Nitrate in groundwater of the midwestern United States: a regional investigation on relations to land use and soil properties

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1 Impact of Land-Use Change on Nutrient Loads from Diffuse Sources (Proceedings of IUGG 99 Symposium HS3, Birmingham, July 1999). IAHS Publ. no. 257, Ill Nitrate in groundwater of the midwestern United States: a regional investigation on relations to land use and soil properties DANA KOLPIN US Geological Survey, 400 S. Clinton Street, Iowa City, Iowa 52244, USA dwkolpin@usgs.gov MICHAEL BURKART National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011, USA DONALD GOOLSBY US Geological Survey, Box 2506, Denver Federal Center, Lakewood, Colorado 80225, USA Abstract The intense application of nitrogen-fertilizer to cropland in the midwestern United States has created concern about nitrate contamination of the region's aquifers. Since 1991, the US Geological Survey has used a network of 303 wells to investigate the regional distribution of nitrate in nearsurface aquifers of the midwestern United States. Detailed land use and soil data were compiled within a 2 km radius of 100 unconsolidated wells in the regional network to determine relations to nitrate concentrations in groundwater. For land use, the amount of irrigated land was directly related to nitrate concentrations in groundwater. For soils, the general water table depth and soil factors associated with rates of water movement were directly related to nitrate concentrations in groundwater. INTRODUCTION Nitrate may be the most widespread contaminant affecting the water quality of the world's aquifers (e.g. Meinardi et al., 1995; Nolan et al., 1997; Zhang et al., 1996). Nitrate can be derived from a variety of natural and anthropogenic sources (Madison & Brunett, 1985) such as septic systems, animal manure, and atmospheric deposition. The most extensive sources of nitrate to groundwater in the United States are the transformation of soil organic matter to nitrate (Schepers & Mosier, 1991) and the application of inorganic nitrogen fertilizer to crops to increase yields (Hallberg & Keeney, 1993). Nitrate is highly soluble and can be readily transported to groundwater. If nitrogen loadings to soils repeatedly exceed what can be used in the system, nitrate concentrations in groundwater can build to problem levels (Steinheimer et al., 1998). Excessive nitrate in drinking water can cause an oxygen deficient condition (methemoglobinemia) in infants (Fan & Steinberg, 1996). For this reason, the US Environmental Protection Agency has established a maximum contaminant level for nitrate at lomgl" 1 nitrate as nitrogen. Ward et al. (1996) suggests that ingesting drinking water with nitrate concentrations of 4 mg l" 1 or more increases the risk of non- Hodgkin's lymphoma in adults. Furthermore, nitrate has been documented to have deleterious effects on amphibians (Hecnar, 1995; Oldham et al., 1997). Nitrate

2 112 Dana Kolpin et al. concentrations in groundwater may impact aquatic ecosystems receiving groundwater discharge. It has been suggested that nitrate concentrations exceeding 2 mg F 1 in groundwater indicate anthropogenic sources of nitrate (Mueller & Helsel, 1996). The midwestem United States is the largest and most intensive crop-producing region of the country. This region comprises about 21% of the Nation's land, but accounts for about 60% of the Nation's nitrogen fertilizer use. Thus, the extensive application of nitrogen fertilizer to cropland in the Midwest has created concern about nitrate contamination of the region's groundwater. In response to this concern, the US Geological Survey (USGS) designed a monitoring network in near-surface (top of aquifer material within about 15 m of land surface) aquifers in the maize- and soybeanproducing regions of the midwestem United States (Kolpin et ah, 1994). These nearsurface aquifers represent hydrogeologic settings most likely to be affected by chemical applications at the land surface. The purpose of this paper is to evaluate the statistical correlations between nitrate concentrations in groundwater and detailed land use and soil data from 100 randomly-selected unconsolidated wells from the USGS regional network. METHODS The original USGS network (Kolpin et al, 1994) consisted of 303 randomly selected wells (from a population of existing production and monitoring wells) located in 12 midwestem states (Fig. 1). Selection criteria for these wells included having at least Base from U.S. Geological Survey digital data, 1:2,000,000, 1972 Albers Equal-Area Conic Projection Standard parallels and 43 30, Central meridian Fig. 1 Location of wells in the USGS groundwater reconnaissance network.

3 Nitrate in groundwater of the midwestern USA: relations to land use and soil properties % of the land use within a 3.2 km radius of the well location being in maize or soybean production during the 1990 growing season. The selection criteria, however, may have decreased the variance (increased homogeneity) in land use surrounding sampled wells. Relations to land use and soil factors may vary among fractured bedrock and unconsolidated deposits. Therefore, for this research it was decided to focus only on wells completed in aquifers that consist of unconsolidated deposits. A stratified, random process was used to select 100 wells completed in unconsolidated aquifers (Fig. 1). Stratification was by state to maintain the broad geographic distribution of wells similar to that present in the original network. The selection strategy greatly reduced the range in well depths encountered, from 2 to 229 m in the original network to 2 to 37 m for the subset of 100 wells for this study. Thus, the selection strategy has not only limited the scope of study to unconsolidated aquifers, but also may have at least partially controlled for the effects of groundwater age (related to depth of sampled groundwater). All water samples were collected by USGS personnel trained in a variety of waterquality sampling procedures. Representative samples were collected after an adequate volume of water was purged from each well (as determined by stable measurements of water temperature, ph, specific conductance, and dissolved-oxygen concentration). Nitrite plus nitrate as N (hereafter, referred to as nitrate) was determined with an automated colorimetric procedure (Fishman & Friedman, 1989). The analytical reporting limit for this method was 0.05 rugi" 1. A series of field blanks and field duplicates verified effectiveness of the sampling protocol. Detailed land use within a 2 km radius of each of the 100 wells in this study was defined on the basis of low-altitude aerial photography. A procedure was developed (Harvey et al., 1996; Kolpin, 1997) to transform the aerial photography into a GIS coverage of detailed land use. Detailed soils data within a 2 km radius of each of the 100 wells in this study were derived from US Department of Agriculture County Soil Survey Maps. Individual map sheets (scales ranged from 1: to 1:24 000) were scanned, converted to GIS coverages, registered to geographic coordinates, and edited where necessary to match the original soil polygons from the map sheets. The various map sheets were merged to produce a 2 km buffer for each well. The soil GIS polygons were attributed with the proper map unit identifier (MUID) (US Soil Conservation Service, 1993). The MUID is used to relate to the soils attribute table containing all the available soils information for that soil. RESULTS Land use The amount of irrigated land within a 2 km radius of a sampled well was directly related (p ; Spearman rank correlation) to nitrate concentrations in groundwater (Fig. 2). The highest nitrate concentrations generally corresponded with the greatest amounts of irrigated land. Previous research also has shown irrigation to increase nitrate transport to groundwater (Hubbard et al., 1984; Timmons & Dylla, 1981).

4 114 Dana Kolpin et al. Irrigation artificially increases recharge to shallow aquifers and thus, potentially increases nitrate transport from the unsaturated zone to the aquifer. The relation between irrigation and nitrate concentration may not be purely causative. Irrigated areas typically are characterized by soils with low water-holding capacities (Hallberg & Keeney, 1993) and higher rates of fertilizer application (Hamilton & Helsel, 1995). Thus, irrigation also could imply hydrogeologic settings with rapid groundwater flow (i.e. rapid nitrate transport) and/or high fertilizer applications (i.e. greater nitrate loadings). Somewhat unexpected, the land-use factors thought to best reflect the amount of fertilizer use (such as amount of maize production) did not show significant relations to nitrate concentrations in groundwater. The relative homogeneity in land use (Kolpin, 1997) for these wells may have caused the lack of significant relations to nitrate. EXPLANATION < to 9.9 (26) O # T X Number of observations Outlier data value more than 3 times the interquartile range outside the quartile Outlier data value less than or equal to 3 and more than 1.5 times the interquartile range outside the quartile Data value less than or equal to 1.5 times the interquartile range outside the quartile 75th percentile Median 25th percentile Nitrate Concentration (mg/l and N) Fig. 2 Relation between nitrite plus nitrate as N (nitrate) concentration in groundwater and amount of irrigated land within a 2 km radius of sampled wells. 2.0 < to 2.0 to > Nitrate Concentration (mg/l as N) Fig. 3 Relation between nitrite plus nitrate as N (nitrate) concentrations in groundwater and general water table depth within a 2 km radius of sampled wells. Symbols as for Fig. 2.

5 Nitrate in groundwater of the midwestern USA: relations to land use and soil properties 115 Soils The general water table depth was directly related (p< 0.001; Spearman rank correlation) to nitrate concentrations in groundwater (Fig. 3). The highest nitrate concentrations generally corresponded to areas with deeper water tables. Initially, this relation may appear opposite to expected results (decreasing nitrate with increasing general water table depth). However, shallow water tables generally reflect poorly drained soils and anaerobic conditions (Fig. 4). Under these conditions, denitrification of nitrate can occur in the presence of organic carbon and denitrifying bacteria (Korom, 1992). Similar trends between water level and nitrate concentration in groundwater have been noted in the literature (e.g. Kolpin et al., 1994; Mueller et al., 1995). General Water Table Depth (m) Fig. 4 Relation between dissolved-oxygen concentration in groundwater and general water table depth within a 2 km radius of sampled wells. Symbols as for Fig. 2. < to 2.0 to > Nitrate Concentration (mg/l as N) Fig. 5 Relation between nitrite plus nitrate as N (nitrate) concentrations in groundwater and amount of soils with slow infiltration rates within a 2 km radius of sampled wells. Symbols as for Fig. 2.

6 116 Dana Kolpin et al. The remaining soil factors significantly related (p < 0.05; Spearman rank correlation) to nitrate concentrations in groundwater were all associated with rates of water movement. Of these, the factor with the strongest relation to nitrate (p < 0.001; Spearman rank correlation) was the area of soils within a 2 km radius of the sampled well with slow (US Soil Conservation Service, 1993) soil-infiltration rates (Fig. 5). These types of soils transport water (and thus, nitrate) at a slower rate than those with faster soil infiltration rates. Furthermore, poorly drained soils tend to be more oxygen deficient, potentially leading to denitrification. A further consequence of slow soil infiltration rates is that these soils are more likely to be artificially drained for improved crop production, diverting nitrate to nearby streams rather than infiltrating to groundwater. REFERENCES Fan, A. M. & Steinberg, V. E. (1996) Health implication of nitrate and nitrite in drinking water: an update on methemoglobinemia occurrence and reproductive and developmental toxicity. Regut. Toxicol. Pharmacol. 23, Fishman, M. J. & Friedman, L. C. (eds) (1989) Methods for the determination of inorganic substances in water and fluvial sediments. US Geol. Survey Techniques of Water Resources Investigations, book 5, chapter Al. Hallberg, G. R. & Keeney, D. R. (1993) Nitrate. In: Regional Ground-Water Quality (ed. by W. M. Alley), Van Nostrand Reinhold, New York. Hamilton, P. A. & Helsel, D. R. (1995) Effects of agriculture on ground-water quality in five regions of the United States. Groundwater 33, Harvey, C. A., Kolpin, D. W. & Battaglin, W. A. (1996) Using a geographic information system and scanning technology to create high-resolution land-use data sets. US Geol. Survey Wat. Resour. Investigations Report Hecnar, S. J. (1995) Acute and chronic toxicity of ammonium nitrate fertilizer to amphibians from southern Ontario. Environ. Toxicol. Chem. 14, Hubbard, R. K., Asmussen, L. E. & Allison, H. D. (1990) Shallow groundwater quality beneath an intensive multiple cropping system using center pivot irrigation. J. Environ. Qual. 13, Kolpin, D. W. (1997) Agricultural chemicals in groundwater of the midwestem United States: relations to land use. J. Environ. Qual. 26, Kolpin, D. W., Burkart, M. R. & Thurman, E. M. (1994) Herbicides and nitrate in near-surface aquifers in the midcontinental United States, US Geol. Survey Wat. Supply Pap Korom, S. F. (1992) Natural denitrification in the saturated zone: a review. Wat. Resour. Res. 28, Madison, R. J. & Brunett, J. O. (1985) Overview of the occurrence of nitrate in ground water of the United States. In: National water summary 1984 hydrologie events, selected water-quality trends, and ground-water resources. US Geol. Survey Wat. Supply Pap Meinardi, C. R., Beusen, A. H. W., Bollen, M. J. S., Klepper. O. & Willems, W. J. (1995) Vulnerability to diffuse pollution and average nitrate contamination of European soils and groundwater. Wat. Sci. Technol. 31, Mueller, D. K., Hamilton, P. A., Helsel, D. R., Hitt, K., & Ruddy, B. C. (1995) Nutrients in ground water and surface water of the United States an analysis of data through US Geol. Survey Wat. Resour. Investigations Report Mueller, D. K. & Helsel, D. R. (1996) Nutrients in the Nation's water too much of a good thing? US Geol. Survey Circular Nolan, B. T Ruddy, B. C, Hitt, K. J. & Helsel, D. R (1997) Risk of nitrate in groundwaters of the United States a national perspective. Environ. Sci. Technol. 31, Oldham, R. S., Latham, D. M., Hilton-Brown, D., Towns, M., Cooke, A. S. & Burn, A. (1997) The effect of ammonium nitrate fertilizer on frog (Rana temporarid) survival. Agric, Ecosystems and Environ. 61, Schepers, J. S. & Mosier, A. R. (1991) Accounting for nitrogen in nonequilibrium soil-crop systems. In: Managing Nitrogen for Groundwater Quality and Farm Profitability (ed. by R. F. Follett, D. R. Keeney & R. M. Cruse), Soil Science Society of America, Madison, Wisconsin. Steinheimer, T. R., Scoggin, K. D. & Kramer, L. A. (1998) Agricultural chemical movement through a field-size watershed in Iowa: subsurface hydrology and distribution of nitrate in groundwater. Environ. Sci. Technol. 32, Timmons, D. R. & Dylla, A. S. (1981) Nitrogen leaching as influenced by nitrate management and supplemental irrigation level. J. Environ. Qual. 10, 421^26. US Soil Conservation Service (1993) State Soil Geographic Data Base (Statsgo) Data Users Guide. Miscellaneous Publ. no Ward, M. H., Mark, S. D., Cantor, K. P., Weisenburger, D. D., Correa-Villasenor, A. & Zahm, S. H. (1996) Epidemiology 7, Zhang, W. L., Tian, Z. X. & Li, X. Q. (1996) Nitrate pollution of groundwater in northern China. Agric. Ecosystems and Environ. 59,