Wastewater Irrigation Impacts on Soil Hydraulic Conductivity: Coupled Field Sampling and Laboratory Determination of Saturated Hydraulic Conductivity

Here we present a methodology which matches a soil sample size and a hydraulic conductivity measurement device to prevent the so-called wall flow along the inside of the soil container from being erroneously included in water flow measurements. Its use is demonstrated with samples collected from a wastewater irrigation site.

Transcript

This method can help answer key questions in environmental fields such as whether a particular land-based activity will result in greater surface water or ground water contamination. The main advantage of this technique is that it prevents us from overestimating the rate of water movement through soil as a result of what we call wall flow. After I demonstrate the field work at the drill rig Melissa Miller, a grad student in our laboratory, will be demonstrating the lab activities.

To collect soil samples first put on hard hat, gloves, and protective goggles prior to starting the drill rig. Power up the drill rig and lower the rotary head sufficiently to allow the installation of the Kelly bar. The Kelly bar is the metal rod that connects the drive head of the drill rig to the sampler.

Insert the Kelly bar into the rotary head. Insert a plastic liner sample tube into the metal sample tube with the cutting head attached to the bottom of the metal sample tube. For the application described here use a 150 millimeter long plastic liner with a 90 millimeter outside diameter fitted into a 200 millimeter long metal sample tube with a 100 millimeter outside diameter and 90 millimeter inside diameter.

Attach the metal sample tube to the Kelly bar using a drive head fitted to both. Operate the drill rig to move the sample tube approximately 150 millimeters into the soil. Remove the metal sample tube from the soil using the hydraulic system of the drill rig.

Now remove the metal sample tube from the drive head. Remove the plastic sample tube holding the soil sample from the metal sample tube. Take care not to lose soil from inside the plastic sample tube and not to compact the soil or squeeze the sides of the plastic sample tube.

Place the end caps on each side of the plastic sample tube using red for the end of the top of the soil sample and black for the bottom of the soil sample. Tape the end caps to the sleeve to avoid contamination or loss of water from the sample. Finally place the sample standing upright for transport back to the laboratory.

Construct the saturated hydraulic conductivity apparatus as detailed in the text protocol. To prepare the water source first connect the plastic tubing to the nylon-barbed end adapters in both the supply gutter and the drainage gutter. Place a large tub on the floor adjacent to the conductivity device set up to serve as the supply container.

Next connect the large plastic tubing to the inlet of a small submersible pump and place the pump into the bottom of the tub. Then connect the small plastic tubing to the barbed end adapters and put the non-connected end of the tubing into the drainage gutter. Now fill the supply container with water.

Plug in the pump and run it to fill the supply gutter. Ensure the rate of water being pumped into the supply gutter is adequate to keep the supply gutter nearly full without overflowing. Prepare a practice soil sample to identify any modifications needed by placing a soil sample into a plastic sampling sleeve leaving about 50 milliliters of space between the top of the soil and the top of the plastic sleeve.

Cover the lower end of the sample and sleeve with a double layer of cheese cloth. Hold the cheese cloth on the sampling sleeve with a sufficiently sized rubber band. So it's also important to ensure that the samples are pre-wetted from below rather than from above, as wetting from above will result in air entrapment in the soil pores and the results then will not be representative.

Place the practice soil sample and sleeve into a tub of water filled to about one third of the height of the sleeve with the cheese cloth and being in the water. Several hours later raise the water in the tub to approximately two thirds of the height of the sample. After allowing the sample to set overnight fill the tub to just below the top of the soil sample.

Now place the soil sample on top of the 89 millimeter outer diameter PVC tube. Gently press it onto the tube allowing the sharpened edge of the PVC tube to press into the soil a few millimeters to permit the bottom of the soil to rest on the screen. It's important that the sharpened edge presses into the soil a few millimeters but that it isn't pressed so strongly that it compacts the soil sample.

Provide water to the top of the soil sample. Turn on the pump and fill the supply gutter. Ensure that the end of the drainage tube is placed into the drainage gutter and that the outlet from the drainage gutter is tightly connected to the plastic tubing which is placed into a drain or container at a lower elevation.

Using six millimeter tubing create a siphon from the supply gutter to the top of the soil. So it's important that the depth of the water above the soil in the sample tube does not overtop the tube. If it does an extension will be needed to be added to either the top or the bottom.

Now collect water samples from the soil core which drains from the funnel. Check for leaks or unanticipated problems. Determine the approximate length of time needed to collect an adequate amount of water based on the time needed to fill approximately half a 100 milliliter beaker with water.

Create a simulated wall flow by inserting a small screwdriver along the inside of the plastic soil sample container to confirm that the excess flow created by this passageway flows to the drainage gutter through the drainage tube. Proceed to obtain soil hydraulic conductivity values as detailed in the text protocol. Representative results of the decreases in hydraulic conductivity observed as soil depth increased in both irritated and non irrigated areas are shown.

It was expected that the irrigated areas would reflect a somewhat higher conductivity near the surface compared with non irrigated areas. This is due to the observed increase in plant and plant root density in irrigated areas creating larger pore spaces for water to move through decayed root channels in the soil. Although no differences were observed between soil hydraulic conductivities in irrigated versus non irrigated areas there were clearly differences in the sodium absorption ratios between those areas.

It is clear that the SAR was a very important factor that impacted hydraulic conductivities. This figure demonstrates that there is a strong relationship between soil depth and soil solution SAR for irrigated depressional areas. This is likely due to both a strong correlation between soil depth and soil solution SAR and to some extent to a decrease in Ksat with an increase in depth.

Once the apparatus is built to contain typical samples this technique can be carried out for whatever length of time is required to reach steady water flow conditions without the need to be present the entire time. While conducting this procedure it's important to remember to match the water quality to the conditions that the soil's exposed to in the natural environment. For example rain water has a much different chemical composition than waste water.

After watching this video you should have a good understanding of how to prevent so called wall flow from creating non representative and artificially high estimates of how fast the water moves through soil. Don't forget that working with heavy equipment in the field can be extremely hazardous. Precautions such as always having a second person accompany you as well as using personal protective equipment is always standard procedure.

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Journal (Environment)

Journal (Environment)

Wastewater Irrigation Impacts on Soil Hydraulic Conductivity: Coupled Field Sampling and Laboratory Determination of Saturated Hydraulic Conductivity

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