Document Type

Thesis

Degree Name

Master of Environmental Studies (MES)

Department

Geography & Environmental Studies

Faculty/School

Faculty of Arts

First Advisor

Michael English

Advisor Role

Thesis Supervisor

Abstract

Agricultural practices may result in nitrate losses, diminished surface water quality and widespread groundwater aquifer contamination. Many works have identified preferential flow through macropores as an important feature that may increase nitrate losses. Subsurface drainage collectors may intercept preferential flow and indicate changing unsaturated/saturated processes. Antecedent conditions are important in determining runoff response. It is unclear how tile drains modify the antecedent soil moisture regime and water tables in near stream areas. Vegetated riparian buffer zones found beetween agricultural fields and watercourses may attenuate nitrate in shallow groundwater pathways. However, little work recognizes the close proximity of buffer strips and tile drainage systems nor studied their interactions. This thesis provides results from sstudy at two sites within the 3.5 km2 Strawberry Creek Agricultural Watershed in southern Ontario. The goal of the study was to identify some of the features controlling runoff and nitrate export from tile drained fields in agricultural watersheds with macroporous soils and narrow vegetated riparian zones. Evaluation of a two-year data set including meteorological data, water table levels, nitrate chemistry and tile/stream discharges indicate that antecedent moisture conditions exhibit control on watershed runoff response and nitrate exports. Initial wetting periods are needed to promote large-scale nutrient export. Greater moisture deficits and preferential flow mechanisms are thought to reduce nitrate export during drier periods. Moist soils and high water tables allowed better transport pathways and consequently greater runoff and nitrate export. Maximum export rates from the study basin were 282 kg NO3 -N d-1 (0.8 kg NO3 -N ha-1 d-1) during a mid-winter rain/melt event in 1998. At one site near the basin outflow, an investigation of tile flow and nitrate export during a single storm detected the effects of preferential flow in upland tile drains. Bypassing of event water caused a marked dilution in both tile drainage and basin outflow. This bypassing event occurred when soils had significant soil moisture deficits (>50%), water tables were below and/or at tile drain level (0.8 m) and soil nitrate levels within plough horizons were moderate, estimate point measurements to be 76 kg NO3 -N ha-1. The sequence of dilution ended with cessation of precipitation and concentrations of tile drains and stream returned to background levels, an indication that groundwater inputs dominated post event flows. At headwater site, seasonal patterns in groundwater/nitrate pathways, tile and stream nitrate export were examined in a gently sloped field. Bypassing is frequently detected, provided vadose zones reach tile drains. Under wet antecedent conditions, tiles nitrate export increased. Drains may help create near stream groundwater ridges in the near stream zones effecting groundwater pathways. Lateral groundwater flow paths were found to deliver little water or nitrate to the stream. Removal of nitrate in the buffer and hyporheic zones was very evident through the fall, winter and early spring. Large vertical hydraulic gradients and hydraulic conductivity helped deliver this dilute water to the stream channel. This study highlights the fact that in some periods tile drains are well connected with preferential flow paths. Preferential flow delivered only dilute event water to tiles in this study. In wet, high water table conditions, tile drains export major portions of basin nitrate exports. As a result of effective nitrate removal in buffer and hyporheic zones, dilute water is discharged through the streambed. The nitrate rich tile drain flux and discharge from hyporheic zones probably compose the chief sources of streamflow generation through much of the dormant season.

Convocation Year

2000

Convocation Season

Fall

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Hydrology Commons

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