Nitrate is a major contaminant and threat to groundwater quality in Texas and around the U.S., so knowing where this chemical tends to pool will be a help in controlling potential damage, according to a Texas AgriLife Research study.
Dr. Srinivasulu Ale, AgriLife Research geospatial hydrologist at Vernon, and his post-doctoral research associate, Dr. Sriroop Chaudhuri, completed a study of groundwater nitrate concentrations and recentlyhad their results published in the Journal of Environmental Quality.
The research paper was co-authored by Dr. Paul DeLaune, AgriLife Research environmental soil scientist, and Dr. Nithya Rajan, AgriLife Research agronomist, both at Vernon.
Results indicated that groundwater nitrate concentrations have significantly increased in several Rolling Plains counties since the 1960s. In 25 counties, morethan 30 percent of the groundwater quality observations exceeded the U.S. Environmental Protection Agency maximum contamination level for nitrate in the 2000s as compared to eight counties in the 1960s, they said.
“This suggests that more counties in the state are getting affected by high nitrate levels in the groundwater over time,” Ale said.
This groundwater, if used for irrigation with accounting for the high nitrate concentration and domestic purposes, could have serious environmental and health implications, he said. Ingestion of high nitrate groundwater can cause methemoglobinemia, commonly known as ‘blue baby syndrome,’ in infants less than six months of age.
Although earlier studies reported high levels of nitrate, exceeding the maximum contaminant level for drinking water, in different parts of Texas, a comprehensive statewide assessment of the groundwater nitrate contamination over a longer time period was lacking, Ale said.
“We assessed 50 years (1960 to 2010) of groundwater nitrate data, as available from the Texas Water Development Board, and employed different statistical and geospatial techniques to study long-term trends in groundwater nitrate contamination across Texas,” Ale said. “We also identified the major factors affecting nitrate contamination.”
A distinct spatial clustering of high nitrate counties was observed in the Rolling Plains and parts of the Southern High Plains in recent times, Chaudhuri said.
“In the course of our study, we found that counties that have high nitrate contamination are generally associated with or surrounded by counties having similarly high nitrate levels in the groundwater,” he said. “The same was true with low-nitrate counties clumping together with other low-nitrate counties.”
Identify problem areas
The main idea behind this research, the two scientists said, was to identify regions where nitrate problems can be expected as well as where they might not occur.
“The Rolling Plains has been identified as the nitrate hotspot of the state since the 1960s,” Ale said. “In Haskell and Knox counties, all observations exceeded the maximum contaminant level in the 2000s. In addition, in Wilbarger, Wichita, Baylor and Fisher counties, the median nitrate concentrations exceeded the maximum contaminant level in the 2000s.”
This indicates substantial groundwater quality degradation in recent times, he said.
Various factors influence the origin and migration of nitrate in Texas, Chaudhuri said. It can originate from natural (soil nitrogen and atmospheric deposition) and anthropogenic (mostly mixture of nitrate and ammonium fertilizers) sources. Due to its solubility and mobility, nitrate can easily leach to groundwater and persist for decades depending on the hydrologic regime.
“For the Texas Rolling Plains, we found a close association between nitrate contamination and agricultural activities, such as fertilizer application and irrigation with high-nitrate groundwater,” he said.
A variety of factors influence nitrate entry to groundwater, including climate, land use, aquifer characteristics and groundwater-table depth and recharge patterns, the researchers said.
A critical review of potential factors that affect fate and transport of nitrates in soils is essential to address groundwater nitrate contamination issues and determining corrective actions, they said.
“We want to emphasize the need for accounting for nitrogen present in irrigation water, soil, manure applied and crop residue recycled, while deciding the fertilizer application rates for various crops in this region to reduce nitrate leaching to groundwater systems,” Ale said.
Ale said the highly transmissive geologic and soil media in the Rolling Plains has also facilitated faster movement of nitrate, causing the widespread groundwater contamination.
They said while groundwater nitrate concentrations continue to increase in different parts of the state, adequate groundwater quality data necessary to support research or decision making is significantly lacking as indicated by lack of nitrate data from seven counties in the 2000s.
As an example, in the Rolling Plains region, nitrate concentration data was available for about 2,400 and 1,800 wells in the 1960s and the 1970s, respectively, but that dropped to 422 and 213 wells in the 1990s and the 2000s, respectively.
Similarly, in the 1970s, about 440 and 480 groundwater wells were monitored for water quality in Haskell and Knox counties, which recorded the highest nitrate concentrations in the state, respectively, as compared to only 19 and 60 wells, respectively, in the 2000s.
This indicates a significant reduction in the intensity of nitrate monitoring in recent years, Ale said.
With the importance of groundwater as a major water source continuing to increase in Texas, more frequent and spatially intensive groundwater quality monitoring and more critical review of the groundwater resources in different parts of the state will be necessary, he said.