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National Soil Tilth Laboratory, USDA Agricultural Research Service, 2150 Pammel Drive, Ames, IA 50011
* Corresponding author (tomer{at}nstl.gov).
Received for publication November 4, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: Ks, saturated hydraulic conductivity • TU, tritium units
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Ground water quality in agricultural areas of the Midwest has been particularly affected by NO3–N leaching (Burkart and Stoner, 2001). Changes in cropping practices including multicrop rotations and N fertilizer management may help reduce this contamination, but the changes in ground water quality may substantially lag changes in management. Underestimating the time required for water quality improvement could cause incorrect assessments of the effect of new practices on water quality, if the assessments are based on short-term studies. The potential lag in water quality response also has implications for the setting of realistic timelines to achieve targeted water quality improvements in watersheds. Unfortunately, documenting the timing of water quality responses to new management practices has received little attention, and there are few reports of watershed- or landscape-scale changes in ground water quality resulting from specific changes in agricultural management.
Long-term studies under documented management are needed to develop knowledge on the timing of ground water quality response to new practices. Opportunities to examine these responses with field data should be identified and exploited. This paper presents a comparison between two small watersheds (30 and 34 ha) for which there is nearly a 40-year record of management history and stream flow. The hydrology and geology of these two adjacent watersheds are essentially identical. Agricultural management practices were also identical, except for two periods between 1969 and 1974, and from 1996 until present. Both of these experimental periods were hypothesized to cause a difference in NO3–N concentrations in ground water and stream baseflow between the two catchments. Could an experimental effect from the first treatment period possibly persist and confound a comparison between the current experimental practices? The objective of this study was to determine if any current differences in NO3–N concentrations could, at least in part, be attributed to a residual effect from the 1969–1974 experiment.
This study included several techniques that included estimating the travel time of ground water through the watershed with physical and isotopic methods. Isotopes of water (18O, 2H, and 3H) have often been used to help interpret hydrologic systems, both surface water (Genereux and Hooper, 1998) and ground water (Gonfiantini et al., 1998). Changes in stable isotope (18O, 2H) composition occur due to changes in water phase (evaporation, sublimation, condensation) in the atmosphere (Ingraham, 1998), at the land surface (Walker and Krabbenhoft, 1998), and in the shallow subsurface (Barnes and Turner, 1998). Spatial and temporal changes in isotopic composition can be used to determine relative water sources and dominant hydrologic processes, and thus aid interpretation of storm flow pathways (Buttle and Sami, 1990; DeWalle et al., 1997; Genereux and Hooper, 1998), ground water flow (Gonfiantini et al., 1998; Hendry, 1988; Kehew et al., 1998; Matheney and Gerla, 1996), and plant water use (Burgess et al., 2000; Dawson and Ehleringer, 1998).
Tritium (3H) is an unstable isotope, with a half-life of 12.43 years, that increased in the atmosphere and in precipitation following aboveground testing of nuclear weapons during the late 1950s and early 1960s, then decreased rapidly after 1963 after testing was banned (Ingraham, 1998). This pulse of tritiated waters has allowed relative aging of ground water, essentially into three classes: "pre-bomb" (before 1953), "bomb-peak" (from the mid 1950s to the late 1970s or early 1980s), and more recent waters (after about 1980). These three age classes are relevant to this study given the dating of the two experiments. Discerning these relative ages is becoming difficult as isotopic decay of "bomb" tritium proceeds. Further discussion and case studies are provided by Bradbury (1991), Daniels et al. (1991), Gonfiantini et al. (1998), and Ingraham (1998).
Nitrogen isotopes (15N/14N) of natural abundance have also been used in environmental studies; however, diagnostic use is typically restricted to distinguishing animal- (and/or human-) waste sources from soil organic matter (SOM) or fertilizer sources (e.g., Spalding et al., 1982; Kendall, 1998). Soil organic matter and fertilizer sources can become difficult to distinguish in ground water because their ranges of isotopic ratios overlap, soil N-cycle processes (i.e., immobilization, mineralization) in time mix soil and fertilizer-applied N, and NO3–N in deep soils and ground water could be subject to denitrification, which results in 15N enrichment of residual NO3–N (Kendall, 1998). The two watersheds in this study never received manure applications, and the only known sources of NO3–N are SOM and applied fertilizer. Therefore, natural abundance N isotopes would not be expected to answer the question being asked in this study.
Nitrate originating from geologic parent materials can occur, but these have only been reported as important sources in more arid areas to the west. Boyce et al. (1976) reported that NO3–N concentrations in Nebraska loess were diminished in eastern portions of that state's deep-loess region. Regardless, any geologic source for the NO3–N should not cause a difference in NO3–N concentrations between these two watersheds, which border one another and have virtually identical geology, pedology, and hydrology. The duration of cultivation is also believed to be similar. Both watersheds were probably cultivated since about 1880, based on Potawattamie County records of cropland acreage (Larry Kramer, personal communication, 2002).
In this paper, the hydrology, water-isotope chemistry, and NO3–N concentrations of ground water and unsaturated sediments are compared between two adjacent first-order watersheds in the Loess Hills of southwestern Iowa. Hydraulic and isotopic data are used to evaluate the subsurface flow system. It is hypothesized that isotopic signatures will help interpret relative ages and pathways of ground water in the two small watersheds at this southwestern Iowa site. If both the physical and the isotopic methods indicate that rainfall predating 1980 still resides within ground water of these watersheds, then any differences in NO3–N concentrations between these two watersheds could result, at least in part, from the 1969–1974 experiment. The NO3–N concentrations themselves were also evaluated and compared with historical data to provide an additional line of evidence for evaluating the possible persistence of NO3–N from the first experiment.
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SETTING AND BACKGROUND |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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New crop rotations were established in both watersheds beginning in 1996 (Table 1). Watershed 1 was placed under a corn–soybean [Glycine max (L.) Merr.] rotation, with each crop covering either the eastern or western half of the watershed every year (Fig. 1). Watershed 2 was placed under a six-year rotation of corn, soybean, and corn, followed by three years of alfalfa (Medicago sativa L.). The six-year rotation was implemented in narrow contour strips (Fig. 1), with all six years of the rotation always present (about 1/3 in corn, 1/6 in soybean, and 1/2 in alfalfa with all three stand ages represented). With both rotations, only the corn received N fertilizer, which was on an alternate-year basis in Watershed 1, and two out of six years in Watershed 2. After the new rotations were established, N fertilizer rates applied to corn from 1996 to 2001 averaged 151 kg N ha-1 in Watershed 1 and 117 kg N ha-1 in Watershed 2. The fertilizer N applications are determined using soil testing; larger carryover of legume-fixed N and differences in timing of soil tests may have contributed to smaller rates being recommended in Watershed 2.
The change in farming practices since 1996 is hypothesized to cause a difference between the two catchments in ground water and stream-baseflow NO3–N concentrations. One question that has arisen, however, is whether the effect of the excessive nitrogen fertilizations between 1969 and 1974 has persisted in ground water. Thus, to discern the effects of recent management changes, we must determine if current NO3–N concentrations in ground water and baseflow in Watershed 1 could still be influenced by these large N applications that date back about 30 years.
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METHODS |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Monitoring Installations
In each watershed, a transect of four piezometer nests was installed in 1996, at divide (D), mid-slope (S), toe-slope (T), and riparian valley (R) positions. The nests are identified by watershed number and landscape position (e.g., 1D, 2S; see Fig. 1). In 1999, three additional riparian-zone piezometer nests were installed in Watershed 1, including 1B, and two nests installed near 1R, denoted 1Rb and 1Rc (the 1996-installed 1R nest was denoted 1Ra; see Fig. 1). The 1999 installations were a part of separate research on a riparian buffer planted in Watershed 1 during 2000. Piezometer nests comprise three transects, including two long transects identified by watershed (e.g., W1, W2), and a short riparian transect in Watershed 1 identified as W1rip (Table 2). Each transect portrays an expected path of ground water flow based on field interpretation of the terrain.
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Materials encountered during coring were recorded with depth, which at upland (D and S) positions included Peoria and Pisgah loess of Wisconsinan age, underlain by Loveland loess of Illinioan age, and then pre-Illinoian till. Sand lenses were common at the till–loess interface. Buried paleosols known as Farmdale, Sangamon, and Yarmouth soils occurred at the upper surface of the Pisgah, Loveland, and till units, respectively. The statigraphy and paleosols of the Loess Hills are described by Prior (1991), Ruhe (1969), and Kay and Graham (1943). In the riparian valleys, alluvial deposits composed of reworked loess were described following a scheme proposed by Bettis (1990). A 50-mm-i.d. piezometer was then set just above the till, with a 0.6-m-long screen. Additional boreholes were also drilled to install shallower piezometers (Table 2) at depths above and below the Sangamon paleosol (a possible impedance to vertical water flow), and/or at the observed depth of the water table (i.e., where water-saturated sediments were first encountered). One or two suction-cup lysimeters were also installed above the saturated zone at several locations (Table 2). Installations at each landscape position were located within a few meters of one another, usually in line along the topographic contour. Piezometers were gravel-packed to at least 0.2 m above the screened interval and grouted to the land surface with bentonite. Lysimeters were similarly installed but were set in a silica–flour slurry to provide hydraulic contact between the ceramic cup and the surrounding unsaturated sediments. Steel casings were set to cover the polyvinyl chloride (PVC) plastic piezometers and lysimeters. Map coordinates and elevations of all piezometers were determined by a global positioning system (GPS) survey with relative errors of 0.01 m in horizontal and vertical dimensions.
Hydrologic Measurements
The hydraulic conductivity (Ks) of the saturated zone was measured by conducting slug tests in each piezometer. A solid PVC rod was lowered and later raised to conduct slug-down and slug-up tests. Hydraulic head was measured during each slug test using a pressure transducer and data logging system. The slug test data were analyzed using the Hvorslev (1951) method. At each piezometer, Ks was taken as the mean of the values measured by rising and falling head. The Ks data were sorted by type of deposit (till, sand at the till interface, loess, and alluvium), and the geometric mean was calculated for each unit. For several of the deep piezometers completed in till or its Yarmouth paleosol, measurements of Ks were not made because there was no response in the water levels after slug addition. Water levels were measured in each piezometer on a monthly basis.
Positional survey data, coring descriptions, water levels, and hydraulic conductivities were used to estimate times required for ground water to travel along the three transects. The saturated zone dominantly occurred within the Loveland loess above the toeslopes, and in alluvium below. Given the uniformity and common origin of these two deposits, the ground water flow system is simple in its hydrogeology, and simple calculations were used to evaluate lateral flow along these transects. Hydraulic gradients and conductivities were used to determine ground water velocities using the Darcy equation. Survey data and water levels were used to calculate hydraulic gradients for June 2001 and April 2002, which were the months with the highest and lowest average water levels since the second set of piezometers was installed in 1999. The average head of all piezometers at each nest was calculated for both times. Hydraulic gradients were calculated by taking the difference in average head between adjacent nests, divided by the distance between them. The geometric mean Ks calculated for the loess and alluvium deposits was then used to calculate flow velocities. The flow rates were next divided by an estimated effective porosity of 0.2 m3 m-3 to calculate ground water velocities, which were finally divided into the distances between transect locations and summed to obtain an estimated travel time along each transect. The effective porosity was estimated to be about half of the average porosity of 0.42 m3 m-3, which was estimated from bulk densities of cores collected during piezometer installation. This was deemed an appropriate value to estimate the travel of a solute's center of mass. Also, water contents of the loess soils at 1500 kPa, near a 1-m depth, are about 0.20 m3 m-3 (Rob Malone, personal communication, 2003), suggesting that little flow occurs in about half the pore space.
Isotope Chemistry
Input Record for Tritium in Precipitation
A record of annual tritium activities in precipitation was constructed to represent past concentrations of tritium that have recharged ground water at the site. We obtained tritium units (TU) data for annual precipitation published by the International Atomic Energy Agency (1992), which covered the period from 1953 through 1986, and monthly data from the Global Network for Isotopes in Precipitation database (International Atomic Energy Agency and World Meteorological Organization, 2001) from 1987 though 1999. Annual weighted averages were calculated from the latter source. Records from three monitoring stations were used to compile a continuous tritium input record from 1953 to 1999. A station at Lincoln, Nebraska, located 90 km southwest of these watersheds, was operated from 1962 to 1986 and provided a data record considered to be local for those years. Annual TU values before 1962 and after 1986 were estimated based on relationships between records at Lincoln and St. Louis, Missouri, which covered from 1987 to 1993, and between records at Lincoln and Ottawa, ON, Canada, to estimate from 1953 through 1961 and from 1994 through 1999 (Table 3). These relationships were calculated using a power function:
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Isotope Sampling and Analyses
Between 13 and 15 June 2001, 33 water samples were collected for isotopic analyses of 
18O, 2D, and 3T (TU). The samples were taken from 30 piezometers (Table 2) and from the stream baseflow passing the weir of each watershed, plus the combined sample from four 1.8-m-depth lysimeters described above. The isotope analyses were conducted at the University of Waterloo Environmental Isotope Laboratory using methods described by Coleman et al. (1982) and Drimmie et al. (1991) for deuterium, Epstein and Maeda (1953) for 18O, and Drimmie et al. (1993) and Taylor (1977) for enriched tritium. Tritium results had a detection limit of 0.8 TU, and analytical errors that varied from 0.5 to 1.4 TU and increased with tritium activity. Isotope samples were submitted in three sets to spread the incurred financial costs, and turn-around times from the lab varied depending on its workload. Consequently, the samples were analyzed either 90, 120, or 321 d after collection, corresponding to 0.020, 0.026, or 0.071 fractional half lives of tritium (12.43 yr), respectively. Accordingly, tritium values were back-corrected from the date of analysis to the sampling date. While these were minor corrections, ranging from 0.0 to 0.7 TU, they did account for known errors and were greater than half the analytical error in four of the last-analyzed samples.
Stable isotope data, expressed as 2D
and 18O, were plotted and a local meteoric water line (LMWL) was estimated using the reduced major axis method (Mann, 1987). This regression method was applied to stable isotope data by the International Atomic Energy Agency (1992), but was named orthogonal regression in that document. The method was chosen because it reduces slope attenuation that can be important when the range of observations is limited and the X variate is subject to measurement error.
A single index was calculated to indicate the isotopic enrichment or depletion of any single sample relative to others at the site, by scaling all the 2D and 18O values between -1 and 0 (values were all negative), and then adding one to their sum. This scaled the data between maximum and minimum possible values of 1 and -1, with positive values being relatively enriched, negative values relatively depleted, and near-zero values close to a mean isotopic condition for the site. This index facilitated simple graphics to evaluate spatial patterns of the isotopic data, which were confirmed by t tests on the original isotope values, assuming equal variances.
Nutrients in Water and Deep-Core Sediments
Upon completion, the installations were sampled on a monthly basis; water samples from lysimeters and piezometers were analyzed for NO3–N concentration using an autoanalyzer technique with a 1.0 mg L-1 detection limit that is described by Hatfield et al. (1999). At least 1.5 water-column volumes were purged from each piezometer before sampling. Recharge to several deep piezometers was too slow to allow this purging, and sampling of these installations (identified in Table 2) was done either less frequently (e.g., twice per year), or, where water levels did not recover within a month of sampling (e.g., at Nest 2D), was suspended. Stream baseflow was also sampled at the outlet of both watersheds for NO3–N, on a monthly basis from autumn 1996 through summer 2000, and then on a weekly basis after spring 2001. Baseflow concentrations between the two watersheds were compared by a paired t test, under the null hypothesis that there was no difference in NO3–N concentrations.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Estimates of ground water velocity between piezometer nests averaged 13.5 m yr-1, and varied from 5.3 to 27.1 m yr-1. The estimates are based on a mean Ks (Table 4) applied for flow through loess above the toeslopes and through alluvium below the toeslopes. Estimated travel times (Table 5) varied according to the length of each transect and changes in hydraulic gradients. Along the W2 transect, larger gradients occurred with higher water levels measured in June 2001. But along W1, the larger gradients occurred during April 2002. Between 64 and 82% of the travel times occurred above the toeslope positions. Because vertical and horizontal gradients are both present, actual travel distances of ground water may be greater than horizontal distances and travel times are considered conservative. Results therefore suggest that land management effects on ground water could persist for decades in these watersheds (Table 5).
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Tritium in Ground Water
After correction for varying sampling-analysis time lags, tritium activities ranged from 0.8 to 18.5 TU. Small tritium activities (<3 TU), indicating water predating any research at the site, occurred in 6 of the 33 samples and were always in the deepest ground water (Fig. 4)
. Tritium values exceeding 12 TU, showing an influence of 20- to 40-yr-old precipitation, occurred in 9 of the 33 samples. These larger tritium activities always occurred below midslope (1S, 2S) and toeslope (shallow at 2T, 1Rc) positions (Fig. 4). Values of intermediate tritium activity (4–12 TU) were most frequent below riparian and toeslope positions (1T, 1B, 1Ra, 1Rb, deep at 2T, 2R), indicating recent or mixed-age waters. Samples collected from the weirs had tritium activities of 11.1 and 12.5 TU for Watersheds 1 and 2, respectively, which are not dissimilar given analytical errors near 1.0 TU. The baseflow samples would be considered of mixed origin, with possibly a weak influence of 20- to 40-yr-old waters, and were consistent with TU values from the shallowest riparian-valley piezometers.
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for 18O and from -39.6 to -60.9 for 2H. Ground water contains a mix of waters from multiple precipitation events, and therefore the range of data was smaller than is typically observed in precipitation samples. When the data were fit to estimate a local meteoric water line (Fig. 5)
, the limited range resulted in a fairly low precision (r2 = 0.72) and the local meteoric water line (LMWL) was not statistically different from the global meteoric water line (GMWL), or other LMWL published from the region (Harvey and Welker, 2000; International Atomic Energy Agency, 1992; Matheney and Gerla, 1996; Simpkins, 1995). The average 18O was -7.1 and the average 2H was -49.2. While some evaporative enrichment of ground water relative to local precipitation could occur (Gonfiantini et al., 1998), these averages are similar to those published for precipitation at Ames, IA (Simpkins, 1995) and Chicago, IL (International Atomic Energy Agency, 1992). The International Atomic Energy Agency did not monitor these stable isotopes at Lincoln, NE, or St. Louis, MO. There are data from central Nebraska (Harvey and Welker, 2000) showing isotopic depletion in precipitation relative to Ames and Chicago, probably resulting from less influence of moisture from the Gulf of Mexico (Simpkins, 1995; Harvey and Welker, 2000). The central Nebraska site is more than 400 km west of the Deep Loess Research Station, across a steep transition in climate and native vegetation from semiarid short-grass prairie (west) to humid tall-grass prairie (east).
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18O averaging -6.4 in upper landscape positions, -7.1 at the toeslope positions, and -7.5 in the riparian positions, and 2H averaging -42.4 in upper landscape positions, -50.4 at the toeslope positions, and -52.6 in the riparian positions. Also, classification of the data by landscape position accounted for 52% of the variance in the 18O data and 60% of the variance in the 2H data, based on a single-factor analysis of variance. This indicates there are differences in processes or sources affecting ground water according to landscape position. Three possible mechanisms causing this difference are upward movement of deep ground water, seasonal infiltration patterns, and landscape differences in water vapor transport in soil.
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A second and more likely possibility is that seasonal changes in runoff and infiltration act to segregate recharge waters. Snowmelt and cold spring rains are isotopically depleted, and occur when there is little plant cover. These depleted waters would be most prone to runoff from upper slopes and then infiltrate near the toeslope. Winds can also redistribute snow toward lower landscape positions. Warmer summer rains, on the other hand, would be enriched and occur when there is crop cover that increases interception, infiltration, and transpiration, and reduces runoff. Also, rain occurring during small precipitation events is likely to infiltrate where it falls, and this rain could, on average, be enriched by the "amount effect." That is, falling raindrops become more enriched by evaporation during small events than during large events, because the atmosphere is likely to be saturated with water vapor during large events (Ingraham, 1998).
A third potential cause could be related to movement of water vapor under winter conditions. Loess soils are known to exhibit frost heave, which results from upward movement of water vapor from depth toward frozen surface soils. Vapor movement through soil is known to be fractionating (Barnes and Turner, 1998), and in frozen soils would lead to enrichment of the deeper soil water. In the lower landscape position, the water table would provide a shallow lower boundary for this process, and therefore upon melt the depleted soil ice would readily remix with the originating water. In the uplands, however, the process occurs under drier conditions with no fixed lower boundary. Complete remixing of melted soil ice to depth would not occur as readily and deeper soil water that recharges ground water could remain enriched. Such a phenomenon has not been documented, but is plausible.
In sum, seasonal runoff and infiltration mechanisms are thought to be most responsible for the relative enrichment of ground water below upland positions and depletion in ground water below the toeslopes (Fig. 6). Large relative depletion in the shallowest piezometers at Positions 2R and 1B further support this. Processes of soil freezing and thawing may also contribute to the observed pattern.
Nitrate Nitrogen Concentrations
Deep Sediments
A large increase in sediment NO3–N concentrations (mg kg-1) was observed in the deep cores taken during 1996 at Position 1D (Fig. 7)
, centered near the 17.5-m depth. This pattern was not observed in any of the other deep cores, and only 25 individual samples out of 323 collected at the other coring positions showed NO3–N concentrations exceeding 4 mg kg-1. Total N and C data from these cores did not indicate this NO3–N could originate from the sediments or buried paleosols (i.e., 1D and 2D showed similar carbon contents that decreased similarly with depth). If this NO3–N resulted from large N fertilizer applications between 1969 and 1974, then there would be some consistency with historical data on sediment NO3–N collected in 1972, 1974–1976, 1978, and 1984 (Schuman et al., 1975; Alberts et al., 1977; Alberts and Spomer, 1985). Depths of increased NO3–N in sediments reported for each of these years showed consistent downward movement. These depths were plotted against the cumulative amounts of water that percolated though these watersheds since 1969 to each year of sampling (Fig. 7). Baseflow provides a surrogate measure of this percolation, and was taken from stream discharge records separated into runoff and baseflow components (Kramer et al., 1999). The result (Fig. 7) showed a strong linear relationship (r2 = 0.98) between depths of increased NO3–N and cumulative baseflow, showing the consistent movement of a pulse of NO3–N through the deep unsaturated zone since the first experiment, in response to hydrologic fluxes through the watershed's subsurface. If the baseflow resulted from deep percolation that occurred at spatially uniform rates, then this relationship suggests that 1 m of deep percolation caused NO3–N to percolate about 5.6 m through the unsaturated zone. This ratio leads to an estimated "mobile" water content of about 0.18 m3 m-3. This is about the difference between the average volumetric water content found during bulk density determinations on the deep cores (0.37 m3 m-3) and estimated 1500 kPa water contents (about 0.20 m3 m-3 at a 1-m depth), and therefore seems reasonable. A similar relationship with time suggests that the excess NO3–N percolated through this 20-m unsaturated zone at an average annual rate of 0.67 m yr-1 (r2 = 0.99; data not shown). These relationships provide a third line of evidence that some of the N from large fertilizer applications between 1969 and 1974 resided in the subsurface of Watershed 1 until 1996. Given the inferred rate of movement of this NO3–N pulse to depth, one would anticipate that this NO3–N would have percolated into the saturated zone before 1996 at lower landscape positions. This would explain why large sediment concentrations were not observed at depth at lower W1 positions.
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Ground Water
The largest concentrations of NO3–N in water were found within the deep suction-cup lysimeters installed in upland positions (Fig. 8)
. The smallest concentrations occurred in the deepest ground water. Concentrations in the saturated zone decreased not only with depth, but also with distance downslope. This could result from an increasing effect of denitrification with residence time, increased carbon availability and denitrification rates in alluvial sediments, and/or dilution from greater infiltration of runoff waters near the toeslope position, which bear small NO3–N concentrations. These hypotheses are the subject of ongoing research at this site.
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Large changes in NO3–N concentrations have occurred in upland (S and D) positions, particularly in the lysimeters in both watersheds (i.e., 6- and 12-m depths at 2D, 6- and 9-m depths at 2S, 12 m at 1D, and 11 m at 1S; see Fig. 9). These variations in the lysimeter waters are not well explained by profiles of NO3–N in core samples, and are asynchronous among each other and with ground water recharge events that caused increased water levels in early 1998 and 1999. Temporal changes in mineralization of organic matter and unsaturated water contents may contribute to the patterns. There are unpublished data that indicate large decreases in soil organic matter occurred in the surface soils of these watersheds during the past 30 years (Tom Moorman, personal communication, 2002). Piezometers in Watershed 1 also showed significant temporal variation in NO3–N (i.e., 21-m depth at 1D; 14.5- and 16.5-m depth at 1S), including the recent entry of a large pulse of deep NO3–N to ground water at 1D. Note that piezometers at 2D were not sampled due to low sampling yield and slow recovery (>30 d) of water levels after sample withdrawal.
In the R and T positions, NO3–N concentrations in ground water are smaller and most stable in Watershed 2, with slow increases evident in Watershed 1 (Fig. 9). The larger NO3–N concentrations in Watershed 1 are reflected in the baseflow concentrations measured at the watershed outlet weirs (Fig. 10) , which averaged 20 mg L-1 in Watershed 1 and 12 mg L-1 in Watershed 2. A paired t test showed this difference to be significant (p < 0.01). Given Watershed 1's average annual baseflow of 131 mm between 1975 and 2001, each 10 mg L-1 of NO3–N would equate to 13.1 kg N ha-1 yr-1 exported in baseflow. Therefore, very large concentrations would be needed over a prolonged period to export an additional 1656 kg N ha-1 (i.e., the excess amount of N applied to Watershed 1 from 1969 to 1974).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Isotopic data suggest that ground water recharge in these watersheds may frequently occur by infiltration of runoff water at lower landscape positions. Results of stable isotope analyses (Fig. 6) suggest that runoff from colder rains infiltrates at and below toeslope positions. Tritium data (Fig. 4) also support recent and/or mixed origin waters at these landscape positions.
Concentrations of NO3–N in ground water beneath upslope positions in Watershed 1 are still influenced by an experiment conducted from 1969 to 1974. However, in lower landscape positions, runoff from upslope infiltrates and mixes into ground water, and therefore historical and recent land use practices affect current NO3–N concentrations. Travel times across the riparian zone are estimated at around two or three years. But, due partly to focused recharge below the toeslope, it appears that upslope ground water is being contributed to the riparian areas very slowly. Overall, it will be difficult to clearly discern the effect of recent cropping system changes between these two watersheds by monitoring ground water or baseflow for many years. Therefore, shallow monitoring of unsaturated-zone waters may be the most reliable means to examine effects of crop rotation on water quality. Ongoing research will assess the denitrification of ground water beneath this site's riparian zone.
In conclusion, multiple lines of evidence gathered in these small watersheds suggest it takes at least several decades for subsurface water to travel from the divide to the stream. In many watersheds, changes in agricultural practices may take several decades to fully effect improvements in ground water quality.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONSETTING AND BACKGROUNDMETHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Vadose Zone Journal | ||||
| Soil Science Society of America Journal | Journal of Plant Registrations | The Plant Genome | |||
