Transport of Solutes Through a Vegetative Filter Strip Under Shallow Groundwater Conditions Hailie Snyder, Rebecca Purvis, Dr. Garey Fox Department of Biosystems and Agricultural Engineering
Pollution from pesticide runoff can be lessened if contaminants are separated from the water source during transport. The purpose of this experiment was to test if vegetative filter strips, which act as a buffer between farmland and water sources, have an influence on the dilution and sorption rates of pesticide runoff by using a tracer dye to track the transport of solutes across shallow groundwater at high and low water tables. If a delay is seen in the initial outflow of the tracer dye, then it can be concluded that the vegetative filter strip adequately slows the transport of solutes across shallow groundwater conditions. A simulation was set up using a vegetated soil box that allowed water flow to mimic that of field irrigation. A tracer dye was introduced and samples were collected from both the inlet and outlet flows. The samples were tested using a fluorometer and it was determined that both water levels had the same travel time, and although there was a slight delay, but not one significant enough to prove that vegetative filter strips adequately slow the transport of solutes.
The experiments were ran at high and low water tables under shallow groundwater conditions. This means that water is easily accessible through wells from water sources near the surface, such as rainwater, river and lake water instead of water that has percolated underground in aquifers and other structures. Most water used for irrigation and human consumption is drawn from shallow groundwater.
Research Question • Are the dilution and sorption rates of contaminants affected by the vegetative filter strip? • Will the height of the water table cause a difference in dilution and sorption rates?
What is a Vegetative Filter Strip? Vegetative filter strips are strips of land, normally with grass or another form of vegetation planted between a water source and farm land to trap sediments and act as a buffer for water pollutants.
Figure 1. Diagram of a vegetative filter strip, Purdue University.
Figure 2. Inlet flow.
Figure 3. Outlet flow.
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Figure 5. The pump transferred water into the box. Below is the Rhodamine.
Figure 6. The bottles above are the water samples taken from outlet flow .
Figure 7. The fluorometer measured the RFU levels of the samples. Figure 8. A high water table allows for less dilution and sorption of solutes because there is less room for infiltration into the soil.
Figure 4. The experiments were ran using this system, with the water source the large tank on the left, and the soil box being the large black container. The water poured into the bucket on the right.
Figure 9. A shallow water table should allow for higher dilution and sorption rates as water moves across vegetation.
Methods The research was split into separate experiments, one with a high water table at 60 cm, and one with a low water table at 25 cm. A soil box was used to replicate a vegetative filter strip, and was filled with soil and grass was planted on top. The system was set up so that water was pumped from a 200 gallon tank into a trough at the front of the soil box that allowed water to flow at a controlled rate over the vegetation and then poured out of a second trough located at the end of the soil box into a bucket that was placed on a scale to measure water flow and emptied in 10 kg intervals. The constant flow rate of the inflow was measured by determining the amount of time it took for 1 kg of water to reach steady state. Once steady state was reached, the Rhodamine was introduced into the water through a separate container and samples were taken in thirty second intervals until the water ran clear. Samples were taken from both the inlet and outlet flow, and tested in a fluorometer to measure the Raw Fluorescence levels of the water and diluted as necessary. The sample data was then analyzed and compared in displayed in the graphs.
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Results The fluorometer determined the level of raw fluorescence left in the water samples after they were ran through the soil box. The fluorescence level represents the amount of tracer dye still left in the water. Because the samples were taken every thirty seconds starting from the beginning sample where there was no tracer dye, we were able to determine the rate at which the dye traveled across the strip. Based on the data obtained from the fluorometer, the transport rates were established to have a two minute travel time for both the high and low water table runs. Because the shallow water table has more room between the soil and water surface, we should have seen a greater delay in the presence of the tracer dye as it should have been able to absorb into the soil more. However, the sorption and dilution rates for the two water heights were almost the same. The graphs display the concentration measured over the peak inflow versus time. They show that the data travels a very similar path.
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While there is a high peak in the in outflow shows relatively no change there a delay for either inflow or ou 800 700 600 Concentration (ppb)
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There is not a significant variation i except around the peak, nor is there the beginning, where there should h higher sorption rate.
Conclusions
The vegetative filter strip was suppo contaminants as they traveled to the source, but our results determined th expected delay did not occur. The re that perhaps the sorption rates are af by the vegetation rather than the soi will have to be ran to see if the trace be delayed more with a slower flow without vegetation. By doing so the have an increased opportunity for so runoff.
Figure 12. Depicts cl from the outflow. The this experiment on th in making a step clos ensuring clean, sustai for all.
A big thank you to Dr. Garey Fox a Purvis for assistance with the resea the Biosystems and Agricultural En department for the use of its labs.
