Capturing Flow-weighted Water and Suspended Particulates from Agricultural Canals During Drainage Events

Nutrients present in particulate form can contribute significantly to the overall loads in agricultural drainage waters. This study describes a novel method to capture flow-weighted water and suspended particulates from farm canal drainage over the entire duration of the drainage event.

The overall goal of this procedure is to provide a novel method for capturing in situ composite flow-weighted farm canal discharge water and suspended particulates during drainage events for their physical chemical characterization. This method will help address the most fundamental step in the field of environmental hydrology, which is collecting a water sample that fully represents a flow event in its entirety. The main advantages of this technique are that it is automated and that it provides a composite sample that is highly representative of typical discharge material present during flow events.

This method can provide insight into water quality parameters during routine drainage events as well as during storm events when large volumes of water are discharged over multiple days. Designing monitoring methods that capture flow-weighted composite water samples could help avoid errors associated with complex, high-intensity rain events. Demonstrating the procedure will be research associate, Doctor Timothy Lang, along with field technicians, Pablo and Johnny.

After identifying a study farm, install a datalogger near the pump station that triggers an auto-sampler to collect composite flow samples on a flow proportional basis. Use the pressure transducers installed in the inflow and outflow canals, adjacent to the pump station, to monitor the farm canal levels and use the proximity switches installed on the pump heads to monitor the pump speed. To calculate the drainage, use a datalogger and a pump calibration equation for each pump.

To program the in situ automated sampler located at the pump station to capture daily composite water samples, first press Enter Program. Then press Program, Flow-based Sampling, Sample every one pulses, 30 composite samples and Sample Volume of 130 milliliters. Press Enter for calibrate sample volume and for Enter Start Time.

Then press Program Sequence Complete. Then store the flow-weighted composite samples in an onsite refrigerator at four degrees Celsius until their analysis. To capture suspended particulates, position two, 200 liter PVC settling tanks a few feet from the datalogger to collect farm canal discharge water over the duration of the drainage period.

To program the datalogger to collect the particulate samples, press Program, Time-Based Sampling and Sample every two minutes. For composite samples, select 200 and select No for calibrate sample volume and enter start time. Then press Sequence Complete and push Sampling button.

Two liters of flow-weighted drainage water will be collected in the settling tanks, every two minutes. As the particulates begin to settle in the tanks, use a hose to siphon off the excess water and transfer the particulates into five-gallon buckets for transport back to the lab for 24-hour equilibration at four degrees Celsius. When the particulates have settled, siphon off the excess water and transfer the particulates into Imhoff settling cones.

After one hour, siphon off the excess water one last time and transfer the settled particulates into pre-weighed 500 milliliter screw top jars. Be sure to collect the water below the surface and away from the sides of the cone to reduce the number of aspirated particulates. In this representative experiment, suspended particulates from three farm canals within the Everglade Agricultural Area were collected as demonstrated, and determined to be highly organic, to possess a low bulk density and to be rich nutrients like phosphorous, properties that are highly representative of the water and sediments typically found for the Everglades Agricultural Area soil types and surrounding land use practices.

While attempting this procedure, it's important to remember to allow the particulates to fully settle before removing excess water. Following this procedure, other methods like phosphorous fractionation using the Hedley et al. method, can be used to evaluate how much labile versus recalcitrant phosphorous is present in the particulates.

After watching this video, you should have a good understanding of how to collect a composite flow-weighted discharge water as opposed to grab sampling. By having a refrigerator onsite, it is possible to preserve the water samples at four degrees Celsius. An added advantage of using this method for collecting water samples is that it is automated and it can be used to capture discharge events associated with storms and hurricanes that can last up to a few days.