Tile Effluent Research Articles

Our national nutrient reduction needs: Applying a conservation prioritization framework to US agricultural lands

Targeted conservation approaches seek to focus resources on areas where they can deliver the greatest benefits and are recognized as key to reducing nonpoint source nutrients from agricultural landscapes into sensitive receiving waters. Moreover, there is growing recognition of the importance and complementarity of in-field and edge-of-field conservation for reaching nutrient reduction goals. Here we provide a generic prioritization that can help with spatial targeting and applied it across the conterminous US (CONUS). The prioritization begins with identifying areas with high agricultural nutrient surplus, i.e., where the most nitrogen (N) and/or phosphorus (P) inputs are left on the landscape after crop harvest. Subwatersheds with high surplus included 52% and 50% of CONUS subwatersheds for N and P, respectively, and were located predominantly in the Midwest for N, in the South for P, and in California for both N and P. Then we identified the most suitable conservation strategies using a hierarchy of metrics including nutrient use efficiency (proportion of new nutrient inputs removed by crop harvest), tile drainage, existing buffers for agricultural run-off, and wetland restoration potential. In-field nutrient input reduction emerged as a priority because nutrient use efficiency fell below a high but achievable goal of 0.7 (30% of nutrients applied are not utilized) in 45% and 44% of CONUS subwatersheds for N and P, respectively. In many parts of the southern and western US, in-field conservation (i.e., reducing inputs + preventing nutrients from leaving fields) alone was likely the optimal strategy as agriculture was already well-buffered. However, stacking in-field conservation with additional edge-of-field buffering would be important to conservation strategies in 35% and 29% of CONUS subwatersheds for N and P, respectively. Nutrient use efficiencies were often high enough in the Midwest that proposed strategies focused more on preventing nutrients from leaving fields, managing tile effluent, and buffering agricultural fields. Almost all major river basins would benefit from a variety of nutrient reduction conservation strategies, underscoring the potential of targeted approaches to help limit excess nutrients in surface and ground waters.

Using artificial neural networks to improve phosphorus indices

The phosphorus index (PI) was developed as a field-scale assessment tool used to identify critical source areas of phosphorus (P) loss, thus most US states have adopted the PI as their strategy for targeted management and conservation practices for effective mitigation of P loss from agricultural landscapes to surface waters. Recent studies have focused on evaluating and updating PI weighting factors (WFs) to ensure agreement between PI values and measured losses of P. Given that the WF of each site characteristic is usually determined individually without considering possible interactions, the goal of this study was to demonstrate how artificial neural networks (ANNs) that consider real-world interdependence can be used to determine WFs. Our specific objectives were to evaluate ANN performance for predicting soluble P (SP) concentrations in tile effluent using site characteristics as predictor variables, and to evaluate whether ANN-generated WFs can be used to improve PI performance. Garson’s algorithm was used to determine the relative importance of each site characteristic to SP loss. Data from a monitored in-field laboratory were used to evaluate the ability of a PI with no WFs (PI_NO), a PI with WFs as proposed in the original Lemunyon and Gilbert PI (PI_LG), and a PI with ANN-generated WFs (PI_ANN), to estimate SP loss potential in tile discharge. Simulation results showed that the ANN model provided reliable estimates of SP in tile effluent (<i>R²</i> = 0.99; RMSE = 0.0024). The relative importance analysis underscored the value of routine soil P testing for agronomic sufficiency for environmental stewardship, and highlighted the necessity of prioritizing both contemporary and legacy P sources during P loss risk assessments. Unlike the other PIs, PI_ANN was able to provide reasonable estimates of SP loss potential as illustrated with significant exponential relationships (<i>R²</i> = 0.60; <i>p</i> &lt; 0.001) between PI_ANN values and measured mean annual SP concentrations in tile effluent. These findings demonstrate that ANNs can be used to develop PIs with a strong correlation to measured SP.

Biochar acting as an electron acceptor reduces nitrate removal in woodchip denitrifying bioreactors

Woodchip denitrifying bioreactors mitigate nitrate loading from agricultural tile effluents. Biochar is considered as an amendment to woodchips. Research findings are contradictory regarding the effect of biochar on nitrate removal. The current study is aimed at investigating the effect of biochar on the removal of nitrate in denitrifying bioreactors. For this purpose, laboratory and field experiments were carried out. In the laboratory, cylindrical up-flow bioreactors were used that were filled with woodchips and oxidized biochar at rates of 0%, 12.5%, 25%, 50% by volume. The removal of nitrate was investigated at two stages: unaged and aged for 16weeks. The bioreactors were operated at four hydraulic retention times of 2, 4, 8, and 24h. Additionally, we measured the concentrations of greenhouse gases in the laboratory bioreactors. In the field, nitrate removal over a 6-year period was investigated in two denitrifying bioreactors, one of which amended with 10% fresh biochar.The laboratory experiments showed that, on average, oxidized biochar (OB) amendment reduced nitrate removal by 9% in unaged and 13% in aged bioreactors while the respiration was equal or greater. This led us to conclude that oxidized biochar acted as an electron acceptor. In addition, oxidized biochar increased greenhouse gas concentrations.In the field bioreactors, fresh biochar initially increased nitrate removal. However, biochar became less effective over six years. In the final year, the biochar-amended bioreactor removed less nitrate than the woodchip bioreactor, which was ascribed to the electron-accepting ability of biochar due to aging. In sum, our study showed that biochar's electron acceptability significantly reduced denitrification in denitrifying bioreactors.

Controlling tile drainage during the growing season in Eastern Canada to reduce nitrogen, phosphorus, and bacteria loading to surface water

Drainage water management such as controlled tile drainage (CTD) is one means to help meet pollution mitigation targets and boost crop yields. In this study, CTD was retrofit to existing tile drained fields in eastern Ontario, Canada (humid continental climate) to study water quality benefits. A suite of paired field systems were used to compare CTD tile drainage quality with conventional tile drainage quality for nine growing seasons (2005–2013), translating to 35 field-crop years. Field crops were corn, soybean and forage. For CTD fields, controlled tile drainage was employed only during the growing season (time period when comparisons were made) due to surface runoff/erosion and growing season length concerns associated with non-growing season flow control. Water quality targets in tile effluent included: nitrate, ammonium, total phosphorus, dissolved reactive phosphorus, and fecal indicator bacteria such as E. coli, and Enterococci. Respectively, there were 60, 51, 58, 66, 66, 76, and 25% reductions in above noted drainage water fluxes and water quality targets as a result of CTD (for all 35 field-crop years combined). Concurrent environmental and potential public health benefits of managing tile drainage during the growing season were demonstrated; moreover, over the course of the study, corn and soybean yields were significantly boosted by CTD.

Modeling the impacts of tillage practices on water table depth, drain outflow and nitrogen losses using DRAINMOD

Water pollution by nitrate removed from agricultural systems through subsurface tile drains is a major environmental concern in Ontario. The control and reduction of the loading of chemicals to water bodies requires knowledge on the contribution of agricultural systems to water pollution under different cropping management practices. In this study, DRAINMOD model was implemented to predict the effects of conventional tillage (CT) and no tillage (NT) treatments on water table depth, tile effluent and NO3–N losses through subsurface drainage from a 14ha agricultural field at the Green Belt Farm of Agriculture and Agri-Food Canada in Ottawa, Ontario. During the study period, flow was significantly higher under NT than CT mainly during the snowmelt and spring periods. Total nitrate loss under NT was higher than CT but the difference was not significant during the 4years of study period. The climate change scenario of PRECIS was then applied to assess the effect of tillage practices under future climate change. In the future, the changes in climate will result in a significant increase in flow and nitrate under both NT and CT practices. It was also noted that, in future, NO3–N losses under no till treatment are expected to be significantly higher than those under conventional tillage practices in future.

Dual permeability modeling of tile drain management influences on hydrologic and nutrient transport characteristics in macroporous soil

Summary Tile drainage management is considered a beneficial management practice (BMP) for reducing nutrient loads in surface water. In this study, 2-dimensional dual permeability models were developed to simulate flow and transport following liquid swine manure and rhodamine WT (strongly sorbing) tracer application on macroporous clay loam soils under controlled (CD) and free drainage (FD) tile management. Dominant flow and transport characteristics were successfully replicated, including higher and more continuous tile discharge and lower peak rhodamine WT concentrations in FD tile effluent; in relation to CD, where discharge was intermittent, peak rhodamine concentrations higher, and mass exchange from macropores into the soil matrix greater. Explicit representation of preferential flow was essential, as macropores transmitted >98% of surface infiltration, tile flow, and tile solute loads for both FD and CD. Incorporating an active 3rd type lower boundary condition that facilitated groundwater interaction was imperative for simulating CD, as the higher (relative to FD) water table enhanced water and soluble nutrient movement from the soil profile into deeper groundwater. Scenario analysis revealed that in conditions where slight upwards hydraulic gradients exist beneath tiles, groundwater upwelling can influence the concentration of surface derived solutes in tile effluent under FD conditions; whereas the higher and flatter CD water table can restrict groundwater upwelling. Results show that while CD can reduce tile discharge, it can also lead to an increase in surface-application derived nutrient concentrations in tile effluent and hence surface water receptors, and it can promote NO 3 loading into groundwater. This study demonstrates dual permeability modeling as a tool for increasing the conceptual understanding of tile drainage BMPs.

Tile Drainage Management Influences on Surface-Water and Groundwater Quality following Liquid Manure Application

This study investigated the potential for controlled tile drainage (CD) to reduce bacteria and nutrient loading to surface water and groundwater from fall-season liquid manure application (LMA) on four macroporous clay loam plots, of which two had CD and two had free-draining (FD) tiles. Rhodamine WT (RWT) was mixed into the manure and monitored in the tile water and groundwater following LMA. Tile water and groundwater quality were influenced by drainage management. Following LMA on the FD plots, RWT, nutrients, and bacteria moved rapidly via tiles to surface water; at the CD plots, tiles did not flow until the first post-LMA rainfall, so the immediate risk of LMA-induced contamination of surface water was abated. During the 36-d monitoring period, flow-weighted average specific conductance, redox potential, and turbidity, as well as total Kjeldahl N (TKN), total P (TP), NH-N, reactive P, and RWT concentrations, were higher in the CD tile effluent; however, because of lower tile discharge from the CD plots, there was no significant ( ≤ 0.05) difference in surface water nutrient and RWT loading between the CD and FD plots when all tiles were flowing. The TKN, TP, and RWT concentrations in groundwater also tended to be higher at the CD plots. Bacteria behaved differently than nutrients and RWT, with no significant difference in total coliform, , fecal coliform, fecal streptococcus, and concentrations between the CD and FD tile effluent; however, for all but , hourly loading was higher from the FD plots. Results indicate that CD has potential for mitigating bacteria movement to surface water.

Effect of tile effluent on nutrient concentration and retention efficiency in agricultural drainage ditches

Abstract Tile drainage is a common water management practice in many agricultural landscapes in the Midwestern United States. Drainage ditches regularly receive water from agricultural fields through these tile drains. This field-scale study was conducted to determine the impact of tile discharge on ambient nutrient concentration, nutrient retention and transport in drainage ditches. Grab water samples were collected during three flow regimes for the determination of soluble phosphorus (SP), ammonium nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 -N) concentrations and their retention in three drainage ditches. Measured nutrient concentration indicated lower SP and NH 4 + -N, and greater NO 3 -N concentrations in tile effluents compared to the ditch water. Net uptake lengths were relatively long, especially for NO 3 -N, indicating that nutrients were generally not assimilated efficiently in these drainage systems. Results also indicated that the study reaches were very dynamic showing alternating increases or decreases in nutrient concentration across the flow regimes. The drainage ditches appeared to be nutrient-rich streams that could potentially influence the quality of downstream waters.

Assessment of a Large Subsurface Controlled Drainage and Irrigation System: III. Water chemistry of the tile effluent and its potential impact on surface water resources

Abstract Controlled subsurface drainage irrigation systems promote crop productivity; however, these land management systems also allow an efficient pathway for the transport of elements from soils to surface water resources. The nitrate and macro-element effluent concentrations from tile-drainage involving a 40 ha controlled subsurface drainage irrigation system are described and compared to soil nitrate availability. Soil nitrate concentrations generally show an increase immediately after soil nitrogen fertilization practices and are sufficiently abundant to promote their transport from the soil resource to the tile-drain effluent waters. The data indicates that: (1) the transport of nitrate-N in tile-drain effluent waters is appreciable; (2) denitrification pathways effectively reduce a portion of the soil nitrate-N when the controlled drainage system establishes winter-early spring anoxic soil conditions, and (3) the best strategy for reducing nitrate-N concentrations in tile-drain effluent waters is adjusting N fertilization rates and the timing of their application. The development of bioreactors for simulating wetland conditions may further limit nitrate concentrations in surface waters because of soil drainage.

Pharmaceutical and personal care products in tile drainage following land application of municipal biosolids

Land application of municipal biosolids (sewage) is a common farming practice in many parts of the world. There is potential for transport of pharmaceuticals and personal care products (PPCPs) from agricultural fields to adjacent surface waters via tile drainage systems. In this study, liquid municipal biosolids (LMB) (total solids = 11,933 mg L − 1 ), supplemented with selected PPCPs and the fluorescent dye tracer rhodamine WT (RWT), were applied to tile drained fields using two land application approaches. Objectives included evaluating the relative benefits of land application practices with respect to reducing PPCP loadings to tile drains, evaluating PPCP persistence in tile water, and determining whether rhodamine WT can be used to estimate PPCP mass loads in tile. The PPCPs examined included an antibacterial agent used in personal care products (triclosan), a metabolite of nicotine (cotinine), and a variety of drugs including two sulfonamide antimicrobials (sulfapyridine, sulfamethoxazole), a β-blocker (atenolol), an anti-epileptic (carbamazepine), an antidepressant (fluoxetine), analgesic/anti-inflammatories (acetaminophen, naproxen, ibuprofen), and a lipid-regulator (gemfibrozil). Maximum observed PPCP concentrations in the spiked LMB were about 10 3 ng g − 1 dry weight. PPCPs were shown to move rapidly via soil macropores to tile drains within minutes of the land application. Maximum observed PPCP concentrations in tile effluent associated with the LMB application-induced tile flow event were ~ 10 1 to 10 3 ng L − 1 . PPCP mass loads, for the application-induced tile-hydrograph event, were significantly ( p < 0.1) higher for surface spreading over non-tilled soil (incorporation tillage occurring 20 h post-application), relative to aerating soil immediately prior to surface spreading using an AerWay® slurry deposition system. PPCP concentrations that were detected above the limit of quantitation (LOQ) in tile water during several precipitation-induced tile flow events that occurred post-application, included: triclosan (max. ~ 1.5 × 10 2 ng L − 1 ), carbamazepine (max. ~ 7 × 10 1 ng L − 1 ), atenolol (max ~ 4 × 10 1 ng L − 1 ), and cotinine (max ~ 2 × 10 1 ng L − 1 ). In spite of their presence in biosolids, the other PPCPs were not observed above LOQ concentrations during these events. PPCP concentrations were predicted from RWT concentrations over a 40 day study period. Tile mass loads as a percent of PPCP mass applied to soil ranged from 4.2% ± SD of 9.2% to 7.1% ± 10.9% for the AerWay® system and surface spreading plus incorporation treatments, respectively.

Intra-annual variability in the contribution of tile drains to basin discharge and phosphorus export in a first-order agricultural catchment

This study examines spatiotemporal variability (event-based, seasonal) in the contribution of drainage tiles within a basin to basin hydrologic discharge and soluble reactive phosphorus (SRP) and total phosphorus (TP) export over a period of 1 year. Tile discharge was highly variable at both moderate (wet versus dry periods) and smaller (within-event) temporal scales, accounting for 0–90% of basin discharge at any given time. An estimated 42% of basin annual discharge originated from drainage tiles, the majority of which occurred during the winter and spring months. Concentrations of SRP and TP in drainage tile effluent were also highly variable in space and time (1–2850 μg SRP L −1, 5–8275 μg TP L −1). Higher concentrations of SRP and TP were linked to fields receiving manure compared to fields receiving inorganic fertilizers. SRP export from tiles accounted for 118% of basin SRP export on average, although their contribution to basin SRP export ranged from 4 to 344% on 32 discrete dates during which all tiles in the basin were sampled for hydrochemistry. On the same 32 dates, tiles accounted for an average of 43% of basin TP export, although this ranged from 0 to 200%. Management options such as tile plugs and optimizing the timing and application rates of fertilizer should be explored to minimize nutrient export from tiles.

Modelling Approaches for Subsurface Drainage Water Quality Management

Abstract Contamination of surface waters by agricultural activities is a serious problem. Two different modelling approaches to simulate nutrient and pesticide transport in subsurface drained soils were investigated in this study. First, artificial neural network (ANN) models, a trainable fast back-propagation (FBP) network and a self-organizing radial basis function (RBF) network, were developed for simulation of NO3--N concentration in tile effluent. Second, a hydrologic model, DRAINMOD, was linked with a chemical transport model, GLEAMS, to simulate chemical transport of atrazine through the soil into subsurface drain outflow. The ANN models and linked DRAINMOD-GLEAMS model were calibrated and validated against experimental data collected at the Greenbelt Research Farm of Agriculture Canada during the years 1988, 1989 and from 1991 to 1994. Several statistical parameters were calculated to evaluate model performance. A comparison of results indicated that the RBF neural network model was superior to the FBP model in predicting drain outflow and NO3--N concentration. Results obtained from the linked DRAINMOD-GLEAMS model demonstrate that atrazine simulations were underpredicted in subsurface drain outflows for spring and fall seasons. Both modelling approaches provide a useful tool for management of fertilizer/manure and pesticides transport through soil and crop root zones into surface water.

Dual-permeability modeling of preferential bromide leaching from a tile-drained glacial till agricultural field

A tile-drained agricultural field can be regarded as a ‘field-scale’ lysimeter that may be used to study soil water and chemical transport under relatively natural conditions. Tile discharge and effluent bromide concentrations measured in a previous field tracer experiment for a structured clayey loam at Bokhorst, Northern Germany, indicated strong preferential flow. Simulation using single domain HYDRUS numerical flow and transport model could nevertheless explain water outflow, however, completely failed to describe tile-drain leaching patterns of the conservative tracer. The objective of this paper was to analyze whether the nonequilibrium-type dual-permeability model concept could better capture soil structure related principle mechanisms of preferential leaching in the unsaturated soil at that study site. The dual-permeability model (DUAL) describes for soil matrix and fracture pore systems Darcian flow with coupled Richards' equations and convective–dispersive (CD) solute transport with coupled CD equations. The hydraulic parameters of the dual-permeability model were obtained from standard soil hydraulic measurements by adopting a bimodal fitting procedure, whereas transport parameters were inferred from soil column tracer experiments and geometrical transfer term parameters were derived using qualitative soil structure descriptions. The hydraulic conductivity K a in the inter-domain water transfer term and the effective diffusion coefficient D a in the solute mass transfer term were calibrated by comparing simulated with measured tile flow and effluent concentrations. The DUAL approach described water flow similarly well as the single-domain model. Bromide concentrations in the tile effluent could be approximated with DUAL when decreasing the K a and D a values by three orders of magnitude compared with the values of the soil matrix domain. The dual-permeability approach seems to reflect nonequilibrium transport mechanisms at this structured soil since it not only predicts concentration peaks in initial leaching phases during early time events of the experiment but also the observed reverse reaction of decreased effluent concentrations in initial phases of leaching events during later times of the experiment. The latter occurs when bromide-free infiltrating water is diluting bromide concentrations in the fracture domain such that less concentrated water is preferentially draining from tiles in initial phases of infiltration. Bromide effluent concentrations gradually increase in later phases since vertical flow velocities decrease while ‘diffusive’ solute transfer from the soil matrix continues. Mass transfer restrictions (e.g. due to soil aggregate coatings) may effectively be controlling preferential leaching at this site.

Neural networks for predicting nitrate-nitrogen in drainage water

Two artificial neural network (ANN) models, a trainable fast back-propagation (FBP) network and a self-organizing radial basis function (RBF) network, were developed for simulation of subsurface drain outflow and nitrate-nitrogen concentration in tile effluent. Experimental data collected at the Greenbelt Research Farm of Agriculture Canada over a 40-month period were used to train and validate the two models. The available field data were divided into training and testing scenarios, with the training file consisting of eight inputs and two outputs. A sensitivity analysis was performed by varying the network parameters to minimize the prediction error and determine the optimum network configuration. The best architecture for the FBP model comprised of 20 neurons in the hidden layer and a learning rate of 0.02, while the RBF network with a tolerance of 20 and a receptive field of 15 yielded 547 neurons in the hidden layer. Overall, the performance of the RBF neural network was superior to the FBP model in predicting the concentration of nitrate-nitrogen in drain outflow due to the application of manure and/or fertilizer. This information, in turn, can be used for proper fertilizer management; thereby, reducing not only the loss of valuable nitrogen fertilizer but also the potential for pollution of subsurface water by nitrate.

The effect of irrigation on tile sediment transport in a headwater stream

A field-scale no-till corn plot (120 m×90 m) located on a tile drained silt loam soil near Kintore, Ontario was irrigated with 2.5 cm of water over a 3 h period to examine the effects of irrigation on tile sediment transport in a headwater stream. Flow characteristics and the composition, concentration and size distribution of suspended solids were measured at the tile outlet, an upstream reference site and three sites located downstream of the tile drain. Results show that tile sediments at the study site are fine-grained ( D 50≈5.0 μm) and consist primarily of quartz, anorthite/albite, dolomite and calcite. Sediment concentrations in tile effluent increased from 8 to 57 mg L −1 after 1.5 h of irrigation and reached a maximum of 72 mg L −1. The sediment yield from the tile drain for the irrigation event was 4.6 kg ha −1. An unsteady, mobile boundary flow model (MOBED) was used to predict flow characteristics in the stream. According to the MOBED model, bed shear stress in the stream was approximately 6 N m −2. This value is significantly greater than the critical shear stress for complete suspension of 1 N m −2 for tile sediments as determined from laboratory experiments using a rotating circular flume. Grain size distributions of suspended solids in the stream were close to the dispersed size distribution because of the high shear stress in the receiving stream.

PHOSPHORUS TRANSPORT INTO SUBSURFACE DRAINS BY MACROPORES AFTER MANURE APPLICATIONS: IMPLICATIONS FOR BEST MANURE MANAGEMENT PRACTICES

Land application of liquid manure can result in nutrient enrichment of subsurface drainage effluent when conditions promote leaching or macropore flow. This contamination is most likely to occur when precipitation follows manure application closely and may cause environmental impacts to receiving waters. Field and column studies were initiated in New York to investigate the impact of manure applications on phosphorus (P) transport through the soil into subsurface drains. Field studies evaluated tile effluent contamination from liquid manure under wet and dry antecedent soil moisture conditions (year 1) and under disk and plow tillage practices (year 2). In year 1, liquid dairy manure was broadcast on the surface and the field was then irrigated. Though the tile drains in the wet plots flowed much earlier and in greater volume than the drains in the dry plots, both wet and dry plots produced similar average peak total phosphorus (TP) concentrations. Irrigation 6 days later produced similar tile discharges, but the peak TP concentrations were about one-third of the earlier values. Cumulative TP loss was significantly higher from wet than dry plots. In year 2, manure was tilled into the soil via one-pass disking or plowing before irrigation commenced. The disking did not incorporate the manure into the soil as effectively as did plowing and exhibited one order of magnitude higher effluent TP concentrations and cumulative TP loss. The timing of P transport in tile effluent relative to the tile flow is consistent with macropore transport as the primary mechanism moving TP through the soil. Column studies utilizing packed soil and artificial macropores were used to examine further the role of macropore size on P sorption to pore walls. Dissolved P was added directly to the macropore, and the effluent from the macropore showed that soluble P may be transported through macropores 1 mm or greater with negligible P sorption to pore walls. In the absence of macropores, no measurable P was transported through the soil columns. Consequently, high P concentrations observed in the tile drain effluent soon after manure application during the field studies can be attributed to macropore transport processes. Even small continuous macropores are potential pathways. Plowing-in manure apparently disturbs these macropores and promotes matrix flow, resulting in greatly reduced P concentrations in the drainage effluent.

Tile Effluent Quality and Chemical Losses Under Conventional and No Tillage — Part 1: Flow and Nitrate

The effect of no-tillage (NT) and conventional tillage (CT) treatments on tile drain flow, and nitrate concentration and loss in tile effluent was studied over a 40-month period in loam soil, corn fields (Zea mays L.) of approximately 3 ha each. Specific conductivity and pH of tile effluent, and corn silage yield were also determined. During the January 1991 to early May 1994 study period, flow was significantly (p < 0.05) higher under NT than CT mainly due to flow difference during the snowmelt and spring periods. The drinking water limit of 10 mg/L for nitrate-nitrogen (NO3-N) was exceeded in over 90% of the 1010 tile effluent samples that were analyzed. Flow-weighted average NO3-N concentrations during tile flow events tended to be higher under CT than NT but treatment effect was significant (p < 0.05) only in one crop-year, 1993-1994. The nitrogen lost as nitrate during the study period represented more than 20% of the amount applied as anhydrous ammonia. Annual losses ranged from 10% to 30%. Most loss occurred during the spring period when tile flow was highest. Over the 40-month study period, NO3-N loss in tile effluent was not significantly different for the two treatments. It was significantly (p < 0.05) higher under NT than CT only in one crop-year due to significantly higher flow under NT. There was no tillage treatment effect on tile effluent pH and specific conductivity. Corn silage yield was significantly higher (p < 0.01) under CT than NT during two years only when a burnout herbicide was not used prior to planting.

Tile Effluent Quality and Chemical Losses Under Conven-tional and No Tillage — Part 2: Atrazine and Metolachlor

The long-term effect of no-tillage (NT) and conventional tillage (CT) on the concentration and loss of the atrazine, deethylatrazine and metolachlor in tile effluent was studied over a 40-month period in four loam soil, corn (Zea mays L.) fields of approximately 3 ha each. Atrazine and deethylatrazine were detected at low concentrations in most of the 773 samples collected between January 1991 and early May 1994. Under both tillage treatments, atrazine concentrations were mostly below the USEPA advisory of 3 mg/L while concentrations of atrazine plus deethylatrazine were generally below 5 mg/L, the Canadian Interim Maximum Acceptable Concentration (IMAC) for the sum of atrazine and its metabolites. Concentrations exceeding these limits were mainly observed during rainfall-induced flow events within a few days or weeks following herbicide application. The flow-weighted average concentrations of atrazine during flow events were significantly (p < 0.05) higher under NT than CT. Metolachlor was also detected at low concentrations, and in relatively few flow events. Concentrations were always well below the USEPA advisory of 100 mg/L or the Canadian IMAC of 50 mg/L. Annual loss of herbicides in tile effluent ranged from 0.02% to 0.34% of the amount applied. Most loss occurred during spring flow events when tile flow was the highest. Atrazine and deethylatrazine losses under the NT treatment were significantly (p < 0.05) higher than those under the CT treatment.

Modification of DRAINAGE Model by Using the Nitrogen Component from the GLEAMS Model

The NITRO subroutine of the DRAINAGE model (Kanwar et al., 1983) was modified using the nitrogen transformation components of the GLEAMS (Leonard et al., 1987) model to predict more accurately the leaching of NO3-N to subsurface drainage water. Predicted values of tile flows and nitrate concentrations in tile effluent have shown a good agreement with observed data for the period from 1984 to 1992. There were some discrepancies between the predicted and observed values in the beginning of the simulation period resulting from lack of field data for soil-profile initialization. Despite the assumed steady-state condition within each time increment (one day) and the complexity of the drainage system, the modified DRAINAGE model has shown the capability to reasonably estimate long-term N loss with tile effluent. Average deviation and standard error between the predicted and observed NO3-N concentrations in the tile water indicated that the modified DRAINAGE model developed in this study resulted in better predictions of NO3-N concentrations in the drainage water than the original DRAINAGE model.

Herbicide and tracer movement to field drainage tiles under simulated rainfall conditions

The extent of herbicide and tracer leaching to field drainage tiles may help to predict chemical movement to deeper groundwater systems. Field experiments were conducted in 1988 and 1989 to measure herbicide and tracer movement to tile lines during and immediately after a simulated heavy rainstorm. The eight tile lines monitored were 1.2 m deep and 3.4 m long. In 1988, alachlor (2-chloro- N-(2,6-diethylphenyl)- N-(methoxymethyl)acetamide), cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile) and Rhodamine WT dye were applied to 4.5-m 2 plots directly over field drainage tiles. In 1989, alachlor, cyanazine, Rhodamine WT, and pendimethalin ( N-(1-ethylpropyl)-3,4 dimethyl-2,6 dinitrobenzenamine) were used. Chloride or bromide tracers were also soil applied. The plots were irrigated 24 h after chemical application with a rainfall simulator. In 1988, 53 mm of rainfall was applied, whereas 81 mm of rain was applied in 1989. Tile effluent was intensively sampled for 8 h after irrigation. In 1988, herbicide and Rhodamine dye concentration peaks ranged from 1 to 38 μg l −1. In 1989, concentrations were higher than in 1988, with alachlor and cyanazine concentration peaks exceeding 500 μg l −1 from one tile line. In contrast, pendimethalin was found in only one sample, barely above the detection limit. Rhodamine WT was found to be useful as a tracer to study the movement of alachlor and cyanazine in the soil profile. In all tile effluent samples containing Rhodamine WT, alachlor and cyanazine were also detected. In both years, herbicide and dye concentrations peaked within 130 min after the start of rainfall. The rapid solute movement to the 1.2 m tile depth suggests that preferential flow is an important mechanism affecting chemical transport through structured soils.