Seasonal variation in herbicide levels detected in shallow Alberta groundwater
A three-year study was conducted at Lethbridge, on a 1-ha field (clay loam soil) with a history of previous herbicide applications, to determine herbicide levels in shallow (0.3-5.8 m) groundwater and the effect of seasonal events such as herbicide application and rainfall/irrigation. A mass-selective detector, gas chromatographic method was used to analyze for seven different herbicides. Samples were collected from 6-m monitoring wells at various intervals (9-32 d) after herbicide applications made in 1991-1993. 2, 4-D (0.05-5.2 ppb), bromoxynil (0.03-8.4 ppb), diclofop (0.1-11 ppb), MCPA (0.2-0.3 ppb) and triallate (0.1-0.5 ppb) were detected in 17-61% of the wells sampled. Herbicide levels depended upon the interval and the timing and amounts of rainfall/irrigation after spraying. All levels were below the Canadian drinking water guidelines except for one detection of bromoxynil (8.4 ppb) and diclofop (11 ppb). We concluded there are large seasonal and spatial variations in herbicide levels in shallow Alberta groundwater. Levels are highest when the first moisture event after a herbicide application is a heavy rainfall/irrigation ('worst case' scenario). Levels are lower when the applied herbicides are first 'set-in' by gentle rains, then a drying period occurs and the residues adsorb to the soil. Subsequent heavy rainfall/irrigation does not flush large amounts of these adsorbed residues into the groundwater. This latter 'best case' scenario probably occurs more often than the former 'worst case' scenario.