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A three year field study was undertaken to assess the influence of water table management, namely subsurface irrigation and controlled drainage, on the movement of pesticides through the soil profile into ground water. The herbicides under investigation were prometryn on an organic soil, and metolachlor on a sandy soil. Both soil and groundwater were collected and analyzed. The results presented are those obtained in the first two years of the project. / In the organic soil, herbicide leaching was greatly reduced due to the management of the water table. The pesticide remained higher in the soil profile, preventing leaching to the drains and allowing degradation. / The opposite effect appeared to have occurred in the sandy soil. The higher water table resulting from subsurface irrigation may have induced the leaching of the contaminant into lower soil levels and into the ground water. The high water solubility of the herbicide metolachlor, in conjunction with low microbial activity, may have played a role in this phenomenon.

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The North American cranberry plant (Vaccinium macrocarpon Aiton) is a wetland crop species grown commercially in natural or constructed peat or sandy soil basins. Since production is highly water dependent, water requirements are very significant and have prompted growers to explore new water management practices to improve irrigation efficiency and protect water resources. One way of conserving and better managing water, given the infrastructure in place, would be to develop sub-irrigation. / The design of a subsurface irrigation system requires the evaluation of various soil properties. This was undertaken at four established cranberry production sites, situated near Saint-Louis-de-Blanford, Quebec. Two of the fields contained a sandy soil, and the other two were peat soils. Soil physical properties measured included: saturated hydraulic conductivity, bulk density, porosity, soil moisture characteristic curves and particle size distribution.

Increasing levels of nitrate (NO3-) in groundwater have become a major environmental and health concern. In situations where NO3-concentrations in the soil-water system pose an environmental hazard, water table management may be a desirable practice to reduce such pollution. Careful management of N applications is also believed to reduce NO3- levels. / A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Quebec, about 30 km South-West of Macdonald Campus of McGill University, to investigate the effect of water table management (WTM) and N fertilizer combinations on potentially leachable NO3- and denitrification rates in the top soil layer (0--0.15 m). The field was planted with monocrop corn (Zea mays. L) in both years. Treatments consisted of a factorial combination of two water table managements, free drainage (FD) and subirrigation (SI) (about 1.0 m and 0.6 m, respectively, below the soil surface) and two N fertilizer rates, 200 kg ha-1 (N200) and 120 kg ha-1 (N120). / Water table management had a significant effect on reducing NO3 - concentrations in the soil profile. Subirrigation treatment reduced NO3- in the top soil layer by 41% and 15% in 1996 and 1997, respectively. Similarly, NO3 - levels were 50% and 20% lower in N120 compared to N200 treatment. / Climatic conditions (rainfall and temperature) played a large role in regulating denitrification rates. Due to drier and cooler conditions in 1997, denitrification rates were lower compared to 1996, leaving more NO3 - in the soil profile. Following harvest, this high NO 3- concentration may be subject to leaching.

The use of agricultural chemicals, such as nitrogen fertilizers in corn production, often results in water pollution. This research, comprising three parts, was designed to investigate the effects of nitrogen fertilizer application rates, water table management, and corn cropping systems on drainage water quality. The first part was a field study, to investigate the impact of two cropping systems and water table management on nitrate loss through tile drainage. The considered water table treatments were free drainage, and subirrigation with target water table depths at 0.5 m or 0.75 M below the soil surface. Corn (Zea mays L.) monoculture and corn intercropped with annual ryegrass (Lolium multiflorum Lam.) were investigated. The highest annual tile drainage losses of 21.9 kg N/ha were measured in monocropped, freely draining plots. Subirrigation with a water table depth of 0.5 m reduced tile drainage loss of N by over 70%, and intercropping corn with ryegrass under free drainage reduced leaching losses by 50%. / The second part of the research was a simulation study with the water quality model, DRAINMOD-N. The water quality impact of fertilizer application rate under free drainage, subirrigation and controlled drainage was evaluated. Leaching losses, denitrification and N accumulation in the soil profile were investigated. Using data obtained from the field experiment, the performance of DRAINMOD-N was evaluated. DRAINMOD-N assumes that denitrification follows first order kinetics, contrary to field measurements which showed little correlation between denitrification rate and NO$ sb3 sp-$-N concentration. Therefore, DRAINMOD-N was modified by replacing the original denitrification function with the Michaelis-Menten relationship. In so doing, denitrification is expressed as a first order process when NO$ sb3 sp-$-N concentration limits denitrification, and as a zero order process for non-limiting NO$ sb3 sp-$-N concentration. / For denitrification to be a decision making criterion of water table management, inexpensive but reliable measurement techniques are required. Thus, the purpose of the final part of this research was to formulate a technique for measuring real-time denitrification rate. Denitrification rate could be expressed as a function of soil redox potential (Eh) and temperature. Laboratory and field studies showed that factors such as soil nitrate and organic carbon had negligible effect on denitrification rate. Therefore, it can be concluded that for most agricultural soil, Eh and soil temperature will satisfactorily describe denitrification variation.

Water table management (WTM), consisting of controlled drainage and subirrigation, can significantly reduce nitrate losses through subsurface drainage. However, recent research showed that this system could increase phosphorus (P) losses in tile drains. The cause of this increase, whether related to enhanced P solubility due to shallow water tables seen with WTM or to the addition of P by the subirrigation water, has not been investigated. / The goal of this research was to evaluate the impact of WTM on P losses and on soil P concentration, soil P saturation and soil pH with a field study. Furthermore, a laboratory soil column study was conducted to better understand the results of the field study. / The field study was carried out in 2004 and 2005 in Coteau-du-Lac (Quebec). The WTM system, with a design water table set at 0.60 m below the soil surface, was compared to conventional free drainage (FD). The results showed that WTM had no effect on the soil P concentration and saturation. However, soil pH was slightly increased in WTM plots, which may demonstrate a greater P solubility. Total outflow volumes from WTM plots were reduced by 19% and 27% in 2004 and 2005, respectively. However, P loads in drainage water from plots under WTM were greater than from FD plots due to increased P concentrations, which always exceeded Quebec's surface water quality standard of 0.03 mg/L. Of the total P concentration, around 96% was dissolved P in both treatments. / The laboratory soil column experiment, in which the two drainage treatments were simulated, also showed that P concentrations were greater under WTM than FD. This suggests that the increased P losses in the field with WTM were most likely caused by the increased P solubility due to a shallow water table rather than by the addition of P by the water used for subirrigation.

Subirrigation systems are generally used in humid areas to provide suitable moisture conditions for plant growth. These systems can also be used to reduce pesticide loadings from agricultural lands, since they tend to keep the discharging waters within farm boundaries for extended periods of time. This allows for greater pesticide microbial and chemical degradation. / A three-year field lysimeter study was initiated to investigate the role of subirrigation systems in reducing the risk of water pollution from the three most commonly used herbicides in Quebec, namely atrazine (2-chloro-4[ethylamino]-6[isopropylamino]-1,3,5-triazine), metribuzin (4-am ino-6(1,1-di meth yl eth yl)-3-(meth yl thio)-1,2,4-tria zin-5(4H)-one), and meto lach lor (2-chlo ro-N-(2-eth yl-6-methyl phen yl)-N-(2-meth oxy-1-meth yl eth yl)acet amide). Eighteen PVC lysimeters, 1 m tall x 0.45 m diameter, were packed with a sandy soil. Three water table management treatments, i.e. two subirrigation treatments with constant water table depths of 0.4 and 0.8 m, respectively, and a free drainage treatment in a completely randomized design with three replicates were used. Grain corn (Zea mays L.) and potatoes (Solanum tuberosum L.) were grown on lysimeters, and herbicides were applied each year at the locally recommended rates at the beginning of each summer. Soil and water samples were collected at different time intervals after each natural or simulated rainfall event. Herbicides were extracted from soil and water samples and were analyzed using Gas Chromatography. / From the three years results (1993--1995), it has been concluded that all three herbicides were quite mobile in this sandy soil, as they leached to the 0.85 m depth below the soil surface quite early in the growing season. This suggests that if the drainage effluent or seeping waters from sandy soils of agricultural lands in southern Quebec drain freely, they may be considered to be a serious non-point source of pollution to the water bodies. The results have also shown that herbicide concentration decreased with soil depth as well as with time, meaning that the higher herbicide residues were found at top layers, and soon after the herbicide application. The herbicide mass balance study revealed that when the drainage effluent was kept within the lysimeters under the subirrigation setup, there was a statistically significant reduction of atrazine and metribuzin residues (shorter half lives) in the adsorbed and liquid phases. However, the reduction in metolachlor concentration under the subirrigation system was not statistically significant. These findings suggest that subirrigation, combined with certain herbicides can significantly reduce the herbicide loadings from corn and potato farms in southwestern Quebec, and become environmentally beneficial. / A computer simulation model (PRZM2), was used to simulate atrazine, metribuzin, and metolachlor leaching in the lysimeters, under subsurface drainage conditions. The simulated values for all three chemicals in most of the cases followed the leaching pattern of observed data. But the model either under- or over-estimated the herbicide concentrations in the soil. This could have been caused by simplistic instantaneous linear adsorption/desorption of herbicides, and inadequacy of conventional Darcian approach for the treatment of matrix flow.

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