The LUBGWMA Well Network Groundwater Nitrate Trend Analysis summarizes groundwater nitrate concentrations and trends from a network of 33 wells within the Lower Umatilla Basin Groundwater Management Area (LUBGWMA) sampled over a 32-year monitoring period. The previous trend analysis was conducted in 2012.
This analysis is the first step in the groundwater quality trends objectives identified in the Oregon Nitrate Reduction Plan. The next step will be to evaluate trends using data from permitted sites and develop a well network capable of monitoring the entire LUBGWMA.
Trend Analysis Results
Generally increasing nitrate concentrations with some decreases observed.
Over the years, the average, median and maximum concentrations have all increased across the 33-well network. Additionally, there are more numerous and steeper increasing trends than decreasing trends at individual wells. Finally, the network-wide trend is slightly increasing.
These results are unsurprising since there are still sources of nitrogen in the LUBGWMA.
Current nitrate leaching is likely.
The continued increase of nitrate concentrations across the network indicates current nitrate leaching is likely. Nitrogen loading from land application can persist and leach into groundwater for decades and accumulate to high levels.
No evident geographic pattern to nitrate concentrations or trends.
While the highest concentrations and steepest trends occur in the western part of the well network, high concentrations are found throughout the area. The geographic extent of the 33-well network limits its usefulness as a predictor in other parts of the LUBGWMA
Changes in nitrate concentration suggest the possibility of changes in land use.
Steep trends, single abrupt changes, or multiple changes in nitrate suggest the possibility of changes in land use and indicate the potential for groundwater quality improvements based on changes in surface activities.
Nitrate was typically higher in Spring than in Fall.
About half of the wells exhibit nitrate seasonality. However, not all wells showed the same timing.
Weak negative correlation between nitrate concentration and depth.
There is a weak negative correlation between nitrate concentration and depth. This correlation may not be observable with the existing well network.
Contact
Antony Sparrow, Department of Environmental Quality
antony.sparrow@deq.oregon.gov