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Fact Sheet 2009–3042

Pubs Warehouse

### Prepared as part of the National Water-Quality Assessment Program, Agricultural Chemicals Team

## A Whole-System Approach to Understanding Agricultural Chemicals in the Environment

| ***The effects of the use of agricultural chemicals and otherpractices associated with agriculture on the quality of streamsand groundwater is well known; however, less is knownabout how those effects may vary across different geographicregions of the Nation. Scientists at the U.S. Geological Survey(USGS) are conducting studies on the transport and fate ofagricultural chemicals in diverse agricultural settings acrossthe country using comparable and consistent methodologyand study designs (fig. 1; Capel and others, 2004; Capel andothers, 2008). Assessments in five study areas have beencompleted, and the results highlight how environmentalprocesses and agricultural practices interact to affect themovement and transformation of agricultural chemicals inthe environment. The studies address major environmentalcompartments, including surface water, groundwater, theunsaturated zone, the streambed, and the atmosphere, as wellas the pathways that interconnect these compartments. Thestudy areas represent major agricultural settings, such asirrigated diverse cropping in the West and corn and soybeanrow cropping in the Midwest and, therefore, findings arerelevant throughout much of the Nation.***
Major Findings
Findings from these studies show how environmental processesand agricultural practices act together to determine the transportand fate of agricultural chemicals in the environment. Selectedhighlights from USGS studies featured in the Journal ofEnvironmental Quality are presented here:
- Hydrologic and landscape modifications—such as irrigation andenhanced soil drainage through tile drains and drainage ditches—change the magnitude and timing of water flow and agriculturalchemical transport into and through the hydrologic system.
- Agricultural chemicals, including nitrogen compounds, pesticides, andpesticide breakdown products, commonly are present in precipitation,surface water, unsaturated-zone soil water, and groundwater inagricultural areas.
- Differences in the rates of agricultural chemical movement from landsurface to groundwater and streams are controlled by many factors,including the characteristics of the chemical (such as solubility, sorptioncharacteristics, and susceptibility to biochemical transformation), thetiming of chemical application relative to irrigation and precipitation,the volume of water inputs relative to evapotranspiration rates, and theflow paths that water follows from fields to groundwater and streams.
- Nitrate is the predominant form of nitrogen present in ground andsurface waters of the agricultural watersheds studied.
- Attenuation of nitrate in groundwater and the streambed occurs to somedegree in all watersheds. However, this attenuation is limited where (1)flow rates are fast relative to biochemical reaction rates, (2) the volumeof water moving through reactive zones is small, or (3) the supply oforganic substrates is not adequate to support biological processes.
- Instream processes such as photosynthesis and respiration can changenitrate loads in surface water.
- Differences in the concentrations and loads of nitrogen among streamsreflect differences in the hydrologic characteristics of these watersheds.
- The load of nitrogen leaving the watersheds was closely correlated withthe amount of water flowing out of the watershed: Stream exports ofnitrogen were greatest where surface-water outflows were greatest.
USGS Studies Featured in Journal of Environmental Quality
Results from five studies on the transport and fate of agriculturalchemicals were published as a “Special Submissions” section inthe May–June 2008 volume of the Journal of EnvironmentalQuality (volume 37, issue 3). Study findings highlighted in thisfact sheet are available in more detail in selected papers fromthat volume:  Bayless and others (2008),Capel and others (2008), Domagalski and others (2008), Essaidand others (2008), Fisher and Healy (2008), Green and others(2008a, 2008b), Hancock and others (2008), Puckett and others(2008), Steele and others (2008), Vogel and others (2008), andWebb and others (2008).

**Figure 1.***Research was conducted in five study areas: (a) Lower Merced River basin,California; (b) Granger Drain basin, Washington; (c) Maple Creek basin, Nebraska; (d) SugarCreek basin, Indiana; and (e) Morgan Creek basin, Maryland. Similar studies are underwayin two additional study units: (f) South Fork Iowa River basin, Iowa, and (g) Bogue Phaliabasin, Mississippi.*
Characteristics of the Study Areas
The studies were designed at nested scales with most workconducted within a focus watershed nested within a larger study basin(table 1). Each study basin was nested within a larger study unit whereprevious investigations have been conducted by the USGS NationalWater-Quality Assessment Program (Capel and others, 2004).

**Table 1.** Drainage areas for study basins and focus watersheds.[**Abbreviation:** km2, square kilometer]
State StudybasinDrainage   area   (km2)Focus   watershedDrainagearea   (km2)CaliforniaLower Merced River831Mustang Creek17.5WashingtonGranger Drain160DR2 Drain5.5NebraskaMaple Creek955unnamed tributary1.5IndianaSugar Creek240Leary-Weber Ditch7.2MarylandMorgan Creek31Morgan Creek31.0
The California and Washington study areas (fig. 1) arecharacterized by semiarid to arid climates and, in both, more than 95percent of crops are irrigated. The sources of irrigation water in thesetwo areas, however, differ substantially. Most irrigation water in theCalifornia study area is pumped from deep groundwater, whereas inthe Washington study area an extensive network of water deliverycanals distribute surface water from the Yakima River. Agriculturecomprises predominantly orchards, vineyards, row crops, dairies, andcattle feedlots in both study areas.

The study areas in Nebraska, Indiana, and Maryland (fig. 1) arecharacterized by humid climates, each receiving 30 to 40 inches ofprecipitation each year. These three study areas are dominated bycorn and soybean row cropping, but several differences exist that canaffect the movement of agricultural chemicals. Specifically, irrigationis used to augment water needs in the Nebraska study area, where30 percent of the farmland is irrigated from deep wells. Subsurfacedrains, used to move excess water from the root zone, are widelyused in the Indiana study area, where topography is flat and soils arerelatively high in clay content. In contrast, soils in the Maryland studyarea are permeable and well drained, and precipitation and naturalsoil drainage are adequate for agricultural activities.

**Figure 2.***Various relatively short flow paths may connect agricultural fields to streams inareas where the water table is shallow: (a) infiltration to shallow groundwater, (b) shallowgroundwater flow, (c) intermediate groundwater flow, (d) seepage to tile drains, and (e)overland flow, which may result in (g) temporary ponding.*

**Figure 3.***Many flow paths from agricultural fields may bypass local streams in areaswhere the water table is deep: (a) infiltration to deep groundwater, (e) overland flow to (g)temporary ponds, and (f) deep groundwater flow.*
How Agricultural Fields are Linked to Groundwater and Streams
The right amount of water at the right time is critical to successful agriculture. When the water supply is too abundant, engineereddrainage systems, such as the subsurface “tile” drains and ditches at the Indiana and Washington sites, are used. When the natural watersupply is too scarce, such as at the California and Washington sites, water is added by engineered irrigation systems. These modifications tothe natural hydrology affect not only the amount of water in a watershed at any given time, but also the timing and manner in which watermoves through and out of the watershed to larger streams, lakes, estuaries, and, eventually, the ocean.

Typically, at least one-half of the water that reaches agricultural fields from precipitation and irrigation is released to the atmospherethrough evapotranspiration (the combined processes of evaporation and plant transpiration). When precipitation or irrigation exceedsevapotranspiration, excess water can follow a variety of pathways (flow paths) from fields. The predominant pathways vary amongenvironmental settings, depending on local conditions such as soil characteristics, land slope and topography, precipitation (or irrigation)patterns, depth below the land surface to the water table, and the presence of artificial drainage systems such as tiles (figs. 2 and 3).

Where the water table is shallow (fig. 2) and no artificial drainage system is present, excess water at the land surface moves downwardthrough the unsaturated soil zone and enters the groundwater system (fig. 2a). If a nearby stream intersects the water table, groundwaterdischarges into the stream via short, shallow flow paths (fig. 2b). If the stream is more distant, groundwater follows somewhat longer, deeperflow paths to the stream (fig. 2c). Where artificial drains are present, water is intercepted by the drainage system and carried quickly to thestream (fig. 2d). If downward water movement (infiltration) into the subsurface is hindered by low permeability soil or when the infiltrationcapacity of the soil is exceeded due to an intense rainfall or irrigation event, water flows across the land surface as overland flow (fig. 2e) andmay accumulate to form temporary ponds or streams in topographic lows (fig. 2g).

At locations where the water table is deep (fig. 3), flow paths through the unsaturated soil zone to groundwater are longer (fig. 3a)and groundwater follows deeper, longer flow paths to more distant streams or lakes (fig. 3f). When precipitation or irrigation exceed theinfiltration capacity of the soil, water may accumulate to form temporary ponds or streams in topographic lows (fig. 3g).

As water moves along various flow paths from fields, dissolved and suspended materials, such as agricultural chemicals, are transportedalong its route.
Hydrology Is Key
An accounting of local hydrology—the amountof water that enters the watershed, moves withinthe watershed, and leaves the watershed—formedthe foundation for understanding the movement ofagricultural chemicals in each of the focus watersheds.Stream hydrographs, which show the amount andtiming of surface water flowing out of each ofthe focus watersheds, illustrate some of the largedifferences in hydrology among these five study areas(fig. 4).

Despite similarities in climate and croppingsystems at the California and Washington sites(figs. 4*A* and 4*B*), differences in irrigation practiceshave resulted in large differences in streamflow atthe catchment outflows. Long-term irrigation fromgroundwater has lowered the water table substantiallyin the Mustang Creek catchment, increasing thedistance between the stream and the underlyinggroundwater system. Groundwater is pumped asneeded and is applied to crops using mostly drip andsprinkler methods, which generate little runoff. Asa result of the deep water table at Mustang Creek,(1) the time it takes for water to travel downwardto groundwater is considerable, (2) there is nogroundwater discharge to the creek, and (3) streamflowtypically occurs only as a short-term response towinter or early spring precipitation (fig. 4*A*). Incontrast, long-term irrigation with imported surfacewater at the Washington site has raised the local watertable and created a perennial stream from year-roundgroundwater base flow (fig. 4*B*).

Hydrographs for the three catchments whereirrigation is limited or not used at all (fig. 4*C-E*) showsimilar patterns of base flow, with intermittent spikesin discharge in response to precipitation or occasionalirrigation. Similar to the California site, the water tableunderlying the Nebraska watershed is more than 70meters below land surface and therefore too deep toprovide flow to the stream. Streamflow occurs only asa result of overland flow or shallow soil drainage fromprecipitation or occasional irrigation events (fig. 4*C*).Overland flow and drainage from subsurface drainsaccount for nearly all streamflow at the Indiana site(fig. 4*D*). During mid to late summer, streamflow inLeary-Weber Ditch generally ceases when precipitationno longer exceeds evapotranspiration requirements.At the Maryland site (fig. 4*E*), more than one-half ofthe streamflow comes from the discharge of shallowgroundwater to the stream and the remainder comesfrom overland flow from precipitation, which generallyoccurs throughout the year. Together, these twosources provide substantial year-round flow in MorganCreek.

**Figure 4.***Hydrographs for the surface-water outflows of the five focus watersheds show large differences inthe magnitudes and annual distribution of streamflow in these watersheds. [Note scale for stream discharge isdifferent for each graph.]*

*A vineyard grows on an irrigated hillside in the Granger basin, Washington.*
Excess Fertilizer in the Environment
Nitrogen is a critical nutrient for crop growth, and large amounts—from both commercial fertilizer and livestock manures—are appliedto agricultural lands to increase crop quality and productivity. However, when more nitrogen is supplied than crops can utilize, surplusnitrogen—often in the form of nitrate—is transported to groundwater and streams. Excessive nitrate in drinking water poses a risk to humanhealth and in streams can stimulate excessive growth of algae and other aquatic plants. Besides the nuisance these plants can pose, decay of theincreased organic matter consumes available oxygen in the water and leads to low-oxygen conditions that threaten aquatic life. The harmfuleffects of nutrient enrichment can be carried downstream and are now known to be affecting a considerable portion of the Nation's fresh-waterresources (Dodds and others, 2009) and coastal zones, particularly in the Gulf of Mexico (U.S. Environmental Protection Agency, 2008).

*USGS scientists collect hydrologic data in theLower Merced River basin, California.*

*Excess water produces overland flow in an almond orchard in theLower Merced River basin, California.*

*Surface runoff flows from a corn field in the Sugar Creek basin, Indiana.*

**Figure 5.***Concentrations of total nitrogen measured at the focus watershed outflows during 2004 showdifferences in both the magnitudes of peak concentrations and the range of concentrations. Annual yields ofwater (annual watershed outflow/watershed area) and nitrogen (annual load exported in the stream/watershedarea) from the focus watersheds were closely correlated and show that stream exports of nitrogen are greatestwhere surface-water outflows are greatest. [Insufficient data to calculate nitrogen yield for the Nebraska watershed.]*
Nitrogen Concentrations in Streams andNitrogen Exports From Catchments ReflectDifferences in Hydrology
Comparison of the concentrations of total nitrogen measured  in the streams of the focus watersheds illustrates the influence of  hydrologic conditions on the transport of nitrogen from fields (fig.  5). In addition to the concentrations in these streams, the total load  of nitrogen exported from each watershed (fig. 5) has important  consequences for downstream receiving waters (see sidebar, Excess  Fertilizer in the Environment). The load of nitrogen exported in  surface water is the product of the volume of streamflow leaving the  watershed and the nitrogen concentration in that water. Both nitrogen  concentrations and streamflow, as well as how each varies with  time, determine the total annual load of nitrogen exported from each  catchment.
- Nitrogen concentrations at the outlet of the California watershed  were moderate relative to the other study areas, largely because  streamflow generally occurs only during the winter and early  spring, when no fertilizer is applied. In addition to the limited  time frame during which most runoff occurs, the total volume of  surface runoff is small because of the limited precipitation, the use  of relatively efficient irrigation methods, and the disconnection  between the stream and the deep water table. Moderate  concentrations and small flows constrain the load of nitrogen  leaving the watershed in surface water. At this site, excess nitrogen  leaches into the unsaturated zone, and the small fraction of nitrogen  that eventually reaches the deep groundwater is either transformed  or eventually discharges to distant streams.
- Nitrogen concentrations also were moderate at the outlet of the  Washington watershed. As described above, groundwater base  flow at this site contributes a substantial part of the streamflow  throughout the year, and moderate nitrogen concentrations can  be attributed to conditions along groundwater flow paths that  allow for some removal by denitrification to occur. Rill irrigation,  the practice of delivering water by flooding furrows between  crop rows, is still common in the area, and the practice results in  considerable excess water that has, over the past century, caused  a rise in the water table to just below land surface. Although peak  discharge in DR2 is low relative to streams in the other study  watersheds (fig. 4), groundwater discharging to the stream sustains  year-round base flow with moderate concentrations of nitrogen. So,  despite the arid climate in the Washington study area, the steady  base flow results in higher basin yields of both water and nitrogen  than any of the other study watersheds.
- Of the five watersheds studied, nitrogen concentrations were  highest at the outlet of the Nebraska watershed. Water in this  watershed is transported predominantly by overland flow and very  shallow subsurface flow derived from precipitation and irrigation,  and much of the flow occurs during periods when nitrogen  fertilizer is applied to fields. In contrast to groundwater flows,  surface-water and very shallow subsurface transport is relatively  rapid and direct, allowing less opportunity for biochemical  transformations or mixing of different waters. As a result, nitrogen  concentrations are relatively high and vary widely.
- Nitrogen concentrations were relatively high at the outlet of  the Indiana watershed, but not as high as those measured at the  Nebraska site, probably because higher precipitation at the Indiana  site dilutes agricultural runoff. Overall, the relatively high rainfall  and the efficient transport of water and its nitrogen load by the  subsurface drainage system results in moderately high basin yields  of both water and nitrogen.
- Of the five watersheds studied, nitrogen concentrations were  lowest at the outlet of the Maryland watershed. At this site, water  moves predominantly along groundwater flow paths, similar to  the Washington site. However, although there is ample time for  removal by denitrification to occur, conditions throughout much of   the aquifer are not favorable and only limited denitrification  occurs. This limited denitrification, coupled with dilution from  infiltration of abundant precipitation, acts to lower nitrogen  concentrations in groundwater. Steady groundwater discharge to  Morgan Creek results in moderately high basin yields of both water  and nitrogen.
Implications For Management
Effective management of agricultural chemicals requires the  application of chemicals at the appropriate time and at the appropriate  rate, and knowledge of how these chemicals are most likely to be  transformed and(or) transported in the range of environmental  settings that comprise the Nation’s agricultural lands.

Knowledge about the predominant water flow paths (figs. 2 and 3)  and understanding their characteristic transport time scales can guide  selection of the most effective management practices for particular  environmental settings (see sidebar, How Agricultural Fields are  Linked to Groundwater and Streams). For example, buffer zones  around streams can be effective where overland flow is important, but  would be of little value where most water in a watershed travels to the  stream through subsurface drains or by deep groundwater flow paths.  Knowledge about which water flow paths predominate in particular  settings also helps managers develop appropriate monitoring plans.  In situations where most water in a watershed is transported along  deep groundwater flow paths, responses to management changes may  not be apparent in stream-water quality for several years. In contrast,  responses in stream chemistry may be apparent almost immediately  in areas with short flow paths, such as watersheds with extensive  overland flow and/or tile-drain systems.
Ongoing Work
The USGS is currently studying two additional agricultural  watersheds using the same whole-watershed approach (fig. 1).  Results from the South Fork Iowa River watershed will add to our  understanding of the movement and fate of herbicides and nutrients in  Midwestern watersheds dominated by corn and soybean row cropping  with extensive surface and subsurface drainage networks. The Bogue  Phalia watershed in northwestern Mississippi is dominated by cotton,  rice, and soybean row cropping, and the results of work there will  increase our knowledge of the movement and fate of pesticides in  humid, subtropical agricultural settings.  
References Cited
Bayless, E.R., Capel, P.D., Barbash, J.E., Webb, R.M.T., Hancock,  T.L.C., and Lampe, D.C., 2008, Simulated fate and transport of  metolachlor in the unsaturated zone, Maryland, USA: Journal of  Environmental Quality, v. 37, p. 1064–1072.

Capel, P.D., Hamilton, P.A., and Erwin, M.L., 2004, Studies by  the U.S. Geological Survey on sources, transport, and fate of  agricultural chemicals: U.S. Geological Survey Fact Sheet  2004–3098, 4 p., available at https://pubs.usgs.gov/fs/2004/3098/.

Capel, P.D., McCarthy, K.A., and Barbash, J.E., 2008, National,  holistic, watershed-scale approach to understand the sources,  transport, and fate of agricultural chemicals: Journal of  Environmental Quality, v. 37, p. 983–993.

Dodds, W.K., Bouska, W.W., Eitzmann, J.L., Pilger, T.J., Pitts,  K.L., Riley, A.J., Schloesser, J.T., and Thornbrugh, D.J., 2009, Eutrophication of U.S. freshwaters—Analysis of potential  economic damages: Environmental Science and Technology, v.  43, no. 1, p. 12–19.

Domagalski, J.L., Ator, S., Coupe, R., McCarthy, K., Lampe, D.,  Sandstrom, M., and Baker, N., 2008, Comparative study of  transport processes of nitrogen, phosphorus, and herbicides  to streams in five agricultural basins, USA: Journal of  Environmental Quality, v. 37, p. 1158–1169.

Essaid, H.I., Zamora, C.M., McCarthy, K.A., Vogel, J.R., and  Wilson, J.T., 2008, Using heat to characterize streambed water  flux variability in four stream reaches: Journal of Environmental  Quality, v. 37, p. 1010–1023.

Fisher, L.H., and Healy, R.W., 2008, Water movement within the  unsaturated zone in four agricultural areas of the United States:  Journal of Environmental Quality, v. 37, p. 1051–1063.

Green, C.T., Fisher, L.H., and Bekins, B.A., 2008a, Nitrogen fluxes  through unsaturated zones in five agricultural settings across the  United States: Journal of Environmental Quality, v. 37,  p. 1073–1085.

Green, C.T., Puckett, L.J., Böhlke, J.K., Bekins, B.A., Phillips,  S.P., Kauffman, L.J., Denver, J.M., and Johnson, H.M. 2008b,  Limited occurrence of denitrification in four shallow aquifers in  agricultural areas of the United States: Journal of Environmental  Quality, v. 37, p. 994–1009.

Hancock, T.C., Sandstrom, M.W., Vogel, J.R., Webb, R.M.T.,  Bayless, E.R. and Barbash, J.E., 2008, Pesticide fate and  transport throughout unsaturated zones in five agricultural  settings, USA: Journal of Environmental Quality, v. 37,  p. 1086–1100.

Puckett, L.J., Zamora, C., Essaid, H., Wilson, J.T., Johnson, H.M.,  Brayton, M.J., and Vogel, J.R., 2008, Transport and fate of  nitrate at the ground-water/surface-water interface: Journal of  Environmental Quality, v. 37, p. 1034–1050.

Steele, G.V., Johnson, H.M., Sandstrom, M.W., Capel, P.D., and  Barbash, J.E., 2008, Occurrence and fate of pesticides in four  contrasting agricultural settings in the United States: Journal of  Environmental Quality, v. 37, p. 1116–1132.  

U.S. Environmental Protection Agency, 2008, Mississippi    River Gulf of Mexico Watershed Nutrient Task Force: U.S.    Environmental Protection Agency Office of Wetlands, Oceans,    and Watersheds website, accessed March 24, 2009, at http://  www.epa.gov/msbasin/.

Vogel, J.R., Majewski, M.S., and Capel, P.D., 2008, Pesticides in  rain in four agricultural watersheds in the United States: Journal  of Environmental Quality, v. 37, p. 1101–1115.

Webb, R.M.T., Wieczorek, M.E., Nolan, B.T., Hancock, T.C.,  Sandstrom, M.W., Barbash, J.E., Bayless, E.R., Healy, R.W., and  Linard, J., 2008, Variations in pesticide leaching related to land  use, pesticide properties, and unsaturated zone thickness: Journal  of Environmental Quality, v. 37, p. 1145–1157.                                                            | -  **Report** PDF (1.6 MB) 

**USGS promotes public access to water-quality information.**

This fact sheet and other publications from the National Water-QualityAssessment Program are available on the World Wide Web athttp://water.usgs.gov/nawqa/bib/.

**Contacts for additional information:**
        Kathleen McCarthy, U.S. Geological Survey(503) 251-3257,mccarthy@usgs.gov
 Paul Capel, U.S. Geological Survey(612) 625-3082,capel@usgs.gov

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