Figure 1. Location of solution sampler site and tile drain recharge sites on BMP quarter section. T03 and T04 are tile drain access manholes located down gradient from tracer plots.
Additional Notes: Poster
Nathan E. Derby and Raymond E. Knighton
Agriculture has been named as a non-point source of groundwater contamination for many years. With the advent of Best Management Practices (BMP's) and Site Specific Variable Rate Technology, more emphasis is being placed on field variability. One area which needs to be investigated further is the spatial variability of groundwater recharge and contamination relative to ground surface topography. A tracer study was initiated on a pre-existing BMP research site to monitor solute movement through the vadose zone into the groundwater relative to ground surface topography. The quarter section research site contains primarily sandy soils with a shallow watertable and groundwater and leachate quality have been monitored at the site since 1990. Granular potassium chloride (KCl) was surface applied to two areas overlying subsurface tile drains and to one area instrumented with soil solution samplers and monitoring wells. One of the tile drain tracer plots was located on level ground while the other two sites were in small topographic depressions which held spring snowmelt and were ponded after heavy rains. The applied chloride was found to move rapidly to the shallow groundwater under the depressional areas after infiltration of spring snowmelt. Excessive rainfall events were also responsible for the rapid transfer of the applied chloride tracer. The site situated on level ground contributed little or none to groundwater contamination. The findings of this study indicate that it may be necessary to delineate areas of a field based on topography. These locations can then be integrated into a Site Specific Farming plan and chemical applications can be managed to reduce groundwater contamination.
Potassium chloride was surface applied at a rate of 448 kg ha-1 on two plots overlying subsurface tile drains and to one site instrumented with solution samplers and observation wells in November, 1993. The solution sampler site was located on the "E" transect in a small depression while the two tile drain sites, one in a depression and one on level ground, were located on the "G" transect (Figs. 1 and 2). Chloride concentration in the tile drain effluent was measured weekly at access manholes T03 and T04, down gradient of the tracer plots. During times of expected recharge events, samples were taken with an automated waste water sampler at 6 hour intervals. Soil solution and groundwater chloride was monitored approximately weekly at the solution sampler site. Instrumentation at the solution sampler site (Fig. 3) consisted of duplicate sets of solution suction samplers at three locations and observation wells at 20 m intervals. The solution samplers were constructed of 30 cm lengths of 3.8 cm diameter PVC pipe with a 1 bar ceramic cup attached to the bottom. Tubing was inserted through a rubber stopper in the top of the samplers to allow application of tension and evacuation of collected solution. Soil moisture was measured hourly at the site by TDR and a CR-10 datalogger. Soil temperature was also measured hourly with thermocouple temperature probes. Figure 4 indicates the type of ponding responsible for focusing groundwater recharge and tracer movement. As the snow melts in the spring, the underlying soil is still frozen for a period of time. As the soil begins to thaw, the ponded water infiltrates rapidly, carrying with it any mobile chemicals in the soil. Ponding also occurs after intense rainfall events or irrigation.
Figure 1. Location of solution sampler site and tile drain recharge sites on BMP quarter section. T03 and T04 are tile drain access manholes located down gradient from tracer plots.
A watertable elevation contour with time at the "E" transect solution sampler site is shown in Figure 5. Watertable mounds form under the depression after spring thaw and after intense precipitation events. This mounding is indicative of focused recharge. As these groundwater mounds recede, the watertable level in adjoining wells is seen to rise. Figures 6, 7, and 8 are time series contours of chloride concentration measured at the three sampler locations at the solution sampler site. Chloride is seen to move rapidly to the groundwater at the bottom of the depression (E7.6). Chloride is only detected at 30 cm at E7.2 until after the large rainfall event in early July when chloride is detected at 90 cm. The chloride never moves past 30 cm at E7.8. The watertable elevation, also included in Figures 6, 7, and 8 is much more variable at E7.6 due to focused recharge. Another indication of the time period when the soil was thawing is illustrated in Figure 9. Time Domain Reflectometry (TDR) measures only liquid water. As a result, frozen soil appears to have lower soil moisture. When the soil thaws, approximately Julian day 75 of 1994, there is a marked increase in soil moisture. Increases in soil moisture due to water applications are also shown. Chloride in tile drain effluent as well as drain flows and cumulative precipitation at T03 and T04 are included in Figures 10 and 11, respectively. Chloride concentration does not vary much under the level tracer plot while a large spike of chloride is measured after the spring thaw under the depressional area. This also corresponds to a period of increased flow volumes measured at T04. This one large spike accounts for 9.2 percent of the applied chloride.
It is evident from this study that topography plays a major role in the movement of surface applied chemicals to the groundwater, especially in areas of sandy soils with shallow watertables. As is indicated from the solution sampler site, mobile chemicals such as chloride and nitrate will be moved almost immediately to depth after infiltration of spring snowmelt. Peaks in chloride concentration at the bottom of the solution sampler site and the depressional tile drain site correspond to the spring thaw, while there is little of no movement of chloride at the level tile drain site. This along with the rapid response of the watertable to water applications, supports the hypothesis that recharge is indeed being focused in small topographic depressions throughout the field. These depressional areas, once identified, can be integrated into a Site Specific Farming management plan so that chemical applications can be managed to reduce the risk of groundwater contamination.