UA Researchers Find Flaw In Water Quality Assessment Method

FAYETTEVILLE, Ark. - Current sampling methods result in pollution loads for streams being underestimated by as much as 20 percent, according to researchers at the University of Arkansas.

"The problem arises because samples are commonly taken on a monthly basis," explained Marc Nelson, director of the Arkansas Water Resources Center Water Quality Lab. "But we found that the pollution load increases dramatically during storms, which are rarely accounted for by monthly sampling."

Nelson and Thomas Soerens, assistant professor of civil engineering, and presented their findings at a joint meeting of the Geological Society of America and the Arkansas Water Resources Center on April 4.

Soerens and Nelson, along with, Jean Spooner of North Carolina State University based their findings on data acquired through two years of intensive sampling of the Illinois River and Moore’s Creek, located in northwest Arkansas. Their database includes 1,200 discrete samples of six different water quality parameters and covers base flow conditions as well as 19 storm events.

The current monthly sampling method assumes that the sample concentration represents the concentration in the stream between samples. This leads to errors when concentrations are not steady, as in a storm event.

"Much or most of the load in a stream is transported during storms," explained Soerens. "This load is missed unless storms are intensively sampled. For example, during one storm in January, 1998, the measured total phosphorus concentrations on the Illinois River surpassed the phosphorus concentration estimated for the entire previous year, which was based on monthly collection data."

By sampling storm runoff events at 30-minute intervals on both waterways, researchers were able to develop sub-sampling and data analysis techniques. They used these results to determine the minimum sample interval required to accurately determine pollutant loads during storms.

"We found that the optimum sample interval depends on the type of pollutant being measured and the size of the drainage basin," said Soerens. "As a whole, we found that as the time between samples increased, the error in the load estimate increased."

On the Illinois River, a 3^rd order stream, the optimum sampling interval for total suspended solids (TSS) during a storm event is 2.5 hours. However, at Moore’s Creek, a 1^st order stream subject to flash flooding, the optimum sampling interval for TSS is 45 minutes. For the Illinois River, TSS peaked at about 10 hours after the storm began, dropped sharply, then spiked up again to nearly the highest level at 28 hours. Because it is prone to flash, Moore’s Creek data did not show this double spike.

"The results indicate that pollutant load determinations are very sensitive to the frequency of sampling during storm events," said Nelson. "All of the parameters except nitrate have most of their concentrations peak during the first part of a storm."

In the future, the researchers will use data from these sites to investigate the role of non-point sources in calculating the total daily maximum load (TMDL) for a body of water. Non-point source pollution is the accumulation of sediments, nutrients, or other contaminants that originate from the surface of the land, often as a result of agriculture or forestry operations. These sources usually enter streams as a result of storms. Stormwater moving rapidly over land or concrete and asphalt can pick up pollutants that diminish water quality.

"Accurate measurement of pollution loads in streams is critical for determining the impact of non-point source pollution," said Soerens. "Since non-point sources are explicitly included in proposed regulations, it is important to determine their impact on calculating TMDL."

Both the Illinois River and Moore’s Creek are subject to non-point source pollution. Moore’s Creek is a small stream that empties into the Illinois River. The Illinois River, which originates in Arkansas, flows into Lake Tenkiller reservoir in Oklahoma before emptying into the Arkansas River. A lawsuit over water quality brought by Oklahoma went to the U.S. Supreme Court. As a result, the Illinois River is subject to an intensive monitoring regimen, where flows are continuously monitored and samples taken every 30 minutes during storms.

During the two-year data collection period, gauges at both sites continuously measured and recorded water levels. As the water level began to rise in a storm event, automatic samplers installed at the sites were triggered to take samples at 30-minute intervals. All samples were collected within 24 hours and analyzed by using EPA-approved analysis and procedures.