seven small agricultural watersheds through- out Kentucky in 1989 and 1990 that they had previously sampled in 1971 to 1972 doi:lo.2489/jswc.64.3.190 and found that NO3 -Nconcentrations were almost the same despite the near doubling of N fertilizer use. The geology of these water- Nitrate-nitrogen patterns in the Raccoon sheds is sedimentary and ranges from sand- stones to shales, the annual precipitation for River Basin related to agricultural practices these watersheds ranges from 1,11(1 to 1.330 nun (43.7 to 52.4 in), and the annual runoff J.L. Hatfield, L.D. McMullen, and C.S. Jones ranges from 371 to 481 mm (14.6 to 18.9 in). Thomas et al. (1992) concluded that the Abstract: Nitrate-N concentrations in the Raccoon River have increased beginning in the parent rock of the soils within the watersheds early 1970s. Since this river is the predominant water supply for the City of Des Moines in had a greater effect on NO3 -Nconcentrations Iowa, there is concern about the potential long-term impacts of these trends. Improvements than agricultural use patterns, and there was no in water quality from agricultural watersheds are critical to protect the water supply, and evidence across these watersheds that fertilizer understanding the factors affecting water quality will lead to potential changes in agricul- use was detected in NO-N or PO4-P levels in tural management to improve water quality.The historical database of nitrate-nitrogen (NO3_ the streains.Variation in the results across differ- N)concentrations sampled at the Des Moines Water Works were combined with observa- emit watersheds suggests that there is no single tions on N fertilizer use, animal production, crop yields, land-use changes, and precipita- answer for the changes in NO3-N concentra- tions froni agricultural watersheds over time. tion patterns to evaluate these interrelationships. Mean annual NO3 -Nconcentrations in the Raccoon River watershed have been increasing since 1970 in spite of no significant After repeatedly violating the US change in N fertilizer use for the past 15 years. There have been three years with maximum Environmental Protection Agency's drinking NO3 -Nconcentrations above 18 nig L. However, these spikes occurred throughout the past water standard of 1(( mg L for NO 3-N, and ll 30 years and are not isolated to the last 10 years of record. Nitrate-N loads from the Raccoon faced with increasing levels of nitrate i its River watershed have shown a slight decrease iii the past ten years because of the increased source water, the Des Moines Water Works crop yields and increased removal of N in the corn (Zea mays L.) and soybean (Gylcine max (DMWW) constructed the world's largest [L.] Merr.) grains. Production numbers for cattle have decreased by 63% since the early 1980s, ion exchange nitrate removal facility in 1991 while hogs have shown a 20% decrease over the same time period. Therefore, N available for (McMullen 2001). Since the facility became application Into the basin has decreased by 25%. Annual variations in NO 3-N loads are sig- operational in 1992, the nitrate standard has nificantly related to precipitation in the first five months of the year. A significant correlation not been violated. However in June of 2005, was found between the land area within the watershed cropped to small grains and hay crops the utility again nearly violated the nitrate and the increase of NO 3-N since 1970 (r = —0.76).This relationship was caused by alteration standard due to very high water demand in the seasonal water-use patterns and loss of N during the fall or early spring in the water concurrent with very high source water movement in contrast to corn or soybean, which have a limited N uptake pattern concen- nitrate levels, a heretofore rare circumstance. trated between June and early September. Changes in the water-use patterns caused by shifts This motivated DMWW staff to extensively in cropping patterns provide an explanation for the positive correlation between precipitation review long-term flow and nitrate data for its and flow during the early part of the year. Development of agricultural management practices primary water source, the Raccoon River. that can potentially affect water quality will have to be more inclusive of all components ill United States Geological Survey flow data agricultural systems, rather than only changing fertilizer rate or timing. dating hack to 1919, along with nitrate data generated froni DMWW's testing labora- Key words: cropping practices—farming practices—nitrogen management—water tory dating to 1931, are a unique data source balance-water quality—watershed to evaluate the interrelationships among water quality and agricultural practices. Understanding the interactions between Nitrogen loss from agricultural water- primary source of NO3 -Nin the river that NO3 -Nlevels in water supplies and agricul- sheds has become a topic of great inter- offset the increase in commercial N fertilizer tural production systems will provide insights est in the past 20 years. Detections of use in the 1970s. Mclsaac and Libra (2003) into efforts to reduce N levels in drinking nitrate-nitrogen (No -N) in water above assessed the nitrate concentrations in the water. There have been several studies that the drinking water standard (10 mg L) Des Moines River using current analytical lisc 5111)\\IJ thc 1113k 1),'t\\ M) ,IFl31Ilt3l1I] raise questions about the source of N and methods and found that there were increases the potential impact of agricultural prac- in the mean NO 3 concentrations from 1945 tices. Keeney and Deluca (1993) evaluated to 2001 periods. Schilling and Libra (200) NO3 -Nm the Raccoon River watershed evaluated 15 Iowa watersheds and concluded from the early 1940s through the 1970s and that NO 3-N concentrations were directly concluded that the intensity of tillage and related to the percentage of row crops in the the subsequent N mineralization was the watershed. Thomas et al. (1992) re-sampled Igo MAY/JUNE 2009—VOL. 64, NO. 3 JOURNAL OF SOIL AND WATER CONSERVATION Figure a Raccoon River watershed location within Iowa and the main tributaries in the watershed. land use and N concentrations in adjacent water bodies. Studies by Hill (1978), Mason et al. (1990), and Jorden et al. (1997) have all shown that as the intensity of agricultural practices increases, the amount of N moved .. .., .. to adjacent streams also increases. Nitrate-N concentrations in the Raccoon River and surrounding areas in Central Iowa ..J._ have been the subject of several studies over past 20 years. Lucey and Goolsby (1993) reported that NO -N concentrations could ' be explained by a four-variable model based t on streamow from the previous seven days, soil moisture condition, and the cosine of 7 the day of year (DOY) and sine of the DOY. They estimated the soil moisture condi- tion from a simple hydrologic model for the watershed. This model explained 70°A of the variation in the N03 -Nconcentra- tions (Lucey and Goolsby 1993). Schilling and Zhang (2004) evaluated basefiow in - the Raccoon River watershed and found that baseflow contributed nearly two-thirds Iowa, and travels approximately 300 km (186 Installation of subsurface drainage has greatly of the NO 3-N export front watershed, nu) to its confluence with the Des Moines altered the hydrology of the watershed by which suggested that seasonal patterns in the River in the City of Des Moines. The main- providing a more direct conduit front fields enrichment of baseflow with N0 1-N were stem of the Raccoon River, also known into adjacent surface water streams. Because linked with seasonal crop N uptake patterns, as the North Raccoon (Hydrologic Unit of subsurface drains, soluble nutrients, e.g., Zhang and Schilling (2005) completed a Code 07100006) in its upper stretches, has N0 1-N are being readily transported from more detailed analysis of the temporal and two main tributaries: the Middle and South fields into surface water sources. spatial patterns of flow in streams contrib- Raccoon Rivers. The Middle Raccoon The Raccoon River as a Source of uting to the Raccoon River watershed and River rises in northwest Carroll County and Drinking Water. The Raccoon River and haseflow in the river and found NO -Nflows 120 kill (74.5 mi) to join the South shallow groundwater influenced by it is concentrations and loading have half-year Raccoon near Redfield, Iowa. The South the primary source of drinking water for cycles, while precipitation, streamfiow, and Raccoon River rises near the Guthrie- Des Moines Water Works' (DMWW's) two baseflow have yearly cycles. Schilling (2005) Audubon County line and flows 80 kill (49.7 treatment plants that serve the Des Moines discovered that the increase in row crop nii) until its confluence with the Middle Metropolitan area. Front until 1948, intensity increased the baseflow and base- Raccoon.The combined flows of the Middle the utility's Fleur Drive Treatment Plant flow percentage in the Cedar and Raccoon and South Raccoon (Hydrologic Unit Code obtained water exclusively from all Basins. This study addressed the 07100007) join the North Raccoon near ground collection system known locally as factors affecting daily N0 5-N concentra- Van Meter, Iowa, a few kilometers down- the infiltration gallery. This is a 5 km (3.1 tions; however, a detailed analysis of the stream from Redfield. The entire Raccoon mi) long, 91 cm (35.8 in) internal diameter effects of changes in the agricultural practices River watershed drains land from 17 coun- pipe that runs parallel to the Raccoon River, within a watershed overlong periods of time ties and 9,324 km2 (2,304,010 ac), 6.4% of 10 ni (32.8 ft) below grade.