Environ. Sci. Technol. 1998, 32, 1048-1052
Agricultural Chemical Movement through a Field-Size Watershed in Iowa: Surface Hydrology and Nitrate Losses in Discharge
LARRY A. KRAMER
surface water component may be responsible for transport of the largest portion of fertilizer and pesticide chemicals load from the field into water resources. From a water quality standpoint, concerns remain about how much dissolved N moves from the soil surface via runoff and what the off-site effects are on human health and on populations of aquatic biota (1). Many reports, recently summarized in comprehensive reviews, document that conservation tillage practices do influence N losses in surface runoff (2, 3). In this paper, we illustrate another aspect of the “water quality profile” conceptual model (4) for assessing impacts of farming systems using an analysis of the surface hydrology and nitrate distribution in surface runoff, headcut seepage, and basin drainage from a high relief landscape.
USDA-ARS, NSTL, Deep Loess Research Station, 28498 Beechnut Road, Council Bluffs, Iowa 51503
Site Description
THOMAS R. STEINHEIMER* AND KENWOOD D. SCOGGIN USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, Iowa 50011
Nonpoint source pollution of surface water by nitrate from agricultural activities is a national problem. An agricultural watershed in the Iowa Loess Hills with a 23year history of annual corn production with average N fertilization is studied. Headcut seepage is transported through a natural riparian zone and observed as weir baseflow; surface runoff is measured separately. Nitrate runoff graphs illustrate the importance of high-frequency sampling of each event to permit quantitative estimation of chemical loss. The concentration of nitrate carried from the field in basin drainage steadily increased from 20 mg L-1 in 1991. The rate of cumulative increase in the amount of applied N is greater than the rate of removal by the crop. Over the 23-year record, 23% of the mean annual application of N remains stored and available for leaching or chemical conversion by soil microbes. Nitrate removal during early spring snowmelt surface runoff shows a diurnal pattern that corresponds to the daily freezing and thawing of the surface soil in early March. Contribution to the load of nitrate deposited on the soil surface by rainfall is very small in comparison to the amount applied by fertilizer application. Measurable changes in water quality within various hydrogeologic compartments are seldom observed in just a few years of monitoring. Therefore, these results emphasize the importance of long-term data sets incorporating temporal variability when evaluating the impact of agricultural practices on surface water resources.
Introduction Most studies addressing surface water issues on agricultural landscapes have emphasized runoff hydrology and/or soil loss by water erosion. With the exception of the eastern United States, more recent studies addressing water quality issues associated with farming practices have placed an emphasis on the groundwater component. However, on many agricultural landscapes of relatively high relief that are impacted by heavy rainfall, the surface water quality must also be considered. In the Loess Hills of western Iowa, the * Corresponding author phone: 515-294-2952; fax: 515-294-8125; e-mail:
[email protected]. 1048 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 8, 1998
Field Characteristics. The general field conditions and the loess soil properties have been described previously (4). Topography. Figure 1 illustrates a cross-section in landscape at the Deep Loess Research Station near Treynor, IA, and how it affects hydrogeologic boundary conditions that delimit these small watersheds. They range in cultivated areas between 30 and 60 ha. Watershed 3 (W3) consists of approximately 40.5 ha (37.6 ha cultivated) that has been under ridge tillage with continuous corn since 1972. The topography of W3 dictates its drainage pattern for both direction and magnitude of surface water movement. Total elevation relief is more than 25 m. The soil association is MononaIda-Napier with the Monona predominating on the ridge tops and upper side slopes, the Ida predominating on the steeper side slopes, and the Napier predominating on the toe slopes and in the valleys. At the base of W3 in an incised gully about 200 m below the seepage face (headcut), the outlet is instrumented with a broadcrested V-notch weir and water-stage recorder, which continuously measures storm surface runoff from the field and seepage/spring flow from the gully. The top of the headcut lies approximately 3-4 m below field elevation with a depth of 4-6 m. Vegetation in and along the incised gully varies from perennial grasses to mature trees. The gully bottom with a slope of about 2% is mainly covered with reed canarygrass (Phalaris arundinacea), bromegrass (Bromus tectorum), and nettles (Urtica dioica). Young willow trees (genus Salix) predominate on the upper third of the gully and are also found on the bottom, side walls, and tops of the banks. Downstream are older trees: large cottonwoods (Populus deltoides) and American elms (genus Ulmus) to medium size mulberry (genus Morus) and willows (genus Salix). Sumacs (genus Rhus) grow along the banks. Reeds canarygrass, bromegrass, and nettles are also found along the banks and sidewalls not shaded by the trees. Approximately 94% of the watershed is cropped. Perennial grass waterways are located in the main valley drainage way and on some valley side slopes where surface runoff flow concentrates.
Experimental Section Meteorological Monitoring. Since 1964, precipitation across W3 has been measured with three universal recording rain gauges (Belfort Instrument Co., Baltimore, MD) located along the northern, southern, and western boundaries. Beginning in 1965 pan evaporation and wind run were observed during the growing season at a weather station along the western boundary. In 1991, the weather station was automated to record hourly air temperature and soil temperature at 10, 20, 50, and 100 cm depths. S0013-936X(97)00728-1 Not subject to U.S. copyright. Publ. 1998 Am. Chem.Soc. Published on Web 02/28/1998
FIGURE 1. Surface topography showing the riparian zone for watershed 3 and its relationship to the sampling locations for surface runoff, headcut seepage, and discharge at the weir. Surface Water Sampling. W3 is instrumented for collection of rainwater, surface runoff, headcut seepage, and basin drainage. Rainwater samples were collected in 19-L buckets positioned on a wet-dry Model 301 precipitation collector (AeroChem Metrics, Inc., Bushnell, FL) located at the southwestern corner of W3. With the onset of a surface runoff event, automatic sampling of surface water begins and continues at 10-min intervals for a maximum of 4 h using a Model 3700FR refrigerated automatic sampler (ISCO Environmental Division, Lincoln, NE). The intake is positioned in the drainage waterway along the thalweg of the valley approximately 3 m above the elevation of the incised channel at the headcut (see Figure 1). Perennial spring discharge at the headcut (headcut seepage) and discharge at the weir (weir seepage baseflow) are sampled monthly. All grab samples were collected in prepared bottles, stored in insulated carriers, and promptly refrigerated on site. Following transport to the laboratory, holding time for water samples never exceeded 14 days before analysis; typically, they were analyzed within 3-5 days of receipt. All nitrate determinations employed the methods previously described (4). These include steam distillation TKN, ISE, and flow-injection colorimetry.
Results and Discussion Rainwater. Atmosphere-borne N contributes to the nitrate loading of W3 as washout from rainfall. Between 1971 and 1973, mean annual nitrate concentrations in rainwater collected at two sites near Treynor, IA, ranged from 0.32 to 0.69 mg L-1, with over 70% deposited during spring and summer months (5). This may be due to seasonal agronomic activity corresponding to N application and attendant loss to the atmosphere. In more recent analyses of rainwater
samples collected during the growing seasons of 1992-1994, the concentration of nitrate averaged 1.8 mg L-1. The highest concentration of 3.8 mg L-1 was measured in the spring, usually within 30-45 days of fertilizer application at planting. Annual contribution of nitrate from precipitation is approximately 7 kg ha-1 (4.2% of mean annual N loading). This has implications for agricultural production operations, which may be a contributing factor to changing global climate. Chemical forms of oxidized N, other than nitrate, produced by atmospheric chemical processes have not been measured. Generally, the amount of nitrate-N added to the soil by precipitation is very small as compared to that added as fertilizer. Headcut Seepage and Riparian Zone Effect. The contribution of intensive annual N fertilization to the chemical quality of water leaving W3 is shown in Figure 2. Fertilization averaging about 168 ( 28 kg of N ha-1 yr-1 over 26 years has resulted in a widely distributed nitrate load that is continuously flushed from the field as an integrated flow composed of perennial seepage and event surface runoff. This rate is the typical agronomic recommendation for corn production in Iowa. The early record of cultivation and fertilizer use shows relatively little impact since the concentration of nitrate leaving the field between 1968 and 1978 never exceeded 10 mg/L. Beginning around 1978-1980, the cumulative effects of excessive fertilization become apparent. Substantial increases in nitrate concentration are seen in the weir seepage baseflow. This excess N is not utilized by the crop, incorporated into humus as organic N, or lost through volatilization. As a consequence of groundwater recharge from percolated rainfall, this stored N is slowly transported from the field, largely as subsurface flow. Thus, observations at the seepage face/headcut represent an integration of all pulsed flows together with contributions from the storage VOL. 32, NO. 8, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Monthly weir seepage baseflow hydrograph and nitrate concentration in discharge from W3; 1968-1993. within the unsaturated loess. The measured value in 1969 was only 0.7 mg L-1 (6). Between 1964 and 1967, prior to the use of fertilizers in the watershed, nitrate concentrations in combined baseflow/surface runoff probably never exceeded 1 mg L-1. The recent higher concentrations are the direct result of the influence of long-term N fertilization on soil-N transformations. Monthly sampling for nitrate of W3 drainage at the weir began in 1969. During the first 4 years, the monthly concentration steadily increased from 5 to 10 mg L-1; by 1977 it had increased to 15 mg L-1 and was 23 mg L-1 by the end of 1993. The headcut and the weir are separated by approximately 200 m of vegetation growing along and within an incised channel. This represents a riparian zone with the potential to function as an environmental buffer by restricting the loss of nitrate from the field. Buffering processes may include both plant assimilation and denitrification as well as physical trapping (7-10). Concurrent samples from the headcut and the weir were analyzed for the period 19811989 to determine the effectiveness of this area. Generally, the differences between headcut seepage and weir seepage baseflow concentrations correlated negatively with rate of discharge. Thus, at lower base flow rates, the riparian zone removed more nitrate than during higher base flow. The capacity of the vegetation to either remove or contribute to the nitrate load in this small stream system is related to its seasonal growth, which is influenced by rainfall input. However, the mechanisms controlling “riparian zone function” as a nitrate scrubber are very complex and not well understood. Between 1981 and 1982, the mean weir concentration was 14.7 mg L-1, and the riparian buffer was effectively removing 1.4 mg L-1 from headcut concentrations. During 1983-1989, while the weir concentration increased to 18.8 mg L-1, the apparent capacity for removal was exceeded, resulting in an increase of 2.9 mg L-1 above headcut concentrations. Thus, the riparian zone appeared to be releasing more nitrate into the drainage than that emerging from the headcut. It appears that the size and water displacement functions of this mature riparian gully are 1050
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insufficient to substantially reduce nitrate concentration, even under low flow conditions. At present, the flow is too high and the plant retention/uptake is too low. Surface Runoff. Fertilization of W3 with combinations of anhydrous ammonia, urea, and ammonium nitrate began in 1968. By 1971, the mean annual N load leaving W3 in surface flow was
