Occurrence of Nitrate in the Nebraska's Groundwater System: Identifying Factors, Examining the Best Management Practices, and Analyzing Costs

Abstract

Nebraska, an agriculturally intensive mid-western state of the United States (U.S.), has a large number of wells with nitrate concentrations above the drinking water standard (10 mg NO₃-N/L). Large amounts of nitrogen fertilizers and irrigation water are applied in Nebraska to increase and maintain agricultural production. Consequently, nitrate contamination in Nebraska’s groundwater continues to occur from leaching through the vadose zone. Understanding the occurrence of nitrate in the Nebraska’s groundwater system and the protection of groundwater from contamination is among the challenges in the conservation of Nebraska’s drinking water resources.

The objectives of this research were to identify the influential factors associated with groundwater nitrate concentrations in Nebraska using the Classification and Regression Tree (CART) model, examine Best Management Practices (BMPs) and climatic scenarios affecting soil water and nitrate-N transport in the deep vadose zone using the Root Zone Water Quality Model (RZWQM2), and assess costs of nitrate contamination in Nebraska’s domestic wells using the Multiple Logistic Regression Model (MLRM) and Life Cycle Costing (LCC).

The results of the CART model indicated that groundwater nitrate concentrations correspond with vadose zone thickness, hydraulic conductivity, and saturated thickness in the west. In eastern Nebraska, concentrations are correlated with average percent sand in the topsoil (0-150 cm), well depth, and hydraulic conductivity.

Using RZWQM2 to examine the BMPs and climatic scenarios, the results indicated that the conversion of gravity to sprinkler irrigation can reduce nitrate-N losses to groundwater. Rotating corn and soybean is an efficient management practice to lower nitrate-N concentrations in the deep vadose zone, compared with continuous corn. Transitioning from dryland production to irrigation can lead to increased nitrate-N concentrations. Splitting nitrogen fertilizer application can reduce nitrate-N losses to groundwater. Nitrate-N transport in the deep vadose zone is higher at the irrigated site than non-irrigated site under the future climatic scenarios.

The results of assessing costs of nitrate contamination in domestic wells indicated that a reverse osmosis Point-of-Use (POU) treatment is the option with the lowest costs for a household, but the biological denitrification Point-of-Entry (POE) treatment system is most likely to be suitable with lower treatment costs for a community.