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TECHNICAL REPORTS

Surface Water Quality

Phosphorus Transport Pathways to Streams in Tile-Drained Agricultural Watersheds

^a Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois at Urbana-Champaign, 1102 S. Goodwin Ave., Urbana, IL 61801
^b School of Public and Environmental Affairs, Indiana Univ., 1315 E. Tenth St., Bloomington, IL 47405
^c current address, Dep. of Crop and Soil Sciences, Michigan State Univ., 572 Plant and Soil Sciences Bldg., East Lansing, MI 48825

^* Corresponding author (gentryl{at}msu.edu)

Received for publication March 9, 2006.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Agriculture is a major nonpoint source of phosphorus (P) in the Midwest, but how surface runoff and tile drainage interact to affect temporal concentrations and fluxes of both dissolved and particulate P remains unclear. Our objective was to determine the dominant form of P in streams (dissolved or particulate) and identify the mode of transport of this P from fields to streams in tile-drained agricultural watersheds. We measured dissolved reactive P (DRP) and total P (TP) concentrations and loads in stream and tile water in the upper reaches of three watersheds in east-central Illinois (Embarras River, Lake Fork of the Kaskaskia River, and Big Ditch of the Sangamon River). For all 16 water year by watershed combinations examined, annual flow-weighted mean TP concentrations were >0.1 mg L^–1, and seven water year by watershed combinations exceeded 0.2 mg L^–1. Concentrations of DRP and particulate P (PP) increased with stream discharge; however, particulate P was the dominant form during overland runoff events, which greatly affected annual TP loads. Concentrations of DRP and PP in tiles increased with discharge, indicating tiles were a source of P to streams. Across watersheds, the greatest DRP concentrations (as high as 1.25 mg L^–1) were associated with a precipitation event that followed widespread application of P fertilizer on frozen soils. Although eliminating this practice would reduce the potential for overland runoff of P, soil erosion and tile drainage would continue to be important transport pathways of P to streams in east-central Illinois.

Abbreviations: BDO, Big Ditch of the Sangamon River • DAP, di-ammonium phosphate • DRP, dissolved reactive phosphorus • EMC, Embarras River • LFK, Lake Fork of the Kaskaskia River • P, phosphorus • PP, particulate phosphorus • TP, total phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
PHOSPHORUS (P) concentrations in surface waters are considered elevated above background concentrations throughout much of the United States due to anthropogenic sources. During the past several decades, P inputs from point sources have been reduced through the NPDES permit system, required monitoring of wastewater discharges, and general regulatory action. In addition, P removal from soap and laundry detergents has reduced P inputs from waste water treatment plants (Litke, 1999). Although sewage effluent from metropolitan areas is still an important P source, the USEPA has identified agriculture as the major nonpoint source of P to surface waters, and the greatest impediment to achieving water quality goals stated in the Clean Water Act (Parry, 1998; USEPA, 1998).

In many areas of the Midwest, corn and soybean production relies on commercial fertilizer, rather than animal manure, to enhance soil fertility. In regions of the Midwest where livestock production has decreased over the past 50 yr, nonpoint source pollution of P remains a pervasive and difficult water quality problem. As agriculture has become the focal point for P contribution to surface waters, there has been a proliferation of monitoring research that has been summarized in several review papers (Carpenter et al., 1998; Sims et al., 1998; McDowell et al., 2001; Hart et al., 2004).

Numerous studies have documented the importance of overland runoff in dissolved and particulate P transport from agricultural fields to streams (Schuman et al., 1973; Kronvang, 1992; Sharpley et al., 1994; Svendsen et al., 1995; Sharpley et al., 1999). The transport of P can be reduced through various cultural and erosion management practices, such as regular soil testing, variable rate fertilizer application, form and timing of P fertilizer application, incorporation of P fertilizer, tillage practice, contour farming, grassed waterways, and riparian buffer strips (Daniels and Gilliam, 1996; Sharpley et al., 2000; Bundy et al., 2001; McDowell and Wilcock, 2004). In general, overland flow is the dominant P transport mechanism from agricultural fields to offsite locations (Sharpley et al., 1994); however, P transport in artificial drainage (subterranean pipes called tiles) is also an important transport pathway (Sims et al., 1998; Stamm et al., 1998; Xue et al., 1998; Dils and Heathwaite, 1999; Heathwaite and Dils, 2000; Chapman et al., 2001). Although P transport has been widely studied, there is a lack of long-term intensive data needed to provide a complete understanding of P transport from fields to streams (and resulting forms, concentrations, and loads) under a range of flow conditions (and controlling weather patterns) in heavily tile-drained agricultural watersheds.

Our overall objective was to determine the dominant P form (dissolved or particulate) and to identify the primary P transport pathways in tile-drained agricultural watersheds using intensive long-term data. Select stream and tile flow events were closely examined to investigate the source of P in ditches and streams. Due to the lack of animal production or significant municipal wastewater discharges, watersheds in east-central Illinois are well-suited for investigating the transport of P from agricultural sources to streams.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Study Site Description
Sampling stations were established to investigate P concentrations and loads of three streams in east-central Illinois, USA: the Embarras River (EMC), Lake Fork of the Kaskaskia River (LFK), and Big Ditch of the Sangamon River (BDO). Stream discharge was available at the U.S. Geological Survey (USGS, 2006) National Water Information System for EMC (USGS site no. 03343400) and LFK (USGS site no. 05590800). Stream discharge for BDO was available from the Illinois State Water Survey (station 106). Annual stream discharge is based on a water year (the period from October through September, designated by the calendar year in which it ends). We sampled eight additional locations in central Illinois that were used to investigate water quality before and during a rain-on-snow event in 2001. These sites included: Black Slough and the East Branch of the Embarras River upstream of EMC; an unnamed tributary upstream of BDO; and Big Creek, Long Creek, South Branch of Salt Creek, North Fork of Salt Creek, and the main stem of Salt Creek (Fig. 1).

View larger version (34K): [in this window] [in a new window]   Fig.1. Map of east-central Illinois indicating three agricultural watersheds and sampling sites used in the study. Watersheds include: (A) Big Ditch, (B) Lake Fork, and (C) Embarras River; and sites are (1) North Fork of Salt Creek, (2) main stem of Salt Creek, (3) South Branch of Salt Creek, (4) Big Ditch (BDO), (5) unnamed tributary of Big Ditch, (6) Long Creek, (7) Big Creek, (8) Lake Fork (LFK), (9) Black Slough, (10) East Branch of the Embarras River, and (11) Embarras River (EMC).

Land use in the three watersheds is predominantly row crop agriculture with nearly an equal amount of corn (Zea mays L.) and soybean (Glycine max L.) production during the study period (Table 1). Soils in this area consist of approximately 0.5 m of loess and contain 4 to 5% organic matter. Phosphorus deficiency in crops can occur without addition of fertilizers and it is recommended to maintain Bray-P1 values above 50 kg ha^–1 in this region of Illinois (University of Illinois Extension Service, 2003). To accomplish this producers commonly apply P fertilizer as di-ammonium phosphate (DAP) every other year following harvest of fields planted to soybean.

Water Sampling and Analysis
Water samples for nutrient analysis were collected weekly or more frequently during storm events by manual grab sampling into flowing water. To supplement grab samples at LFK and BDO, an automated water sampler (model 2900, Isco, Lincoln, NE) was used to collect discrete samples every 6 h during high flow events in the winter and spring. During the period of record for each site, we collected 515, 389, and 245 water samples at EMC, LFK, and BDO, respectively.

Four tiles (A, B, C, and D) in the Embarras River watershed were monitored during the 1995–1996 water years using a weir structure, pressure transducer, and an automated water sampler (Xue et al., 1998). Three tiles (1, 2, and 3) in the Big Ditch watershed were monitored during the 2001–2002 water years using a Sigma 900 MAX area velocity sampler (Hach Company, Loveland, CO). All tiles were sampled on a flow proportional basis with sampling intervals from 40 000 to 120 000 L depending on the maximum discharge of each tile.

All water samples were placed in a cooler in the field and were filtered and preserved on returning to the laboratory, generally within 4 h of collection. During the past 10 yr, our laboratory methods for sample preservation and P analysis have changed. Water samples analyzed for dissolved reactive phosphorus (DRP) and ammonium (NH₄–N) were filtered through Whatman GF/C glass fiber filters (pore size 1.2 µm) until March 1999, after which they were filtered through Fisherbrand cellulose acetate/cellulose nitrate filters (pore size 0.45 µm). Tests were conducted to compare effect of filter pore size and no significant differences in DRP or NH₄–N concentrations were detected.

Dissolved reactive P concentration was measured manually using the ascorbic acid reduction method with colorimetric analyses until November 2002 after which a Lachat QuikChem 8000 Flow Injection Analysis system (Hach Company, Loveland, CO) was used. Before November 2002, unfiltered aliquots for total P (TP) were preserved by freezing; after that time unfiltered aliquots were acidified with H₂SO₄ to pH < 2 and stored at 4°C. Unfiltered water samples were digested using ammonium persulfate and H₂SO₄ and then analyzed for TP as described earlier for each period (APHA, 1998). Particulate P (PP) was estimated as TP minus DRP. Ammonium concentration was measured using the automated phenate method on a Technicon segmented flow auto analyzer until November 2002. Thereafter, samples were analyzed for NH₄–N using an automated sodium salicylate method on the Lachat QuikChem 8000 system. We used a full QA/QC program, including external quality control samples, to ensure high quality data throughout the study.

Load Calculations and Data Analysis
Daily stream P loads were determined by multiplying mean daily discharge by nutrient concentration. Linear interpolation in SAS was used to estimate P concentrations between sampling dates (SAS Institute, 1990). Annual P loads were determined by summing the daily loads for each watershed for each water year during the period of record. Tile P loads were estimated by multiplying discharge by the P concentration, assuming the concentration was constant from a time halfway between the previous sample and the present sample to a time halfway between the present sample and the subsequent sample (Gentry et al., 2000).

Cumulative error in the measurement of stream and tile discharge, sample handling, and sample analysis contribute to overall uncertainty in load estimates (Harmel et al., 2006). Given the quality of the discharge data, our intensive sampling regime and focused sampling during periods of changing discharge, and our QA/QC program, we believe the uncertainty in our load estimates to be <10% based on the analysis of Harmel et al. (2006).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Precipitation and Discharge
Average annual precipitation for the 1994–2003 water years was 97, 96, and 92 cm for the Embarras River, Lake Fork, and Big Ditch watersheds, respectively (Table 2). Based on (±) 5 cm from the average annual precipitation, water years 1994, 1996, 1998, and 2002 had above average precipitation and 2000, 2001, and 2003 were below average. The interaction of precipitation, infiltration, runoff, and evapotranspiration (ET) created a seasonal pattern of low discharge during the summer and fall and multiple high flow events during the winter and spring months (Fig. 2, 3, and 4). Annual discharge ranged from 52 to 290, 31 to 187, and 8 to 38 million m³ at EMC, LFK, and BDO, respectively, during the 10-yr period (Table 2). Average annual discharge represented 33, 31, and 28% of the annual precipitation at EMC, LFK, and BDO.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Stream hydrograph and P concentrations for Embarras River site (EMC) during the 1994 through 2003 water years. TP, total phosphorus; DRP, dissolved reactive phosphorus.

View larger version (19K): [in this window] [in a new window]   Fig. 3. Stream hydrograph and P concentrations for Lake Fork site (LFK) during the 1998 through 2003 water years. TP, total phosphorus; DRP, dissolved reactive phosphorus.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Stream hydrograph and P concentrations for Big Ditch site (BDO) during the 2001 through 2003 water years. TP, total phosphorus; DRP, dissolved reactive phosphorus.

Dissolved Reactive Phosphorus and Total Phosphorus Stream Concentrations
Annual flow-weighted mean DRP and TP concentrations showed less year-to-year variability compared with annual P loads, but generally corresponded with annual precipitation (Tables 2 and 3). For example, the smallest annual flow-weighted mean DRP and TP concentrations for each watershed occurred during the year with the smallest P loads (EMC in 2000; LFK and BDO in 2003). In contrast, years with above average annual precipitation did not always produce flow-weighted mean concentrations greater than those occurring in years with average precipitation. Furthermore, for the two wettest years (1998 and 2002) LFK had similar water yields but quite different flow-weighted mean concentrations. The difference in discharge patterns between these 2 yr greatly affected loads and flow-weighted mean P concentrations at EMC and LFK (Fig. 2 and 3).

Similar to other studies, we found DRP and TP concentrations in streams were elevated (>0.2 mg L^–1) during storm events and reduced (<0.05 mg L^–1) during base flow in the summer and fall (Schuman et al., 1973; Sharpley et al., 1993; Sharpley et al., 1999; Borah et al., 2003). Total P concentrations occasionally exceeded 0.5 mg L^–1 during storm events at each site (Fig. 2, 3, and 4). McDowell and Wilcock (2004) showed that TP concentrations >0.2 mg L^–1 were associated with intense rainfall events that could result in higher flows with increased sediment transport capacity. For example in the spring of 2002, approximately 10 cm of rain fell on saturated soils in the Embarras River and Lake Fork watersheds and produced the largest discharge events during the 10-yr period at EMC and LFK (Fig. 2 and 3). During this event, PP concentrations at EMC and LFK were two to five times greater than DRP concentrations which resulted in annual flow-weighted TP concentrations that were threefold greater than the annual flow-weighted DRP concentrations (Table 3). Although large discharge events exacerbate bank erosion and potentially entrain benthic sediments, the 2002 event at EMC and LFK likely transported large amounts of eroded soil from fields to streams. Other studies have shown a similar pattern in PP and attributed it to overland runoff with a heavy silt load (House and Warwick, 1998; Bowes et al., 2005). In general, extreme discharge events that originate from overland runoff appear to play a disproportionately large role in P export, particularly in the Corn Belt region of the Midwest (Borah et al., 2003; Royer et al., 2006).

Water years without extreme discharge events had annual flow-weighted mean DRP concentrations that were 50 to 73% of the corresponding TP concentrations. More evenly distributed precipitation such as during 1998 created numerous moderate flow events with similar DRP and PP concentrations at peak discharge. When precipitation rates did not exceed infiltration rates and surface runoff was minimal, DRP concentrations were greater than PP concentrations at peak discharge. Therefore, in dry years without overland runoff (e.g., 2000 and 2003) tile drainage was assumed to be the dominant transport pathway of P to ditches and streams (both DRP and TP). Although entrainment of benthic sediments and bank erosion may have contributed to P loads in all years, we believe these mechanisms to be relatively minor compared with tile drainage during dry years.

Watershed Comparisons
Annual flow-weighted mean DRP and TP concentrations were less at LFK than at EMC or BDO (Table 3). All three watersheds are dominated by conventional row crop agriculture, suggesting the differences in flow-weighted mean P concentrations were likely due to other watershed or stream channel characteristics. Although all three streams are highly modified to enhance drainage, Lake Fork appears to be entirely channelized. Dredge spoils along channelized streams in central Illinois often create a barrier to direct inputs of overland runoff, except during extreme events. With its extensively channelized stream network, Lake Fork watershed may generate less overland runoff, soil erosion, and river sediment load. This hypothesis is supported by the observation that high flows resulted in elevated concentrations of PP at EMC and BDO, but to a lesser extent at LFK (Fig. 5).

View larger version (16K): [in this window] [in a new window]   Fig. 5. Relationship between discharge and particulate phosphorus (PP) concentrations for three streams in central Illinois. See text for the period of record for each site. EMC, Embarras River site; LFK, Lake Fork site; BDO, Big Ditch site.

Phosphorus in Tile Drainage
Agricultural tile drainage water has been shown to transport elevated DRP concentrations, and Xue et al. (1998) reported annual flow-weighted mean DRP concentrations of 0.08 to 0.20 mg L^–1 for four agricultural drainage tiles in the Embarras River watershed. As in ditch and stream water, DRP concentrations in tile water increased with discharge. However, contrary to leaching patterns of nitrate or herbicides during a first flush following application (Gentry et al., 2000), DRP continues to be present at elevated concentrations in successive tile flow events (Fig. 6). As shown by Gächter et al. (2004), these data suggest that there was an available pool of soil P that readily desorbed during preferential flow of solutes through the soil and into tiles.

View larger version (23K): [in this window] [in a new window]   Fig. 6. Discharge and dissolved reactive phosphorus (DRP) concentrations from two agricultural tile drains in the Embarras River watershed showing three successive rain events during June 1997.

View larger version (16K): [in this window] [in a new window]   Fig. 7. Discharge, particulate phosphorus (PP), and dissolved reactive phosphorus (DRP) concentration from an agricultural tile drain in the Big Ditch watershed showing two successive rain events during June 2002.

Concomitant with elevated stream P concentrations, NH₄–N concentrations on that day were also the greatest observed for each watershed during the period of record (data not shown). As part of a larger water quality study, we sampled 11 stream locations in the Embarras, Kaskaskia, and Sangamon River watersheds (including EMC, LFK, and BDO) on 30 Jan. 2001 (Fig. 1). There was a significant increase in DRP (Mann–Whitney Test, W = 66.0, p < 0.001) and NH₄–N (W = 67.0, p < 0.001) in streams throughout east-central Illinois (Fig. 8). The highest concentrations observed occurred in the smallest watershed (the unnamed tributary in the Big Ditch watershed) measuring 5.0 mg L^–1 of DRP and 9.1 mg L^–1 of NH₄–N.

View larger version (11K): [in this window] [in a new window]   Fig. 8. Box-and-whisker plots of dissolved reactive phosphorus (DRP) and NH4–N concentrations (log10) from 11 streams throughout central Illinois before and during a rain-on-snow event that occurred 30 Jan. 2001. Horizontal lines in the box indicate the 25th, 50th, and 75th percentiles; the 10th and 90th percentiles are indicated by the whiskers and solid circles indicate concentrations outside of the 10th and 90th percentiles.

Following this rain-on-snow event, there was a substantial P load (80% in the form of DRP) exported from the three watersheds (4.6, 3.8, and 1.7 Mg at EMC, LFK, and BDO, respectively) in a 7-d period, which represented approximately 40% of the annual TP load for each stream. This single event increased the annual flow-weighted mean DRP and TP concentrations for all three streams during the 2001 water year. These data likely document a worst-case scenario for DRP transport from fields to streams and highlight how agricultural practices, climate, and precipitation interact to influence stream nutrient concentrations.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Extreme discharge events accounted for the majority of annual stream TP load as both DRP and PP concentrations increased with stream discharge, especially during high sediment loads in these tile-drained watersheds. Although there were several years with less than average precipitation, flow-weighted mean concentrations of TP in surface waters of central Illinois exceeded 0.1 mg L^–1 in all 16 watershed by water year combinations. Total P loads were greatly increased by overland runoff and extreme discharge in some years; however, tile drainage was likely an important contributor of DRP every year, beginning in late fall or early winter and ending in early summer. Overall, these data further our understanding of P transport pathways from agricultural fields to surface waters and underscore the difficult challenges facing the agricultural sector in reducing nonpoint source pollution of P to streams in extensively tile-drained watersheds. In addition, we observed conditions that created a worse case scenario for widespread P transport from agricultural fields to streams. The practice of applying P fertilizer on frozen soils or on top of snow can result in large inputs of P to surface waters when rain-on-snow events generate overland runoff which transports unincorporated P fertilizer to streams.

	ACKNOWLEDGMENTS

 
Funding was provided by the state of Illinois through the Council on Food and Agricultural Research (C-FAR) Water Quality SRI. We thank L. Keefer and E. Bauer for Big Ditch data, and S. Duke and R. McKeehan for Villa Grove sewage effluent data. We thank C. Davidson and J. Benson for field sampling, and S. Sippel for the site map.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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