Environmental Consulting Firm Performs Regulatory Compliance
Approximately 2,000 gallons of urea ammonium nitrate (UAN) solution were released from a leaking pipeline at a bulk handling terminal. Immediately after the release, most of the contaminated soil was excavated. However, some of the contaminated soil was inaccessible because of the pipeline and other structures. In response to an enforcement action by the state regulatory agency, a new non-corrosive pipeline was installed and pipeline inspection procedures were modified to meet environmental regulatory compliance standards. Monitoring wells were also installed to evaluate the levels and migration of residual contamination.
The primary contaminant of concern was nitrate. The state was also concerned with urea and ammonium because they convert to nitrate via ammonification and nitrification, respectively. Monitoring documented a rapid decline in concentrations of all three constituents in the release area. Analytical modeling determined that nitrate levels were unlikely to exceed the drinking water standard (10 mg/L) at the site boundary.
In UAN solution, nitrate-N comprises 25% of the total N, ammonium-N comprises 25%, and organic-N (urea) comprises 50%. Figure 1 shows how cumulative percentages of the three constituents varied with time in groundwater samples collected just downgradient from the release zone.
Initially, nitrate-N comprised about 25% of the total-N, indicating that there had been little conversion of ammonium to nitrate via nitrification. Organic-N comprised about 30%, which is less than occurs in UAN product. Ammonium-N comprised about 45% of the total-N, which is consistent with the fact that some of the urea had already been converted via ammonification. As sampling continued, the proportions of organic-N and ammonium-N decreased until nitrate-N comprised approximately 90% of the total-N about two years after groundwater sampling began.
Groundwater Sampling Reveals Potential Nitrification
Both nitrification and ammonification occur primarily in oxidizing conditions such as those in unsaturated soil. Being an anion, nitrate-N is not subject to adsorption. Therefore, in the absence of nitrification, the proportion of nitrate-N would be expected to decrease with time as the nitrate-N is transported away more rapidly than the other compounds. The fact that the proportion of nitrate-N increased in groundwater beneath the release zone provides strong evidence for nitrification.
The site is located near a large river and is underlain by about 90 feet of sand, with discontinuous silt layers. The direction of groundwater flow is influenced by the river stage and several pumping centers in the valley. It was necessary to determine the extent to which nitrate concentrations varied with the groundwater flow direction. Figure 2 includes two rose diagrams that show the range of flow directions based on 68 sets of water-level measurements in two sets of monitoring wells. The rose diagrams show the number of measurements within 15o ranges of compass bearings. A flow direction was calculated for each set of measurements using a simple trigonometric algorithm. Nitrate concentrations did not show a close correlation to the groundwater flow direction, or to other potential controls such as precipitation and water-table stage.