 2 Soc. Chem. Industry; Blackwell: London, 1980. (17) Holford, I. Phosphate behaviour in soils. Agric. Sci. 1989, 2, 15− 21. (18) Price, R. M.; Savabi, M. R.; Jolicoeur, J. L.; Roy, S. Adsorption and desorption of phosphate on limestone in experiments simulating seawater intrusion. Appl. Geochem. 2010, 25 (7), 1085−1091. (19) Drew, D. P.; Daly, D. Groundwater and Karstification in Mid Galway, South Mayo and North Clare; Geological Survey of Ireland Report series 93/3 (Groundwater); 1993; ISSN: 0790-0279. (20) Kelly, F.; Champ, T.; McDonnell, N.; Kelly-Quin, M.; Harrison, S.; Arbuthnott, A.; Giller, P.; Joy, M.; McCarthy, K.; Cullen, P.; Harrod, C.; Jordan, P.; Griffiths, D.; Rosell, R. Investigation of the Relationship between Fish Stocks, Ecological Quality Ratings (Q-values), Environmental Factors and Degree of Eutrophication; EPA, Ireland, 2007; Vol. 6, pp 99−115. (21) Jordan, P.; Arnscheidt, J.; McGrogan, H.; McCormick, S. Characterising phosphorus transfers in rural catchments using a continuous bank-side analyser. Hydrol. Earth Syst. Sci. 2007, 11 (1), 372−381. (22) Cassidy, R.; Jordan, P. Limitations of instantaneous water quality sampling in surface-water catchments: Comparison with nearcontinuous phosphorus time-series data. J. Hydrol. 2011, 405 (1−2), 182−193. (23) Kisters, A. G. Wiski 7 Water Management Software; Kisters Pioneering Technologies: Aachen, Germany, 2011. (24) Morgan, M. F. Chemical Soil Diagnosis by the Universal Soil Testing System. Conn. A.E.S. Bull. 1941, 450. (25) Sims, J. T. Soil test P: Mehlich 3. In Methods of Phosphorus Analysis for Soils, Sediments, Residuals and Waters; Pierzynski, G. M., Ed.; North Carolina State University: Raleigh, NC, 2000; Vol. 396. (26) Maguire, R. O.; Sims, J. T. Measuring agronomic and environmental soil phosphorus saturation and predicting phosphorus leaching with Mehlich 3. Soil Sci. Soc. Am. J. 2002, 66 (6), 2033−2039. (27) Tunney, H. Phosphorus needs of grassland soils and loss to waters. In Agricultural Effects on Ground and Surface Waters: Research at the Edge of Science and Society; Steenvoorden, J., Claessen, F., Willems, J., Eds.; IAHS 273 CEH, Wallingford, England, 2002; pp 63−69. (28) Wall, D. P.; Murphy, P. N. C.; Melland, A. R.; Mechan, S.; Shine, O.; Buckley, C.; Mellander, P.-E.; Shortle, G.; Jordan, P. Evaluating nutrient source regulations at different scales in five agricultural catchments. Environ. Sci. Policy 2012, 24, 34−43. (29) Daly, D.; Warren, W. P. Mapping groundwater vulnerability: The Irish perspective. In Groundwater Pollution, Aquifer Recharge and Vulnerability; Robins, N. S., Ed.; Geological Society: London, 1998; Special publication 130, pp 179−190. (30) Daly, D.; Dassargues, A.; Drew, D.; Dunne, S.; Goldscheider, N.; Neale, S.; Popescu, I. C.; Zwahlen, F. Main concepts of the “European approach” to karst-groundwater-vulnerability assessment and mapping. Hydrogeol. J. 2002, 10 (2), 340−345. (31) Fitzsimons, V.; Daly, D.; Deakin, J. GSI Guidelines for Assessment and Mapping of Groundwater Vulnerability to Contamination; Draft; GSI: Dublin, June 2003. (32) DoELG/GSI/EPA. Groundwater Protection Schemes Guidelines; Department of the Environment and Local Government, Environment Protection Agency and Geological Survey of Ireland, 1999. (33) Melland, A. R.; Mellander, P.-E.; Murphy, P. N. C.; Wall, D. P.; Mechan, S.; Shine, O.; Shortle, G.; Jordan, P. Stream water quality in intensive cereal cropping catchments with regulated nutrient management. Environ. Sci. Policy 2012, 24, 58−70. (34) Mellander, P.-E.; Melland, A. R.; Jordan, P.; Wall, D. P.; Murphy, P. N. C.; Shortle, G. Quantifying nutrient transfer pathways in agricultural catchments using high temporal resolution data. Environ. Sci. Policy 2012, 24, 44−57.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is a part of the Agricultural Catchments Programme funded by the Irish Department of Agriculture, Food and the Marine. We thank the Irish Department of Environment, Community and Local Government for additional support with mapping and tracing work. We acknowledge farmers for cooperation and access to their land. We thank Mr. Donal Daly from the Irish Environmental Protection Agency for valuable discussion and suggestions regarding the specific P vulnerability map. Discharge data were provided by Environmental Protection Agency hydrometric staff.
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