PUBLICATIONS Earth and Space Science RESEARCH ARTICLE 10.1002/2015EA000117 Key Points: • Groundwater in claypan watersheds was more susceptible to NO3-N contamination • Preferential pathways were the primary mechanism for NO3-N movement to groundwater • Hydraulic conductivity controlled NO3-N distributions over strata in the aquifer
Correspondence to: O. M. Al-Qudah,
[email protected]
Citation: Al-Qudah, O. M., F. Liu, R. N. Lerch, N. Kitchen, and J. Yang (2016), Controls on nitrate-N concentrations in groundwater in a Missourian claypan watershed, Earth and Space Science, 3, 90–105, doi:10.1002/2015EA000117. Received 20 MAY 2015 Accepted 22 OCT 2015 Accepted article online 28 OCT 2015 Published online 6 MAR 2016
Controls on nitrate-N concentrations in groundwater in a Missourian claypan watershed Omar M. Al-Qudah1, Fengjing Liu1, Robert N. Lerch2, Newell Kitchen2, and John Yang1 1
Department of Agriculture and Environmental Science and Cooperative Research Program, Lincoln University, Jefferson City, Missouri, USA, 2Cropping Systems and Water Quality Research Unit, USDA-ARS, Columbia, Missouri, USA
Abstract Nitrogen (N) fertilizer applications have resulted in widespread groundwater nitrate-N (NO3-N) contamination in the U.S. Corn Belt. Goodwater Creek Experimental Watershed (GCEW) is an agricultural watershed in the claypan soil region of northeastern Missouri with a network of 96 wells at depths of 2.7–15.7 m. The objectives of this study were to (1) inspect the spatial and temporal variations of NO3-N concentrations in GCEW’s groundwater, particularly with well depth at scales ranging from individual well, well nest, and field to the entire watershed during the period 1991 to 2004; (2) understand the processes controlling the variability of NO3-N concentrations in groundwater at various scales within GCEW; and (3) compare groundwater NO3-N concentrations in GCEW to other agricultural watersheds in the U.S. Nitrate-N concentrations were determined in more than 2000 samples collected from 1991 to 2004. Despite the low hydraulic conductivity of the claypan soils, considerable NO3-N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10 mg L 1. Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO3-N movement to the aquifer. Changes in concentration with depth steadily increased to 8.5–10 m and then decreased with further depth. This pattern was consistent with decreased hydraulic conductivity in the Paleosol layer at 8.5–10 m, denitrification below this layer, and mixing of recent contaminated water with older uncontaminated water in the lowest strata. Only 19–23% of sampled wells exceeded 10 mg L 1 in nonclaypan agricultural watersheds over the continental U.S., suggesting that groundwater in GCEW was more susceptible to NO3-N contamination than nonclaypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO3-N transport in this claypan watershed. 1. Introduction Withdrawals of groundwater have increased and are expected to continue rising as the population increases and the availability of surface water becomes increasingly limited with a changing climate [Healy, 2010; Follett, 1989]. Not only does quantity but also the quality of groundwater affects the long-term sustainable use of groundwater resources, especially in intense agricultural regions, where the urban and rural population, irrigation, and industries have relied on groundwater as their major water supplies [Noland and Stoner, 2000]. The intense applications of fertilizers in agricultural regions have resulted in severe groundwater contamination, particularly nitrogen [e.g., Burow et al., 2010; DeSimone, 2009; Lerch et al., 2005; Kitchen et al., 1997; Wilkinson and Maley, 1996]. Therefore, understanding the controls on nitrogen concentrations in groundwater is very important for protecting groundwater resources.
©2015. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
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Nitrogen (N) is an essential element of aquatic ecosystem, but excessive levels in groundwater, particularly in the form of nitrate, can reduce the quality of water for human uses and lead to many environmental and health problems. The deleterious effects of excessive environmental N include the following: (1) an oxygen deficient condition referred to as “blue baby syndrome” in infants under the age of 6 months [Fan and Steinberg, 1996], (2) the risk of non-Hodgkin’s lymphoma in adults and reduced stomach acidity [Ward et al., 1996; Washington State Department of Health, 2005], and (3) acidification of soils and water resources [Motavalli et al., 2008]. Because of these problems, the U.S. Environmental Protection Agency (EPA) has established a maximum contaminant level (MCL) for NO3-N in drinking water at l0 mg L 1 [http://water.epa.gov/drink/ contaminants/basicinformation/nitrate.cfm; Ferrier and Jenkins, 2010; Kolpin et al., 1999]. In 5101 wells sampled throughout the continental U.S., 20% of wells in the agricultural land use setting had NO3-N concentrations higher than the MCL, while 3% of wells in the urban land use setting and 4% of wells in major aquifers exceeded the MCL [Burow et al., 2010]. Madison and Burnett [1985] have suggested a value
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of 3 mg L 1 as a possible division between natural and anthropogenic (human-related) sources of NO3-N in groundwater. The fate and transport of NO3-N in groundwater have been very well studied over the world [e.g., Burow et al., 2010; DeSimone, 2009; Lin et al., 2007; Noland and Stoner, 2000; Adamski and Pugh, 1996; Wilkinson and Maley, 1996; Mueller et al., 1995; Kolpin et al., 1994; Wilkinson and Maley, 1994; Ziegler et al., 1994; Sievers and Fulhage, 1992; Mesko and Carlson, 1988; Madison and Burnett, 1985]. However, our understanding of the processes or factors controlling NO3-N concentrations in groundwater of claypan soil areas is lacking. Claypan soils cover about 41,000 km2 of Illinois, Indiana, Kansas, Missouri, Ohio, and Oklahoma [Pomes et al., 1998; Lerch et al., 2008]. The smectitic mineralogy of the claypan and subsoil horizons results in restricted downward movement of air and water, slows root development, and promotes surface water runoff [Hjelmfelt et al., 1999; Blanchard and Donald, 1997; Blevins et al., 1996; Jamison et al., 1968]. However, a number of studies conducted over the last two decades in Goodwater Creek Experimental Watershed (GCEW) in Missouri have indicated that NO3-N leaching to shallow groundwater aquifers has occurred in the claypan soils. [Donald et al., 1998; Kelly and Pomes, 1998; Wilkinson et al., 2000]. Groundwater in GCEW is much more vulnerable to contamination by NO3-N than by common soil-applied herbicides [Blanchard and Donald, 1997; Pomes et al., 1998; Wilkinson et al., 2000; Lerch et al., 2005]. Despite all the efforts to establish an effective management system that can protect water quality in GCEW, more than 25% of the groundwater wells in the watershed had NO3-N concentrations greater than the MCL [Kitchen et al., 1997]. In 2006 and 2007, there were serious taste and odor problems for the water in the Mark Twain Lake (receiving water body for most of the Salt River Basin) due to excess nutrient loading [Arabi et al., 2012]. Even though water quality has been very well studied at GCEW as discussed above, these studies have not investigated the factors or specific processes (e.g., land uses, topography, soils, climate, crops, hydrologic pathways, chemical mixing, and hydrogeologic characteristics) affecting watershed vulnerability to NO3-N transport. These studies either did not encompass an area with enough variation in these factors or covered such a large area that the variability in climate, soil mineralization potential, types of soils and crops, and sample collection timing (e.g., limited to short periods) masked the impact of these factors. Our study reported here was aimed at understanding the susceptibility of groundwater in claypan watershed to nitrogen-fertilizer applications and the processes or factors that control NO3-N concentrations in groundwater within GCEW. Specifically, the objectives of this study were to (1) inspect the spatial and temporal variations of NO3-N concentrations in GCEW’s groundwater, particularly with well depth at scales ranging from individual well, well nest, and field to the entire watershed during the period 1991 to 2004; (2) understand the processes or factors that control the spatial and temporal variability of NO3-N concentrations in GCEW’s groundwater; and (3) evaluate NO3-N concentrations in groundwater at GCEW by comparing with other agricultural watersheds throughout the continental United States.
2. Study Area GCEW is located in the central claypan Major Land Resource Area 113—an area of about 30,000 km2—in Audrain and northeast Boone Counties, about 45 km north of Columbia, in north-central Missouri (Figure 1). The watershed covers 77 km2 and is a subwatershed to Youngs Creek within the larger Long Branch watershed of the Salt River, draining to Mark Twain Lake, a major public water supply and recreation area. The study area has significant heterogeneity in soils, hydrology, and land uses (intensive row-cropping) [Blanchard and Lerch, 2000; Lerch et al., 2008]. The watershed’s topography is characterized by broad, nearly flat divides, gentle side slopes (0–3%)—elevation from the divide to the outlet ranges between 235 and 271 m above sea level (asl) (Figure 1)—and broad alluvial valleys often dissected with small streams [Alberts et al., 2003]. The long-term water-budget analyses (from 1948 to 2003) of the GCEW indicate that the mean annual precipitation was about 997 mm [Alberts, 2003; Arabi et al., 2012], of which 18%, 36%, 28%, and 18% occurred between January–March, April–June, July–September, and October–December periods, respectively. Spring rainfall (May to June) during seedbed preparation and planting is short, but intense [Alberts et al., 2003; Arabi et al., 2012]. Fall rainfall often occurs as low intensity, long-duration events due to slow moving cold fronts [Alberts et al., 2003]. Mean annual streamflow was 292 mm, which is about 30% of mean annual precipitation [Alberts et al., 2003; Arabi et al., 2012], whereas evapotranspiration accounted for about 70% of the mean annual precipitation [Alberts et al., 2003]. Baseflow accounted for about 15% of streamflow
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Figure 1. (top) Maps showing the location and boundary of Goodwater Creek Experimental Watershed (GCEW) in Audrain and Boone Counties, MO, elevations (meters asl), and the locations of groundwater wells, weather stations, rain gauges, and streamflow gauges in the watershed (modified after Blanchard and Donald [1997]). (bottom) Maps showing the locations of well nests and fields (I) and the arrangement of well nests in well fields (II to VI), along with elevation contours (meters asl) and soil series.
discharge, and surface runoff accounted for about 85% [Alberts et al., 2003; Arabi et al., 2012]. Maximum and minimum daily mean temperatures were 17.1 and 6.3°C [Alberts et al., 2003; Arabi et al., 2012]. The land uses in this watershed include (a) row crops with an area of about 54 km2, 69–78% of the watershed area; (b) pasture and other grassland (10.4 km2, 12–17%); (c) woodland (4.5 km2, 6%); and (d) a small town at the upper end of the watershed (3.3 km2, 4%) [Arabi et al., 2012; Baffaut et al., 2009]. The predominant crops in the watershed follow the ranking of soybeans ≫ corn = hay > wheat > sorghum, and the typical crop rotations are corn/soybeans, corn/soybeans/wheat, soybean/soybeans/wheat, and sorghum/soybean/wheat [Alberts et al., 2003]. Historical agriculture applications in the Goodwater Creek Experimental Watershed are shown in Table 1. Quaternary stratigraphy for north-central Missouri was adapted for the GCEW based on core samples collected in previous studies [Blanchard and Donald, 1997; Sharp, 1984; Guccione, 1983]. Figure 2 presents
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a
Table 1. Historical Land Uses and Fertilizer Applications Over the Three Farm Fields in the GCEW Cropping History Location
Manure Applications
Applied Nitrogen (kg/ha)
1930–1960
1960–1980
1980–1990
1990–2004
1930–1960
1960–1980
1980–1990
1990–2003
Field 1
Cultivated-unsure of crops
42% Corn 50% Soybean 8% Sorghum 50% Soybean 50% Sorghum
Few animal manure
No animal manure
Total (13 years) = 1250 Annual mean = 96
Cultivated-unsure of crops
Few animal manure
Few animal manure
No animal manure
Total (11 years) = 749 Annual Mean = 68
Field 3
70% Corn 20% Oats 10% Wheat
40% Soybean 20% Wheat 40% Sorghum 50% Soybean 40% Wheat 10% Sorghum 50% Soybean 50% Wheat
No animal manure
Field 2
10% Corn 85% Soybean 5% Sorghum 20% Corn 40% Soybean 40% Wheat 50% Corn 30% Soybean 20% Wheat
Animal manure
1960–1965 Animal manure 1965–1975 No animal manure
1975–1982 Animal manure 1982–1990 No animal manure
Total (11 years) = 890
a
35% Corn 35% Soybean 30% Wheat
Annual mean = 81
Arabi et al. [2012], Baffaut et al. [2009], Lerch et al. [2005], Ghidey et al. [1997], and Kitchen et al. [1997].
a modified version of the strata based on current studies of Brooks and Saia [2012] and Pagan [2009], which shows that the total thickness of the till strata is around 16 m at the summit landscape position. The till is especially vulnerable to soil erosion, which has degraded the upper or lower soil units at some locations throughout the basin [Arabi et al., 2012]. Wisconsin and Illinoian loess deposits are separated by a claypan layer, and the three layers that overlie the till are about 3 m thick at the summit position and are thin or absent near streams. At most locations, a paleosol in till is present between the upper glacial till unit and Illinoian loess, at the bottom of the lower till unit, and separating the two pre-Illinoian glacial till units. The glacial till overlies Pennsylvanian shale and Mississippian limestone, and in some locations preglacial sediments including peat. Alluvial deposits are present near streams, particularly near the outlet of the watershed, where the loess, till, and Pennsylvanian deposits have been completely eroded and alluvium directly overlies the Mississippian
Figure 2. (top) Sketch of generalized cross-section and stratigraphy of geological deposits in GCEW (modified after Pagan [2009], Blanchard and Donald [1997], and Guccione [1983]). (bottom) Variation of accumulative fraction of clay, sand, and silt contents with depth from drilled cores at selected nests (e.g., F1-C means well nest C in Field 1). Note that the darker shaded areas near the bottom mark the depths that were screened and the last four profiles show fractions averaged to field and watershed scales.
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Burlington Limestone [Alberts et al., 1995; Blanchard and Donald, 1997]. The loess and glacial till have sufficient water yield for domestic use and together will be referred to as the glacial aquifer. Soils within GCEW are distinguished by the presence of a naturally formed argillic horizon (claypan layer about 0.15 to 0.45 m thick) located 0.13–0.37 m below the soil surface [Anderson, 2011; Jamison et al., 1968]. The clay content ranges from 350 to 600 g/kg [Blanco-Canqui et al., 2002]. The clay contents of the argillic horizon is generally greater than 50%, and consisting of 38%, 34%, 21%, and 7%, respectively, of montmorillonite, quartz, kaolinite, and illite [Blanco-Canqui et al., 2002]. The claypan soils are predominantly classified as hydrologic soil groups C and D [Arabi et al., 2012], which have moderately to high surface runoff potential when thoroughly wet [U.S. Department of Agriculture, 2003]. Table 2 shows the spatial distribution of some hydraulic conductivity and effective porosity over field and entire watershed scales in GCEW.
3. Methods 3.1. Distribution of Wells and Information on Well Drill Cuttings Ninety-six groundwater wells—ranging in depth from 2.7 to 15.7 m with a screened interval of 0.61 and 1.2 m for shallow and deep wells, respectively—are distributed in three farm fields and other locations throughout the watershed at GCEW. In three farm fields (Fields 1–3), the monitoring wells were organized by five- to eight-well nests (e.g., A, B, C, D, and E in Figure 1 and Table 3). The number of wells for each nest ranges from 1 to 5 and the wells within a nest are within about 25 m2 of each other (Table 3 and Figure 1). All monitoring wells were furnished by 5.08 cm diameter polyvinyl chloride pipes and equipped with water pumps. Twentyfour production wells located throughout the watershed (watershed wells) were also sampled. Soil extracts were obtained from core cutting samples by the University of Missouri-Columbia Soil Characterization Laboratory during the drilling of the deepest wells at Field 1 (nests C, D, and E), Field 2 (nests A and C), and Field 3 (nests A, B, C, D, and E). These extracts were collected with a 1.6 m long, 7.62 cm diameter core barrel, which had a removable acetate liner [Kitchen et al., 1997]. Drill cutting samples were divided horizontally according to soil horizons or visible textural or color changes. Where the core appeared uniform in texture and color, it was divided into 30 cm increments [Kitchen et al., 1997]. Soil samples were divided into two subsamples, one of which was oven dried to determine water content, and the other to obtain soil texture (Figure 2). 3.2. Water Quality Data The water quality data of rainfall, stream water, and groundwater was obtained from U.S. Department of Agriculture Agricultural Research Service (USDA-ARS’s) Cropping Systems and Water Quality Research Unit (CSWQRU) through Sustaining the Earth’s Watersheds, Agricultural Research Data System [Sustaining the Earth’s Watersheds, Agricultural Research Data System, 2013, http://www.nrrig.mwa.ars.usda.gov/stewards/ stewards.html]. Groundwater wells were sampled quarterly (March, June, September, and December) from 1991 through 1996, and semiannually (March and September) from 1997 through 2004. Because of low recharge rates, some deeper wells were only sampled annually. Prior to sample collection, three volumes of water were purged out from each well. Samples were collected in 900 mL amber glass bottles, placed in a cooler, and transported to the laboratory at CSWQRU in Columbia, MO, at the end of the day. Additional details of well installation and sample collection procedures were previously documented [Blanchard and Donald, 1997; Kitchen et al., 1997]. Stream water was sampled (through concrete V notch weirs) weekly during the growing season and every 2 weeks in winter. Precipitation was sampled after each storm event from rain gages installed over the watershed. From April 1991 to September 2004, 2583, 4572, and 442 samples were collected from groundwater, stream water, and rainfall, respectively. Water samples were filtered through 0.45 μm nylon filters within 48 h of sampling. Samples were preserved by lowering pH to approximately 2.0 using sulfuric acid and refrigerated prior to analysis. Samples were analyzed for NO3-N within 5 days if applicable, and if not, they were frozen and the analysis was done within 30 days of collection [Blanchard and Donald, 1997; Kitchen et al., 1997]. Nitrate-N analyses were determined colorimetrically using a Lachat flow injection system (Lachat Instruments, Milwaukee, Wisconsin), and the detection limit (DL) for this method was 0.05 mg L 1. 3.3. Statistical Analysis The Kruskal-Wallis analysis of variance by ranks and median test (H test) is a nonparametric method (does not assume a normal distribution of the data or equal number of observations of treatments) for testing whether
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a
Table 2. Spatial Distribution of Hydraulic Conductivity and Effective Porosity, as Well as the Number of Wells in Each Depth Over Field and Entire Watershed Scales Field 1 Stratum Series Top of claypan Wisconsin loess Within claypan Wisconsin loess Base of claypan Wisconsin loess Illinoian loess Paleosol in till Glacial till Paleosol in till Glacial till Pennsylvanian shale
Depth Range (m) 0.0–0.46 0.46–0.71 0.71–1.1 1.1–2.85 2.85–3.24 3.24–8.5 8.5–10 10–13 13–15.2
a Blanco-Canqui et al. [2002]: Ksat = 72 mm/h b Kelly and Pomes [1998]. c Blanchard and Donald [1997]. d
Hydraulic Conductivity K (m/s) b
6
b
6
1.68 × 10 1.98 × 10 b 9.87 × 10 c,d 5.0 × 10 b,c,d 5.0 × 10 c,d 4.5 × 10 c,d 5.0 × 10 c,d 4.5 × 10 c,d 1.9 × 10
7 7 7 6 7 6 8
Field 2
Field 3
Watershed Wells
Watershed GCEW
ne
NW
ne
NW
ne
NW
ne
NW
ne
NW
48 47 43 40 38 35 30 34 31
0 0 0 5 0 11 3 34 31
47 44 40 38 41 36 27 32 -
0 0 0 1 0 15 1 3 -
46 41 39 36 40 33 27 32 35
0 0 0 5 2 14 2 3 1
52 48 43 -
0 0 0 3 0 19 2 0 0
48 44 41 38 39 35 28 33 32
0 0 0 14 2 59 8 9 4
in the depth (0–0.46 m). ne: effective porosity; NW: number of wells.
Sharp [1984].
samples originate from the same distribution. As the data presented in this study were likely not normally distributed, Kruskal-Wallis approach was used to conduct an analysis of variance on the ranked data to examine the significance of the spatial and temporal variations of median NO3-N concentrations using software developed by StatSoft Inc. (1984–2010). The a priori level of significance (α) was chosen to be 0.05 for all statistical analyses. Spearman’s rank order correlation (S-R) was used to obtain the correlation coefficients. Spearman’s rank is a nonparametric correlation, where chi-square statistic computed for two-way frequency tables, and provides a careful measure of a relation between the two (tabulated) variables. Moreover, Spearman’s rank assumes that the variables under consideration were measured on at least an ordinal (rank order) scale, that the individual observations can be ranked into two-ordered series [Siegel and Castellan, 1988]. The Spearman’s rank order correlations (S-R) were considered to be positively and perfect positive correlated if its value >0.1 and 1, respectively, and negatively and perfect negative correlated if its value < 0.1 and 1, respectively [Siegel and Castellan, 1988].
4. Results 4.1. Nitrate-N Concentrations in Groundwater, Stream Water, and Precipitation Within GCEW, median nitrate-N concentrations were in the order groundwater > stream water > precipitation (Table 4 and Figure 3). The nitrate-N concentrations in groundwater for the entire watershed varied from DL to 46.0 mg L 1, with a median of 7.8 mg L 1. About 29% of the 2583 groundwater samples (38% of the 96 wells) had NO3-N concentrations greater than the MCL. The nitrate-N concentrations in stream water ranged from DL to 145.0 mg L 1 with a median of 1.3 mg L 1. About 7% of the 4572 stream water samples exceeded the MCL. The nitrate-N concentrations in rainwater were lower than 6.0 mg L 1 with a median of 0.3 mg L 1. About 99.8% of the 442 samples had NO3-N concentrations varying between 0.05 and 3.0 mg L 1. 4.2. Spatial Variation of Nitrate-N Concentrations in Groundwater The median nitrate-N concentrations in groundwater over well nests and fields (Figure 3 and Table 4) were also used to examine the spatial variation. Results of the H test indicated that the median NO3-N concentrations were significantly different at all scales within and among nests, fields, and the watershed (p < 0.001). The median NO3-N concentrations were also significantly different over individual wells except for wells in nest B of Field 1, nests C and G of Field 3, and nest E of watershed wells. The nitrate-N concentrations in wells of Field 1 varied between 0.3 and 46.0 mg L 1 with a median of 4.6 mg L 1 (Table 4). Nest A had the greatest median concentration among all nests in Field 1 (Figure 3). About 7% of the 589 samples collected from Field 1 (about 20% of the 25 wells) had NO3-N concentrations >10 mg L 1, with the majority of those wells from nests A, D, and E. In Field 2, the nitrate-N concentrations varied between 0.1 and 11.7 mg L 1, a much narrower range than in Field 1, with a median of 8.9 mg L 1.
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Table 3. Hydrologic, Soil, and Topographic Characteristics of the Areas Occupied by Wells Organized by Fields and Nests Soil Type Distributions Field ID Field 1
Field 2
Field 3
Watershed wells
Nest ID
No. of Wells
1A 1B 1C 1D 1E 2A 2B 2E 2C 2D
5 5 5 5 5 5 3 4 4 4
3A 3E 3H 3B 3C 3D 3F 3G 0A 0B 0C 0D 0E 0F 0G 0H
5 5 4 3 4 2 3 1 4 3 2 4 2 2 3 4
Latitude
Longitude
39.2259079 92.1171863 39.2274985 92.1173205 39.2312061 92.1194111 39.2319456 92.1188754 39.2328156 92.1183258 39.2530602 92.1238151 39.2530003 92.1247928 39.2518823 92.1238273 39.2525977 92.1256610 39.2518819 92.1229436 Area Not Occupied With Wells 39.2323376 92.1490584 39.2312136 92.1502471 39.2304786 92.1489512 39.2304760 92.1471665 39.2305235 92.1520808 39.2310289 92.1523987 39.2305979 92.1534198 39.2307563 92.1533490 39.2553900 92.1506538 39.2552315 92.1451503 39.2551066 92.1397131 39.2551785 92.1268072 39.2681910 92.1183250 39.2827462 92.0789218 39.2833124 92.0813832 39.2827267 92.0843113
Elevation Well Depth (m) Range (m) 265.5 264.6 263.7 263.5 262.7 262.4 262.1 262.7 261.5 263.1
2.7–13.4 2.7–14.5 2.7–14.2 2.7–11.5 2.7–9.7 2.7–12.1 3.5–6.5 4.8–10.3 3.5–8.3 6.7–10.6
262.4 260.9 262.1 263.7 259.4 259.1 259.1 259.1 261.2 260.3 253.6 260.9 255.7 248.7 251.8 254.8
6.5–11.7 2.7–9.9 2.7 8.0–14.0 3.0–8.5 3.4–6.6 3.0–5.0 2.8–7.4 4.1–10.0 6.0–10.0 2.2–4.1 3.75-10.3 2.29-4.13 2.3–4.2 3.4–7.2 3.8–8.4
a
Soil Series
a
Adco Adco Adco Mexico Mexico Mexico Mexico Mexico Leonard Putnam Adco Mexico Mexico Mexico Adco Leonard Vesser Vesser Vesser
Percent of Area %
a
Slope Percent
a
b
a,b
Area 2 (km )
Drift Thickness (m)
Hydraulic Gradient
0.355
16
0.004
0.242
13
0.006
0.204
15
0.009
70
0–2
30
1–3
30
1–3
25 15 30
1–4 0–1 0–2
65
1–3
10 15
0–2 1–4
10
0–1
NA
NA
NA
0.12
11
0.008
NA
NA
NA
0.1
11
0.01
a Blanchard b
and Donald [1997]; Sharp [1984].
About only 1% of the 433 samples collected from Field 2 (10% of the 20 wells) had NO3-N concentrations >10 mg L 1, mainly from wells in nest D. In Field 3, the nitrate-N concentrations varied from DL to 26.4 mg L 1, with a median of 11.4 mg L 1. About 58% of the 969 samples collected from Field 3 (82% of the 27 wells) had NO3-N concentrations >10 mg L 1, with nests A, B, and F having the most wells with high concentrations. In watershed wells, about 24% of the 592 samples collected from these locations (67% of 24 wells) had NO3-N concentrations >10 mg L 1, with a range from DL to 26 mg L 1, and a median of 6.5 mg L 1. Nitrate-N concentrations were highest in Field 3 (Figure 3). Also, nitrate-N concentrations that exceed the MCL in Field 3 represent the greatest percentage among all the fields (Table 4). 4.3. Variations of Nitrate-N Concentrations in Groundwater With Well Depth The relationship between NO3-N concentrations and well depths at entire watershed scale (Figure 4) indicates that the median nitrate-N concentrations were high near the surface (~8 mg L 1), decreased from near surface to about 3.5 m below the surface (to ~4.5 mg L 1), increased with depth below 3.5 m to peak concentrations between 8.5 and 10 m (~12 mg L 1) and then decreased significantly with depth below 10 m (10 (mg L )
Range of NO3-N 1 (mg L )
1991–2004 1992–1995 1970–1992 1991
2205 926 2012 303
3.1 (M) 3.4 (M) 3.4 (M) 0.2 (M)
-
-
63
29
23 19 21 13
0.02–16
