Environ. Sci. Technol. 1998, 32, 1039-1047
Agricultural Chemical Movement through a Field-Size Watershed in Iowa: Subsurface Hydrology and Distribution of Nitrate in Groundwater THOMAS R. STEINHEIMER* AND KENWOOD D. SCOGGIN USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011 LARRY A. KRAMER USDA-ARS, NSTL, Deep Loess Research Station, 28498 Beechnut Road, Council Bluffs, Iowa 51503
A 40-ha field is under study in the loess hills of southwestern Iowa to determine the impact of corn production in ridge-tilled soils on the nitrate-nitrogen loading in groundwater. Within the vadose zone, nitrate concentration between June 1989 and December 1991 ranged from 80 mg/ L. Well water concentrations increased from 60 mg/L in 1994. In both hydrogeologic compartments, time of sampling and landscape position are important factors influencing concentrations. The unsaturated zone groundwater system has a high potential for storage of unutilized nitrogen as nitrate. Leaching resulted in the drinking water MCL being exceeded for several wells screened within the saturated loess, which is characterized by relatively high hydraulic conductivity. Concentrations within and below the loess-glacial till interface did not exceed the standard. A conservative solute transport model was used to predict the concentration of nitrate exiting the field in basin drainage. Denitrification in which nitrate is reduced to nitrite by autotrophic bacteria and then further reduced geochemically to nitric oxide, nitrous oxide, or nitrogen may be an important mechanism for reducing the nitrate concentration within selected landscape positions, especially those in near proximity to the water table. Due to its relatively rapid conductance of both water and applied agchemicals, the loess hills represent a vulnerable agricultural landscape on which nitrogen fertilization impacts groundwater quality.
Introduction Agribusiness is the largest sector in the U.S. economy accounting for more than 30% of the Gross National Product. One factor responsible for such growth has been the successful application of chemical-based fertilization and pest-control strategies to farming. In 1991, Congress funded the Presidential Initiative to Enhance Water Quality that was implemented nationwide. Two central objectives are to measure the impact of farming systems on groundwater and * Corresponding author fax: 515-294-8125; e-mail: steinheimer@ nstl.gov. S0013-936X(97)00598-1 Not subject to U.S. Copyright. Publ. 1998 Am. Chem. Soc. Published on Web 02/28/1998
surface water chemistry and other agroecosystem resources and to identify the factors and processes controlling the fate and transport of fertilizers and pesticides. In Iowa and four other midwestern states, field research is conducted under the Management System Evaluation Area (MSEA) Program, a federal interagency, state, academia, cooperative study of best management practices (BMP) and water quality. In Iowa, BMP are defined in the context of combinations of tillage, crop rotation, or sequencing and both fertilizer and pesticide usage; whereas, in NE and MN, BMP also include water application. Within the Iowa MSEA program, three areas are under study that represent diverse scales, landscapes, soil associations, and management practices. In each, fields under controlled agronomic practices are extensively instrumented for monitoring environmental conditions. Throughout the year, both soil and water samples are collected for laboratory determination of chemical residues as the legacy from both fertilizer and pesticide-derived formulations. Detailed studies of the extent of agrichemical contamination across varied geohydrologic settings in the upper midwest are best described only in recent reports (1-5). When enhanced with long-term records of weather patterns and agronomic practices, the environmental impact at the field scale of a farming system can be defined in terms of a “water quality profile” (WQP). This conceptual model defines water quality in terms of a common suite of chemical properties together with the fate and distribution of agchemicals in each hydrogeologic compartment. The impact of the farming system on the environment can be quantified in terms of the cropping system, tillage practice, and fertilizer/pest management chemicals strategy. In this paper, we illustrate one aspect of the WQP using the analysis of the subsurface hydrology and nitrate distribution beneath continuous ridgetill corn production that has been in place on a research watershed since 1972. Water movement patterns and nitrate concentrations in both the unsaturated and saturated zones of the groundwater system are presented. Site Description. The Deep Loess Research Station (DLRS), a field station of the National Soil Tilth Laboratory, is located 3.9 km southwest of Treynor in Keg Creek township, Pottawattamie County, in southwestern Iowa. Established in 1964, the station consists of four small watersheds within the Deep Loess Hills of the Central Feed Grains and Livestock Major Land Resource Area (MLRA). This MLRA encompasses an area of more than 1.3 × 105 km2 (6) across Indiana, Iowa, Illinois, and Missouri. Watershed 3 (W3) consists of a 40-ha field that has been under continuous corn production since 1972. The Deep Loess Hills of Iowa and Missouri are characterized by gently sloping ridges, steep side slopes, and well-defined alluvial valleys often with incised channels that usually terminate at an active gully head, commonly referred to as the “headcut”. At the base of the headcut, upgradient groundwater emerges at the surface as spring seepage. Slope gradients of 2-4% on the ridges and 12-16% on the side slopes are common. Loess-derived soil is relatively porous, with low bulk density, and easily eroded. Thickness of loess material within 3-16 km of the Missouri River valley is typically >20 m, with some deposits of 50-60 m recorded locally (7). Ridge-tillage is a farming practice in which 6-10 in. ridges of soil are built up over the seedbed in 38-in. width corn rows. The result is the creation of microterraces along the landscape contour, which increase infiltration and decrease erosion. Ridge-tillage is a soil conservation practice that leaves more than half of the plant residue on the field each year. Figure 1 illustrates a representative cross-section VOL. 32, NO. 8, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Landscape profile and well water sampling placements on watershed 3 at the Deep Loess Research Station, Treynor, IA. of the watersheds at the DLRS together with the landscape profile and well water sampling placements for watershed 3.
Experimental Section Groundwater Sampling. Vadose-zone water is retrieved from ceramic porous-cup suction lysimeters installed at four locations near existing wells and clustered at depths ranging from 2.4 to 8.5 m and with 0.6 m horizontal spacing. These samplers, which have been in place since 1989, are installed at positions to capture water movement below the root zone in ridge top, side slope, and valley landscape positions. Lysimeters are sampled monthly, except in the winter. Well water is collected from 5 cm i.d. iron pipe open piezometers installed within the loess. Five installations at ridge top, side slope, and toe slope positions were completed between September 1964 and July 1969. Additional wells with 5 cm i.d. plastic casing and screens were installed through the saturated loess and into the glacial till at two locations in October 1991. Groundwater levels representative of the water table are measured to an accuracy of (0.03 m. Well water samples are collected monthly, except in the winter, in accordance with U.S. Geological Survey protocols for purging, sampling, storage, and transport of samples for groundwater quality studies (8). Spring seepage below the headcut is manually sampled monthly. Ammonium and Nitrate Ion Determination. Methods used during more than 25 years of analysis of groundwater from W3 for nitrate and ammonium ion included distillation/ titration (9), flow-injection colorimetry (10), and ammoniumand nitrate-specific ion-selective electrode (ISE). ISEresponse baseline data were corrected to reference values obtained by autoanalyzer measurements. The ISE for nitrate determination expresses a much poorer response for nitrite present in the same sample but does not distinguish between 1040
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the two anions. Ammonium ion determinations were made at least monthly between 1968 and 1983 and extrapolated from 1983 through 1990. While several different methods were used to generate the 25-year record, all are comparable (11). Depending upon the matrix background, the minimum detectability by each method was approximately 0.5 µg/mL at signal-to-noise ratio >3. Data Analysis. Hydrologic records of precipitation, runoff, and groundwater level have been maintained continuously at the research station since the mid-1960s. Frequency analysis of monthly water table measurements were used to plot the periodicity of groundwater-level responses across this field. The measurements were autoand cross-correlated (12), and statistical factors were computed using Minitab Statistical Software (v8). Discrete Fourier transform analysis on this stationary time series was done using Mathematica (v2) without detrending or filtering. Modeling of groundwater flow was done by finite difference 3-D transient flow simulation using JDB-3D (JDB, Menlo Park, CA) and transport of nitrate by the method of characteristics using JDB-MOC (JDB, Menlo Park, CA).
Results and Discussion Hydrology. Major components of the long-term water budget are precipitation, evapotranspiration, infiltration, percolation, recharge, and discharge. As used here, infiltration is considered the difference between rainfall and runoff; percolation is the difference between infiltration and evapotranspiration; and recharge is percolation reaching the water table. Factors that greatly influence surface and groundwater flow patterns are amount, seasonality (timing), and intensity of precipitation. The box plot in Figure 2a for the monthly precipitation, averaged over 30 years, is weakly bimodal with maxima occurring during May-June and again during August-September. Historically, July represents the driest
FIGURE 2. Box plots for W3 of (a) annual precipitation by month, 1964-1993; (b) annual pan evaporation by month, 1966-1993; (c) annual infiltration by month, 1965-1993. month of the crop growing season. Wet precipitation is the only source of recharge into the groundwater system. Pan evaporation data were collected as an estimator of water loss to the atmosphere from both soil and plant surfaces by evapotranspiration. Evaporation of water from a standardized pan is generally greater than that from a well-wetted vegetative surface because of the pan’s larger exposure and its lower reflectance of solar radiation. Thus, a large disparity between calculated pan evaporation and actual water loss
from a soil or plant surface may exist. Figure 2b shows the 26-year record of monthly pan evaporation data calculated from daily measurements for the April-October period. Maximum potential evaporation occurs during June-July. This loss process reduces the amount of water available for percolation. The 30-year record of infiltration (precipitation less surface runoff) is shown in Figure 2c. The similarity of the precipitation and infiltration records indicates that most VOL. 32, NO. 8, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Groundwater elevation response to monthly wet precipitation on W3, 1972-1991. rainfall infiltrates into the soil, especially during the warmer months of the growing season. Mean monthly infiltration was about 88% of precipitation during May-September; whereas, during December-February it was only about 2%. Estimated annual percolation was about 15% of annual precipitation beneath grass on watershed 3 in 1969 and about 20% beneath corn on watershed 4 in 1970 (13). Groundwater movement patterns are determined by the hydraulic conductivity of the aquifer material and by the landscape position defining the gradient. As illustrated in Figure 1, both the till surface and the shallow groundwater table generally conform to the surface topography (14, 15). The unsaturated zone of the loess material varies in thickness from 2 to 20 m with an average wetting-front velocity on the order of 10-3-10-4 m s-1. These values are computed from time-domain reflectometry, ponding-infiltration tests, and cross-correlation of groundwater table lag measurements. Well slug tests resulted in hydraulic conductivity of approximately 10-5 m s-1 above the loess-till interface and 10-7 m s-1 below it (Figure 1). Annual corn cropping together with the precipitation cycle result in periodic recharge impulses to groundwater. The shallow wells on W3 exhibit different degrees of response to percolation as compared to the deeper wells. Figure 3 illustrates the 20-year record of monthly precipitation and groundwater level measurements recorded for W3. Water levels in wells with greater depth to groundwater (W3-1, W3-4, W3-6) show negligible short-term change in response to either seasonal or event-driven percolation. However, these wells show a long-term response to seasonal percolation. Time-series analysis suggests the ridge top wells exhibit water level response frequencies of 100, 66, and 40 months. These results would be associated primarily with hydraulic 1042
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gradients controlling percolation to and flow from groundwater under the ridges. The shallower wells (W3-2 and W3-3) in the valley landscape positions show a strong response to precipitation at an 11-month frequency. The shallow groundwater at these wells receives percolation from overland stormwater flow and direct infiltration of local rainfall as well as groundwater inflow from upgradient storage in this 8-14% sloped landscape. Frequency analysis of the shallow well groundwater levels also indicated response frequencies of 100 and 40 months. These results are influenced by contributions from the upgradient storage in addition to local percolation and drainage. JDB-3D was applied to the landscape of W3 (13) in order to estimate the time required for a parcel of groundwater to move across the field and reach a discharge point. Input parameters included rainfall, headcut discharge flow rates, and estimates of hydraulic conductivity across the watershed. Simulation results from JDB-MOC showed that just under 10 years is required for a parcel of saturated-zone groundwater traversing from the most remote and highest elevation release point to reach the headcut. This is consistent with the 100-month average water table recharge frequency computed for the wells located within ridge-top positions. Groundwater movement is directly related to the slope of the groundwater table gradient and the distance to the nearest discharge point. Nitrate Distribution. N Fertilization Record. Long-term records of early land use together with more recent agronomic practice on W3 indicate that the field was seeded to bromegrass and used as pasture for grazing from 1964 to 1971. Nitrogen fertilization began in 1968 and has averaged 168 kg of N ha-1 as an annual application in the spring (19681994). The watershed was planted to corn each year since
FIGURE 4. Annual nitrogen fertilizer application on W3, 1964-1994. 1972. Figure 4 shows the 27-year record of annual amount and chemical form of N fertilizer applied to W3. Unsaturated-Zone Water. The rainfall record for the period of June 1989-December 1991 and the concentration of nitrate in pore water from each lysimeter site are shown in Figure 5. Monthly samples were taken each year during the growing season between April and October. Subsurface isoconcentration plots result from linear extrapolation of monthly concentration data for each lysimeter depth at each landscape position. Superimposed above is the corresponding rainfall record. Nitrate concentration from the two ridgetop positions (L3-4 and L3-6) ranged from 15 to 80 mg L-1 for the 30-month period. At both sites, the maximum concentration at the 6-8 m depth was nearly 2 times greater than that at the 2-4 m depth. The volume of water recovered in sampling was 60% less at the 7 m lysimeter depth versus shallower placements, indicating a zone of restricted conductivity due to a Sangamon paleosol unit (15). The high concentration of nitrate at the 6-8 m depth in the unsaturated loess of the ridge top suggests that substantial leaching occurs beneath these landscape positions. The mid-valley, toe-slope position (L3-3) generally shows the highest concentration nearer the surface at the 3 m depth. At L3-3 the soil-pore water nitrate concentration was appreciably lower than that at the ridge tops at the 4-6 m depth. The lysimeter nest in the valley position (L3-2), which is nearest to the shallow water table, reveals the lowest nitrate concentration. At depths below the root zone, the distribution of nitrate in soil pore water varies by landscape position. Maxima at depths below 6 m on both ridge tops suggests that soil pore water is a major source of the nitrate-nitrogen load transported to the headcut in groundwater. During the past 25 years, numerous studies have shown a direct relationship between nitrate concentration in groundwater and nitrogen fertilization rates and/or fertilization history on agricultural landscapes (16). Nitrate concentration in groundwater under forest, unfertilized (or lowlevel fertilized) landscapes, pastures, meadows, and grasslands are generally cited as 100 mg/L. Denitrification Processes. Mitigation of the nitratenitrogen load in soil and groundwater may be achieved through combinations of heterotrophic denitrification, autotrophic denitrification, or dissimilatory nitrate reduction to ammonium ion. Heterotrophic and autotrophic bacteria are distinguished by their source of electron-donor species; heterotrophs requiring organic carbon and autotrophs utilizing inorganic species. Dissimilatory nitrate reduction produces ammonium ion as the end product. It is regulated by oxygen and usually only observed in electron-rich environments (17, 18). This process is presumed to be unimportant because no increase in ammonium ion concentration in baseflow leaving W3 was observed. Even though ammonium ion is not very mobile in soils, it could have been produced but not transported. In addition, the low organic carbon content in the loess does not suggest an electron-rich environment. There is no published information describing the microbial ecology of loess that enumerates denitrifying bacteria and very few reports discussing the mineralogy or the chemical composition of loess soil. However, measurement of nitrous oxide concentrations above ambient levels in several soil gas wells installed on W3 during the early 1990s does offer presumptive evidence for some denitrification. Heterotrophic denitrification, with its requirement for soluble and bioavailable organic carbon, could remove nitrate from the soil pore water. However, organic carbon content in the deep loess is low, 2-3% at the surface, declining rapidly to
