Real-time Nitrate Monitoring Provides A Window on Nutrient Loading
Louisiana's Lake Pontchartrain Basin lies at the mouth of the Mississippi Delta and squarely in the sights of water quality experts from around the world. The basin is an esturarine system that has been significantly disturbed by the construction of the Mississippi River Gulf Outlet (MRGO) in the 1960's a shortcut for ships headed to and from the Gulf, which bypasses the lake and deprives it of fresh water and sediment as well as agriculture, logging and development in the area.
The lakes increasing salinity and shrinking wetlands have garnered significant attention, as has the nitrate-laden water rushing past the basin through the MRGO just below Pontchartrains southern shore.
Complicating matters, the basins own wetlands contribute nutrients, and the lakes high salinity complicates measurement of many water quality parameters.
Meanwhile, tidal effects, the Gulf Coasts flashy storm events, and the lakes 625-sq-mile size have made reliable grab-sample data collection during storm events extremely difficult. While recreational users, shrimp boat captains, wildlife conservationists and regulators grapple with the impacts, Dennis Demcheck and Kevin Grimsley of the U.S. Geological Surveys Louisiana Water Science Center in Baton Rouge, La., are using cutting-edge technology to collect data that could help guide decision-making in the basin.
"This is the first study ever to get real-time loadings into the lake," explained Demcheck, as he described an 18-month study of nutrient loadings in the lake. "There have been models, but they have been estimates based on a limited number of samples. We can finally document and quantify the short-term events we couldnt quantify. We've gone from estimating general principles we knew to finally being able to get data to refine our models based on real-life information."
The team installed four data gathering stations around the lake in April 2004, one in each of the major arteries feeding the lake, Grimsley said. Each station features cable-mounted monitoring equipment a YSI 9600 Nitrate Monitor, a YSI 6600 Extended Deployment System (EDS) multiparameter sonde, and a SonTek/YSI Argonaut-SL acoustic Doppler current meter configured to measure 10 water quality parameters in real time. The monitoring units were connected to a Vaisala 555A datalogger with satellite telemetry hookups, broadcasting data back to Grimsley and Demcheck in Baton Rouge and updating the teams web site with real-time data.
"The four sites capture the great majority of the contributions to the lake," Demcheck said. "We put the stations as far down the stream as possible to capture as much nitrate as possible coming into the lake."
The flood of data stage, direct velocity, temperature, specific conductivity, salinity, pH, dissolved oxygen (DO), dissolved oxygen percent saturation, turbidity and nitrate proved so massive at a sampling interval of one hour that the USGS team reduced its interval to two hours. The longer interval had other practical benefits, Demchek added. "We found that we could conserve our reagents using two-hour intervals, and we weren't missing anything."
Although use of the monitor does not imply endorsement by the USGS, Demcheck said he is fully satisfied with the accuracy of the flow injection/chemical reaction technology of the YSI 9600 Nitrate Monitors, which mixes sample water with reagents to create a colored solution. The monitor uses an LED and a photodiode sensor to read the intensity of the color, which corresponds to nitrate concentration in the sample. Demcheck said the flow injection system in the 9600 outperforms ion probe monitoring, especially in saline conditions. This technology opened the world of real-time data monitoring for nitrates to saltwater systems like Lake Pontchartrain.
"An ion probe does not work well in this environment" Demcheck explained. "Ion probes can give false positives in estuarine systems, caused primarily by chlorides. By using wet chemistry [in the 9600], we're able to get concentrations we can have confidence in. We routinely look at environmental concentrations of 0.5 mg/L and lower, as low as 0.05 mg/L. Ion probes can't read at 0.2 mg/L or below reliably."
For all the cutting-edge science represented by the monitoring stations, the equipment turned out to be quite reliable, Grimsley said at least after the inevitable challenges of getting four monitors to feed volumes of data through another manufacturers telemetry system. "With something collecting this many parameters, we had to work through some communications protocol issues how the datalogger talks to the equipment and vice-versa," Grimsley noted. "But it's nice when you get going we've gotten a lot of really good data, and the nitrate monitor is simpler to use than I expected."
The nitrate monitor self-calibrates every 12 hours in the field, so maintenance is limited to replacing reagent and waste bags, and then running a pre-deployment test. Grimsley said the secret to easy monthly maintenance visits is doing as much as possible in the lab rather than in the field.
"I don't try to do any of the reagent filling and cadmium column activation in the field we do all of that in the office," he explained. "When you get out into the field, it's just a matter of switching out the bags and resetting the machine, and basically you've got a brand-new machine."
Regular maintenance is a must for equipment deployed in the challenging environment of an estuary. "It needs to be re-emphasized that as with all water quality instrumentation, biofouling is a serious issue," Demcheck said. "This is particularly true in estuarine systems. The hardware performs well, but fouling of intake ports and a general buildup of biological growth requires a continuing commitment to maintenance."
After putting the suite of real-time monitors to the test in Lake Pontcahrtrain, Demcheck has set his sights on a real-time monitoring study of perhaps the highest profile nutrient loading issue in in the region nitrates in the Mississippi River as it feeds into the Gulf of Mexico the infamous hypoxic zone. The findings, now technologically within reach, could shape the debate on the health of the Mississippi and have consequences from Minnesota to the mouth of the river.
"An important long-term monitoring goal is to have the Mississippi/Atchafalaya system instrumented in this manner, with the goal of monitoring and ultimately predicting the size of the hypoxic zone," Demcheck said. "We know there is a lot of nitrate in the Mississippi and it's going out into the Gulf we've had monthly samples in the Mississippi for decades that show nitrates at median concentrations of 1.4 to 1.6 mg/L. But the amount of nutrient processing in the swamps is the subject of a lot of debate. By having unbiased data collected for everyone to use, theoretically, there would finally be some consensus. We could determine whether wetland processing could really reduce the amount of nutrients going into the Gulf. Can there be processing at the end of the pipe, or must it all be managed up in the center of the country? This would have national implications."