Frozen soil and topography can have a profound impact on the spatial and temporal variation of water and solute movement. Frozen soil impedes the infiltration and vertical movement of water and surface applied chemicals while topographic lows can collect a relatively large volume of runoff. A tracer study was initiated in SE North Dakota to investigate the depression focused movement of water and a surface applied tracer material to a shallow unconfined aquifer. Potassium chloride (KCl) tracer was applied at a rate of 450 kg ha-1 to both depressional and topographically level areas on an irrigated quarter section in September 1993. These areas were instrumented with soil solution samplers, ground water observation wells, and subsurface tile drains. Soil moisture was logged hourly with time domain reflectometry, as were tile drain flow rates and soil temperature, throughout the study period which spanned from October 1993 through August 1994. During the study, water samples for potassium and chloride analysis were taken approximately weekly and more frequently during times of expected ground water recharge. It was found that frozen soils caused water from snowmelt the following spring to runoff and collect in small depressions. This water remained ponded until the underlying soil thawed. Immediately after the thaw, the ponded water rapidly infiltrated and carried with it the surface applied chloride tracer material to the shallow ground water. Elevated chloride concentrations were detected in the soil solution samplers and the ground water as early as March 25 under the depression. In contrast, no deep movement of tracer was observed at the edges of the depression until mid August after a 3.7 cm rainfall event, and then only to a depth of one meter. No elevated chloride levels were observed under the topographically level site throughout the study period. The rapid formation of a large water table mound under the depression after the thaw is another indication of the extent of the depression focused recharge. This type of focused infiltration of water and solute through previously frozen soils can be viewed as field scale preferential flow. The areas subject to snowmelt accumulations over frozen soils should be taken into account when planning agricultural chemical applications and precision farming practices.

Nathan E. Derby
Dept. of Soil Science
Walster Hall 114
North Dakota State Univ.
Fargo, ND  58105-5638
Phone: (701) 231-7555
Fax: (701) 231-7861
E-mail: derby@badlands.nodak.edu