Continuous Instream Monitoring of Nutrients and Sediment in Agricultural Watersheds

The overall goal of this procedure is to accurately measure the water quality of agricultural watersheds in situ, in a chosen time interval with high temporal resolution. This method can help answer key questions in the water resources field about spatial and temporal variations in pollutant loads, their drivers, and hydrologic and water quality processes. The main advantage of this protocol is that users can obtain high temporal resolution data on nutrients and sediments to study complex environmental problems.

Accurate and timely water quality information is a prerequisite to making informed water management decisions because non point source pollution is effected by water shed characteristics and rain fall timing and intensity. Though this method can provide insights into agricultural water sheds, it can also be applied to other types and sizes of water sheds such as urban, forested, and mine impacted water sheds. First, thoroughly clean the sonde sensor surfaces with soft brushes, pads, and soap or all purpose cleaning solution.

Rinse the sensors with deionized water. Remove and clean the turbidity sensor wiper and sensor cleaning brush. Then unscrew the cap from the PTFE reference junction of the pH sensor.

Discard the electrolyte solution. Add a potassium chloride salt pellet to the sensor and fill the sensor with fresh electrolyte solution. Screw the cap of the reference junction to form an airtight seal, and rinse away the displaced electrolyte solution.

Suspend the sonde 20 to 30 centimeters above the work surface with steel wire and a caribiner. Connect the sonde to the control computer. Start the software and select the sonde to be calibrated.

Under the Parameter Setup tab, set the number of standards to use for each calibration. Prior to calibrating each sensor, thoroughly rinse the sensor with deionized water and dry the sensor with lab wipes. To begin calibrating the conductivity sensor, first ensure that the oval sensor surface is completely dry.

Under the Calibration tab, select specific conductance and micro siemens per centimeter. Set the dry sensor to zero. Then immerse the sensor in the appropriate calibration standard.

Once the reading has stabilized, set the sensor to 1, 412 micro siemens per centimeter. Rinse the calibrated sensor with deionized water. Next, immerse the pH sensor in pH 7 standard solution.

Select the pH tab, wait for the reading to stabilize, and set the sensor to 7.0. Thoroughly rinse and dry the sensor and repeat the process with pH 10 solution. Rinse the pH sensor and check the reading in pH 4 standard solution to verify the calibration curve is linear.

Then rinse the residual standard solution from the sensor with deionized water. Next, fill the sonde calibration cup with temperature stabilized, air saturated, deionized water, and fit the cup onto the sonde. Invert the sonde to ensure that the temperature and dissolved oxygen sensors are covered by the water.

Select LDO in percent saturation, wait for the reading to stabilize, and set the barometric pressure to the local pressure in millimeters of mercury. Next, right the sonde and remove the calibration cup. Place fresh deionized water in the calibration cup, ensuring that no air bubbles form, and fit the calibration cup on to the sonde.

Select Turbidity in NTU. Once the reading has stabilized, set the turbidity to 0.6 NTU for the first calibration point. Repeat this process for the three turbidity standards to set the remaining calibration points, and then rinse the sensors and dry them.

Next fill the calibration cup to three quarters with a 50 milligrams per liter nitrate standard solution and fit the cup onto the sonde. Invert the sonde so that the temperature and nitrate sensors are covered. Select Nitrate in milligrams per liter of Nitrogen.

Once the readings have stabilized, record the temperature in voltage. Set the first calibration point to 46.2. Rinse the sensors thoroughly with deionized water, and dry the sensors with lab wipes.

Repeat the process with the five milligrams per liter Nitrate standard solution for the second calibration point. Verify that the voltage difference is between 50 and 65 millivolts, and that the temperature difference does not exceed five degrees Fahrenheit. Calibrate the ammonium sensor in the same way.

Once calibration is complete, reinstall the clean wiper and brush. Select SelfClean in the software, and calibrate the wiper and brush with one rotation. Synchronize the sonde clock and delete the oldest sonde log file.

Create a new log file and select the desired parameters to be measured. Fill in the monitoring duration and interval, and then save the log file. Before installing the sonde, check the sonde battery and replace it if necessary.

Wrap the sonde surfaces in all weather adhesive tape. And cover the sensor guard with copper tape or mesh to simplify cleaning an to reduce microorganism growth. To begin the installation, drive in a telescoping square mounting tube at the thalweg of the stream to be monitored.

Securely mount an area velocity sensor on the tube using a steel plate and an L bracket, so that the sensor tip faces upstream along the flow lines. If an area of velocity sensor is not available, install a pressure transducer inside the telescoping post with the sensor tip just touching the stream bed. Set the pressure transducer to measure the water depth every 15 minutes.

Use the pressure transducer readings and portable flow meter measurements to generate a stage discharge curve over a range of flows. Next, use ferrules and a caribiner to mount the sonde on the downstream side of the telescoping post, with the bottom of the sonde at least one to 10 centimeters above the stream bed. Ensure that the sonde is always submerged in water.

Connect the sonde to an external power source if necessary. Then, position an autosampler in a weather protective housing, installed on stable, level ground at the top of the stream bank. Then, secure a strainer pipe under water on the upstream side of the telescoping post.

Connect the strainer pipe to the autosampler with a hose. Set the desired sampling interval and start the autosampler. Clean and calibrate the sonde at intervals appropriate for the stream conditions.

Regularly download the acquired data, collect water samples, and replace the collection jars with clean, dried, 10 liter jars. Transport the samples to the laboratory, on ice, for further analysis. Examine the data for gaps, drifting, or erratic measurements indicating that maintenance is required.

The recorded water quality data, when compared with rainfall data from local weather stations. Each rainfall event resulted in increased discharge. The multiple local discharge peaks within the two major peaks were attributed to spatial variability in rainfall, and the drainage patterns of the rice and soy bean fields contributing to the flow.

Turbidity also increased with discharge. The highest turbidity was observed during periods of increasing discharge on the hydrograph. The Nitrate concentration increased during initial flushes, indicating that recently applied soluble Nitrogen had been washed from the fields.

Small Ammonium increases during initial flushes were also observed. An inverse relationship was observed between conductivity and discharge, suggesting that Nitrate and Ammonium were not major contributors to water conductivity. The decreased conductivity was attributed to dilution by rain water.

Pollutant loads varied over time with the loads being highest in early summer and late fall. The low pollutant loading in September and October was attributed to an overall low flow. High rain fall on recently disturbed fields in November and December was reflected in higher and more variable pollutant loads.

The pollutant loads also varied as the water traveled downstream. Bio fouling and sediment accumulation in the sonde and the surrounding sensors are really the biggest challenge in the agricultural water sheds, mostly due to high sediment loads and nutrient levels. Especially after large precipitation events.

Correct sensor and sonde positioning using an external battery, wrapping the sonde with copper mesh, frequently downloading the data, frequently cleaning the sonde and sensor surfaces at the site, and regularly calibrating the sonde at the laboratory are all important for obtaining high water quality data. Data quality can be compromised by power failure to the sonde, or by bio fouling and sediment accumulation on the sonde. When attempting this procedure, it is important to make and follow a quality assurance project plan.

Don't forget, field work can have safety concerns such as flooding, snakes, inclement weather, and lightning. Take care to devise a safety, health, and welfare plan, identifying safety concerns and emergency preparedness. These should be devised and followed.

Once mastered, this procedure can produce good in situ continuous water quality data that is required for making water resource management decisions in agricultural and other water sheds.