FACTORS INFLUENCING NITRATE DISTRIBUTION IN GROUNDWATER OF THE COLUMBIA BASIN GROUND WATER MANAGEMENT AREA

The Columbia Basin Groundwater Managament Area (GWMA) was formed in 1998 in response to concerns over elevated nitrate levels in groundwater in Franklin, Adams, Grant, and (since 2005) Lincoln counties. Aquifers underlying the GWMA are hosted in the Columbia River Basalts and the overlying sediments, the latter being primarily used for domestic water supply. Since 1998, biennial monitoring of several hundred wells in the basalt and sediment aquifers has been performed to establish baseline nitrate concentrations across the GWMA. Monthly sampling of selected shallow sediment aquifer wells was also performed in 2006-2007 to characterize seasonal and other factors influencing nitrate concentrations in the shallow system.

Data from the biennial sampling was analyzed to assess spatial distributions of nitrate concentrations, and data from the 2006-2007 sampling was used to more closely evaluate the influence of factors such as canal recharge, lithology, and well depth, as well as to provide a baseline for future assessments of the effectiveness of Best Management Practices. The monthly sampling program targeted two geographic areas characterized by relatively high rural population density and abundant irrigated agriculture – the Pasco Basin in Franklin County and the Quincy Basin in Grant County. Seventy domestic wells with known lithology and construction were monitored in each basin.

The median nitrate concentration for the overall GWMA sampling (1998-2006) was ~4 mg/L. Median nitrate concentrations from the monthly sampling program were 9.9 mg/L in the Pasco Basin, and 4.4 mg/L in the Quincy Basin. In the Pasco Basin, 69% of the wells exceeded the federal maximum contaminant level (MCL) of 10 mg/L as N, compared to 14% in the Quincy Basin. In both areas, shallower wells and wells completed in finer-grained formations were more likely to have elevated nitrate concentrations. Seasonal variability in nitrate concentration was observed for the Quincy Basin, indicating the importance of local dilution by canal recharge during the irrigation season. In the Pasco Basin, nitrate levels appear to be lower in areas characterized by higher hydraulic gradients, indicating higher groundwater through-flow results in greater dilution of nitrate.