Montana Department of Agriculture Groundwater Protection Program

Policy of Montana: (1)protect ground water and the environment from impairment or degradation due to the use of agricultural chemicals (2)allow for the proper and correct use of agricultural chemicals (3)provide for the management of agricultural chemicals to prevent, minimize, and mitigate their presence in ground water (4)provide for education and training of agricultural chemical applicators and the general public on ground water protection, agricultural chemical use, and the use of alternative agricultural chemicals

41 wells sampled regularly Expected background nitrate 2 ppm Most wells are located in agricultural land use Irrigated and non-irrigated Various crops

Sample Schedule: Samples collected twice a year when possible in spring and summer SOP: Well casing volumes purged three times Field pH, specific conductance, dissolved oxygen, and temperature Handling: Samples stored in labeled amber glass bottles at 4°C for ≤10 days Analysis: MDA Analytical Laboratory Bureau ion chromatography or electrode Nitrate measured as Nitrate-N

21 wells sampled every year Dataset assessed for outliers based on standardized residual values PMWs with median GW nitrate >2ppm assessed for identifiable trends Concentrations assessed in context of: Accepted background nitrate level of 2 ppm (Mueller and Helsel, 1996) Montana numerical standard for drinking water of 10 ppm NO 3 - -N (MT DEQ, 2012)

243 total PMW GW samples Statewide median 2.9 ppm nitrate in PMW GW Maximum 150 ppm nitrate in PMW GW Most GW nitrate concentrations were in background level range of ≤2 ppm

-N

Regression analysis with trend lines illustrating NO 3 - -N concentration changes in PMWs MAD-1, SHE-1, T-1, and ROS-1 over time ( ). An alpha level of 0.05 was used to determine a significant response in NO 3 - -N with time.

Upward trend Downward trend Dryland small grains with nitrogen management plan Irrigated small grains and potatoes Irrigated small grains and potatoes Dryland small grains and pulse

Three primary factors over longer time intervals: (1) Amount of source nitrogen available (2) Water infiltration and percolation through surface and subsurface materials (3) Potential for NO 3 - reduction and/or denitrification Land use: summer fallow practices with associated mineralization of organic matter and excessive irrigation on well-drained soils

Most of the MDA PMW network has consistent accepted background levels of nitrate (≤2 ppm) Statewide MDA PMW median nitrate (2.9 ppm) indicates influence of human activities Elevated levels of PMW nitrate may be linked to land use practices in small grains

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