|Mississippi Delta MSEA Volume 2, Issue 2, Page 2
|Volume 2, Issue 2, Page 2
|Second Quarter 1996
|MSEA Reporter Archive
Many of you are probably wondering what those little "out-houses" will be used for that are set up in the three watersheds of the MSEA project. Those houses belong to the U.S. Geological Survey, which will be sampling runoff from the fields during storms. The instrumentation that collects water samples and measures the amount of runoff from fields is installed in the houses. To describe the equipment, it is probably best to give an explanation of what we are trying to find.
We know that chemicals such as pesticides and fertilizers are applied to fields at a certain rate. For example, the herbicide Cotoran? is applied at a rate of pounds per acre. By multiplying the rate by the number of acres, we can then know how many pounds of Cotoran? were applied for that field. In order to understand how well a BMP is working to keep Cotoran? on the field, we need to determine how many pounds of Cotoran? , if any, actually ran off during a storm. The amount of chemical or sediment that is determined in the runoff is called chemical or sediment "load." Therefore, if we applied 100 pounds of Cotoran? to a field, and we were able to quantify a load of 25 pounds in the runoff, then the BMP on that particular field retained 75 percent of the Cotoran? that was applied.
How do we determine the load of chemical or sediment that is in the runoff? We first have to collect two basic pieces of information: chemical (or sediment) concentration and streamflow rate. Concentration data is fairly easy to collect. During storm events, an automatic sampler will pump water into bottles. The bottles are sent to a lab to be analyzed for pesticide, fertilizer, and sediment concentrations in the storm water. Concentration data are reported in weight per unit volume. For example, the concentration of Cotoran? in storm runoff at a particular site could be reported in terms of pounds per gallon. However, we generally report concentrations in terms of milligrams per liter or micrograms per liter in metric units.
There are several ways to set-up an automatic sampler to collect samples. The two most common ways are discrete and composite sampling. Discrete sampling is where the sampler collects many samples during a storm and puts each sample in a separate bottle. Each individual bottle is then sent to the lab to be analyzed. For instance, if the sampler is programmed to collect a sample every 5 minutes, then 24 bottles will be sent to the lab for a 2-hour storm event. Composite sampling is where a small portion of water is collected at various intervals during a storm, and then these portions are deposited into one large bottle. The large bottle is sent to be analyzed for average concentrations for the entire storm event. There are advantages and disadvantages to both types of sampling. Discrete sampling is more precise than composite sampling but is also very expensive. Composite sampling is less expensive than discrete sampling but may also be less precise. We will be using a combination of discrete and composite sampling at the MSEA runoff sampling sites.
The other piece of information that we collect is streamflow rate. Most of the MSEA runoff sites are located on drainage canals that lead to the lakes. These canals are very "flashy" during storms making it difficult for field personnel to measure. We have installed structures, called controls, such as weirs and flumes, that have already been calibrated at a laboratory to deal with these flashy conditions. At one of the sites at Beasley Lake watershed, we have constructed a very large weir that is about 20 feet wide and 10 feet high. It has been nicknamed, the "Hoover Dam," by the landowner. Because these controls have been pre-calibrated, it is not necessary for field personnel to be at each site at every storm to try to measure streamflow rate. Streamflow data are reported in terms of volume per time. For example, wells for drinking water or irrigation are set up to pump at rates of gallons per minute. Streamflow rate can also be reported in terms of gallons per minute; however, the USGS typically reports streamflow rate in terms of cubic feet per second.
Once we have collected chemical (or sediment) concentration and streamflow rate data, we can then multiply the two pieces of information together to determine chemical or sediment load for a particular storm. We will collect these data for the life of the MSEA project to see if the BMP fields have less chemicals or sediment running off during storms than the fields with no BMP's. If you have additional questions about the storm runoff sampling program, please contact:
Richard A. Rebich
U.S. Geological Survey, WRD
308 S. Airport Rd.
Pearl, MS 39208-6649
Phone: 601-965-4600, ext. 5583
|MSEA Reporter Archive